EPA-600/8-81-017

                               August 1981
          LIMESTONE FGD SCRUBBERS:

               USER'S HANDBOOK
                     by

        D. S. Henzel and 6. A. Laseke
          PEDCo Environmental, Inc.
           Cincinnati, Ohio  45246
                   x  and

        E. 0. Smith and 0. 0. Swenson
     Black & Veatch Consulting Engineers
        Kansas City, Missouri  64114
           Contract No. 68-02-3173
                 Task No. 13
                Project Officer

              Robert H. Borgwardt
     Emissions/Effluent Technology Branch
Utilities and Industrial Processes Division
 Industrial  Environmental Research Laboratory
      Research Triangle Park, N.C.  27711

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                                TECHNICAL REPORT DATA
                         (Please read Instructions on the reverse before completing)
1. REPORT NO.
 EPA-600/8-81-017
                                                      3. RECIPIENT'S ACCESSION-NO.
                                       PB82   1 Q 6 2 1  9
4. TITLE AND SUBTITLE
Limestone FGD Scrubbers: User's Handbook
                                 5. REPORT DATE
                                 August 1981
                                                      6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
David S. Henzel and Bernard Laseke*
                                                      8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
 PEDCo Environmental, Inc.
 11499 Chester Road
 Cincinnati, Ohio  45246
                                  10. PROGRAM ELEMENT NO.
                                  CAAN1D
                                  11. CONTRACT/GRANT NO.

                                  68-02-3173, Task 13
12. SPONSORING AGENCY NAME AND ADDRESS
 EPA, Office of Research and Development
 Industrial Environmental Research Laboratory
 Research Triangle Park, NC 27711
                                  13. TYPE OF REPORT AND PERIOD COVERED
                                  Handbook; 6/80-2/81	
                                  14. SPONSORING AGENCY CODE
                                   EPA/600/13
is. SUPPLEMENTARY NOTES JERL-RTP project officer is Robert H.  Borgwardt, Mail Drop
62, 919/541-2336. (*) Authors included E.G.Smith and D.O.Swenson, Black and
Veatch, Kansas City. MO 64114.   	
IB. ABSTRACT Tne handbook, intended for use of utility project managers and project
engineers, provides guidance in selection,  installation, and operation of a limestone
FGD system, covering all phases from inception of the project through design, pro-
curement, operation, and maintenance of the system. It gives detailed accounts of
utility experience with operational limestone scrubbing systems.  It also deals exten-
sively with optional process features and with recent innovative process modifica-
tions that enhance the efficiency of the system.  Among the many available processes
for desulfurization of flue gas from utility boilers, the limestone wet scrubbing pro-
cess is widely used and is being enhanced by numerous technological advances.
17.
                             KEY WORDS AND DOCUMENT ANALYSIS
                DESCRIPTORS
                                          b.IDENTIFIERS/OPEN ENDED TERMS
                                              c. COSATI Field/Group
 Pollution
 Flue Gases
 Desulfurization
 Limestone
 Scrubbers
 Coal
 Engineering Costs
Boilers
Utilities
Design
Procurement
Operations
Maintenance
Pollution Control
Stationary Sources
13 B        13A
21B
07A,07D   14G
08G    15E,14A
131
21D
15A
18. DISTRIBUTION STATEMENT
 Release to Public
                                          19. SECURITY CLASS (This Report)
                                          Unclassified
                                                                   21. NO. OF PAGES
                                                  527
                      20. SECURITY CLASS (This page)
                      Unclassified
                                              22. PRICE
EPA Form 2220-1 (9-73)

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                             ABSTRACT
   Among the many available processes for desulfurization of flue gas
from utility boilers, the limestone wet scrubbing process is widely used
and is being enhanced by numerous technological advances. The "Lime-
stone FGD Scrubbers: User's Handbook" is intended for use by utility
project managers and project engineers. It provides guidance in selection,
installation, and operation of a limestone FGD system, covering all phases
from inception of the project through design, procurement, operation, and
maintenance of the system.  The Handbook gives detailed accounts of
utility experience with operational limestone scrubbing systems. It also
deals extensively with optional process features and with recent innovative
process modifications that enhance the efficiency of the system.
                                 ii

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                                  CONTENTS
List of Figures                                                         v
List of Tables                                                          x
Metric Conversions                                             '       xiv
Acknowledgement                                                        xv

1.   Introduction                                                     1-1

          Purpose, scope, and structure of the handbook               1-1
          Overview of the limestone scrubbing process                 1-9
          Process chemistry and operational factors                   1-23

2.   Overall System Design                                            2-1

          Powerplant considerations                                   2-1
          Design basis                                                2-16
          Material and energy balances                                2-19
          System configuration options                                2-22
          Computerized design guides                                  2-31

3.   The FGD System                                                   3-1

          Scrubbers                                                   3-1
          Mist eliminators                                            3-22
          Reheaters                                                   3-44
          Fans                                                        3-68
          Thickeners and dewatering equipment                         3-77
          Sludge treatment                                            3-96
          Limestone slurry preparation                                3-104
          Pumps                                                       3-112
          Piping, valves, and spray nozzles                           3-127
          Ducts, expansion joints, and dampers                        3-132
          Tanks                                                       3-142
          Agitators                                                   3-145
          Materials of construction                                   3-146
          Process control and instrumentation                         3-154

4.   Procurement of the FGD System                                    4-1

          Competitive bidding                                         4-1
          The purchase specification                                  4-3
          Procurement planning                                        4-3
          Preparation of specifications                               4-9

                                     iii

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                            CONTENTS (continued)
          Evaluation of proposals
          Installation, startup, and testing

5.   Operation and Maintenance

          Standard operations
          Initial operations
          Startup, shutdown, standby, and outage
          System upsets
          Operating staff and training
          Preventive maintenance programs
          Unscheduled maintenance
          Maintenance staff requirements

Appendix A     Chemistry of Limestone Scrubbing

Appendix B     Operational Factors

Appendix C     Computer Programs

Appendix 0     Innovations in Limestone Scrubbing

Appendix E     Material and Energy Balances

Appendix F     Limestone Utility FGD Systems in the United States

Appendix G     Materials of Construction
4-29
4-36

5-1

5-2
5-13
5-14
5-16
5-17
5-19
5-24
5-31

A-l

B-l

C-l

D-l

E-l

F-l

G-l
                                      iv

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                                   FIGURES
Number                                                                 Page
 1-1      Some Alternative FGD Processes Available for Commercial
            Application                                                1-3
 1-2      FGD Project Coordination Sequence                            1-4
 1-3      Limestone FGD Decision Sequence                              1-6
 1-4      Limestone FGD Process:  Basic Process Flow Diagram           1-10
 1-5      Limestone FGD Process:  External Presaturator                1-13
 1-6      Limestone FGD Process:  Venturi Scrubber                     1-14
 1-7      Limestone FGD Process:  Venturi Spray-Tower Scrubber with
            a Charged Particulate Separator                            1-16
 1-8      Limestone FGD Process:  EPA Two-Loop Scrubbing               1-17
 1-9      Limestone FGD Process:  RC Double Loop® Scrubbing            1-18
 1-10     Limestone FGD Process:  Hydrocyclone Used to Provide Mist
            Elimination Wash Liquor                                    1-20
 1-11     Limestone FGD Process:  Forced Oxidation of Scrubber Hold
            Tank in a Single-Loop System                               1-21
 1-12     Limestone FGD Process:  Chemical Additives                   1-22
 2-1      Elements of Overall System Design                            2-2
 2-2      Influence of Sulfur in Coal on Required S02 Removal
            Efficiency                                                 2-13
 2-3      Overall Inputs and Outputs                                   2-21
 3-.1      Basic Venturi Scrubber and Design Configurations             3-7
 3-2      Basic Spray Tower Designs                                    3-10
 3-3      Sieve Tray Scrubber and Detail of Tray                       3-11
 3-4      Packed Tower and Packing Types                               3-13
                                      v

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                             FIGURES (continued)

Number                                                                 Page
 3-5      Baffle-Type Mist Eliminator Designs                          3-26
 3-6      Relationship of Penetration and Gas Velocity                 3-28
 3-7      Effect of Gas Velocity and Liquid Loading on Performance
            of Baffle-Type Mist Eliminators                            3-30
 3-8      Horizontal and Vertical Mist Eliminator Configurations       3-31
 3-9      Schematic of One-Stage Mist Eliminators with Two-, Three-,
            and Six-Pass Arrangements                                  3-33
 3-10     Mist Eliminator Vane Angle Configurations                    3-35
 3-11     Original Mist Eliminator Configuration at Shawnee Test
            Facility                                                   3-38
 3-12     In-Line Reheat System                                        3-46
 3-13     Direct Combustion Reheat Systems                             3-48
 3-14     Indirect Hot Air Reheat System                               3-49
 3-15     Waste Heat Recovery Reheat System                            3-51
 3-16     Exit Gas Recirculation Reheat System                         3-53
 3-17     "Hot-Side" Flue Gas Bypass Reheat                            3-54
 3-18     "Cold-Side" Flue Gas                                         3-55
 3-19     Schematic of Heat Balance Around Downstream System with
            Indirect Hot-Air Reheater                                  3-57
 3-20     Schematic of Heat Balance Around Downstream System with
            In-Line Reheater                                           3-60
 3-21     Effect of Reheat Increment on Ground-Level  S02 Concentra-
            tion                                                       3-63
 3-22     Relative Improvement in Ground-Level S02 Concentration
            as a Function of Degree of Reheat                          3-65
 3-23     Typical Dry ID Fan Applications                              3-70
 3-24     Wet ID Fan Installation                                      3-71
 3-25     Relationship Between Sludge Volume and Solids Content        3-79
                                      vi

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                             FIGURES (continued)

Number                                                                 Page
 3-26     Cross-Section of a Conventional  Gravity Thickener            3-81
 3-27     The Lamella® Gravity,Settler Thickener                       3-83
 3-28     Cutaway View of a Rotary Drum Vacuum Filter                  3-84
 3-29     Cross-Section of Solid-Bowl Centrifuge                       3-86
 3-30     Hydrocyclone Cross-Section                                   3-88
 3-31     FGD Sludge Disposal Alternatives                             3-97
 3-32     Sludge Treatment Flow Diagram                                3-99
 3-33     Typical Closed Circuit Grinding System                       3-105
 3-34     Limestone Slurry Feed Arrangements                           3-107
 3-35     Erosion/Corrosion as a Function of Location within a
            Limestone FGD System                                       3-152
 3-36     Limestone Feed Control Loop                                  3-158
 3-37     Slurry Solids Control Loop                                   3-159
 3-38     Makeup Water Control                                         3-161
 3-39     Bypass/Reheat Control Loop                                   3-162
 4-1      FGD System Procurement Sequence                              4-2
 4-2      Primary Components of Purchase Specifications                4-4
 4-3      Bidding Requirements for Proposal Preparation                4-11
 4-4      Technical Specifications Consisting of General, Equipment,
            and Erection Requirements                                  4-13
 4-5      Checklist for Preparation of System Design Basis to
            Bidders                                                    4-15
 4-6      Checklist for Specification of Guarantees Required from
            Prospective Bidders                                   "    4-18
                           p
 A-l      Effect of pH and  S02 on S02 Absorption                      A-8
 A-2      Effect of Total Dissolved Sulfite-Concentration on Mass
            Transfer Enhancement at pH 4.5, 55°C, with 1000 ppm
            S02, 0.3 Molar Ionic Strength                              A-11
                                      vii

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                             FIGURES (continued)


Number                                                                 Page

 A-3      Dissolved Alkalinity Generated by Addition of MgO            A-12

 A-4      Dissolved Alkalinity Generated by Addition of Na2C03         A-13

 A-5      Effect of Organic Acids on the Enhancement Factor,  pH 5,
            0.3 M CaCl2, 55°C, 1000 ppm S02, 3 mM Total Sulfite        A-16

 A-6      Effect of Scrubber Inlet pH and Adipic Acid Concentration
            on S02 Removal                                             A-18

 A-7      Dissolution Rate of CaC03 in 0.1M CaCl2, 25°C                A-24

 A-8      Precipitation Mechanism and Rate as a Function of
            Relative Saturation                                        A-29

 B-l      Effect of Gas and Liquid Flow Rates on Gas Pressure Drop
            in a Packed-Bed Scrubber                                   B-10

 B-2      Total Energy Consumption by Typical Limestone FGD Systems    B-22

 C-l      Growth in the Number of FGD Systems Which are Planned,
            Under Construction and Operating                           C-27

 C-2      Computerized Data Base Structure, FGDIS Program              C-29

 D-l      Jet Bubbling Reactor                                         D-2

 D-2      Cocurrent Scrubber Used for Forced Oxidation Tests with
            a Single Effluent Hold Tank                                D-5

 D-3   .   Cocurrent Scrubber Used for Forced Oxidation Tests with
            Multiple Effluent Hold Tanks                               D-6

 D-4      Typical Flow Diagram of the Dowa Process                     D-8

 D-5      Packed Mobile-Bed Scrubber Used for Forced Oxidation
            Tests                                       .               D-12

 D-6      Construction of an FGD Gypsum Stack                          D-15

 E-l      Model Example Limestone FGD System or 500-MW Plant,
          3.7 Percent Sulfur Coal                                      E-2

 E-2      Schematics of Basis for Material Balance Calculations        E-8

 E-3      Psychrometric Chart                                          E-13

 E-4      Material Balance Around Thickener and Solids Dewatering
            System                                                     E-19
                                     viii

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                             FIGURES (continued)

Number                                                                 Page
 E-5      Material Balance Around Scrubbers                            E-20
 E-6      Overall Water Balance                                        E-24
 E-7      Fan Energy Requirement                                       E-27
 E-8      Recirculation Pump Energy Requirement                        E-28
 E-9      Model Limestone FGD System on 500-MW Plant,  0.7 Percent
            Sulfur Coal                                                E-29
 E-10     Material Balance, Required Thickener, and Dewatering
            Device Solids Dewatering System                            E-38
 E-ll     Material Balance Around Scrubbing Modules                    E-39
 E-12     Overall Water Balance                                        E-42
 G-l      Effects of pH and Chlorides on Stainless Steel Alloys        G-4
 G-2      Effect of Molybdenum Content on Resistance to Pitting and
            Crevice Corrosion                                          G-ll
 G-3      Effect of Molybdenum and Chromium Content on Corrosion
            Resistance (Inco Tests)                                    G-12
 G-4      Effect of Molybdenum and Chromium Content on Corrosion
            Resistance (Shawnee Tests)                                 G-12
 G-5      Weighted Price Ratio and Corrosion Resistance                G-14
                                      IX

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                                   TABLES

Number                                                                 Page
 2-1      Major Powerplant Factors that Influence FGD System Design    2-4
 2-2      Fuel Properties of Four Representative Coals                 2-5
 2-3      Combustion Characteristics for a 500-MW Case                 2-6
 2-4      Site Evaluation Factors for a 500-MW Case                    2-10
 2-5      NSPS and Emission Regulations Applicable to a Coal of
            Specified Properties                                       2-14
 2-6      Typical Makeup Water Analyses                                2-20
 3-1      Basic Scrubber Types for Commercial  Limestone FGD Systems    3-4
 3-2      Numbers and Capacities of Limestone  Scrubber Types           3-5
 3-3      Limestone Scrubber Design L/G Ratios                         3-16
 3-4      Limestone Scrubber Design Gas Velocities                     3-17
 3-5      Limestone Scrubber Design Gas-Side Pressure Drops            3-18
 3-6      Scrubber Design and Operating Characteristics for Oper-
            ational Limestone FGD Systems                              3-21
 3-7      Basic Mist Eliminator Types                                  3-24
 3-8      Mist Eliminator Wash Protocols at Shawnee Test Facility      3-40
 3-9      Mist Eliminator Design and Operating Characteristics for
            Operational Limestone FGD Systems                           3-42
 3-10     Basic Reheater Types                                         3-45
 3-11     Examples of Operational Stack Gas Reheaters                  3-67
 3-12     Limestone Slurry Preparation at Operational Facilities       3-111
 3-13     Specifications of Recirculation Pumps in Operational
            Limestone FGD Systems                                      3-123

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                             TABLES (continued)

Number                                                                 Page
 3-14     Typical  Ductwork Applications and Materials  of Construction  3-135
 3-15     Basic Damper Types                                           3-138
 3-16     Basic Types of Construction Materials                         3-147
 3-17     Organic  Linings:   Base Materials and Resistance to Some
            Environments                                               3-149
 3-18     Major Variables in a Limestone FGD Control  System            3-155
 3-19     Some Manufacturers of Extractive S02 Monitors Based on
            Various Operations Principles                              3-169
 3-20     Current  EPA Performance Specifications for Continuous
            Monitoring Systems and Equipment                           3-171
 4-1      Specification Guidelines:  Scrubbers                         4-20
 4-2      Specification Guidelines:  Limestone Receiving and
            Conveying Equipment                                        4-23
 4-3      Specification Guidelines:  Limestone Feedbins                4-23
 4-4      Specification Guidelines:  Limestone Feeder                  4-25
 4-5      Specification Guidelines:  Limestone Ball Mill               4-25
 4-6      Technical Data Summary Sheet                                 4-31
 4-7      Commercial Data Summary Sheet                                4-33
 5-1      Methods  of Improving pH Sensor Reliability                   5-9
 5-2      Preferred FGD System Instrumentation                         5-29
 5-3      Operational Troubleshooting Checklist                        5-30
 A-l      Typical  Solution Compositions                                A-4
 A-2      Concentrations of Buffer Additives Required to Achieve
   .         Enhancement Factor of 20                                   A-15
 C-l      Required Input Parameters, TVA/Bechtel Program               C-5
 C-2      Basis for Major Variables, TVA/Bechtel Program               C-7
 C-3      Model Options, TVA/Bechtel Program                           C-8
                                      xi

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                             TABLES (continued)

Number                                                                 Page
 C-4      Input Data Report                                            C-9
 C-5      Process Parameter Reports                                    C-ll
 C-6      Pond Size and Cost Parameters                                C-17
 C-7      Equipment Sizes and Cost Report                 -             C-18
 C-8      Capital Investment Report                                    C-22
 C-9      First Year Annual Revenue Requirement Report                 C-23
 C-10     Lifetime Annual Revenue Requirement Report                   C-24
 C-ll     Major Data Fields, FGDIS Program                             C-30
 D-l      Process Design Conditions for Limestone Systems              D-19
 D-2      Capital Investments for Limestone Systems                    D-20
 D-3      Annual Revenue Requirements for Limestone Systems            D-21
 E-l      Design Premises:   High-Sulfur Coal Case                      E-4
 E-2      Typical Pressure Drop Data                                   E-5
 E-3      Limestone Analysis                                           E-5
 E-4      Composition of Available Limestone for S02 Absorption        E-10
 E-5      Composition of Cleaned Flue Gas (Stream 3)                   E-15
 E-6      Waste Sludge Solids for High-Sulfur Coal                     E-17
 E-7      Energy Requirement Calculations                              E-25
 E-8      Design Premises:   Low-Sulfur Coal                            E-32
 E-9      Waste Sludge Solids for Low-Sulfur Coal Case                 E-37
 E-10     Energy Demand for FGD on a 500-MW Plant                      E-44
 F-l      Wet Limestone Utility FGD Systems in the United States  -     F-2
 G-l      Nominal Chemical  Composition of Base Metals                  G-2
 G-2      Service Application Guidelines Based on Stellite Test
            Results                                                    G-5
                                     xii

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                             TABLES (continued)

Number                                                                 Page
 G-3      Operating Conditions During Corrosion  Tests  at the
            Shawnee Test Facility                                      G-7
 G-4      Data From Corrosion Tests at the Shawnee Test Facility       G-8
 G-5      Prices of High-Grade Alloys and Stainless Steels              G-13
 G-6      Typical Characteristics of Resins                            G-16
 G-7      Characteristics of Black Natural Rubber and  Neoprene
            Rubber                                                     G-17
 G-8      Coating Application Techniques                               G-19
 G-9      Causes of Organic Lining Failures and  Procedures
            Recommended to Prevent Failure                             G-21
                                     xm

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                             METRIC CONVERSIONS
     This handbook expresses measurements  in  English units so that informa-

tion is clear  to  intended readers in the United States.   The following list

provides factors for conversion to metric units.
     To convert from

     Btu
     Btu/lb
     cfm
     °F
     ft
     ft/h
     ft/s
     ft2
     ft2/ton per day
     ft3
     ft3
     gal
     gal/ft3
     gal/min
     gal/min per ft2
     gr
     gr/scf
     hp (mechanical)
     hp (boiler)
         H20
         H2
in
in
in
in.2
in.3
Ib
Ib
lb/106 Btu
lb/ft3
Ib/gal
lb/in.2
lb-ft/s-ft2
Ib-mol
Ib-mol/h
Ib-mol/h per ft2
Ib-mol/min
oz
scfm (at 60°F)
ton
                                To
kWh
kJ/kg
m3/h
°C
m
m/h
m/s
m2
m2/Mg per day
liters
m3
liter
liter/m3
liter/min
liter/min per m2
9   0
g/Nm3
kW
kW
cm
kPa
mm Hg
m2
m3
g
kg
g/kJ
kg/m3
kg/m3
kPa
Pa-s
g-mol
g-mol/min
g-mol/min per m2
g-mol/s
kg
Nm3/h (at 0°C)
kg
Multiply by

   0.0002931
   2.326
   1.70
 (°F -32)/1.8
   0.305
   0.305
   0.305
   0.0929
   0.102
  28.32
   0.02832
   3.785
   0.134
   3.79
  40.8
   0.0648
   2.29
   0.7457
   9.803
   2.54
   0.2488
   1.87
   0.0006452
   0.00001639
 453.6
   0.4536
 429.9
  16.02
 119.8
   6.895
  47.89
 453.6
                                                          7.56
                                                         81.4
                                                          7.
                                                          0.
                                                          1.
     56
     02835
     61
                                                        907.2
                                     xiv

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                               ACKNOWLEDGEMENT
     This handbook was  prepared  under the sponsorship of the EPA Industrial
Environmental Research Laboratory at Research Triangle Park, North Carolina.
The Project  Officer  who provided overall guidance and  coordination  was Mr.
Robert H. Borgwardt.   The prime contractor was PEDCo Environmental, Inc., in
Cincinnati, Ohio.  The subcontractor was Black & Veatch Consulting Engineers
in Kansas City, Missouri.  The PEDCo Project Director was Mr. William Kemner
and the  PEDCo Project  Manager was Mr.  David Henzel.   The  Senior Technical
Reviewer for  PEDCo was  Mr.  Bernard Laseke.   The Project Manager for Black &
Veatch was  Mr.  Earl Smith,  and  Mr.  Donald  Swenson was  Managing  Project
Engineer.

     A central  element  of the  project  was  a  Review Panel,  who provided
comment  and  guidance  as to  content and emphasis.   In addition to  those
mentioned above,  the members of the Review Panel were:

                    Dr.  Gary Rochelle
                    University of Texas  at Austin

                    Dr.  Norman Ostroff
                    Peabody Process Systems

                    Dr.  Nicholas Stevens
                    Research Cottrell, Inc.

                    Mr.  Philip Rader
                    Combustion Engineering, Inc.

                    Mr.  Earl Smith
                    Black & Veatch Consulting Engineers

In addition  to his  role as a  reviewer,  Dr.  Rochelle was the primary author
of Appendix A, Chemistry of Limestone Scrubbing.

     Other authors are  Messrs.  Henzel,  Laseke, and  Avi  Patkar  of PEDCo and
Messrs. Swenson and John Noland of Black & Veatch.
                                      xv

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                                  SECTION 1
                                INTRODUCTION

PURPOSE, SCOPE, AND STRUCTURE OF THE HANDBOOK
     Since the early  attempts  to control emissions of sulfur  dioxide  (S02)
in the  flue  gas  of power plants, there has been a pronounced preference for
limestone wet scrubbing systems.  About 63 percent of the flue gas desulfur-
ization  (FGD)  systems  that  are  now  operational,  under construction,  or
planned  for  the next 5 years  in the utility industry use limestone as the
absorption reagent (Laseke, Devitt, and Kaplan 1979).
     The  widespread  acceptance of  limestone  systems  is   due   to  several
factors.  Detailed cost studies by TVA (McGlamery et al.  1980) indicate that
both the  capital  and  the  annual operating costs  are  competitive  with  those
of other  FGD  systems  designed for high-sulfur coal applications.   Addition-
ally, the on-line experience of full-scale limestone units at utility plants
has generated  a  wealth  of operational  data, which are being  used to enhance
system  reliability.   Advances  in sludge disposal technology, such as forced
oxidation of the sludge, have enabled utility operators to reduce the volume
of sludge to be disposed of and to improve its handling and disposal proper-
ties.  With continuing technological advances and increasingly wide utiliza-
tion, limestone  scrubbers  are  a major means of  compliance with  regulations
promulgated by the U.S.  Environmental Protection Agency (EPA) for control of
S02 emissions from power plants.
     The  performance  of early  commercial  systems often  did  not  reach full
potential because  design  and  operational experience was limited.   Ten  years
have passed  since  the first full-scale systems went into operation, and the
experience over  that  period has generated much  information.    Our intent is
to Ibring together in one volume—clearly and  concisely—the best guidance
available for achieving optimum performance from limestone scrubber systems.
                                     1-1

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INTRODUCTION: Purpose and Scope	1-2
     Since 1972,  an  intensive research and development program sponsored by
the  EPA  has been  under  way in prototype scrubbers at  the  Tennessee Valley
Authority's  Shawnee  Station.   That  program has produced a  large  amount of
accurate  data  on  scrubber performance,  design,  operating  parameters,  and
reliability.  In this volume the Shawnee work is drawn on, together with the
full-scale commercial experience, for information regarding best engineering
practice.
Purpose
     The  information  presented here provides guidance to those  involved in
selecting, installing, and  operating a limestone FGD system.  For  the util-
ity Project Manager, it provides the background needed to select a  limestone
scrubber,  develop  the   configuration  most  appropriate  for  the site,  and
prepare  specifications   for procurement  of system  hardware and  services.
Additionally, the  handbook gives information on  installing,  testing,  oper-
ating, and maintaining the system.
     Figure  1-1  shows   the principal  alternative  FGD  processes.   It  is
assumed throughout the remainder of this handbook that the process selected
from among those  shown  in Figure 1-1 is the limestone wet scrubbing process
that generates wet  sludge as  a "throwaway" product or  gypsum as a recover-
able byproduct.
     The emphasis throughout is on practical applications.  For example,  the
discussion of system design and performance provides the kind of information
routinely  requested  by  regulating  agencies  in permit  applications.   This
information can also be applied in evaluating preliminary studies and recom-
mendations of  a consulting architectural/engineering  (A/E)  firm.   Further,
it can be used in developing detailed equipment specifications and  assessing
the performance predictions of various scrubber suppliers.
Scope
     The scope of the handbook is the entire limestone FGD system,  including
the procedures and processes  involved in selecting and operating-a success-
ful  system.   Figure  1-2  illustrates a project  coordination sequence,  from
the  initial  system concept  through the purchase of equipment, installation,
startup, operation, and  maintenance of the limestone scrubbing system.   The

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                 PROCESSES WITH A THROHAHAY PRODUCT
                                         SLUDGE OR
                                        FILTER CAKE
Until ONI

SODIUM
CARBONATE
(TRONA)
fc LIQUID
HASTE

DUAL-
ALKALI
(SODIUM/LIME)


SPRAY DRYER
(SODIUM CARBONATE,
TRONA, OR LIME)
                                         FILTER CAKE
                                        '    HASTE

                                           DRY SOLID
                                        *"  HASTE
                   PROCESSES HITH GYPSUM BYPRODUCTS
                    LIMESTONE
                     CHIYODA
                     T-121
                  (JET BUBBLER)
    GYPSUM
  (WALLBOARD OR
  CEMENT CLINKER)
                                         GYPSUM
                                        (WALLBOARD OR
                                       CEMENT CLINKER)
                                              ^  GYPSUM
                                                (WALLBOARD OR
                                               CEMENT CLINKER)
               PROCESSES HITH SULFUR PRODUCT RECOVERY
                 |CITRATE
                                      SULFURIC ACID
                                       SULFUR OR
                                      SULFURIC ACID
  SULFUR OR
SULFURIC ACID
Figure  1-1.   Some  alternative FGD processes available  for
                    commercial  application.
                                 1-3

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      ASSIGNMENT OF  LIMESTONE FGD PROJECT
OVERALL SYSTEM DESIGN (SECTION
POWER PLANT CONSIDERATIONS
DESIGN BASIS
MATERIAL AND ENERGY BALANCES
SYSTEM CONFIGURATION OPTIONS
COMPUTERIZED DESIGN GUIDES
l
EQUIPMENT DESIGN (SECTION
SCRUBBER MODULES
LIMESTONE SLURRY PREPARATION
LIQUID FLOW EQUIPMENT
FLUE GAS FLOW EQUIPMENT
SLUDGE PROCESSING EQUIPMENT
PROCESS CONTROL AND INSTRUMENTATION
i
PROCUREMENT (SECTION
0 PREBID CONSIDERATIONS
0 PREPARATION OF SPECIFICATIONS
0 EVALUATION OF PROPOSALS
0 ENGINEERING DESIGN, INSTALLATION, STARTUP, AND 1
i
OPERATION AND MAINTENANCE (SECTION
0 STANDARD OPERATIONS
0 INITIAL OPERATIONS
e SYSTEM STARTUP AND SHUTDOWN
0 SYSTEM UPSETS
0 PREVENTIVE MAINTENANCE PROGRAMS
0 UNSCHEDULED MAINTENANCE
2)

3>

4)
rESTING

5)
Figure 1-2.   FGD project coordination sequence.
                      1-4

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INTRODUCTION:   Scope	1-5
goal is successful  performance,  as  measured by compliance with S02 emission
regulations and by highly reliable day-to-day operation.
     Figure 1-3 amplifies the decision factors to be dealt with by a utility
Project Manager.  Notice  that  these decision factors are based on an under-
standing of the  basic  powerplant design,  which affects  all  of the systems,
components, and operations that are to be  applied in S02  control.   Awareness
of the optional  process  features will figure importantly  in  development of
the scrubber system design.   The information presented here as an aid to the
Project Manager is based in part on current engineering design practices and
in part on  the records and experience of  operational limestone FGD systems.
It includes concise  guidelines  and  a detailed review  of proven operational
procedures and maintenance practices.
     Following are  some of  the  important questions  to  be  answered by the
utility project management team:
     0    What degree  of  control  is needed to satisfy the emission regula-
          tions?
     0    What  criteria  should  be  established for  disposal  of  solids and
          wastewater?
     0    What are  the  flue  gas  composition and  flow  rate generated by the
          "worst case" coal fired at the plant?
     0    What  variability   is  expected  in  the  coal  as  received  from the
          supplier?
     0    What are the availability, chemical composition, and reactivity of
          the limestone reagent?
     0    What variability is expected in  the limestone as received from the
          supplier?
     0    What  is  the source of  the  makeup water and what  is its expected
          chemical composition?
     0    What variability is expected in  the makeup water?
   -  °    Should the sludge be disposed of as a wet slurry or processed into
          a dry solid?
     0    How should wastewater effluent be treated?
     0    How  can  the system  be designed for a  closed-loop water balance?

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a\
             REGULATIONS

             e EMISSION LIMITATIONS
             0 DISPOSAL CRITERIA
BASIC POWER
PLANT DESIGN
0 PRELIMINARY
STUDIES




MASS AND
ENERGY
BALANCE



DESIGN BASIS
0 FLUE GAS FLOW/COMPOSITION
0 REAGENT SUPPLY
e MAKEUP WATER SOURCE
0 SLUDGE/BYPRODUCT DISPOSAL
                DETAILED
                DESIGN
                CRITERIA
  SYSTEM
SELECTION
                BID SPECIFICATION

                0  SCOPE OF  SUPPLY
                0  MATERIALS OF CONSTRUCTION
                0  FLEXIBILITY
                0  REDUNDANCY
                0  O&M PROGRAM
SYSTEM
CONFIGURATION
OPTIONS

0 PARTICULATE COLLECTION
e FAN LOCATION
0 FLUE GAS BYPASS
0 REHEAT
0 ADDITIVES
0 FORCED OXIDATION
0 FINAL SLUDGE DISPOSAL
                        SYSTEM
                        SUPPLIER
                        BID LIST
SPACE CONSTRAINTS

' SCRUBBER EQUIPMENT
• SLUDGE DISPOSAL
                        0 QUALIFICATIONS STATEMENT
                        0 EXPERIENCE AND REPUTATION
                                   Figure  1-3.  Limestone  FGD  decision  sequence.

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INTRODUCTION:   Scope	1-7
     0    Is land  available  at  the  site  for  location of  a final  sludge
          disposal  area?
     0    What  are  the spatial  constraints  for placement  of the  scrubber
          system  equipment?
     0    What  should the  process design configuration  consist of?
     0    Where will  the scrubber system fan  be  placed?
     0    How  will   fan  placement  affect the  final  design  requirements?
     0    Should  the  scrubbing system follow  equipment  for collection  of dry
          fly ash?
     0    What  type  of  scrubber  and  scrubber  internals   should  be  used?
     0    Should  the  system use reheat?
     0    Can the system operate  as  a partial  scrubbing unit  that  bypasses
          some  of the flue gas to provide reheat?
     0    What  provisions  can  be  made for  reheat  if the environmental  regu-
          lations will  not permit bypassing?
     0    Should  the  system use chemical additives  (adipic acid or magnesium
          oxide)  to enhance the limestone scrubbing performance?
     0    Should  the  system include forced  oxidation to yield a more manage-
          able  solid material  for disposal  or a saleable gypsum byproduct?
     0    What  will be the process control  philosophy?
     0    What  generic  types  of equipment  have   been proven in  limestone
          scrubbing systems?
     0    What  are the best  materials of construction  for various  equipment
          items?
     0    How  can  flexibility be  designed into the system  to  accommodate
          future  needs?
     0    Which items should be provided as redundant spares?
     0    What  should  be   the scope  of supply  of  the various contracts?
     0    Who  are  appropriate suppliers of major  subsystems  and equipment?
     0    What  operating  and maintenance  practices should   be  established?

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INTRODUCTION:  Organization
These questions  and  related matters are dealt with  in  detail  in this hand-
book.  The  answers  to such questions will dictate the  configuration of the
limestone scrubbing system and its mode of operation.
Organization
     The handbook is  structured  in accordance with the project coordination
sequence shown  in Figure 1-2.   We assume that the Project Manager is under-
taking  his  first  assignment  involving  an S02  control  system.  The  first
step, therefore, is a survey of the major elements of the project.   For this
purpose  we  provide  in  this  introduction  a review  of  the  fundamentals  of
limestone  scrubbing,   including   recent  advances  in scrubbing  technology,
process chemistry, and key operational factors.
     In  succeeding   sections,  we address  the  subject matter  in  greater
detail.    The  handbook  format reflects the  probable  sequence of  project
events,   from planning/design/procurement  to  installation/operation/mainte-
nance.
     Section  2  deals with  overall  design  considerations, starting  with
factors   that  affect  selection of an S02  control  system.    This  discussion
includes  a  detailed  account  of  the options  for system  configuration  and
describes some of the tools available to a Project Manager in conducting the
preliminary study; e.g.,  pertinent computer programs and experience records
of operational systems.
     Section  3  analyzes  the  FGD  system,  the  process control  options,  and
criteria for  selecting ancillary  equipment.   This section provides  detailed
guidelines  for  completion  of the  engineering effort.   It  also  can  serve as
reference material  for  the  Project Manager in  overseeing the major  engi-
neering  aspects  of  the  project.   The discussion of  equipment  in Section 3
identifies  generic  and  specific  scrubber  system components  and  is supple-
mented by operational records.
     Section  4  describes the purchase documents required  to obtain compet-
itive proposals from qualified bidders and the procedures for evaluating the
proposals.   Guidance is given also on the project activities that immediate-
ly  follow  the  award of  contract;  these  include  liaison  with vendors  and
consultants during installation,  startup, and performance testing of the FGD
system.

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INTRODUCTION:  Overview	1-9
     Section 5  deals with  system  operation and maintenance  (O&M),  both  of
which are  critical  to  successful  performance and thus are  significant for
the entire  project  staff.   Experience  to date  has  shown that many  of the
major  operational   problems with  limestone  FGD  scrubbing   systems  relate
directly to inadequate operation and maintenance practices.   The discussions
encompass establishment  of O&M  protocols,  relationships  of  preventive and
reactive  maintenance,  troubleshooting  operations,   staffing,   methods  of
training, and other major facets of an effective O&M program.
     The appendixes consist of supplementary reference material, giving more
specific details concerning the topics treated in Sections 2  and 3,  together
with other  useful reference materials.   Appendix A is a detailed discussion
of  process  chemistry, addressing  the  theoretical  aspects of scrubber per-
formance.  Appendix B focuses  on process operation,  emphasizing the effects
of  the  key  operational  factors on system reliability.   Appendix C describes
three pertinent computer  programs:   the  Tennessee  Valley Authority (TVA)
design  and  cost-estimating  program;  the  PEDCo  FGD  Information  System
(FGDIS), which  constitutes  an  experience record data bank;  and the Bechtel-
Modified Radian Equilibrium Program for monitoring  gypsum relative satura-
tion  levels and  gypsum  scale  formation  potential.   Appendix  D discusses
further  the  potential of  advanced limestone scrubbing processes and inno-
vative designs; Appendix  E gives examples of detailed  calculations of mass
and energy  balance;  Appendix F lists the operating limestone FGD systems in
the United  States;  and  Appendix G gives detailed guidance on  materials  of
construction.

OVERVIEW OF THE LIMESTONE SCRUBBING PROCESS
     A brief description of the basic limestone FGD process is followed by a
review of some of the process options that are now available.   This overview
also briefly  summarizes process  chemistry and  the  key operational factors
that affect limestone scrubber performance.
Basic Limestone FGD Process
     The basic  limestone  FGD process is shown  schematically  in Figure 1-4.
The  process incorporates  proven  equipment components  and   utilizes  well-
established technology.  The throwaway process considered here is relatively

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                  CLEAN FLUE GAS
  FLUE GAS-
      GROUND
    LIMESTONE-
      SLURRY
                      A  A  A  A  A
                   /\  A A

                     SCRUBBER
                      NODULE
                            A  A
                                         MIST ELIMINATOR
                                            HASHHATER
 TO DISPOSAL
           —[
                MIXER     VACUUM FILTER
THICKENER
 OVERFLOW
   TANK
Figure 1-4.   Limestone  FGD process:   basic process  flow diagram.
                                  1-10

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INTRODUCTION: Overview	;	1*11
simple In comparison with the chemically more complex processes that yield a
recoverable product or with  other advanced S02 control  processes_now avail-
able.  Limestone scrubbing systems of this basic type have provided the high
S02  removal  efficiencies  that are  needed  to meet  the current New  Source
Performance  Standards  and have  operated reliably  at power plants.   These
systems have successfully  treated flue gas generated from the  combustion of
coals  having  wide ranges  of sulfur and ash  contents.   Additionally,  lime-
stone  systems are  the  least  expensive to maintain  and  operate among all of
the commercially available systems.
     A  goal  of  all  wet  scrubbing S02 control  processes 1s  "closed  loop"
operation, in  which fresh water  1s  added  to the system  only  as makeup for
water  lost by evaporation and that lost with the sludge.  EPA personnel have
conducted extensive  pilot-scale  development work on  closed-loop systems at
the Agency's Industrial  Environmental  Research Laboratory/Research Triangle
Park  (IERL/RTP)  and  have used  the  Shawnee test  facility  to  demonstrate
closed-loop operation  of limestone  systems.   Commercial systems  now being
installed and planned will operate in a closed-loop mode.
     The basic limestone  process  flow diagram shows  incoming  flue  gas from
which  fly ash  has been  removed  by  treatment  in a  particulate  collection
device such as an electrostatic  precipitator (ESP) or a fabric filter.  The
flue  gas  is  brought into  contact  with  the  limestone  slurry in  a  simple
scrubber  tower.   Chemical reaction  of limestone with  S02  in  the  flue gas
produces waste solids, which must be removed continuously from  the  scrubbing
loop.  These waste solids are concentrated in a thickener and then  dewatered
in a  vacuum  filter to produce a filter "cake," which is mixed  with fly ash.
The  resulting  stabilized mixture  is then transported  to a landfill.   The
limestone  scrubbing  system   is  called a  "throwaway"  process because  the
product sludge  is disposed  of rather than  regenerated  to  recover sulfur.
     The  limestone F60 process has been enhanced with  the  advent  of recent
technology  improvements.   The  utility Project  Manager  can consider these
innovations as  options to the basic  limestone  process.  Selection  of the
final  system obviously depends greatly on factors specific to  an individual
project.

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INTRODUCTION:  Overview	1-12

Optional Process Features
     The  ability  of  optional  process  features  to  enhance the  limestone
scrubbing  process  is being verified by  current commercial  experience.   The
options  discussed  here are  offered by  manufacturers  of  scrubbing systems;
they do not constitute an  inclusive  list but represent a  cross  section of
systems  that can be  purchased  currently.   Many have  been  developed  on the
basis of research  and development at the Shawnee test facility.   The Intro-
duction of  these process  features has led to higher S02 removal  efficiency,
greater  reliability  of operation in a scale-free mode,  reduced  consumption
of  limestone reagent, and  production  of smaller quantities of  sludge  that
can  be  more easily handled  and disposed of.   Some of these  Improvements
involve  additional  equipment  items or  reagent additives.   To the  extent
possible, schematic  diagrams  of these  optional features are superimposed on
the  basic  process  flow diagram to  indicate  the location and nature  of the
process change.  Most  of  the equipment items in these figures  are discussed
in detail in later sections of the manual.
     Presaturation is  a process  feature that is required  in some  cases to
cool the hot  flue  gas  to  protect the  materials  that  line  the  scrubber
vessel.   Cooling  is  accomplished  by  quenching the  hot  gas with  scrubber
slurry.   Evaporation of the water cools the  flue gas to approximately 125°F
and  saturates  it with water vapor.   When saturation is effected  in an inlet
section of the main scrubbing vessel,  that portion of the vessel  is known as
the  quencher.  When  saturation  takes  place in an external  vessel  or section
of the  incoming  flue gas  ductwork,  the  external vessel or  ductwork section
is called  the presaturator.  Figure  1-5 depicts a  presaturator.   The  pre-
saturator can  also  serve  as a prescrubber for secondary particulate collec-
tion and  removal  of  sulfur trioxide  and/or  hydrogen  chloride ahead  of the
main scrubbing vessel.  The prescrubber then  can serve as  a  separate loop to
collect  the chloride  and  isolate  it  from  the principal  scrubbing  loop.
     Figure  1-6 shows  a  venturi scrubber placed ahead  of the  main scrubber
module.   The venturi  scrubber is an efficient device  for removing particu-
lates but  not-for  removing S02  from a  flue  gas  stream.  Use of  the venturi
provides a  means  of  removing  any  particulates remaining  in  the  flue gas
after it has passed through the particulate collection system.   Installation

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Figure 1-5.   Limestone FGD process:   external  presaturator.
                            1-13

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Figure 1-6.   Limestone FGD process:   venturi  scrubber.
                            1-14

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INTRODUCTION:  Overview	1-15
of a venturi  scrubber  is  often recommended as best available retrofit tech-
nology (BART) for effective control  of participate emissions.
     Collection  of  fine participates  is difficult  for  a venturi  scrubber
operating  at  a  reasonable pressure  drop.    A  venturi  scrubber  can  serve
efficiently, however,  as a primary collector followed by  an S02  scrubber and
a charged  particulate  separator.   Figure 1-7 shows such a  system,  which is
offered by at least two system suppliers (Combustion Engineering and Peabody
Process Systems).  The low-pressure-drop venturi  provides primary collection
of particulates  (approximately 90 percent removal).   The spray  tower scrub-
ber then  removes the  S02,  and the  charged particulate separator  provides
final  collection of particulate and scrubber carryover.
     Experimentation at the Shawnee test facility has shown that utilization
of  the limestone  reagent is  greatly  improved when  an  additional  tank is
placed  in  the  process flow  to collect and  recycle the venturi  scrubber
slurry and to keep this slurry separate from the  main scrubbing  vessel (Head
1977).   Figure  1-8 shows  the  type  of installation that constitutes  a two-
loop limestone FGD system.
     Concurrently with  the experiments  at Shawnee, Research-Cottrell devel-
oped their Double Loop  Limestone Scrubbing Process, as shown in Figure 1-9.
Unlike  the EPA  two-loop  system,  this process uses only  a  single scrubber
module.  The incoming flue gas enters the bottom  section of the  tower, where
it  is  quenched;  the  cooled  flue gas  passes through  the  tower,  where  S02
removal occurs.
     An important innovative  feature of both two-loop systems is separation
of the  prescrubber/quencher  from the  scrubber slurry loop,  which enhances
limestone  utilization  by  promoting  low pH in the prescrubber/quencher loop.
The two-loop  process  confines  chloride ions to the  prescrubber/quencher
loop;  thus  special  materials  of construction are  needed only in this loop.
Additionally, separation of the loops permits operation of the scrubber loop
in  a  gypsum-subsaturated  mode and  enhances  oxidation in  the  prescrubber/
quencher loop, thus permitting production of a gypsum byproduct.
     The equipment  item that  permits  this separation in the system is the
hydrocyclone, which feeds  the solids forward to  the  quencher loop from the
scrubber effluent hold tank  and returns the clear liquor  to  the scrubber
loop.    This procedure  applies  to  high-sulfur applications; in  low-sulfur

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                              CHARGED PARTICIPATE SEPARATOR
Figure 1-7.   Limestone FGD process:  venturi  spray-tower scrubber
             with a-charged participate separator.
                                    1-16

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           FLUE GAS
VENTURI SCRUBBER
                                           MIST ELIMINA
                                             UASHUATER
Figure 1-8.   Limestone FGD process:  EPA  two-loop scrubbing.
                                  1-17

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                               MIST ELIMINATOR
                                 HASHHATER
Figure 1-9.   Limestone FGD  process:  RC  Double Loop scrubbing.
                                1-18

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INTRODUCTION:  Overview	1-19
applications the discharge mode  is  reversed.   A two-loop system  thus  main-
tains a closed-loop water  balance while improving limestone utilization and
achieving high S02 removal  efficiency.
     Peabody Process Systems  has  recently  applied for a patent on  a system
that uses  a hydrocyclone  to  increase process stability and  reliability by
permitting  the  system  to  "control"  the  chemistry  in the  critical  mist
elimination area.   Figure  1-10 shows the Peabody  system. .  The  hydrocyclone
separates unreacted limestone in the recycle  slurry from the calcium sulfite
and calcium sulfate solids.  Because no limestone is available to  react with
S02, plugging  cannot  occur.   Thus, recycled  liquor can be  used  for  mist
eliminator  wash without  upsetting   the  closed-loop water  balance.   Addi-
tionally, the hydrocyclone is used to achieve 100 percent limestone  utiliza-
tion (Johnson  1978).  Research Cottrell  has  recently been  awarded  a patent
on a similar system.
     Other development work at the  Shawnee test facility has shown  that the
volume of sludge is substantially reduced by  forced oxidation.  In addition,
forced oxidation  improves  the physical  properties of the sludge  so that it
need not be mixed with fly ash for disposal (Head and Wang 1979).  Secondary
benefits  of forced  oxidation are  increased  utilization  of limestone and
improved  control  of  scaling in  single-loop  limestone scrubbing  systems
(Borgwardt  1978).   Figure 1-11  shows  a  scheme  for  forced oxidation  by
aerating the scrubber effluent hold  tank of a single-loop system.
     Forced oxidation can  be  incorporated  into a two-loop scrubbing process
to produce gypsum for use in wallboard manufacture.  In this system  a second
hydrocyclone concentrates  the solids to produce a wet  gypsum underflow and
an overflow that is returned to the  quencher  (Braden 1978).
     Limestone  scrubbing technology has been improved not  only with equip-
ment changes but also by the use of  chemical  additives.   Figure 1-12 shows a
process  in which  either  adipic  acid  [HOOC(CH2)4COOH]  or magnesium  oxide
(MgO) is  added to the scrubber slurry.  Recent  EPA  tests  indicate  that low
concentrations  (600 to  1500  ppm) of adipic acid will improve S02  absorption
(Head et  al.  1979).   Addition of adipic acid, which  is an advanced process
option, has been shown  to  be effective in conjunction with forced oxidation
and in the presence of chloride ions.  It improves scrubber operating condi-
tions  and  can  be  used  with  forced  oxidation  in single-loop  scrubbers.
Because   there  is  no  chloride interference, addition of  adipic acid is

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Figure 1-10.   Limestone FGD process:   hydrocyclone used to
          provide mist elimination wash  liquor.
                           1-20

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Figure 1-11.  Limestone FGD process:   forced  oxidation  of scrubber
               hold tank in a single-loop  system.
                               1-21

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                                 CHEMICAL )*MgO
                                 ADDITIVES )*ADIPIC ACID
Figure  1-12.  Limestone  FGD process:  chemical additives.
                            1-22

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INTRODUCTION:   Chemistry and Process Operation	1-23
effective  in  systems  with the most  tightly closed  loops.   To verify  the
commercial viability of adipic acid as a process additive,  the EPA  is demon-
strating this process  on  the  limestone FGD system in  service at the 200-MW
Southwest  No.  1  Station  of City  Utilities,  Springfield,  Missouri.   Early
results of  this  demonstration have  verified results from the  Shawnee  test
facility.   The use of adipic acid has been cost-effective because the higher
utilization of  limestone  and lower  sludge  volume offset the  cost  of  the
additive (Burbank 1980).
     Alkali additives  like  magnesium  oxide also enhance the  capacity of the
scrubbing slurry  to  absorb S02 by buffering the pH.   Experiments  with mag-
nesium additives  at the Shawnee facility have resulted in higher S02 removal
efficiencies  in   both  single-  and  two-loop systems  (Head  et al.  1977).
Pullman Kellogg offers commercial,  magnesium-buffered limestone FGD systems
that use the Weir horizontal crosscurrent spray  scrubber.
     The optional  features discussed here represent some of the more promis-
ing of  current process modifications.   The basic  limestone  FGD process  is,
of  course,  amenable to  further variations  that may  substantially enhance
overall operation  and  reliability.   Moreover,  the use of several  of these
features  in  combination   may  be  practicable,   depending  on  site-specific
factors.

PROCESS CHEMISTRY AND OPERATIONAL FACTORS
     Process  chemistry and  operational  factors  are  closely interrelated.
For example,  the  ready availability of dissolved alkaline  species  (neutral-
izing  capacity)   in  the   scrubbing  liquor  can  yield  an energy savings  by
reducing  the  required  liquid-to-gas  (L/G)  ratio.   In  operation  at a  low
stoichiometric ratio,  the resulting  high utilization of  limestone reduces
plugging  and  fouling of  mist eliminators.  Control  of solids concentration
in  the slurry  reduces the potential  for scaling  in the scrubber vessel.
Scaling potential  is  reduced also  by  establishment of  optimum  residence
times of slurry in the scrubber effluent hold tank to dissolve the  limestone
and precipitate the calcium solids.  These and other important relationships
between process chemistry and operational factors are  described briefly in
the  following discussion,  which  also  provides guidelines  for design  and
operation.

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INTRODUCTION:  Chemistry and Process Operation	      1-24

Type and Grind of Limestone
     The  promotion  of good  process  chemistry  begins  with selecting  the
limestone  reagent and  determining  how  fine to  grind  it.   The  limestone
recommended  for  scrubber applications is a  high-purity  material  containing
90 percent or more  calcium carbonate (CaC03) and less than 5  percent mag-
nesium  carbonate  (MgC03).   Scrubber  operation  has  demonstrated that  the
finer the grind,  the better is the utilization of limestone,  especially when
the  goal  is 90  percent S02  removal  (Rochelle  1980).  The  grind should be
selected to  maximize the  limestone utilization,  i.e.,  the  portion of  the
limestone  fed  to the  scrubber  that is actually used to neutralize  the  S02
and  other  acidic species  absorbed from the  flue gas.   Recent work at  the
Shawnee test facility  has  shown  that a  good limestone  grind for use in a
typical limestone wet  scrubbing system would be one  in  which  90 percent by
weight passes  through  a 325-mesh  screen.   Decreasing the limestone particle
size increases the  rate  of  dissolution of  solids  in the scrubber  and  the
overall rate of  S02  removal.   For a  given degree of S02 removal, reduction
of the  limestone particle  size decreases the  required  pH  setpoint of  the
scrubber recirculation slurry liquor and improves the limestone utilization.
     Good  limestone  utilization (> 85 mol percent)  is related  to the  effi-
ciency  of  S02 removal by  the limestone  scrubbing  unit.   High  limestone
utilization is generally attained at relatively low  pH's, and scrubbers must
therefore be designed for efficient mass transfer.
Stoichiometric Ratio and pH
     Stoichiometric  ratio  (SR)  is  defined as the ratio of the actual amount
of S02  absorbing reagent,  usually calcium carbonate (CaC03),  in the  lime-
stone fed  to the scrubber to the  theoretical amount  required  to neutralize
the S02 and other acidic species absorbed from the flue gas.   The neutraliz-
ing  capacity of  the scrubbing  liquor,  which is directly related  to the pH
level,  can be  increased by increasing the SR up to  a limit of about 1.2 and
maintaining pH at 5.8.   Beyond this limit scaling can occur in the scrubber.
     Control of  pH  is  essential to reliable scrubber operation.   Appendix A
explains in detail that the chemical reactions in a  scrubbing system must be
confined to separate parts of the system.   The absorption of S02 in the flue

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INTRODUCTION:  Chemistry and Process Operation	1-25
gas must take  place  in the scrubber vessel, whereas  the  neutralization and
precipitation  reactions  must  occur primarily  in the  effluent hold  tank.
Stoichiometry and Mist Eliminator Fouling
     Stoichiometric  ratio  has  a great  impact on performance  of the  mist
eliminator.    Low SR  values  reflect  high limestone  utilization;  a low  SR
prevents fouling and  plugging  of the mist eliminators and thus  enhances the
system  reliability.   Testing  at the Shawnee facility has  demonstrated  this
relationship.   Operation has  been most successful  at  SR values not exceeding
1.18.
SO? Removal  and pH
     Commercial  experience has shown  that  5.8 is the  maximum  practical  pH
level  for high S02 removal  efficiency in a single-loop system.  When this pH
level  is exceeded, the greater quantity of limestone  dissolved in the system
can lead  to problems  with  process chemistry.   Increasing the  pH  increases
the neutralizing capacity  of  the slurry, but use  of  too much limestone is a
principal  cause of scale formation.   An operating  limestone scrubbing system
must achieve  proper  balance  of limestone consumption, S02  removal, and pH
level.
SO? Removal  and L/G
     The major  operational  means  of  achieving the  required degree of S02
removal is  maintenance  of  proper L/G ratio.  The  ratio of liquid flow rate
to gas  flow rate in  the S02  scrubber is expressed as gallons of slurry flow
per 1000 actual  cubic feet of  flue gas  flow at scrubber outlet conditions.
The primary effect of  higher  liquid  flow  rate,  or  higher  L/G ratio  at a
given gas flow rate,  is to increase the S02 removal efficiency.   The minimum
L/G ratio  required for  a  given degree  of S02 removal can  be  decreased by
increasing  the  neutralizing capacity  of the recirculation slurry,  which can
be  effected,   within  limits,   by  increasing  the limestone  Stoichiometric
ratio.
     A  high L/G ratio  is  normally  needed for  efficient S02  removal at the
low pH  control  setpoint (< 5.8) and low SR (£ 1.18)  that must be maintained
to minimize fouling of the mist eliminator.

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INTRODUCTION: Chemistry and Process Operation	1-26

L/G Ratio and Liquid Holdup
     The operational L/G ratio is affected by holdup of liquid in the scrub-
bing vessel.   Liquid holdup  is  achieved by  placement of  various  internal
structures such as  packing,  rods,  or trays in the  scrubber.   All scrubbers
except  venturi  and  open  spray  towers  incorporate some  type of  internal
device to  impede  the free fall of slurry and thereby prolong the contact of
slurry  with  the  gas stream.  The  term "tower  internals" refers  to  these
internal structures.
     Increasing  the  holdup   of  liquid  slurry  can  increase  S02  removal.
Alternatively, operators  can  achieve  a required level of  SOg  removal  effi-
ciency by  providing  a means  for slurry  holdup and  reducing the liquid flow
rate.   This procedure, however, increases the gas-side pressure drop and the
horsepower requirements of system  fans.   The gas pressure  drop  through the
scrubber  depends  on  the  operating gas  velocity and  the  type  of  scrubber
design.   The pressure  drops  in  the  scrubber  and  in the  mist  eliminator
usually constitute most  of the total  system gas-side  pressure drop and are
major factors in the energy consumption of the system.   A sudden increase in
pressure  drop  across  the scrubber,  mist  eliminator,  or  reheater usually
indicates the deposition of solids on scrubber internals.
     The trade-off between gas-side pressure drop and L/G ratios has a major
impact  on  the total  energy  demand of the  scrubber system.   Sulfur dioxide
removal can  be  increased  by  increasing the system  power consumption,  which
is achieved  by  increasing the liquid flow rate and/or the  gas pressure-side
drop.
     A  well-designed limestone FGD system  must be able  to operate  over a
wide range of gas flow rates.  The ratio of maximum to minimum gas flow that
a  scrubber  can  handle without  unstable operation or  a  reduction  in S02
removal efficiency  is known  as  turndown capability.   The use  of  parallel
scrubber modules  provides  overall  stepwise  turndown capability.   The use of
staging pumps and installation of multiple sprays,  gas dampers, and baffles
can provide turndown capability for individual modules of the system.
Liquid Phase Alkalinity and L/G
     The available alkalinity  in  the  scrubbing liquor, which is supplied by
the dissolving limestone, continuously neutralizes the absorbed S02-

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INTRODUCTION:   Chemistry and Process Operation	1-27
Removal  of the  S02  as a  neutralized product  allows for  the  continuing
transfer  of  additional  S02  from  the  gas  phase  to the  liquid phase.   The
available  liquid-phase  alkalinity  (LPA) thus  provides a driving force  for
the absorption and neutralization  of S02.   The higher  the  LPA,  the higher is
the driving force for S02  removal.
     The availability of alkaline  species  in the liquid phase  is critical.
In addition to absorbing S02 from  the flue  gas, the  scrubbing liquid absorbs
hydrogen chloride (HC1), which  is  formed from combustion  of  the chloride in
the  coal.   When  the  scrubber liquid accumulates  substantial  amounts  of
chlorides, less alkalinity is then available for  S02 removal.   The impact of
HC1 absorption  on limestone  consumption is minimal except when  the sulfur
content of the  coal  is  very low and the chloride content  is  high.   Addition
of soluble alkalis  or buffering agents such as MgO  or adipic acid increases
the available LPA at  a  given pH level  and  thus  reduces the  impact  of chla-
rides on the S02 removal process.
     The primary operational  means  of increasing available alkalinity is to
increase the  liquid flow rate.   Increasing the  volume  of  liquid  per unit
flow  of gas  allows more  alkaline neutralizing  species  to  contact acidic
components of the flue  gas.   Increasing the liquid  flow  rate,  however, also
increases the energy  demand  of the scrubbing system.   Therefore,  an optimum
balance of the  available  alkalinity,  L/G ratio,  energy consumption, and S02
removal must be determined.
Scrubber Effluent Hold Tank and Relative Saturation
     Design of  the  scrubber effluent  hold tank is  very important  in  the
control of relative saturation.  All of the precipitation  chemical reactions
should occur in the hold tank, as  well as the dissolution  of  limestone.  The
hold  tank  in  a single-loop system  should  be sized  to allow a  minimum of 8
minutes residence  time  of  the  recirculation slurry flow to  ensure optimum.
limestone  utilization and precipitation of solids  as gypsum.   A residence
time  of 5 to  7  minutes in  a double-loop   hold  tank  will achieve  the same
objective.
     In limestone FGD systems,  the term "relative saturation" (RS)  pertains
to the degree of saturation (or approach to the solubility limit) of calcium
sulfite and sulfate in the scrubbing liquor; RS is important  as an indicator

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INTRODUCTION: Chemistry and Process Operation	   1-28
of scaling potential, especially of gypsum scaling, which can become severe.
Relative  saturation  Is defined as  the  ratio of the product of  calcium and
sulfate ion  activities  to  the solubility product constant.   The  solution is
subsaturated when  RS is less than 1.0,  saturated when RS equals  1.0,  and
supersaturated when RS is greater than 1.0.   Generally a limestone scrubbing
system will operate in a scale-free mode when the RS of gypsum is maintained
below a  level  of  1.4 and the  RS  of  calcium sulfite is maintained  below a
level of approximately 6.   Operation below these levels provides  a margin of
safety to ensure scale-free operation.
Prevention of Scale Formation
     Scale  formation can  lead to  the accumulation  of solids  on  scrubber
internals and ultimately can  lead  to  shutdown of  a  module  or of the entire
system.   Scaling can  also  cause instrument malfunction and loss  of process
control.
     Scaling can  be  prevented  by  close control  of  the pH of the  scrubber
inlet  liquor,   stoichiometric  ratio,  and  relative  saturation  of  calcium
sulfate  in  the  scrubber   inlet  liquor.   As  indicated  earlier,  control  of
these parameters is  achieved  by providing sufficient residence  time  in  the
effluent  hold  tank,   optimizing the  L/G ratio,  and maintaining  a  proper
solids content in the scrubbing slurry.   The solids content  of recirculation
slurry should be not less  than 8 weight percent in a fly-ash-free limestone
scrubbing system and  15 weight percent in a  scrubbing  system  that simulta-
neously removes fly ash.
Degree of Oxidation
     The degree of oxidation of sulfite to sulfate in the FGD  system affects
the  precipitation  of  the  solid  reaction  products.    In  general,  gypsum
(CaS04-2H20) begins to  precipitate  in  limestone FGD systems when the degree
of oxidation exceeds  about  16 percent  (sulfite to sulfate on a molar basis).
     The degree  of oxidation  generally  increases with  an  increase in  the
ratio of 02  to  S02 in the  inlet flue  gas,  an increase in the  concentrations
of certain trace metals (such as manganese)  in  the  scrubbing  liquor,  and a
decrease in the  pH level  of the scrubbing  liquor.   Various techniques have
been developed  for forced oxidation  of scrubber  systems to  improve sludge

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INTRODUCTION: Chemistry and Process Operation	1-29
dewatering characteristics and  thereby  to reduce the total  volume of sludge
generated.
Chloride Removal
     Because of  operational  and regulatory  considerations,  scrubber system
designers  should  consider  the  available options for  reducing  high chloride
concentrations  in  the scrubbing liquor.  The  use  of a prescrubber  or  of  a
two-loop  system can  confine the chlorides  to  a single  loop,  so that  the
concentrated  chlorides  can  be  removed  from  the  scrubbing system.   Some
methods currently  used to  permit chloride liquor blowdown are  effective but
are  not  recommended "closed-loop"  practice; these  methods  include sluicing
the blowdown through the bottom-ash pond, or diluting it in the plant waste-
water system, or  controlling the dewatering of sludge so as to maximize its
liquor content.
     In the  future, more  stringent  enforcement of  zero-effluent discharge
regulations may necessitate  the  use  of feasible but costly control options,
such as vapor-compression  evaporation.   Vapor  compression can  evaporate the
concentrated stream of chlorides and reduce them to a soluble  salt product.
Appendix B describes this technique,  which is currently used in power plants
to evaporate cooling  tower blowdown  and will soon  be tried on the blowdown
liquor from FGD systems.
Equipment Considerations
     Some  of  the  equipment  considerations  that should be  addressed in the
design phase to facilitate maintenance of the scrubbing system  are access to
internals and effluent hold  tanks  and to the spray headers and nozzles.   In
addition,   the  design  should incorporate such features as  sootblowers  near
the  wet/dry  interfaces   and   in-line  reheater  tubes,   sprays  for  mist
eliminator  wash,   and strainers  in  the  effluent  hold  tanks  to  prevent
plugging and erosion of piping  and  spray nozzles.

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INTRODUCTION:  References                                      	1-30
                          REFERENCES FOR SECTION 1


Borgwardt, R.  H.   1978.   Effect of Forced Oxidation on Limestone/SO  Scrub-
ber Performance.  In:  Proceedings of the Symposium on Flue Gas DesuTfuriza-
tion, Hollywood,  Florida,  November 1977.   Vol. I.  EPA-600/7-78-058a.  NTIS
No. PB-282 090.

Braden,  H.  B.   1978.   Double-Loop Operating  Offers  "Best of  Both Worlds"
Approach to  Sulfur  Dioxide Scrubbing.   Public Utilities Fortnightly, August
17, 1978.

Burbank, D.  A.,  S.  C.  Wang,  and  R.  R.  McKinsey.   1980.  Test  Results on
Adipic Acid-Enhanced  Limestone Scrubbing  at the EPA Shawnee Test Facility--
Third  Report.    Presented  at  the  Symposium  on  Flue Gas  Desulfurization,
Houston, Texas, October 28-31, 1980.

Head, H.  N.    1977.   EPA Alkali Scrubbing  Test  Facility:   Advance Program,
Third Progress Report.  EPA-600/7-77-105.   NTIS No.  PB-274 544.

Head, H.  N.,  and  S.  C.  Wang.   1979.   EPA  Alkali  Scrubbing  Test Facility:
Advance  Program,  Fourth  Progress  Report.    EPA-600/7-79-244a.   NTIS  No.
PB80-117906.

Head,  H.  N.,  et al.   1977.   Results  of  Lime  and  Limestone  Testing With
Forced Oxidation  at  the EPA Alkali Scrubbing  Test  Facility.   In:  Proceed-
ings  of  the  Symposium on  Flue  Gas  Desulfurization, Hollywood,  Florida,
November 1977.   Vol. I.  EPA-600/7-78-058a.  NTIS No. PB-282 090.

Johnson, C.   1978.   Minimizing  Operating Costs  of Lime/Limestone FGD Sys-
tems.  Power Engineering, 82(2):62-65, February 1978.

Laseke, B. A., T.  W. Devitt,  and  N.  Kaplan.  1979.  Status of Utility Flue
Gas  Desulfurization  in  the United States.   Presented  at the  72nd Annual
AIChE Meeting, San Francisco,  California, November  1979.

McGlamery, G.  G., et  al.   1980.   FGD Economics  in 1980.   Presented at the
Symposium on Flue Gas Desulfurization,  Houston, Texas, October 28-31, 1980.

Rochelle, G.  T.  1980.   May Monthly Progress Report on  Limestone Type and
Grind Project.  EPA  Grant R.  806251 done  at University of Texas at Austin.

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                                  SECTION 2
                            OVERALL SYSTEM DESIGN

     This section describes  a  systematic approach to design of  a  limestone
FGD  system.   Consideration of the  major design factors outlined  here  will
enable the  utility project  team,  together with  their  A/E consultants,  to
formulate an  overall  system  configuration.   Detailed analysis  of the system
components and related equipment is given in Section 3.
     Figure 2-1  shows  the  major  elements to be  considered  in  design of the
overall system.  The discussion that follows is presented in the order shown
in the figure.   Analysis  of  design factors begins with  those related to the
powerplant:    coal   properties  and  supply,  steam  generator  design,  power
generation demand,  site-related  factors,  and  environmental  regulations.  In
establishment  of the design basis,  the  size  and configuration of  the FGD
system are determined  by  such  parameters as flue  gas flow  and composition,
pollutant  removal   requirements,  reagent  stoichiometric  ratio,  limestone
composition,   and makeup water.   Calculation of material and energy balances
provides  further basis  for  finalization  of  the  FGD  system  design.   The
design details are  refined by  evaluating and  selecting  from among a number
of system configuration options,  such as reheat versus no reheat.
     Throughout  the  design process,  the project team may apply  some of the
computerized  design tools  that  are  readily  accessible for this  purpose.
Three major FGD  computer  programs  are outlined  briefly  in  this  section and
are discussed in detail in Appendix C.

POWERPLANT CONSIDERATIONS
     Design of the limestone FGD system will be strongly affected by certain
conditions related  to  the  powerplant and its  operation.  Ideally,  an emis-
sion  control  system  is  planned  and  developed as an integral  part  of the
                                     2-1

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  POWERPLANT  CONSIDERATIONS
      0 COAL PROPERTIES AND SUPPLY
      0 STEAM  GENERATOR DESIGN
      0 POWER  GENERATION DEMAND
  •    ° SITE-RELATED  FACTORS
      0 ENVIRONMENTAL REGULATIONS
  DESIGN BASIS
     0 FLUE GAS FLOW AND COMPOSITION
     0 POLLUTANT REMOVAL REQUIREMENTS
     0 REAGENT STOICHIOMETRIC RATIO
     0 LIMESTONE COMPOSITION
     0 MAKEUP WATER
  MATERIAL AND ENERGY BALANCES
  SYSTEM CONFIGURATIONS OPTIONS
     0 SEPARATE VERSUS INTEGRAL PARTICULATE
       REMOVAL
     0 FAN LOCATION
     0 FLUE GAS BYPASS VERSUS NO BYPASS
     0 REHEAT VERSUS NO REHEAT
     0 SLUDGE DISPOSAL
     0 FLEXIBILITY AND REDUNDANCY
   COMPUTERIZED DESIGN GUIDES
     0 TVA LIME/LIMESTONE SCRUBBING COMPUTER MODEL
     0 PEDCo FLUE GAS DESULFURIZATION  INFORMATION
       SYSTEM
     0 BECHTEL-MODIFIED RADIAN EQUILIBRIUM PROGRAM
Figure 2-1.   Elements of overall system design.

                        2-2

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SYSTEM DESIGN:   Powerplant Considerations	2-3
powerplant, with the  system  design  based on economic,  geographical,  govern-
mental, and  other  factors that  affect the overall plant operation.   These
factors are considered here  in  five major categories:   (1)  the powerplant1s
coal  supply,  (2)  the  steam generator  design,  (3) the  power  generation
demand, (4) the powerplant site,  and (5) the applicable environmental  regu-
lations.    Table  2-1  lists  the  major  powerplant-related  considerations.
Coal-Related Factors (Properties and Supply)
     Properties of the coal to be burned affect the design of components and
operational characteristics of a powerplant.   The combustion characteristics
and physical  properties  of the coal  affect the design of  the  steam gener-
ator,  air  heaters,  primary air  fans, pulverizers, and  coal-handling system,
as well as  the maximum volume of flue  gas  to be treated in the FGD system.
The chemical composition  of  the coal affects the ash-handling system,  elec-
trostatic  precipitators,  limestone  scrubbers,  limestone handling  systems,
sludge-handling systems,  and miscellaneous  equipment  (Galluzzo and Davidson
1979).
     The ash and sulfur  contents  of the  coal,  together with the applicable
environmental  regulations, determine  the required efficiency of particulate
and S02 control; the  coal's  chloride content affects  the materials  of con-
struction and  the  ability of a limestone scrubber to  operate in the closed-
loop mode.
     Table 2-2  lists  the  properties of  four widely used types of coal from
different  locations  in the  United  States.   This table  indicates  the  broad
range  of  properties  characteristic  of  these  coal   supplies  (note,  for
example,  the   ranges  of  heating  values  and of  sulfur, ash,  and  moisture
contents).    Table   2-3 lists  the  combustion  characteristics  of the  four
representative coals described in Table 2-2 for a hypothetical 500-MW gener-
ating station.
     The diversity  of coal  supply sources for  a  coal-fired powerplant must
be  considered  in terms of the  performance of both the  steam generator and
the associated limestone  FGD system.  Building  flexibility into the scrub-
bing  system is  of utmost  importance  at the  design  stage.   Although the
properties of  coals  are  diverse,  designers-can incorporate a high degree of

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   TABLE 2-1.  MAJOR POWERPLANT FACTORS THAT INFLUENCE
                    FGD SYSTEM LCSIGN
Coal Properties and Supply

  Sulfur content
  Ash content
  Fly ash composition
  Chloride content
  Moisture content
  Heating value
  Availability of coals
  Transportation considerations
  Flexibility for firing alternative coals

Steam Generator Design

  Type of steam generator
  Size of steam generator
  Flue gas (Note that the following items are also related to
            properties of the coal)
    weight flow rate
    volume flow rate
    temperature
    dewpoint
    fly ash loading
  Additional control equipment

Power Generation Demand

  Base load
  Cycling load
  Intermediate load
  Peak load

Site Conditions

  Land availability
  Soil permeability
  Disposal facility
  Climatic and geographic effects
  Quality and availability of limestone and makeup water

Environmental Regulations

  New Source Performance Standards
  Disposal site standards
    Resource Conservation and Recovery Act (sealed site)
  Effluent discharge standards
    Clean Water Act (closed loop)
                           2-4

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                          TABLE 2-2.   FUEL PROPERTIES OF FOUR REPRESENTATIVE  COALS'

Proximate analysis
Moisture, X
Volatile matter, X
Fixed carbon, X
Ash, X
Heating value, Btu/lb
Ultimate analysis (as n
Carbon
Hydrogen
Nitrogen
Sulfur
Chlorine
Oxygen (difference)
Ash analysis, X
S10,
A120S
Fe203
T102
Ms
CaO
MgO
Na20
K20
S03
Undetermined
GHndablllty.
Hardgrove i ndex
Wyoming subbltumlnous
Average

30
32
32
6
8,000
celved). X
48
3
0.6
0.5
0.03
11.87

32
15
5
1
1
23
5
1
0.4
16
0.6

64
Ranqe

28-32
30-33
30-34
5-8
7,800-8,200

46-49
3-4
0.5-0.7
0.3-0.7
0.00-0.05
9-12

28-36
14-17
4-5
0.9-1.4
0.2-1.3
19-27
4-6
1.0-1.6
0.3-0.6
14-19


60-70
Montana subbltumlnous
Average

24
32
40
4
9.200

55
4
0.9
0.5
0.01
11.59

32
17
6
1
1
15
5
7
0.5
15
0.5

48
Range

23-25
30-32
40-41
3.5-6.0
8,300-10,000

53-56
3-4
0.8-1.0
0.4-0.7
0.00-0.02
11-12

25-40
15-20
5-9
1.0-1.1
0.5-1.0
10-20
3-5
6-9
0.4-0.6
10-20


45-50
Illinois (raw) bituminous
Average

11
37
42
10
11,000

60
4
1.0
3.9
0.10
10.0

47
23
16
1
0.1
6
0.1
1.0
1.6
4
0.2

56
Range

10-14
32-42
37-47
10-11
10,800-11,200

57-63
4-5
1.1-1.2
3.7-4.1
0.04-0.60
10-11

42-51
18-24
15-21
0.8-1.2
0.0-0.1
2-6
0.1-0.5
0.9-1.3
1.5-1.7
3-5


54-58
Gulf Coast lignite
Average

32
29
27
12
6,800

41
3
0.7
0.8
0.1
10.4

49
21
7
1.5
0.3
9
2
2
0.7
7
0.5

55
Range

30-35
b
b
10-15
6,200-7,000

b
b
b
0.6-0.9
0.0-0.2
b

b
b
b
b
b
b
b
1-3
b
b
b

50-60
t\>
I
C71
           Galluzzo and Davidson 1979.
           Data not available.

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                        TABLE  2-3.   COMBUSTION  CHARACTERISTICS FOR A 500-MW CASEC
ro

Slagging potential
Fouling potential
Erosion potential
Combustion air, 106 acfm
Flue gas, 106 acfmc
Maximum ash, tons/h
Maximum ash, lb/106 Btu
Minimum sulfur, lb/106 Btu
Maximum sulfur, lb/106 Btu
Wyomi ng
subbi luminous
High
Medium
Low
1.49
2.24
26.9
10.3
0.4
0.9
Montana
subbituminous
High
Severe
Low
1.57
2.24
17.2
6.7
0.4
0.6
Illinois (raw)
bituminous
High
High
Low
1.34
2.01
26.1
10.2
3.3
3.8
Gulf Coast
lignite
Severe
High
Medium
1.57
2.39
64.9
24.2
0.9
1.5
            .  Adapted from Galluzzo and Davidson 1979.
              At 30 percent excess air.
              At 15 percent air heater leakage.

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SYSTEM DESIGN:   Powerplant Considerations	2-7
flexibility  so that  substantially  different coals  may  be  fired  without
diminishing the efficiency of the S02 control  system.
Steam Generator Design
     The scope of this handbook does not include  guidelines for selection of
the most  suitable steam generator  for  a powerplant.   The type and  size of
steam generator are determined  by numerous factors specific to the  overall
service function  of the  powerplant.   The type of coal  burned  in  the gener-
ator does,  however,  affect  the arrangement and operation of  the limestone
FGD system.
     The combustion characteristics  of  a coal impact not only the potential
for  slagging,  fouling,  and  erosion  of  the  steam  generator  but also  the
composition  and  quantity of flue gas  introduced  to the  scrubbing  system.
Coal properties influence  the  weight and volume  flow rates,  the temperature
and dewpoint,  and the  fly ash loading of the  flue gas.   They also determine
the need  for additional  control  equipment ahead of or  after  the scrubbing
system.   Because  of  these  considerations,  design  of  the steam generator
should be  integrated with  selection of the coal  supplier and with design of
the FGD system.
Power Generation Demand
     The  powerplant1s  electric  generation demand  affects  both  the  basic
powerplant  and the associated  limestone  FGD  system.   The utility  Project
Manager, working  with a  consulting A/E firm,  may be  required  to  design for
operation at the following loads, either individually or in any combination:
     0    Base load—operation  to take  all or part of a minimum load over a
          given period of time; consequently,  operation would be essentially
          at a constant output.
     0    Cycling  load—operation to provide power  during extended  periods
          of  low  power  demand   (25  percent   of station  rated  capacity),
          followed by  cycling  to  high  power  demand  (full  rated  capacity).
     0    Intermediate load—operation  to provide power for a  portion  of a
          day at  full rated  capacity as a base load and for the rest of the
          day at a reduced (50 to 67 percent range) constant load.
     0    Peak load—operation  to provide  power during periods  of  maximum
          demand.

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SYSTEM DESIGN:  Powerplant Considerations	2-8
     Additionally,  future power demand  situations could  necessitate other
modes of  operation  of both the powerplant and  the associated limestone FGD
system.    These situations  could  include  daily  startup  and  shutdown with
operation  at  station rated  capacity,  or weekly  startup  (following weekend
shutdown) with  operation  at full  rated capacity.   Because of the possibili-
ties for  diverse  power  generation  demands,  it is important that flexibility
of response be built into the limestone FGD  system.   The following are the
kinds of design provisions that will enhance system flexibility:
     0    Provision  of  space in the process  layout for  future installation
          of additional  equipment.
     0    Provision  for  fitting  of the  individual  scrubber modules  with
          additional  internal  gas-liquid contacting devices to increase S02
          removal efficiency.
     0    Provision  for fitting  of an  open spray  tower scrubber with addi-
          tional spray bank headers.
     0    Provision for handling of additional scrubber slurry by the slurry
          recirculation pumps,  pipe headers,  and  spray nozzles  to increase
          the slurry flow rate.
     0    Provision  of  additional   horsepower capabilities  for  the  fans to
          move  the  flue gas through the scrubbing system  at a higher pres-
          sure drop or to handle a greater gas flow.
     0    Provision for future use of chemical additives.
     0    Provision  for  future  use of  forced oxidation  to  reduce  sludge
          volumes.
     0    Provision  for  future incorporation of  a chloride removal  system.
Site Conditions
     Several  site-specific  factors will strongly influence  the  FGD  system
design.   Among the most significant are those related  to sludge handling and
disposal.   The availability of land for disposal and the permeability of the
soil are  two   key parameters.   Containing untreated  sludge  onsite could be
the preferred sludge disposal option where land is available, where the soil
properties tend to  prevent  leaching,  or where any leaching that occurs will
not pollute local ground waters.

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SYSTEM DESIGN:  Powerplant Considerations	2-9
     In many  Instances,  local  requirements  might dictate such sludge treat-
Ing  operations  as forced  oxidation,  chemical  stabilization, and  the like,
either singly or In combination, for preparation of a nonpolluting, ecologi-
cally acceptable  landfill  material.   When some form of  sludge  treatment Is
needed, the  utility should  consider  the proprietary sludge  fixation tech-
nologies that are available.
     Final  decisions regarding  sludge  disposal  are affected  by  economic
considerations, local regulations,  and provisions of the Resource Conserva-
tion  and  Recovery  Act  of 1976  (RCRA),  which  establishes  guidelines  for
disposal of solid  wastes.  Where direct disposal is desired but the permea-
bility of the soil  could allow ground water pollution,  direct disposal into
a  pond is  a possibility.   Guidelines  for pond  disposal  indicate  that  a
coefficient of  permeability  of 1 x 10~7  cm/sec  (0.1 ft/yr)  is necessary to
adequately isolate the  disposal  site  from the surrounding environment.  The
effective permeability of an area can be reduced by the  use of a liner.  The
choice of liner materials includes a polymeric or elastomeric sheet covering
the entire area of the disposal facility, or an impermeable shell of natural
clay or chemically stabilized scrubber sludge.
     Climatic and  geographic conditions  at the  site also affect  design of
the FGD system.   The designer must anticipate the effects of such factors as
mean  evaporation  rate  and rainfall, temperature  range,  maximum wind veloc-
ity,  and  seismic  phenomena.   Local  atmospheric conditions, together with
local  regulations,  often  dictate  the  need  to reheat the flue  gas prior to
venting.  These factors  can  directly  affect the  cost,  operation,  and main-
tenance of the planned system.   Table 2-4 shows site evaluation factors that
were  used as  a  design  basis for a limestone scrubber at a 500-MW powerplant
(Smith et al.  1980).
     Another site-specific design consideration is development of a workable
layout.  The  designer  should attempt  to minimize the distance between major
process operations and at the same time allow adequate space at ground level
and  overhead  for access  to  the equipment  for  maintenance,  inspection,  and
testing.  These factors must  be considered by the  A/E  consultants as they
work  with  the  utility  project team to  develop  preliminary  design studies.

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            TABLE 2-4.  SITE EVALUATION FACTORS FOR A 500-MW CASE
                                                                 a
                                                     Site conditions
Land availability
  Total area available for
   site development, acres
  Disposal area, acres
  Scrubber module area, ft
  Sludge preparation area, ft
Guidelines for sealing disposal
 site, gal/ft2/yr

Natural clay availability
Grade elevation, ft
Barometric pressure, in.  Hg
Ambient temperature, °F
  Minimum
  Maximum
Mean avg. rainfall, in./yr
Mean avg. evaporation, in./yr
Maximum wind velocity, mph
Seismic activity
             965
             480
         ~ 200 x 200
         ~ 250 x 150
Ensure that leakage does not
  exceed_0.75 (equivalent to
  1 x 10"7 cm/sec permeability)
Available near site
             540
            29.4
              -5
              95
            46.5
              36
             100
  Smith et al. 1980.
                                     2-10

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SYSTEM DESIGN:   Powerplant Considerations	2-11
     Additional  site-related  factors  that may affect system design  are the
quality  and availability  of  limestone  and  of  makeup water.   Maintaining
consistent  and  uninterrupted  supplies of these  materials is a  key  to  suc-
cessful  long-term performance of  the  FGD system.   An FGD  system may be  able
to accept cooling tower  blowdown  water as process makeup.   If  the blowdown
water  is compatible with the  FGD  process chemistry,  then  the FGD system can
dispose  of  at  least  a portion of this contaminated  water'that  would other-
wise require expensive treatment  prior to discharge.  Details  regarding the
analysis of  limestone  and makeup  water supplies are  outlined later in  this
section (Design Basis).
Environmental Regulations
     The applicable environmental regulations exert  an obvious  influence on
FGD system  design, particularly with  respect to particulate and S02 removal
capabilities  and the sludge  disposal  system.    The following  discussion
places primary  emphasis  on Federal  regulations.   Where state or local regu-
lations are more stringent, they would govern the system design.
     New Source Performance Standards.   New  Source   Performance  Standards
(NSPS) were  issued on June 11,  1979,  for implementation by coal-fired elec-
tric utility steam generating units capable of firing more than 250 million
Btu/h  heat  input (about  25 MW electric output), and  upon which  construction
commenced after  September 18,  1978.  Following is a brief  summary of these
standards:
     (1)  S02 emissions are subject to the following:
          (a)  If the  uncontrolled  emissions are less than 2 Ib/million Btu
          of heat release,  the  S02  removal efficiency of the system may not
          be less than 70 percent.
          (b)  If the  uncontrolled  emissions are between 2 and 6 Ib/million
          Btu,  the  emission  from  the scrubbing  system  may not  exceed 0.6
          Ib/million Btu, and S02 removal must be between 70 and 90 percent,
          depending on the uncontrolled emissions.

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SYSTEM DESIGN:   Powerolant Considerations	__	2-12

          (c)  If the uncontrolled emissions are between 6 and 12 Ib/million
          Btu,  the S02  removal  efficiency of the scrubber system must be 90
          percent.
          (d)  If the uncontrolled  emissions  are greater than 12 Ib/million
          Btu,  the  emission  from  the  scrubber system  may not  exceed  1.2
          Ib/million Btu.
     These regulations  are coal-related  in  that S02  removal  rate  require-
     ments are affected by sulfur content and heating value.   Note that at
     no time may the  overall  system S02  removal efficiency be less than 70
     percent, nor may the  actual  emission exceed 1.2  Ib/million  Btu.   Com-
     pliance is determined  on a continuous basis by  using  continuous moni-
     tors to obtain a  30-day rolling average.   Figure  2-2  delineates  the
     NSPS, requirements  relating to S02 emissions as  required removal  effi-
     ciency.
     (2)  The NSPS particulate  standard  limits emissions to 0.03 Ib/million
     Btu heat input.  The  opacity standard limits the opacity of emissions
     to 20 percent (6-minute average).
     (3)  Instrumentation installed to verify compliance must meet perform-
     ance specifications  and  must exhibit general characteristics  as given
     in Appendix B of Part 60, Title 40,  Code of Federal  Regulations.
     (4)  A  spare   scrubber  module  must  be  provided  if  emergency  bypass
     capabilities are  desired  at  the  plant without  a  commitment to  load
     reduction.   A spare  scrubber module is required for any facility whose
     capacity is greater than 125 MW electric.
     Table 2-5 shows  the  NSPS parameters applicable to  a hypothetical  coal
of  specified properties.   The  design  of  a  limestone  FGD  system can  be
affected by  the  S02  limitations  and by  the  degree  of redundancy required.
     Owners of new  powerplants  are required under NSPS to notify regulatory
officials of the state in  which  the proposed plant will be located  before
beginning construction.   They are also required to give notification-before
startup and to  submit operating data after startup.

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                                        ERMISSIBLE OPERATING REGION
           4         6         8         10       12
           SULFUR CONTENT IN COAL (AS HINED),*b SO-/106 Btu
14
16
Figure  2-2.   Influence of sulfur  in coal on required
                  S02 removal  efficiency.
                            2-13

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           TABLE 2-5.  NSPS AND EMISSION REGULATIONS APPLICABLE TO
                       A COAL3 OF SPECIFIED PROPERTIES0
Emission limitations:

  Particulates


  Opacity

  S02


  Redundancy
NSPS continuous emission
monitoring requirements:

  Opacity
  SO,
  Oxygen (02) or carbon dioxide (C02)
0.03 Ib particulate/106 Btu
(corresponds to 99.7% removal)

Limited to 20%    >

0.6 Ib S02/1Q6 Btu (corresponds to
90% removal)0

A spare scrubber module is required
for a facility with capacity
exceeding 125 MW electrical output
if emergency bypass capabilities are
desired
Continuous measurement of the
attenuation of visible light by
particulate matter in stack effluent
with an averaging-time interval of
6 minutes

Continuous measurement of S02 in
stack effluent (30-day rolling
average)

Monitoring required to determine
appropriate conversion factors for
flue gas stream dilution
a Coal analysis:

  Sulfur content, %          3.2
  Chlorine, %                0.06
  Carbon, %                 60.0
  Oxygen, %                  7.5
  Heating value,  Btu/lb   11,000
 Ash content, %
 Moisture, %
 Hydrogen, %
 Nitrogen, %
16.0
 8.0
 4.2
 1.0
0 Adapted from Smith et al.  1980.

c Assuming 80% of the coal ash leaves the boiler as fly ash.

  Assuming 100% of the coal  sulfur leaves the boiler as S02.

                                     2-14

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SYSTEM DESIGN:   Powerplant Considerations	2-15

     Prevention of Significant Deterioration.   New  or modified  powerplants
are  also  subject  to preconstruction  review and  to  the  requirements  for
Prevention of Significant  Deterioration (PSD).   These requirements are cur-
rently being implemented  by  the  U.S.  EPA  while  the  States  are incorporating
them into their implementation plans.
     Under the  PSD program,  clean  areas  of  the nation [i.e.,  those  whose
pollutant  levels  are  below  the  National   Ambient  Air  Quality  Standards
(NAAQS)] are  classified  as Class I,  II,  or III, each class  representing  a
specific amount or increment of allowable  deterioration.   Class I increments
permit  only  minor  air  quality  deterioration,   Class II  increments  permit
moderate deterioration consistent with normal growth, and Class  III  incre-
ments permit considerably  more  deterioration; in no  case, however, can the
deterioration  reduce the  area's air  quality below  that  permitted by the
NAAQS.   Except  for  certain wilderness  areas designated  as  Class I,  the
entire country is designated as Class  II.
     In addition to the  increment concept  and classification  system, the PSD
regulations require that each major new or modified  source apply Best  Avail-
able Control Technology  (BACT).   BACT is  determined on a case-by-case basis
for  each  pollutant; it  must  represent an emission limitation  based  on the
maximum achievable degree of reduction, taking into  account energy, environ-
mental, and economic  impacts.   At a minimum, BACT  must  result in emissions
not  exceeding  any applicable  NSPS  or National   Emission Standards for Haz-
ardous Air  Pollutants (NESHAP).  The  PSD regulations also  provide further
protection for Class  I  areas in terms of "air quality related values," such
as visibility.
     Other Pertinent Regulations.  In  addition to the PSD requirements, EPA
has  recently  published   regulations   to  protect visibility  in  designated
national parks  and wilderness  areas.   Generally,   these  visibility regula-
tions  require  the  affected states to formulate provisions  for installing
Best  Available  Retrofit  Technology  (BART)  at  existing  powerplants  whose
emissions  impair visibility,  to  consider further  controls  beyond BACT for
new  sources,  and  to develop  a  long-term  strategy  to  remedy  and prevent
visibility impairment.  These  regulations apply to  36 states containing 156
areas designated for protection as "mandatory Class  I areas."

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SYSTEM DESIGN:  Design Basis	2-16
     Local,  state,  and Federal  regulations  relating to water  pollution or
land use will  also  affect the overall system design.   The RCRA regulations
mentioned  earlier provide  guidelines for preventing the contamination of
groundwaters.  The Clean  Water Act of 1977 further  restricts the  discharge
of  powerplant  effluents  into  any  natural water bodies.   These regulations
provide a  strong impetus  to  development of closed-loop limestone  FGD  sys-
tems.
     The major effect  of  closed-loop operation  is the  buildup  of  dissolved
solids,  especially   chloride,  in  the  circulating  scrubber  liquors.   The
chloride enters  the  scrubber  in the  makeup water  and with the flue  gas as
HC1.   It  is converted  to calcium  chloride by reaction with  the  limestone.
The  dissolved  chloride accumulates  in  the scrubber  liquor because,  unlike
S02 and S03, it forms no insoluble  calcium compounds  and therefore  can leave
the  system only  with  the liquid  fraction  of the sludge.   The problem is
exacerbated  when low-sulfur,  high-chloride  coals are  used  in conjunction
with tightly closed  loops,  forced  oxidation,  and high percentages  of solids
in  the  final  sludges.   Chloride  removal   techniques  are   described  in
Appendix B.
     New limestone scrubbers  designed to incorporate chloride removal tech-
niques, with or  without  some of  the  available modifications in  scrubber
technology, could treat flue  gases from combustion of high-sulfur  and high-
chloride coals without  damage  to the environment (Borgwardt 1978).   In  such
cases,  economic factors must be considered very  seriously.

DESIGN BASIS
     The primary  factors  in  determining the  size and  configuration  of the
FGD system  are the  flow rate, composition, pressure, and temperature of the
flue gas  from the boiler.   These  parameters,  together with the  pollutant
removal requirements, reagent  stoichiometric  ratio,  and compositions of the
limestone and makeup water  provide the basis  for FGD system design.  Estab-
lishment of the  overall  system  design  is  also  based upon the  available
configuration options.   Material  and energy balances  are performed  to estab-
lish water and reagent requirements and to allow estimates  of the amounts of

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SYSTEM DESIGN:  Design Basis	-	2-17
sludge production  and energy  consumption.   The following  discussion deals
with  each  of these  major  factors that establish the overall  system design
bas is.
Flue Gas Flow and Composition
     The  flue  gas   entering  the  FGD  system  contains  combustion  products
together with sulfur oxides,  nitrogen oxides, HC1,  and some  fly  ash.   The
volume of  the flue  gas and the design velocity determine  the number of FGD
scrubber modules required.  The  amount of S02 determines the liquid pumping
rate  and  the volume of the effluent  hold tank.  A substantial part of the
fly ash generated  in combustion  is removed upstream of the FGD system, most
commonly by  electrostatic  precipitators,  and to a lesser  extent  by fabric
filters and  wet scrubbers.   Chlorides  from the coal  are absorbed by the FGD
system, and   attention  must be  given  to  the  possible  development  of high
concentrations  of corrosive chlorides.   Temperature  and moisture content of
the  flue  gas  entering  the FGD  system determine the  amount of water that
evaporates when the  gas  is cooled (adiabatically) in the scrubber.  Adi aba-
tic saturation  is  discussed in Appendix E in relation to material  balances.
Pollutant Removal Requirements
     It is  essential to  know the S02  and particulate  removal efficiencies
required  for  compliance   with NSPS  regulations.    The percentage  of S02
removal varies with  the  type of  coal fired,  its  sulfur  content,  and the
heating value,  as discussed earlier.   The S02 removal requirement determines
the amount of limestone  required and  the  quantity of  sludge produced.  The
required S02  removal  may  also be important when bypass reheat is considered
(see Appendix E).
Reagent Stoichiometric Ratio
     In its  broadest sense,  stoichiometry is a system of accounting applied
to the materials participating in a process that involves physical or chem-
ical change (Benenati 1969).  As outlined briefly in Section 1, the Stoichi-
ometric ratio (SR) in a limestone scrubbing system is the number of moles of
CaC03  fed  to the  limestone  scrubber  required to neutralize  a mole of S02;
the  theoretical value  is 1.   In practice,  however,  the actual  amount of

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SYSTEM DESIGN:  Design Basis	;	2-18
limestone required to neutralize the S02 exceeds the theoretical value.   The
ratio  of  the  actual  to  the  theoretical  amount  is  the  SR.   In  most
limestone-based FGD systems, the SR value, based on S02 removal, ranges from
1.02 to 1.30.
     The SR  depends  upon  the mass transfer capability  of the scrubber, the
quality of limestone, and the hold tank design.   Some scrubbers provide good
capability  for  mass  transfer,  and  thus  operate  at  relatively lower  SR
values.  It  should be noted that stoichiometry and pH  are not independent.
A  higher SR  may be  needed when  the  inlet S02  loading is  variable.   The
limestone requirement for an  FGD  system can be  estimated on  the  basis  of
limestone quality and the requirements for both  S02 and HC1 removal.
Limestone Composition
     Limestone  consists  primarily  of  calcium  and  magnesium  carbonates
(CaC03, MgC03) and inert material.   On the basis of their chemical analyses,
limestones are classified into three grades:
     1.   High-calcium  limestone,  containing at  least  95 percent  calcium
          carbonate.
     2.   Magnesia limestone, containing 5 to 15 percent magnesium carbonate
          and 80 to 90 percent calcium carbonate.
     3.   Dolomitic  limestone,  containing 15 to  45 percent  magnesium car-
          bonate and 50 to 80 percent calcium carbonate.
     High-calcium limestone  is  the most widely  used for  S02  absorption.   A
typical composition of such a limestone is 95 percent calcium carbonate, 1.5
percent magnesium carbonate, and 3.5 percent inert materials.   Small  amounts
of magnesium  ions  have  a beneficial effect on scrubbing chemistry (Appendix
A), and  the  use  of  magnesia  limestone  is  gaining favor.  Very  often, the
magnesium fraction of dolomitic limestone is unavailable to become magnesium
ions for FGD chemistry.
     The  utility  Project Manager,  together with  the  A/E  consultant and
potential scrubber suppliers, must determine how much limestone of what size
and grade is needed and the source of supply.   Design of an efficient system
will require  that  all project participants have a  working knowledge of the
chemical  composition, reactivity,  and grindability  index of  the  selected

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SYSTEM DESIGN:   Material  and Energy Balance	2-19
limestone.   If  possible,  pilot plant  tests should  be  conducted with  the
candidate  limestone  to determine  its  performance characteristics.   Recent
experience  in  commercial  FGD  operation  has demonstrated that  a  continuing
supply of  the  specified  grade of limestone is important to reliable system
performance.
Makeup Water
     Water  is consumed in  the scrubbing process  during the  quenching of the
incoming  flue  gas  and by  chemical and  physical  binding of  water  in  the
sludge.  Knowledge of the chemical  composition of the makeup water is impor-
tant  in  FGD system  design.   Sources of  makeup  water are raw  water,  fresh
water, and cooling tower blowdown.
     Raw  water  may  come  from a river,  lake, or  an untreated well;  fresh
water  comes from a municipal  water  system.   Filtered water  is  usually
required for pump seals.   Cooling tower blowdown  can sometimes  be  taken from
the powerplant water inventory for use as makeup  water.  This  water should
be  closely monitored,  however, because  only limited  use can be  made  of
cooling  tower  blowdown that contains high concentrations of sodium, magne-
sium, or chloride ions, as discussed in Appendix  A.
     Table 2-6 shows typical makeup water analyses.

MATERIAL AND ENERGY BALANCES
     A material balance and energy requirements should be calculated so that
the specific configuration of the FGD system can  be established.  Appendix E
presents  detailed procedures for calculating material and  energy balances,
giving  example  calculations  for  a  hypothetical   500-MW  plant  firing  an
eastern high-sulfur and a western low-sulfur coal.
     Figure 2-3  depicts  the overall inputs and  outputs  associated  with the
boiler-furnace system,  the  ESP, and the FGD system.  Flue gas leaving the
boiler-furnace system  passes  through  the ESP,  which  removes  enough parti-
culate to bring the flue gas content below the maximum allowable particulate
emission limit (0.03 ID/million Btu heat input for a new coal-fired boiler).
Inputs to  the  FGD system include the S02-laden  flue gas,  limestone slurry,

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                  TABLE 2-6.  TYPICAL MAKEUP WATER ANALYSES'
Sources of makeup water
  Normal fresh makeup water will be
  cooling tower blowdown and filtered
  well water.  Additional makeup water
  may be obtained from the recycle basin,
  which contains the following:
    0 Decanted bottom ash sluice water
    0 Plant equipment, floor, and roof
       drainage
    0 Coal pile runoff
    0 Area drains
Typical chemical composition,
  mg/liter as CaC03 except as noted
 Filtered
well water
Cooling tower
  blowdown
Calcium
Magnesium
Sodium
Total alkalinity
Sulfate
Chloride
Silica, as Si02
Orthophosphate, as P04=
Total phosphate, as P04=
Total dissolved solids, as such
Total suspended solids, as such
pH
Conductivity, pmho/cm
  200
   55
   30
  225
   25
   20
   15
  315
   <1
  7.5
  505
    800
    220
    120
    200
    800
     80
     60
      2
      6
   1375
      7
  7.5-8.0
   2200
  Smith et al. 1980.
                                     2-20

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COAL-

 AIR-
                                                    CLEANED
                                                   FLUE GAS
BOILER
FURNACE
SYSTEM


ESP


FGD
SYSTEM
              BOTTOM
                ASH
FLY
ASH
SLUDGE
                                 -LIMESTONE


                                 •WATER
                  Figure  2-3.   Overall  inputs and outputs.
                                     2-21

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SYSTEM DESIGN:  Configuration Options	    2-22
and makeup water.   The outputs Include cleaned flue gas (with residual S02,
particulate, and evaporated moisture) and dewatered sludge.
     Beginning with the  known amount and composition of the flue gas enter-
ing the  FGD  system and the emission regulations,  the  material  balance cal-
culations are performed in five steps,  as follows:
     0    S02 removal  requirement
     0    Limestone requirement/slurry preparation
     0    Humidification of flue gas
     0    Recirculation loop and sludge production
     0    Makeup water requirement
     Once the makeup  water requirement is known, the overall water utiliza-
tion is established on the basis of the mist eliminator (ME) wash procedure.
The important interplay of the ME wash requirements and water balance in the
limestone scrubbing system is discussed in Appendix E.
     Within  the  FGD  system,  energy  is consumed by the fans  that drive the
gas through  the  system,  by recirculation and transfer  pumps  that'drive the
slurry and liquor,  and by the reheater  (if  selected).   Comparatively small
amounts of energy  are also consumed by the ball mill,  thickener, dewatering
devices,  agitators,   conveyors,  bucket  elevators,  and  similar  components.
The energy demands of the fans, slurry recirculation pumps,  and reheater are
calculated separately (as shown  in  Appendix E for the  hypothetical  500 MW
powerplant).   Energy consumption by the other parts of the system is assumed
to be  20  percent of that consumed by flue gas fans and slurry recirculation
pumps.

SYSTEM CONFIGURATION OPTIONS
     After  determination  of   the  major  design  basis and  material/energy
factors,  several  system  configuration  options must  be considered.   Most
critical  among  these  are the options  for  (1) particulate removal (separate
versus  integral),  (2) location  of  the  flue gas  fan, (3) flue  gas  bypass
(versus no bypass), (4)  reheat (versus no reheat), and (5) sludge disposal.
Redundancy  and  system  flexibility,  which  are  closely interrelated,  are

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SYSTEM DESIGN: Configuration Options	2-23
needed to  some extent  In  every  limestone  FGO system.   They  are discussed
here  as  options because decisions must  be  made as to the  degree of redun-
dancy and flexibility to be incorporated into the system design.
Participate Removal
     One of the major decisions required in preliminary planning is whether
to  assign  any particulate  removal  function  to  the limestone  scrubbers.
Slack (1977) outlines the following options:
     1.   Separate high-efficiency particulate removal.
     2.   Partial  low-efficiency particulate removal.
     3.   Integral particulate removal.
     Most of the current systems operate with a separate high-efficiency ESP
upstream of the scrubber.   Fabric filters have been used to a lesser extent
in FGD systems  but are becoming  increasingly  popular.   In  these configura-
tions  there  is  no need for  additional particulate  collection   in  the  FGD
system.  The combination ESP/FGD system offers the advantages of simplicity,
segregation  of  functions,  and  relatively  low  costs   for  operation  and
maintenance.  The  ESP's afford  high reliability and can allow for emergency
flue gas bypass around the scrubbing modules without load reduction or unit
shutdown.   The  ESP/FGD system offers  further  benefits  (Devitt,  Laseke,  and
Kaplan 1980):
     0    Exotic construction materials  can be used more selectively and in
          lesser amounts.
     0    Induced-draft and booster  fans can precede rather than follow the
          FGD  system, and  thus  are  less  subject  to fouling and erosion.
     0    Sludge stabilization  and  fixation processes that use  dry fly ash
          as an additive can make use of the precollected ash.
     0    Less  total  waste  volume  is  produced by mixing  dry fly  ash  and
          dewatered sludge.
     Partial  low-efficiency particulate  removal  can be achieved by use of a
mechanical  collector  or  low-efficiency precipitator (90  to  95% removal)
upstream  of  the  scrubber.   Under  this option the  S02 scrubbers  must  be

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SYSTEM DESIGN:  Configuration Potions	            2-24

capable of  removing  the residual  particulate.  A low-efficiency participate

collector may be combined with the  FGD  system when there  is  no  system by-

pass.   This  method  has particular  advantages when additional  particulate

collection  is  achieved  by  means of a wet ESP downstream of the scrubber, as

now provided commercially (see Appendix D).

     Integral  particulate   removal   is  done  with  a venturi   or  mobile-bed

scrubber that accepts all of the particulate and S02.   Some designs have two

scrubbers in  series.   Low  capital  cost  has  been  cited as an advantage of

this option, but there are drawbacks (Slack 1977):

     0    The  product  solids cannot  be  dewatered  to  the  degree  attainable
          when dry ash is mixed with dewatered sludge.

     0    The  system bleed  must be increased because  the  effluent contains
          ash plus sludge with  higher water content than  the  sludge alone.
          The  higher blowdown reduces the  steady-state  chloride  concentra-
          tion and thereby minimizes corrosion.   This advantage, however, is
          offset by  a  larger amount of effluent that  is subject to leaching
          at the final  disposal  site.

     0    Ash  in  the   scrubber  increases  erosion  and  could   increase  the
          potential  for corrosion caused by  the accumulation  of  solids at
          wet/dry interfaces.  Careful engineering  can  effectively eliminate
          these  problems.   If the  fly  ash has  high  alkalinity,  it can be
          used for S02  absorption and can considerably reduce  the limestone
          requirements.

     0    Full particulate  removal  in the FGD system  eliminates  the option
          for bypass of  the  system.   Many utilities have  a market for some
          or all of  the ash, depending on ash properties  and  site-specific
          conditions.   In  these  cases,   full  ash  removal  in  the scrubber
          would not be feasible.

     0    The higher pressure drop  required for particulate scrubbing (com-
          pared  with S02 removal only)  increases  power  consumption.   With
          low-sulfur coal, the power consumption penalty is reduced because
          of the relatively high power requirement  of  the precipitator.  The
          use of a wet ESP downstream of the scrubber may further reduce the
          power penalty.

     When fly  ash  resistivity is  very high or when outlet particulate emis-

sions must be very low, a fabric filter may become  a viable option for total

particulate removal.

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SYSTEM DESIGN:  Configuration Options	;	2-25

Fan Location
     Fans are  used  to  overcome the pressure drop associated with the scrub-
bing system  by pushing  or  pulling the  flue gas through the  system.   This
pressure drop  may be  overcome by induced-draft (ID) fans in the main boiler
or by  booster fans in the  FGD  system.   Booster fans  generally supplement
existing boiler  ID  fans  and may be  located upstream or downstream of the
scrubbers.    Fans  located  upstream of  the  scrubbers  or downstream  of the
reheaters are  considered  dry fans;  they do not handle saturated gas and are
not sprayed  with wash water.   Fans  located  downstream of  scrubbers  where
reheat is not provided are wet fans.
     Wet booster fans  offer a size advantage because  the  cool,  wet gas has
less volume  than dry  gas.   Because of numerous operating problems, however,
the trend is to  the use of  dry  booster  fans upstream of the scrubbers.  In
view of  the  poor performance record of wet  fans, downstream fans should be
specified only with a rationale for circumventing known problems of erosion,
corrosion,  and solids deposition.
     Abrasion  effects  of particulate matter in a gas stream  require a dry
fan to be placed before a scrubber only when it is preceded by a particulate
removal device.
     The location and  operational  characteristics of boiler  ID  fans and/or
scrubber booster  fans  will  determine  whether the  FGD system  operates at
positive or  negative pressures with respect to the  atmosphere.  Location of
fans upstream  of the  scrubbers is  currently  favored  in  most installations.
It should be noted,  however, that  with upstream  fans  the  scrubbers will be
under positive pressure  and any leaks in the  system  will  allow emission of
flue gas to  the  local  environment.  Any leakage problem would be aggravated
where the FGD  system  is  located in an  enclosure.   For this reason, special
consideration  should   be  given to  ensuring  a  leak-proof  design.   Specific
areas of concern include seal welding,  access  doors,  dampers,  and penetra-
tions of the scrubber shell and ductwork.
     Where fans are  located downstream  of the FGD system,  the scrubbers and
adjacent ducts operate at  a negative pressure with respect  to the atmos-
phere.    If  leaks occur,  the  result is  inleakage of air to  the  FGD system,

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SYSTEM DESIGN:  Configuration Options	'    •  '  .  .:- .  - :..   •.  2-^26
which has  caused  high natural oxidation (see Appendix A)  but generally has
no adverse  Impact on scrubber operation.  The fans, however, are subject to
potential  corrosion  and  Imbalance caused by contact with saturated flue gas
and  entrained slurry  droplets.   Although  downstream fans  have  been used
successfully,  their  use demands close  attention to mist  eliminator  and
reheat design.
     Regardless of the  type  of  fan selected,  It  1s critical  to maintain a
balanced draft between the  boiler and the FGD  system.   This stabilizes the
boiler flow and  thereby stabilizes the flame.   A balanced  draft also pro-
vides sufficient  energy  to  move the flue gases through the FGD system In an
efficient, stable manner.
Flue Gas Bypass (Versus No Bypass)
     There  are  two basic types  of  flue  gas bypass:  emergency bypass,  in
which the  total   flue gas flow bypasses the FGD system; and continuous by-
pass, in  which part  of  the  gas  bypasses the  system.   Emergency bypass  of
flue gas  is  used only  when  the  FGD system  is  forced to shut down  and the
boiler must  continue  to  operate.   The current NSPS regulations require that
a  redundant  module  be  provided  for the FGD  system when emergency bypass
ductwork is  installed.   Thus  emergency bypass would occur only upon failure
of the total  FGD  system,  including the redundant module.
     The bypass system should be designed to prevent  undesirable eddies  or
unintended reversal of flow.  Safety in boiler operation is always a primary
consideration.  During a period  of emergency  bypass, the main  concern  is
whether the  bypass damper  will  open when the  scrubber  dampers are  closed.
Most current  systems appear acceptable in this respect.
     When bypassing of flue gas around the scrubbing system is not possible,
the boiler  operation  is  dependent on the performance  of the scrubbing sys-
tem; i.e.,  if the  scrubbing  system shuts down, so must the boiler.   Hence
the reliability of the boiler, an important consideration in utility opera-
tion, is directly linked with that of the scrubbing system.
     Continuous bypass of portions  of the flue gas is permissible only when
the S02 emissions to the atmosphere and the S02  removal efficiency conform

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SYSTEM DESIGN:  Configuration Options	2-27
with NSPS regulations.  If bypass is selected, the S02 removal efficiency of
the treated portion of the gas must be significantly higher than that deter-
mined by  simple  material  balance so as to  compensate for the untreated gas
(see  Appendix E).   Operation under the  performance standard  requiring 90
percent S02 removal efficiency, as when firing high-sulfur coal, essentially
rules out continuous  bypass.   Continuous  bypass of gas usually necessitates
that a particulate  control  device (ESP) precede the scrubbing system.   In a
system with  integral  particulate-S02   removal,  this device  may be in the
bypass duct.  Bypassing  hot gas around the scrubber  provides an economical
means of  reheat.   In addition to providing reheat energy, continuous bypass
allows the  use of smaller scrubbing modules and reduces the amount of water
evaporated by the flue gas.
Reheat (Versus No Reheat)
     At this  time,  a  preference  for stack gas reheat  is  evident  among the
systems now in service and some that are planned for the future.  Incorpora-
tion of reheat capability into the system design does offer several signifi-
cant advantages.
     First, the  greater  buoyancy of reheated gas tends  to reduce  pollutant
concentrations at ground level near the plant.  Unheated flue gas returns to
ground level  more quickly  than does reheated gas and  thus produces poten-
tially higher ground-level concentrations.
     Reheat also  helps to prevent condensation and the formation of a heavy
steam plume.   When  the  wet,  cool  gas  from  the  scrubber  is  not  reheated,
condensation  may take place,  possibly  in the exit duct  or the stack.   The
effects of  local  stack fallout are  under  intensive  investigation.   In cold
weather,  operation without reheat can generate a heavy steam plume.
     Finally, reheat  can  reduce downstream corrosion of scrubber components
by preventing or minimizing condensation of the sulfurous acid produced from
residual  S02  in the gas.  Under most FGD conditions, the inlet concentration
of  sulfur  trioxide  (S03)   is  1 to 2 percent of  the  S02  concentration.
Although  the  scrubber  removes much of the S03, nearly  half of  it passes
through as  acid  mist  (Slack 1977), which  can also  condense in the outlet
ductwork.

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SYSTEM DESIGN:  Configuration Options	                     2-28
     Some  of  the methods  available  for Increasing  the temperature of  gas
from a scrubber prior to discharge to the stack are indirect In-line reheat,
direct combustion reheat, direct hot-air reheat, gas bypass reheat,  exit gas
recirculation  reheat,  and waste  heat  recovery reheat.   The first  four  of
these methods  have  been applied in commercial limestone FGD systems operat-
ing in the United States, and waste heat recovery is planned for two future
installations.   Among  the systems that have operated  or are currently  in
service, indirect in-line reheat  has proved to be the  most popular method,
although not the most reliable.
     The various  reheat  systems  can be evaluated  in terms of  capital  and
operating  costs,  but  it is very difficult to evaluate the intangible relia-
bility factors.   Bypassing flue  gas to the degree feasible is  the lowest-
cost approach; a  problem, however,  is that designers often bring the bypass
gas back into  the system at a point near  the stack, in which case much  of
the duct from  the scrubbing modules to the  stack  is exposed to  wet gas and
is  subject to corrosion.   An  in-line  reheat  system is the next  lowest  in
cost, but tube corrosion and fouling have been major problems.
     Recently,  designers  of   some  powerplants  have  selected  a  no-reheat
system and have  designed  for  condensation  in  the outlet  ductwork,  the  ID
fan, and  the   stack.   An important  feature  of the no-reheat system is  the
low-velocity stack, in  which  the  gas velocity  is  about 30 ft/s  as  compared
with a conventional stack velocity of about 90 ft/s.   At this  low velocity,
mist droplets  can settle out  and be collected in hoppers at the bottom  of
the  stack.   Such  an  installation  requires  larger,  corrosion-resistant
stacks, with   resultant  increases in  costs and  plume opacity.   The  plume
opacity, which is due  to liquid  water droplets rather  than solid  particu-
late, could be a problem under some local regulations.
     To limit  corrosion  in a  no-reheat operation, the  designer may either
select materials  that  are inherently resistant to corrosion, such  as  high-
alloy steels and  acid-resistant brick mortar, or provide protective linings
in the ductwork  and the stack.  Many installations with wet stack operation
have problems  with stack linings,  which tend to blister and flake off.   Once
this happens,  stack corrosion begins (PEDCo Environmental 1979b).

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SYSTEM DESIGN:   Configuration Options	2-29

Sludge Disposal
     The handling and disposal of the waste or sludge generated in limestone
FGD  systems  has been  of  great concern.   The sludge can be  converted  into
gypsum for use  in  manufacture of wallboard or portland cement; however, the
presence  of  fly  ash  in  the  sludge  and the  abundance of relatively  pure
natural  gypsum  have  kept  U.S.   utilities  from  full-scale  commitments  to
produce  gypsum  from  FGD  sludge.   Several  installations  are  undertaking
gypsum production,  but none  are  yet in commercial  operation  (see Appendix
D).   As  a  result,  most  of the  sludge  is discarded,  usually in  ponds  or
landfills  (Jones  1977).   There  is, however,  an  increasing trend  toward
dewatering and/or  stabilization of  FGD  wastes for disposal  in managed land-
fills; correspondingly,  the  trend   is  away  from  pond disposal,  which has
served the  functions of  clarification,  dewatering, and temporary or final
storage of the sludge.  The increasing emphasis on disposal  in landfills and
structural fills,  in conjunction  with the desire for closed-loop operation,
has  necessitated the use  of thickeners, centrifuges, and vacuum filters for
sludge  treatment.    In addition,  several  installations  are using  forced
oxidation to enhance solids settling and filtration properties,  to improve
the sludge quality, and to reduce the land requirements for sludge disposal.
Added  benefits cited  for forced  oxidation  are  a  general  improvement  in
process  chemistry  and a  reduction  in scaling potential.  Details of these
and  other aspects  of  FGD sludge disposal  are presented  by  Knight  et al.
(1980).
     There  are  four  principal  options  for  landfill disposal:   landfill
without  treatment,  treatment by  blending  with  fly  ash  (stabilization),
fixation with  a  chemical  additive such as  lime,  and forced  oxidation.   The
first three options do not require modification of process chemistry (Ansari
and Oren 1980).
     In  landfill  without  treatment  the  sludge is  thickened  from  about  15
percent solids to about 30 percent and dewatered in a vacuum filter to about
60 percent solids.   The thickener overflow and the filtrate are returned to
the  FGD  system.   The  filter cake is  conveyed to a landfill  with no addi-
tional treatment.   Alternatively, the  waste  slurry or the  thickened waste
slurry can be pumped directly to a disposal pond.

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SYSTEM DESIGN:  Configuration Options       ;                       ...    2-30
     In a  stabilization  process,  the dewatered FGD sludge is mixed with fly
ash, soil, or other similar material to induce only physical changes without
chemical  interaction  between the  additive  and the  sludge.   Fixation  is  a
type of stabilization involving the addition of reagents that cause chemical
reactions  with  the sludge.   The  better known processes use  lime  and other
alkaline  materials such  as  blast  furnace slag  or alkaline flyash,  which
cause cementitious reactions in the sludge (Knight etal.r 1980).         - f^
     As an alternative to these landfill options, the  limestone  FGD system
can  be modified  to  include  a step that forces  the oxidation of calcium
sulfite  sludge  to  calcium sulfate  (gypsum)  by  the  introduction  of  air.
Gypsum is a more desirable waste product than calcium sulfite because of its
greater  chemical   stability  and better  settling  properties,  and  because  a
lower volume  of  material  is generated for disposal.  Gypsum  can  be readily
dewatered  to  greater than  80 percent solids,  and the  process reduces the
thixotropic potential that is inherent in calcium sulfite sludge.   In gypsum
"stacking" (see Appendix  0) the bleed slurry is merely pumped to the stack,
where the gypsum settles  to a dry state without use of either a thickener or
filter (Ansari and Oren 1980).
     If it is not to be  marketed, the gypsum sludge can have a high fly ash
content.  With forced oxidation,  the thickener can be  much  smaller, or may
not  be  needed at  all.   Gypsum  sludge can be  dewatered  satisfactorily  by  a
centrifuge without previous thickening  or  by a  hydrocyclone/vacuum filter
system.
Flexibility and Redundancy
     Flexibility and redundancy are considered as options to the extent that
decisions  are  needed concerning  the degree  to which they will be designed
into  the   system.   Flexibility  and  redundancy are  interrelated,  and  both
contribute to total system reliability.  A new FGD system should be designed
with  the  greatest possible  degree  of flexibility  that is consistent  with
cost and with site-specific considerations.   This can be done by providing a
spare  scrubbing  module and spares of  various  components,  especially pumps,
and  by  providing  surge capacity in tanks.  Under  the  current NSPS, a spare
scrubbing  module  must be  provided  in order to permit  emergency  bypass, so
that no violation occurs  in a full-demand power supply situation.

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SYSTEM DESIGN:  Computerized Guides	                2-31
     The design should provide  for expected increases in the  flue  gas flow
over the service life expectancy of the FGD system and the remaining life of
the boiler.  As the  boiler ages and leaks  occur,  the volume of flue gas to
the FGD system will  increase as a result  of  increasing  excess air  require-
ments by the boiler or air leakage into the ductwork.
     Much depends on  the  relative complexity of the  FGD  system.  A complex
system  is  more  likely to need spares  than a  simple one.  . If  the design is
based upon  the  firing of an average  coal, a spare  scrubber  module  may be
needed to  accommodate the use of other coals with  higher sulfur content or
lower heat content.
     System  breakdowns  are  usually  due  to the  failure  of pumps,  valves,
piping,  and  other such ancillary components  rather than to failure  of the
scrubber vessel  itself.   Hence proper redundancy of this  ancillary equipment
contributes to system reliability.
     Other flexibility provisions could include the  availability of  a site
for additional  S02  scrubbing modules,  or  a  built-in capability for addi-
tional S02 removal to accommodate a future boiler  unit.   The  system should
be  flexible  enough  also  to  handle the  use of additives that increase S02
removal  efficiency,  to allow modifications for byproduct utilization, and to
permit the implementation of energy conservation measures.

COMPUTERIZED DESIGN GUIDES
     Three major computer programs are available for use in the planning and
operational  stages  of a  limestone FGD project.   These  programs and their
potential  uses  are  described briefly in the  following paragraphs;  detailed
information is given in Appendix C.
TVA Lime/Limestone Scrubbing Computer Model
     The TVA and  Bechtel  National, Inc.,  have  jointly developed a  computer
model for  use in  calculating major design  parameters and costs of  lime and
limestone  FGD  systems.   The model  is  based on  results of  tests  at the
Shawnee test facility, which was constructed by the TVA with Bechtel serving
as test contractor.   The test facility is owned by the U.S.  EPA and  operated

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SYSTEM DESIGN:  Computerized Guides	          t 2-32
by the TVA  to determine the effects of various process parameters on system
performance.
     The model  is  structured to generate a complete conceptual design pack-
age for either a lime or a limestone scrubbing system.   The'package includes
a detailed  material  balance, a detailed water balance, equipment specifica-
tions and quantities,  and  a breakdown of capital investment and annual cost
requirements  for the system.   The model output  is  presented in the form of
the following individual reports:
     1.    Input data.
     2.    Process parameters.
     3.    Pond size parameters and costs.
     4.    System equipment sizes and costs.
     5.    Capital costs.
     6.    First-year annual revenue requirements.
     7.    Lifetime annual revenue requirements.
   .  Any or all of  these  reports  can  be obtained  by entering appropriate
values on the input  cards.   Also available are  several  system options such
as the  number of  scrubber  trains, number  of spare scrubbing modules, and
indirect cost percentages.
     The input for the model is of two types:
     1.    Input  needed  to  calculate  process  stream rates  and sizes  of the
          equipment  items.    This  information  consists  of basic plant param-
          eters  such  as system  gas  flow rates,  fuel  analysis,  and S02
          removal requirements.
     2.    Input  parameters  controlling the program  options.   This  informa-
          tion is derived by multiple choice from among entries given in the
          users manual.
     This model  is  intended  for use by project managers  and project engi-
neers during the  planning  and  decision-making stages.   It  is  useful for
comparison  of various FGD  scenarios.   The  model is equally useful  for the
manager and  the  engineer in that each  can  obtain  the  reports that meet his
needs; for  example,  the manager might  request summary reports,  whereas the

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SYSTEM DESIGN:  Computerized Guides	..      	2-33
engineer  might request  process parameter  reports  for  analysis of  stream
compositions and flow rates.
  -  The  model  is developed  for  an IBM 370/165 computer system. -  The  pro-
spective  user  can obtain a copy of the  program from the TVA or  can  supply
TVA staff with  the input needed for analysis of cases.   The model is  struc-
tured to  accept  input  through  punched cards  for  batch runs or  through an
interactive terminal.
PEDCo Flue Gas Desulfurization Information System (Experience Records)
     For  the  past 6  years PEDCo Environmental  under sponsorship  of EPA has
conducted an EPA Utility FGD Survey Program, whose  purpose is to  monitor and
report on the  nationwide status of utility FGD systems.   The ongoing  survey
is  conducted  through   telephone  contacts  with the system  operators  and
suppliers, visits  to the operating plants, written transmittals,  and  use of
in-house  data files.   The goals  are  to maintain  current  design  and  per-
formance  information on  the operating  FGD systems and to obtain  design and
progress  information on  systems under  construction or in various stages of
planning.    In addition  to the design  and  performance  data,  capital  and
annual cost  information is  sought regarding both the operational  and  non-
operational  systems.
     These data are  then reduced,  verified, and loaded  on a continued basis
into the  Flue Gas Desulfurization Information  System (FGDIS), a  collection
of data  base files  stored at  the  National  Computer Center  (NCC)  in North
Carolina.   The system is used to generate a quarterly EPA Utility FGD  Survey
Report,  which  summarizes current  FGD  developments.   The report  is distri-
buted worldwide  to recipients  who are directly or indirectly involved in
development of FGD technology.
     In addition  to  EPA's  printing of the  report,  the National  Technical
Information  Service  now  makes  the  FGDIS available for  on-line access by
interested users.   The  more detailed design and performance  data  that  cannot
be conveniently included  in the survey report are  available for  examination
and analysis.  Users  thus have access  to current and detailed information in
the interim periods between quarterly reports.

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SYSTEM DESIGN:  Computerized Guides	     :            2-34
     Access  to  and manipulation  of data within  the FGDIS  is  accomplished
through  SYSTEM  2000,  a general  data-base  management  system.   The  system
contains  a  complete set of user-oriented commands  that offer  flexible and
extensive data retrieval capabilities.   Both design and performance data can
be  accessed  and tabulated  in  such a manner that  virtually any information
re.quest can be satisfied.
     Because of these flexible and comprehensive data retrieval  capabilities
and the  extensive  data base,  the FGDIS  is a valuable design tool.  In pre-
liminary  design stages, the predominant FGD design schemes can  be tabulated
on the basis of current operating experience.  Various design parameters can
also be  evaluated by  analogy  to those of operating  systems.   The designer
can identify  current problems  as they are reported,  e.g.,  problems of com-
ponent failure  or  chemical  imbalance,  and can evaluate the effectiveness of
the solutions.  Additional  means of using the FGDIS in technology transfer,
together  with details  of the  system,  are  given in  Appendix  C and  in the
system users manual (PEDCo Environmental, Inc.  1979a).
Bechtel-Modified Radian Equilibrium Computer Program
     The Bechtel Corporation has fitted data to a computer program developed
by  Radian Corporation  to  predict  the  calcium sulfate (gypsum)  saturation
level  of  slurry at the scrubber outlet  on the basis of measured concentra-
tions  of  calcium,  chloride, magnesium,  and sulfate  ions (Head  1977).   This
program for monitoring of gypsum saturation was developed because testing at
EPA's  Shawnee facility has  shown that scaling usually occurs  whenever the
saturation level exceeds 130 percent.
     Simplified equations  for calculating the degree of gypsum saturation in
scrubbing liquors at 25° and 50°C were fitted to predictions of  the modified
Radian program  based on data  from  long-term  reliability  tests at Shawnee.
Results obtained with the  equations differed only slightly from  the computer
results  of  the  Radian  program.   The equations are  accurate for concentra-
tions   of total  dissolved   magnesium and  chloride ions  up to  15,000  ppm.
Additionally, they have been  expanded  to include operation with adipic acid
additive  (Burbank  and  Wang 1980).   They provide a  convenient  means  for

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SYSTEM DESIGN:  Computerized Guides	2-35
simple  and  accurate  prediction of  gypsum saturation  levels by  those  not
having access to the modified Radian program.
  --  Details  of  this  computer program and the  simplified  gypsum saturation
equations are also presented in Appendix C.

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SYSTEM DESIGN:  References                                               2-36
                          REFERENCES FOR SECTION 2


Ansari, A.,  and  J.  Oren.  1980.   Comparing  Ash/FGD Waste Disposal Options.
Pollution Engineering, 12(5):66-70.

Benenati, R. F.  1969.  Industrial Stoichiometry.   In:  Kirk-Othmer Encyclo-
pedia of  Chemical  Technology.   2d ed. Vol. 9.  John Wiley & Sons, New York.

Borgwardt, R. H.   1974.  EPA/RTP Pilot Studies Related to Unsaturated Opera-
tion  of  Lime and  Limestone  Scrubbers.  Symposium  on Flue Gas Desulfuriza-
tion, Atlanta,  Georgia,  November  4-7, 1974.   EPA-650/2-74-126a.   NTIS No.
PB-242 572.

Borgwardt, R.  H.   1978.   Effect of Forced Oxidation on Limestone/SO  Scrub-
ber Performance.   In:  Proceedings of the Symposium on Flue Gas Desutfuriza-
tion, Hollywood,  Florida,  November 1977.   Vol. I.  EPA-600/7-78-058a.  NTIS
No. PB-282 090.

Bunicore,  A.  J.   1980.   Air  Pollution  Control.   Chemical  Engineering,
87(13):92-94.

Burbank,  D.  A.,  and S. C.  Wang.  1980.  EPA Alkali Scrubbing Test Facility:
Advanced Program  Final Report (October 1974 to June 1978) EPA-600/7-80-115.
NTIS No.  PB 80-204 241.

Devitt, T. W., B.  A. Laseke and  N.  Kaplan.   1980.   Utility Flue Gas Desul-
furization in the U.S.  Chemical Engineering Progress, 76(5):45-57.

Galluzzo, N. G.,  and P.  G. Davidson.   1979.   Effects of Coal Properties on
the  Installed  and  Operating Costs  of Power  Plants.   Presented at  the 2d
International Coal  Utilization  Conference  in Houston, Texas, November 1979.

Head, H.  N.   1977.   EPA Alkali Scrubbing Test Facility;  Advanced Program,
Third Progress Report, EPA-600/7-77-105.  NTIS No.  PB-274 544.

Hudson, J.  L.   1980.  Sulfur Dioxide Oxidation  in Scrubber Systems.  EPA-
600/7-80-083.  NTIS No. PB-187 842.
                               t
Jones, J. W.   1977.  Disposal of  Flue-Gas-Cleaning Wastes.   Chemical Engi-
neering,  84(4):79-85.

Knight, R.  G., E.  H.  Rothfuss,  K.  D.  Yard, and D.  M.  Golden.   1980.  FGD
Sludge Disposal  Manual.    2d ed.   EPRI-CS 1515.   Research  Project 1685-1.

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SYSTEM DESIGN:  References	2-37

PEDCo  Environmental,   Inc.   1979a.   Flue  Gas  Desulfurization  Information
System Data Base User's Manual.  Cincinnati, Ohio.

PEDCo Environmental, Inc.  1979b.  The Lime FGD Systems Data Book.- Prepared
for  the  Electric  Power  Research  Institute  under Research  Project 982-1.
EPRI FP-1030.

Slack,  A.  V.   1977.    Design  Considerations  in  Lime/Limestone  Scrubbing.
Proceedings:    The  2d  Pacific  Chemical  Engineering  Congress (Pachec  '77)
Denver, Colorado.   AIChE, New York.

Smith, E. 0.,  W.  E. Morgan, J.  W.  Noland,  R.  T. Quinlan, J. E.  Stresewski,
D. 0.  Swenson,  and C.  E. Dene.  1980.   Lime FGD System and Sludge Disposal
Case Study.   EPRI CS-1631.  Research Project 982-18.

Stephenson,   C. D., and R.  L.  Torstrick.   1979.   Shawnee Lime/Limestone
Scrubbing Computerized  Design/Cost-Estimate  Model Users  Manual.   EPA-600/
7-79-210.  NTIS No. PB-80-123037.

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                                  SECTION  3
                               THE FGD SYSTEM
                                                                           :.l
     Major emphasis  in  this section  is  given to  the equipment items  that
most strongly affect the  operation  and performance of a limestone  FGD  sys-
tem:   the scrubber,  mist  eliminator, reheater,  and fans,  together  with
equipment used  in slurry preparation  and sludge treatment.   Each  of these
major equipment items is  presented  in a similar format:   first, a  descrip-
tion of  the  unit and  its  function,  followed by discussion  of the basic
equipment types,  including  major  design  variations.   Then follows  a  review
of the principal design  considerations, and  finally a  brief survey of  actual
applications  in operational  systems  in the United States.
     The remaining items  of equipment that  make up the scrubber system are
treated  in   less  detail.   These  items—pumps,  piping,  ductwork,  and  the
like—receive less emphasis not because  they are  considered unimportant in
scrubber operation but  because  they are  common to many other types  of major
engineering  systems  and  thus  the  functional  operations  and  basic  design
options are  better  known.  The discussions of  these equipment items  deal
primarily with specific  application to limestone scrubbing.
     Additionally, this  section gives an  overview of process  control and its
application  to  the  operation  of  a limestone scrubber system.    Once  again,
the  emphasis  is on  those aspects of the control  loops and  instrumentation
that are peculiar to the scrubber system.   An overall control  philosophy is
described, and  examples  of  reliable operational  process control  systems are
presented.                                                    •

SCRUBBERS
Description and Function
     The principal unit  operation involved  in a wet  limestone  FGD  system is
absorption of S02 from  the  flue gas  stream.   The  scrubber  is  the  principal

                                     3-1

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FGD SYSTEM:  Scrubbers	;	3-2
component  of the  system because It provides the  means  of bringing the S02-
laden  flue gas into  contact with the limestone  slurry  to  undergo chemical
reaction.
     The scrubber promotes  the transfer of energy and material  between the
flue gas and the  liquid portion of the slurry.  Within the scrubber vessel,
the following transfers occur:
     0    Transfer of heat
     0    Transfer of water vapor
     0    Transfer of gas-entrained solids
     0    Transfer of water-soluble gases
     Transfers  of  heat and  water  vapor  occur as  entering  flue gas  is
quenched and  cooled to approximately  its  adiabatic  saturation temperature.
In a well-designed  scrubber,  these transfers occur rapidly; the exit gas is
saturated with water  vapor,  and its temperature  is the  same as the average
temperature of the scrubbing slurry.
     The transfer of  gas-entrained solids (particulates) occurs  as the gas
and liquid become completely mixed.  The particles become wetted and trapped
in the  liquid  phase.   This transfer occurs more slowly than the transfer of
heat  and water  vapor.   In  a  well-designed  scrubber,  the discharged  gas
stream  contains  virtually  no  unwetted particulates (fly  ash);  essentially
all of  the particulates that enter the  scrubber  become  constituents of the
scrubbing  slurry.   In  a  scrubber designed primarily  for  gas  absorption,
there is no net increase in particulate loading.
     The transfer of  water-soluble gases,  the slowest transfer step, is the
major concern in limestone  scrubbing because it is the mechanism for removal
of S02  from the flue gas.    This discussion focuses on the  design and per-
formance aspects  of scrubbers  as  they relate  to S02  removal.   Particulate
removal  is  addressed  only  in relation to  scrubber designs  in  which primary
or  secondary  particulate   removal  is  performed simultaneously  with  S02
removal.

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FGD SYSTEM:   Scrubbers	    3-3

Basic Scrubber Types (PEDCo Environmental.  Inc.  1981;  IGCI  1976;  Calvert
1977; Saleem 1980)
     Many scrubber  designs have been  developed for removal  of  participate
and gaseous pollutants  from waste  gas  streams.   A number  have been adapted
or developed exclusively  for  removing  S02  (and particulates) from  the  flue
gas  of  coal-fired  boilers.   Table  3-1  summarizes  the  various  scrubber
designs currently used  or planned  for  use in commercial limestone  FGD  sys-
tems for coal-fired utility boilers.  The generic types  listed in this table
represent a grouping  of scrubber designs according to the  basic collection
mechanism.  The  specific types represent distinct design  variations  within
each generic grouping.   The trade  names or common names have been  assigned
to  special  or proprietary scrubber designs.   Five  generic  scrubber types
encompassing 17  specific designs  have  been developed for  commercial lime-
stone  FGD systems in  service  or  planned  for  future service  on coal-fired
utility boilers.
     Table 3-2 summarizes the  numbers of units  and the equivalent electrical
generating capacities  (MW) associated  with each  limestone  scrubber design.
The data  indicate that spray  towers are the predominant scrubber design for
application to wet  limestone  FGD systems.   It should be noted that all the
venturi scrubbers identified  in  Table  3-2, except for one  on a 235-MW unit,
provide primary  or  secondary  particulate control and remove  only a portion
of the inlet S02.  These are two-stage  scrubbing systems in which additional
scrubbers provide primary S02  control.
     The  following  paragraphs  describe in more detail the  various  scrubber
designs  used  or  planned for  use  in commercial  limestone FGD  systems for
coal-fired utility boilers.  These detailed descriptions are presented  in a
manner consistent with the classifications in Table 3-1.
     Venturi Scrubbers.  The most prominent feature of the  venturi design is
a  converging  throat,  which causes  acceleration of the  inlet flue  gas  to
velocities between 150  to 400 ft/s.   The  scrubbing  slurry,  which is intro-
duced  at  the  inlet  of the throat,  is sheared into fine  droplets  of approxi-
mately  25  to  100  microns diameter  by the  high-velocity  gas  stream.   A
turbulent  zone  created immediately   downstream of  the  throat   promotes

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    TABLE 3-1.  BASIC SCRUBBER TYPES FOR COMMERCIAL LIMESTONE FGD SYSTEMS
Generic type
          Specific type
Trade or common name
Venturi
Spray

Tray
Packed
Combination
Variable-throat/bottom-entry liquid
  distribution disk
Vari able-throat/s i de-movable blades
Variable-throat/side-movable blocks
Variable-throat/vertically adjust-
  able rod decks
Variable-throat/adjustable drum
Open/countercurrent spray
Open/crosscurrent spray
Sieve tray
Static bed
Mobile bed

Rod deck
Grid
Spray/packed
Venturi/spray
Flcoded-disk scubber
                                                     Rod scrubber
Radial flow venturi
Vertical spray tower
Horizontal spray chamber
Marble-bed scrubber
Turbulent contact
  absorber (TCA)
Ventri-sorber
                                      3-4

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TABLE 3-2.   NUMBERS  AND  CAPACITIES8 OF LIMESTONE SCRUBBER  TYPES

Scrubber type
generic/specific
Ventur1b
Variable-throat/bottom-entry
liquid distribution disk
Variable-throat/slde-novable
blades
Var i abl e-throat/s 1 de-Bovabl e
blocks
Variable- throat/vertical ly
adjustable rod decks
Subtotal
Spray
Open/countercurrent
Open/crosscurrent
Subtotal
Tray
Sieve
Subtotal
Packed
Static bed
Mobile bed
Rod deck
Grid
Subtotal
Combinations
Spray/packed
Venturi /spray
Subtotal
Total c

Operational
No.

2

1

2

4

9

11
2
13

2
2

2
3
3
1
9

9

9
42
MW

383

550

1,155

2,025

4,113

4,728
700
5,428

1,100
1,100

1,480
1,176
816
550
4.022

3,927

3,927
8,590
Under
construction
No.





1

1

2

7
2
9







2
3

3
2
5
19
MW





235

575

810

2,721
1,380
4,101







980
1,380

1,475
1,408
2,883
9,174

Planned
No.











7
3
10










1

1
11
MW
-










3,760
1,280
5,040










166

166
5,206

Total
No.









11



32


2





12



15
72
MW









3,921



14,569


1,100





5,402



6,976
31,968
 ? Gross unit generating capacity, MW.
   One unit (235 MW) uses venturi scrubbers for S02 control only.   All of the others
   use Venturis as part of a scrubbing train to provide primary or secondary particulat
   removal  and remove only a portion of the inlet  S02.
   Totals include venturi scrubbers.  Except for the 235-MW unit,  all venturi scrubbers
   •re followed by another scrubber that provides  primary S02 control.
                                      3-5

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FGD SYSTEM:  Scrubbers	 3-6
thorough mixing of the gas and slurry droplets.  Large differences in veloc-
ities of the slurry and gas occur in the turbulent zone and cause impaction.
As. the  slurry droplets and  gas  decelerate in the divergent  section of the
venturi,  the  droplets agglomerate  through collisions and a  portion of the
energy required to  accelerate the gas through the throat is recovered.  The
solids (fly ash, limestone slurry, S02 reaction products) are separated from
                                                          ....        .*.,.,*
the gas  stream by gravity as  the stream moves to the  next  scrubber stage.
     Venturis  are  considered  high-energy  devices  because  they  normally
operate  in the pressure  drop range of  10 to 20 in. H20  (in limestone FGD
systems).  They can  remove  submicron participate with increasing efficiency
as a function  of  pressure drop.   They also provide effective S02 absorption
because the finely  atomized  slurry droplets present a  large  liquid surface
area for  contact  with the gas stream.   Absorption of  S02,  however, is much
less efficient  than  in other scrubber designs because the contact time is
short and the slurry is introduced into the gas stream cocurrently.
     Variable-throat  designs  offer  the  option  of  changing  the  cross-
sectional  area  of the throat to accommodate  varying flue gas  flow rates.
With a  variable-throat design a constant  pressure  drop can  be maintained
across  the  throat,  and  thus  a  relatively constant  removal  efficiency
(particulate and  S02) can be maintained even with  widely  fluctuating flue
gas flow  rates  (turndown ratio  of  approximately 2:1).  Designs  currently
used in   limestone  scrubbers  to adjust the  throat  opening  include liquid
distribution disks,  blades,  blocks, rod decks, and adjustable drums.  Figure
3-1 depicts a  basic venturi  scrubber  configuration  and  the  van"able-throat
design options.
     Spray Tower Scrubber.   A  spray tower is an open gas  absorption vessel
in which  scrubbing  slurry is introduced into  the  gas  stream  from atomizing
nozzles.    The  relative velocities  of  the  gas  stream and the slurry spray
allow intimate gas/liquid contact for S02 absorption.
     In typical spray towers,  the pressure imparted  to the scrubbing slurry
discharged from the  spray nozzles,  together with the velocity of the incom-
ing gas  stream, produces  fine liquid droplets as  sites  for  S02 absorption.
Nozzle pressures of  15 to 20 psig are normally utilized to produce droplets
of  2500  to 4000  microns  diameter.   Droplets in this size  range provide

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 FIXED THROAT
  OPENING
Figure 3-1.   Basic venturi  scrubber and design configurations.

                               3-7

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 FGD SYSTEM:  Scrubbers	_	            3-8
 sufficient  surface  area  for S02 absorption,  and the  entrainroent problems
 normally  associated witlk_smaller droplets  are minimized.   Relatively low
                                       -j/ .-*&[*   •                 :
 gas-phase  pressure  drops,   approximately^ 1  to  4  in.  ;H20,  are normally
 encountered because  the  spray tower includes no  internals  other than spray
 headers.                ~&"
      Important design  features of a  limestone slurry  spray tower  are L/G
*.    - -~3g% ,-;«.• ,,1 ,jt          •                          ' ,*sSs2'i-~",™23i*"''''"
 ^atio, "g«"distributiorr^iand liquid distributionr^ffign^P^ratios improve
 S02 removal by increasing the surface area for mass transfer and by reducing
 the  high  Vfijuid  film  resistance  to  S02   absorption.   Moreover,  because
 calcium sulfite and  sulfate  tend to form highly supersaturated solutions by
"virtue  of their  low solubilities in  water, operation  at high  L/G ratios
 prevents  any  instantaneous  saturation  beyond 30 percent of normal solubil-
 ities.   This  reduces  the potential  for scale  formation   in the scrubber.
      Gas  distribution  is  a  critical  feature with  respect to  spray tower
 performance.   Uniform  distribution  of  gas  in  the tower  is   achieved  in
 vertical  towers  by  action   of  the sprays  on  the  rising  gas;  the sprays
 apparently impart enough  energy to the gas  stream to  distribute  it evenly.
      The  distributed  liquid  must completely cover the  cross-section of the
 tower.  The tower must  include enough spray nozzles  to provide a  spray zone
 of  uniform  density.   Placement  of  nozzles  so as to provide  a  considerable
 overlap of slurry spray reduces the problems associated with nozzle failure,
 which could  create  a path of least  resistance to gas  flow.  The number of
 spray  headers (spray  banks) through  which  slurry  is  fed to  the nozzles
 varies with  the  amount  of  S02  loading  and the  required  S02  removal effi-
 ciency.    One  to  six  spray   headers are used in  limestone spray  towers  in
 commercial FGD systems operating on coal-fired flue gas.
      Spray towers currently used in commercial limestone FGD systems include
 open/countercurrent and open/crosscurrent designs.   The open/countercurrent
 spray tower  (vertical  spray  tower)  is  a simple  configuration  in which the
 gas stream passes vertically upward  through the tower, and the slurry drop-
 lets  fall by  gravity countercurrently  to  the  gas  flow.   The open/cross-
 current spray tower  (horizontal  spray  chamber) is a variation  of the open/
 countercurrent design,  in which the gas stream passes  horizontally through

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FGD SYSTEM:  Scrubbers	^	3-9
the  vessel  and the  slurry droplets  fall  by  gravity.   Figure 3-2  shows
simplified diagrams of these basic spray tower designs.
  -  Tray Scrubber.  A tray tower incorporates a tray internal consisting of
a  horizontal  metal surface  perforated  with holes  or slots  mounted  trans-
versely  across  the vessel.  In  this  scrubber,  flue gas enters  at  the base
and passes  upward through  the holes  while slurry is sprayed  onto  the tray
from above.  The  slurry builds  up on the  tray  until  it has enough pressure
to overcome  the pressure of the gas passing up through the holes.   An equi-
librium  is established  between  the gas and  slurry  on the  tray.   The slurry
is vigorously agitated to a froth, which provides a large gas/liquid contact
area.   Absorption of S02 into the slurry occurs in drops suspended on froth.
     The sieve tray is the simplest tray scrubber design.  With the use of a
conventional  sieve tray, the gas velocities are  such  that  gas passing up
through  the  holes  bubbles  through the liquid on the tray and provides inti-
mate gas/liquid contact.  Overflow pipes  or weirs divert  a  portion  of the
slurry through a "downcomer" to preceding stages in the scrubber or directly
to the effluent hold tank.
     A principal  disadvantage  of  the sieve tray  is  its  extremely limited
turndown capability.  A stable  froth  layer  can be  maintained only within a
narrow range of gas  flow rates.   At  rates  below  the  lower limit, "weeping"
may occur, whereas at rates above the upper limit the gas/slurry mixture is
blown out  the  scrubber.   Another disadvantage is the potential for plugging
of  the  tray  holes  with  accumulated reaction  products  (scale deposits),
unreacted limestone, and/or fly ash.  Such plugging can cause an increase in
pressure drop  across the  scrubber,  sometimes  severe  enough that  the unit
must be  shut down for manual removal of the deposits.
     Pressure  drop across  each  tray  varies with  the open  tray area, hole
diameter,  slurry  recirculation  rate,  and  overflow rate.   Typical  pressure
drops  are  1 to 4 in.  H20.  Typical  hole  diameter is  1-3/16  in.   Typical
superficial gas velocities through the holes are 15 to 20 ft/s.
     A simplified  diagram  of a  sieve tray  scrubber  is  shown in Figure 3-3.
     Packed Tower Scrubber.  A packed tower incorporates a bed of stationary
or mobile  packing, which  is mounted  transversely across  the  vessel.   In a

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                                         HIST A
                                       ELIMINATOR
                          ZS   A  A
                          .'/i»' .  viv. VK • -
                         75A  A
                          7\> .  '/»»   f -.
- SCRUBBING

  SLURRY .
                      Open/countercurrent
GAS
                           SCRUBBING
                            SLURRY

                             M.\

                           -rrr
           MIST

        ELIMINATOR
                      Open/crosscurrent
           Figure 3-2.   Basic spray tower  designs.
                           3-10

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      MIST  ELIMINATOR
      TRAY
MIST PRECOLLECTOR
             SUMP
                                                        SIEVE
                                                        TRAYS
                 _7
       Figure 3-3.  Sieve tray scrubber and detail  of tray.
                                 3-11

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FGD SYSTEM:  Scrubbers     	3-12
packed scrubber,  flue  gas  enters the base of the tower and flows up through
the packing  countercurrent to the slurry, which is introduced at the top of
the scrubber by  low-pressure  distributor nozzles.  The  packing  provides a
large  surface  area for  gas/slurry  contact.   The  greater  the  contact area,
the longer is the holdup time and the more effective is the mass transfer of
SOj into the slurry.
     Depending  on  packing  type  and configuration,  packed towers  usually
operate at gas-side pressure drops  of 3  to  10  in. H20, L/G ratios of 20 to
50, and gas velocities of 5 to 12 ft/s.
     Three packed  tower  designs currently  used  in  limestone FGD  systems
include static bed, mobile bed, and rod deck.  A static bed is an essential-
ly immobile  bed  of a  packing material such  as  glass  spheres.   A mobile bed
consists of a highly mobile bed of solid spheres, which are fluidized by the
gas stream.   A  rod deck  tower consists  of a  series  of decks  of  closely
spaced immobile  rods placed  on staggered centers.  Figure  3-4 is a simpli-
fied diagram of a packed bed tower,  showing the design variations.
     Combination Scrubbers.    Combination  towers  currently used  or  planned
for use  in  limestone  FGD  systems include the  spray/packed design  and the
venturi/spray design.   In  the  spray/packed  tower, the flue gas  first con-
tacts  the limestone slurry in an open/countercurrent spray zone.   The second
stage   consists  of  a fixed  bed  of  stationary rigid packing  of  honeycombed
material.   Typical design parameters for this scrubber are gas-side pressure
drops  of 3 to  4 in. H20, L/G  ratios of  40 to 80,  and  gas velocities of 10
ft/s in the  spray zone and 15  ft/s  in  the packed zone.  A slurry with low
solids content  (5  to  10 percent)  and low pH (approximately 5)  is  recircu-
lated   in the spray zone and consists of  spent  slurry from the packed zone.
     In the combination venturi/spray tower,  the flue gas first contacts the
limestone slurry in a variable-throat venturi.  The second stage consists of
an open/countercurrent spray zone.    The  venturi  removes  primarily  partic-
ulate  and  a  portion of the inlet S02, and spent slurry from the spray zone
is used  to  improve limestone utilization while  controlling  sulfite oxida-
tion.   The spray  tower removes primarily S02 and  a portion of the residual
particulate  remaining  from the venturi.   Typical design parameters for this

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                                          OVERFLOW
                                        STATIC BED
A £>  A
'II' 'll%  Ml*
          MIST
       ELIMINATOR
          SCRUBBING
           SLURRY
       PACKING
        ZONE
        /c, DIRTY GAS
        V   INLET
                                                        SOLID
                                                        SPHERES
                                       MOBILE  BED
                                           RODS
Figure 3-4.  Packed tower and packing  types.
                        3-13

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FGD SYSTEM:  Scrubbers 	3-14
scrubber are a gs -side pressure drop of 10 in.  H20  an L/G ratio of 80, and
gas velocities  o  150  ft/s  in the  venturi  and 10 ft/s in  the  spray zone.
Design Considerations
     Important mechnical  design features  to be considered  when evaluating
the various types  of scrubbers for limestone FGD systems include L/G ratio,
gas velocity and pressure drop, residence time,  gas/liquid distribution, and
turndown capability.   A detailed  discussion of the  theoretical  aspects of
these  design  considerations  is presented  in Appendix  B.   Following  is  a
brief  review  of  these design  considerations  as they  relate  to the major
aspects of limestone scrubber selection.
     L/G Ratio.   The scrubber  L/G ratio  represents  the  primary mechanical
means  of  achieving the required S02  removal.   The  L/G ratio is  one  of the
more variable parameters in scrubber design.   For example,  the  L/G ratios of
limestone  scrubbers  in  commercial  service or planned for commercial service
on coal-fired utility boilers range from 20 to 80 gal/1000 acf.   Pilot plant
systems have been tested at L/G ratios of 5 to 120 gal/1000 acf.
     With  respect  to mechanical design  considerations, selecting  the most
appropriate L/G  ratio  for  a  specific application must take into account the
gas/liquid contact  area.   In some scrubber  designs,  the  gas/liquid contact
area is directly proportional to the L/G ratio.   In a venturi or spray tower
design,  for  example,  the  only contact  between the  gas  and liquid  phases
occurs as  the liquid droplets  pass  through  with the gas  (venturi) or fall
through the gas  (spray  tower).   In these designs, the L/G ratio affects the
quantity of S02  absorbed,  typically  in direct proportion to a  change in L/G
ratio.    In other scrubber  designs (tray, packed),  the  relationship between
L/G and  S02  is  not so direct because  of  variations  in  other  mechanical
design  factors  that  influence  S02  absorption  (e.g.,   the   presence  of
internals that provide intimate mixing of liquid and gas).
     Although  precise  L/G  ratios  cannot be  specified  to define optimal
values  for all  scrubber  designs,  the  following   generalizations  can  be
applied:
     0    If the scrubber design is one in which the gas/liquid contact area
          is directly proportional  to L/G, any increase in L/G  will  increase
          total   absorption  of  S02,  independently  of  S02  concentrations.

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FGD SYSTEM:   Scrubbers	3-15
     0    An increase in L/G will  increase total  absorption of S02,  indepen-
          dently of scrubber design.
     Table 3-3  summarizes  the  design L/G ratios of the  limestone, scrubbers
currently operating  or planned for  service on  coal-fired  utility  boilers.
     Gas Velocity and Pressure Drop.   The  superficial gas  velocity of  the
flue gas  in  the scrubber is determined by  volumetric  flow rate through the
vessel  and  by the cross-sectional  open area  of the  vessel.   Typically,  a
higher superficial gas velocity is preferred because a lower cross-sectional
area  is  then required, and  the scrubber vessel  can be  smaller.  The  upper
limit on  gas  velocity is  determined by the  flooding  potential  (or  pressure
drop) of  packed and  tray  towers  and by the mist  eliminator re-entrainraent
potential of venturi  and spray towers.
     The gas-side pressure  drop through the scrubber depends on superficial
gas velocity  and  scrubber  design;  it is increased, for example, by  scrubber
designs  that include  internal structures  to impede  the  flow of  gas  and
slurry.
     Tables  3-4  and  3-5 summarize  the design gas velocities  and  pressure
drops of current and planned limestone FGD scrubbers.
     Residence Time.   Residence time  is the amount of time  that  the slurry
is  in  contact with  the gas in the  scrubber vessel.   Generally,  increasing
the  residence time  increases S02  removal.  Thus,  a required S02  removal
efficiency can  be  achieved by increasing the scrubber residence time,  which
is  done by  promoting  slurry  holdup through  the use of internals and by
reducing  the  slurry  flow rate  (L/G   ratio).    This  alternative,  however,
increases the gas-side  pressure  drop,  which increases the  system energy
consumption because of higher demand on the fans.
     The  relationship  between residence  time  and  S02  removal  would  be
directly proportional if the S02 transfer rate were to remain constant.  The
relationship  is  not  linear,  however,  because of  changes  in composition of
the  flue  gas  and the  slurry.   Moreover,  since  residence  time  is also  a
function of equipment size, a larger scrubber tends to capture more  S02 than
a smaller one.

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            TABLE 3-3.   LIMESTONE SCRUBBER DESIGN L/G RATIOS
Scrubber type
Venturi
Van' able- throat/bottom-entry
liquid distribution disk
Van' abl e- throat/ s i de-movabl e
blades
Variable-throat/side-movable
blocks
Van' abl e-throat/vert i cal ly
adjustable rod decks
Spray
Open/countercurrent
Open/crosscurrent
Tray
Sieve
Packed
Static bed
Mobile bed
Rod deck
Grid
Combination
Spray/packed
L/G ratio, gal/1000 acf (inlet)
Operational
No.a

1
1
2
4

9

2

2
3
3
1

1
Range

10
10
12-14
16-18

19-70

27-48

10
41-60
50
60

20-60
Avg.

10
10
13
17

49

37

10
50
50
60

40
Planned
No.a






3
1



1

1
Range






10-80
54



45

60
Avg.






48
54



45

60
Represents number of systems for which design scrubber L/G is known.
                                    3-16

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          TABLE 3-4.   LIMESTONE SCRUBBER DESIGN GAS VELOCITIES
Scrubber type
Venturi
Vari abl e-throat/si de-movabl e
blocks
Van' able- throat/vertically
adjustable rod decks
Spray
Open/countercurrent
Open/crosscurrent
Tray
Sieve
Packed
Static bed
Mobile bed
Rod deck
Grid
Combination
Spray /packed
Gas velocity, ft/s
Operational
No.a

2
1

2
1

2

1
2
1
1

2
Range

90-130
80

10
22

10-15

31
13-15
13
12

7-10
Avg.

110
80

10
22

13

31
14
13
12

8
Planned
No.a




3
2





1
Range




8-10
22





10
Avg.




9
22





10
Represents the number of systems for which design scrubber L/G is known.
                                   3-17

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      TABLE 3-5.  LIMESTONE SCRUBBER DESIGN GAS-SIDE PRESSURE DROPS
Scrubber type
Venturi
Van' able- throat/bottom-entry
liquid distribution disk
Van' abl e- throat/ s i de-mpvabl e
blades
Variable-throat/side-movable
blocks
Variable-throat/vertical ly
adjustable rod decks
Spray
Open/countercurrent
Open/crosscurrent
Tray
Sieve
Packed
Static bed
Mobile bed
Rod deck
Grid
Combination
Spray /packed
Gas-side pressure drop, in. H20
Operational
No.a

2
1
2
4

9
2

2

2
3
3
1

8
Range

15
5
3-7
9-13

1-7
2

4-6

2
6-12
8
2

1-6
Avg.

15
5
5
11

5
2

5

2
8
8
2

4
Planned
No.a






5
1



1


Range






3-8
2



14


Avg.






7
2



14


Represents the number of systems for which design scrubber L/G is known.
                                   3-18

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FGD SYSTEM:  Scrubbers	3-19

     Gas/Liquid Distribution.   Gas/liquid distribution refers  to  the inter-
mixing of gas and slurry in the scrubber vessel.   Proper  distribution of gas
and  liquid is  essential  for maintaining  design  S02 removal  efficiencies.
Improper  distribution reduces  both the  effective  residence  time  and  the
effective interfacial mass transfer area.
     Uniformity of gas  flow  across the scrubber can be  enhanced  by the use
of  distribution vanes  and by  provision of  sufficient freeboard  distance
between  the  gas  inlet and first  stage (tray, packing,  spray header)  and
between the stages.
     .Uniformity of scrubbing liquor flow is achieved by  atomizing  the liquor
into  fine  droplets  to  increase  the interfacial  area and by  optimizing the
angle  of  the spray  from  the  nozzles.   In venturi and spray  tower designs,
atomization of liquid is an important consideration; atomization is affected
by  pressure  drop in  the  throat in  a  venturi scrubber,  or by nozzle type,
nozzle opening,  and   nozzle pressure  drop in  a  spray tower  scrubber.   The
fineness of droplets  in these designs is limited  by considerations of energy
consumption and entrainment.
     Turndown Ratio.   Turndown capability is the  ratio of maximum  to minimum
gas flow that  a scrubber can handle without reducing S02 removal  or causing
unstable  operation.    This ratio  is  dependent   on scrubber  design.   For
example,  although a  reduction  in gas flow rate should generally improve S02
removal  efficiency,   it  also  reduces   the  gas/liquid  interfacial   area  by
decreasing the gas dispersion in tray towers,  the liquid agitation in packed
towers, or the  pressure drop  in venturi towers.    In spray  towers, the gas/
liquid interfacial area  does  not depend on gas flow rate  or pressure drop.
     Turndown capability  is also a function of the mechanical  design of the
scrubber.   For  example, "weeping"  may occur  in  tray towers at reduced gas
flow rates because of the decrease in  gas pressure.   "Channeling" may occur
in certain packed towers at low gas flow rates.
     The range  of turndown capability  therefore  depends  on scrubber design.
In  general, the  turndown  capabilities  of spray towers (4:1) are superior to
those  of tray  towers (3:1),  packed towers (2:1),  and venturi  towers (2:1).

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FGD SYSTEM:  Scrubbers	3-20

Operational Systems
     Table  3-6 summarizes  data on selected  utility limestone  FGD  systems
according  to   scrubber  design  and  operating  parameters.   The  systems  are
representative  of  the  major  scrubber  types  described.   One  facility  is
listed  for each  major scrubber  type;  high-sulfur  coal  applications  were
selected  preferentially because  they  represent  the  greatest  severity  of
design and performance conditions.

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                             TABLE 3-6.   SCRUBBER  DESIGN AND OPERATING CHARACTERISTICS
                                         FOR OPERATIONAL  LIMESTONE FGD  SYSTEMS
Generic type
Spray tower3
Tray tower
Packed tower0

Combination
Specific type
Open counter-
current spray
Sieve tray
Mobile bed
packing
Grid packing
Spray /packed
Nodules/
unit
2
8
1
3
2
Dimensions,
ft
NR
32x16x65
30x16x34
30x16x34
30x93
(dia.xheight)
Gas flow.
acfm
296,000
238,500
400,000
400,000
387,500
Gas temp. ,
°F
129
122
280
280
280
L/G, gal/
1000 acf
74
38
60
60
NR
gas velocity,
ft/s
NR
15
12
Nff
9
AP,
1n. H20
6.0
6.0
2.0
2.0
0.7
S02
removal, X
89
80
70
70
95
ro         * Built by Babcock & Wilcox; operated on Marlon 4,  Southern Illinois Power Coop;  boiler output 173 MW gross; coal  sulfur
~*          content 3.75 percent.

            Built by Babcock & Wilcox; operated on La Cygne 1, Kansas City Power & Light; boiler output 820 MW gross; coal sulfur
            content 5.39 percent.

           c Built by TVA; operated on Widows Creek 8, TVA; boiler output 550 MW gross; coal sulfur content 3.70 percent.

            Built by Research Cottrell; operated on Dallman 3, Springfield Water, Light & Power; boiler output 290 MW gross; coal
            sulfur content 3.30 percent.
           NR  - Not reported.

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FGD SYSTEM:  Mist Eliminators	3-22
MIST ELIMINATORS
Description and Function (Conkle et al.  1976)
     A mist  eliminator collects  slurry  droplets entrained  in  the scrubbed
flue  gas  stream  and returns  them to the  scrubbing liquor.   In  limestone
scrubbing, small droplets  of liquid are formed and carried out the scrubber
with  the  gas.   These mist  droplets generally  contain both suspended  and
dissolved  solids.   The  suspended solids are derived  from  particulates  (fly
ash) collected in the scrubber, from limestone introduced into the scrubbing
liquor,  and  from  products  of  chemical   reactions  occurring  within  the
scrubber.   Similarly,  the  dissolved  solids  come  from  gaseous  species
absorbed from the  flue  gas,  from limestone and  reaction products, and from
the system's makeup water  (e.g., cooling tower  blowdown,  well  water,  river
water).
     Carryover of mist  from  the scrubber can affect both the FGD system and
the ambient  atmosphere.   It can collect  on  the downstream equipment—the
reheater ID fan, ductwork,  dampers, and stack.   Where an in-line reheater is
used,   the  reheat  energy  requirement  to  effect a  given  temperature  rise
increases  as  mist  carryover increases.   Moreover,  the mist carryover  can
collect on the  heat exchange surfaces of the  reheater  and eventually cause
plugging,  which can reduce  the  reheater1s  heat  transfer  capability  and
contribute to corrosion.
     Droplets can collect  on the blades of an ID fan.  The solids deposited
from these droplets can  cause vibration, possibly leading to failure of the
blades, rotor, housing,  and/or support structure.
     Solids from the  mist  carryover can also be  deposited in  the ductwork,
dampers,  and stack,  where  they can accumulate and  break off in chunks  that
are eventually blown through the system and out the stack.   Moreover, if the
equipment  downstream of the mist  eliminator is  constructed  of  materials
dependent on dry gas service, an increase in energy demand or a reduction in
heat transfer capability may reduce the efficiency of the reheater and  lead
to corrosion attack on these components.
     With respect to the ambient atmosphere, mist carryover can increase the
particulate loading  in  a discharge gas stream to the extent that emissions
do not comply with particulate and opacity regulations.

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FGD SYSTEM:  Mist Eliminators	3-23
Basic Types (Conkle et al.  1976; PEDCo Environmental.  Inc.  1981;  IGCI 1975)
     Two basic types  of  mist eliminator designs have  been  developed for use
in'limestone scrubbing systems:   the primary collector and  the precollector.
A primary  collector sees the heaviest duty with respect to mist  loading and
required removal efficiency.  A precollector precedes the  primary collector
and is designed to remove most of the large entrained  mist  droplets from the
gas stream before  it  passes through the primary collector.   Most limestone
FGD  systems  are  equipped  solely with  primary collectors;  only a  few are
equipped with precollectors.
     Table 3-7  summarizes   the  various designs  for primary  collectors and
precollectors  used or planned   for  use in  commercial-scale  limestone FGD
systems  for  coal-fired utility boilers.  The  generic  classification desig-
nates the  basic collection  mechanism by which the entrained mist droplets
are removed from  the  gas stream.  The  specific  classification includes the
distinct  design  variations  within  each  generic grouping.   The  common  or
trade  name classification  lists trade  names or common names  assigned  to
special  or proprietary designs.  This  classification also  includes varia-
tions of specific design  types.
     The four generic groupings established for primary collectors are based
on  collection  by  impingement,  electrostatic,  centrifugal,  and  cyclonic
mechanisms, briefly defined as follows:
          Impingement  (or  inertial  impaction) effects mist  removal  by col-
          lection on surfaces placed in the gas stream.  The liquid droplets
          thus collected coalesce and fall  by gravity (or  drain) back into
          the scrubbing liquor.
          Electrostatic deposition  effects  mist removal through  the use  of
          an electrostatic  field.  The particles entrained  in the gas stream
          are exposed  to an electrostatic  field, become charged, migrate to
          an  oppositely  charged  surface,  are  collected,  and  then  are
          returned to the scrubbing liquor.
          Centrifugal   separation  is  based  on the use  of baffles  that impart
          a centrifugal  force to the gas.   The mist  droplets entrained  in
          the gas  are  spun out to the walls of the  separator chamber, where
          they collect and drain by gravity back  to  the  scrubbing liquor.

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                   TABLE 3-7.  BASIC MIST ELIMINATOR TYPES
       Generic type
      Specific  type
  Trade  or  common  name
Primary collectors
  Impingement
  Electrostatic deposition
  Centrifugal separation

  Cyclonic separation
Mesh
Tube bank

Curved deflector plate
Baffle

ESP
Radial vane

Cyclonic separator
 Knitted wire mesh pad
 Vertical parallel tube
 bank
 Gull wing
 Open vane (slat)
 Closed vane
 Wet ESP
 Spin vane
 Radial baffle
 Cyclonic tower
Precollectors
  Bulk separation
  Knock-out collection
Baffle slats
Perforated plate
Gas direction change

Wash tray
Trap-out tray
Bulk entrainment separator
Sieve tray
90°
180°
Valve (bubble cap) tray
Irrigation tray
                                     3-24

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FGD SYSTEM:   Mist Eliminators	3-25
          Cyclonic separation  involves  the use  of tangential  inlets  which
          impart a swirl or  cyclonic  action to the gas  as  it passes through
          the  separator  chamber.   The  mist droplets entrained in the  gas
          stream  are  spun  out to  the  walls  of  the  chamber,  where  they
          collect and drain to the scrubbing liquor.
     Of these four generic  types,  impingement collectors have been used most
extensively  in  limestone scrubbers.   Virtually all of  the  impingement col-
lectors used  to date have relied on  the baffle design, through  the use of
either open slats or continuous chevron  vanes.
     Of the two generic types of precollectors, bulk separation devices have
been  used  most extensively  in  limestone scrubbers.  These  devices involve
the use of slats (similar to the open-vane primary collector design),  trays,
or  gas changes  prior to  passage through  the  primary collector.  In  the
knock-out devices, trays are used  to  collect and recirculate wash water in a
separate recirculation loop.
     The balance  of  this  discussion  focuses on  primary collectors.   (Where
precollectors  are  discussed,  they   are designated  as  such.)   Moreover,
because only  the baffle type is  in  use or planned  for use  in commercial
systems, this  discussion is  limited  to  the two design variations of baffle-
type mist eliminators:  open vane  and chevron vane.
     Open Vane.  The  open-vane,  baffle-type mist  eliminator consists  of a
series of disconnected  slats.   Two  variations  of this design are the zigzag
baffle  and  the  open  louver.   These design variations are  illustrated in
Figure 3-5.
     The open-vane design  has received  recent interest  in  limestone  appli-
cations because of ease  of washing and  the absence of "dead corners,"  which
can become clogged.   Recent  model  tests have  shown that,  during washing of
the open louver  unit, water actually circulates around  the  blades and thus
cleans both sides.
     Chevron Vane.  The  chevron-vane, baffle-type  mist eliminator consists
of  a   series  of connected  slats.   Two  variations of  this design are  the
sharp-angle  (Z-shape)  chevron  and   the  smooth-angle  (S-shape)  chevron.
Figure  3-5   illustrates  the  difference  between  the  Z-shaped  and S-shaped
bends.   Basically,  sharp-angle chevrons provide  greater  collection  effi-
ciency  and  construction  stability.   With  smooth-angle chevrons, however,

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\\\\\\\\
\\\\\\\\
    SLATS
             GAS
            DIRECTION
          OPEN-VANE DESIGN
A-V
AY
   LOUVERS
             GAS
            DIRECTION
          CLOSED-VANE DESIGN
 Figure 3-5. Baffle-type mist eliminator designs.
           3-26

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FGD SYSTEM:  Mist Eliminators	3-27
there  are fewer  problems associated  with plugging  in "dead corners"  and
reentrainment.
  -  Among all  of the baffle-type mist eliminator designs,  the  sfiarp-angle
chevron mist  eliminator is  the  predominant type currently  used  or planned
for use in limestone FGD systems.
Design Considerations (Conkle et al.  1976; PEDCo Environmental. Inc. 1981)
     Calvert  et  al.  (1974) developed theoretical equations  for  baffle-type
mist  eliminators.  The  equations pertaining  to primary collection  effi-
ciency, pressure drop, and reentrainment potential  are not rigorously appli-
cable  to  the  high-solids  slurry environment typical  of limestone scrubbers,
but they  do  provide  insight  into the relative importance of specific design
variables.
     Figure 3-6  shows the calculated relationships between  penetration and
gas velocity  with diameter of the mist droplets and  baffle angle.  Penetra-
tion is defined  here  as the  ratio of the mass of drops at the outlet to the
mass at the  inlet of the mist eliminator;  that  is,  it represents the quan-
tity that escapes collection  and thus is a measure of collection efficiency.
At moderate gas  velocities,  where collection is high and reentrainment low,
this theoretical  relationship  agrees  well with experimental  data on overall
collection efficiency.   As velocities  increase, reentrainment becomes sig-
nificant  and  this relationship no longer adequately  describes the observed
collection efficiency.   Since  high gas velocity is desirable for efficient
mist collection, designers must consider a trade-off  between high gas veloc-
ity and reentrainment.   Ostroff  and  Rahmlow (1976)  have measured gas veloc-
ities  in  chevron mist  eliminators and have  suggested a  method  for deter-
mining the point of failure.
     The pressure  drop  that  can be tolerated before  reentrainment occurs is
determined experimentally.   Then,  the maximum  gas   flow  rate can  be cal-
culated  with  known   physical  constants,  gas  properties,  mist  eliminator
dimensions, a  correction  factor,  and the experimentally determined pressure
drop.  Alternatively, if  the flow rate is  dictated by the application, the
appropriate dimensions of the mist eliminator can be  determined.

-------
      1.0
      0.1
0>
  O)
(SI
     0.01
    0.001
         6
                                            I         I         I
                                            BAFFLE  ANGLE
                                            DIAMETER OF MIST DROPLETS
8       10       12      14       16
            GAS VELOCITY,  ft/s
18
20
         Figure 3-6.   Relationship of penetration and gas velocity.
                                    3-28

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FGD SYSTEM:  Mist Eliminators	3-29
     Factors affecting  reentrainment include wash water rate  and quantity,
turndown  ratio,  gas velocity,  and  scrubber L/G ratio.  Of these,  the  most
important  are  gas velocity  and L/G ratio.   In relating these  two factors,
Figure  3-7  displays  in  the  shaded area  the  region where  reentrainment
becomes  observable.   In  that  area,  reentrainment  constitutes   0.5  to  1
percent of  the  inlet  loading to the mist eliminator.   Thus,  to maintain the
scrubber  in or below the  desired  low-reentrainment  range,.superficial  gas
velocity  and L/G ratio cannot  be  set  independently.   Clearly,  if the gas
velocity is  increased, the L/G ratio must be reduced and vice versa.
     In the  design  of a mist elimination system, several  conflicting objec-
tives must be considered.   The theoretical considerations  of  high collection
efficiency  and  reduction  of  reentrainment must  be  weighed   against  such
mechanical  factors  as washability and susceptibility of the  unit to scaling
and plugging.  The balance of this discussion of design considerations deals
with  the  following  significant  design  factors:   configuration,  number  of
stages,  number  of  passes,  freeboard  distance,  distance between stages,
distance between vanes,  vane angle,  and use of precollection  devices.
     Configuration.   The two  basic  mist eliminator configurations are hori-
zontal  and  vertical.   In the horizontal  configuration, the gas  flows  in a
vertical direction  and  opposes  the  path of drainage.   In  the  vertical  con-
figuration,  the  gas  flows  in  a  horizontal  direction through  vertically
arranged  vanes.   Figure 3-8 illustrates  these  basic  configuration arrange-
ments.
     In the horizontal  configuration,  the  mist droplet must  overcome  drag
forces  exerted  by the gas  stream before  it falls from the  mist  eliminator
blade.  The  balancing of  drag and  gravitational forces results in a longer
residence  time  of  droplets  on the  blade,  which  increases  the  chance  of
solids  deposition  and reentrainment.   This is one of  the disadvantages  of
the horizontal  configuration.  Another problem is that the direction of wash
water is limited.  Whereas the most effective washing is achieved when water
flows longitudinally along the  length of the vane,  the horizontal  configura-
tion admits  wash  water  only from the top  or  bottom of the mist eliminator.

-------
    100
     10
ro
 4->
 H-

 O
 O
 o
 10
 CD
    1.0
 CO
 CD
    0.1
                I       I        I        I
       :  NUMBER OF ROWS              6
       -  BAFFLE ANGLE              30°
          STAGGERING OF ROWS        1 1n.
          DISTANCE BETWEEN ROWS     1 in.
       h  SPACING BETWEEN BAFFLES
            IN A ROW                2.75  1n.
          WIDTH OF BAFFLE           3 1n.
DESIRABLE OPERATIONS
RANGE (SHADED AREA)
              A   A
              *   A
              O
       -O
            A SOME  REENTRAINMENT  (
-------
               HORIZONTAL ARRAY OF
                 MIST ELIMINATOR
                      VANES
               VERTICAL GAS FLOW
 HORIZONTAL/GAS FLOW
                                  VERTICAL ARRAY OF
                                   MIST ELIMINATOR
                                        VANES
Figure 3-8.   Horizontal  and vertical  mist eliminator
                    configurations.
                       3-31

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FGD SYSTEM:  Mist Eliminators	'  3-32
     In  the  vertical  configuration,  the  mist  eliminators  are  normally
installed  in a  separate  chamber  after the  scrubber.   Sometimes  they  are
mounted in the top section of the scrubber in a "suspended box" arrangement.
Removal  of  droplets  is  efficient  in  the  vertical  configuration  primarily
because  "piling  up"  of  the collected  liquor is  avoided.   Also,  in  the
vertical  configuration  the  unit can  be  operated  at  higher gas  velocity
without reentrainment. Even  though  it is more expensive,  the  vertical  con-
figuration  reduces  the  load on the  reheater because  of its  higher effi-
ciency-.
     Number  of Stages.   Both  single-  and  multiple-stage  (two-stage is  most
common) mist eliminators are used in limestone scrubbers.   The efficiency of
a  single-stage  mist eliminator  can be  increased  with the use  of  a precol-
lector.  Although the two-stage  mist  eliminator is more expensive and some-
what  more complex,  it   offers  several  advantages.   The  first stage of a
two-stage system can be washed rigorously from the front as well as from the
back.  Mist  generated in the washing operation is collected in the normally
unwashed second stage.  A two-stage unit makes possible the use of a greater
quantity of wash water at higher pressure for a longer period of washing; it
also affords greater operating flexibility.
     Number  of Passes.   The  number of  passes  in  a  mist  eliminator corres-
ponds to  the number of  direction changes the gas  stream must make before it
exits.  The  greater the  number of passes, the greater  the collection effi-
ciency and the  gas-side  pressure drop.   Because of the high-solids environ-
ment of a  limestone scrubber,  however, the likelihood of plugging increases
with  the  number  of passes.   Figure  3-9  shows  two-pass, three-pass,  and
multiple-pass  chevron mist  eliminator configurations.    Three-pass collec-
tors, the most  commonly  used in limestone FGD systems,  provide good collec-
tion efficiency (>90 %)  with adequate washability.
     Freeboard Distance.   Freeboard distance is the distance between the top
of  the  scrubber section  and the mist  eliminator.   It varies  widely among
installations, ranging from 4 ft to more than 20 ft.   In the freeboard area,
entrained  particles can  coalesce   and  return to  the scrubber  solution by
gravity  before  encountering  the mist  eliminator.   Most  particles usually

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    TWO-PASS
THREE-PASS
.SIX-PASS
        t
        GAS
        FLOW
     GAS
     FLOW
    GAS
    FLOW
Figure 3-9.   Schematic of one-stage mist eliminators  with  two-,  three-,
                       and six-pass arrangements.
                                  3-33

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FGD SYSTEM:  Mist Eliminators	3-34
drop out  in  the first 8 to  10  ft;  additional  freeboard is not effective in
removing the  smaller  entrained  particles before they contact the mist elim-
inator.                                                           -
     Distance Between Stages.   First-generation  designs  of  multiple-stage
mist eliminators  typically provided  less  than 3  ft between  stages.   With
such short distances, deposition of  solids  occurred frequently.   Designers
have  subsequently achieved  higher  collection  efficiency  by allowing  for
better-washed first  stages and  including  enough spacing between stages to
allow entrained liquid droplets  to settle out before they contact the second
stage.   The minimum  requirement for distance between stages  is  6 ft.   This
also allows sufficient space for personnel  to walk between stages for clean-
ing and maintenance.
     Distance Between Vanes.   The spacing between individual baffle vanes is
an important  factor  in  mist eliminator design.   The closer the spacing,  the
better the collection efficiency,  but the greater the  potential  for solids
deposition.  In a single-stage mist eliminator,  the spacing typically ranges
from 1.5  to  3  in.   If a  second  stage  is  used, the spacing  is  usually  the
same as that  in the  first stage.   It can,  however,  be  reduced to as low as
7/8  to 1  in.  to provide  higher efficiency for  collection of  the  smaller
droplets that pass through the first stage.
     Vane Angle.  Both  sharp and  smooth vane bends  are  used in limestone
scrubbers, but  sharp-angle vanes predominate.   Figure  3-10 illustrates  the
difference between the  120-degree  and the 90-degree bends  in  a  three-pass,
continuous-chevron mist eliminator.   Sharp-angle bends  cause  more sudden
changes in direction  of the gas and provide greater collection  efficiency,
but they  also  are conducive to reentrainment and provide  convenient  sites
for the deposition of solids.
     Precollectors.   As indicated earlier,  a precollector removes large mist
droplets from the gas stream before it passes through the primary collector.
Precollectors available for  use in limestone scrubbers  include bulk separa-
tion and knockout devices.   The  bulk separation devices  are characterized by
a  low  potential for  solids  deposition, a  low  gas-side pressure drop,  and
simplicity.   The  knockout  devices  are  characterized by a  higher potential

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                                 120*
GAS DIRECTION
GAS DIRECTION-
                  Cross section of 120-degree
                  bend chevron mist eliminator
                   Cross section of 90-degree
                  bend chevron mist eliminator
   Figure 3-10.  Mist eliminator vane angle configurations,
                             3-35

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FGD SYSTEM:  Mist Eliminators	   3-36
for solids deposition,  higher  pressure drop (~3 in. H20),  and greater com-
plexity.   Because of  these characteristics,  bulk  separation devices  are
favored  exclusively  over knock-out  devices  in limestone  scrubber applica-
tions.
Mist Eliminator Wash
     Design of  the mist  eliminator  wash system has advanced  greatly since
scaling  and  plugging problems  first became evident.  Factors  important in
the design and  operation of a mist  eliminator wash  system include the wash
water  type,  the direction,  duration,  flow rate,  and  pressure of the wash
water, and the maintenance provisions.
     Wash Water Type.  Since the  main  purpose  of mist  eliminator  washing is
to  remove accumulated  solids, fresh  water is  preferred.  In  closed-loop
operation, washing with 100 percent fresh water is not  possible.   The normal
procedure  in  limestone scrubbing  systems  is  to introduce  all  makeup fresh
water through the mist eliminator wash system.   Additional  wash water, which
is sometimes required,  is obtained by recycling clear  water from  the solids
dewatering system  (thickener,  vacuum filter,  and/or sludge pond)  and blend-
ing it with the fresh makeup water.
     Wash Direction.   The direction  of wash water flow depends on the mist
eliminator configuration  and on  the number of  stages.   With a  horizontal
configuration,  washing  is possible only from  the bottom and the  top of the
column.  With a  vertical  configuration, wash  water can be  directed horizon-
tally  from  the front or  back  and vertically  from the top.  Experience has
shown  that  washing in  a  direction countercurrent to the gas  flow,  or from
the top  in a  vertical  mist eliminator, generates  large  quantities of mist.
A  second-stage  mist  eliminator  is  therefore  desirable when  long-duration
countercurrent washing  is planned.   When a single-stage mist  eliminator is
specified, countercurrent washing is normally limited  to  short duration at
high pressure and high volume.
     Wash Duration and Flow Rate.   The mist eliminator  wash  system  can be
operated  in  a  separate  loop.   This  is possible with  horizontal  mist elim-
inators  (vertical  gas  flow)  when  knockout devices are used.   These devices

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FGD SYSTEM:  Mist Eliminators	;	3-37
can  significantly  increase  the  total  quantity  of wash  water  available,
allowing the use, when necessary,  of continuous,  high-volume sprays.
     Wash Water Pressure.   Wash water pressure,  important to design  of the
wash system, varies widely.  Operational  limestone scrubbing systems  use an
intermittent flush spray at  pressures ranging from 20 to 100  psig.   Incor-
poration of  high-pressure  washing procedures requires the use  of stainless
steel or  reinforced plastic baffles  to withstand the additional  stress of
high-pressure washing.
     Shawnee Experience (Burbank and Wang May 1980).    Much  of   the   early
effort at  the  Shawnee  test facility was devoted to  achieving  reliable mist
eliminator operation.   This  work  concentrated on the resolution of problems
with  solids deposition  that  often  arose  in  the  first  generation  lime/
limestone  FGD  systems.   Methods   initially  investigated to control  these
problems, which were caused  by scaling and plugging,  included  operation at
reduced gas  velocity  using various combinations of mist eliminator washing
techniques and hardware configurations.
     The original mist eliminator used in the spray tower prototype FGD unit
was a baffle-type,  open-vane assembly consisting of one  stage of vanes with
three passes (see Figure 3-11).  This same mist eliminator  configuration was
later  used  in the  TCA prototype.   Other  mist eliminator  configurations
tested  at  Shawnee  included  the  following:   (1) a  one-stage,  six-pass,
baffle-type, closed-vane mist  eliminator,  which was later equipped with an
upstream wash  tray;  (2)  a  two-stage, three-pass unit, also  of baffle-type,
closed-vane  design;  (3)  a  cone-shaped, one-stage, four-pass unit,  again of
baffle-type, closed-vane configuration; and  (4)  a  mesh pad  unit placed atop
the open-vane mist eliminator in the spray tower.
     Because the experiences with these units were not completely favorable,
a  wide  range  of  operating conditions  was   explored  in an Advanced  Test
Program, in  which open-vane mist eliminators were  used in both the spray
tower and  TCA  prototypes.   The most significant finding  involved the effect
of  alkali  utilization on  mist eliminator cleanliness.   It was  determined
that the mist eliminator could be kept clean  much more easily at high alkali
utilization  rates (>  85  percent)  than  at'lower  utilization rates.   The

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                                         ONE STAGE
                                        THREE  PASSES
                                  HORIZONTAL CONFIGURATION
        GAS  FLOW  (VERTICAL)
                SCALE
                           6 in.
Figure 3-11.   Original  mist eliminator configuration at
                Shawnee test facility.
                          3-38

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FGD SYSTEM:  Mist Eliminators	3-39
residual alkali  in  slurry  solids  deposited on the mist  eliminator  surfaces
continues to react  with  the S02 (and 02)  in  the  exit gas,  forming  reaction
products that are difficult to remove.   In limestone scrubbing,, which typi-
cally involves operation at low utilization values (< 85 percent),  the mist
eliminator was kept clean  by operating at a  lower pH level,  which  improved
utilization to the required minimum level  but was  accompanied  by a reduction
in S02 removal efficiency.
     Other significant findings included relationships between gypsum satur-
ation level  in the  wash  water and  the incidence of gypsum  scaling  in the
mist eliminator, between superficial  gas  velocity and loss  of mist  elimina-
tor efficiency due  to reentrainment,  between solids  levels  in the scrubbing
slurry and mist  eliminator loading,  and between the  quality and quantity of
wash water and cleanliness of the mist eliminator.
     As  a  result of  these findings, mist  eliminator wash procedures  were
developed  that  maintained  clean mist  eliminators  while  minimizing  any
impacts on process chemistry, S02 removal, and closed-loop operation.   These
procedures, for  bottom-side and top-side wash, are  summarized  in Table 3-8
as a function of high alkali utilization and low alkali utilization.
     In  operation  at high  alkali  utilization rates, a  periodic  flush with
makeup water  was all that was required to keep the  mist eliminator clean.
The bottom side  was flushed intermittently for 6 minutes every 4 hours with
makeup water at  a  specific wash rate of  1.5  gpm  per ft2 of mist eliminator
cross-sectional  area.   The top  side  was  sequentially washed by  one  of six
spray nozzles  at a  time for 4 minutes  every 80 minutes  at a specific wash
rate of  0.5  gpm/ft2.   Total  makeup  water consumption with this  scheme was
equivalent to approximately 2.3 gpm  on a  continuous  basis, well  within the
requirements for closed-loop operation.
     In  operation  at lower  utilization rates,  the bottom side  of  the mist
eliminator was  washed continuously  at  a  specific wash  rate  of  0.4 gpm/ft2
with a blend  of  clarified  scrubbing liquor and fresh water.   At a  blend of
80  percent clarified  liquor  and  20 percent fresh  makeup water,   all  the
makeup water available  to  maintain closed-loop operation was  used.   The top
side was washed with each of the six spray nozzles activated in sequence for
3  minutes  every 10  minutes at a  specific wash  rate of 0.5  gpm/ft2.   All

-------
                TABLE 3-8.   MIST ELIMINATOR WASH PROTOCOLS AT SHAWNEE TEST FACILITY
->
O
Scrubber system
Maximum flue gas rate, acfm at 300°F
Bottom- side wash
Wash scheme
Number of nozzles
Nozzle location, in. below ME
Nozzle "On" time, min
Nozzle "Off" time, min
Total on/off sequence, h
Nozzle AP, psi
Flow rate per nozzle, gpm
Total flow rate, gpm
Specific wash rate, gpm/ft2
Makeup water (continuous basis), gpm
Top-side wash
Wash scheme
Number of nozzles
Nozzle location, in. above ME
Nozzle "On" time, min
Nozzle "Off time, min
Total on/off sequence, win
Nozzle AP, psi
Coverage area per nozzle, ft2
Flow rate per nozzle, gpm
Specific wash rate, gpm/ft2
Makeup water (continuous basis), gpm
Alkali utilization >S5 percent
Spray tower
35,000

Low intermittent
10
10
6
234
4
50
7.5
75
1.5
1.9

Low intermittent
6
16
4
76
80
13
15
8
0.53
0.4
TCA
30,000

Low intermittent
2
31
6
234
4
41
37.5
75
1.5
1.9

Low intermittent
6
15
4
76
80
13
14.5
8
0.55
0.4
Alkali utilization <85 percent
Spray tower
35,000

Continuous
4
20
Continuous


21
5.0
20
0.4
20

High intermittent
6
16
3
7
10
13
15
8
0.53
2.4
TCA
30.000

Continuous
2
31
Continuous


20
10
20
0.4
20

High Intermittent
6
15
3
7
10
13
14.5
8
0.55
2.4

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FGD SYSTEM:   Mist Eliminators	3-41
available makeup  water  was  used  with  2.4  gpm  as  top-side  wash and  the
remainder as clarified  liquor  diluent for bottom-side wash.   With this wash
scheme,  some  minor  restriction of the  mist eliminator  was  caused by  the
deposition of  soft  solids (plugging).  The restriction,  however,  reached a
steady-state level  rarely above 10 percent  and usually below  5  percent of
the mist eliminator open area.
Operational  Systems (PEDCo Environmental. Inc.   1981)
     Table 3-9 lists  selected limestone FGD  systems currently  in  service
according to mist  eliminator design and operating parameters.   The  systems
listed in this  table are representative of the major designs.

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                   TABLE  3-9.   MIST ELIMINATOR DESIGN AND OPERATING CHARACTERISTICS FOR
                                     OPERATIONAL LIMESTONE FGD SYSTEMS
Coapany MM/unlt MM
Arizona Public Service
Choi la 1


Central Illinois Light
Duck Creek 1
Colorado UTE Electric Assn.
Craig 1
Kansas City Power t Light
La Cygne 1
Kansas Power & Light
Jeffrey 1
Kansas Power & Light
Lawrence 4
Northern States Power
Sherburne 1
Salt River Project
Coronado 1
South Mississippi Electric Pwi
R.O. Morrow 1
Tennessee Valley Authority
Widows Creek 8
Coal,
X S

0.50



3.66

0.45

5.39

0.32

0.55

0.80

1.0

1.30
3.70
Generic design

Spray tower



Packed tower

Spray tower

Tray tower

Spray tower

Spray tower
-
Packed tower

Spray tower

Packed tower
Packed tower
Primary collector type
generic/specific

\wf\nqement/
closed vane (1st stage)
open vane (2nd stage)

[epingeaent/closed vane

laplngeaent/closed vane

I*p1nge*ent/closed vane

I«p1nge*ent/closed vane

Iep1nge**nt/closed vane

Iiplngeaent/closed vane

IiplngeMnt/closed vane

laplngenent/closed vane
{•plngeaent/ciosed vane
Configuration

Horizontal



Vertical

Horizontal

Horizontal

Horizontal

Horizontal

Horizontal

Vertical

Vertical
Vertical
No. of
stages

Two-



Two

One

Two

Two

Two

Two

One

Three
Two
No. of passes/
stage

Two (1st stage)
Four (2nd stage)


Three

Three

Three

Two

Two

Three

Three

One
Three
Freeboard
distance, ft

13.5



12.0

5.0

12.0

4.0

3.5

14.0

29.0

KR
14.0
Distance between
stages/vanes, in.

1.0/1.5
1.0/7.1


0.8/2.5

NA

0.8/3.0

NR/2.0

1.0/3.5

10.0/4.0

0.7/1.2

m
i.b/i.s
00

*»
rvj
     (continued)

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     TABLE 3-9 (continued)
Company Mac/unit MM
Arizona Public Service
Choi la 1
Central Illinois Light
Duck Creek 1
Colorado UTE Electric Assn.
Craig 1
Kansas City Power & Light
La Cygne 1
Kansas Power & Light
Jeffrey 1
Kansas Power & Light
Lawrence 4
Northern State* Power
Sherturne 1
Salt River Project
Coronado 1
South Mississippi Electric Pwr.
R.D. Morrow 1
Tennessee Valley Authority
widows Creek 8
AP.
In. H20
O.S
1.0
1.0
1.4
NR
1.0
0.5
0.5
NR
1.0
Precol lector type
generic/specific
NA
NA
Sieve tray
Bulk separation/
sieve tray
NA
Bulk separation/
bulk entra invent
separator
NA
NA
NA
NA
Wash systeai type
Makeup water
from well
Fresh water and
Fresh water
Blended pond and
lake water (under-
spray); pond water
(overspray)
Pond water
Cooling tower blow-
down, pond over-
flow, flash water
Thickener overflow
and cooling tower
blowdown
Cooling tower
» < 	 *jll_m
DIUWQOWn
Supernatant and
fresh water
Fresh water
Hash system direction
Vertically downward
(1st stage)
Front spray/backspray
pond overflow
Overspray
Overspray/underspray
NR
1st stage: vertically
upward and downward
2nd stage: vertically
upward
1st stage: vertically
upwards, 180° roto-
table; between stages,
360° rototable lance
NR
NR
Vertically downward
wash and horizontal
front water
Hash system duration
Intermittent
Continuous
Intermittent
Continuous underspray/
intermittent over-
spray (every 8 hrs. )
NR
2 minutes every 8
hours
2 minute* every 24
hours
NR
Continuous,
Continuous front;
Intermittent top
Wash water
pressure, psig
60
Low pressure
70
80
150
150
120-150
70
NR
1
'20
4k
00
           NA - not applicable.
           NR - not reported.

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FGD SYSTEM:  Reheaters	3-44
REHEATERS (Choi et al. 1977)
Description and Function
     In its most  fundamental  sense,  stack gas reheat  involves  the addition
of thermal  energy to the gas  stream  discharged  from the scrubber  so  as to
raise its  temperature.   No  equipment  item, subsystem, or unit  operation of
the scrubbing  process is more controversial  than stack gas  reheat.   Ques-
tions are raised concerning the need for reheat and the effectiveness of the
various strategies for achieving the desired level of reheat.   The following
reasons are generally advanced for using reheat:
     0    To prevent condensation  and subsequent  corrosion in  downstream
          equipment such as ducts, dampers, fans, and stack.
     0    To prevent formation of a visible plume.
     0    To enhance plume rise and pollutant  dispersion.
     The mechanisms  by which  a reheat system achieves these  objectives are
discussed  in the  following discussions  of equipment types and  design con-
siderations.
Basic Types
     A systematic  grouping of the various stack gas reheat  methods available
for use  in limestone  FGD  systems is  provided  in Table 3-10.   The generic
classification  represents a grouping  of reheat methods according  to varia-
tions in  configuration or  energy source  needed  to  raise the  gas stream's
temperature.  The specific  classification  represents distinct design varia-
tions within each generic  grouping,  and the common classification indicates
further design  variations within each  specific class.
     In-line Reheat.   In-line  reheat  involves  the  installation of a heat
exchanger in the  flue gas  duct downstream of  the mist eliminator.   The heat
exchanger is a  set of tubes or tube bundles through which the heating medium
is circulated.   The gas passes  over  the  tubes  and  picks up  thermal  energy
from the surfaces of the heating tubes.   These tubes are typically arranged
in banks.   Figure  3-12  is  a  simplified  diagram  of an in-line  reheater.

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                      TABLE 3-10.   BASIC REHEATER TYPES
       Generic type
     Specific type
     Common designs
In-line



Direct combustion



Indirect hot air


Waste heat recovery


Exit gas recirculation

Bypass
Steam
Hot water
In-line burner
External combustion
 chamber

Boiler preheater
External heat exchanger

Gas/gas
Gas/fluid

Steam heat exchanger

Hot side
Cold side
Bare tube
Fin tube
Shell and tube

Natural gas
or
Oil

Boiler combustion air
Steam tube bundles

Wheel type
Tube bundles
                                     3-45

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co
                                                                        REHEATER
BOILER
1


AIR
PREHEATER


ESP


SCRUBBER



STEAM
^

1
*

i
*




STACK

                                                                  OR
                                                               HOT  WATER
                                 Figure 3-12.   In-line reheat system.

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FGD SYSTEM:   Reheaters	3-47
     In-line reheaters  can  be classified  according to the  heating  medium:
steam or  hot water.  When steam  is  used,  the inlet steam temperatures  and
pressures  range  from 350°  to  720°F  and  115  to  200  psig,  respectively.
Saturated  steam  is  preferred  because the  heat  transfer  coefficients  of
condensing  steam  are  much  higher  than those  of  superheated steam.   The
configuration of  a reheater  using hot water  is  similar to that of  the  one
using steam.  Inlet temperature of  the hot water ranges from. 250°  to 350°F,
and the temperature drop over the  heat exchanger  is 70°  to 80°F.
     Fin-tubes are  used  in some  applications  because of their superior heat
transfer capability.  Soot blowers are generally  used to periodically remove
deposits that build up on the tube surfaces.
     Because  in-line reheaters are  situated  in the gas stream,  they  are
prone to corrosion and plugging,  the latter due to the  reheater's dependence
on  proper  operation of  the upstream  mist   eliminator.   Moreover,  solids
deposited on the tubes can reduce  heat transfer considerably, increasing the
potential for corrosion of downstream equipment.
     Direct combustion—A direct combustion reheater eliminates the need for
heat exchangers.   As shown  in  Figure 3-13,   gas or oil  is  burned  and  the
combustion  product  gas (at 1200° to  3000°F)  is  mixed  with the flue  gas to
raise its temperature.
     Direct firing  requires  some  care in mixing  the hot combustion gas with
the cool  scrubbing  gas.   If  mixing is not effective, hot spots can develop
downstream  from the heater,  causing damage to the duct  lining.  Also, main-
tenance of  flame  and flame  stability are required  for  effective operation.
The main  problems  with direct firing  are  the availability and cost  of  gas
and oil.
     Indirect hot air reheat—As   shown in  Figure  3-14,  indirect hot  air
reheat  is  achieved  by  heating  ambient air with an  external  heat  exchanger
using  steam  at  temperatures  of  350° to 450°F.   The  heating  tubes  are
arranged  in  two to three banks in the heat exchanger.   Hot air and flue gas
may be  mixed by  use of a device such as a  set of nozzles  or a  manifold.
     The  advantage  of indirect hot  air reheat over in-line  reheat  is that
the indirect system involves  no  corrosion  or  plugging because it is located

-------
                                                                         REHEATER
BOILER



AIR
PREHEATER



ESP



SCRUBBER





r"


STACK
A IN-LINE
f BURNER
                                                                      FUEL AND AIR
                                   a. SYSTEM WITH AN IN-LINE BURNER
00
                                                                         REHEATER
           BOILER
   AIR
PREHEATER
ESP
SCRUBBER
STACK
                                                                                                  FUEL
                                                                                                 AND AIR
                                    b. SYSTEM WITH AN EXTERNAL COMBUSTOR
                                                         EXTERNAL
                                                        COMBUSTION
                                                         CHAMBER
                             Figure  3-13.  Direct combustion reheat systems.

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GJ
i
                                                                        REHEATER
                                                                                                  AMBIENT

                                                                                                    AIR
                                                                               EXCHANGER
                              Figure 3-14.   Indirect hot air reheat system.

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FGD SYSTEM:  Reheaters	'     	3-50
outside of  the  scrubber gas discharge duct.  The  disadvantages  include the
need for an additional fan to convey hot air; the relatively large amount of
space required  for  the  reheat system (compared with  other  reheat methods);
the  increase  in stack  gas  volume,  which may be undesirable because  of the
limited capacities of ID fans and stacks; and the higher energy consumption,
which is  needed to  heat air from  the  ambient  temperature  level.   Another
advantage,  however,  offsets the  higher  energy requirement to  some extent:
the dilution by the added air reduces the incidence of steam plume formation
and gives better plume dispersion.
     Another variation of the indirect hot air reheat method involves use of
the  air  preheater associated with  the  boiler to provide hot air for stack
gas  reheat.   In this case,  the temperature of the combustion  air entering
the boiler would be lower than usual because part of the heat content of the
flue gas  is used  to provide the hot air for stack gas  reheat.   Also,  the
preheater should be  designed to heat a  greater-than-normal  amount of air to
a lower-than-normal exit temperature.
     Waste heat recovery—In  the waste   heat  recovery  method  (see  Figure
3-15),  the  sensible heat of unscrubbed  flue gas, which  would  otherwise be
released  in  the scrubber,  is  recovered  in an in-line  heat  exchanger.   The
heat is  used  to reheat  the scrubbed  stack  gas.   A  direct gas/gas  heat
exchanger, especially the wheel  type (e.g., Ljunstrom),  can  be used for this
purpose.   This  type  of  reheat system requires a large heat-transfer area in
the  in-line  heat  exchangers  because   of  the  narrow  temperature  range
involved.   Another method involves  gas/fluid heat exchange  through a medium
such as   water   or  a  fluid of  high heat capacity  (e.g.,  a  glycol-water
mixture),  which  is  circulated  through  in-line  heat  exchangers  (tubes)
located upstream and downstream of the scrubber.   With this  method, there is
a potential for corrosion due to acid condensation when the hot flue gas is
cooled.
     Exit gas recirculation—In reheat by  exit  gas recirculation, a portion
of  the  heated stack gas is  diverted, heated further to  around 400°F by an
external  heat exchanger, and injected into the flue gases.   An advantage of
this type  of  reheat  system  over indirect hot air reheat is  that it does not

-------
co
i
en
BOILER


AIR
PREHEATER


ESP
                            Figure 3-15.  Waste heat recovery reheat system.

-------
FGD SYSTEM:  Reheaters	3-52
iii.rease the total  stack gas flow rate.  Moreover,  the reheat operation is
less influenced by ambient air conditions.
  -  A  simplified diagram  of recirculation  reheat  is presented  in Figure
3-16.  This type of reheat system can be used in combination with an in-line
reheat  system  to  evaporate  the  mist  droplets  in  the flue  gas  from wet
scrubbers before they impinge on the heat exchanger tubes.
     Bypass reheat--In the  bypass  reheat system,  a portion of  the hot flue
gas from the boiler is allowed to bypass the scrubbing system and is mixed
with flue  gas  that has been processed through the scrubber.   Two variations
of  this method   are  "hot-side"  bypass, in  which  the flue  gas  is  taken
upstream of the  air preheater  (Figure 3-17),  and  "cold-side"  bypass,  in
which the  flue gas is taken downstream  of  the  air preheater (Figure 3-18).
In the  former, a  separate particulate removal device (ESP or fabric filter)
is required for fly ash control  if an upstream particulate collector is not
used.  The  limiting  conditions  for application of this sytem are determined
by:  (1)  the  properties  of  the  boiler fuel, such as  heating  value, sulfur
content, and ash  content;  (2) particulate control (ESP) ahead of the scrub-
bing system; (3)  the  temperature of hot flue gas; (4) the efficiency of the
scrubbing system;  and (5) emission regulations.
Design Considerations
     In  new plants,  and whenever possible  in  retrofit  installations,  the
steam for  reheat  should be taken from a point in the boiler cycle where its
withdrawal  will not derate the electrical generating capacity.
     Additionally, several  important  design factors  should be considered in
evaluation of the various stack gas reheat methods for a particular applica-
tion:
     1.    The heat and energy requirements needed to effect normal operation
          with an  external  source of reheat energy,  and at  the same time
          0    to prevent downstream condensation
          0    to prevent formation of a visible plume
          0    to enhance plume rise and pollutant dispersion.

-------
                                                                          REHEATER
            BOILER
   AIR
PREHEATER
ESP
SCRUBBER
CO
I
en
CO
STACK
                                                STEAM
                                                                           HEAT
                                                                         EXCHANGER
                          Figure 3-16.  Exit gas  recirculation  reheat  system.

-------
1
01
*»
BOILER



ESP




AIR
PREHEATER


SCRUBBER
PORTION OF

FLUE (


3AS 1


STACK

Fiqure  3-17.  "Hot-side" flue gas bypass reheat.

-------
                                      REHEATER
co
tn
en
BOILER


AIR
PREHEATER


ESP


1
P
SCRUBBER

ORTION OF

FLUE (

5AS 1


STACK

Figure 3-18.  "Cold-side" flue gas.

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FGD SYSTEM:  Reheaters	3-56
     2.   Constraints  Imposed  by  S02  standards  when  bypass  reheat  is
          selected.
  -  3.   The  feasibility  of using  no reheat and  the resultant, impact on
          system design.
     These considerations  are  discussed briefly in the following paragraphs
with  respect to reheat  methods that  are  currently in use  or  likely to be
used in commercial limestone FGD systems.
     Definitions of  pertinent  terminology (from Perry 1973)  are given here
as an aid in understanding of the discussions.
     Absolute humidity:  The amount  of water vapor carried by one unit mass
     of dry air.
     Relative humidity:  The partial  pressure of water vapor in air divided
     by the vapor pressure of water at a given temperature.
     Dew point or saturation temperature:  The temperature  at which a given
     mixture of water  vapor and air is saturated.   Dew point represents the
     temperature at which the relative humidity is 100 percent.
     Wet-bulb temperature:   The equilibrium  temperature  attained by a water
     surface when  the  rate of  heat transfer  to  the liquid surface from the
     air-water vapor mixture equals the rate of energy carried away from the
     surface by the diffusing vapor.
     Prevention of Downstream Condensation.  The reheat  requirement to pre-
vent  downstream condensation  can be  estimated by making  a  heat balance
around the downstream system, including the stack.   Condensation takes place
when  the  vapor pressure of water in  the stack gas  exceeds the saturation
value at a  specific  stack  gas  temperature and pressure.   To prevent conden-
sation (1) the  gas temperature  is raised above the dew point or (2) the gas
is diluted with another gas so that the relative humidity of the mixture is
less  than  that of the  original  stack gas.  A combination  of both measures
can be used.
     1.   In-line reheat:   A schematic of the heat balance around the down-
stream system,  including an in-line  reheater, is shown in Figure 3-19.  The
gas temperature at the top of the stack should be at the dew point or higher
to prevent  condensation  before  emission.   Because entrained moisture (mist)
carried over from  the  scrubber is vaporized  as  the flue gas temperature is
increased during reheat, the dew point at the  top  of the stack is slightly

-------
I
in
         FLUE GAS FROM
         SCRUBBER, T
                    1
 STEAM IN-



STEAM OUT-
                                         HEAT LOSS
                                      FROM DUCTS, QLD
                                 HOT AIR, Tha
                                    J
       HEAT LOSS
    FROM STACK, QL$
                                                                      I.
HEAT REQUIRED TO
 EVAPORATE MIST
  CARRYOVER, L
                            STACK GAS TO
                           •ATMOSPHERE, T
                            DEWPOINT, T
                           AMBIENT AIR

                              IN* Tca
                            Figure 3-19.   Schematic of  heat balance around
                           downstream system with  indirect hot-air reheater.

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FGD SYSTEM:  Reheaters	        3-58
higher than the  temperature  of gas exiting the  scrubber.   The  minimum heat
requirement from steam or hot water is determined as follows:
Heat       Heat required      Heat      Heat          Heat      ~     Heat
input  =  to raise gas to  +  loss   +  loss   +   required to    -   gain
           its dew point      from      from      evaporate mist     due to
                              ducts     stack       carryover         fan

°r            FC_m
          Q = -£- (Td - Tx) + Q,D + Q,s + L - QF                  (Eq.  3-1)
where:         k     a          LU    Li        I-

     Q = minimum heat requirement from steam or hot water,  Btu/h
   Cm = mean heat capacity of flue gas, Btu (lb-mol)°F
     F = flue gas flow rate, scfh
    T. = dew point of stack gas at top of stack, °F
    T! = temperature of flue gas exiting wet scrubber,  °F
   Q,D = heat loss from ducts, Btu/h
   Q,s = heat loss from stack, Btu/h
     k = 379 ftVlb-mole
     L = heat required to evaporate mist carry-over, Btu/h
    Qc = heat gain from ID fan, Btu/h
For  estimation  purposes,  the  overall  heat-transfer  coefficient  can  be
assumed to be  10 Btu/ft2 per °F per h for steam/gas (condensing steam)  heat
exchange and to  be  6 Btu/ft2 per °F  per  h for hot water/gas heat exchange.
Because of the  very small difference between T. and Tl5 the amount of  heat
required to raise gas to its dew point is very small.   The  method for calcu-
lating dew point is shown in Appendix E.
     The values  of QLD,  Qp,  and QLS vary with the specific  situation;  Q.p
could be significant, depending on whether the duct is  insulated, the length
of duct,  and the  difference  between stack gas temperature  and  ambient air
temperature.   The value  Q.$ depends on the height of the stack,  the materi-
als  of  construction, and  the difference between stack  gas  temperature and

-------
FGD SYSTEM:  Reheaters _ 3-59
ambient air temperature.   In  stacks  with an annular space between the stack
and liner, the temperature drop should not be significant.
  -  The  heat required  to evaporate  mist carryover  (L) depends  upon  the
amount of carryover from the mist eliminator to the reheater.
     2.   Indirect hot air reheater:   Figure 3-20 is a schematic of the heat
balance  around  the  downstream  system,  including  an  indirect  hot  air
reheater.   In this system, the  flue gas  from a wet scrubber  is  heated by
addition of hot  air.   The minimum heat requirement from steam is determined
as follows:
Heat    Heat required   Heat loss    Heat loss    Heat required     Heat gain
input = to raise gas  + from ducts + from stack + to evaporate   = from heated
        temperature                               mist carry-           air
         to its dew                                   over
        temperature
or
                              Heat required by amount
             Net heat input = of ambient air to reach
                              temperature T.
          or
where:
    F  = ambient air flow rate, scfh
     B
   C a = specific heat of air, Btu/(lb/-mole)°F
    h
       = temperature of hot air, °F
   T   = temperature of ambient air, °F
    Col
    QN = net heat input, Btu/h
The total volume flow rate of stack gases is F + F ,  and the other symbols
are as previously defined.
     Equation  3-2  is used  to determine the  temperature of  hot air (T.  ),
then Equation 3-3 is used to determine the net heat input.

-------
                                              HEAT GAIN FROM
                                                ID FAN, Qc
                               HEAT LOSS FROM
                                 DUCTS, QLD
                                               HEAT  LOSS  FROM
                                                 STACK, QLS
                                                                    1
               FLUE GAS FROM
CO
crv
o
SCRUBBER, T
           1
 STACK GAS TO
ATMOSPHERE, T2
 DEWPOINT, Td
                                       NET  HEAT  INPUT,  Q
                                                             HEAT REQUIRED TO
                                                              EVAPORATE MIST
                                                               CARRYOVER, L
                            Figure  3-20.  Schematic of heat balance around
                              downstream system with in-line reheater.

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FGD SYSTEM:  Reheaters       	3-61


     Prevention of Visible Plume.   The following methods of determining heat

requirement are  currently applied in most reheat applications.   For normal

operation  and  prevention of  visible  plume,  the temperature desired at the

top of  the stack is  selected  by  the designer.   It is  usually  between 125°

and 220°F.   The reheat  requirement  for normal  operation  and prevention of

visible plume can be estimated by making heat balances as follows:

     I.    In-line reheat:  The minimum  heat  requirement with in-line reheat

can be calculated as follows:

Minimum      Heat required     Heat     Heat      Heat     Heat required
 heat     =  to raise gas   +  loss  -  gain   +   loss  =   to evaporate
required     from tempera-     from     due      from     mist carryover
             ture T! to T2     ducts    to       stack
                                        fan

or
         F C m
     Q = -*£- (T2 - TO + QLD -  Qp + QLS + L                      (Eq. 3-4)

where,

     Q = minimum heat required, Btu/h

    T2 = stack gas temperature at the top of  the stack, °F

     All other symbols have been  defined.

     2.    Indirect hot  air:   The  minimum heat requirement with indirect hot

air reheat can be calculated as follows:

Minimum        Heat          Heat          Heat        Heat    Heat   Heat
 heat    =   required    + required  =   required   - loss +  gain - loss
required     to raise      to evapo-     by amount    from    due    from
            temperature    rate mist    of ambient    ducts    to    stack
           from Tj to T2   carryover   air to reach           fan
                                       temperature
                                       Tha from
or
                                               • QLD + QF - QLS    (Eq. 3-5)

     Net heat input = Heat required by F  amount of ambient air to reach
                                        Q
                      temperature Th

-------
FGD SYSTEM:  Reheaters _ ^_ _ 3-62
or
     Equation 5  is  used to determine the temperature of hot air (T.  ); then
equation 6 is used to determine the net heat input.
     Enhancement of Plume Rise and Dispersion of Pollutants.  Using  a lime-
stone scrubber without reheat could result in poor plume rise and dispersion
characteristics,  which  in  turn could  cause undesirably  high  ground- level
concentrations  of  pollutants  (residual  parti cul ate,  SO  ,  and NO  ).   The
                                                         A         A
computation  of   reheat  requirements to  enhance  plume  rise  and dispersion
requires a quantitative  analysis  of plume behavior in the atmosphere, which
is  a  function of  such  variables as meteorological  conditions,  stack size,
and characteristics of the stack gas at the emission point.
     A family of curves was developed using mathematical models that predict
maximum ambient  concentrations  under certain given conditions.   As shown in
Figure 3-21,  each curve  represents a given percentage of  S02  removal from
the gas.   The reduction  in  ambient S02  concentration  is  equal to  the S02
removal when  the stack  gas  is reheated to  the  scrubber  inlet  temperature,
because ground-level concentration is directly proportional to the amount of
S02 emitted from the  stack.   Because the temperature of the scrubber gas is
lower  (unless reheated  to scrubber inlet temperature), the ambient  concen-
tration will  be a higher value.
     The  curves  show  that at  any  S02  removal  efficiency, the  amount of
reheat has  a pronounced  effect on  ground-level  S02 concentration.    At 70
percent S02 removal and a stack gas temperature  of  175°F, for example, the
ground- level  concentration can  be  reduced from 50 percent with no reheat to
36 percent of the maximum values — a 28 percent  improvement.  With the same
amount of reheat at 90  percent S02  removal,  the  ground-level concentration
of S02 is  reduced from  17 percent  to  12.5  percent  of the maximum values — a
26.5 percent  improvement.  Thus it can be concluded that even with increas-
ing S02 removal  efficiency,  reheat definitely offers a substantial benefit.
A  much greater  benefit  can  be obtained,   however,  by increasing  the S02
removal efficiency of the scrubber.

-------
            25
REHEAT INCREMENT,^
50     75     100    125
             150    175
                                 I      i       I
                                 LEGEND:
                                       S02 REMOVAL, %
      -NOTE:   MAXIMUM GROUND-LEVEL CONCENTRATION IS
              THAT ATTAINED WITHOUT A WET FGD SYSTEM
              AND WITH A STACK GAS TEMPERATURE OF
            I  300°F.|
        I
I
I
I
    125    150    175    200    225    250
                 STACK GAS TEMPERATURE, .°F
                            275   300
Figure 3-21.   Effect of reheat  increment on ground-level
                        S02  concentration.
                          3-63

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FGD SYSTEM:  Reheaters
                                                                        3-64
     The relative improvement in the ground-level air quality remains almost
identical for  various  S02  removal  efficiencies over a wide  range of reheat
increments, as  indicated  in  Figure 3-22.   At a  reheat  increment of 150° to
175°F (stack gas temperatures of 275° to 300°F),  the curves approach a value
of about  40 percent  relative  improvement asymptotically.   This  means that
theoretically a maximum  relative  improvement of 40 percent can be attained.
The decision  on amount  of reheat to  be  used,  however, must  depend on the
economic  considerations  as  well   as  plume  rise and pollutant  dispersion.
     It  should be  noted that  the higher water  vapor  content  (15  percent
versus 6  percent)  in  the gas offsets to  some  extent the adverse effects of
gas cooling.   Since the  water vapor  is  of lower  density than  other con-
stituents of the gas,  it makes the plume more buoyant.   The effect is small,
however, and has been omitted in developing the curves.
     Constraints on Bypass Reheat.   Bypass  reheat offers  the  advantages of
low capital investment and simple operation.  The maximum amount of reheat
that can  be obtained, however,  is limited by the constraints of pollutant
emission  standards.   A  regulation requiring 90  percent S02  removal  effi-
ciency could  limit or completely  rule out  the  bypass  reheat option.   The
limitation on  use  of  bypass  reheat to meet the emission standard for sulfur
dioxide can be written as:
(Fraction of bypass
  flue gas stream)
                    _ ,
  fractional
sulfur removal
  efficiency
                                                (fuel
                                               required)
                                                          (sulfur  (fractional
                                                          content  sulfur re-
                                                          in fuel) moval effi-
                                                                     ciency)
or


where:
     X
     E
     A
         fraction of bypass flue gas stream
         fractional sulfur removal efficiency of the wet scrubbing system
         maximum allowable S02 emission, lb/106 Btu

-------
                   REHEAT  INCREMENT,°F
                 50     75     TOO    125
150    175
             S02 REMOVAL
                         RELATIVE IMPROVEMENT  IS THE
                         MARGINAL GAIN  OVER A  BASE
                         CASE OF NO REHEAT, EXPRESSED
                         AS A PERCENTAGE.
                 175    200    225    250
                STACK GAS TEMPERATURE,  CF
Figure 3-22.   Relative  improvement  in ground-level S0?
  concentration  as  a  function of degree of reheat.
                        3-65

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FGD SYSTEM:  Reheaters	3-66
     W = amount of fuel required to generate 1 million Btu, 1b
     S = weight fraction of sulfur in the fuel
     Details of the  heat balance around a bypass reheat system are given by
Choi et al. (1977).
     The No-Reheat Option.   As  indicated earlier,  the  use  of stack  gas
reheat  in  wet scrubbers  is  not  universally  accepted   in  the  industry.
Results of a recent survey on reheat practices indicated that most designers
and suppliers  who  responded  to the survey do  not  think reheat is necessary
and  generally  do  not  recommend  the  use of  reheat (Nuela  et  al.  1979).
Therefore, alternative  design  strategies  have been developed to handle such
consequences as  condensation and subsequent corrosion  in  downstream equip-
ment.  Basically,  these strategies  require the use  of  fans  upstream of the
scrubber  (unit or  booster  fans)  and  the use  of a suitable  construction
material  to  protect the  downstream  ductwork and  stack from  acid corrosion
attack.
     Most  systems  with  wet  stacks  have  reported  problems  with the  stack
linings, which tend to blister and eventually flake off.   Once this happens,
stack corrosion begins.   To  limit corrosion in no-reheat operation, one may
either select  materials that are inherently resistant to corrosion, such as
high-grade alloys, or  use organic or inorganic  linings to cover corrosible
base metals.   These  options  are addressed in the  Materials  of Construction
subsection.
Operational Systems
     Table  3-11  summarizes  selected   limestone  FGD  systems currently  in
service according  to reheater  design  and operating  parameters.   These FGD
systems are representative of  the stack gas reheat methods discussed in the
foregoing.

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                           TABLE 3-11.   EXAMPLES OF OPERATIONAL STACK GAS REHEATERS
Power plant
Cholla 1
Arizona Public Service
Cholla 2
Arizona Public Service
La Cygne 1
Kansas City Power and Light
Lawrence 4
Kansas Power and Light
Lawrence 5
Kansas Power and Light
Sherburne 1
Northern States Power
Sherburne 2
Northern States Power
Widows Creek 8
TVA
Petersburg 3
Indianapolis Power and Light
Type
In-line
In-line
In-line
In-line
In-line
In-line
In-line
Indirect
Indirect
Heat
source
Steam
Steam
Steam
Hot water
Hot water
Hot water
Hot water
Steam
Steam
AT,
°F
40
40
65
50
50
40
40
50
30
Tube
type
Circular
Circular
Circular
Finned
Finned
Finned
Finned
Finned
Finned
No. of
tube
banks
2
2
4
1
1
3
3
NR
NR
No. of
tubes
per bank
NR
NR
8
66
66
15
15
80
65
Soot blowing
Steam, once
per shift
Steam, once
per shift
Steam, every
4 hours
Two to three times
per shift
Two to three times
per shift
Once per shift
Once per shift
None
None
oo
en
     NR -  Not reported.

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FGD SYSTEM:  Fans	3-68
FANS
     The fans  In  a limestone scrubbing system are used to move the flue gas
through  the system.   They  are  designed  to  generate  a static  head great
enough  to  overcome resistance to  flow of  the flue gas  within  the system.
The criteria  for  specification  and  selection of fans  for  this  service are
outlined in this section.  Because the fan applications in a scrubber system
are very similar to those in a boiler system,  most power plant engineers are
familiar with  the  operation and  the basic mechanical  design  of  these fans.
This  discussion  therefore   emphasizes  features  that  are  specific  to  FGD
system applications.
Description and Function
     The fans  in  an FGD system are  required  to handle gas flow rates typi-
cally ranging  from 200,000  to 800,000 acfm.   Gas temperatures, which depend
on  fan  location in  the system,  range  from 200° to 320°F  after the boiler
preheater and from 120° to 180°F  after the scrubber exhaust reheater.  Along
with moisture,  S02,  and acid mist, the gases  may contain particulates that
are usually abrasive and tend  to accumulate  in the  form  of scale  on fan
blades.
     The fans  used in  a scrubber system may  be either axial  or centrifugal
types.   Axial fans have been used in FGD systems at only two plants, whereas
centrifugal fans are used  in many FGD  installations.   The  major reason for
this predominance  is that  the centrifugal fan  with  radial  blades is better
suited to operation with gas streams containing particulate matter.
     Because the  fans  are  seldom  able to operate continuously  at  constant
pressure and volume, some convenient means of controlling the volume of flow
through  the fans  is needed to  meet the demands  of variable scrubber and
boiler  loads.   Control  is  commonly achieved  with variable-inlet  vanes  or
dampers, and with variable-speed  controls on the fans.
     The centrifugal action of  a  fan  imparts  static  pressure  to  the gas.
The diverging  shape  of  the  scrolls (the curved portion of the fan housing)
also converts a portion of the velocity head into static pressure.  Although
the normal  static pressure requirement is approximately 20 in.  H20,  scrubber
system  designers  commonly add 15  to 25 percent to  the  net static  pressure

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FGD SYSTEM:  Fans	3-69
requirement as  a  margin  of safety to allow for buildup of deposits in duct-
work and for inherent inaccuracies of calculation.
Design Parameters
     Location.   In  this  manual  fans are called  either boiler induced-draft
(ID) fans  or scrubber booster  fans,  depending on the  location of the fans
with respect to the boiler.   Location will determine whether the FGD system
operates at positive or negative pressure with respect to the atmosphere and
whether the fans operate in a wet or dry gas environment.
     The main  boiler ID  fan must produce enough pressure  to overcome flow
resistance downstream of the boiler and maintain adequate gas flow until the
point  at  which the  natural  stack draft provides the  energy  to  exhaust the
gas.   When the  ID fan is located ahead  of the scrubbing system,  it is con-
sidered a  dry  fan and it provides  a  positive pressure that pushes the flue
gas through the scrubbing system.  Any leaks that develop are easily detect-
able by  the  escaping  flue  gas.  The  leaking gas can be  hazardous  if the
scrubber is in an enclosure.
     An ID fan  located  after the scrubbing system and the reheater is also
considered to be  a  dry fan because it  does  not handle saturated gas.  This
fan  operates  at  negative pressure  relative  to  the atmosphere  because  it
pulls  the  flue gas  through  the  FGD system.   If leaks  occur,  the result is
inleakage of air into the FGD system;  such inleakage has caused high natural
oxidation but essentially  no adverse  impact on scrubber operation.  Inleak-
age of air can  increase the volume of  gas that must be handled,  and detec-
tion of  inleakage  is  very  difficult.   Figure  3-23  depicts  typical  ID fan
applications for limestone scrubbing FGD systems.
     Scrubber booster fans are  ID fans that  supplement the existing boiler
fan and  their  location  and  functions are the same  as the  boiler  ID fans
previously described.
     When the ID fan is located immediately after a scrubber, it operates in
a wet  environment  because the  gas is  saturated with  condensing water and
contains extra  moisture  caused  by entrainment.  A typical wet ID fan appli-
cation is  shown in  Figure 3-24.  The gas  from the prescrubber is saturated
with water and  contains  acid mists and  abrasive carryover materials.   Even

-------
FLUE
 GAS
    PARTICULATE
      REMOVAL
       DEVICE
                                               REHEATER
MIST
ELIMINATOR
                        QUENCH
                            POSITIVE PRESSURE MODE
      FLUE  GAS-
                                      REHEATER
                                                    ID FAN
                                MIST
                                ELIMINATOR
                            NEGATIVE PRESSURE MODE
                Figure 3-23.   Typical  dry ID fan applications.
                                       3-70

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FLUE GAS
                          MIST
                          ELIMINATOR
WET ID
  FAN
          Figure  3-24.   Wet ID fan installation.
                            3-71

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FGD SYSTEM:  Fans	3-72
though water  sprays are  Installed to clean  the fan  internals,  corrosion,
erosion,  and  scaling can  occur.   Buildup of  solids of the  fan  blades can
cause severe corrosion and rotor imbalance, which in turn lead to high noise
levels,  excessive vibration,  and fan failure.   Wet ID  fans offer  a  size
advantage  because the  cool,  wet gas has  less  volume than  dry gas.   Because
of numerous operating problems, wet fans  have been in disfavor and the trend
is to  use dry ID fans upstream of the scrubbers.  Wet fans should be speci-
fied only with a rationale for circumventing these known problems.
     Temperature Increase.  The adiabatic  gas  compression  caused by the use
of  a fan  increases the  gas  temperature  through  the system, typically  by
about 0.5°F per  inch of water pressure  increase.  The  temperature  increase
due to the fan operation is an advantage  when the fan is  located after the
scrubber  in that  it supplements the stack gas reheat (Green Fuel  Economizer
Co. 1977).
     Materials of Construction.   Fans  operating  on   hot  flue gases  can  be
constructed of carbon steel because they  are not subjected to corrosive con-
ditions.    They may  be  subjected  to  erosion  by  particulate,  however,  if
particulate removal  is  inefficient or nonexistent.   Carbide wear plates are
used to protect against such erosion.
     Where fans are  subjected  to  a wet gas environment, with high levels of
acidity  and  chlorides,   corrosion-resistant  alloys,  polymer coatings,  or
rubber linings should be  used for  the  fan  components.   The major problem
with rubber-lined housings  has been damage by impingement of loosened scale
deposits from ductwork.
     Cleaning and Inspection.   All fans should have adequate cleanout doors.
This  is  especially  important on  dry  ID  fans.   Inspection ports  are  also
useful for checking possible accumulation of deposits.
Operational Systems
     Following  is a  discussion of  fans  used  at some  limestone scrubbing
facilities, and their performance histories.
     Cholla.   The two  booster  fans provided for Choi la  1  maintain  constant
gas stream pressure to the scrubber and eliminate the need for extra loading

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FGD SYSTEM:  Fans	    3-73
on the  ID boiler  fans.   The mild  steel  booster fans are rated  at 240,000
acfm  of flue  gas at  276°F.   Operation  of  the FGD  system  began  in  1973.
During  the  period December  1973  to April 1974,  vibration occurred  in the
Module  B  booster  fan  as a  result of  uneven  buildup of  scale  on the fan
blades  caused  by intermittent  operation.   The  blades  were  sandblasted,
cleaned, and  rebalanced to  eliminate  vibration.   No further  problems have
been reported (Laseke 1978a).
     Petersburg.   Petersburg  3  of Indianapolis Power and  Light Company has
four  scrubber modules.   Four booster  fans  (one per module), each with  a
pressure drop  of 18  in. H20, force the gas  from the ESP's through the FGD
system.    Each fan handles   a  gas  flow of 475,000  acfm  at 279°F.   Louver
dampers permit  isolation of one  or more FD  fans and crossover ducts.  The
FGD system was  put into operation in  October 1977, and  through July 1978 no
fan problems were reported (Laseke 1978b; Laseke et al.  1978).
     La Cygne.   La Cygne  1  of Kansas  City Power and Light Company is fitted
with eight scrubbing modules for control of particulate  and  sulfur dioxide
emissions.   Flue  gases  at 285°F from  the boiler are pushed through a common
plenum  by  three  fans.    The  gases  then enter the  eight  scrubbing modules.
After passing  through  the FGD system, including a reheat section, the gases
enter a common  plenum  at a temperature of 172°F and are fed to the stack by
six ID  fans  through  six ducts.   The system cannot be bypassed.  Each of the
three FD  fans has a  design  capacity  of 765,000 acfm of  gas flow at 105°F;
the six ID fans are each rated at a gas flow of 445,200  acfm at 172°F with a
7000-hp motor drive (Laseke 1978c).
     Problems have occurred  with  the  ID fans since they were first balanced
in September  1972.   Initially the fans were prone  to imbalance,  and opera-
tion  at close  to the  critical  speed caused  severe vibration  in  the fan
housing, resulting in cracks in the inlet cones.  Additional  stiffeners were
installed to strengthen the housing.
     The temperature  was finally  controlled by cutting  oil grooves  in the
thrust  collar and installing forced-lubrication  systems  on  all  the  fans.
These modifications  reduced the  thrust collar temperatures  to  a  range of
140° to 160°F.

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FGD SYSTEM:  Fans	  -        3-74
     Problems with  the  ID fans began with the initial firing of the boiler.
Fly  ash and  slurry,  carried over  from the  scrubber  and deposited  on the
blades  of  the impeller,  aggravated  the tendency of the  fans  to  become un-
balanced and  promoted  fan blade erosion.  Examination of all  the blades by
magnaflux testing revealed  several  cracks,  indicating a need for reinforce-
ment.   By  June 1974 all  ID  fan rotors were replaced with units  of heavier
design.   Shaft diameter  was  increased,  and  thickness   of  .the  radial  tip
blades  and  side  plates  of the wheels was increased.  The thick center plate
was  scalloped to reduce  its weight,  and  the fan  blades were modified to
reduce  the  tendency to  vibrate.  The leading edge of each blade was covered
with a stainless steel clip to deter erosion.  Although fly  ash carryover
still  necessitates  intermittent washing of  the fans, the cleaning frequency
has been reduced.
     Lawrence.  Lawrence 4 of Kansas Power and Light Company operates with a
retrofitted  limestone  FGD system that began  operation  in 1977.*   The FGD
system  consists  of  two 50  percent  capacity scrubber modules,  to which the
flue gas is conveyed through new ductwork.   Flue gas from each module exits
through a mist eliminator and reheater and  is conveyed  by two ID fans (one
per  module) to  the stack.   A  bypass duct  is  provided for  each module.
Isolation dampers are  located at the inlet and outlet of each module and in
the  bypass  ducts.   Each  fan handles  a  gas flow  of 181,500  acfm  at 144°F
(Green and Martin 1978).
     A new FGD system has been installed on Lawrence 5, identical  to that on
Lawrence 4 except in flue gas handling capacity.   It is equipped with two ID
fans  (one  per module),  each with  a design capacity  of 600,000  scfm.   No
qualitative  evaluation  of fan  operation on these  units is yet  available.
     Sherburne.  Sherburne  1 and 2  of  Northern  States   Power  Company have
limestone FGD  systems  containing 12 scrubbing modules each.   Flue gas from
the  boiler  passes   through  the  scrubber,   the  mist  eliminator,  and  the
reheater before reaching the ID fan.   Only 11 of the 12 scrubber modules are
* The modified limestone venturi/spray tower FGD system was placed in  ser-
  vice in 1977.  The original limestone injection marble-bed scrubbers were
  in service from 1968 to 1976.

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FGD SYSTEM:  Fans	^	3-75
required for  full-load  operation.   Reheat provides protection  to  the down-
stream ductwork  and ID  fans  and controls potential plume  formation before
final discharge of flue gas to the atmosphere.   The scrubbing modules cannot
be bypassed.   Each  of  the boiler units is equipped  with six fans:  two FD,
secondary-combustion-air  fans  upstream, and  four ID fans downstream of the
scrubbing  system.   The  ID  fans  are  30  percent  capacity,   axial-flow,
variable-drive fans.   The flow capacity  of  each ID fan is  702,000  acfm at
171°F (Laseke 1979).
     Since startup  of  the FGD system in  1976,  no  problems  with the ID fans
have been reported (Melia et al.  1980).
     Winyah.   The FGD  system  on  Winyah 2 of South  Carolina Public  Service
treats  only 50  percent  of the  flue  gas.  The  remainder  bypasses  the FGD
system and is recombined  with the scrubbed portion for reheat before dis-
charge  to  the  stack.    After  passage  of the  flue  gas through an  ESP,  a
booster fan drives 50 percent through the  scrubber.   The booster fan handles
407,000 acfm  of  flue gas at 270°F  with a design pressure drop of 13.5 in.
H20.   The  FGD system was  placed  in service  in 1977.  No problems with the
scrubber FD fan have been reported (Melia et al. 1980).
     Southwest.  The FGD system at Southwest 1 of Springfield City Utilities
consists of  two   scrubber  modules  of 50 percent capacity each.   The boiler
flue  gas  first passes  through an  ESP  and is  then  driven  through  the two
modules by  booster  fans.   The cleaned flue gas then passes through a set of
mist  eliminators  before  it  is discharged to  the  main stack.   One  or both
modules can be bypassed during emergency or malfunction periods by the use
of seal-air gas dampers.
     Design capacity  of  the  booster  fans, manufactured by the Green Fuel
Economizer Co.,  is  454,200 acfm of flue gas at 335°F,  with 4390-hp, 800-rpm
motors.   No fan problems have been reported (Melia et al. 1980).
     Widows Creek.  The  limestone  FGD system on Widows Creek 8 consists of
four  equal  capacity scrubbing  trains.   Flue gas from ESP's  enters  the FGD
system through four fans, each with  a  flue  gas capacity of 400,000 acfm at
280°F.  The fans, manufactured by the Green Fuel Economizer Co., are powered
by 4000-hp, 890-rpm motors.

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FGD SYSTEM:  Fans	     3-76
     Extensive problems were  encountered  after startup of the FGD system in
August 1977.  These problems included erosion of fan blades and trouble with
the  drive   units.   Guillotine dampers became  inoperable because  of  jammed
gear boxes.  The seals around the dampers corroded, allowing leakage of flue
gas and particulate matter.  Serious erosion of the FD fan rotors continued.
All of these fans have been rebuilt.
     The problems at  Widow's  Creek stemmed from the operation of old exist-
ing ESP's,  which  collected only 50 percent of  the inlet fly ash.  As a re-
sult, large pieces of fly ash literally sand-blasted the fan blades,  result-
ing in erosion and failure (Wells et al.  1980).

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FGD SYSTEM:  Thickeners and Dewatering Equipment	  3-77
THICKENERS AND MECHANICAL DEWATERING EQUIPMENT
     This  section  describes  the  types  of equipment  used to  dewater  the
slurry generated  by  limestone  scrubbing.   Dewatering  is  classified  as either
primary or secondary.  The  primary dewatering process takes the  slurry  bleed
stream from  the  scrubber at 5 to  15  weight percent solids content and per-
forms  liquid/solid  separation  as  an  initial step, typically increasing  the
solids content to 20 to  40  weight  percent.   The secondary  dewatering process
can dewater  sludge  solids fed from a primary dewatering device to a solids
content  of  60  to  85  weight  percent.   Thickening  and  dewatering  can  be
accomplished  by   any  one or  a  combination of  the  following mechanisms:
     0    Settling ponds
     0    Thickeners
     0    Vacuum filters
     0    Centrifuges
     0    Hydrocyclones
     Settling ponds  are  not considered here because  the trend  is toward the
use  of  mechanical   dewatering  equipment.   Additionally,   plants  that  burn
high-sulfur coal  must  use thickeners  in combination with  secondary dewater-
ing equipment.  These  plants  do not generate enough fly  ash to  mix  with only
primary  dewatered sludge  to make  a  stabilized material  for dry  landfill.
Because  of  their simplicity,  thickeners  are  almost always  justified  and
hence  are  now used  in more than  half of  the  limestone  scrubbing systems.
Although more  expensive  than  a  settling pond,  a  thickener is  usually pre-
ferred because  it removes  solids  more  easily,  can be located more flexibly
in the plant environs, and requires less space.
     A  recent EPRI  publication  (Knight et  al.  1980)  provides additional
information on the  details  of  thickeners and dewatering  equipment as applied
to limestone  scrubbers.   Also,  the EPA has published  an excellent  manual on
the treatment and disposal  of  sludge  from wastewaters that includes a review
of mechanical dewatering  equipment (EPA 1979).

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FGD SYSTEM:  Thickeners and Dewaterlng Equipment     ~J**&*^''-'	3-78
     The main purposes of dewatering are:          _
     To  reduce the  volume  of  sludge, for transportC^&cL disposal,  thereby
     reducing  transport costs,  land area  requirements, land final  disposal
     To recover the liquid for further use.    .
     To reduce  the  moisture  content so the waste cafl^b^fhandled and disposed

     To minimize leaching and pollution of ground water.
     To recover sludge for utilization.                !
Depending  on  the sludge  characteristics,  the thickening/dewatering  process
may also  be  beneficial  to sludge blending, equalization  of  sludge flow,  and
sludge  storage, as  well  as  reducing the  chemical  requirements  for  sludge
conditioning.   Figure  3-25   shows  the  effect  of  dewatering  on  the  total
sludge volume.  As  the  water is taken out and the  solids content  increased,
the volume for  disposal  is  substantially reduced.   It is evident,  therefore,
that transport  and disposal  costs  can be  reduced by reducing  the  volume  of
water carried to the disposal site.
     On the other  hand,  the  size and cost of  dewatering equipment increases
as the  degree  of  dewatering increases.    As  a result, there is an economic
trade-off  between  disposal  cost and  dewatering cost.   Similar  trade-offs
occur in  other aspects of sludge  thickening  and dewatering.   For  any  given
situation, an optimum system can be determined by evaluation of the alterna-
tives.
     Dewatering  facilities  should  be placed  as close  as  possible to  the
scrubber  facilities.  This will  minimize the  length of large-diameter piping
required  to  convey scrubber  bleed  to  the dewatering facility  and  to  return
the portion of  the  liquid fraction  that  is removed.   Placing a  settling pond
close to  the  scrubber might  not be possible because  of  the  relatively large
amount of land required for a pond.
     Dewatering  facilities   may  be  installed  singly,  in  parallel,  or  in
series,  depending upon  the degree  of treatment  required.  Mechanical  equip-
ment should be  sized  to allow sufficient storage, or sufficient surge capac-
ity  should   be provided to  permit  flexible  operation.   Enough equipment

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   2.0
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        SPECIFIC  GRAVITY
        OF SLUDGE =2.5
                                I
                                      I
              30      40       50       60
                            SOLIDS CONTENT, %
                                              70
80
90
              Figure 3-25.  Relationship  between  sludge
                     volume and solids  content.
                                  3-79

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FGD SYSTEM:  Thickeners and Dewatering Equipment	3-80
should be  provided  to ensure uninterrupted operation of  the  scrubber in the
event  of  failure in the sludge  processing  system.   Usually this is  done  by
providing  redundant or oversized  equipment  or by  installing a bypass  with
emergency storage facilities.
Description and Function
     Thickener.  A  thickener is  a sedimentation device that concentrates  a
slurry  by  gravity  so that  the  settled solids  may be disposed  of  and  the
clarified  liquid recycled.   Thickeners  (also known as clarifiers)  have  been
used  in many  industries,  from  which conventional design  has  been  applied
successfully to limestone scrubbing.
     A typical thickener (Figure 3-26)  consists of a large circular holding
tank  with  a  central  vertical  shaft that  is  supported  either by  internal
structural  design,  by a center  column, or  by a  bridge.   Two  long,  radial
rake  arms  extend from  the  lower  end of the vertical  shaft;  two short  arms
may be added when  necessary  to  rake  the inner area.   Plow blades  are mounted
on the  arms  at  an  oblique  angle  (with the  trailing edge toward the center
shaft), with a clearance of  1.5 to 3 in.  from the bottom of  the  tank.   They
can be arranged  identically  on  each arm or  in an  offset  pattern  so that the
bottom is  swept  either once  or twice during  each  revolution.   The  bottom of
the tank  is  usually  graded  at a  slope of  1:12 to 1.75:12 from the  center.
The settled  sludge  forms  a blanket on  the  bottom  of the thickener  tank  and
is pushed  gently toward the  central  discharge outlet.   Center scrapers clear
the discharge  trench and move the solid  deposits  toward the underflow  dis-
charge point.
     The scrubber  slurry  is  fed through a  feedwell  into  the thickener at  a
concentration of 5  to 15  percent  solids, and  the  underflow is  discharged at
a concentration of 30 to 40 percent solids.
     The use of  inorganic  polymer flocculant can reduce solids  concentration
in the  overflow  from a range of  50  to  100 ppm to  a  range of 10 to  70  ppm.
Additionally, the size of  a  thickener  in which a  flocculant  is used may  be
only about half  that of one  without  flocculation  because  the sludge settles
much  more  rapidly.   Flocculants in powder  form are dissolved to make solu-
tions  of   0.5  to 1 percent  concentration   for  addition   to  thickener  feed
slurry.                        *

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             LIFT
           INDICATOR
    LAUNDER
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                          CENTER DRIVE
                        UNIT AND LIFTING
                             DEVICE
DRIVE MOTOR AND
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WALKWAY
                                          TORQUE AND
                                           RAKE ARMS
              HIGH PRESS BACK
            FLUSHING WATER LINE
                           DISCHARGE TRENCH
                                                                                                   THIXO POST
                                           PLOW BLADES
          CENTER SCRAPERS
                        Figure 3-26.   Cross-section of a conventional  gravity thickener.

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FGD SYSTEM:  Thickeners and Dewaterlng Equipment	3-82
     When  space  is limited, a  high-capacity  thickener may be used.   Such  a
unit is  the  Lamella  plate-type thickener that has been used successfully in
the  phosphate industry.  The  Lamella® thickener  (Figure  3-27)  consists  of
two tanks:   an  upper  rectangular unit contains a series of parallel,  slotted
plates  inclined at a  55-degree angle  and serves  as  a  high-rate  gravity
settler;  a  lower  circular  tank functions  as  a  thickener  with  a  picket-
fence-type sludge  rake  and  houses the underflow outlet.   The arrangement of
plates in  the rectangular tank increases the  settling capacity of  the thick-1
ener and substantially reduces space requirements.
     A  plate-type  thickener  has  been  tested  at  the  Shawnee   facility.
Results  indicate  that the  Lamella   thickener can  be used  at installations
where a  dilute  or  rapidly settling  slurry such as  gypsum requires  clarifica-
tion.
     Continuous  Vacuum Filters.    Vacuum  filters are  economical  for  contin-
uous service  and are widely used because they can be  operated  successfully
at relatively high  turndown ratios  over  a broad range of feed solids  concen-
trations.  A vacuum  filter also  provides more operating  flexibility  than
other types  of  dewatering  devices,  and a  drier product.   Because a vacuum
filter will  not yield an acceptable  filter cake if the feed solids  content
is  too   low,  it is  frequently  preceded  by a  thickener or  a hydrocyclone.
     Four  types of  vacuum filters  are  applicable  to  limestone-generated
sludge systems:   drum,  disk,  horizontal  belt, and pan.   Each has  different
characteristics  and applicability.  The  drum  type, which  is the most widely
applied, is discussed here.
     A rotary-drum  vacuum  filter is  depicted  in  Figure 3-28.  The  drum is
divided  into  sections,  each connected through ports  in the  trunnion to  the
discharge  head.    The  slurry  is  fed to a  tank in  which the  solids are  held
uniformly  in  suspension by an agitator.   As  the drum rotates,  the faces of
the  sections  pass  successively  through  the slurry.   The vacuum in the  sec-
tions draws  filtrate  through  the  filter  medium,  depositing the  suspended
solids on  the filter drum  as cake.   As the  cake leaves the slurry, it is
completely saturated  with  filtrate  and  undergoes  dewatering by the simul-
taneous  flow   of   air   and  filtrate in  the cake drying zone.  Drying is

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                                                   OVERFLOW

                                                   FEED
         FLOW  DISTRIBUTION
            'ORIFICES
                                                          PICKET-FENCE-
                                                           TYPE SLUDGE
                                                             SCRAPER
                                                               MOTOR
 LAMELLA
PLATE PACK
          Figure 3-27.   The Lamella gravity settler thickener.
                                   3-83

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                                                       CLOTH CAUtKINQ
                                                           STRIPS
                                  AUTOMATIC VALVE
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                   AIR BLOW-BACK LINE
                                                                                             DRUM
                                                                                                     FILTRATE PIPING
                                                                                                        CAKE-SCRAPER
                                                                                             SLURRY AGITATOR
                                                                                                VAT
                                                                              SLURRY FEED
                                Figure  3-28.   Cutaway view of  a rotary drum vacuum filter.

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FGD SYSTEM:   Thickeners and Dewatering Equipment	3-85
negligible  when  the  air  is  at  room temperature.   Finally,  the  cake  is
removed  in  the discharge  zone  either by  a  scraper or  a string  discharge.
     A vacuum  filter  produces  filter  cake of  45  to 75 percent solids  from
feed slurries  containing 20  to 35 percent solids.  The  filtrate  containing
0.6 to 1.5 percent solids is recycled to the  thickener.
     The  filtration rate in  limestone scrubbing applications in which  some
calcium  sulfate  solids are present ranges between  150  and 250 Ib/h per ft2
(Heden and Wilhelm 1975).
     Normally, the filters  are  installed at  an  elevated  location so  that the
cake solids  discharging  from the filter can drop  into  a chute leading  to a
storage  hopper  for easy  loading into a  truck.   If an elevated position is
undesirable, a  belt  conveyor may  be  used to  collect the solids  discharged
from the filter and  carry them  to  a  raised  storage hopper,  again to  permit
easy loading.
     Centrifuge.   Centrifuges  are  widely used  for separating  solids  from
liquids.   They  effectively  create  high centrifugal forces, about  4000 times
gravity.   The  equipment  is  relatively  small  and  can  separate bulk  solids
rap.idly  with a short  residence  time.  A  centrifuge is a reliable and effi-
cient  machine,  yielding products  that are  consistent,  uniform,  and  easily
handled.
     Centrifugal  separators  are of  two  types:   those that settle and those
that filter.   A  settling  centrifuge  uses centrifugal  force to increase the
settling  rate  over that obtainable  by  gravity settling; this  is  done by
increasing  the  apparent difference   between  densities  of   the  phases.   A
filtering centrifuge generates  by  centrifugal  action the pressure needed to
force the liquid  through a septum.   This discussion describes the  continuous
settling  centrifuge, which  separates  a slurry  into a clear liquid  and  a very
thick sludge.
     Figure  3-29  shows  a continuous settling,   solid-bowl  dewatering centri-
fuge.  The  principal  elements  are  the rotating bowl,  which  is the  settling
vessel,  and  the  conveyor,  which discharges  the settled  solids.   Adjustable
overflow  weirs  at the  larger  end  of  the bowl  discharge  the clarified
effluent,  and  ports   on the  opposite end  discharge  the  dewatered  sludge
cakes.    As the  bowl rotates, centrifugal force  causes  the  slurry  to form an

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                                                                COVER
        DIFFERENTIAL SPEED
            GEAR BOX
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                                                                                FEEDPIPE
                                                                  BASE NOT SHOWN
                            CENTRATE
                            DISCHARGE
                                                      SLUDGE CAKE
                                                       DISCHARGE
                                                                                                           •ouoMimctn
                                                                                                      (ttTTUED AGAINST BOWL WALL)
LIQUID SURFACE
(CLEAR LIQUOR)

 BOWL WALL
                                                                                                            RADtAL CROSS-SECTION
                                       Figure  3-29.   Cross-section of solid-bowl  centrifuge.

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FGD SYSTEM:  Thickeners and Dewaterlng Equipment	3-87
annular pool, the  depth  of which is  determined by adjustment of the  effluent
weirs.  A  portion of  the  bowl  is of reduced diameter to prevent its  being
submerged  in the  pool;  thus it forms a drainage  deck for the solids as they
are conveyed  across  it.   Feed enters through  a  stationary  supply  pipe  and
passes through the conveyor hub into the bowl.   As  the  solids settle to the
outer  edges  of  the  bowl,  they  are  picked  up by  the  conveyor scroll  and
continuously overflow the  effluent weirs.   In a recent study with  feed of 16
to 20  weight percent solids, the resultant  solids concentrations ranged from
0.7 to 4 weight  percent in the  effluent and  from  60 to 70  percent  in  the
cake.    The concentrations  were  dependent  on  feed  rate  and  bowl  rotation
speed  (3300 to 5400 rpm) (Wilhelm and Kobler 1977).
     Hydrocyclone.   A  hydrocyclone  is  a  small,   simply constructed  device
used extensively  for the classification  and dewatering  of slurries.   It can
rapidly  separate  bulk  solids  from  process  streams.    Although  it   is  an
excellent  low-cost dewatering  device,  it  is not  very  effective in producing
clarified overflow.  The product of  a hydrocyclone, therefore,  is often fed
to another unit,  such  as a centrifuge or filter,  for additional dewatering.
The .hydrocyclone  also  requires frequent maintenance  because  of high  rates of
wear,   erosion, and corrosion by fly ash and  slurries  from the S02 scrubbing
system.  A hydrocyclone  also can be  installed in  a slurry recycle  line for a
venturi  scrubber  to prevent  plugging  of  the  nozzles by   removing  large
particles.
     A  typical  hydrocyclone (Figure  3-30)  consists  of  a vertical  cylinder
with  a conical  bottom,  a  tangential inlet  near  the top, and  an  outlet  for
solids  at  the  bottom  of  the cone.  The  fluid  overflow  pipe is  usually
extended into  the cylinder  to prevent  short-circuiting of  fluid  from inlet
to overflow.
     The inlet piping  imparts  a rotating motion  to  the  incoming fluid.  The
path of the  fluid follows  a downward vortex,  or spiral,  adjacent to  the wall
and reaching to  the  bottom of the cone.  The fluid  stream moves upward in a
tighter  spiral,   concentric with the  first,  and  leaves,   still  whirling,
through the  outlet pipe.   Both spirals  rotate in the  same  direction.   The
particles  settle  to  the side walls and slide down the  inclined wall  to the
apex  of  the  cone.   They are  then  discharged through an  underflow  orifice.

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                 FEED
                                                     OVERFLOW
              FEED  INLET
                 CROSS  SECTION
                 FEED CHAMBER
                   APEX OPENING
                                  UNDERFLOW
VORTEX FINDER
                                                     CYCLONE  DIAMETER
                                                     CONE SECTION
             Figure  3-30.  Hydrocyclone cross-section  (Dorrclone~),
Courtesy:  Dorr-Oliver, Inc.

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FGD SYSTEM:  Thickeners and Dewatering Equipment	3-89
     Basically,  a  hydrocyclone  is  a  settling device  in  which  a strong
centrifugal force is  used  for separation.  The  centrifugal force  in  a hydro-
cyclone  ranges from  5  times gravity  in  large, low-velocity  units  to 2500
times gravity in small, high-pressure units.
Design Considerations
     Thickener.  The  usual  criteria  for sizing  of a clarifier/thickener unit
are  solids  loading  and  hydraulic  surface  loading.    For  a  thickener and
clarifier/thickener,  the  solids  loading  factor is more important.   Solids
loading  without  polymer flocculant may range from 12  to 30 ft2/ton per day
to  achieve 25  to  35 percent solids  in  the underflow.  To  achieve higher
underflow  solids  concentration,  in the range of 35 to  45 percent,  the solids
loading without polymer ranges  from  8 to 12 ftVton per day.  With  addition
of polymers, the  solids  loadings are reduced to ranges  of 3.5 to  11 and 1.7
to 4.5 ftVton  per  day,  respectively, for the  two ranges of  underflow  solid
concentrations.
     Recommended  surface loadings  range from  300 to  4000 gal/ft2  per day  of
granular solids;  800 to 2000 gal/ft2 per day  of slow-settling solids; and
1000 to  2000 gal/ft2  per  day of flocculated particles.  The  latter  range  is
normal for the thickeners  used in dewatering  FGD scrubber sludge.
     In  limestone scrubbing  systems,  the  thickener diameters  typically  range
from  50  to 100 ft; sidewall heights  typically  range  from  8  to 14  ft.  The
rake arms  are  driven  by a  2- to  5-hp motor through a worm gear connection  at
a  speed  of 10  to 20 min/rev.   The  thickener is usually a  lined  or painted
(mild) steel tank with  a  steel or concrete bottom.  Because  the  pH  level  of
slurry in  the  thickener tank varies  and  chloride  content  may be  high, most
submerged  parts are protected from corrosion by an epoxy or  rubber  coating.
     Rotary Vacuum Filter.    A number  of  variables affect  the operation  of
the filter system:
     Feed solids concentration
     Filter cycle time
     Drum submergence

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FGD SYSTEM:  Thickeners and Dewaten'ng Equipment	 3-90
     Agitation
     Filter vacuum requirements
     Filter media
     The size  of  a filter for a given  application  is inversely proportional
to  the  solids concentration  of  the slurry  feed.   Thus,  if  a  thickener  is
installed  upstream,  it  is important to determine the  minimum  solids concen-
tration in the underflow that occurs in average operation of the unit.   When
the  filter is  sized to  accommodate this minimum  solids concentration,  it
will  have  adequate  capability to  dewater the solids output  of the  plant.
     If large  variations in  solids handling capability  are expected,  it  is
often more desirable to  install two smaller filters than one  large filter.
Then when  the  amount of solids is  reduced substantially  over  a long period,
one  of  the units  may be shut down; the  other unit may  then operate  at the
proper  submergence  level  and thereby  optimize performance.   In  addition,
vendors  recommend  installation  of  a   spare  to ensure  uninterrupted  plant
operation.
     A  standard  rotary  drum  vacuum  filter  can  be  purchased  from  many
suppliers.   Correct  sizing,  i.e.,  determination of the correct  filter  area,
is  important  economically because  size usually accounts for an appreciable
portion of both  capital  and  operating  costs.   Enough filter  area must  be
provided to  maintain a  rate  of solids  removal that will prevent  excessive
accumulation of solids in the plant.
     To prevent  corrosion in  limestone slurry applications,  the piping and
support members in  a vacuum filter should be  made  of stainless steel.   Drum
heads in the filter  can  be made  of carbon  steel  if corrosion  allowance  is
provided.   Polypropylene is the customary filter medium.
     Centrifuge.    The  following   design  parameters  must be  considered  in
design of a centrifuge:
     Bowl   diameter:   A  centrifuge with  large bowl  diameter  and low  speed
     (rpm)  may  produce  the  same particle settling velocity as  a high-speed
     machine with a small bowl diameter.
     Bowl   length:   Increasing  the  length of  a  bowl will  result in  a propor-
     tional increase in retention time.

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FGD SYSTEM:  Thickeners and Dewaten'ng Equipment	3-91
     Bowl  speed:   Bowl  speed controls  the centrifugal force  that will  be
     produced in a machine with  a fixed bowl  diameter.   The  general  trend  is
     to select machines  that  operate at low speeds  to  reduce operating costs
     and   increase  machine   life.    Recent  application  of  special   wear-
     resistant  materials  such  as  tungsten carbide  and  ceramic  tile  with
     long-life  bearings   has  reduced  the  maintenance  needs  significantly.
     Conveyor speed:    Increasing  the conveyor speed allows more rapid move-
     ment  of solids through the  machine.   With most sludges,  higher conveyor
     speeds will  produce a dryer cake by  causing  the  wetter  solids  to  be
     left behind.
     Conveyor pitch:   In fluidlike  and sulfite-rich FGD  sludge,  increasing
     the  scroll  pitch will   produce  results  similar  to  those produced  by
     increasing the conveyor speed.
     Because scrubber  sludge  is  erosive and sometimes corrosive,  all  liquid
contact materials in  the centrifuge should be made  of  316L  stainless  steel.
The tips  of the  conveyor should be  made  of tungsten carbide to reduce abra-
sive wear.
     Hydrocyclone.   The   chief  factors  that  influence  hydrocyclone  design
include  hydrocyclone   diameter,   cone  angle,  size   of  orifices,  length  of
cylindrical section,  feed pressure,  feed concentration,  and particle size.
Because of the  number of significant variables, a  hydrocyclone manufacturer
is usually consulted regarding particular applications.
     The most important  factor  influencing the application and  efficiency  of
a  hydroclone  is  diameter.    The  smaller  the  particles to  be removed, the
smaller must be the diameter  of  the hydrocyclone.   For any specific applica-
tion, the  hydrocyclone manufacturer will  determine  the appropriate diameter,
height, and cone angle.
     When  size  and  cone  angle are established,  sizing of the  flow orifices
for feed,  overflow, and  underflow is the  next most important factor.   Plant
operating  personnel   can  change  the  orifices  as   needed to  suit process
requirements.
     By virtue of the high velocities within the device, the portions  of the
hydrocyclone exposed  to  the  high velocities must be fabricated of erosion-
resistant  metal  alloys.   The  bulk  of  the device can be constructed of type
316L stainless steel  to resist chloride corrosion.

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FGD SYSTEM:  Thickeners and Dewatering Equipment      	3-92

Operational Systems
     Thickener.  More  than half  of the operational  limestone  scrubbing  FGD
systems  in the United  States  use thickeners.   Some  operational  systems  are
described below.
Lawrence—
     At  Lawrence 4,  the dewatering system was designed with  separate slurry
hold  tanks  for the  combination  venturi  rod and  spray tower  scrubber  .
Staging  of the  liquid  side  of  the  system  enables the  addition of  fresh
limestone  to  the  scrubber  (spray  tower) effluent  hold tank (EHT).   The
solids content in  the EHT is controlled  at  5  percent.   Slurry is bled from
the EHT  to the collection tank.   The  solids  content of the  collection tank
is  controlled  at  8 to 10 weight  percent  by  varying the effluent  bleed pump
flow.   This pump delivers  the  effluent bleed  to  the system  thickener (40 ft
diameter),  where   the   slurry  is  concentrated to  about  50  percent  solids
before being pumped  to  a disposal pond.   The  pH  in the thickener is  6.5 to
7.0, and  the pH of the slurry  leaving  the thickener is  about 8.5.  The inlet
temperature is 115°  to  120°F,  and the  outlet temperature  is  80° to  90°F.
Water from the pond  is  returned to the makeup water surge tank,  where it is
combined  with  the  thickener overflow water  to provide  makeup water  for  the
scrubber system.
Sherburne—
     Units  1  and  2  of  Northern  States  Power  Company's Sherburne  County
generating plant are basically  the same.   For control  of  the solids  content
in the EHT,  an effluent bleed flow of  160 gpm is drawn from the  spray water
pump discharge and sent to the  thickener through the  slurry transfer tank.
The  flow  is  controlled by  a  nuclear density  meter,  which  maintains  the
solids content at  10 percent.   Of this  10  percent  solids,  2 to 3  percent
consists of calcium  sulfate seed crystals.  The  continually  forming  calcium
sulfate  adheres  to the  seed  crystal.   As  the crystals start to  grow,  they
precipitate out of the solution.
     Air  is introduced  into the  EHT to  provide enough oxygen to complete  the
oxidation  of calcium sulfite to  calcium  sulfate.   The air enters near  two
horizontal-entry mixers,  which  ensure  that the  air is mixed  with   all  the

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FGD SYSTEM:  Thickeners and Dewatering Equipment	3-93
available  sulfite.  In  the  thickener,  the slurry is  concentrated and sent to
a  fly  ash pond.  Clarified water  from both the thickener and  settling  pond
is recycled as makeup water.
Shawnee—
     The   EPA   facility  at  Shawnee  tested  a  Lamella®  Gravity   Settler
Thickener, 20  ft  high,  consisting of two tanks  constructed of epoxy-painted
carbon  steel.   Solids  content of  the underflow  is 40 percent and of  the
clarified  liquid, 0.5  percent.   The tests were  done with  oxidized limestone
with fly ash.
     Vacuum Filter.   Both rotary  drum  and horizontal belt filters have  been
used successfully as  secondary dewatering devices  for  limestone sludge.   The
following  plants  are  using or  plan  to  use rotary drum  vacuum  filters:
          San Miguel 1, San Miguel Electric Coop
          Paradise 1 and 2, TVA
          Widows Creek 7, TVA
     The following plants are using horizontal belt vacuum filters:
          R.  D. Morrow 1 and 2, Southern Mississippi  Electric  Power
           Association
          Southwest 1, Springfield City Utilities
     Much  of  the  experience  with vacuum  filters   has  been   with  lime  wet
scrubbing  systems  or  with  the dual  alkali process.  The  Lime  FGD Systems
Data Book  (Morasky 1979)  gives  details  on use of vacuum filters with  lime
scrubbers.
     Rotary  drum  vacuum  filters  have  been used as  the  secondary dewatering
device by  International  Utilities Conversion System (IUCS) as  the principal
means of  drying sludge (Morasky 1978)  for mixing lime  with fly ash to fixate
FGD sludge.   To date,  IUCS has a total of 16 FGD systems either in operation
or  under  contract  for  treating  waste sludge  by use of  their  rotary  drum
vacuum filter process (Knight et al. 1980).
     Centrifuge.   The  use  of  a  centrifuge  for   secondary  dewatering  is
becoming more prevalent.

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FGD SYSTEM:  Thickeners and Dewaterlng Equipment	    3-94

Shawnee—
     The  Shawnee  test facility  operates  a continuous  centrifuge as  one  of
several processes  used to dewater  scrubber waste sludge and to  recover  the
dissolved  scrubbing  additives.   Normal operating conditions with an  unoxi-
dized  slurry  are a  feed  stream flow  of  15  gpm at 30 to 40 weight  percent
solids, a  centrate  of 0.1 to 3.0 weight  percent solids,  and a  cake  of 55 to
65 weight  percent  solids.   About 30 percent  of  the total solids  is  fly ash;
the remainder is predominantly calcium sulfate and sulfite.
     The  machine  is a  Bird  18-  by 28-in.  solid-bowl  continuous  centrifuge,
which  operates  at  2050 rpm and  requires  30 horsepower.   It is made of 316L
stainless  steel with stellite  (Colmanoy)  hardfacing  on   the  feed  ports,
conveyor  tips,  and  solids  discharge ports.   The bowl head plows and case
plows  are  replaceable.  The  pool depth is  set at 1-1/2 in.   No cake  washing
is performed in this machine.
     The  centrifuge  was inspected  in  June 1978  after 6460 hours of  opera-
tion.   The inspection was prompted  by  a  gradual and continuing  increase  of
suspended  solids  in the centrate to a  level  of approximately 3  weight per-
cent.   The machine was  judged to be in  generally fair condition,  but  certain
components were in  need  of  factory repair.  Serious wear  was observed at the
conveyor  tips  on the  discharge  end at the junction  of  the cylinder  and a
10-degree  section  of conveyor.  The  casing head plows and  solids discharge
head  near the discharge  ports  were also  worn.   The bowl and  effluent head
were  in  good condition (Rabb  1978).  The machine has required little main-
tenance.
Mohave—
     An early experimental   investigation  of  limestone  scrubbing  was  con-
ducted at Southern  California  Edison's  Mohave  Station,  and the scrubbing
units  have since  been dismantled.   The  centrifuge at  Mohave  was  of  the
solid-bowl type,  having a bowl  2 ft in  diameter by 5 ft long.  The  auger/
bowl  speed  ratio  of  40/39  was  established with  an eddy current  clutch
device, and  the centrifuge  drive was  designed  to  allow evaluation  of  de-
watering  characteristics  over a range  of  rotating  speeds.   The device  was
designed  to  handle  120 Ib/min of solids  (dry basis), but was tested  at  160
Ib/min and could have gone still  higher.

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FGD SYSTEM:  Thickeners and Dewaterlng Equipment	3-95
     The  centrifuge  was controlled  with  relative ease.   With inlet  solids
concentrations of  10  to 15 percent and 30  to  35 percent, solids content  of
the centrifuge  cake  ranged from 68  to  72  percent at approximately 2000  rptn
and various flow rates.  The  only  operating problems were high torque  during
the feeding  of 5  percent  slurry solids in one  test,  and cementing  of  the
auger  to  the bowl  with  dried solids  during  outages.   Normal  centrifuge
operation  with  solids  between 25  and 35  percent  in  the  feed resulted  in
average  solids content  of 71 percent  in the  cake and average  suspended
solids  of about  0.15  percent in  the  centrate.   Suspended  solids  in  the
centrate decreased when  the  feed solids content was reduced  (Robbins  1979).
     Centrifuges are also in use or planned for use  at the following plants:
          Marion 4, Southern Illinois Power
          Martin Lake 1, 2, 3, and 4, Texas Utilities
          Thomas Hill  3, Associated Electric Coop
          Laramie River 1 and 2, Basin Electric Power Coop
          Craig 1 and 2, Colorado Ute Electric Association
     Hydrocyclone.    Both  Peabody  Process  Systems  and  Research  Cottrell
Corporation  use hydrocyclones to  separate solids  from  slurry.   A  hydro-
cyclone  system  supplied by Research Cottrell   is being  used  in  conjunction
with  forced   oxidation  at  the  Dal 1man  3 unit,  Springfield  Water  Light  and
Power.

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FGD SYSTEM:  Sludge Treatment	     3-96
SLUDGE TREATMENT
Treatment Processes
     The method selected for sludge handling and disposal must be economical
and must  result  in a properly placed, structurally  stable  material  that is
environmentally  acceptable and  complies with  provisions  of the  Resource
Conservation and Recovery Act (RCRA).
     Detailed guidance that will direct a Project Manager through the sludge
treatment  decisions  is not within the scope of this book.  A  recent EPRI
publication (Knight  et  al.  1980) provides such  guidance details.  The brief
introduction presented here is adapted from that publication; it is intended
to indicate the  basic considerations  involved in the sludge treatment deci-
sion.   This  decision should  be  made early,  because delay  will  reduce the
number of options available.
     Since it is very likely that today's waste  may be a natural  resource of
the future, strong  consideration should  be given to reclaiming FGD scrubber
wastes as well as the fly ash generated in burning the coal.  Therefore, the
first decision is  whether  to  co-dispose  the fly ash with scrubber sludge or
to stockpile each separately to facilitate future reclamation.
     Decision  paths   lead  the Project  Manager   through  the steps shown  in
Figure 3-31, which are required for all systems, wet or dry.
     The first step  is  selection of the  processing steps, which include the
options of whether to (1) change the sludge characteristics by forced oxida-
tion  to  the  calcium sulfate  or  gypsum  form,   (2) dewater by  gravity  or
mechanical means,  or (3)  stabilize by the  addition  of  fly ash or fixate by
the addition  of a  fixating agent with  or without fly  ash.   Each  of these
options has disadvantages,  ranging from added cost of equipment or operation
to increased solubility of calcium sulfate over  calcium sulfite.   Advantages
may include  better water  separation,  less volume  of waste  to  handle, and
better structural properties.
     Transportation  is  the  next  step in the  decision  path.  There  is  no
choice when  wet disposal  is  selected because  the  only  practical transport
mode  is by pipeline, although specially  equipped trucks  or rail cars are a
possibility  but  appear  impractical.   For  dry  transportation a  variety  of
motorized  earth-moving  equipment  and  rail  cars are  available and  in use.

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                                                                                            IMHIKHfO Ij

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                          Figure 3-31.   FGD sludge disposal Alternatives (Knight et al.  1980),

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FGD SYSTEM:  Sludge Treatment	  3-98
     Dispos  /utilization is tna  final  step in the system; this factor must
be  considered early  in the  decision  process  so as  to allow  the  primary
choice  of wet  or  dry  disposal.   Cost  is  a very  important factor  in  the
decision.  To  this point,  utilization  is  not  yet a  viable alternative to
disposal.  With more  interest  in and use of forced oxidation, the potential
for  utilization  has increased.   Calcium sulfate or  "abatement  gypsum"  has
found  limited  experimental  use  in  agricultural applications,  in wallboard
manufacture,  and  as a  set-retarding agent  in  cement.   Some  systems under
construction are  based  on  utilizing abatement gypsum as a saleable  byprod-
uct.  A new method for storage/disposal  of abatement gypsum has been used in
a  prototype  scrubber   test  under  EPRI  sponsorship.    The  method,  called
"stacking,"  has   been  used  for  over 20 years  by the  phosphate fertilizer
industry  in  Florida.   The gypsum is transported as a  slurry  to the stack,
for dewatering by gravity.
     All sludge disposal systems  can be categorized as wet (ponding) or dry
(landfilling).  Wet disposal has  been used for years for disposal of bottom
ash  and  fly ash.   Many  utility  operators have  naturally selected  this
familiar method  for sludge disposal  because of ease and  economy of opera-
tion.   Practice   is  now tending  toward dry disposal,  however,  because of
lower capital costs and environmental pressures.
     Sludge  processing  to  produce  a dry material is  done by stabilization
and  fixation.  Stabilization  improves the physical properties of sludge by
blending the material  with dry solids such as fly ash, bottom ash, or earth.
The  resultant  moisture content  of  the  mixture  is reduced,  resulting  in a
stable material for landfill disposal.  Fixation involves chemical reactions
between  the  fixative  and the  scrubber  sludge.   Figure  3-32 shows a sludge
treatment  flow  diagram that depicts the processing  of the  scrubber bleed
through the  primary dewatering and/or the secondary dewatering equipment in
conjunction  with  mixing  of  fly  ash  and/or  fixatives to  produce a  dry
material for landfill  disposal.
     The terms  "stabilization" and  "fixation"  are used  interchangeably in
the  industry,  and the  terminology is somewhat  confusing.   Stabilization is
generally  understood  to mean  addition  of  dry  fly ash, soil, or other dry

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GO
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so
io
                          PRIMARY  DEWATERING
SCRUBBER
              SLUDGE
                                SETTLING
                                 PONDS
                             SEDIMENTATION
                                BASINS/
                              CLARIFIERS
                                GRAVITY
                               THICKENERS
                              CENTRIFUGE
                              HYDROCYCLONE
CHEMICAL
ADDITION
IF NEEDED
                                                I
                                            DISPOSAL
                                                      SECONDARY DEWATERING
                                                            SETTLING
                                                             PONDS
                                                             VACUUM
                                                           FILTRATION
                                                          HYDROCYCLONE
                                                          CENTRIFUGE
DRY MATERIAL
                                                                            DISPOSAL
                                                                                         LANDFILL
                                   Figure  3-32.   Sludge  treatment  flow diagram.

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FGD SYSTEM:  Sludge Treatment	3-100
additive to reduce the moisture content and improve handling characteristics
without a  chemical  interaction between the sludge components  and  the addi-
tive.  The term is used in this context throughout this manual.
     In fixation processes, the chemical reactions bind the sludge particles
together, thus increasing shear and compressive strength and reducing perme-
ability.  The  structural  stability and environmental characteristics of the
waste  product  are thereby  improved.   The most  widely practiced  method  of
fixation involves the  blending of lime, fly ash, and vacuum filtered sludge
in a pug mill under controlled conditions.  The resultant mixture is stacked
for  2   to  3 days,  during  which  time  pozzolonic (cementitious)  reactions
responsible  for  improving the  physical  and  chemical  properties   of  the
mixture occur.   The fixated product is ultimately disposed of in a landfill.
Landfill produced by such a process is chemically and physically stable, and
generally  characterized by  low permeability.   This approach  is  currently
offered by several scrubber vendors as well as companies such as IUCS, which
specialize in  FGD  sludge  treatment.   The process has the advantage of being
nonproprietary  and   utilizes  a  generic   fixative  (lime).    Alternative
approaches such as  the Dravo  process, in which thickener underflow is mixed
with a proprietary fixative (Calcilox) are also available.
     An important preliminary  step in any sludge disposal system is consid-
eration  of  the chemical/physical  characteristics  of  the material.   The
predominant  solid  components  are calcium sulfite, calcium  sulfate,  and fly
ash.   The  quantities  of fly  ash  can  vary  considerably, depending  on  the
presence and efficiency of upstream fly ash removal devices and, in the case
of alkaline  fly ash,  the use  of the  fly  ash as all  or part of the scrubber
reagent.  The  proportions  of  calcium sulfite and  calcium sulfate  depend  on
many process factors.   Estimates  of relative quantities of these components
can  be made  from a  knowledge of the  boiler system,  upstream particulate
removal equipment,  fuel and  reagent analyses,  and the  liquid  cycle of the
scrubber/disposal  system.  These  data can be obtained  from  pilot  or proto-
type operation, from  similar  full-scale systems, or from equipment/material
specifications.

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FGD SYSTEM:  Sludge Treatment	3-101
     Likewise, the  quantities of  sludge  to be processed,  transported,  and
disposed of  can  be  estimated by a series of assumptions  and calculations.
     With this type  of  information and other pertinent data, a decision can
probably be made  regarding  wet or dry disposal.  Consideration  should then
be given to disposal site and design,  which will involve a thorough explora-
tion of environmental requirements.
Regulating Considerations
     Because  FGD  sludge is  a  relatively  new material,  many  regulatory
agencies are  not  familiar with its properties and characteristics,  nor are
they  familiar with  disposal  technologies.   As a  result,  few  regulations
pertaining specifically to the disposal of FGD sludges have been issued.   In
the absence  of specific Federal  guidance,  state and local  agencies  often
rely  on  standards and  regulations developed around  the disposal  of  other
types of waste, many of which are inappropriate or irrelevant in relation to
FGD sludges.   This situation, however, is rapidly changing.
     The  first two  major   pieces  of  Federal  legislation  addressing  solid
wastes were the  Solid  Waste Disposal  Act  of 1965 and the Resource Recovery
Act  of 1970,  both aimed primarily  at municipal solid waste,  with little
direct applicability to  FGD sludges.   Furthermore,  there was little regula-
tory  authority  centered in  these  laws.    The Federal   government issued
general guidelines  and  provided technical  assistance, but  had little  power
to regulate operations.   This situation has now changed.
     The  Resource  Conservation and  Recovery  Act  of  1976  (RCRA)  greatly
expanded  the   role   of   the  Federal   government in  regulating  solid  waste
handling and  disposal.   Subtitle C of RCRA  addresses  hazardous  wastes,  and
Subtitle D focuses  on  nonhazardous wastes.  As required by the Act, the EPA
is developing a  detailed program  for the regulation of  hazardous wastes.
Wastes defined as hazardous  are subject to "cradle-to-grave" considerations.
     In EPA's regulations of May 19,  1980,  for identifying hazardous wastes,
large-volume  utility  wastes,   including   FGD  sludge,  were  specifically
excluded from  regulation under Subtitle  C.  With this exclusion, FGD sludge
will be primarily  regulated as a  nonhazardous waste  under Subtitle D.  The

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FGD SYSTEM:  Sludge Treatment	          3-102
nonhazardous waste regulations,  however,  are quite general and not specifi-
cally oriented  toward  FGD wastes.   This generalized approach to the regula-
tion of  utility wastes  may change  in  the near future as  the  EPA develops
standards  that  are specifically applicable  to  large-volume utility wastes.
The regulations evolving  from  this program will probably not be promulgated
for  several  years.   Therefore,  while  nonhazardous waste  regulations  will
govern current  FGD disposal  activities, compliance with more detailed stan-
dards will likely be required in the future.
     Subtitle D of RCRA  prohibits open dumping and requires  that environ-
mentally acceptable practices  be utilized.   As provided for in the Act, the
EPA  has   developed  criteria  for  classifying  solid waste disposal  sites.
Facilities that cannot meet the criteria will be  considered  "open dumps,"
which are  illegal.  The  EPA has also proposed  guidelines  for  landfill  dis-
posal of  solid  waste, describing  waste disposal  practices recommended for
attaining compliance with the aforementioned criteria.   Although the EPA has
issued  criteria  and  guidelines  for  nonhazardous waste  management,  the
primary  authority  for implementing  nonhazardous  waste  regulatory programs
will continue to be with State agencies.
     Current state regulations applicable to FGD  sludge vary  widely across
the country.  Most  states have  sufficiently broad  legislative  authority to
regulate FGD sludge disposal  to some degree.  Some state  laws  allow direct
control   of solid  wastes.   Other  states  achieve   indirect  control  through
implementation  of regulations  designed  to protect the  ground water or limit
activities in floodplains.  Most states proceed cautiously with FGD wastes,
issuing permits  on a case-by-case basis.
     The  primary  concern  with  pond  disposal   of   FGD  wastes  has  been the
integrity of dikes and impoundments.   Regulations pertaining to such struc-
tures are  concerned with physical  damage to persons or property and effects
on  ground  or surface  waters.   Primary concerns with  landfilled wastes are
those of structural  stability  and  leachate/runoff effects  on  ground and
surface water.
     As a result of RCRA, many state programs are being revised.  With EPA's
encouragement and financial support,  numerous  state agencies are developing
new regulations and plans  to  implement them.  Most state programs are being

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FGD SYSTEM:  Sludge Treatment	3-103
modeled after the Federal program so as to minimize complications in obtain-
ing authorization from the EPA.  Therefore, delays in finalizing EPA regula-
tions are, in turn, postponing the promulgation of state regulations.
     In  carrying out  this  new  Federal  program,  the  state agencies  will
continue  to  play an  important role in issuing permits for  FGD sludge dis-
posal sites.   In spite of the increased Federal role, state requirements and
procedures may  differ substantially from state to state.   For  this reason,
operators of scrubbing  systems are advised to contact the appropriate state
agencies to determine the prevailing permit requirements.

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FGD SYSTEM:  Slurry Preparation	     3-104
LIMESTONE SLURRY PREPARATION
     The bulk  material  handling properties of limestone are  very  similar to
those of coal,  and the conventional powerplant procedures  for  coal  handling
and storage  can be easily adapted to  limestone.   Because  of  the close simi-
larity,  this  section emphasizes  those aspects  of  limestone  slurry  prep-
aration that apply specifically to limestone scrubbing.
     All operating scrubbing milling facilities use wet grinding  equipment,
specifically some version of the wet ball mill.
     In  closed-circuit  grinding,  as  shown in  Figure 3-33,  all   oversized
particles  are   recycled  until   they  are ground away.   Therefore,  feed  rock
should  contain the minimum  possible level of  siliceous  impurities.   It is
not necessary  to  include facilities for slurry dilution in a closed-circuit
installation;  the  15 to  30  percent slurry  from  the classifier is  suitable
for direct feed to the scrubber.
     The classification  device  used most often  in wet closed  circuits is the
hydraulic  cyclone  (hydrocyclone).   This  is  a  static apparatus  that  uses
energy  from  a   circulating pump to  develop a centrifugal  force  that concen-
trates particles of  higher  mass into a  fraction  of  the flowing stream.   The
underflow stream from the hydrocyclone is a thick slurry  containing most of
the larger  (heavier) particles;  this  stream is  recycled  to the  ball  mill.
The overflow stream,  containing most of the water and  smaller particles, is
sent to a  storage  tank.   A classifying  hydrocyclone  can make a fairly sharp
separation if  slurry composition and  flow rate  and pressure at  the hydro-
cyclone  inlet  remain  reasonably  constant.    In  plants   where  feed  rock
composition  is expected  to be inconsistent,   automatic  instrumentation  to
control density and  flow rate  should  be included  to  ensure proper hydro-
cyclone performance.
Slurry Storage
     Finely  ground  limestone   slurry  does  not  settle  quickly and can  be
stored  for  long  periods  in properly  designed,   agitated  storage  vessels.
Since limestone does  not react chemically either with  air  or with constitu-
ents in recycled water,  no  scale is formed and there are  no  special consid-
erations other  than  the  application of proVen engineering  design  principles
for the storage of viscous slurries.

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  LIMESTONE
MAKEUP
 WATER
FEED
BIN
                     WET BALL MILL
                     1  OPERATIONAL
                     1  SPARE
                           MILL SLURRY TANK
                           1 OPERATIONAL
                           1 SPARE
                                                    TO SLURRY
                                                   STORAGE TANK
                                  MILL SLURRY PUMP
                                  1 OPERATIONAL
                                  1 SPARE
    Figure 3-33.  Typical closed circuit grinding system.
                             3-105

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FGD SYSTEM:  Slurry Preparation	3-106
     Each  storage tank  must be equipped with a constantly operating agitator
to  prevent  settling  of  the  solids.   Agitators  should  be  vertical-shaft,
top-mounted  units  located  axially  within  the  vessel.    Agitators  with
internal  bearings  should  be  avoided  because  they present maintenance  and
repair  problems.   Turbine  impellers are best for  slurry  agitation,  usually
with  motors  connected  to  the shafts  through speed-reduction gears.   High-
speed agitation is not needed.
     Use  of  a pump to  move  slurry  from the  bottom of the tank to  the  top
also  improves  vertical  mixing  and  thereby maintains an even  slurry  concen-
tration at all levels within the storage tank.
Slurry Feed
     Limestone slurry is pumped  from the storage tank to  the scrubber vessel
at varying rates to maintain  proper pH in the scrubber.   This  operation is
accomplished  by  a  combination  of  pumps, piping,  and controls designed to
minimize  erosion of  the  transfer   equipment and  solids  deposition  during
transfer.   Four  basic arrangements, shown in Figure 3-34,  have been devel-
oped to realize these design objectives.
     Arrangement  1  utilizes  a  variable-speed  pump  drive  to  deliver  the
required  volume  of  slurry.   In this arrangement, there are no control valves
and  no  piping restrictions.   Operating pressure  is therefore minimal,  and
the system consumes the least amount of  pumping  energy.   Separate  pumps and
pipelines are  needed, however, for  each point of  limestone  application,  and
the system is consequently  expensive  for plants  that have  multiple scrub-
bers.    The  system is not  applicable to  plants that  have  significant turn-
downs in scrubber operating rate.
     Arrangement 2  is used  in other industries that  handle  heavy  slurries.
A pump  delivers  slurry  through a piping loop that is routed to pass close to
each point of  application,  and the   slurry returns  through  a piping restric-
tion to the  storage  tank.   At each  point of application,  valves are used to
control   the  slurry  flow.    Success  with  this  arrangement  depends  upon
accurate  hydraulic calculations  and proper pump selection.   The pump must be
a  "flat  head"  design,  in  which the  discharge  pressure remains  relatively
constant  regardless  of  pumping  rate.   The  piping  restriction is  usually a
"flow choke,"  a  block  of  hardened metal  drilled  through to a  calculated

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                          DIRECT PUMPING
     PUMP WITH
     VARIABLE-SPEED
     DRIVE MOTOR
                   -D-
                   RECYCLE WITH PIPING RESTRICTOR
                  -It	
        PIPING RESTRICTION
   •s
          •S	1
RECYCLE WITH CONTROL VALVE
t	
              CONTROL
              VALVE
                                      CONTROL
                                      VALVE
                                      CONTROL
                                      VALVE
                   RECYCLE WITH HEAD BOX
               -tt-
                              0
                                                  \
                                 HEAD
                                 BOX
                                 CONTROL
                                 VALVE
                                                               \
Figure 3-34.   Limestone  slurry feed  arrangements.
                           3-107

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FGD SYSTEM:  Slurry Preparation	    3-108
diameter.   Control  valves,  often  angle-type  with hardened trim,  are  close-
coupled  to the piping  loop and are  located  so  as  to discharge  vertically
downward to the application point.
     Arrangement 3  is  the same as  Arrangement 2,  except that  a control  valve
is substituted  for  the static flow restrictor.   This system provides greater
rangeability  in the  slurry  loop  and  is  used where  the piping  hydraulics
cannot  be  calculated   accurately.   The  best  applications,  of this  system
incorporate  instrument-activated  control  devices   to  throttle  the  return
valve and  thereby maintain  a calculated minimum flow rate in  all  sections of
the piping loop,  regardless  of rate of  slurry usage.   The  instrumentation
may be complex.
     Arrangement 4  is  used  in plants  where there  is  sufficient difference in
elevation  of  the  slurry storage  and  point  of feed  and no   great  distance
separates  them.   In this arrangement,  slurry  is  pumped continuously  to an
elevated  head  box.   Control  valves  below the  box  allow  flow of slurry as
needed, and excess  slurry overflows  a weir in the box and returns by gravity
to the  storage  tank.   Control valves in this  system are quite large but are
long-lasting  because  they  operate with  only a  slight pressure  drop.   Any
type of pump  can  be used, providing  it delivers  slightly more slurry to the
head box than the maximum total rate of usage.
     Velocity  of  flow  in all  arrangements must  be  high enough  to  provide
turbulence to  keep  solids from settling.   A frequent error is to provide too
much velocity;  the  amount needed is  less  than  many  piping designers assume.
Shawnee experience  indicates  that  a  slurry velocity  of 8 to 10 fps should be
maintained in piping.
Design Considerations
     Following  are  two  basic  considerations  for  selection and  handling of
dry limestone for scrubbers:
     Procure  the  grade  of  rock that is most  inexpensively  ground to very
     small size.
     Design materials-handling  equipment  to permit the handling of unclassi-
     fied or waste limestone.

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FGD SYSTEM:  Slurry Preparation	      3-109
     At least two  feed  bins or "day tanks"  are  needed  to  provide flexibility
in the  filling  schedule and to allow partial drying of excessively wet  feed
rock before use.   This  improves the feeding characteristics  of  the material.
Limestone  feeders  should also  be  duplicated,  since they are  usually high-
maintenance items  and spares may  be  needed to  ensure continuity of  produc-
tion.
     The size of a ball  mill is determined by four factors:   -
     1.   The mill capacity,  usually  based  on required tonnage  of feed  rock
          per 24-h day and hours per day of mill operation.
     2.   The  size  analysis  of  the  feed  rock,  including  both   maximum
          particle size and size distribution.
     3.   The required size of product from the  mill.
     4.   The Bond Work Index (grindability) of the  rock, which is a  measure
          of its resistance to grinding.
     The  size  of  a  ball mill  and to a  large  extent its  initial  cost  are
established by  the degree of size reduction necessary.   The larger the  feed
size and  the smaller the required particle  size,  the  larger a  mill must be.
For a  given  ball mill,  the  throughput capacity is reduced  by  20 percent if
the feed  size   is  increased from  0.25  to 1  in.   Capacity   is  reduced by 65
percent if product requirements are changed from 200 to 325  mesh.
     The most fundamental  and difficult  procedure in  selection  of limestone
milling  facilities  is  the  development  of  specifications  for  the finished
slurry.  These  specifications  significantly affect the initial  and operating
costs  of  the scrubber  installation.   There is,  however, very  little basic
information that defines  the most economical degree of grinding.  The trend
is  apparently  toward  finer  grinding:   one  major  equipment  manufacturer
reports  that  specifications  now  range  from 70  percent passing through  a
200-mesh screen  to 95 percent passing through a 325-mesh  screen; most are in
the range of 60 to 80 percent passing through 325 mesh.
     A  large  volume  of  slurry storage, divided into at  least two tanks,
should  be  provided  to  permit maintenance of equipment without interrupting
the scrubbing operation.   Storage  for 72 hours  of operation may be necessary
to  permit major  maintenance,  such as  replacement of the  ball  mill  liner.

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FGD SYSTEM:  Slurry Preparation	3-110
Because agitation  equipment Is  costly  and consumes power continuously,  the
best  compromise  may be  to  provide a portion  of the storage with only  com-
pressed air agitation (sparging), which Is used only when needed.
     The arrangement of  pumps  and piping to deliver slurry  to  the scrubbing
system  should  be based  on  engineering  comparisons of all possible  arrange-
ments.
     The main experience  factor  gained  from current limestone milling opera-
tions  has  been the  effect  of silica  and other impurities contained  in  the
supply  limestone on  the  ball  mill and  its  associated  classification system.
The quality  of delivered limestone  must be continuously checked to  ensure
that  it meets  the  scrubber  design  specifications.    Table 3-12  presents
design  specifications  for  several  representative  FGD  installations.   For
details, the reader  is  referred  to EPA's quarterly utility survey report and
to Appendix  F,  which lists the  U.S.  plants with operating  limestone  scrub-
bing systems.

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                         TABLE 3-12.   LIMESTONE SLURRY  PREPARATION AT OPERATIONAL  FACILITIES
co
i

Ball Mills
Number
Capacity, ton/h
Limestone slurry
concentration, X
Size (diameter x
length), ft
Motor, hp
Product size, X
through required
mesh
Type of circuit

Slurry handling
Number of slurry
tanks
Capacity of storage
tanks, gal
Agitator motor, hp
Number of dilution
tanks
Dilution tank capacity,
gal
Storage
Mode


Number of silos
Storage capacity,
No. of days
Conveying
No. of feed bins
Capacity of feed
bins, (ton) each
Type of feeders

No. of. feeders
Duck Creek 1

2
40

65

NR
NR

90
(200 mesh)
Closed


1

79,500
NR

NA

NA

Open
stockpile

NA

NA

1

NA
Weigh

1
LaCygne 1

2
110

66

NR
2,000

95
(200 mesh)
Closed


2

3,000
12

2

186,000

Open
stockpile

NA

30

2

1,200
Belt
conveyor
2
Lawrence 4 & 5

2
6

NR

6x12.5
150

NR

Closed


1

3,000
NR

1

1,000

Open
stockpile

NA

NR

1

30
Belt
conveyor
1
Sherburne 1 & 2

2
24

60

7x30
500

80
(200 mesh)
1-closed
2-open

2

270.000
1

2

4,200

Open
stockpile

NA

100

2

600
Belt
conveyor
2
Southwest 1

2
8

40

NR
NR

NR

Closed


1

30,000
NR

NR

NR

Silo


1

NR

1

NR
Belt
conveyor
1
Widow's Creek 8

1
40

40

NR
NR

90
(200 mesh)
Closed


1

200,000
NR

NR

NR

Open
stockpile
and silo
1

NR

NR

NR
Weigh
feed
1
               NR - Not reported.

               NA - Not applicable.

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FGD SYSTEM:  Pumps	3-112
PUMPS
     A  coal-burning  power plant  that  installs a limestone  FGD  system must
provide  for  handling of  the  slurries  generated in removal  of S02  from the
flue gas.  Because experience with slurry pumping is the basis for achieving
successful performance, the  utility  should obtain performance data from A/E
firms,  consultants,  and  pump  manufacturers.   Even  with  these  helpful
records,  however, the  project manager  must be aware that his slurry pumping
application is unique and must weigh all potential operating factors when he
assembles the fluid handling system.
     Operating records of the early  limestone FGD units show numerous fail-
ures  of pumps,  valves, piping,  and instrumentation  (O'Keefe  1980).   These
failures  can  be traced to the overconfidence of designers,  who envisioned
the  FGD  slurry conditions  as being  fairly  easy  compared  with those  of
mining,  metallurgical,  and  many chemical  processes.   Additionally,  the
operating  experience  with pumps  was  not well delineated  because the early
FGD systems  performed at  low availability and  reliability.   Shutdowns for
various reasons allowed ample time for maintenance of slurry-fluid handling
systems.  As the  overall  system performance improved, deficiencies in pumps
and  piping  became  obvious.   These deficiencies  are  now  being  corrected
through cooperation among operators,  A/E firms, and equipment manufacturers.
     The  following  discussion  gives  design  information  on pumps  for the
handling  of scrubber  recirculation slurry, limestone slurry feed, thickener
overflow,  thickener  underflow, and sludge,  and also on pumps that service
clear liquid streams.

Description and Function
     Recirculation pumps.   The recirculating  pumps  are  the largest pumps in
the  limestone  slurry  system.  They  receive  the  slurry directly  from the
bottom  of the  scrubber effluent hold tank.  The discharge slurry is contin-
uously  recirculated  through   the  scrubber.   Normally,  a  portion   of the
recirculation stream is bled to the solids disposal  system.
     Many features of the recirculation pump distinguish it from  the typical
centrifugal  pump  used  for clear  liquids.   Wall  thicknesses  of wetted-end
parts (casing,  impeller,  etc.) are greater than those  used  in conventional

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FGD SYSTEM:  Pumps	.	3-113
centrifugal pumps.  The  cutwater,  or volute tongue (the point on the casing
at which  the  discharge  nozzle diverges from the casing), is less pronounced
so as  to  minimize the effects of abrasion.   Flow  passages  through both the
casing and  impeller are  large enough to permit solids to pass without clog-
ging the  pump.   Because  the gap between the impeller face and suction liner
will increase  as  wear occurs, the rotating assembly of the slurry pump must
be capable  of axial  adjustments to maintain  the manufacturer's  recommended
clearance.  This is critical to the maintaining of  design heads,  capacities,
and  efficiencies.   Other  special   features   include  extra-large  stuffing
boxes, replaceable shaft sleeves,  and impeller back-vanes that  act  to keep
solids away from the  stuffing box.   Although the  impeller  back-vanes also
reduce axial  thrust by  lowering the stuffing-box pressure,  these  vanes can
wear  considerably in  abrasive service.   Hence,   both the  radial  and  the
axial-thrust bearings on the  slurry pump are heavier than those  on standard
centrifugal pumps (Dalstad 1977).
     Slurry pumps are available in a variety of materials of construction to
meet the  requirements  of various  applications for  withstanding abrasion and
corrosion.  A comprehensive service description must be developed as a guide
to selection of recirculation pumps.   This necessitates detailed  analysis of
slurry characteristics,  including composition, pH, specific  gravity,  vis-
cosity, and other factors, all of which are discussed briefly below.
     Composition of the  slurry is  critical to pump  selection.   The erosive
circulating  fluid contains  many  solid  and  dissolved species.   The major
solids are  undissolved  limestone,   fly  ash,  calcium sulfite (CaS03),  and
calcium sulfate  (CaS04).  Solids  content ranges  from 10 to  20 percent by
weight.   The  dissolved  species are calcium,  magnesium, sodium,  sulfite,
sulfate,   chloride, and  carbonate  ions.   Chemical  analysis of  the  slurry is
particularly important with closed-loop operation,  because some species such
as chloride  ions  can  build up to high levels.  Information about concentra-
tions and particle size of the solids will help determine abrasion-corrosion
resistance and the mechanical  strength required of  the pump.
     The pH of the slurry in the scrubber is maintained between 5.0 and 5.8.
The pH of a given solution varies with temperature, especially in the range
from 50° to 150°F.

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FGD SYSTEM:  Pumps	3-114
     The  specific gravity  of the  recirculating  slurry is  usually between
1.05 and 1.14.  The system incorporates automatic solids control to keep the
specific gravity of the slurry constant.
     Knowing  details  of  the rheology  of the slurry  makes it  possible to
evaluate  the reduction  in  pump  performance  due to  the  viscosity  of  the
mixture and  the  added slip between the fluid and the solid particles as the
mixture  accelerates   through  the pump  impeller.   This  slip is  greater in
mixtures with high settling velocities.
     The slurry may contain significant amounts of fly ash, depending on the
coal and  on  whether  an ESP mechanical collector precedes the scrubber.  The
amount and composition of the fly ash must be determined before equipment is
specified.
     Gas entrainment  in  the recirculating slurry could cause fluctuation of
the slurry in the pump from all liquid to essentially all  gas.   These vari-
able  conditions   can  cause  deflection  of  pump   shafts, which may  lead to
bearing failure and abnormal packing wear.  Gas entrainment also reduces the
liquid flow and thus reduces S02 absorption.
     Limestone Slurry Feed Pumps.    Concentrated  slurry   feed   is  usually
handled by rubber-lined centrifugal  pumps.  Positive displacement pumps with
variable-speed drives are  also  applicable.   Because the basic design of the
centrifugal  limestone slurry feed  pumps is the same as that of the recircu-
lation pumps, only the salient features of the positive displacement (screw)
pumps  are  discussed.   Cast  iron, erosion-resistant  alloy,  and rubber-lined
pumps  are  common in  limestone  scrubber systems.   Some  utilities prefer to
use rubber-lined  pumps  from a single manufacturer for uniformity throughout
the plant.
     The  limestone as   received  contains  tramp  materials  such  as  rocks,
metal, and wood.   With  proper  design and  operation,  the  screening process
before the ball  mills  should remove these  impurities so  that  they do not
enter the limestone slurry preparation system.

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FGD SYSTEM:   Pumps	3-115
     The  limestone slurry  is alkaline.   Following  are  typical  limestone
slurry conditions:

               pH                            8
               Solids, wt.  percent           20 to 60
               Solids                         CaC03
               Temperature,  °F               50 to 104
               Specific gravity              1.1 to 1.3
     Although centrifugal pumps are widely used, the screw pump that handles
limestone slurry feed  is a special type of rotary positive displacement pump
in which the  flow through  the pumping elements  is  truly  axial.   The liquid
is  carried  between screw  threads on  one  or more  rotors and is  displaced
axially as the screws  rotate and mesh.   In all other rotary pumps the liquid
is forced to  travel circumferentially; thus the screw pump, with its unique
axial  flow  pattern and  low internal  velocities, offers  a  number  of advan-
tages in those few applications where centrifugal pumps cannot be used.  The
screw pump can handle  liquids with viscosities ranging from that of molasses
to  that  of  gasoline.   Because  of  the   relatively  low  inertia  of  their
rotating parts,  screw  pumps can  operate at higher  speeds than other rotary
or reciprocating  pumps  of  comparable displacement.   Screw pumps, like other
rotary  positive  displacement  pumps,  are   self-priming  and have a delivery
flow characteristic that is essentially independent of pressure.
     As with any rotary pump, the arrangement for sealing the shafts is very
important and often is  critical.   All  types of screw pumps require at least
one  rotary  seal  on the drive shaft.   For drive  shafts,  rotary mechanical
seals as well  as stuffing  boxes or packings  are used.   Double back-to-back
arrangements with  a flushing liquid are sometimes  used for very viscous or
corrosive substances.
     The mechanical seal is gaining wider use with  the  advent of new elas-
tomers such as Viton,  Butyl, and Nordel.   The rotary components are made of
carbon, bronze, cast iron,  Ni-Resist, carbides, or ceramics.  The mechanical
seal can  be designed  to be  completely or partly  independent of  the fluid
pressure to which  it  is exposed, and  it  also can operate at subatmospheric
pressure without drawing in air.

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FGD SYSTEM:  Pumps	3-116
     Although It Is the maximum viscosity and the expected suction lift that
determine  the  size  of the pump and set the speed, it is the minimum viscos-
ity  that determines  the  capacity.   Screw pumps must always  be selected to
give  the specified capacity  when handling fluids  of the  expected  minimum
viscosity  since this  is  the point  at which  maximum  slip,  hence  minimum
capacity, occurs.
     Viscosity  and  speed  being closely linked, it is impossible to consider
one without  the other.   The basic speed that the manufacturer must consider
is the internal axial velocity of the liquid going through the rotors.   This
is a function of pump type, design, and size.
     Rotative speed should be reduced for handling of  liquids of high vis-
cosity.  The reasons for  this are not  only  the difficulty  of filling the
pumping elements, but also the mechanical losses that result from the shear-
ing action of the rotors on the substance handled.   Reducing these losses is
often more important  than obtaining relatively high speeds, even though the
latter might be possible  given positive pressure inlet conditions (Karassik
1976).
     The actual delivered capacity of any screw pump is  theoretical capacity
less  internal   leakage  or  slip  when  handling vapor-free  liquids.   For  a
particular speed, the actual  delivered capacity of any specific rotary pump
is reduced by  a decrease in viscosity and an increase in differential  pres-
sure.   The actual   speed  must always  be known; often  it differs from the
rated  or nameplate  specification.   Actual speed  is the  first item to  be
checked and verified in analyzing pump performance.
     Thickener Overflow Pumps.  Thickeners  are normally  arranged  for  over-
flow  by  gravity to a collection  tank,  from which one or  more pumps return
the  clarified  overflow  to  the system.   Thickener overflow may  be used for
preparing  the  limestone slurry,  as  prequencher water,  as part  of  a  blend
with fresh water for  mist eliminator wash, as  level  makeup to the effluent
hold tank,  or as makeup  for an ash disposal system.   The FGD system supplier
should select the pump to satisfy the capacity and head requirements  for the
intended use.   If some  other use of the thickener overflow is contemplated,
the  utility  must determine the  end  use  before  the pump  specification  is

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FGD SYSTEM:  Pumps	3-117
prepared.  The  pump head will  vary considerably, depending upon where  the
thickener overflow is used.
     Although largely  free  of suspended solids, the thickener  overflow  may
still contain some suspended solids.   Since corrosive ions such  as  chlorides
may  build up  in the  liquid  in  a closed-loop  operation,  the main  design
consideration for wetted parts of the pumps is corrosion resistance.
     Following are typical properties of thickener overflow Liquid:

               pH                            7 to 8 (normal)
               Solids                        500 ppm
               Temperature,  °F               100
               Specific gravity              1.0
     The  thickener   overflow pumps  are  usually  centrifugal,  with  direct
drive.
     Thickener Underflow Pumps.    Underflow  from  the   thickener  is  pumped
either  to a dewatering system or  to  a sludge pond, which  is  often located
thousands of feet and sometimes miles from the thickener.   The pump may also
recirculate  solids  to  the  thickener  to maintain  solids  concentration.
Depending  on  the  sludge destination or whether it must feed directly to the
downstream equipment, pumping head requirements may vary significantly for a
slurry  having uniform  solids content.   If solids  are  recirculated  to main-
tain  solids  concentration in the thickener, an oversized pump must be used.
     The  thickener  underflow is  a thick  slurry  containing all the  solids
species present in the scrubber (e.g., calcium sulfate, calcium sulfite, fly
ash, excess limestone).
     The  concentration  of solids  in  the  thickener  underflow is  limited to
about 40  percent  maximum because a centrifugal pump could not handle higher
concentrations without causing nonuniform  flow.  Because the slurry contains
high  concentrations of  abrasive solids,  the  main design  consideration is
erosion.

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FGD SYSTEM:  Pumps	  3-118
     Following are typical thickener underflow characteristics:
               pH                            7 to 8
               Solids, wt. percent           40
               Solids                        Fly ash, calcium sulfate,
                                              calcium sulfite, excess
                                              limestone
               Temperature, °F               100
     Thickener underflow  pumps  are  either centrifugal or positive displace-
ment  types,  with  belt drive.   The  positive  displacement  pumps may  have
neoprene stators and  high-chrome-alloy rotors.   Discharge from the positive
displacement pumps  is nonpulsating,  uniform,  and  reversible.   Rubber-lined
centrifugal pumps  provide extra  protection  against  corrosion  during upset
conditions.  The  underflow  pump must be specified  to handle  high solids
concentrations  with  abrasive  components and  under corrosive  conditions
during upsets.
     Sludge Disposal Pumps.   The distance to  the  disposal  site determines
the  type  of pump  selected.   The  sludge may  have  to  be  pumped as  far as
several  miles  to  remote  areas  for  disposal  or processing.   If  this is the
case, a  high head  (700 psi)  is  required  and reciprocating pumps are neces-
sary.   If  the  sludge disposal  is  close to the utility  site,  centrifugal
pumps similar to thickener underflow pumps can be used.
     Where the  sludge has a high solids content and contains  abrasive fly
ash, pumps must be rubber-lined or made of erosion-resistant alloy.
     Pond Water Return Pump.   In  systems where the sludge  does  not undergo
secondary  dewatering  treatment  prior to landfill,  large sludge ponds are
used to  separate suspended solids.   Clarified water from the sludge pond is
recycled by pump to the scrubbing system to balance the overall water usage.
The  pond water return pump  should be specified to take a pH  range of 6.5
to 8.  Since  chloride levels can  become high  in  closed-loop  systems,  the
pump  should  be chloride-corrosion  resistant.   Chloride corrosion  is  best
deterred by using rubber-lined pumps.

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FGD SYSTEM:  Pumps	3-119
     With good pond  design  the pond return water should contain few solids;
however, the pond  water  pumps return should be designed to accommodate some
suspended solids.
     Filtrate or Centrate Pump.   Clarified  filtrate  or  centrate  from  a
vacuum  filter or centrifuge  is  pumped  back into  the scrubbing  system,
usually  to  the  thickener.   In  either  case,  the  solids  content of  the
filtrate or  centrate  should be low and the  pH should be 7 to 8 or greater.
This pump  sees the  mildest duty of  any process pump  within  the  scrubbing
system.
     Operating upsets in the vacuum filter or centrifuge may allow solids to
contaminate the  filtrate  or centrate.   Although these occurrences are rare,
the filtrate or  centrate  pump must be designed  to  protect against possible
corrosion and  erosion.   Erosion-resistant alloys and rubber-lined pumps are
suitable.
     Fresh Water Pump.  In a limestone FGD system, the most likely points at
which  fresh  water  would  enter are the ball  mills,  pump seals, and the mist
eliminator wash  system.   A  fresh water pump for this service can be a stan-
dard centrifugal pump.   The important items to  be  specified are properties
of the  service water, available net positive suction head (NPSH), materials
of construction,  type of drive, and type and size of motor.
Design Considerations
     The  following discussion  of pump  design  parameters  is based  on the
service  functions  of  the  slurry recirculation pumps.  Many of these consid-
erations are applicable  also  to the other  types  of  pump service in the FGD
system.
     Flow/Head.   Low-speed  operation  is  one  of  the most  important wear-
reducing  features  of a  recirculation slurry  pump;  abrasive wear increases
proportionally  to  the  third power  of  rpm.    The   impeller  tip  speed  of
rubber-lined  pumps is  limited  to  3500  to 4500  ft/min.    This  limitation
restricts the  rpm of  rubber-lined pumps to 400 to 600 rpm, which corresponds
to  a  maximum  discharge  head of  about  100  ft.   Additionally the discharge
slurry  velocity  must be  maintained at  7 to 11 feet per  second  (fps) as  a
design compromise between abrasion and settling in the piping.

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FGD SYSTEM:  Pumps	_	3-120
     Total  liquid  flow rate required for the  scrubber  is  determined by the
design L/G ratio.  The normal recirculation flow range,  corresponding to the
rpm and  head  limitations,  is 6000 to 10,000  gpm.   Capacities, however, can
go to  20,000  gpm at present, and  30,000  gpm  is seen as a  possible  peak in
the near future.  Larger pumps are needed because larger scrubbers are being
installed to  reduce  the number of modules required and thus improve overall
system reliability  (O'Keefe 1980).   The number of pumps required per scrub-
ber, other  than spares,  is determined by the need for redundancy to achieve
system reliability.
     Net Positive Suction Head (NPSH).   The  available  NPSH must  be deter-
mined accurately.  More pump troubles result from incorrect determination of
available NPSH  than  from any other single cause.  As the available NPSH for
a  given  pump decreases,  its capacity  and  efficiency decrease,  and a low-
suction  pressure is developed  at  the  pump  inlet.   The pressure decreases
until a  vacuum  is  created and the liquid  flashes  to vapor (if the pressure
is lower than the liquid vapor pressure).  This condition,  which can lead to
cavitation  damage,  can be  avoided by  ensuring  that the  available  NPSH is
greater than the required NPSH.
     Pump Efficiency and Energy Requirements.   The  pump  manufacturer should
be given data  regarding energy costs, service conditions,  and flow and head
requirements.    For example,  an  energy cost of 3$/kWh and  a penalty of $750
per additional  kW  can  be specified as the basis for preliminary comparisons
with the  most  efficient  pump  (Dublin  1977).   Variations   in  pump size and
efficiency result  from each manufacturer's effort to choose, from his stan-
dard line of  pumps,  the one that  most  closely meets the specified require-
ments.    Hence,  specifications  should not be  so  restrictive as  to  exclude
high-efficiency pumps.   Finally, when the efficiency penalty is less than 10
percent,  the  pump  with  lower  speed  should  be selected because  the longer
pump life will  compensate  for  the slightly higher operating costs (Reynolds
1976).
     Drives.  Slurry  pumps operate  at  relatively  low  speeds,  ranging from
400 to 600  rpm.  Since the motors operate at either 1200 or 1800 rpm, some

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FGD SYSTEM:  Pumps	3-121
type of speed  reducer  must be used.   The most common way of driving a lime-
stone slurry pump is by using a V-belt drive with a fixed ratio;  this method
has the  advantages  of  flexibility  and  Tow  cost.   The V-belt drive  can  be
overhead-mounted or can be side-mounted  on horizontal pumps.   Because it  is
difficult  to  determine friction values  of certain slurries for which data
are  not  readily  available,  it  is  advisable  to  use  V-belt drives  with
variable-pitch diameters.   Without  greatly  increasing the  initial  purchase
cost, these  drives  simplify  balancing of the system at  startup and enable
the pump  to accommodate  future  changes   in  flow rate and  head.  They also
reduce deterioration of pump performance due to wear, and  allow correction
to initial system design for pumping of a particular slurry.
     Seals.   In  horizontal centrifugal  slurry pumps the shaft  that passes
through  the pump  casing   must  be  sealed to  prevent leakage.   Mechanical
seals, which  are used for handling  clear liquids, should  not be used with
slurries.    Packed  stuffing boxes are customarily used  to  seal  the shafts
because  they  cost  less,   allow  faster  repair,  and  usually last longer  in
abrasive service.
     At an  intermediate position in the  packing, a continuous  flow  of clear
water should  be  introduced into a lantern ring.   This  flush water  prevents
abrasive  solids  from entering  the  critical  stuffing  box and shaft sleeve
area  and  greatly extends  the  life  of the packing and the  sleeve.   Because
abrasive  solids may  enter the packing during  a  shutdown or upset,  the pump
should be  designed  with  a shaft sleeve of hardened alloy.  Even in  the best
operations, abrasive slurry may enter the packing.
     The  flush water  flows  past the packing into the  process  or  out the
stuffing  box.  The  volume of flush water  that mixes  with the  recirculating
slurry does affect  the scrubber system water  balance.   In  closed-loop sys-
tems, however, the  sump water is reused within the system,  depending on its
quality.  As  a  housekeeping measure, a  large-diameter drain line should  be
provided to carry the leakage from the stuffing box to the sump.
     The  flush water  supply system must  be  external  to  the scrubber system
and  reliable  enough to deliver a minimum quantity and  pressure.   Process
water can be used  if  the  suspended  particles are less  than  40 microns  in
diameter  and  the   maximum  particle  concentration is  1000  ppm by weight

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FGD SYSTEM:  Pumps	   3-122
(Wilhelm  1977).   The required  clarity  may be  achieved with  addition  of a
filter in  the  water line.   Slurry pump design requires that the pump impel-
ler  have  back-vanes,  which remove  slurry from  the  stuffing box  region.
Because this makes the pressure in the stuffing box assembly essentially the
same as the suction pressure to the pump, the sealing fluid must be supplied
at a pressure  at least 5 to 10 psi above the suction pressure.  When a pump
lacks such back-vanes  or is excessively worn, the  pressure .in the stuffing
box  region may  rise  to the discharge  pressure.   Then  it is  necessary  to
supply the sealing  fluid at 5 to 10 psi above the discharge pressure.   As a
precautionary measure, an alarm may be installed on the seal  water feed line
to indicate low pressure or low flow of seal  water.
     Wear  Rings.  A wear  ring provides  an  economically  renewable  leakage
joint between  the impeller and casing.   The purpose  of  the  wear ring is to
minimize  leakage from  the  discharge to the suction of the impeller by main-
taining  a  close  running  clearance  between  the  vanes  and  the  wear  face.
Running clearance has  a profound effect on the head capacity and efficiency
of the  pump.   To  reduce  the  rate of  wear  of the  wear rings and  thereby
extend the life of the pump,  the designer must consider  the  corrosion and
wear characteristics of the ring material.
Operational Systems
     Table  3-13 lists  specifications  of  recirculation pumps  installed  at
operational limestone FGD  facilities.   The following is a brief  summary of
performance histories of limestone slurry recirculation pumps.
     Cholla 1.    Rubber   linings  in the  recirculation  pumps  at Cholla  have
been damaged  many  times,  primarily  because  of  frequent  plugging  of  the
strainer at the suction end of the pump  (Laseke  1978a).   As plugging stops
the  flow or drastically reduces  it, the  pump cavitates  and  the liner  is
sucked into the path  of the impeller and shredded.   Initially the  suction
strainers  were  located  inside  the recirculation tank and could  be  cleaned
only by draining the tank.   Pump problems were solved, however, by installa-
tion of  a  hydrocyclone  in  the recirculation  line of each module,  an instal-
lation made for  other  purposes  as well.   The hydrocyclone  separates  the

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            TABLE 3-13.  SPECIFICATIONS OF RECIRCULATION PUMPS IN
                      OPERATIONAL LIMESTONE FGD SYSTEMS

Cholla No. 1
Arizona Public Service
Petersburg No. 3
Indianapolis P & L
La Cygne No. 1
Kansas City P & L
Lawrence No. 4
Kansas P & L
Lawrence No. 5
.Kansas P & L
Sherburne No. 1
Northern States Power
Winyah No. 2
South Carolina Public
Service
Southwest No. 1
Springfield City
Utilities
Widow's Creek No. 8
Tennessee Valley
Authority
Martin Lake No. 1
Texas Utilities
Individual
pump ratings
Flow,
gal /mi n
2,670
9,300
3,000

5,000
9,000
5,300
3,600
NR
NR
5,500
8,000
6,600
13,700

13,800


10,400
3,380

3,150

Head,
ft
90
100
NR

98
114
83
NR
NR
NR
95
NR
130
84

100


100
85

125

Speed,
rpm
NR
NR
NR

500
550
550
NR
480
480
500
500
500


500


500
500

500

Motor,
hp
NR
NR
NR

350
400
200
200
500
500
200
NR
NR
NR

NR


NR
NR

500

No. of pumps
Total
2
2
12

8
8
2
2
2
2
12
12
1
1

4


10
6

18

Spare
1
1
4

1
1
1
1
1
0
1
1
0
. 1

2


4
1

NR

NR - Not reported.
                                     3-123

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FGD SYSTEM:  Pumps	3-124
larger particles of scale from the main slurry stream by centrifugal action.
The  strainer therefore  is  no  longer  needed,  and  its removal  eliminates
plugging problems.
     Another preventive  measure at  Choi la  is rubber lining  of  the process
piping network  to protect the carbon steel  base from abrasive slurry.   This
has been generally successful,  although a few incidents of wear in the area
of the  spent slurry valves  have  been reported.   The wear  is attributed to
the  throttling  action of the valve  to modulate the  flow of slurry.   The
problem  was solved  by  operating  the  valve  only in  a completely  open or
completely closed position.
     La Cygne 1.   Problems at  La  Cygne have been more  in  the piping system
than  in  the recirculation pumps.   Sediment built up several  times  in dead
spaces in  the pipelines  and  valves of idle  pumps and also in process lines.
This  buildup  occurred when  slurry velocities in the pipe  were  low (during
periods of reduced operating rate).  The problem was  resolved by redesigning
some pipes  to eliminate  potential  dead pockets.   To prevent  valve freezing
due to  sediment  buildup,  some  valves were repositioned and  flushout  lines
were installed.
     Some  pipe   liners  eroded  (e.g.,  in  the   scrubber  tower  pump  inlet
piping).   Some  of the  erosion was caused by  unsatisfactory liner materials
and some  by high  flow velocities through  pipes and fittings.   The rubber
lining in  some  pipes  cracked,  primarily because of  defects in fabrication.
Piping modifications helped to reduce the erosion (Laseke 1978b).
     Lawrence 4 and 5.   The  original  FGD  system  on the Lawrence  No. 4 unit
entailed  limestone  injection  into  the  boiler  along  with   a  marble  bed
scrubber.   Limestone   injection has  been discontinued,  and the  marble  bed
scrubber was replaced by a rod scrubber/spray tower system.
     The original  system  was plagued with erosion, corrosion, and plugging.
The modifications involved  replacement of  all   steel  pipes  with  FRP  and
rubber-lined  pipes,  and  installation  of  new  slurry  pumps   and  strainers
(Teeter 1978).   In the new system, the recirculation  pumps at first required
packing  replacement  every 2 or  3  days.   The  gland  seal arrangement  was
redesigned, and the seal  water  flow was  increased  from  7 gpm  to  15 gpm.

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FGD SYSTEM:  Pumps	3-125
     Pump problems at  Lawrence  5,  Installed In 1971, were  similar  to those
at Lawrence 4.   This  unit was also modified to  a  rod scrubber/ spray tower
design.
     Sherburne County 1 and 2.   The  most  significant   problem  affecting
availability  of  the  overall  scrubber  system  has  been  plugging  of  spray
nozzles, originally caused  by failures  in the Zurn duplex  strainers  on the
discharge of  the  spray water  pumps.   The strainers were  designed  to  remove
solid  particles  greater than 1/4  in.  to prevent  plugging of  the  nozzles.
Frequent plugging  of the  strainers,  mechanical  failures,  and  bypassing of
solids large enough to plug the spray nozzles  led to reduced availability of
the scrubber unit and high maintenance requirements.   Maintaining the  duplex
operation  of  the strainers proved  to be impractical;  subsequent  operation
with a  single  strainer basket required shutdown  of the module for cleaning.
Extensive efforts to correct the problem were  not successful (Kruger 1978a).
     When  the  duplex strainers were  abandoned,  Combustion  Engineering in-
stalled  new  in-tank  strainers  consisting  of  a  large,  perforated,   semi-
circular plate  installed over  the  spray water pump suction  with  an  oscil-
lating and  retracting  wash  lance  for periodic  backwashing.   After installa-
tion of  one strainer  in  September  1976, plugging of the spray nozzles was
significantly reduced.   Because of this successful  operation,  all modules on
Sherburne  1 and  2 were converted by  March 1977.   Plugging  of nozzles has
continued,   however.   The apparent cause is formation  of scale inside the
perforated plate  and piping headers;  the scale  then breaks off and plugs the
nozzles (Kruger 1978b).
     When  plugging  occurred,  the  blower was  used  to  clean  the  strainer
perforated  plate.   Because  the cleaning was  not  complete,  the plate was
plugged  again and  the blower  used  again.   As  this  continued, the  slurry
level  in the  reaction  tank became too high and  the  solids  content too low.
The  problem is believed to lie in  the supply  pressure and capacity  to the
blowers, and a new system for the  supply has been designed.
     Failures  of  the LaFavorite rubber  lining downstream  of orifices have
occurred in the main recirculation and effluent bleedoff piping.  When these
failures occur,  the  rubber breaks  off  in  chunks  and plugs  the downstream
nozzles or headers.   Such failures were minor  on  Unit 1 but major on Unit 2.

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FGD SYSTEM:  Pumps	3-126
The  rubber  lining  has been  removed to prevent  further plugging.   A test
program was undertaken to evaluate the performance of reinforced fiberglass,
stainless steel, and rubber-lined spool pieces.
     Operation  of  the  recirculation  pumps  indicates  that  the  Ni-Hard
impeller must  be replaced  about every 6000  hours and  the  Ni-Hard suction
side  wear  plate about  every 4000  hours.   Pump internals  of 28  percent
chrome-iron  and  rubber-lined internals  have  been  installed on  selected
modules for evaluation.
     Operation of Slurry Feed Pumps.    Following   is   a   brief  summary  of
reported experience with operating limestone slurry feed pumps.
     The positive  displacement pumps  at  Sherburne 1  and 2  (Kruger  1978b)
were underdesigned,  and capacities were upgraded from 1 to 12 gpm to 2 to 20
gpm.  The positive  displacement  pumps at Lawrence 4 and 5  require replace-
ment of the stator and the rotor every 10 to 15 weeks (Teeter 1978).   Though
this  life  expectancy  is short,  the  wear  is predictable  and  control  of the
slurry feed  rate  is excellent.   For these  reasons, the  company has decided
to  continue operations with  the same type of pump.   At most other sites
there have been no major problems in feed pump operation.

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F6D SYSTEM:  Piping. Valves. Spray Nozzles	3-127
PIPING, VALVES, AND SPRAY NOZZLES
     This  discussion  emphasizes  design  details  related  to  the  piping,
valves, and  spray nozzles  in  limestone scrubbing applications.  Since many
of the  design considerations of  these  components are integrally related  to
their  materials  of construction,  this  discussion focuses  on materials and
summarizes  some   of  the  information  presented  later in  Section  3  and  in
Appendix G.   This information provides  guidance for the project manager  in
reviewing the proposals of scrubber system suppliers.
     Piping,  valves,   and nozzles  have been  subject to  moderate material
failures and  mechanical  problems,  mostly attributable to the  mode  of use  of
the component,  difficulty of repair,  unavailability of materials, and  lack
of standby units  or bypass capability  (Rosenberg et  al.  1980).   Although the
"best"  designs  for this  application  are not  yet identified, much has  been
learned by review  of the  experience  records  of  full-scale operation and
correlation of reported failures  with the service environment.
Piping
     The choice for  slurry piping has  been  predominantly rubber-lined carbon
steel  pipe.   Alloyed metal pipe  is expensive  and often not  as  resistant  as
the carbon steel  pipe to erosion and corrosion.   Reinforced  fiberglass  pipe
has been  discarded  in some plants,  although  it  is  being used in  lieu  of
rubber-lined  carbon  steel at other  plants  that have been  dissatisfied  with
metals or elastomers (O'Keefe 1980).
     Piping constructed  of various materials has proved moderately success-
ful in  limestone  scrubbing  systems,  and with  proper maintenance  has given
fairly reliable service at a reasonable investment cost.
     Typical piping applications  in  a  limestone scrubbing  system are listed
below:
          Type                                    Service
Carbon steel                                 °  Steam and condensate return
                                             0  Instrument air
                                             0  Industrial/service water
                                             0  Noncorrosive slurry
                                             0  Vacuum piping
                                             0  Miscellaneous water service
                                             0  Lube oil piping

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FGD SYSTEM:  Piping, Valves. Spray Nozzles	.	3-128
Rubber-lined carbon steel                    °  Severely abrasive slurry
                                                 with acid corrosives
Reinforced plastic, typified by fiber-       °  Nonabrasive or mildly abra-
 glass reinforced polyester (FRP)                sive slurry. Severely
                                                 abrasive or corrosive con-
                                                 ditions in straight piping
                                                 runs
316L or 317L stainless steel                 °  Mildly abrasive slurry with
                                                 acid corrosives
     Rubber-lined  carbon steel  piping  is  used  frequently  in the  scrubber
area and  the limestone  slurry preparation  area.   It is used because  of  its
superior  resistance  to  abrasion  by  solid particles carried in the  slurries
and because  it  is  resistant to corrosion by acid liquids and by solutions of
certain inorganic  salts.   Rubber-lined steel  pipe is not easily damaged,  but
care must be taken not  to  heat  the  rubber above its maximum operating  tem-
perature.
     FRP  pipe is  used in similar service because it is  resistant  to  chemical
attack  and  is  moderately  resistant  to  erosion by  solid  particles.   This
piping also  can  be damaged by overheating.   It  is  used at most locations as
reclaimed water piping  either  for pond return or thickener overflow  service.
These streams are  essentially  solids-free.   Some problems with  FRP pipe  have
been reported:   it has  been difficult to heat trace (although this  problem
has recently been  corrected),  it is  subject  to  rupture,  and it will  leak if
threaded  fittings  are used.   Flanged  or shop-fabricated  joints  are  prefer-
able to  field-cemented  joints  because many  pipefitters  are not skilled in
making FRP joints.
     Type 316L stainless steel  piping  along with FRP is primarily chosen  for
spray  headers  located  inside  scrubbers.   Stainless steel  317L is  gaining
favor because  it provides  greater corrosion  resistance  as  a result  of  its
additional molybdenum content.
     Some  important  design considerations  for slurry  piping are increasing
the sidewall  thickness  at  points of  high  erosive  wear and the  use  of  long
radii  for pipe  turns.    The  use  of  too  many reducers  and sharp  bends  in
piping  lines are  design errors  that  have accelerated  wear and failure  at

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FGD SYSTEM:  Piping. Valves. Spray Nozzles	3-129
these points.  The  abrasive slurry can  shear  off  the  rubber  lining  in  chunks
that can  be carried  to  the spray  nozzles  and spray headers (Laseke  1979).
The bottoms  of the pipe sections  tend  to erode first, but  rotation extends
pipe  life fourfold  or  more.   Therefore  the  joint and  pipe support  should
permit  easy  turning.   Provisions  for  automatic  clear water  flushing  and
drainage  are absolutely necessary  for  good  operability.  All  slurry  lines
must be sized  to  maintain  a minimum velocity  (7-11 fps) adequate to prevent
solids  from  settling in the  lines during  minimum flow and abrasion  during
maximum flow.
Valves
     Frequent problems with isolation and control  valves have been  caused by
mechanical or  material  failures.   Therefore,  most designers agree that  the
number  of valves  should be kept  to a minimum.  Rubber-lined valves are  the
most  common, although many stainless   steel  valves  are used  and  trials  of
polyethylene  have  been  promising.   Designers  have  successfully  modified
conventional  valves  used in mining and mineral processing  for the erosive/
corrosive  service  of limestone  slurry  systems, but their size, weight,  and
price are substantially greater than those of conventional  valves.
     Deep  erosion  can occur where a valve  causes an abrupt change in flow
direction.   Areas  that  can fill  with  slurry will  block  the  opening  and
closing of valves.   The  scouring effects  on seats and disks  make valves leak
quickly.   These  characteristics  have ruled out the  use  of several conven-
tional  valve  types  in  this  application.    A variety  of  valve  types  and
materials  of construction   are in use,  however,  and  the chief factors that
dictate  valve  selection  are  corrosion  and  erosion  resistance, minimum
resistance  to flow,  and positive  shutoff  in long-term use.   Hand-operated
valves are of  the overhead chain wheel  type  or of the gear  assistance type.
Following  are typical  hand-operated valves  used  in a limestone  scrubbing
system:
               Eccentric plug valves
               Lined butterfly valves
               Flex check valves
               SJ"

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FGD SYSTEM:  Piping. Valves. Spray Nozzles	3-130
     Air-operated or electric-operated  control  valves  are very Important for
control of  the  various  liquor streams in accordance with the process control
scheme.   They  are  also  used  for  isolation of  equipment  such  as  pumps.
Typical control valves  used in a limestone scrubbing system are pinch valves
and plug valves.
     Globe  valves  are  used to  control the  service water, mist  eliminator
wash, and miscellaneous water  flows.   A few globe-type valves with favorably
formed  body passages  simulating plug  valves have seen  service  in  slurry
control.
     Butterfly  valves  are used  as  on-off control  valves for  slurry service
lines and  in  recirculation  slurry pump discharge piping.   They range in size
from 2  to  20  inches and can handle flow rates  up  to  10,000  gpm.   When used
as  isolation  valves in abrasive slurries,  they are  typically  rubber-lined
carbon steel.    In the  throttled position, butterfly valves  can  damage down-
stream piping because their discharge flow is asymmetric.
     Knife-gate valves are  used  for shutoff and also  for modulating service
for clear  liquor like  streams.   The knife-gate  valve  uses  a shearing action
in which  its  thin  disk knifes through  any deposit on  the valve seats.   The
seat  rings  can  be  elastomeric,  but  the disk  is  usually stainless  steel.
     Plug  valves  are  used  for  isolation  service  in water and/or  slurry
lines.  Eccentric plug  valves,  in which the plug wipes  past the seat and is
clear of  flow  in the  open position,  have given excellent results  in lime-
stone scrubbing slurry service.
     Quarter-turn valves,  in which  the stem rotates without the sliding that
permits  slurry  into  the  stem  packing,  are preferred  for  slurry  service.
Spray Nozzles
     Spray nozzles provide  the  liquid distribution pattern that is essential
to scrubber operation,  especially  in a spray tower scrubber.   A  sufficient
number of  spray nozzles and source spray  banks  and headers must  be  used to
cover the  cross section of the  scrubber.   Spray nozzles should be  selected
to provide  droplets  of about  2500  microns (mass  median diameter),  which
results  in  minimum  droplet entrainment while providing  enough  surface area
for S02 absorption  (Saleem 1980).

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FGD SYSTEM:  Piping. Valves. Spray Nozzles	3-131
     The nozzle must  be  nonclogging and abrasion-resistant.   Hollow-cone and
full cone  nozzles  cast  of silicon carbide and/or ceramic meet these require-
ments.   Liquid  is  atomized by centrifugal action induced by tangential  entry
or  swirl   vanes.   The  nozzle chamber  has few  internals  which  can  become
clogged.  They  can  be made in various  sizes  to  suit liquid flow  requirement
at  a  normal  operating pressure of  10  psig.   Full cone  nozzles are normally
used in tray or packed scrubbers.
     Wear, plugging,  and  installation  problems with spray  nozzles  have been
reported.   Wear occurred with plastic nozzles made  of Noryl  resin  (Laseke
1979),   but  replacement  nozzles of  ceramic proved successful.  Nozzles made
of  extremely  hard alumina  and silicon carbide have also been  used success-
fully.   Stainless  steel  appears to  be preferred  for  wash spray nozzles used
in mist eliminators (Rosenberg et  al. 1980).
     The number of  sprays and spray banks installed in spray towers can vary
depending  on  the  required S02  removal  efficiency.   One  to six  banks  are
used.    When  each spray  bank  is   fed by a separate  pump, a  great  degree of
flexibility can be  built into the scrubber by allowing the use of individual
pumps depending on need.
     Early experience has  led to a great improvement in the spray nozzles,
spray  banks,  and  headers now being offered.  These  components  now have  a
relatively low  incidence of  problems  and they are amenable  to rapid repair
or replacement.

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FGD SYSTEM:  Ducts. Expansion Joints, and Dampers	3-132
DUCTS, EXPANSION JOINTS, AND DAMPERS
     The ductwork, expansion  joints,  and dampers of the limestone scrubbing
system, along  with the  fans,  scrubber, mist eliminator, reheaters  (if in-
stalled) and stack, constitute  the flue gas handling portion of the system.
A ductwork assembly consists of ducting, dampers, stiffeners, turning vanes,
flanges, transition pieces, access doors, test  ports,  gasketing,  expansion
joints, and duct  support bases.   The assembly is field-erected as part of a
structural  steel  installation contract  or as part of the overall  limestone
scrubber supply contract.  Designs for all components are well standardized.
Design  details  for these  various  components are  briefly discussed  in the
following  subsection.    Again,  as in the  foregoing discussion,  this  discus-
sion  focuses  on  materials of  construction  as  a  critical   design  factor.
Because of the volume  of steel required and the alloys used  for dampers, the
cost  of ductwork is  a  major part  of the total FGD  system  cost,  sometimes
representing as much as 10 to 15 percent.
Ductwork
     Ductwork in  an FGD system  is usually made  of  carbon steel plates 3/16
or 1/4  inch thick,  welded in a  rectangular  cross  section.   The ductwork is
supported  by  angle frames that  are stiffened  at uniform  intervals.   The
following  factors  should  be  considered  in  designing limestone  scrubber
ductwork:
     0    Pressure and temperature
     0    Velocity
     0    Flow distribution
     0    Variations in operating conditions
     0    Materials of construction
     0    Materials thicknesses
     0    Pressure drop (AP)
     The ductwork must  be designed to withstand the  pressures  and tempera-
tures  that occur during  normal  operation and also those that  occur during
emergency conditions such as an air heater outage.

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FGD SYSTEM:  Ducts. Expansion Joints, and Dampers	3-133
     The  maximum  permissible  gas  velocity through  the  ductwork should  be
designed so that  ductwork  furnished with the scrubbing system is compatible
with that  provided  under other powerplant contracts.   Typically the maximun
gas velocity should be 3300 to 3600 ft/min.
     Ductwork associated with  the  scrubbing system is subject  to a variety
of conditions, depending on  location within the system.   The following list
identifies the basic variants:
     0    Inlet ductwork
     0    Bypass ductwork (all  or part of the flue gas)
     0    Outlet ductwork (with reheat and without bypass)
     0    Outlet ductwork (with reheat and with bypass for startup)
     0    Outlet ductwork (without reheat and without bypass)
     0    Outlet  ductwork  (without  reheat  and  with  bypass  for  startup)
     Outlet ductwork  has  been  a  major problem, particularly  in units with
duct sections  that handle both hot  and  wet gas.  Materials  of the outlet
duct  range from  unlined  carbon  steel  to  Haste!loy  C-276.   Environmental
differences related  to the  location and use  of reheaters  and the type of
coal burned are factors that affect the use of different materials.
     The primary  material  of  construction  for ducts  is  carbon steel  plate
with various  linings  selected  for their cost-effectiveness in resisting the
effects of corrosive  or abrasive  gases.  The gas  environments can  be cate-
gorized as relatively  mild,  moderate, and severe.  Typical lining materials
that can adequately serve each environment are as follows:
     Mild:  Wet corrosive  gases with or without  liquids  at  temperatures up
     to 180°F with no abrasion.
     Glass-flake  filled plastic   coating  is  satisfactory.    The  polyester
     resin can  successfully  withstand  the  chemical  and  immersion  environ-
     ment.  The glass  flakes provide mechanical  strength and resistance to
     permeation by creating a labyrinthine path  for liquid.   This lining
     type  requires  sand blasting,  priming,  and two hand-troweled coatings.
     The total thickness is 60 to 80 mils.
     Moderate:  Wet  corrosive gases  and  impingement  by  abrasive slurry
     liquid or sprays at temperatures up to 160°F.

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FGD SYSTEM:  Ducts. Expansion Joints, and Dampers 	3-134
     An  aggregate-filled,  fiberglass-reinforced plastic  resin  system  is
     satisfactory.  It requires sand blasting, priming, a base coat overlaid
     with  fiberglass  woven  cloth,  and  finally  a  top  coat filled  with  a
     sand-like aggregate to resist abrasion.  The total thickness is approx-
     imately  1/3  in.   The  fiberglass  cloth  gives   good   control  of  the
     troweled  thickness,  minimizes  the   thermal   expansion  coefficient,
     resists mechanical drainage,  and  serves as a backup abrasion-resistant
     barrier.  The  resin  used  is the same as that in the glass-flake-filled
     plastic resin systems.
     Severe:   Alternate  exposure  to  hot  dry  gas  (at  temperatures up  to
     700°F) and to saturated gas or liquid.
     A  heavy castable  lining,  such as an  aluminous cement with  silicate
     binders,  is applied  by  guniting  to  a thickness  of  1-1/4 in.   This
     lining  is  resistant to the chemical,   thermal,  and immersion  environ-
     ment.
     In addition  to carbon steel with the various  linings,  ducts  have been
made from high-grade  alloys and from  carbon steel  base plate  cladded  with
the alloys  to serve as a  lining.   Acid-proof brick has been used  in ducts
located in  severely abrasive environments.   Thickness is 2-1/2  in.  or more.
     The  inlet  ductwork from  the  precipitator to the  scrubber is  unlined.
The scrubber outlet  ductwork  is  lined for severe  service because it  is
exposed to  both  hot dry flue gas and saturated gas.   Table  3-14 lists duct-
work applications in  a  typical limestone  system containing four  scrubber
modules, with suggested materials of construction.
     The  choice  of linings  or the  use  of  acid-proof  brick or high-nickel
alloys should be  determined  so as to insure the reliability of  various  duct
service applications.   The project  manager should  seek out the experience
record of the  scrubbing  system supplier and weigh this record against cost-
effective economics of alternative choices.
Expansion Joints
     Two  basic types  of  expansion  joints are available:  metallic  and elas-
tomeric.  The following factors should be considered in designating the type
and construction of scrubbing system expansion joints:
     0    Movements to be accommodated
     0    Temperature
     0    Pressure

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  TABLE 3-14.  TYPICAL DUCTWORK APPLICATIONS AND MATERIALS OF CONSTRUCTION
            Application
         Materials
Fan inlet duct:  conveys flue gas from
discharge duct of boiler ID fan to FGD
booster fan; can contain turning vanes

Fan discharge duct:  conveys flue gas
from booster fan outlet damper to .
scrubber inlet manifold

Scrubber inlet manifold:  receives flue
gas from booster fan outlet duct for
distribution to scrubbers

Scrubber inlet duct:  conveys flue gas
from manifold to scrubber; usually
provided with manholes

Bypass duct:  conveys flue gas from
inlet manifold through bypass damper
Reheat duct:  conveys air from dis-
charge of reheat fan to stack inlet
duct through gas reheater

Stack inlet duct:  conveys gas from
scrubber outlet manifold to stack;
also accepts hot air from reheater
duct and provides chamber for mixing
with wet flue gas; severe service
1/4- to 3/8-in..-thick carbon
steel plate; externally
stiffened, unlined, insulated

1/4-in.-thick carbon steel
plate; externally stiffened,
unlined, insulated

1/4-in.-thick carbon steel
plate; externally stiffened,
unlined, insulated

1/4-in.-thick carbon steel
plate; externally stiffened,
unlined, insulated

1/4-in.-thick carbon steel
plate; externally stiffened,
unlined, insulated

1/4-in.-thick carbon steel
plate; externally stiffened,
unlined, insulated

1/4-in.-thick carbon steel
plate lined with 1-1/4 in.
aluminous cement; covered with
2-in. mineral fiber blanket
insulation
                                    3-135

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FGD SYSTEM:  Ducts. Expansion Joints, and Dampers	_	3-136
     0    Location and attendant variations in operating conditions
     0    Exposure to particulates
     The layout of the ductwork and the location of the expansion joint will
determine  the  magnitude  and type  of movements  a joint must  accommodate.
Metallic joints may be used for axial and/or angular movements.   Elastomeric
joints may be used for all types of movements.
     For determining  the maximum  expansion  and contraction  movements,  the
maximum  and  minimum  ambient temperatures expected and  the  normal  sustained
operating temperature should be considered, as well as the maximum excursion
temperature  and  its  duration.   Maximum positive  pressure  and  negative
pressure (vacuum)  should  be a part of the design basis.  Elastomeric joints
should be  designed for  the normal operating temperature; their service life
is  shortened by exposure  to high temperature  during  emergency  conditions.
Suppliers  can  provide guidance  in determining life expectancy  factors  for
various conditions.
     Elastomeric expansion  joints should  be  provided with  internal  baffle
plates  to  shield  the  joint  fabric  from  impingement  by  particulates.
Metallic expansion joints  do not require baffles and should be left exposed
to facilitate cleaning and inspection.
     Metal  expansion  joints  have been  satisfactory in dry service, but even
stainless  steels  can corrode  if exposed to concentrated acid and/or high-
chloride condensate.   Operators at several installations have replaced metal
expansion  joints   with  elastomer  joints  because  the  metal  has  corroded.
     In a scrubbing system, expansion joints are generally U-shaped and con-
structed of an elastomer with fabric (fiberglass or asbestos) reinforcement.
The greatest problem  with joints downstream from the scrubber has been with
the metals used for  attachment purposes, rather than with the joint fabric.
Selection  of  the  expansion  joint  depends   on   the  service  temperature.
Suppliers suggest the following guidelines:
     0    At 250°F or below, fabric-reinforced neoprene or Viton
     0    At 300°F or below, fabric-reinforced chlorobutyl  rubber or Viton
     0    At 400°F, layered asbestos

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FGD SYSTEM:  Ducts. Expansion Joints,  and Dampers	.	3-137

     A  typical   limestone  scrubber  could be  designed with  the  following
materials of construction for expansion joints  in various  locations:
     0    Asbestos and  Viton in all  ductwork  leading to the  scrubbers  and
          bypass duct
     0    Asbestos and  chlorobutyl  rubber in ductwork between  the scrubber
          and the stack
     0    Asbestos and  neoprene in the  reheat fan outlet ductwork  in cold
          air service
     0    Elastomer/fiberglass  or  Teflon  in  reheat  ductwork in hot  air
          service
Dampers
     Dampers are used for flow control and isolation of the  flue gas stream.
They  are used  in three  locations:    the  inlet duct  to  the  scrubber,  the
outlet duct  from  the  scrubber,  and the bypass  duct.   (In  installations that
have no bypass,  all of the flue gas goes through the scrubbers.)  Table 3-15
lists  the  basic damper  types installed in  scrubbing systems.   Designs  of
isolation  dampers,  which  prevent  the  flow  of the  gas  into  the scrubbing
system, include  the slide gate (guillotine),  the single-blade butterfly,  the
multiblade parallel  (louver), and the two-stage louver with  a pressurized
seal air system to maintain  positive pressure  between stages.   The seal  air
system provides superior sealing  over other damper  designs,  and minimizes
the potential for gas leakage during periods  of scrubber maintenance.
     Control dampers  are  used to balance the flow between the scrubbers and
to  regulate  the amount  of flue gas through  the  bypass duct in systems with
bypass reheat.   Control  dampers are of the multiblade opposed (louver) type.
     Proper  materials of  construction and  adequate mechanical  design  are
necessary to ensure  reliable operation of dampers.    In addition  to materi-
als, the factors  to  be  considered in selecting dampers are  the damper loca-
tion and service function, and the pressure and temperature  of the gas flow.
Care  must  be taken  to  minimize  areas of fly ash  deposition.   Mechanical
problems with dampers caused by deposition of  solids from the flue gas have
outweighed any materials problems.

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                      TABLE 3-15.  BASIC DAMPER TYPES
    Generic
         Specific
  Common designs
Louver
Guillotine
Blanking plate

Butterfly
Opposed blade multilouver
                    Parallel  blade multilouver
Top-entry guillotine
Top-entry guillotine/seal air
Bottom-entry guillotine
Bottom-entry guillotine/seal  air

Isolation

Pneumatic operator
Single louver
Double louver
Double louver/seal air
 compartment

Single louver
Double louver
Double louver/seal air
 compartment
Steel plate
                                    3-138

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FGD SYSTEM:  Ducts. Expansion Joints,  and Dampers	3-139
     The damper frames  are  usually channel-type,  of either rolled or formed
plate.  The  material  and weight of the  frames should be determined  on  the
basis of stress resulting from seismic loading, total  size and weight of  the
damper, and  operating conditions.  The  blade deflection should be less than
1/360 of the blade span.   A detailed  stress analysis should be performed on
the blade  for a  specific  design.   Each  damper requires an  activator that
should be mounted out of the gas stream.
     The  simplest damper  is the  simple isolation  blanking  plate.    It  is
advisable  to  be  sure  that  blanking plates are available and that provisions
are made  in  the  ductwork  design for  their  insertion.  Blanking  plates  are
essential  when ducts must  be  isolated for  protection of the  maintenance
crew.    Should it become necessary for  persons to enter any  section  of  the
ductwork or  scrubber, the  blanking plate will ensure isolation.   When used
in conjunction with positive ventilation air purge, the  blanking  plates  do
protect the safety of personnel.
     The  following paragraphs  describe typical  damper  types and  service
applications  in a limestone scrubber system.
     Precipitator  outlet dampers perform dry  flue gas service.   The usual
design  is  a  top-entry,   carbon  steel, double guillotine damper with  a seal
air blower to maintain  zero flue  gas leakage across the  damper.   Included
with  the  damper   is  the  blower  motor, motor  operator,  torque  limiter,
flexible  317L stainless steel   seals,  and  limit  switches.   This  assembly
serves as an  isolation damper.
     The bypass damper  can  be located in a bypass duct  between the scrubber
inlet and  outlet  manifolds.   If the bypass duct is typically round and < 10
feet  in diameter, a  butterfly-type damper can be used.   Each butterfly is
moved  by  a  pneumatic operator.  This is a  modulating  damper that controls
flue gas flow.
     The reheat damper  is  located  in  the duct  branches  that  come from each
scrubber outlet and from the stack inlet duct.   It  is  situated between  the
reheat  fan and the stack inlet.  This is a modulating damper for control of
flue  gas  flow to  the reheater.  All  materials of construction  in  the  gas
stream  can   be   317L  stainless  steel except seals,  which  need   to   be

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FGD SYSTEM:  Ducts. Expansion Joints, and Dampers    	3-140
Inconel 625.   Again this  service dictates  a butterfly  damper in  a  round
duct.  A  pneumatic  operator that strokes the damper  in  5 seconds- should be
provided.
     The  scrubber  inlet  damper  serves  as  an  isolation  damper.   It  is  a
double-bladed guillotine type, equipped with a seal air blower to pressurize
the sealing space and thus ensure against leakage past the damper.   The dual
blades can  be  carbon  or stainless steel.  The damper seals can be Type 316L
or 317L stainless steel, HasteHoy G, or Inconel  625.   This damper should be
capable  of  being opened or  closed in  approximately 60 seconds by  use  of a
dual chain and sprocket drive.
     The  scrubber outlet damper  may be a  combination isolation and  modu-
lating control  damper:   This unit can be a combination guillotine and multi-
louver damper assembly.  The guillotine is served by a seal air blower.   All
materials of  construction in  the gas stream  are stainless  steel  (316L or
317L) to provide protection against corrosion by  wet flue gas.  Where corro-
sion  has  been a  problem,   high-grade  alloys such  as  904L,  Inconel  625,
Hastelloy  G,   and  JESSOP JS 700  have  been used to  replace  components.
Inconel 625 is  preferred for the seals.   An operator for the dual  chain and
sprocket drive  should  close  the guillotine damper in 60 seconds;  a separate
operator should close the multilouver damper in 30 seconds.
     The  expected  electrical  loads  needed  to energize  the  various  damper
motors and  seal air blowers are as  follows (500 MW, four  scrubber system
basis):
     Scrubber inlet damper motor to drive chain and sprocket     10 hp
     Scrubber inlet damper blower                                20 hp
     Scrubber outlet damper for the guillotine                   10 hp
     Scrubber outlet blower                                      20 hp
     Scrubber outlet louver damper                                1 hp
     The smaller butterfly dampers are moved by pneumatic operators.
     All   dampers should  be  inspected  initially  for proper installation,
e.g., for flow direction and free movement.   During operation of the scrub-
bing  system,  they  should be inspected for  cleanliness,  working  freedom of
positioners, and positive closure.

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F6D SYSTEM:   Ducts. Expansion Joints,  and Dampers	3-141
     On startup and  in  commissioning  of gas flow,  the scrubber inlet isola-
tion dampers  must be opened  and  the  scrubber outlet louver damper  must be
adjusted to balance  the gas  flow equally across as many scrubber modules as
are in  service.   The bypass  and reheat dampers must be balanced to give the
desired exit  gas  outlet temperature.   The amount  of hot air  supplied for
reheat  is  controlled by modulating  the  reheater  inlet butterfly  damper.
     On shutdown  the  scrubber inlet and outlet guillotine isolation dampers
must be activated to shut  off individual scrubbers and  the  damper seal air
blowers must be operating.
     The 60-second interval   for  activation of  the isolation  dampers  will
meet emergency shutoff requirements.

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FGD SYSTEM:  Tanks	__	     3-142
TANKS
     This  subsection  discusses  the  major tanks in the liquid circuit of the
limestone  scrubbing system.  The tanks are used for storage, for mixture and
reaction,  and for collection and recirculation of slurry, makeup water, wash
water, and other fluids.
Scrubber Effluent Hold Tank (EHT)
     The EHT  is  also  known as the reaction, recycle, or recirculation tank.
The EHT must provide sufficient retention time to relieve supersaturation of
the liquor and  thereby minimize scaling.  The EHT may be a separate tank or
it may be the bottom part of the scrubber vessel that serves as a reservoir.
     The critical  design  factor is  to  size  the EHT  so that  it  provides
enough holdup time to ensure optimum limestone utilization and precipitation
of gypsum.   Failure to provide  sufficient holdup time  increases the super-
saturation and  can  cause  scaling.   The  exact holdup  time required  is  a
function of  the  degree of  supersaturation  that is  allowed to  take  place
during S02  absorption and also of the tank design (Saleem 1980).
     If the  tank is an  integral  part of the scrubber,  it  must  be  designed
for the  same  pressure.   Typically  10 minutes is adequate holdup  time  to
ensure that crystallization occurs in the open EHT vessel rather than in the
piping and spray headers.   Eight-minute retention time  has been proven  as
adequate for  slurry holdup at the Shawnee test facility.  Additionally, the
Shawnee work  has  shown that plug flow tanks are  more efficient as  reaction
tanks than single back mixed tanks (Borgwardt 1975).
     In addition to holdup time, control  of the solids content of the recir-
culation slurry  is  essential  to maintain the amount  of seed crystals needed
for scale  control.  Seed crystals provide a large surface area on which the
dissolved gypsum deposits preferentially in the hold  tank rather than on the
walls or  internals of  the scrubbing vessel.   For a  fly-ash-free limestone
scrubber,  normal operation  with 8 to 15 weight percent solids in the EHT is
typical.   Where  the scrubber is part of a system that also removes  fly ash,
a  normal   concentration  of  solids  in the hold tank  would be  15  percent.
     Specifications for construction of the EHT should be in accordance with
applicable portions of recognized tank codes such as  API 650.

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FGD SYSTEM:  Tanks	  3-143
     Some  hold  tanks include  strainers  to prevent  large  chunks and  solid
particles  from  the  slurry circuit  from plugging the  spray nozzles.   For
example, strainers were  installed  after  startup in the  Sherburne County FGD
systems.   These  strainers  consist of large, semicircular perforated  plates
fitted around the  suction  side of the slurry recirculation  pumps  that send
the slurry to the  spray nozzles.   Each strainer is  equipped with an oscil-
lating  and retractable  wash lance for periodic backwashing.. Laseke  (1979)
gives a detailed account of this in-tank  strainer design.
     Effluent hold tank  designs  also have included baffles  to aid in mixing
of  the  slurry  by  the  agitators,  which  are  both   center- and  side-wall
mounted.   Intricate  baffling  configurations are  sometimes   used  to  provide
compartmentalized zones and separate  mixing areas.
     The scrubber  supplier  should  provide design information and background
data to  assure  the project manager  that the EHT is  adequate and correctly
sized.
Limestone Slurry Feed Tank
     The limestone  slurry  feed tank is  designed to  provide surge  capacity
for  the limestone  slurry  before  it is  fed  from the  storage  tank  to the
scrubber.  A typical  design would provide about 2 hours of  retention capac-
ity.  The  slurry  feed is 40 to 60 percent  solids, containing finely ground
particles  of  limestone.   The  current  trend is toward  finer grinding,  with
specifications ranging from 60 to 90  percent through  325 mesh.   Since agita-
tion equipment is costly and consumes power continuously,  some designs equip
a portion  of  the  storage with agitation  only by  compressed air, to be used
only when needed.
     As  with  the  EHT,   the  limestone feed tank  should  be constructed  in
accordance with applicable portions of recognized tank codes.
Thickener Overflow Tank
     The thickener overflow tank  acts  as a surge tank  and  stores the clear
supernatant liquid from the thickener to  be pumped back to the EHT.
     The pH of the thickener overflow should be between  6 and 8.   Because pH
excursions may  occur,   however,  specification  of   proper  materials   of

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FGD SYSTEM:  Tanks	    3-144
construction is  important.   Unless there are upsets in thickener operation,
serious erosion should not occur.
     The  thickener  overflow tank  often  provides surge  capacity to achieve
water balance  in  the system.  Therefore, the tank  should  be sized to allow
for  system swings.   Adequate NPSH for the overflow pump must  also be con-
sidered.
Mix Tank (Optional)
     The mix tank is sometimes  used to  mix  a  fixation agent with the spent
slurry (sludge).  Thickener underflow is fed to the tank, the fixation agent
is added,  and  the mixed product flows or  is pumped to a sludge pond.  Ero-
sion  is  the   major  consideration.   High-torque  agitators  are  needed  to
achieve proper  mixing  of the slurry.   The rapid movement  of the slurry and
fixation agents against  the  tank walls heightens abrasive action.  So as to
minimize  maintenance,  the  tank walls  and  bottom  should  be rubber-lined.
     The  tank  must  be  sized to allow  proper  mixing.    If  slurry is pumped
from the tank,  the NPSH requirements of the pump must be considered.
Mist Eliminator Wash Tank
     In systems  using mist  eliminator  wash tanks,  simple design  and con-
struction materials have been adequate.   The tank should be sized to provide
3- to 6-minute surge capacity.   Nominal sizes are in the 2000 to 6000 gallon
range.

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FGD SYSTEM: Agitators	3-145
AGITATORS
     Agitators are used  to  provide slurry mixing in the  EHT and the slurry
feed tank so as to keep the solids suspended.   Additionally, all other tanks
and/or  sumps  in  the  system  that  serve  as  vessels  for  slurry  should  be
equipped with agitators to keep solids in suspension.
     The agitators are standard models supplied by commercial manufacturers.
Some are  top-entry agitators  center-mounted  in the tanks; some  EHT's  are
equipped with agitators  mounted on the side wall.   Commercial manufacturers
have worked  closely  with the  scrubbing system  suppliers to  perform model
studies  on mixing  characteristics,  especially for  the   EHT; they  have
provided design  details on baffling  and  the  use of both  top and side-wall
entry agitators.
     A limestone scrubbing system in a typical 500-MW case could use as many
as 20 agitators ranging in horsepower demand from 3 to  250 Hp.
     Relatively  few  problems  with materials  or  mechanical  reliability have
been reported.   Additionally,  agitators  can be repaired or replaced rapidly
in limestone slurry service.

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FGD SYSTEM:  Materials of Construction	3-146
MATERIALS OF CONSTRUCTION
     Some  of  the foregoing  discussions  have touched upon materials of  con-
struction  that are  Integral to  the design of  specific components.    This
discussion  summarizes  the  principal  overall  design considerations  in  selec-
tion of  materials for  a limestone FGD  system.   Additional  information and
design guidance are given in Appendix G.
Basic Classification of Materials
     A  systematic grouping  of  the  various   construction materials used  in
limestone  FGD systems  is  shown in  Table 3-16.  The  materials are grouped
into  two  basic  types:  (1) base metals and  (2)  plastics,  ceramics,  and
protective  linings.  Within  each  group  the materials  are further categorized
according  to  generic,  specific,  and common  or trade name  classifications.
The generic classification  of base metals includes carbon  steels,  stainless
steels,  and  high-grade  alloys;  the  plastics,  ceramics,  and  protective
linings  are grouped  generically  as  organic and inorganic materials.   The
specific classifications designate distinct  types  of materials within  each
generic  grouping.   The  common  or  trade  name  classifications  designate
special or proprietary materials within  each  specific grouping.
     It should be emphasized that Table 3-16 lists  only  those materials  that
have been  used or  tested  in limestone  FGD  systems.   For example, the  base
metals grouping  lists only  the wrought  alloys  because they are used  to the
virtual  exclusion of  cast  alloys.  Also, the  martensitic  stainless  steels
are  excluded   because   they  do  not  see  widespread   use  in  limestone  FGD
systems.
Performance Characteristics
     Base Metals.  To  date,  four  major  test  programs  have been  performed for
evaluation  of  base  metals  with  respect  to  performance  and  corrosion-
resistance  in wet  scrubbing  systems.    These  programs  were  sponsored  by
International   Nickel  Company (Inco),  the High Technology  Materials Division
of  Cabot  corporation,   Combustion  Engineering,  and the  EPA/TVA, whose tests
were  done  at  the   Shawnee facility.    Test   results   are  summarized  in
Appendix G.

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   TABLE  3-16.   BASIC  TYPES  OF  CONSTRUCTION  MATERIALS
    Generic
              Specific
Base Mtals*
C«rbon »tt«l


Stainless steel
High alloy
PUstlct, ceranlci

   Organic
   Inorganic
 A1SI 1110
 High strength low alloy  (HSU)

 Ftrrltle
                   Austenltlc
  Iron base/nickel-chroeiluB-copper
  •olybdenun
                   Iron base/nickel-cnrMluK-ewlybdenum
                   Nickel baselchro*>1un-1ron-copper-
                    •olybdenun

                   Nickel base/chron1uB-1ron-Bolybdenumc
,  and protective linings

  Natural rubber
  Synthetic rubber
  Polyester
  M1ca flake-filled polyester
  Alunlna flake-filled
   polyester
  Glass flake-filled polyester
  Fiber-reinforced polyester
  Nat-reinforced polyester

  Vinyl ester
  Inert flake-filled vlnylester
  Glass flake-filled vlnylester
  Epoxy
  Glass flake-filled epoxy
  Nat-reinforced epoxy
  Fluoroelastoaer

  Acid-resistant brick and awrtar

  Abrasive-resistant brick and
   awrtar

  Hydraullcally bonded cement
                   ChMlcally bonded ceecnt
Cor-Ttn

Type 430
E-Brlte 26-1

Type 304
Type 304L
Type 316
Type 316L
Type 317
Type 317L

Carpenter 20
Udthole 904L
FirraUun

Haynes 20
Jessop JS 700
Allegheny AL-6X
Nltronlc 50

Incoloy 825
Hastelloy G

Hastelloy C
Hastelloy C-276
Hastelloy C-4
Inconel 625
Black natural rubber
Neoprene
Chlorobutyl
                                                               Hell  and Cell cote
                                                                linings
Plaslte


Carbollne


Colbrand CXL-2000

Preflred brick

Alualnua oxide
Silicon carbide

CalcluB aluvlnate
 CCMnt

Gelled silicates
  Wrought alloys only.
° Arranged according  to  Increasing Iron content.
d Arranged according  to  Increasing nickel  content.
  The various Manufacturer grades available for certain trade or conon
  MM HtcrlaU are  not listed because of their awnufacturer specificity
  and maerous types.

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FGD SYSTEM:  Materials of Construction	3-148
     Evaluation  of  results of  these  test programs and of  operating  experi-
ence  in  the industry  has  led  to  the widespread use of Type  316L  stainless
steel.   Under  certain stringent  operating conditions, in  which this  alloy
may undergo  localized  attack,  nickel-based alloys with higher molybdenum and
chromium  contents are  superior.   Although more expensive initially,  these
high-grade alloys may  be economically justified in severe  scrubber environ-
ments.  It should be noted that the use of high-grade alloys  demands  careful
fabrication.    The welding recommendations  of the  alloy  producer  should  be
followed precisely.
     Organic Linings.  Among  the  basic organic  linings,  the resins,  poly-
ester, bituminous,  epoxy,  vinyl   ester,  furan,  and  rubber linings are  the
most  commonly   used  in  utility  FGD  systems.    Table  3-17  summarizes  the
resistance properties  of these materials in various  environments.  Programs
of systematic  testing  with the various organic linings have  indicated that
type of coating,  thickness  of coating, application techniques,  and  degree of
surface  preparation  are  important   variables  affecting  performance.    The
quality  of the  coating  application,  especially  rubber  lining,  is critical
for  good  service  and  long  life.    The  applications  contractor must  be
selected very carefully.
     Inorganic  Linings.  Bricks,  ceramics, and concrete  are the  inorganic
linings in  common use.  The  bricks  used  most  often  in FGD systems are  red
shale,  fire  clay,  and  silicon carbide.   Red   shale  should  be  used  where
minimum permeation of  liquor  through  the brick  is required  and thermal  shock
is not a  factor.  Fire clay should be used where minimum  absorption is  not
required and thermal shock  is a factor.   Silicon carbide  brick is used  where
high resistance to abrasion is required.
     Selection  of ceramics  is  governed by their physical  properties,  which
result in  objects of relatively thick cross-section, heavy weight, and lack
of resistance  to impact.   Since ceramic shapes  are usually  made  by  extrusion
or casting,  the  shapes that  are symmetrical (nozzles, cylindrical scrubbers,
ductwork, piping) are  most  suitable.   Where ceramics are used,  it  is nearly
impossible to  make  changes, adaptations,  or  repairs  in the field  that will
provide resistance equivalent to that of the as-fired material.

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           TABLE 3-17,   ORGANIC LININGS:   BASE MATERIALS AND RESISTANCE TO SOME ENVIRONMENTS
Name Base material
Bituminous Coal tar
Chlorinated Chlorine, natural rubber
rubber rubber
Natural rubber
Fluorocarbons Fluorine, ethyl ene
Epoxy Epichlorohydrin, Bisphenol-A
Furan Furfural alcohol
Coal tar Coal tar, epoxy
epoxy »
Phenolic Phenol, formaldehyde
Polyesters Phthalic acid, Bisphenol-A
Polyurethanes Compounds containing
isocyanate and hydroxyl
groups

Vinyls
Resistance to environment
Abrasion
F
F

E
E
G
F
F

F
G
E


F
Heat
P
P

F
E
G
E
F

F
G
G


P
Acid
F
G

G
E
G
F
G

G
E
G


E
Alkali
F
G

G
E
E
G
G

G
F
G


G
Solvent
P
P

P
E
G
P
P

P
G
G


P
Water
F
E

E
E
G
F
G

G
G
G


E
Weather
F
F

E
E
G
F
G

F
G
E

i
E
E - Excellent - May be used under all conditions.
G - Good - May be used under all conditions.
F - Fair - May be used under certain conditions.
P - Poor - Should not be used under any conditions.

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FGD SYSTEM:  Materials of Construction	3-150
     Unlike the  ceramics,  concrete  need  not be  fired as a formed  shape  to
achieve strength and  resistance  but achieves these values by  the-process  of
hydration.  To meet  strength requirements,  concrete  is usually  reinforced
with  steel; more recently,  fibrous  materials have been used  for  reinforce-
ment.  In  a limestone FGD  system, concrete  is  used  in presaturators,  tanks,
and piping.
Basis for Selection of Materials
     The following discussion  briefly reviews the major factors involved  in
selection  of  construction  materials  for  service  in a  limestone FGD system.
The  chief  considerations are  operating  conditions,  location within the  FGD
system, material  characteristics,  safety factors,  and economic  factors.
     Operating Conditions.   The physical  and chemical  parameters of tempera-
ture, pressure,  pH,  flow rates,  and  the  presence of trace elements are  the
important process-related considerations.
     Operating temperature governs  the  selection  of  materials  because  corro-
sion  increases with  increasing  system  temperature.    Nonmetallic  materials
such as plastics are  temperature-limited.   Maintaining system  temperature  as
close as possible to ambient is therefore always desirable.
     System pressure  determines  the requirements for vessel  materials  and
reinforcements.
     The pH  level  of  the  recirculation  slurry  should be known accurately,
because corrosive properties of the fluid depend primarily on its pH.
     The  abrasive  effects   of flue  gas  containing  fly ash  particles  are
proportional  to  velocity of  the  gas.  Where high gas flow rates  are  to  be
maintained, abrasion-resistant lining is  needed,  especially at the  inlet  to
the  scrubber.   High  liquid  flow rates can wear  away  metallic oxide coatings,
exposing bare metal  to  the  corrosive slurry.   Conversely,  low liquid flow
rates  can allow  solids to  settle,  with  consequent  potential for  fouling.
Process design of  ductwork,  piping,  and nozzles should therefore accommodate
optimum fluid velocities.
     A  complete  elemental   analysis  of  the coal,  limestone, and  fly  ash
(especially where  fly  ash   is  to be collected  in the  FGD  system) is  very
important with respect  to  corrosion because of  the presence  of chlorides and

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FGD SYSTEM:  Materials of Construction	3-151
fluorides, which can  cause  pitting of metal surfaces.  The  source  of  makeup
water  to  the  FGD  system  should  also  be  analyzed for  corrosive agents,
especially where cooling  tower  blowdown, well  water,  or brackish  river water
are considered as makeup water sources.
     Location in the System.   The   operating   parameters   just  described,
especially temperature,  pH, and  flow rate, will  vary  with location  in  the
FGD  system and  therefore should be  identified as a function of  location.
Also,  a  range   must  be  specified for  each  variable,  indicating  maximum,
minimum,  and  average  values.    Figure 3-35 is  a  schematic diagram showing
trouble  spots  common to  various  limestone  FGD systems.  Where the  flue  gas
enters and is quenched  by spray nozzles,  localized attack and  erosion  by  fly
ash  are  likely,  particularly when  fly ash  is collected  in the  FGD system.
Erosion-corrosion can  present a  problem  in recirculation pumps.   Corrosion
of  fittings   and crevice  corrosion  may  predominate  inside  the  absorber,
whereas  stress-corrosion  cracking  occurs  in  reheater  tubes.   Corrosion  by
moisture  takes  place downstream  of the  mist  eliminator  and  in  stacks,  and
fatigue cracking has caused the failure of exhaust fans.
     Material  Characteristics.   Temperature limitations  and  resistance  to
corrosion  and   erosion   are   the  primary  process  design considerations.
Fabricability and  material  strength are  the  primary  mechanical considera-
tions.   FGD  system design  may  involve a  variety of shapes,  with  points  of
high  stress.    Selection  of  materials   is  especially  critical  when  two
different metals are  to  be welded.   Quality control of  materials  can prevent
difficulties caused  by  scales,  cracks,  scratches, or rough  surfaces,  all  of
which enhance corrosion.
     Safety Factors.   Fire-retardance,  always   an  important  feature,   is
especially  so  in   systems  using  plastics  and/or  organic  coatings.    Fire
retardance adds to the cost and must be expressly specified.
     Economic Factors.  The chief  economic  factors in selection of  materials
include  the  total   installed  cost (capital  cost),  the  cost of replacements
and  maintenance  (operating  cost), and  the projected  life of  the  system.
Accurate  estimates  of  these  three  variables  are  needed  for  an  economic
evaluation.   The recommended  scheme is  the  "Present  Worth"   method,  which

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     FLUE GAS
TEMPERATURE 250 - 350'F
FATIGUE
DEW POINT
CORROSION
                                            DEW POINT
                                             CORROSION
EROSION LOCALIZED ATTACK 1
PRESCRUBBI
i-ii j i
IMINATOR
\
^ SPRAY
NOZZLES
:R*
SUMP

c
^r
c
\
^^^S6l^l^$6iS5^S^
SCRUBBER
1
* EFFLUENT HOLD TANK
REHEATER (CAh
IN DUCTW
STRESS-CORRO!
CRACKING
LOCALIZED
ATTACK
EROSION-
CORROSION
-£7-^
1 APPEAR
ORK)
5ION
RECIRCULATION
LINE
   QUENCHER, PRESATURATOR,  OR SCRUBBER
                                                 RECIRCULATION
                                                      PUMP
              Figure 3-35.  Erosion/corrosion as a function
               of location within a limestone FGD system.
                                   3-152

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FGD SYSTEM:  Materials of Construction	3-153
essentially combines  the three  variables  into one present worth value.   It
is difficult,  however,  to  determine service life and maintenance costs with-
out many  years of  operating  experience with each material under considera-
tion.   Most of the  experience accrued to date  in limestone FGD applications
is summarized in Appendix G.
     From  the  spectrum of available  materials, only those that  satisfy  the
physical criteria should be  considered for economic evaluation.  The  physi-
cal and economic  factors  are  complementary:   the physical  criteria  eliminate
materials  from the  lower  range  of  the  spectrum  because of  unacceptable
performance, whereas  the economic  criteria eliminate  those  from the  upper
range because of prohibitive cost.

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FGD SYSTEM:  Process Control and Instrumentation	3-154
PROCESS CONTROL AND INSTRUMENTATION
     The primary  functions of  a  limestone FGD process  control  system are:
     0    To hold S02 emissions within the required limits
     0    To achieve continuous and reliable operation
     0    To  achieve optimum  limestone  utilization and  energy  consumption
     0    To ensure boiler safety
Table 3-18  lists  the major variables that are  measured  in limestone scrub-
bing systems  so as  to fulfill these functions.  The  methods  of controlling
these measured variables are discussed later in this section.
     Control of process  chemistry  is a major factor in successful  operation
of a full-scale FGD system.   Following is a  brief  review of some  essential
findings that  have  emerged from  the development  of  process control  tech-
nology.
     0    Virtually all  operating  full-scale systems  regulate  reagent feed
          rate by controlling  slurry pH.   A pH electrode  probe  activates  a
          signal   that  regulates the position of control  valves to  control
          the  rate  of  limestone feed.   This  procedure,  however, has proved
          unreliable at  times  and  fluctuations  in  boiler  load may cause pH
          instability.
     0    The  pH  probe should  monitor  the scrubber EHT  because changes in
          boiler  load  are  first  noted  there.   To  avoid  cracking  of  the
          probe,   a  small  dip  tank  in a  screened location  in  the EHT is
          preferred.
     0    Sufficient operating  experience  has been gained  to identify most
          of the  limestone  feed control  problems.   These problems  have been
          resolved mostly through design modifications or new operating and
          maintenance  procedures.    For   pH   control,  dip-type  electrode
          probes,  which  are inserted into  a slurry tank,  are preferable to
          in-line  probes because they  are easier  to clean and calibrate.
          In-line, flow-through probes, located in  a  section of piping, are
          generally subject to  more  wear  and abrasion and generally  require
          more frequent  maintenance.   Flow-through sensors must be  removed

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       TABLE 3-18.  MAJOR VARIABLES IN A LIMESTONE FGD CONTROL SYSTEM
   Stream/equipment description
       Measured variables
Flue gas at boiler outlet
Flue gas at scrubber inlet
Flue gas at scrubber outlet

Limestone feed to ball mill
Fresh water to ball mill
Limestone slurry to effluent hold tank

Fresh slurry hold tank
Makeup water

Spent scrubbing slurry
Fresh slurry to scrubber
Bleed slurry to thickener
Scrubber EHT

Thickener overflow
Thickener underflow
Sludge to disposal
Flow rate, temperature, pressure
S02, 02

Flow rate, limestone composition
Flow rate, dissolved solids
Flow rate, slurry solids, solids
 size, particle size distribution
Slurry level
Flow rate and water level

pH, slurry solids
pH, slurry solids, flow rate
Slurry solids, flow rate
Slurry level

Flow rate, suspended solids
Flow rate, slurry solids
Sludge solids, flow rate
                                     3-155

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FGD SYSTEM:  Process Control and Instrumentation	      3-156
          and kept wet if the sample line is drained when the module is shut
          down.  The dip sensor stays wet despite shutdown.

     0    Other limestone feed  control  systems have been  used  or are being
          evaluated on  full-scale systems.   One type  involves  feed control
          based on the  inlet flue gas flow rate and S02 concentration, with
          trim  provided by  slurry pH.   Another  type involves  control  of
          limestone feed rate based  on the outlet S02  concentration as the
          control  variable.   Only partial success has been reported for both
          systems  because of the difficulty in obtaining  accurate and con-
          sistent   readings  from  S02  gas  analyzers, particularly  in high-
          sulfur coal  applications.
     An  EPRI  publication   (Jones,  Slack,  and  Campbell   1978)   covers  the
subject of process control  in great detail  and is recommended as a guide in
design of control  systems for limestone scrubbers.
Limestone FGD System Control Loops
     This  discussion  concerns  five  of  the principal  control  loops of  a
typical  limestone scrubber  unit:  limestone  feed  control,  slurry  solids
control, gas flow  control,  bypass reheat control, and  makeup  water control
at the thickener overflow tank.
     Limestone Feed Control  Loop.  Limestone slurry  feed  rate  is one of the
factors that determines  the extent of limestone dissolution in  the EHT and
thus pH  of the slurry in the tank, which in turn affects  the  S02 removal.
The pH  level can  be  used in a  simple  feedback control loop to regulate the
flow of  limestone feed to  the  hold  tank.   The advantage  of this system is
its simplicity.  The  disadvantages are the nonlinearity and sluggishness of
the response and other limitations of pH as the controlled variable.  Proper
pH  control  through  limestone  addition  can prevent scaling  and plugging,
optimize reagent utilization and S02 removal, and thus improve the reliabil-
ity of the scrubbing system.
     All operational  systems control  the limestone feed rate with a feedback
loop  using pH  as the  controlled variable.  Many  of these systems report
problems with arrangements  that rely solely on pH to control limestone feed.

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FGD SYSTEM:  Process Control  and Instrumentation	3-157
An  improved  system incorporates feed-forward control of the  limestone feed
rate by measurement  of  the S02 concentration at the boiler outlet.   Instru-
ments measuring the  S02  content of a dry gas have operated more reliably in
the field than have S02  analyzers on a wet gas stream.   From measurements of
S02 concentration and of the gas flow rate, the mass flow rate of S02 can be
computed.   The limestone feed rate is then set in proportion to the  quantity
of S02 entering the scrubber.
     The primary advantage of this system is that it responds immediately to
changes in S02  concentration and gas flow  rate  and assists the pH  feedback
system in  control  of limestone feed.  Feed forward of the inlet S02 concen-
tration does operate well when properly limited (trimmed) by an indicator of
slurry pH  level.   A schematic of this arrangement  is  shown in Figure 3-36.
     Slurry Solids Control Loop.  The solids content of recirculation slurry
must  be  controlled  to   provide  adequate  crystallization  surface  area  for
precipitation and  to minimize erosion and solids buildup.  A density feed-
back  control  system  for  recirculation  of  slurry  solids  is  shown  in
Figure 3-37.
     In the  primary  loop,  the  density  sensor  activates  the  density con-
troller, which  changes  the flow rate of  the  bleed  stream  to the thickener.
In the independent secondary loop,  the level sensor adds more makeup water/
thickener overflow if the level falls below  a  set  point.   The advantage of
the system is  its  simplicity; the disadvantage  is  a higher rate of wear of
the control valves.
     Gas Flow Control.   Gas  flow rate  to  the scrubber  is   determined  by
boiler  operation,  and  the  scrubbing  system  must  respond  to boiler load
changes.   If multiple scrubbing modules are  used,  a dependable system must
be provided  to  balance  the flow rates to the parallel  modules.  Improperly
balanced gas flow  results in high gas velocities  in one or more scrubbers,
which  will   lead  to  a  reduction  of S02  removal,  an  increase  in  moisture
carryover  to the  mist  eliminator and downstream equipment,  an increase in
particulate  loading,  and possible  erosion damage  to  downstream equipment.
     The most difficult  problems of gas flow control arise in protection of
the boiler/scrubber  system from explosion or implosion damage on tripout of

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PROCESS
CONTROL
FUNCTION
                                                      FROM LIMESTONE
                                                     FEED SLURRY TANK
MAJOR
PROCESS
EQUIPMENT
                                                            SPRAY PUMP
                                     *-TO THICKENER
             Figure  3-36.   Limestone  feed control  loop.
                                    3-158

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PROCESS
CONTROL
FUNCTION
FINAL
CONTROL
ACTION
                                                            THICKENER
                                                            OVERFLOW
                                                              TANK
MAJOR
PROCESS
EQUIPMENT
                                                             SPRAY PUMP
                                         »• TO THICKENER
               Figure  3-37.   Slurry solids control  loop.
                                     3-159

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FGD SYSTEM:  Process Control and Instrumentation	3-160
the boiler or on  loss  of  a  scrubber  or fan.   When the  boiler  shuts down,
there  is  a sudden  increase or  decrease  in gas flow rate,  depending  on the
safety requirements of  the  boiler.   Although interlocks are used to achieve
simultaneous shutdown of  the  boiler and the scrubber,  a  pressure or vacuum
surge can develop in the boiler and ductwork if the dynamic response to this
condition  is unfavorable.   Similarly,  loss of a fan will  produce a pressure
surge  in   the-opposite  dtrl|t4wWiit "will ^rfff^the  boiler but  may also
create potentially damaging surges.
     If the scrubber  system is  designed to scrub  only  part of the flue gas
and the  remainder  is  routed  through a bypass  damper,  connection  of the
bypass damper  and  the  fans  to  the  boiler  flame safeguard  system  is  an
acceptable solution.  Then, in  the event of an  emergency  boiler shutdown,
the fan  can be  shut down  and  the damper opened.  If the fan  fails,  the
damper can be opened and an operator can then conduct an orderly shutdown of
the boiler.
     Makeup Water Control.  Control  of the overall system  water balance is
critical  and  varies significantly  from  site  to site.   The objective is to
maintain  a stable water  inventory  in the  system and not  to  discharge any
excess water.   This is usually accomplished with recycled  thickener overflow
rather than new water.  The scrubber water system should be operating in the
closed-loop mode  so that all the water  discharged from the  system is con-
tained either  in the dewatered sludge  or  as  water vapor  in  the flue gas.
     Figure 3-38  depicts  a  system that uses measurements  of thickener over-
flow tank  level  to control  system makeup  water at the  thickener overflow
tank.   The thickener overflow tank then provides water for makeup at the EHT
to maintain proper slurry solids content and also for use  as mist eliminator
wash water.   By  eliminating the  direct  use of fresh makeup water for mist
eliminator wash,  this control  approach  maintains the  water  balance  during
periods of washing the mist eliminators.
     Bypass/Reheat Control Loop.  Figure 3-39  shows  a combination bypass/
reheat system  in which a portion  of  the  hot  flue gas from  the boiler by-
passes the wet  scrubber  to  be  used  for reheat  and  additional  reheat  is
supplied  by a supplemental indirect hot air reheat system.

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PROCESS
CONTROL
FIMCT10N
                                                                           THICKENER
                                                                           UNDERFLOW
                                                                            PERCENT
                                                                            SOLIDS
 THICKENER
 OVERFLOW
TANK LEVEL
  CONTROL
                                                                                    THICKENER
                                                                                     OVERFLOW
                                                                                   TANK CONTROL
                                                                                      VALVES
FINAL
CONTROL
ACTION
         THICKENER
         UNDERFLOW
          VALVES
                                                                          THICKENER
                                                                          OVERFLOW
                                                                          TANK LEVEL!
PROCESS
MEASUREMENTS
                                        THICKENER
                                      (TTP OF TWO)
                                                                            TO
                                                                        OEWATERING
                                                                        EQUIPMENT
                                            (THREE PER THICK.)
MAJOR
PROCESS
EQUIPMENT
                          (ONE PER BOILER)
           TO SCRUBBERS •*-
                           TO MIST
                           UIMINATOR •
                           WASHERS
                                                TOF
                                                PUMP
                                                (TYP OF TWO)
                                        DEMISTER PUMPS
                                        (TTP OF TWO)
                  Figure  3-38.    Makeup  water control,
                                          3-161

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PROCESS
CONTROL
FUNCTION
MAJOR
PROCESS
EQUIPMENT
                                  SCRUBBER
                GAS IN-
 FROM LIMESTONE
FEED SLURRY TANK
                                                            FROM THICKENER
                                                            OVERFLOW TANK
                                                           £-0
                                        EFFLUENT HOLD TANK
                                                              SPRAY PUMP
                                      TO THICKENER
             Figure  3-39.   Bypass/reheat  control loop.
                                    3-162

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FGD SYSTEM:  Process Control and Instrumentation	3-163
     Bypass  reheat  offers  the  advantages of  low  capital  investment  and
economical operation, but the maximum use of bypass reheat is limited by the
constraints of S02  emission standards.   Therefore, as in this  case,  supple-
mental reheat must be added.
     The  S02  concentration  and temperature  of  flue gas  at the  stack  are
measured  to  balance both the  bypass damper  and  the reheat fan  damper and
control both S02 and temperature at the system outlet.
Instrumentation
     In early  limestone  FGD systems, sensors caused most of the major prob-
lems with control   instrumentation  (Jones,  Slack, and  Campbell  1978).   The
performance of measuring  devices  has been improving  in  newer  limestone FGD
systems and  has contributed to an  increase  in system  availability.   This
discussion presents basic information on common instrument applications in a
limestone FGD system and suggests suitable hardware.   The major applications
include measurements of  pH  level,  slurry solids content, liquid/slurry flow
rate,  liquid/slurry level,  and  S02  in the  flue  gas.   Information  is also
given  concerning  S02  analyzers  for  continuous monitoring  to  meet  the
requirements of NSPS.
     Measurement of pH Level.    Recycle  slurry pH  is  used in  all  limestone
scrubbers to control  the limestone feed rate.  On-line  determination  of pH
in the  slurry  of a limestone scrubber is difficult.   Dependable pH readings
require the  use  of multiple, nonlinear controllers.  The  following  factors
should be considered in the design and specification of a pH electrode probe
and controller:
     0    Probe location and maintenance
     0    Probe type (dip-type or flow-through)
     0    Nonlinearity
     Electrodes  are fragile  devices,  easily damaged  by  the  action  of an
abrasive  slurry.   Scrubber  slurry  can form a deposit on an electrode, which
may act as an electrical insulator and cause false readings.
     Dip  sensors are  easier to maintain and  calibrate  when  installed in an
open overflow  pot  rather than  directly  in  the EHT.   Flow-through  sensors

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FGD SYSTEM:  Process Control and Instrumentation	3-164
are prone  to  higher erosion than are dip-type  sensors;  they must either be
installed  in  a  slipstream  from  the recirculation  pump  or supplied  with
slurry by  a separate pump.   Additionally, flow-through sensors must be kept
wet when  the  FGD  system  is shut  down.   Pressure must  be high  enough  to
produce  a  flow  rate within the range recommended by the electrode supplier.
The piping should  be as short as possible and should be drawn completely by
gravity when the FGD system is shut down.   Flow-through electrodes should be
installed  with valves  to  permit frequent maintenance.   Dip-type sensors are
more widely used in operational limestone FGD systems.
     The pH electrodes  are  available with ultrasonic self-cleaning devices.
Ung et  al. (1979)  report  that the  maintenance required  for  self-cleaning
electrodes is  significantly reduced by removing the screen surrounding the
electrode  or by  increasing  the mesh size of  the  screen.   The self-cleaning
electrodes  generally  require  less maintenance  than  standard  (non-self-
cleanirig)  electrodes  in continuous  service  of more than  2 weeks duration.
     Measurement of Solids Content.   A scrubber  installation should include
instrumentation  for  continuous control  or  recording  of  solids  content,
especially for  the  fresh  limestone slurry,  recycle slurry, and thickener
underflow.   Densinometers are  used  to  monitor and control the percentage of
solids in  slurry streams and the EHT.
     Slurry density  can  be  measured  directly  with  special  differential
pressure instruments, but a minimum liquid depth of 6  ft is needed to mea-
sure  a   span  of 0.1  specific  gravity  unit.   Ultrasonic  devices  directly
measure  the  percentage  of  suspended  solids.   Vibrating  reed  instruments
measure  the dampening effect  of  the slurry on vibrations  from  an electri-
cally driven coil.
     Nuclear density meters, which measure the degree of absorption of gamma
rays from  a radioactive  source, are preferred for this  service.   They have
the minor  disadvantage  of  producing a signal that is not linear with solids
content  unless  the  unit  contains  an electronic  linearizer.  The nuclear
density  meter  can  be  precalibrated by theoretical  calculations  if  an
accurate chemical  analysis  of  the  slurry being metered  is available.   The
main advantage is  ease  of  application.   The meter can be strapped to a pipe
without  insertion into the pipe line.

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FGD SYSTEM:  Process Control  and Instrumentation	      3-165

     Measurement of Liquid/Slurry Flow.    Measurement of  liquid  or  slurry
flow  rates is vital  to  the optimization  of system performance.   The flow
rate  of  fresh  limestone  slurry for pH control  is  an important application,
as are the flow  rates of recycle slurry, slurry bleed to the thickener, and
thickener underflow.
     Mechanical  flowmeters  are not  suitable  for the  abrasive  slurry  of
limestone  scrubbers.  The  most acceptable meters for this service are elec-
tronic devices.   Electronic measurement of flow  rate  can  be  accomplished
with  vortex-shedding  instruments,  ultrasonic transmission devices,  Doppler-
effect ultrasonic meters, and electromagnetic flowmeters.
     The  Doppler-effect  ultrasonic meter is a  fairly  new  development for
slurry applications.   The principal  advantage  of this  device  is  that the
electrodes are attached  to the outside of the pipe through which the slurry
is  flowing;  as  is the  case with  the  nuclear density  meter,  there  is  no
penetration of the pipe.
     The electromagnetic flowmeter, or  "magnetic meter," is the best proven
instrument available  for the measurement of pressurized  slurries.   It con-
sists of  a stainless  steel pipe section lined with an electrically insulat-
ing material.  The magnetic meter does not require installation in  straight
piping.   It  has  no operating parts in contact with the fluid, produces very
little pressure  drop, and  is  fairly  accurate.   A magnetic  meter  should be
recalibrated  periodically.   The only  disadvantage of the magnetic  meter is
the high cost.
     Good  quality  magnetic  flowmeters  give  trouble-free   performance;  in
selection  of a vendor,  the  availability of personnel to inspect  and cali-
brate the  meter  at startup should be considered.  Magnetic  flowmeters come
in sizes from 1/10 in. to 6 ft in diameter.
     Less  expensive  devices  are  also  made that  use  the  electromagnetic
principle.   The  only advantage  of these  instruments  is their  lower cost.
Their disadvantage is uncertainty of the accuracy of calibration.
     Measurement of Liquid/Slurry Level.    Level  controls  in  a  limestone
scrubber serve two  functions:   to feed makeup water  into the system and to
control   the  levels in the various hold  tanks.   The system should be sup-
ported  by  high-   and low-level  signals for  notification   of  malfunction.

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FGD SYSTEM:  Process Control and Instrumentation	3-166
     Controllers that operate with  mechanical  flow sensors are not suitable
for  limestone  scrubber applications  because  the slurry  contains-chemicals
and  abrasive   substances.   Displacement   controllers   and  flange-mounted
differential controllers are best suited for scrubber applications.   Capaci-
tance level controllers,  pneumatic  bubble  tubes, and ultrasonic devices are
also effective.
     Measurement of S02 Concentration.  Jahnke  and  Aldina  (1979)  have dis-
cussed continuous monitors  in  detail; parts of their discussion are briefly
summarized  here.   There  are two  basic types  of continuous  S02  monitors:
extractive  and in-situ.   The extractive  monitor withdraws a  sample  of the
gas from a  stack  or duct and performs analysis in an analyzer usually situ-
ated in a  housing  near the sampling site.   The in-situ  monitor, as the name
suggests,  performs the analysis within the stack or duct.
Extractive monitors—
     The  total extractive  system  must remove  a representative gas  sample
from the stack or duct, maintain integrity of the sample during transport to
the analyzer,  condition the sample so that it is compatible with the analyt-
ical method,  and  provide  a means  for reliable  calibration at the  sampling
interface.
     The  most  consistent  problem  with  operation  of  the  extractive  S02
analyzer has been the  difficulty of withdrawing  a  sample  of  gas,  precondi-
tioning it, and  feeding  it  to the  analyzer  cell.   Sampling  systems  can
become plugged frequently and  corroded  very quickly.  The function  of the
preconditioning system  is to remove  solid particulates and condense  water.
In practice, however,  as  water is collected it  continues  to  absorb S02 and
oxygen and  can create  a  strong sulfuric  acid solution.    Solids  preferen-
tially collect  on other precipitated  solids and form scale.   Careful  design
of the system is therefore required.
     Design of an extractive monitoring system involves  determination of the
gas  stream parameters,  selection   of the  sampling site,  selection   of  an
analyzer,  and  design  of the sampling interface.  Gas temperature  and velo-
city profiles,  the  particulate  loading  and the water vapor content  of the

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FGD SYSTEM:  Process Control  and Instrumentation	3-167
gas, and the absolute pressure should be determined at all  possible sampling
sites.  Selection  of a  sampling  site should be based on  accessibility  and
whether the samples  obtained will  be  representative.   If  the  site  is  at
least 8 or more  duct diameters downstream from a point of  air in-leakage or
mixing of  different gas  streams,  the gases  are generally  mixed  enough to
give a representative sample.  The analyzer selected must be compatible with
the gas parameters, sampling  site,  intended housing  or location,  and  the
sampling interface, which is designed to precondition the gas sample.
     A  typical  sampling  interface  includes a  coarse in-stack  filter,  gas
transport  tubing,  sampling pump,  fine filter, and acid mist removal system.
The coarse  filter,  made  of sintered or fine mesh screen stainless steel, is
located at  the probe tip in the stack  to  prevent plugging of the probe and
the sample lines.   A sample  line  of %-in.-OD Teflon tubing may  be used to
transport the gas to the analyzer over a short run.   Caution should be taken
with longer runs of 50 feet or more.   This and all exposed  components of the
interface  must  be  heat-traced  thoroughly  to prevent  condensation of mois-
ture.    It   is  advisable   to  place a  fine  filter upstream  of this  pump  to
reduce  maintenance.  A  moisture  trap  to  remove  water  droplets  should  be
placed  in  front of the  analyzer cell  and should  be kept  at a controlled
temperature to  ensure consistent  water vapor  content  in  the sample cell.
Some analytical methods require a drying (conditioning) system to reduce the
sample dew  point  to 0°C.   This can consist of refrigerated traps or permea-
tion type dryers.
     The use of absorption  reference filters and the introduction of refer-
ence gas downstream from the sample lines  can  reduce the  cost of automatic
calibration  valving and  the time required  to perform  daily  calibrations;
however, periodic checks involving the introduction of reference  gas through
the entire sampling interface must be performed to ensure accuracy.
     The extractive instruments  in use  today  are based   on a  variety  of
principles of detection and analysis.  They differ in sensitivity, suscepti-
bility to  specific interferences,  complexity, ease of operation,  and other
operational factors, as  well as in initial and operating costs.   Some offer
the flexibility of application to stack gas monitoring,  analysis of process
streams, and  analysis  of in-plant  or ambient  air.   Selection  from among

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FGD SYSTEM:  Process Control and Instrumentation	3-168
these Instrument systems must be based on Information from vendors and users
and on the Intended application.  Table 3-19 lists some of the manufacturers
of these instruments and the principles of operation.
     As a means of verifying the reliability of continuous monitors, the EPA
has  awarded  a contract  to GCA Corporation to  finalize  a monitoring system
design  for use  in  a 1-year  demonstration  program  to  be conducted  at the
Conesvilie Power Station operated by the Columbus and Southern Ohio Electric
Company.  This demonstration  program will  result in a set of guidelines for
specifications,  certification  requirements,  and recommended  operation and
maintenance practices  for the  use  of extractive continuous  S02  analyzers.
In-situ monitors—
     "In-situ" monitors directly measure the pollutant concentrations in the
stack  or  duct.  These  systems do  not modify  the  flue gas  composition  by
dilution and  are designed  to detect gas concentrations  in the presence  of
moisture and participate  matter.   Because  participates cause a reduction in
light transmission, ceramic filters are used to exclude particles.   Several
types of in-situ monitors are available, all based  on  absorption spectros-
copy.
     In comparison  with an extractive monitor, the  in-situ  monitor offers
greater flexibility  in  site selection, reduced  calibration time,  and fewer
components  for maintenance.   Also, a single  in-situ  monitor can measure
several gases  and opacity.   The  disadvantages  of  in-situ  monitors include
problems with  optical  systems and  failure  of complicated electronic  com-
ponents.  An  in-situ monitor can sample only  one stack or duct  at a time.
Where  a number  of stacks  must  be monitored,  the  use of multiple probes
leading into a single  extractive  system might  be a  cost-effective  choice.
     S02 Monitoring for NSPS Compliance.   The foregoing discussion has dealt
with measurement of  S02  as a process  control  factor,  i.e.,  as an indicator
for  use  in process  operation.   The utility  operator should  also  consider
available  instrumentation for  S02   monitoring  within  the  context  of  NSPS
requirements.   Under the  NSPS promulgated  in 1979,  the owner or operator of
a new  electric utility  steam  generating unit must install, calibrate, main-
tain, and operate  a  continuous monitoring system for S02 concentrations and

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              TABLE 3-19.  SOME MANUFACTURERS OF EXTRACTIVE S02
               MONITORS BASED ON VARIOUS OPERATING PRINCIPLES
   Operating principle
          Manufacturer
Nondispersive ultraviolet




Ultraviolet fluorescence


Flame photometry




Conduct!metry

Coulometry


Polarography




Spectrometry
DuPont
CEA Instruments
Esterline-Angus
Teledyne

Celesco Industries, Inc.
Thermo Electron Corp.

Tracer
Melroy Laboratories
Bendix Corp.
Process Analyzers

Calibrated Instruments, Inc.

Barton ITT
Beckman Instruments, Inc.

Dynasciences
Beckman Instruments, Inc.
Theta Sensors
Teledyne

Environmental Data Corp.
Wilks Scientific Corp.
Environmental Research and Technology
Barringer Research, Ltd.
Lear-Siegler, Inc.
                                    3-169

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FGD SYSTEM:  Process Control and Instrumentation	3-170
record the output of the system.  The EPA has presented performance specifi-
cations and  test  procedures for S02 and NO   continuous  monitoring systems.
                                           A
Any  of the  instrument  systems  used for  emissions measurements  must meet
these performance specifications  at the time of installation and throughout
their  operation.   A summary  of the current  specifications  is  presented in
Table 3-20.
Diluent concentrations—
     S02  monitor  data alone  do not give  accurate information  on emission
rates.  If only a volumetric measurement is desired,  such as for control of
limestone feed rate, the monitoring of only S02 concentrations may be suffi-
cient.  If emission rate information is required,  however,  diluent concen-
trations must be measured.   This can be done by measuring the concentrations
of oxygen (02) or carbon dioxide (C02) at the point of S02 analysis.
     If  diluent  monitoring  is  required,  the  extractive  sampling  system
interface used for  S02 sampling can be used  by  taking a side stream of the
conditioned  or preconditioned  sample.   Just as with the  S02 monitors, care
must  be  exercised  in  evaluating the  compatibility of  sample  conditioning
with the analysis  method.  If in-situ S02 monitoring methods are being used,
a separate in-situ diluent monitor will  be needed.
     In evaluation  of  the  monitoring systems, it  should  be  recognized that
multiple use of the extractive monitor interface tends to increase its cost
effectiveness over that of in-situ monitoring.
Recording systems--
     The strip-chart recorder is used most frequently in continuous monitor-
ing applications because of the ease of reading the  recorded data and com-
pactness  of  the  display.   The strip-chart  recorder  provides  a continuous
analog record.   Low-cost digital recording devices  are available for record-
ing and processing of emission data.
Control Philosophy
     In a control loop the objective is to maintain one variable at a fixed
value.  In an  integrated  control  system the  objective  is  to  maintain the
overall output  from  the plant  within limits.  An integrated  FGD control
system consists of  many  control  loops that interact.   In the design of each

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           TABLE 3-20.  CURRENT EPA PERFORMANCE SPECIFICATIONS FOR
                 CONTINUOUS MONITORING SYSTEMS AND EQUIPMENT
          Parameter
        Specification
Accuracy6
Calibration3
Zero drift (2-hour)

Zero drift (24-hour)a

Calibration drift (2-hour)a

Calibration drift (24-hour)'

Response time

Operational period
>_ 20 percent of the mean value of
"the reference method test data

> 5 percent of each (50 percent,
 90 percent) calibration gas mix-
 ture value

2 percent of span

2 percent of span

2 percent of span

2.5 percent of span

15 minutes maximum

168 hours minimum
  Expressed as sum of absolute mean value plus 95 percent confidence inter-
  val of a series of tests.
                                    3-171

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FGD SYSTEM:  Process Control and Instrumentation	3-172
control subsystem, the interactions must be considered as part of an overall
control philosophy.
     The  earliest control  concept  was  to maintain  a slurry pH value that
controlled  the  limestone   feed  rate  and  thus  the  stoichiometric  ratio.
Control  by feedback  of pH  value  alone proved  inadequate because of high
limestone stoichiometric rates and the following limitations:
     0    The response of pH to a change in limestone feed rate is extremely
          nonlinear.
     0    There is  an  inherent time lag caused by obtaining  equilibrium in
          the scrubber EHT.
     0    The pH  value  is  not suitable as a variable on which to base lime-
          stone feed rate when limestone utilization  is  <90 percent.
     Recent commercial  experience  by Peabody Process Systems,  however,  has
shown that  simple feedback  control  of pH has proven  to  be satisfactory (see
process control writeup of Colorado Ute).  The reason the Peabody process is
able  to  overcome  these  pH  feedback  limitations  is  the  unique use  of  the
hydrocyclone in  a  single loop system.   The hydrocyclone  permits  very high
(>90%) limestone utilization to be achieved and this  results in a typical pH
set point value of 5.5 maximum.  Peabody's process has measured low stoichi-
ometric ratios (1.02)  and  this supports the successful  performance of feed-
back pH in their system (Ostroff 1981).   Simple feedback pH can successfully
be used  to control limestone utilization or  limestone  stoichiometric ratio
and thus limestone feed rate.
     The  shape  of  the titration  curve of  an  acid/base  neutralization is
nonlinear.  Greater quantities of a base are  needed  to change the pH of a
solution from 5 to 6 than to change it from 6 to 7.   A  standard controller,
however,   is  a  linear  device.   It  is  adjusted  so that  if the  pH  value is
doubled,  the rate of limestone addition is also  doubled.   If the acid/base
neutralization is  buffered,  as  it  is in a limestone  scrubber, the titration
curve is  even  less regular.   It shows  "plateaus" where  the mixture absorbs
quantities of limestone and  there  is little change in pH.   As the degree of
buffering changes,  the  shape  of the curve changes.   Buffered solution in a
scrubber, therefore, is less  amenable to standard linear control than is an
unbuffered solution.

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FGD SYSTEM:  Process Control and Instrumentation	3-173
     Several  manufacturers  have   recently  produced  nonlinear  controllers
specifically  designed  for  control  of pH.  Within  a pH  band near the  set
point,  the controller  output signal  is amplified  only  slightly.   As  pH
change  increases, greater  amplification  makes  a greater change in limestone
feed  rate to  drive the  pH  back  more  rapidly  into the  acceptable  range.
Controllers of this  type  are more suited to this application than are stan-
dard linear instruments.
     Since  dissolution  of  limestone and  precipitation  calcium  sulfite/
sulfate  are not  instantaneous,  an  EHT  generally  retains  the  mixture  of
scrubber  effluent  slurry  and  fresh  limestone  slurry  for about 10  minutes
before  it is recycled to the scrubbing vessels.   Thus,  a time lag of several
minutes  is  inherent  in  a  system using pH of the recirculation slurry as  the
measured variable.
     The point of pH measurement is an important consideration (Jones et  al.
1978).   The primary  choice for location of the  probe  is in the EHT itself.
     As  a  means  of  improving pH feedback,  a simple  pH cascade control loop
can be  used to  regulate  limestone feed  rate.   The  feed  rate is controlled
primarily  by  pH  level  of  the  spent  slurry.  In a secondary  loop,  the con-
troller  is activated  on   the  basis  of  a  measurement of  pH of  the  fresh
slurry.  In the secondary loop an operator may  manually adjust the set point
according  to  the pH reading.   The overall  control  is  improved  by reducing
the time lag.
     For  a given size  of the EHT and given  L/G  ratio, pH  of  the recycle
slurry  depends  on  the limestone  slurry feed  rate and  the amount  of  S02
removed.  Because of the complex interrelationships among the variables,  the
optimum pH set point for the control system is  not easily identified, and an
operating  range  of pH  values is  usually defined.   The  upper limit corres-
ponds  to the maximum  limestone  feed rate  beyond which  poor utilization of
reagent  and sulfite  scaling may occur.  The lower  limit corresponds to  the
minimum  feed  rate required  for  the desired degree  of  S02  removal.   There-
fore, the  optimum  limestone feed rate should be  determined not only by  the
pH, but also  by  total  mass  flow rate  of S02  at the inlet  or outlet of  the
scrubber.

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FGD SYSTEM:  Process Control and Instrumentation	3-174
     Tne  use  of  outlet  S02  concentration  as  a  controlled variable  for
adjustment of  limestone  feed rate is theoretically advantageous because the
response of limestone feed to a measured error in S02 content is more nearly
linear than the pH response.  The time lag will be the same as in pH control
because it is  set by residence time of  the  EHT.   The sensor used in direct
S02 control  is an S02 analyzer of the types described earlier.   Care should
be  taken  to  assure  the  reliability of  such a sensor for use  in a control
loop.
     Application  of  this   control  method  still  requires  pH control.   As
mentioned earlier, the pH level of recycled slurry must be maintained within
a  range  that  will cause neither  low S02 removal (and  sulfate  scaling)  nor
excessive limestone addition (and sulfite scaling).   Thus, recycle pH can be
used to limit  (or trim) the feedback loop.   The limestone feed rate would be
controlled by  the loop  controlling outlet  S02  so  long  as  the  pH remains
within limits.    If pH reaches its high  limit, the  high-limit pH controller
would prevent  further addition of  limestone.  Similarly, the  low-limit pH
controller would  prevent the S02  controller from closing the limestone feed
valve too far.
     The use  of inlet S02 concentration to improve limestone feed control is
also desirable and is being applied commercially.   This system,  as described
earlier, is  proving   itself  as the  preferred method of  limestone feed con-
trol.   The advantage  of  measuring the S02 content of a dry gas  has enhanced
the reliability of this method.
     Another   technique,  developed  at  the  Shawnee  test facility,  merits
attention.   Both  the  inlet  and outlet S02 concentrations  are measured,  and
the difference is computed.   The  differential,  which  is a  measure  of  the
amount of  S02  removed,  is  then used in a feed-forward control loop (trimmed
by the recycle slurry pH),  similar to the  inlet  S02 feed-forward loop dis-
cussed earlier.   This  control  scheme is superior to the latter  in that both
the inlet  and  the outlet  S02 concentrations are considered  in  the control
loop.   The S02 removal efficiency,  however, is established  by  the initial
design operating  parameters  and  is  not to be considered a control variable.

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FGD SYSTEM:  Process Control and Instrumentation	3-175
     Improvements in the  reliability  of wet S02 analyzers should affect the
manner  in  which scrubbers  are  controlled.   With  the emphasis  on  stricter
emission limits  and shorter  emission averaging time, outlet  S02  emissions
must be monitored continuously.   Thus, the new control system specifications
should  have provisions  for  outlet  S02  feedback  or inlet  S02  feedforward
control.
Operational Systems
     The  process  control   systems  currently  used  at   Sherburne  1  and  2
(Northern  States Power),  Lawrence 4  and 5 (Kansas  Power and  Light),  and
Craig 1  and 2  (Colorado Ute) are described in detail.  The descriptions are
based on EPA-sponsored  survey reports and on information  obtained  from the
scrubber supplier of the Craig system.
     Process Control at Sherburne (Laseke 1979a).     A   dedicated   computer
monitoring  and  control  system is used for process  control  at  the Sherburne
scrubber  plant.    This  system  represents  a  refinement  over  the  original
concept, which  relied on  only enough parameters to  schedule the modules in
and out of  service to meet load requirements.   The old system,  operated from
a separate  control  room for the scrubber system,  monitored 3  analog and 26
digital  inputs with  no  visual display.  The refined system is  operated from
the  main control  room  and is  interfaced  with  the boiler controls.   The
computer now monitors  134  analog  and 48 digital  points, with display  on a
cathode  ray tube.   All  routine operations are performed by the computer and
logic network.
     Process control is maintained by regulating limestone  feed as  a func-
tion of slurry pH, regulating waste discharge as a function of  slurry solids
content, and regulating water  feed  as  a function of liquid  levels  in the
tanks.   The efficiency  of particulate removal  is  maintained by  controlling
gas-side pressure  drop  across  the venturi  rod decks.   Pressure drop of 12
in.  H20  across  the rod  decks is maintained by raising or lowering the lower
rod  decks   with  a manual  scissor  jack.  Regulation  of the vertical space
between  the rods controls the gas-side pressure drop, which in turn affects
the amount  of particulate removed by the scrubber.

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FGD SYSTEM:  Process Control and Instrumentation 	3-176
     The  efficiency  of S02  removal  is maintained by  controlling slurry pH
level.  The pH level is measured with a Universal Interloc pH sensor located
in a  fiberglass  flow-through chamber that receives slurry from a slipstream
tapped off  the  spray pump.   Based on these measurements,  the rate of lime-
stone feed  into  the  reaction tanks is manually  controlled.   The pH control
range is 5.0 to 5.5.   As the pH swings above or below this range, the amount
of fresh  limestone slurry  is  reduced or increased manually.   Thus optimum
removal efficiency is  attained while avoiding the scaling or plugging that
results from loss  of chemical  control.   Addition of limestone to the slurry
circuit ensures  a minimum  S02 removal  efficiency of 50  percent while pre-
venting possible  corrosion  and  scaling  caused by pH  swings.   The additive
slurry feed to each  EHT  is controlled  by  an Invalco valve  with a Norbide
valve plug and seat (boron carbide ceramic)  for abrasion resistance.
     To control the  level of slurry solids circulated through the scrubber
system, an effluent pumping  service removes  a spent slurry bleed stream from
each  EHT.   One effluent  bleed pump  is  provided for  each EHT.   The  spent
slurry is  discharged at  a  rate of  150  gpm  per module and  is  sent to the
thickener via the  slurry  transfer tank.   This bleed stream is automatically
controlled  by  a  Texas Instruments nuclear density meter  (standard clamp-on
type) working in  conjunction with a Leeds and Northrup controller.  A meter
inside the  EHT of each module monitors the solids  level  of the circulating
slurry.   When  the level  exceeds  10 percent,  a  control  valve is activated
that allows a bleed  stream  to flow to the  thickener  until the proper level
is reestablished.   Two  control  valves are  placed in  series in  the  bleed
line, the  first manually  set and the second controlled.   The control is not
critical,  however, and may be made manual.
     Maintenance of  a 10 percent  solids level  in  the scrubbing slurry is
vital to the process  chemistry of this system.   The level of sulfate in the
slurry is controlled by selective desupersaturation in the EHT and continual
bleed from  the spray  pump discharge to the  thickener.   Desupersaturation of
sulfate requires the presence of enough calcium sulfate solids in the slurry
to act as  seed  crystals,  promoting crystal  growth of  calcium sulfate in the
process.   This process is aided by the use of a forced oxidation system that
sparges air into  the  EHT's, oxidizing all  the remaining sulfite to sulfate.

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FGD SYSTEM:  Process Control and Instrumentation	3-177
The sulfate then  crystallizes  on the seed crystals and  is  removed from the
system for disposal  in  the fly-ash pond.  As a  consequence,  a sate sulfite
level is maintained  in  the slurry being returned to the scrubbers; critical
supersaturation,  which  can produce  uncontrolled  gypsum scale  formation on
equipment  internals,  does not  occur.   The amount  of calcium  sulfate  seed
crystals is  2 to  3  percent of  the  10  percent solids level  in the slurry,
which is sufficient to control  sulfate saturation levels.
     A water balance system controls the water returned from the holding and
recycle  basins  to maintain the  proper  liquid levels in the  EHT  and makeup
tanks.  Bubble-tube level controllers are used in these tanks to control the
flow  of  makeup  water  to  the  tanks.   Flow  rates  in  all  slurry  lines are
measured by magnetic flowmeters (Foxboro).
     Concentrations  of  S02 in  the flue gas  are measured  by DuPont ultra-
violet analyzers  (Photometric  460).   Each  scrubbing system is equipped with
four  analyzers,  each  having  a  four-stream  sample   train.    The  monitors
analyze  the  inlet and  outlet  values of S02 for  each module,  the overall
content of each generating unit, and the total flow to the stack.
     Process Control at Lawrence (Laseke 1979b).    The process  control  net-
works of the  Lawrence  limestone  scrubbing  systems  rely on  a significant
amount of  instrumentation  to  provide  total automatic  control  of process
chemistry.   Included  are  S02 gas analyzers (DuPont Photometric 460) for all
gas inlet  and  outlet streams,  magnetic  flow meters (Foxboro) for all liquid
slurry streams  (recirculation, bleed,  and feed lines),  pH meters (Uniloc)
for all  EHT's,  and nuclear density meters for  all  the collection tanks and
EHT's.   This  instrumentation  is  the basis of a control  network that main-
tains  particulate  and  S02  removal  efficiencies   at  desired  levels  while
preventing the loss of chemical control.  The effect of the Lawrence control
network  on  performance of  these major  functions  has  been  very successful.
     Particulate removal is maintained by controlling gas-side pressure drop
across the rod-decks  situated  in  the  throat area of each  module through
regulation of  the vertical  spacing between the two rows of rods in response
to gas flow.   This arrangement maintains  a  set  gas-side  pressure drop (9.0
in. H20) across  the  rods and  ensures particulate  removal  efficiency at 0.1
lb/106 Btu of heat input to the boiler.

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FGD SYSTEM:  Process Control and Instrumentation	3-178
     Sulfur  dioxide  removal is  maintained by regulating the  flow of lime-
stone  to the  scrubbing  systems as  a function of  coal  sulfur  content.   A
signal based on  coal  feed rate is used to indicate the inlet S02 content of
the flue  gas,  and this signal regulates the  limestone  feed rate.  The coal
flow  signal  is  related  to inlet  sulfur  conditions  only  when  the  sulfur
content  of the  coal  is  constant;  an operator-selected  stoichiometry bias
allows correction of the  limestone demand signal to account  for change in
the coal  sulfur content.   The sulfur in the coal is usually fairly constant;
therefore, the coal flow signal provides an accurate indication of the inlet
sulfur for  all boiler  loads.   This allows accurate variation of the lime-
stone  feed rate  for the  correct  stoichiometry  rate  throughout the  load
range.  The  system  can  operate at design  removal efficiencies without loss
of chemical control, which can lead to scale formation or corrosion.
     In the Lawrence 4 scrubbing system, discharge of spent  slurry occurs in
the liquid staging  system,  where the slurry from the EHT's  (one  per module)
is bled to the collection tanks (one per module).   Spent slurry that accumu-
lates  in  the slurry circuit of the rod-deck scrubber is discharged from the
collection  tanks  by variable-drive,  effluent   bleed  pumps.   The  solids
content  in  the EHT's  is  controlled  at  the 5 percent  level  by  a constant
gravity-flow bleed  stream,  which discharges to the collection tanks;  solids
in  the collection  tanks  are  controlled at  the  8 to  10 percent  level  by
varying the flow  of the  effluent bleed pump.  The  effluent bleed stream is
transferred to the  thickener,  where the slurry is  concentrated  to 30 to 35
percent solids before  it  is discharged to the sludge ponds.  Solids content
in the collection tanks and thickener is monitored by nuclear density meters
in the spray lines.
     Spent slurry is  discharged  from the  Lawrence 5 scrubbing  system in a
manner similar  to that described  for Lawrence 4.  Notable differences  are
(1) the lack of selective liquid staging and thickening in Lawrence 5, which
is  equipped  with only one  EHT  for  all  the  scrubbing modules,  and  (2) the
direct transfer of  the  effluent bleed stream to  the  sludge ponds without a
preceding thickening step.   Solids  content of the EHT is controlled  at the
10 percent level by cycling the effluent bleed pump on and off.

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FGD SYSTEM:  Process Control  and Instrumentation	3-179
     In the water  balance  system at Lawrence, fresh water,  thickener  over-
flow water, and pond  return  water are used to  compensate  for water loss in
the process.   Procedures  for maintaining  water balance in  the  two systems
differ because of  the presence  of additional liquid-staging and thickening
equipment  in Lawrence 4.   For Lawrence 4,  fresh water  is used to slurry the
limestone  prepared  in the  ball  mill.   Dilution  water,  which is added to the
slurry to  dilute  the solids  content  of the mill effluent,  .originates  from
the  recirculation   tank,  which  receives  pond  return   water and  thickener
overflow.   This water is used  to  maintain liquid levels in  the hold  tanks
and also for mist eliminator wash and  tank strainer wash.
     The water balance  network  is  essentially  the same as  for  Lawrence 4,
except that because Lawrence 5 contains no thickener, only pond return water
is used as the dilution  water.   As in the Lawrence 4 system, dilution water
is  used  to  maintain  liquid  level in the EHT  and also  to wash  the  mist
eliminator and tank strainers.
     Formation of  surface  scale  is minimized in the  EHT's  of  the Lawrence
scrubbing  systems   by  controlled  desupersaturation, which  is   effected  by
providing  calcium  sulfate  seed  crystals  for crystal growth sites, providing
and maintaining adequate solids  levels in the slurry circuits, and providing
adequate mixing and retention time in  the hold tanks.
     Precipitation of calcium sulfate  is maintained by  providing enough seed
crystals for crystal  growth  sites  in  the  slurry  circuit and by controlling
saturation  below  the  critical  supersaturation  level.   Sufficient  seed
crystals  are  maintained by  controlling  the solids content of  the  slurry
circuits (5 percent solids  in the Lawrence 4 EHT's;  10  percent solids in the
Lawrence 4 collection tanks and the Lawrence 5 EHT).
     Process Control at Craig (Ostroff 1981).   The  system at  Craig differs
from  the   Sherburne  and  Lawrence  installations  in that it is  a  pure S02
removal system; particulate matter is  removed by electrostatic precipitators
upstream of the FGD equipment.   There are two generating units,  each with a
dedicated  FGD  system.   Each  FGD system consists of four  scrubbing modules
complete with  auxiliaries,  one  thickener, and one  thickener overflow tank.
The two FGD  systems share  a common limestone preparation  area and a common
final dewatering (centrifuge) installation.   Each scrubbing module contains

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FGD SYSTEM:  Process Control and Instrumentation	3-180
a liquid  hydrocyclone  system to remove unreacted calcium carbonate from the
process slurry.  The calcium carbonate is returned to the EHT for-reuse; the
decarbonated slurry  is  used for scale control in the scrubber and for waste
slurry  from  the process.   This system provides  sufficient alkali  for S02
removal at a high utilization rate.
     The  process  is controlled  with both  analog  and digital  equipment.
Digital control is accomplished with a programmable system controller (manu-
factured by Modicon), which controls the start/stop sequence of every major
subsystem  within the  FGD  system.   Analog control  is accomplished  with  a
Bailey 820 Analog  Control  System,  which adjusts the controllable parameters
for optional  system operation.
     Sulfur  dioxide  removal   is  controlled  primarily  via the  pH  of the
recirculation  slurry.   The analog  controller  was  calibrated during  unit
startup to provide  the  required  S02 removal.  The  system  can handle minor
perturbations  in entering  S02  concentration.   More  significant  deviations
may call  for a change in the  pH set point or the starting of an additional
recycle  pump.    Changing  the  pH  set  point  and  starting  (or stopping)  a
recycle  pump   are  initiated by the operator; after that,  the  sequence  of
events  involved in  the pump  startup  (or shutdown)  is controlled  by the
digital system.   The coal  used at  this  installation has  been fairly con-
sistent  with  respect to  sulfur  content,  and the pH  control system has
handled adequately the small changes that have occurred.
     The number  of  modules  required is determined  by  boiler  load.   Signals
from the boiler control panel are  converted to an equivalent gas flow and to
the number of required modules.  The result of this "electronic" calculation
is  displayed  on the FGD control  panel.   The starting  (or stopping)  of  a
module and the choice of module are operator-initiated activities, but once
initiated, the  required  sequence   of  events is  controlled by  the digital
system.
     The waste  product  is  removed  by a simple overflow arrangement.  Liquid
is  added  to  the  system as  limestone  slurry in response to  the  pH signal;
liquid  is added  as clear  water  for  density  control, for mist  eliminator
wash,  for pump seals,  and for closed-loop water  balance.   The system over-
flow is the decarbonated product from the liquid hydrocycTones.  It flows by

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FGD SYSTEM:  Process Control and Instrumentation	3-181
gravity from the EHT's and is collected in a common sump,  then pumped to the
thickeners.  This  arrangement precludes the use of  complex,  interconnected
control  loops  for  slurry  density  control and  waste product purge.   The
amount of  waste slurry produced  varies with boiler load and  coal  composi-
tion, and the thickener feed rate is constant (at the pump capacity).   Since
these two flow rates are not equal,  thickener overflow water is added to the
waste  slurry on  the basis  of sump  level  to provide a  thickener feed of
constant  flow  but  variable  density.  This  eliminates   the  necessity  for
frequent pump starts and protects the pump against running dry.
     The  thickener  underflow  pumps  are  operated  continuously to  prevent
plugging of the transfer line.   Removal of thickener underflow is controlled
by  slurry  density.  When  the  density  is  low,  the system is  operated  in a
"recycle mode,"  in which  thickener underflow is returned to  the  thickener
feed well,  where  the  sludge blanket  builds  up to the desired density and
thickness.    When  this  occurs, a signal  alerts the  FGD  operator  that the
thickener  underflow  may be  operated in a  "production mode,"  in  which the
slurry is  transported  to  the centrifuges for final  dewatering and disposal.
The  change  from the  recycle to the  production  mode  is  operator-initiated;
after  that,  the  digital   system  controls  starting  of the centrifuges  and
diversion  of the  flow.  When  the  sludge blanket in the  thickener  has  been
depleted, the thickener  is returned to the recycle  mode  and  the centrifuge
system is sequentially shut down by the digital  controller.
     Three  centrifuges  feed  a common removal   point.  Trucks are  used to
remove the  sludge to  the  disposal   site.   A control  system  at the loading
site synchronizes  the  operation of  the centrifuges with  the availability of
a truck.  When  a truck is  in  position to  be loaded, the  centrifuge accepts
thickener  underflow  and discharges  sludge to a conveyor system and  to the
truck.   When a  truck is not in position  to be  loaded, the thickener under-
flow is  briefly returned  to  the  recycle mode but the centrifuge  system is
not stopped.
     The  process  makeup   water is  cooling tower  blowdown,   introduced in
response to a system-water inventory signal.  Liquid is stored in the alkali
storage tanks, in the EHT's, in the thickener, and in the  thickener overflow
tanks.    Since  the  EHT's  and  the  thickeners are  overflow devices  and are

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F6D SYSTEM:  Process Control and Instrumentation     	3-182
always full,  the water  inventory  is determined  by measuring  the  contents
(level) of the  alkali  storage tanks and the thickener  overflow tanks.   The
cooling  tower  blowdown  water  is  blended with decarbonated  slurry  and
introduced into  the modules.   This  arrangement allows the makeup  water to
equilibrate  chemically with  the FGD  system and  to isolate the  reactive,
alkali-containing,  recycle  slurry  from the  mist  eliminating  equipment.
     Scaling  is  controlled by  (1)  providing sufficient residence  time and
agitation  in  the EHT  to allow desupersaturation to occur; (2)  providing a
high  enough  concentration of  seed  crystals for  nucleation sites;  (3)  pro-
viding enough liquid flow to minimize the S02 make per pass;  (4) controlling
the system pH;  and  (5)  minimizing internals within the  scrubber  modules.

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FGD SYSTEM:  References                                                3-183
                          REFERENCES FOR SECTION 3
SCRUBBERS
Calvert,  S.   1977.   How  to Choose a  Participate  Scrubber.   Chemical Engi-
neering, 48(18):54-68, August 28, 1977.

Industrial  Gas  Cleaning  Institute.   1976.   Basic  Types of  Wet Scrubbers.
Publication No.  WS-3, June 1976.

PEDCo  Environmental,  Inc.   1981.    Flue Gas  Desulfurization  Information
System.   Maintained  for   the   U.S.  Environmental  Protection Agency under
Contract No. 68-01-6310, Task Order No. 6.  February 1981.

Saleem, A.  1980.   Spray  Tower:  The Workhorse of Flue Gas Desulfurization.
Power, 124(10):73-77.


MIST ELIMINATORS

Burbank,  D. A.,  and S.  C. Wang.  1980.  EPA Alkali Scrubbing Test Facility:
Advanced  Program  -   Final  Report (October 1974  to June  1978).   EPA-600/7-
80-115.  NTIS No.  PB80-204241.

Conkle, H. N., H.  S. Rosenberg, and S. T. DiNovo.   1976.  Guidelines  for the
Design  of  Mist  Eliminators  For  Lime/Limestone  Scrubbing  Systems.   EPRI
FP-327.  December 1976.

Industrial  Gas  Cleaning  Institute.   1975.   Scrubber  System  Major Auxilia-
ries.  Publication No. WS-4.  November 1975.

Ostroff,  N., and  T.  D.  Rahmlow.  1976.  Demister Studies in a TCA Scrubber.
MASS-APCA.  Drexel University.   April 23, 1976.

PEDCo  Environmental,  Inc.   1981.    Flue Gas  Desulfurization  Information
System.   Maintained to  the U.S. Environmental Protection Agency under Con-
tract No.  68-01-6310, Task Order No. 6.


REHEATERS

Choi,  P.  S.  K.,  S.  A.  Bloom, H.  S.  Rosenberg,  and  S. T.  DiNovo.   1977.
Stack Gas  Reheat  for Wet Flue Gas Desulfurization Systems.  EPRI report No.
FP-361.   Prepared  for  Battelle  Columbus Laboratories  for  EPRI  Research.
February 1977.

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FGD SYSTEM:  References	3-184

Muela, et  al.   1979.   Stack Gas  Reheat  - Energy and  Environmental  Aspects.
In:  Proceedings:  Symposium on Flue Gas  Oesulfurization,  Las  Vegas,  Nevada,
March 1979.  Volume II.  EPA-600/7-79-167b.  NTIS No.  PB80-133176."

Perry, R.  H.,  et al.   Chemical Engineers Handbook.   Fifth edition.  McGraw
Hill, New York 1973.  p. 12-2.


FANS

Green Fuel Economizer Co.  1977.  "Heavy  Duty Fans."   Technical Bulletin  No.
195.  Beacon, New York.

Green, K., and  J.  Martin.   1978.   Conversion  of Lawrence No. 4  FGD  System.
In:  Proceedings:   Symposium  on Flue Gas Desulfurization  Held at Hollywood,
Florida,  November  1977.   Vol.  1.   EPA-600/7-78-058a.   NTIS  No.   PB-282 090.

Laseke,  B.  A.   1978a.  EPA  Utility FGD  Survey:   December  1977 -  January
1978.  EPA-600/7-78-051a.  NTIS No. PB-279 Oil.

Laseke, B. A.   1978b.   Survey of Flue Gas  Desulfurization Systems:  Cholla
Station,  Arizona Public  Service  Co.   EPA-600/7-78-048a.  NTIS  No. PB-281
104.

Laseke, B. A.  1978c.   Survey of Flue Gas Desulfurization  Systems:   La  Cygne
Station,  Kansas  City  Power  and  Light   Co.   EPA-600/7-78-048d.   NTIS  No.
PB-281 107.

Laseke, B. A.,  et al.   1978.   EPA  Utility FGD Survey:  February-March  1978.
EPA-600/ 7-78-051b.  NTIS No.  PB-287 214.

Melia,  M.,  et  al.   1978.   EPA Utility  FGD Survey:   June-July  1978.    EPA
600/7-78-051d.  NTIS No. PB-288 299.

Miller, D. M.   1976.   Recent Scrubber Experience at Lawrence  Energy  Center,
Kansas Power  and Light Co.   In:  Proceedings:   Symposium  on  Flue Gas Desul-
furization Held  at  New Orleans, Louisiana, March 8-11, 1976.  EPA-600/2-76-
136a.  NTIS No. PB-255 317.

Wells, W.  L.,  G.  T. Munson, and  E. G.  Marcus.  1980.  Actual and Projected
Materials Problems in Limestone and MgO Scrubber Processes.   Paper presented
at 7th  Energy Technology Conference and  Exposition, Washington,  D.C.,  March
24-26, 1980.


THICKENERS AND MECHANICAL DEWATERING EQUIPMENT

Heden,  S.  D.,  and J.  H.  Wilhelm.   1975.   Dewatering of Powerplant  Waste
Treatment Sludges.  Paper presented at the 36th  Annual  Meeting of the Inter-
national Water Conference, Pittsburgh, Pennsylvania.

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FGD SYSTEM:  References	3-185
Knight,  R.  G.,  et  al.   1980.   FGD Sludge  Disposal  Manual, Second Edition.
EPRI-CS-1515.
Morasky,  T.  M.   1978.   State-of-the-Art  of  Sludge  Fixation.  -EPRI-FP67.
Morasky, T. M.   1979.  Lime FGD Systems Data Book.  EPRI-FP-103.
Rabb,  D.  T.   1978.   Selected  Topics from  Shawnee  Test Facility Operation.
EPA Industry Briefing at Research Triangle  Park, North Carolina.
Robbins, J.  1979.   Private communication  to PEDCo Environmental, Inc. froo
BIRD Machine Company regarding centrifuge at Mohave.
U.S.  Environmental  Protection  Agency.   1979.    Process  Design  Manual   for
Sludge Treatment and Disposal.   EPA-625/1-79-011.
Wilhelm, J.  H.,  and R. W.  Kobler.  1977.   Private  communication from EIMCO
Process Machinery, Division of Envirotech.

SLUDGE TREATMENT
Knight,  R.  G.,  et  al.   1980.   FGD Sludge  Disposal  Manual, Second Edition.
EPRI-CS 1515.
PUMPS
Dalstad,  J.   I.   1977.   Slurry Pump  Selection  and  Application.   Chemical
Engineering, 84(9):101-106.
Doplin, J.  H.   1977.   Select Pumps  to  Cut Energy Cost.  Chemical  Engineer-
ing, 84(2):137-139.
Karassik,  J.,  et al.   1976.  Pump Handbook,  McGraw-Hill  Book Co.   pp.  3-47
to 3-69.
Kruger,  R.  J.    1978a.    Experience with  Limestone  Scrubbing:    Sherburne
County  Plant, Northern  States Power  Co.   In:   Proceedings:   Symposium  on
Flue  Gas  Desulfurization, Hollywood,  Florida,  November  1977.   Volume  I.
EPA-600/7-78-058a.  NTIS No. PB-282  090.
Kruger, R. J.  1978b.  Personal communication  to  PEDCo.
Laseke, B.  A.,  Jr.   1978a.  Survey  of  FGD Systems:  Cholla  Station,  Arizona
Public Service Co.  EPA-600/7-78-048a.  NTIS No.  PB-281 104.
Laseke, B. A., Jr.  1978b.  Survey of FGD  Systems:  La Cygne Station,  Kansas
City Power and Light.  EPA-600/7-78-048d.  NTIS No. PB-281 107.

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FGD SYSTEM:  References	3-186

O'Keefe, W.  1980.   Flue Gas Desulfurization and Coal's Upswing Direct Your
Attention to Slurry Pumping.  Power, 124(5):25-35.

Reynolds, J. A.  1976.  Saving Energy and Costs in Pumping System.  Chemical
Engineering.

Teeter, R.  1978.  Personal communication to PEDCo.

Wilhelm,  J.  H.   1977.   Personal  communication  with Mr.  Wilhelm  of EIMCO
Process Machinery, Division of Envirotech.


PIPING, VALVES, AND SPRAY NOZZLES

Laseke,  B.  A.,  Jr.    1979.   Survey  of  Flue  Gas Desulfurization  Systems:
Sherburne  County  Generating  Plant,  Northern  States  Power  Co.   EPA-600/
7-70-199d.  NTIS No.  PB 80-126287.

O'Keefe, W.  1980.   Flue Gas Desulfurization and Coal's Upswing Direct Your
Attention to Slurry Pumping.  Power, 124(5):25-35.

Rosenberg,  H.  S.,  et  al.   1980.   Operating Experience  With Construction
Materials for Wet Flue Gas Scrubbers.  Combustion, 52(l):23-36.

Saleem, A.  1980.  Spray Tower:   The Workhorse of Flue Gas Desulfurization.
Power, 124(10):73-77.
TANKS

Borgwardt, R.  H.   1975.   Increasing Limestone Utilization in FGD Scrubbers.
Presented at the 68th AIChE Annual Meeting, Los Angeles, California.

Laseke,  B.  A.,  Jr.   1979.   Survey  of Flue  Gas  Desulfurization  Systems:
Sherburne  County Generating  Plant of  Northern  States Power  Co.   EPA-600/
7-79-199d.  NTIS No.  PB 80-126287.

Saleem, A.  1980.  Spray Tower:   The Workhorse of Flue Gas Desulfurization.
Power, 124(10):73-76.
AGITATORS

Rosenberg,  H.  S.,  et  al.   1980.   Operating Experience  with Construction
Materials for Wet Flue Gas Scrubbers.  Combustion,   pp.  23-36.   July 1980.


PROCESS CONTROL AND INSTRUMENTATION

Gruenberg, N. R.   1979.   Instrumentation and. Control  for  Double Loop Lime-
stone Scrubbers.   Power Engineering, 83(6)72-75.

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FGD SYSTEM:  References	s	3-187

Jahnke, J. A.,  and G.  J. Aldina.  1979.  Handbook for Continuous Air Pollu-
tion  Source  Monitoring  Systems.  EPA-625/6-79-005.   NTIS No.  PB-300 930.

Jones, D.  G., A. V. Slack, and K. S.  Campbell.  1978.  Lime/Limestone Scrub-
ber Operation and Control Study.   EPRI-RP 630-2.

Laseke, B.  A.,  Jr.    1979a.   Survey  of  Flue Gas  Desulfurization Systems:
Sherburne  County  Generating  Plant,   Northern  States  Power  Co.   EPA-600/
7-79-199d.  NTIS No.  PB-80-126287.

Laseke, B.  A.,  Jr.    1979b.   Survey  of  Flue Gas  Desulfurization Systems:
Lawrence Energy Center, Kansas Power and Light Co.  EPA-600/7-79-199b.  NTIS
No. PB 80-125628.

Ostroff,  N.   1981.  Private  communication to  PEDCo Environmental, Inc. on
Peabody's simple feedback pH control  system.

Ung,  C.,  A.  Acciani,  and R.  Maddalone,  1979.   The Use  of pH and Chloride
Electrodes  for  the  Automatic Control of  FGD  Systems.    EPA-600/2-79-202.
NTIS No. PB 80-138464.

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                                  SECTION 4
                        PROCUREMENT OF THE FGD SYSTEM

     This  section  deals  primarily  with  procurement  of  the  FGD  system,
including  the  basic  procedures  to  be followed  in  preparing the  purchase
documents  and  evaluating  the  respondents  proposals.   Guidelines are  given
for management of the total  project effort following  award  of contract  so as
to ensure that the contract is fulfilled, i.e., management  of the activities
of vendors, A/E  consultant,  and  utility staff in the installation,  startup,
and  testing  of the  system.   The  scope of the section  thus  encompasses  the
major  project  activities  from the  start of procurement to the  point of
system  operation.   The  sequence  is  depicted in Figure 4-1, and  the  text
discussion also follows that sequence.
     Because  each  utility  follows  preferred  or regulated procedures  in
administration of  purchasing activities, the  information  presented here is
intended  to  supplement rather than  to  replace  those  practices.   Although
there may  be considerable  variation in the format and nomenclature  of  docu-
ments generated  by individual purchasers, the overall  procedures described
here are  considered typical  of  those used  in successful  procurement  of a
limestone FGD system.

COMPETITIVE BIDDING
     The  competitive  bidding process  has  become routine  in business  prac-
tice.  So  routine,  in  fact,  that the  purchasers  may  lose  sight of  the real
objective, which  is to obtain the best  possible product from  a qualified
bidder at  the  lowest evaluated cost.   Achieving  this objective  may be easy
in theory but is sometimes difficult in practice.
     The  key  to  successful  procurement  by  competitive  bidding   lies  in
obtaining  comparable  proposals.   For proposals from  competitive  bidders to
be  truly comparable,  each  must  be  prepared against a baseline document,
which is the purchase specification.
                                     4-1

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     PROCUREMENT PLANNING
     e SCOPE OF SUPPLY
     0 ALLOCATION OF PROCUREMENT PACKAGES
     0 SELECTION OF BIDDERS
PREPARATION OF .SPECIFICATIONS

0 REQUIREMENTS:  BIDDING, CONTRACT, TECHNICAL
0 DESIGN BASIS
0 GUARANTEES
0 EQUIPMENT SPECIFICATION
            EVALUATION OF PROPOSALS
            0 TECHNICAL
            0 COMMERCIAL
            0 ECONOMIC
  ENGINEERING DESIGN, INSTALLATION, STARTUP,
  AND TESTING
  0 ENGINEERING DATA
  0 SYSTEM INSTALLATION
  0 SYSTEM STARTUP
  0 PERFORMANCE TESTING
Figure 4-1.  FGD system procurement  sequence.
                      4-2

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PROCUREMENT: Purchase Specification and Planning	  4-3
THE PURCHASE SPECIFICATION
     As the medium  through  which the utility communicates with the prospec-
tive bidders, the purchase  specification  should clearly define the require-
ments  to  which  the bidders will  respond.   The  requirements  are  of  three
basic kinds:
     1.   Bidding requirements, which provide  guidance  and instructions for
          preparation and  submission of the proposal.   Typically,  the  bid-
          ding  requirements   include   detailed   instructions   to  bidders,
          pricing forms, and specification  of  data to be submitted with the
          proposal.
     2.   Contract  requirements,  which  consist  of  contract forms  and  con-
          tract  regulations.   Because  each utility  probably  follows  pre-
          ferred contract  procedures,  including the  forms  and regulations,
          the discussion in this  section  is limited  to  the  significance of
          these documents in the purchase  specification.
     3.   Technical   requirements,   which   identify  the  design basis,  the
          performance requirements,  and the guarantees  for  the FGD system,
          and present  the  detailed equipment  specifications  and erection
          requirements.
These three basic components of the purchase specification  are depicted in
Figure 4-2 and are discussed in detail  later in this section.

PROCUREMENT PLANNING
     Before  the project team prepares specifications  for  purchase   of  a
limestone  FGD  system,   they  must  make  certain  decisions  regarding  the
detailed system design,  scope  of supply,  procurement package breakdown, and
selection of bidders.  As discussed earlier, preliminary engineering studies
must be made  to determine the  conceptual  system  design,  including the most
advantageous  scrubber   type,   additive  type,  and  methods  for processing,
transportation,  and disposal   of  the solid waste.   These  fundamental  deci-
sions must be made prior to requesting proposals.
Scope of Supply
     The scope of supply for a  limestone FGD system encompasses a variety of
equipment.    The following  list  of items  to  be supplied  is  based on the
primary functions of specific subsystems of the FGD system:

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                  BIDDING REQUIREMENTS
                    INSTRUCTIONS TO BIDDERS
                  0 PRICING REQUIREMENTS
                  0 DATA REQUIREMENTS
                          PURCHASE
                       SPECIFICATIONS
                                  TECHNICAL REQUIREMENTS
CONTRACT REQUIREMENTS
  0 CONTRACT FORMS
  0 CONTRACT REGULATIONS
                               0 DESIGN BASIS
                               0 EQUIPMENT SPECIFICATIONS
                                    ERECTION REQUIREMENTS
  GUARANTEES
Figure 4-2.   Primary components of purchase specifications,
                            4-4

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PROCUREMENT:  Planning	    4-5


     0    Scrubber modules

            Presaturators
            Scrubbers
            Scrubber purge system

     0    Limestone receiving, conveying, and storage subsystem

            Hoppers
            Conveyors
            Feed bins

     0    Limestone slurry preparation and storage subsystem

            Feeders
            Ball mills
            Storage tanks
            Mixers
            Classification system

     0    Liquid flow subsystem components

            Scrubber recirculation pumps
            Other pumps
            Piping and piping support
            Valves
            Tanks
            Agitators
            Liquid hydrocyclones

     0    Gas flow subsystem components

            Booster fans
            Ductwork and support
            Dampers  - isolation and control
            Expansion joints
            Hoppers
            Mist eliminators
            Flue gas reheaters
            Soot blowers
            Test ports
            Insulation

     0    Sludge thickening subsystem

            Thickener
            Flocculant feed system
            Thickener overflow tank and pumps

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PROCUREMENT: Planning	    4-6
     0    Sludge dewaterin  subsystem
            Filter or centrifuge
            Filtrate/centrate return system
     0    Sludge stabilization/fixation subsystem
            Lime feeding
            Flyash feeding
            Blending
            Precure
            Transportation
     0    Instrumentation and control subsystem
     In addition  to the  foregoing  subsystem equipment items, the  scope of
supply includes
     0    Foundations, structural support steel, and access provisions
     0    Buildings to enclose equipment
     0    Electrical distribution system
     0    Model and model tests
     0    System installation
     0    System startup
     0    Performance testing
Allocation of Procurement Packages
     The project team  must  allocate these items of  equipment  and materials
into logical procurement  packages.   The allocation depends on the utility's
plant  design  approach  and  on  the  capability  of  the engineering  staff in
scrubber system design.   The  focus  on capability  logically  carries  over to
allocation of the procurement packages to individual suppliers; that is, the
utility project  staff must become  aware of the specific  qualifications of
each potential  supplier.
     Basing the allocation solely on function may not be the most economical
or manageable  procedure.  Depending on the available  resources  and person-
nel, any of the  following approaches may be  used  to produce a workable set

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PROCUREMENT:   Planning	   4-7
of procurement packages.  None  of these,  however,  is considered optimum for
all situations.
     One approach  is to purchase  the  entire FGD system from a  single  sup-
plier, a "turnkey"  or "design-construct"  firm that would be responsible for
all equipment, materials, and work required to produce a complete operation-
al  system.    Essentially the  opposite  approach  is  to  perform  extensive
detailed  engineering design,  purchase the  components,  and construct  the
system, maintaining primary control over the entire effort.
     A  third  approach,  and probably  the  most common,  is  to group  entire
subsystems or major elements  of the subsystems into  several  major procure-
ment  packages  based  on  the technical  expertise  required.    This  procedure
places  responsibility  for the  design  of crucial  components  with  suppliers
who are qualified  by experience and technical competence.   Under  this  plan
the  utility  can  purchase and  install the  less  critical  components  in  a
routine manner.
     A  typical allocation  of procurement packages under  the  third approach
would consist of four packages,  as follows:
     0    Limestone  receiving  and  conveying equipment.   These  would  be
          purchased from a conveying equipment manufacturer.
     0    Scrubber  modules,  mist eliminators, flue  gas  reheaters  and  soot
          blowers  (if required),  booster  fans, limestone slurry preparation
          and storage subsystem,  sludge thickening subsystem and portion of
          liquid flow subsystem.   These would be purchased from a scrubber
          system supplier  (a possible variation would be separate procure-
          ment  of  limestone  slurry  preparation  and/or sludge  thickening
          subsystems).
     0    Sludge dewatering subsystem,  sludge stabilization and/or fixation
          subsystem, and portion  of liquid flow subsystems.  These would be
          purchased  from a  supplier with  established technical  expertise in
          this specialty.
     0    Ductwork, dampers, and expansion joints.   These would be purchased
          from  a  scrubber  system  supplier  or  acquired  under a  separate
          contract  for ductwork or structural steel.
Obviously, other  breakdowns  of the systems  for procurement are  possible.
     The procurement package allocation described above  does not encompass
all items needed to complete the FGD  system.  Additional  services and work
to be procured include the following:

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PROCUREMENT:  Planning	4-8
     0    Site preparation and underground utilities
     0    Electrical construction
     0    Painting (other than special coatings and linings)
     •°    Testing
     0    Underground  piping between  subsystems  (if  not  provided by  the
          system suppliers)
This work is usually performed under construction contracts.
Selection of Bidders
     Selection  of qualified  bidders is  vital  to the  competitive bidding
process.   Selection of  qualified  firms  to receive a request for proposal is
typically accomplished as follows:
     1.    Prepare a list of potential bidders based on FGD surveys (Smith et
          al.  1980b).
     2.    Request that  each bidder  submit  a  pre-bid  qualifications state-
          ment, which  should include related  experience,  a customers  list,
          organization  and  manpower capabilities,  and  financial  status.
     3.    Verify the experience and  reputation of each bidder by inspecting
          the  bidder's  fabrication facilities  and by talking  with utility
          personnel  involved with operating FGD systems.
     4.    Evaluate the  qualifications statements and establish  the list of
          qualified bidders.
     The qualified bidders will  receive  the purchase specifications and will
be asked to submit proposals for evaluation.
     The  following  are  limestone  FGD  system  suppliers  whose units  are
currently in service in domestic utility plants (Smith et al. 1980b).
     Babcock & Wilcox Co.
     Chemico Air Pollution Control
     Combustion Engineering, Inc.
     Peabody Process Systems
     Pullman Kellogg

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PROCUREMENT:   Preparation of Specifications	      4-9
     Research-Cottrell
     Rlley Stoker/Envlroneering
     UOP, Air Correction Division

PREPARATION OF SPECIFICATIONS
     This  subsection  describes a  typical  purchase specification document.
The  guidelines  given  here are  intended to  supplement a utility's  normal
procedures for preparing specifications.
     Prospective suppliers of the FGD system must be given specific informa-
tion so  that  they  can submit proposals that  are  cost-effective  and respon-
sive to  a  utility's  needs.   The bid requests must be specific and detailed,
so that  proposals  received  from the various  vendors are  of  similar content
and scope  and thus are readily comparable.   The specifications must provide
all pertinent available information concerning design  of the  limestone FGD
system.  They should  also allow for enough flexibility that each bidder can
apply  his  own technology;  this  could provide  both technical and economic
benefits for the overall project.
     As  a  minimum, the  following  items must be  transmitted  to  prospective.
system suppliers to provide guidance for proposal preparation.
     0    Scope of supply
     0    System  equipment  redundancy and   interfaces with   other  systems
     0    Site constraints
     0    Requirements  for  performance  guarantees  that  ensure  compliance
          with applicable regulations at all operating conditions
     0    Requirements  that establish quality  baselines  for  equipment and
          materials  to  reflect  the  utility's  operating  and  maintenance
          philosophy
     0    Description  of  construction  site  and restrictions on field activ-
          ities
     0    Schedule and time constraints
     0    Equipment erection  requirements  that  establish quality standards
          for the field construction work

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PROCUREMENT:  Preparation of Specifications	.	4-10
     0    Economic  evaluation  factors (cost  of electricity, water,  steam,
          etc.)
     0    Fuel, makeup water, and limestone composition
     All of the above information must be presented to prospective suppliers
in  a  logical, clear, and concise manner.   As an aid in  formulation  of the
total purchase  package,  the following  discussion deals  in  order with the
three major elements described earlier--the bidding,  contract,  and technical
requirements.
Bidding Requirements
     As shown in Figure 4-3,  general instructions to bidders  should state
the requirements  for  proposal  preparation, including the following types of
instruction:  to  whom,  where, when,  and  in  how many copies to  submit the
proposal;  style and format  to  be used;  the information required;  methods of
indicating  any  intended  deviations  from the specifications;  the utility's
provisions  for  site accessibility;  procedures  for seeking  clarification of
the specification and requirements;  and  the technical and cost factors that
will be used  in proposal  evaluation.
     Pricing  requirements should be set forth clearly, with pricing forms on
which the bidder  will  list  cost information.   Proposal  pricing forms should
provide for a breakdown of the lump  sum price  into material  and erection
prices.   This will  permit  the application of  separate  cash  flow schedules
and/or price  adjustment  indices  during  the utility's economic evaluation of
proposals.   The forms should define  the basis of pricing and should require
an  indication as to whether the prices  are firm or  are  subject  to  escala-
tion.   The  forms  should  also request an exact procedure for calculating the
effects of escalation (if applicable) on the contract amount.
     Proposal data  requirements  should  specify  the  information needed from
bidders for technical and economic evaluation of proposals.   Requested items
should include performance curves,  drawings, equipment data sheets, and such
other supplemental  information as descriptions of support  and maintenance
operations,  anticipated  maintenance schedule,  startup  and shutdown  pro-
cedures, mass balance diagrams, and materials lists.

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BIDDING REQUIREMENTS
                                    INSTRUCTIONS TO BIDDERS
                                   0 UTILITY REPRESENTATIVE, DUE DATE,
                                       NUMBER OF  COPIES,  ETC.
                                     PROPOSAL FORMAT
                                     INSTRUCTIONS RE DEVIATION  FROM SPECS
                                   e ACCESSIBILITY TO SITE
                                   c COST FACTORS
                                    PRICING  REQUIREMENTS
                                    (INCLUDES  PRICING  FORMS)

                                    0 LUMP-SUM BASE BID
                                     BREAKDOWN OF LUMP-SUM
                                     SEPARATE PRICING OF
                                       ALTERNATIVES/OPTIONS
                                     PRICING  BASIS
                                     ESCALATION TERMS
                                    DATA REQUIREMENTS
                                   0 PERFORMANCE CURVES
                                   0 DRAWINGS
                                   0 EQUIPMENT DATA SHEETS
                                   0 SUPPLEMENTARY INFORMATION
      Figure 4-3.  Bidding requirements for proposal preparation.
                                   4-11

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PROCUREMENT:  Preparation of Specifications	4-12

Contract Requirements
     Copies of legal contracts to be signed by the successful  bidder and the
utility  are submitted  as part  of  the  specification  package,  as are  the
contract  regulations  that specify  the  commercial   terms  and  conditions,
liabilities, and other conditions by which the supplier must abide.
     0    Bond—The bonding requirement should include bonding of the equip-
          ment performance  guarantees.   The bond should cover all  perform-
          ance testing requirements.
     0    Guarantee—The  guarantee  period  should  cover  completion  of  a
          performance test to  be conducted some time (for example,  1 year)
          after initial operation and testing of the  equipment.
     0    Payments—Payment provisions may be structured to  provide progres-
          sive payments  at  crucial  stages  of  the  project.   For  example,
          payments can be  made for engineering work, at start of construc-
          tion,  upon receipt  of shipments, at  various milestones in  con-
          struction,  at  start  of  system  operation,  and  upon  passage  of
          performance and compliance tests.
Technical Requirements
     A full statement  of the  utility's technical requirements is the key to
eliciting substantive information from bidders in their proposals.   As shown
in  Figure  4-4, the  technical  requirements may be grouped into  three major
categories:  general, equipment,  and erection requirements.
     General requirements pertain to the overall limestone  FGD system rather
than to  specific  components.   These  should begin with a general  description
of  the  project and  its  scope,  including  the  project  schedule.   Specifica-
tions are  given  regarding shipping procedures and details  of  the mechanical
and  electrical  components,  including  instrumentation  and  other  auxiliary
items.    Instructions  should  be  given  also on procedures for submission of
detailed engineering data  after award  of  contract;  e.g.,  such items  as
performance curves,  drawings, and manuals  that are  developed  during  the
construction/i nstal1ati on phase.
     Equipment requirements are  the  heart of the purchase document.   It is
here that  the  utility  delineates  the  design basis  of the  limestone  FGD
system, the guarantees expected  from suppliers, requirements  for guaranteed

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   TECHNICAL
 REQUIREMENTS
                         |GENERAL REQUIREMENTS
                          0 DESCRIPTION, SCOPE, SCHEDULE
                          0 GENERAL EQUIPMENT SPECIFICATIONS
                              SHIPPING
                              MECHANICAL/ELECTRICAL COMPONENTS
                              INSTRUMENTATION
                              OTHER AUXILIARIES
                          0 ENGINEERING DATA
                              PERFORMANCE CURVES
                              DRAWINGS
                              MANUALS
                              OTHER SUPPORT DATA
                          EQUIPMENT REQUIREMENTS
DESIGN BASIS
GUARANTEES
PERFORMANCE TESTING,
RELIABILITY DEMONSTRATION
DETAILED EQUIPMENT SPECIFICATIONS
                          ERECTION REQUIREMENTS
                          e  SCOPE AND SCHEDULE
                          0  WELDING,  PIPING,  PAINTING,  OTHER
                            STARTUP
Figure 4-4.   Technical  specifications consisting of
   general,  equipment,  and erection requirements.
                        4-13

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PROCUREMENT:  Preparation of Specifications	4-14
performance  testing  and reliability demonstration,  and  detailed specifica-
tions for  system  components.   These critical portions of the purchase pack-
age are discussed in detail in the following subsections.
     Erection requirements outline  the  scope and schedule of field erection
work, describing  the  construction facilities and specifying the services to
be provided by the  supplier  and by the utility.   Specifications  are given
for welding,  piping,  cleaning, painting, and  other installation functions.
Erection activities will culminate in system startup, for which requirements
should be stated.
     Design Basis.  As  a minimum, the Design  Basis  should  include informa-
tion and specifications pertaining to the following:
     0    Equipment arrangement
     0    Scrubber type
     0    Design operating conditions
     0    Composition of fuel, limestone, and makeup water
     0    Equipment sizing criteria
     0    Required redundancy
     0    Construction criteria
     0    Performance data curves
     0    Economic evaluation factors
     Although the Project Manager may not be directly involved with develop-
ment of the detailed information included in the Design Basis portion of the
specifications, the Manager is ultimately responsible for results and there-
fore should  review the  design information provided  for  the  bidders.   As an
aid in ensuring that  all necessary design basis  information is transmitted
to the prospective suppliers,  a checklist of important items is presented in
Figure 4-5.
     Guarantees.   Vague  process guarantees for FGD  system  equipment should
be avoided.   Such guarantees  sometimes  prove  to  be  nonbinding because they
are not  specific  in  covering the  possible  range of  operating conditions.

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                           DESIGN BASIS CHECKLIST

EQUIPMENT ARRANGEMENT

     State whether the unit It new or retrofit.  If retrofit,  specify the
     following:

          Spice limitations (arrangement drawings, elevations, plans, end
          ductwork schematics).
          Locations and sizes of  existing fans, ducts, and stack.
          Materials of construction of existing equipment, 1f  any.

     Instruct bidder to minimize  the ae»unt of FGD systeai ductwork.

DESIGN CONDITIONS AND EXPECTED RANGES

     Coal and ash properties

          ProxlMte and ultlaate  analyses
          Fly ash alkalinity analysis

     S02  Inlet loading and emission Hal tat ions

          S02 content of total flue gas stream
          Minimum design S02 removal efficiency
          Guaranteed maximum allowable S02 content of the effluent gas stream

     Boiler  characteristics

          Boiler type
          Coal burn rate
          Heat Input rate
          Excess air In flue gas

     Flue gas conditions

          Unit generating capacity
          Flue gas flow rate (lb), temperature, and pressure
          Emergency operating conditions
          Variability 1n gas flow and temperature
          Bypass limitations
          Reheat requirement and  mode

     Partlculate control

          Method of participate control
          I-dlet and outlet participate loading
          Partlculate emission limitation and opacity limitation
          Expected maximum partlculate content in the Inlet flue gas stream
          Guaranteed maximun allowable partlculate content of  effluent gas stream
          Possibility of operating at reduce partlculate control efficiency

     Limestone supply

          Limestone analysis
          Quantity required
          Limestone grind

     Makeup  water supply

          Source
          Water analysis
          Allowable water discharge under local regulations

     Waste disposal

          Proposed disposal site
          Desired quality of final product (solids content. pH. leaching
          characteristics, Impact strength)
 Figure 4-5.   Checklist  for  preparation  of  system
                   design  basis  to  bidders.
                                        4-15

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     Minimum number of modules acceptable
     Spare Module requirements
EQUIPMENT CONSTRUCTION CRITERIA
     Scrubbtr design pressure
          Maximum positive
          Mix1BUM vacuum
     Maxima flue QM temperature at scrubber Inlet
     Makeup water pressure
     Seismic criteria
     Location (Indoor end outdoor)
     Wind and snow loads (If applicable)
     Grade elevation
     Barometric pressure
     Ambient temperature
          Minimum
          Maximum
     Indoor temperature
          Mlnlaiun
          Maximun
PERFORMANCE DATA AND CURVES REQUESTED
     Mass balance dlagraas (showing flow rates,  pressures  and  temperatures of
      flue gas, Makeup water, additive, slurry,  sludge,  chemicals, etc.)
          At maxlBu* continuous capacity of steam generator
          At expected average operating capacity
          At reduced conditions
      Scrubber performance curves
           Pressure loss through scrubber versus Inlet gas flow
           Water droplet content In flue gas leaving Bist  ellan'nator  versus  load
           SOj removal efficiency versus S02 concentration and gas flow
      Pump characteristic curves
           Head
           Power requirements
           Efficiency
           Net positive suction head required versus capacity
      Fan performance curves
           Static pressure
           Power requirements
           Fan efficiency
           System resistance versus capacity
                    Figure  4-5  (continued)
                                       4-16

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PROCUREMENT:  Preparation of Specifications	  4-17
     Specific guarantees provide two advantages for the utility.   They allow
an in-depth  comparison  of  the strength and scope of guarantees presented in
the various  proposals and  also allow predictions of the  operating  costs to
be based on guaranteed performance parameters.
     Following is a  suggested list of significant guarantees to be  obtained
for a limestone FGD system:
     0    S02 removal
     0    Particulate outlet loading
     0    Mist in the outlet gas stream
     0    Power consumption
     0    Reheat energy consumption
     0    Limestone consumption
     0    Water consumption
     0    Waste streams
     0    Turndown ratio and rate of unit load change
     0    System availability*
     A checklist  for specifications requesting these guarantees is  provided
in Figure 4-6.
Detailed Equipment Specifications
     This subsection  identifies  some  of the significant  factors  to  be con-
sidered when specifying the  components of the  FGD system.   The equipment
specifications should be organized so as to list  together  the requirements
for  similar   items  insofar as  is  practicable.   For  example, one  section
X
  Availability is defined as the number of hours the FGD system is available
  (whether  operated  or not) divided  by the number of  hours  in  the period,
  expressed as a  percentage.   Operation of the FGD  system is often largely
  outside of  the  system supplier's control.  For this  reason,  flat guaran-
  tees of availability are  rare; an availability guarantee usually contains
  limitations on operating conditions and other factors.

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                                GUARANTEE CHECKLIST
   GENERAL
        Specify EPA requirements  for test port locations.
        Specify stapling procedure.
        Specify analysis procedure.
        Specify data reporting procedure.
        Specify assignment of financial responsibility for sampling and analysis.

   SOj REMOVAL

        Request a guarantee for S02 removal over entire range of scrubber design
            operating conditions.
        Specify EPA test procedures.
        State all conditions  that require compliance testing.

   PARTICIPATE OUTLET LOADING

        Specify that quantity of particulate leaving scrubber system is not to
            exceed the quantity entering scrubber system.

   HIST  IN THE OUTLET GAS STREAM

        Request guarantee for maximum quantity of entrained water droplets leaving
            the Mist eliminators.  Since this is a difficult guarantee to verify,
            adequate design  data should be requested for  use in technical
            evaluation of the design.

   POWER CONSUMPTION

        Request guarantee for FGD system power consumption on a 24-hour
            time-averaged basis.

   REHEAT ENERGY CONSUMPTION

       Request guarantee for maximum energy consumption.
       Specify location and  Methods of energy and gas flow measurement;  specify
            test Interval.

   LIMESTONE CONSUMPTION

       Request guarantee for Ib limestone consumed per Ib S02 removed.
       Specify test method for limestone feed rate.

  WATER CONSUMPTION

       Request guarantee for closed-loop operation:   net  amount of water consumed.

  WASTE STREAMS

        Specify acceptable waste streams (whether plant will use landfill, settling
            pond, or other).
       Request guarantee for limits of wastewater quality.
       Specify the sampling  and analytical methods and level of boiler operation
            at which waste streams will be measured.

  TURNDOWN RATIO

       Request guarantee for ratio of maximum flue gas flow rate to minimum flue
            gas flow rate.
       The ratio should be consistent with the lowest expected steam generator
            load at which power can be produced efficiently.
       Identified by system  as well as Individual scrubber module.
       Request that bidders  state FGO system lag time as  boiler load Increases
            and whether operating conditions Increase in  stepwise or continuous
            manner.

  AVAILABILITY GUARANTEE

       Request demonstration of operation for a specified time period
            (e.g.,  60 days).

       Request demonstration of operation at rated capacity and minimum  load
            capability of FGD system.
Figure 4-6.     Checklist  for  specification  of guarantees
             required  from prospective bidders.
                                       4-18

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PROCUREMENT:  Preparation of Specifications	4-19
should cover all pump requirements, another section all  piping requirements,
and  so  on  for  tanks,  valves, agitators,  ductwork,  and similar  classes  of
equipment.
     The objective of the specifications is to control  the  quality of equip-
ment  and services  without  limiting  the  supplier's  freedom  to  apply  his
expertise.   Specifications  for systems  or subsystems  must  be performance-
oriented.   Where a  component must  interface with  another  .system  or  sub-
system, detailed specifications are needed to ensure that the component does
not prevent the interfacing items from achieving the guaranteed performance.
     Detailed information  is  given in Section 3 regarding the functions  of
FGD  system  equipment and  the preferred materials  of  construction.  That
information  should  be  considered  as the technical basis for formulation  of
the  equipment  specifications.   The  discussion  that follows  considers  the
major equipment items  and  focuses on the means of transmitting the required
technical information  to the  prospective  suppliers.   Guideline  sheets are
given  for  several  of the  major  components,  indicating  what the  utility
should  specify  and what information  the bidder is asked to provide in the
proposal.
     Scrubber.   The chief factors  to be  considered in  specifying require-
ments for the scrubber include the following:
     0    S02 removal
     0    Acceptable scrubber types
     0    Pressure drop
     0    Scaling and plugging
     0    Corrosion/erosion
     0    Maintenance
     Guidelines  for  specifying  the  scrubber  requirements  are  given  in
Table 4-1.
     Provisions that simplify maintenance must be specified because they are
generally extra cost  items that may not be included in competitive bidding.
Repair   of   coatings   or  linings,  for   example,  may  be  a difficult and

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              TABLE 4-1.  SPECIFICATION GUIDELINES:   SCRUBBERS
Utility specifies the following:
     S02 removal requirement
     Acceptable scrubber types
     Acceptable materials of construction
     Acceptable coatings or linings
     Acceptable types of packing materials for mobile beds
     Nozzle materials
     Maintenance provisions
Bidder provides this information:
     Configuration and overall dimensions
     Inlet and outlet dimensions and other internal dimensions as required
     to define cross-sectional areas of various stages
     Materials of construction and thicknesses
     Lining or coating types and manufacturers
     Packing type, size, and material
     Nozzle locations, size, type, and material
     L/G ratio
     Gas pressure drop across scrubber
     Estimated weights empty and for normal operating and flooded conditions
     Detailed maintenance provisions
                                      4-20

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PROCUREMENT:  Preparation of Specifications	    4-21
time-consuming process.   Thus the  use  of highly alloyed  stainless  steels,
though  initially  more  expensive,  may be  justified  in some  applications
because  it  may reduce  maintenance  costs  and the number of  unplanned  shut-
downs.   It  is  essential  that the supplier provide for rapid and easy access
to  the scrubber for  cleanout and  repairs.   There  should  also be  a  drain
system  that allows  complete drainage  of the  scrubber for  inspection  and
maintenance.   Provisions  must  be made  for  removal  of deposits  from  the
scrubber  interior,  including  ready access  as well  as proper  location  of
cleanout doors.
     Mist Eliminator.   The  following factors  must be considered in specify-
ing FGD system mist eliminators:
     0    Droplet and particulate collection efficiencies
     0    Configuration,  bulk separation,  stages, spacing,  and geometry of
          separation devices
     0    Corrosion/erosion
     0    Pressure drop
     0    Structural integrity
     0    Wash system
            Spray pressure
            Top and bottom wash flow rates
            Ratio of fresh to return water
            Number of sprays per unit area
            Intermittent and sequential washing frequencies
            Piping layout
     0    Maintenance features
Associated  factors  to consider  are  the scrubber system design and operating
conditions,  system construction,  scrubbing  medium,  solids content of  the
slurry,  sulfur content  of  the  coal,  and  chloride content of  the  water and
coal.
     Collection  efficiency  is  implicitly specified  when allowable particu-
late emissions are stipulated as part of the design basis requirements.  The
scrubbers must not increase the particulate  load.   Specifying a percentage
elimination efficiency  probably is  impractical because compliance cannot be
readily  determined.  Attempts   to   design  to  a  specified efficiency may

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PROCUREMENT:  Preparation of Specifications	4-22
compromise reliability by a reduction in washability of the mist eliminator,
which could lead to plugging and higher pressure drops.   The supplier should
provide enough  information  on  the proposed design to permit comparison with
units now in operation.
     Where a utility wishes to install vertical mist eliminators (horizontal
flue gas  flow), the  specifications  must clearly so  stipulate.   A vertical
installation probably  will  increase  the initial capital expenditure and may
require more space for ductwork and the scrubber module.
     Flue Gas Reheaters.  The following factors must be considered in speci-
fying FGD system reheating equipment:
     0    Design temperature increment and allowable range
     0    Type of reheat strategy
     0    Energy source or heating medium
     0    Materials of construction (for in-line reheat)
     0    Soot blowing requirements (for in-line reheat)
     Soot Blowers.    Soot blowers  are  required  for removing  deposits  from
ductwork downstream  of the  scrubber and from  the  reheater.   The following
factors must be  considered  in  specifying FGD system soot blowing equipment.
     0    Choice of cleaning medium (compressed air or steam)
     0    Compressor type and receiver capacity, if applicable
     Choice of  the  cleaning medium is determined by economic considerations
because compressed air and steam offer comparable cleaning service.
     Limestone Receiving and Conveying Equipment.   The   following  factors
should be considered  in specifying limestone receiving and conveying equip-
ment:
     0    Primary and backup methods of transporting limestone to the plant-
          site
     0    Relative locations of receiving station, storage piles, and slurry
          preparation area feedbins
     Mechanical conveying systems are used to move limestone.  Specification
guidelines are given in Table 4-2.

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                    TABLE 4-2.  SPECIFICATION GUIDELINES:
                 LIMESTONE RECEIVING AND CONVEYING EQUIPMENT
Utility specifies the following:
     Limestone type, grindability, size range, and analysis
     Conveying rate required
     Conveying distance, elevation change, relative locations
     Type of conveying system
     Emission limits for fugitive emissions from the conveying system;
     type of dust collector
Bidder provides this information:
     Configuration and overall dimensions of major system components
     Manufacturer and model numbers of system components
     Construction details for major mechanical equipment items such as
     conveyors, feeders, and dust collectors
     Power requirements
          TABLE 4-3.  SPECIFICATION GUIDELINES:   LIMESTONE FEEDBINS

Utility specifies the following:
     Required capacity of feedbins in terms of time period of operation or
     other criteria
     Materials of construction and minimum thicknesses
     Configuration criteria such as maximum diameter or maximum height,
     rectangular cross-section, offset hopper construction
Bidder provides this information:
     Configuration and overall dimensions of feedbins
     Materials and thicknesses
                                     4-23

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PROCUREMENT:  Preparation of Specifications	4-24

     Feedbins.   The  following  factors should  be considered  in  specifying
limestone feed bins.
     0    Space available for storage
     0    Configuration of the bins
     0    Feedbin  capacity  considered necessary  to  permit routine  daily
          operating procedures
     Specification guidelines for feed bins are given in Table 4-3.
     Limestone Feeder.   The following factors should be considered in speci-
fying limestone feeding equipment:
     0    Type of  feed  measurement best suited for the system installation:
          volumetric or gravimetric
     0    Required accuracy of measurement of limestone usage
     0    Manual or semiautomatic control
     0    Type  of conveying mechanism  (belt  conveyors,  screw  conveyors,
          oscillating hoppers, or vibrating hoppers)
     0    Provisions for shutdown and necessity for cleaning and maintenance
     Specification guidelines are given in Table 4-4.
     Limestone Ball Mill.  Since  the properties of  limestone  slurry affect
the removal efficiency and economics of the FGD system, specification of the
ball mills is important to the operation of a scrubbing system.   The follow-
ing factors should be considered in specifying limestone ball mills:
     0    Limestone type
     0    Ball mill type
     0    Grinding media
     0    Mean particle size of product
     0    System reliability
     Guidelines  for  specification  of ball  mills are  given in  Table  4-5.
     Thickener and Flocculant Feed.  The  thickener plays an important  role
in water  balance, sludge  characteristics,  and sometimes in chemical  reac-
tions.  The following factors must be considered:

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           TABLE 4-4.  SPECIFICATION GUIDELINES:   LIMESTONE FEEDER

Utility specifies the following:
     Limestone type and size range
     Acceptable type(s) of feeder
     Accuracy requirements for measurement of limestone usage
Bidder provides this information:
     Configuration and overall dimensions of equipment
     Manufacturer, model, and construction features of equipment
     Power requirements
     Control systems
     Maintenance provisions
     Feeder capacity in terms of excess capacity over the anticipated lime-
     stone demand rate or other criteria
         TABLE 4-5.   SPECIFICATION GUIDELINES:   LIMESTONE BALL MILL

Utility specifies the following:
     Limestone type and size range
     Acceptable types of ball mills
     Hours per day of operation
Bidder provides this information:
     Configuration and overall dimensions of equipment
     Manufacturer, model, and construction features of equipment
     Product particle size distribution
     Power requirements
     Reliability of system
     Ball mill capacity in terms of excess capacity over the anticipated
     limestone slurry demand rate              	
                                     4-25

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PROCUREMENT:  Preparation of Specifications	        4-26
     0    Corrosion
     0    Solids concentration
     0    Clarification
     0    Recirculation
     0    Solids removal
     Flocculant is usually  added  to the thickener by a  reciprocating pump,
which can be of a piston or diaphragm type and is invariably associated with
a check valve in the discharge piping.
     Vacuum Filter.  The following  factors  should be considered in specify-
ing a vacuum filter (Knight et al. 1980):
     0    Materials of construction
     0    Drying time
     0    Barometric legs
     0    Filter medium
     0    Final solids content
     A 10-second drying time should be specified to maximize the cake solids
content of  the filter  cake without causing sludge cracking.   To  keep fil-
trate  from  going  through  the  vacuum  pump,   a barometric  leg  should  be
installed to protect  the  pump by trapping  liquid  before  the suction of the
vacuum pump.  The  filter  mediur  should be cheap, durable, and noncorrosive,
and it should allow easy cake discharge.
     Centrifuges.   The  following  factors  should be considered in specifying
a centrifuge for limestone slurry applications.
     0    Materials of construction
     0    Rotational  speed
     0    Conveyor
     0    Final solids content

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PROCUREMENT:   Preparation of Specifications	\	4-27
All  materials  that  contact  liquid  in  the centrifuge  should  be  made  of
corrosion-resistant materials.  The tips  of the conveyor should be made  of
tungsten carbide to reduce abrasive wear.
     Rotational speed should  be midrange,  3000 rpm or less,  to gain some  of
the benefits of high-speed clarification while  preventing excessive  abrasion
and  difficult solids  discharge.   If  centrifuge speeds  are too high, the
conveyor and  the  bowl  will  lock.   The  screw conveyor within  the bowl  should
turn at the  minimum  speed required to remove  solids  without causing  exces-
sive turbulence.  Since the scrubber  caking rates may vary,  a variable-speed
conveyor should be specified.
     Mix Tank or Pug Mill.  The  following  factors  should be  considered  in
specifying a mix tank or a pug mill (Knight et  al.  1980):
     0    Corrosion/erosion
     0    Tank size
     0    Quantity of material
     0    Degree of blending
     0    Additive
In  the  event of  low pH  swings,  the  equipment could be  stopped to prevent
corrosion.    Erosion  is the major specification consideration.   To achieve
proper mixing of the  slurry, high torque agitators  are  needed.  The rapid
movement of  the  slurry and fixation  agents against the tank walls heightens
abrasive action.   For  reduced maintenance, the tank walls and bottom should
be rubber-lined.
     Fixated Sludge Conveying System.   The chief factors to be considered  in
specifying sludge  conveying equipment  are the  type of system (belt or screw
conveyor) and the materials of construction.
     For belt conveying, the weight per cubic foot of material to be handled
should be determined accurately  and  specified  in an "as-handled" condition,
rather than  taken  from published information.   Belts can be quickly damaged
by  high  temperatures; a  high-priced  belt may  prove  most economical  in the
long run.  The  elastomers available  for belt construction include Neoprene,
Teflon or Teflon-coated rubber, Buna-N rubber,  and vinyl rubbers.

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PROCUREMENT:  Preparation of Specifications	4-28
     Capacities of  screw  conveyors  are generally restricted to about 10,000
ftVh.   Serviceable  materials  range  from cast  iron  to stainless  steel.
     Additional information is given by Knight et al.  (1980).
     Additional Equipment Items.   Many  of the other  equipment items  to  be
specified for  the  limestone  FGD  system are common to  most major engineering
installations—pumps, piping,  ductwork,  and the like.  In  specification  of
these  items,  it is  most  important to  define the conditions  of  service  so
that the potential  supplier  has  a firm grasp of the service limits to which
the equipment  will  be exposed.   For a limestone  FGD  system,  a chief objec-
tive is to  provide designs and materials that will  withstand the corrosive/
erosive action  of  gases  or slurries.   The supplier must  be informed of the
range  of pressures  and  temperature  encountered in normal  operation and also
those that could occur during emergency conditions;  the probable duration of
excursion conditions  is  important  also.   These conditions must  be clearly
specified in the design  basis presentation.   Some considerations for speci-
fication of these  additional  equipment  items  are  discussed  briefly  here;
detailed information on design and materials is given  in Section 3.
     Specification  of pumps  and  piping  depends largely  upon the  type  of
service.   Equipment that  handles  slurry transfer and  makeup  must withstand
erosive conditions but need not be acid-resistant.   Slurry recirculation and
discharge service  is  the  most severe;  service involving reclaimed water and
fresh water makeup generally is not severe.  All  pumps, however, should have
capability for remote start/stop and manual local start/stop.
     Valves are used for  isolation  and control functions  within the system,
but  because of potential  plugging and  mechanical  problems their  number
should be limited.   Rubber-lined  valves  are the most  common,  although many
stainless steel valves are  used;  functional  problems  are  seldom  related  to
materials.
     As with other equipment, specifications for tanks are determined by the
service function.    The EHT must  allow adequate retention  time (typically 8
to 10 minutes) for completion of the slurry reaction with  the S02.  Specifi-
cations for the thickener  overflow  tank should allow  for  some system swings
in the demand for  recycle water.

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PROCUREMENT:  Evaluation of Proposals	4-29
     A principal factor  in  specification  of ductwork is compatability among
all parts  of the  system,  regardless of supplier.  The  maximum  permissible
gas velocity  through the ductwork  should be  specified;  typically this  is
3300 to 3600  ft/min.   The  minimum permissible thickness of ductwork materi-
als in a  given  application  must be consistent with that of the remainder of
the unit.   Typical  ductwork thickness ranges from 3/16 to 1/4 inch.   Accept-
able  materials  for  ductwork in  a  limestone  FGD  system are discussed  in
Section 3 (Materials of Construction).
     Specifications for  dampers  should  emphasize the importance of mechani-
cal design provisions  that will minimize  fly ash deposition.   Mechanical
problems with  dampers caused by deposition of  solids have  outweighed  any
problems attributable to materials.
     In  specification  of  expansion  joints  the  utility  should  provide
information with which the supplier can determine the magnitude and types of
movements  that  the  joints  must accommodate.   Maximum and  minimum ambient
temperatures are important,  along with operating and excursion temperatures.
     The chief factors to be considered in specifying system instrumentation
are the overall  control philosphy, the parameters to be measured or used for
control functions,  the acceptable types of instrumentation for each service,
and the degree of redundancy required for reliable operation.

EVALUATION OF PROPOSALS
     The proposal  evaluation  process is considered in terms  of  three cate-
gories:  technical, commercial,  and economic evaluation.   A detailed example
evaluation  of a  proposal for an FGD system  on a hypothetical 500-MW power-
plant is given by Smith et al. (1980a).
Technical  Evaluation
     The technical  portion  of  each supplier's  proposal  should  be reviewed
thoroughly,  mainly to  determine compliance  with the  specification.   When
noncompliance is  indicated, the  supplier  must  be  contacted for clarifica-
tion.

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PROCUREMENT:  Evaluation of Proposals	4-30
     The  important  technical  information  supplied  by each  manufacturer on
the equipment data  sheets  should be summarized in  tabular  form for ease of
comparison.   Table  4-6  is an  example  of a  technical  data  summary  sheet.
     Most of the data furnished are used for determining compliance with the
specification.  Each proposal  is compared with the specifications, and price
additions  or  credits are  applied  where the systems are deficient or over-
designed.   Examples of  deviation  from  specifications  would be  changes in
materials of  construction  or  failure to supply required dampers.   Where the
proposal  is otherwise  adequate,  the reason for such deviations is sometimes
ascertained by contact with the supplier.
     The  technical  expertise  and construction experience of the prospective
suppliers should be evaluated and  compared.   Obvious indexes  of  the  credi-
bility of  potential  system suppliers are the numbers of  limestone FGD sys-
tems each  has placed in operation, those  under construction,  and those for
which contracts have been awarded.
Commercial Evaluation
     Each bidder may take numerous exceptions to the commercial terms  of the
specifications,  such as terms  of payment and escalation  factors.  Any  excep-
tions should  be clearly stated  in  terms  compatible with  those  specified.
For comparison purposes, a summary of significant commercial terms specified
and those offered  by  each bidder  should  be tabulated.  A commercial  data
summary sheet is shown in Table 4-7.
     In addition to the key items shown in the summary sheet, the commercial
evaluation  should  consider in  a qualitative way  the extent  to  which each
bidder defines  responsibilities  for various  aspects of  the contracted work.
Performance guarantees, for example, should not only stipulate the guarantee
period and  completion dates  for performance tests,  but also  should cover
contingencies,  e.g.,  procedures to  be  followed if  the  initial performance
tests are unsatisfactory,  and  at  whose expense  additional tests  would be
run.  Details of  shipping  procedures should be clear-cut:   who pays  trans-
portation  charges,  what is the  F.O.B.   site,  and so on.   Project staffing
proposed  by the supplier  should be identified by function  if  not by name:
technical service rep, erection supervisor, project engineers.

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                                     TABLE  4-6.   TECHNICAL DATA SUMMARY  SHEET

Scrubber AP, in. H20
Through scrubbing stage
Through total all scrubber elements
Superficial velocity, ft/s
Through scrubber
Through mist eliminator
Water droplet carryover past mist eliminator, Ib/h
Overal 1 system S02 removal , percent
Slurry recycle system
Liquid to gas ratio-scrubber,3 gal/min per 1000 acfm
Slurry recycle-scrubber, gal/min
Solids in recycle slurry, percent
Scrubber tank retention time, min
Additive system
Limestone additive, Ib/h
Limestone grind, maximum particle size
Limestone purity, percent
Stoichiometric feed rate (based on S02 removal), ratio
Solids in additive slurry, percent
Additive slurry flow rate, gal/min
Bidders:
A
















B
















C




	










D
















I
CO
       (continued)

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I
CO
ro

Freshwater requirements, gal/ml n
Additive system freshwater
Scrubber makeup freshwater
Vacuum filter freshwater
Mist eliminator wash freshwater
Pump seal freshwater
Total freshwater
Sludge disposal system
Waste slurry from scrubbers, gal/min
Solids in waste slurry, wt. percent
Underflow from thickener, gal/min
Solids in thickener underflow, wt. percent
Filter cake production, dry tons/h
Solids in filter cake, wt. percent
Structural factors
Overall dimensions of each module, ft
Shell material
Mist eliminator material
Temperature increase after reheat, °F (AT)
Heating tube material
Distance between uppermost spray bank and bottom
of mist eliminator, ft
Distance between top of final stage of mist eliminators
and the bottom of the reheater, ft
Bidders:
A

	

















B

	

















C

	












	



D

	

















       Based on saturated flue gas conditions at scrubber outlet.

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                                  TABLE 4-7.  COMMERCIAL DATA SUMMARY SHEET

Payment Terms -- Material and Shop Labor
Percentage paid each month for work performed
Percentage paid after completion of the initial
performance guarantee test
Percentage paid upon satisfactory completion of the
1-year performance guarantee test
Payment Terms - Erection
Percentage paid upon completion of work
Percentage paid upon completion of performance
guarantee tests
Escalation
Portion of material, erection, and labor subject to
escalation
Time period of escalation
Value of each index or the date of the base value of
each index
Bidders:
A









B









C









D









OJ

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PROCUREMENT:  Evaluation of Proposals	4-34
     Delays  in  the  project schedule,  whether caused  by the  utility,  the
supplier,  or other factors, can  lead to substantial expense;  likewise,  as
the project  develops,  certain  work not specified in the contract may become
desirable  or  necessary.   Although  both purchaser and supplier may be reluc-
tant to  make firm commitments  regarding such unknown quantities,  each sup-
plier should address such eventualities as a basis for commercial evaluation
of his proposal.
Economic Evaluation
     Economics  is  a  key element in the evaluation of bids.   Various capital
and operating costs  must be assessed.   One proposal may indicate the lowest
operating  costs  and  another,  the  lowest capital  costs.   A  systematic
approach for evaluating the overall  economic  impact of each proposal  is a
necessity.    The FGD suppliers  should  be provided  with a  list of economic
evaluation factors.
     Capital Investment.  So that  the capital  investment required for each
FGD  system proposal  can be  compared  on  an  equal basis,   the as-received
proposal  prices  for  equipment,  materials, and  erection  are   adjusted  by
adding the following:
     0    Technical cost adjustments
     0    Balance of plant costs
     0    Commercial cost adjustments
     Technical  cost adjustments are  price additions or credits  applied to a
proposal when the systems are technically deficient or overdesigned relative
to the intent of the specifications.
     Balance of plant  costs  are equipment and material  costs that are out-
side of  the manufacturer's  scope  of  supply but  that would be incurred  in
completing the proposed system; therefore, these  costs must  be included in a
comparison  of  prices.   The following  are several  examples of  balance  of
plant costs:
     0    Cost  of material  and labor  for  concrete foundations  and  piling
     0    Erected cost  of  structural  steel  for scrubber module and building
          support, platforms and stairs, and wall  panel  and  roofing material

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PROCUREMENT:   Evaluation of Proposals	4-35
     0    Cost  additions   or  deductions  to  provide  ductwork  to  selected
          reference points on the inlet and outlet of  the  system
     0    Cost additions  for  installed electric wiring,  conduit,  starters,
          cable  trays,  circuit breakers,  and other miscellaneous  electric
          equipment
     0    Cost additions  for  a  comparatively larger induced-draft fan  as  a
          result of greater system static pressure
     The  commercial  costs  calculated  for  each proposal  are  based on the
supplier's terms of payment,  the price adjustment policy, and  the  economic
criteria given in the specifications.  Commercial  costs  include  the costs of
escalation and interest during construction.   The  escalation  should be  based
on each bidder's delivery and erection schedule,  if one  is  given,  or  on an
estimated schedule.  The  escalation  factor should incorporate  the  value of
each  index  or  the date  of  the base  value  of each  index  that the bidder
proposes to use.   The  escalation is  calculated from the base proposal  price
plus  all  differential  technical  adjustment  and  balance of  plant costs.
Interest during  construction  should be  based on  the  escalated bid price.
The interest  is  calculated from the  date of  payment  to the  date of commer-
cial operation.
     Annual  Operating Costs.    Operating   costs   are    levelized   over  the
expected plant  life and  then capitalized for  use in  the evaluation.   The
operating costs include the costs of  additive, air, water, electrical demand
and energy (recirculation pumps,  ball  mills,  and  miscellaneous  pumps), and
sludge disposal.   The  economic  criteria used for this  evaluation  should be
clearly stated  in  the  bid specification so that the bidders  are fully  aware
of the  economic penalties  associated  with the operation of their systems.
This will  enable the bidders to optimize their proposed  systems  and make key
decisions regarding  the tradeoffs associated  with capital versus  operating
costs.
     Comparisons of  operating costs are  normally based  on  the information
provided by the bidders in their proposals.  Suppliers vary,  however,  in the
degree  of conservatism  applied  in  estimating operating cost  parameters.
Therefore, it is  prudent for  the  utility  Project  Manager or  consulting

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PROCUREMENT:  Installation, Startup. Testing	4-36
engineer to  carefully  review,  and if necessary adjust,  the  indicated oper-
ating costs to ensure that they are reasonable.
     Total Evaluated Costs.  The  total  evaluated costs  of  the  FGD  system
should  include the  total  evaluated bid price (technical and  commercial cost
adjustments),  the balance  of  plant costs,  and the differential  operating
costs.
Selection of Successful Bidder
     Selection of the  FGO system  supplier is based on  more  than economics.
It  should  be recognized  that  many factors that contribute  to  a successful
FGO installation can be assigned no specific economic  (dollar) value.   These
are  such items  as  effluent hold tank  retention  time, carryover  of water
droplets through  the  mist eliminator,  and overall system  layout.   Although
such  factors  are difficult  to evaluate economically,  they  should be con-
sidered  carefully  after the economic  advantages  and disadvantages of each
offering are assessed.  Selection of the "best" of the  proposed FGD instal-
lations  is  also  based  on system operation  and maintenance, on the  stated
basis of guarantees,  and  on schedule considerations.    Because  of  these and
other factors significant  to success of the overall project, the economics
are not  necessarily controlling but must be factored into a broader  judge-
mental process.
     Before  a  contract  is  awarded,  all  of  the  technical  and  commercial
exceptions  indicated   by   the  successful  bidder  should be clarified  and
resolved.

INSTALLATION, STARTUP, AND TESTING
     Following the  award  of contracts,  the utility project team  will work
closely with suppliers  on  detailed engineering  design,  fabrication, instal-
lation,  startup,  and  performance  testing—all  of these  activities  being
prerequisite to  operation of  the FGD system.   Good lines of communication
must be established with the system supplier and the A/E consultant.
     The  supplier  provides  engineering  information   for   the  design  and
purchase of  interfacing auxiliary equipment.   Fabrication and erection must
be  closely  monitored  to  ensure  that the  installed equipment conforms with

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PROCUREMENT:  Installation. Startup. Testing	4-37

specifications.  Performance testing  is  conducted to demonstrate compliance

with  applicable  flue gas  emission  regulations and to demonstrate  that the

equipment  meets  the  performance guarantees.   During startup and  initial

operation  of  the system,  procedures are established  for  operator training,

recordkeeping, and maintenance.   Particularly during early operation, system

reliability  must  be  monitored  to  confirm  satisfaction  of  contractual

performance requirements.   Again, it  is  emphasized that  communication among

all parties concerned is essential from preliminary design through operation
of the system.

Engineering Data

     The supplier should  be  required  to provide as a minimum the following
engineering data:

     0    Complete,  detailed mass balance  diagrams for maximum capacity and
          average operating capacity,  as well  as specified intermediate and
          lower  load conditions.  These  conditions should include the mini-
          mum  expected  percentage of steam generator  capacity.   These data
          would  support  detailed  engineering  analysis   of  such items  as
          minimum acceptable flow velocities in slurry pipelines.

     0    Scrubber performance  curves.   These curves  should  include super-
          ficial   gas  velocity  through  the  scrubber  and mist  eliminators
          versus   load.    They  should  indicate  the  recommended  points  of
          changeover to increase or decrease the number of modules in opera-
          tion,  L/G versus  S02  removal  efficiency,  expected S02  removal
          efficiency versus coal  sulfur  content,  and S02  removal  efficiency
          versus  boiler load.  The  supplier should provide a chart relating
          modules and  pumps  in  service  to boiler  load and  sulfur content.

     0    Pump characteristic  curves  for  each  pump furnished  under  the
          specifications.   The  curves  should indicate head,  power require-
          ments,  efficiency, and net positive suction head  required versus
          capacity.

     0    Fan  performance  curves  for  all fans.   The  curves  should indicate
          static  pressure,  power requirements,  fan efficiency,  and system
          resistance to gas flow versus capacity.

System Installation

     Installation of the FGD system, one of the major power plant systems to

be constructed,  should be managed  as  part of the  overall power  plant con-

struction activities.   Written procedures  for system installation should be

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PROCUREMENT:  Installation. Startup. Testing	4-38
prepared In advance of actual Installation and should be used to control the
project  field  organization, project documentation, and  lines  of communica-
tion  with  the  utility and FGD system  construction  contractor.  The  pro-
cedures should include standard forms for use in control  of activities.   The
following are some of the major requirements for effective management of FGD
system installation at a large utility power plant:
     0    Resident  FGD system  management and  field engineering  personnel
     0    Interface with other system construction contractors
     0    Management of startup
     0    Contract administration of the  construction contract and material
          supply contracts
     0    Management of inventory control, storage, and maintenance of mate-
          rial  and equipment
     0    Document control to support field management operations
     0    A quality assurance program to ensure effectiveness and compliance
          with design specification
     0    Progress and status reports to utility management
System Startup
     The FGD system  supplier must  be informed of  the overall  power genera-
tion project schedule  and of all other interfaces  between  his work and the
work of  other  contractors on site.   A detailed schedule should be developed
for  FGD  system erection  and subsystem shakedown.  This  schedule  should be
followed from the start of construction activities.
     Weekly and monthly progress reports  should include information regard-
ing  the  labor  force,  weather,  conference  memorandums,  effects  of  change
orders,  and  other significant  information related to progress  of the  con-
struction.   Also  essential  are  inspection  of  FGD equipment  and  materials
received at  the project  site,  and followup activities such  as  handling of
loss claims and correction of manufacturing errors.
     Well-planned startup and shakedown of the major FGD subsystems and com-
ponents  can  minimize any  adverse  impacts of construction  errors  or design

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PROCUREMENT:  Installation. Startup.  Testing	4-39
deficiencies.   Representatives  of the  major subsystem equipment  suppliers
should be present during initial startup of their equipment.
Performance Testing
     Sampling and  analysis of  the flue gas stream are performed  to  demon-
strate compliance  with emission standards and with performance  guarantees.
Emission compliance  testing consists of measurements for  concentrations  of
S02, particulate,  and  nitrogen  oxides.   Testing  of the FGD  system perform-
ance will depend upon  the  guarantees established in the specifications and
in  contract negotiations with the equipment vendor.  This testing commonly
consists of measurements  of  S02  removal  efficiency,  particulate  emission
rate and/or removal  efficiency,  and  scrubber additive usage;  it may  also
include such items  as flue  gas stream pressure loss, water  usage, and  sludge
generation rate.
     The purpose of  the tests  determines to  some  degree  the selection  of
sampling locations.  The EPA  provides  guidelines for  location of gas  stream
measurements.  These  should be considered prior to construction of the power
generation unit and the FGD system.
     A written test protocol should describe clearly the  responsibilities of
all participants, the operational requirements for the  power  generation unit
and FGD system,  and methods to be used in testing and  analysis.   An unbiased
statistical  method should  be  used to reject poor data.   The  protocol  should
identify the goals  of the test program so that the end  uses of the test data
are well  established.    This  is  especially important in  a   combination  of
tests to determine system  performance  and to demonstrate  regulatory compli-
ance.
     One person should coordinate all field testing activities.   This  person
can thus coordinate  operation of the boiler and FGD system with the testing
schedule.
     All tests  should  be performed at  steady-state boiler load  conditions.
Load should  be  stabilized  for at least an  hour  before testing begins.   The
FGD system operations should be maintained at constant conditions during the
tests,  and  for  gas stream  sampling the FGD  system  operation also should be
stabilized for at least 1 hour before the test.  For sampling of the liquid/

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PROCUREMENT:  Installation. Startup. Testing	4-40
slurry streams,  a  much longer stabilization period may be required to reach
steady-state  conditions.   Testing  should be  interrupted during  any major
upsets of the boiler or FGD system.   Testing need not be interrupted because
of minor deviations from specified conditions because most of the gas stream
measurements are time-averaged.
     Sulfur  dioxide  removal  efficiency  is  determined  by  simultaneously
measuring S02 concentrations at the inlet and outlet of the scrubber system.
Similarly,  particulate  removal  efficiency  is  determined  by. particulate
measurements  at the  inlet  and  outlet  of  the  particulate  removal  system,
which may  also  be  the S02 scrubber.   Where a separate  particulate removal
system is  located  upstream of the scrubber, particulate measurements at the
outlet of  the scrubber may  be required  to ensure that the  scrubber is not
generating particulate.
     Limestone  utilization  or stoichiometric ratio  may  be guaranteed  at
designated emission performance levels, and confirming performance tests are
required.   Limestone  stoichiometry  is  defined  and  described  fully  in
Appendix  A.    The   limestone  stoichiometric  ratio  may  be  calculated  from
measurements  of  the limestone  feed rate and the S02  removal  rate.   Deter-
mination of  limestone  stoichiometries  by performance testing has been found
to be only  ±10 to  20 percent  accurate because of inherent discrepancies in
flow measurement of liquid/slurry  and  gaseous  streams  and because  of the
resulting inaccuracy of sampling techniques.
     The  limestone stoichiometric  ratio can also  be measured  directly by
chemical  analyses  of the  molar ratio of  total sulfur to calcium in the dry
scrubber sludge  and  by analyses of the ratio of  carbonate to calcium in the
sludge.    Because  composition of  the  solids is  time-averaged  over  long
periods  and  can be  determined accurately, chemical  analysis of the sludge
solids provides the best means of establishing limestone utilization.  These
chemical  analyses  also  provide a valuable check  on the measured S02 removal
efficiency  when used  in  conjunction with  data  on  limestone  feed  and S02
concentration of the incoming flue gas.
     Confirming  performance tests of  makeup water usage may also  be needed
when usage  is guaranteed  at designated  emission  performance levels.   Water
usage by the  scrubber  is  determined by measuring selected process stream

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PROCUREMENT:  Installation. Startup.  Testing	4-41
flow  rates  and densities  and  performing a water balance  determination.   A
good  water  mass  balance must  satisfy  the requirement  that  the amount  of
water  in  the incoming flue  gas stream,  the limestone slurry feed,  arid the
makeup water streams, together with any accumulation in the scrubber system,
equals the  amount  of water in the output streams,  consisting  of moisture in
the flue gas outlet and in the scrubber sludge.

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PROCUREMENT:  References                                           	4-42
                          REFERENCES FOR SECTION 4
Knight,  R.  G., E.  H.  Rothfuss,  K.  D.  Yard, and  D.  M.  Golden.  1980.   FGD
Sludge Disposal Manual, 2d ed.  EPRI-CS 1515.

Smith, E. 0.,  W.  E. Morgan,  J. W.  Noland, R. T.  Quinlan, J.  E.  Stresewski,
D. 0.  Swenson,  and C.  E. Dene.  1980s.  Lime FGD  System and  Sludge Disposal
Case Study.   EPRI CS-1631.

Smith, M.,  M.  Melia,  N.  Gregory,  and M.  Groeber.   1980b.   EPA  Utility  FGD
Survey:   July-September 1980.  NTIS No.  PB  81-142655.   EPA-600/7-80-029d.

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                                  SECTION 5
                          OPERATION AND MAINTENANCE

     The mark  of  a successful scrubber installation is  reliable  operation.
Achieving  reliability  in  the chemical  processes involved  in a  limestone
scrubber may present  new and unfamiliar operating and maintenance concepts
for  a utility  staff.   Even  though  redundancy has  been designed  into  the
system, poor operational  practices  and inadequate maintenance could lead to
reduced  unit  output or  could curtail  power production entirely.   Fortun-
ately, together with  the improvements incorporated  into scrubber  design in
recent years,  the experience gained  by utility personnel in  operation  and
maintenance of  limestone FGD units  can further  enhance  system performance
and availability.
     The roles of operating and  maintenance personnel must be clearly delin-
eated, but  with  enough flexibility to optimize overall  system performance.
Operators should be able to perform the more basic maintenance so that down-
time  is  minimized.    The  maintenance  staff  must  be equipped to  respond
rapidly to  component failure.   Just as the design and procurement phases of
the project focus  on  system operability and maintainability, so the goal of
the  operation  and maintenance  functions   is  to  achieve  a  high   level  of
scrubber availability for the life of the unit.
     This  section addresses  the operational  and maintenance requirements
associated  with a limestone FGD system, including standard  operating prac-
tices, routine  startup and  shutdown,  and operating modes for system upset
conditions.  The  discussion  deals  with the major components  of the gaseous
and liquid flow paths.   The size, duties, and training needs  of an operating
crew are reviewed.
     Maintenance  practices  are  reviewed  in  detail.    Even  with a  well-
executed preventive  maintenance  program,   unscheduled  maintenance will  be
                                     5-1

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OPERATION/MAINTENANCE:  Standard Operations	5-2
needed.  Familiarity of the staff with typical malfunctions, proven trouble-
shooting techniques,  and spare  parts  requirements will  reduce  maintenance
time and will  improve the reliability and availability of  the  system.   The
requirements for maintenance personnel, in terms of numbers, duties, experi-
ence level, and training are also discussed.

STANDARD OPERATIONS
     Planning of system  operations  begins when the system is in the concep-
tual design stage and continues through procurement and construction.   Since
the goal  is to achieve  a high degree of reliability  and  availability,  the
equipment and the controls must be arranged in a manner easily understood by
the operating staff.
     With stringent requirements on plant emissions, the utility must make a
strong  commitment  to  scrubber  operations,   including  adequate  staffing.
Operators should be  assigned  specifically and solely to the scrubber system
during  each shift.   Considerations of  scrubber  operation must  be  incor-
porated  into'the unit's  power generation schedules and even into the  pur-
chasing of coal.
     Many of the first-generation FGD installations were required to control
S02 from widely  varying  unit loads (cycling and peak), with different coals
(low-sulfur  western,  high-sulfur  eastern, and blends).    Often,   too,  the
control  systems  in the  early installations demanded  a response  beyond the
capability  of  the  FGD  system.   Resulting variations  in the reagent  feed
rate,   loss  of  chemical  control,  and many chemical  and mechanical problems
caused  numerous  forced outages and low reliabilities.  Building  on experi-
ence gained in operation  of  those first-generation  systems,  suppliers and
designers are  now providing  better design configurations  and  construction
materials.
     Among  several  general  tendencies  in recent FGD  system  designs  are an
increased  degree  of  flexibility  and reliability.   Specifically,  design
trends  that increase  availability  are toward the use of  spare  modules and
spare  ancillary components,  and away from the use of interdependent systems
(i.e.,  systems in  which  major unit operations  are  affected by  difficulties
in upstream components).

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OPERATION/MAINTENANCE:   Standard Operations	5-3
     Some of the  current  difficulties  with limestone FGD systems  relate  to
poor  operating  practices,  unnecessarily  complex  operating procedures,  or
both.  In some  cases,  although the equipment has been correctly  installed,
it rapidly deteriorates and  breaks down.   The user blames  the  supplier for
selling inferior and poorly designed equipment;  the supplier blames the user
for  improper  operating practices; and both  may well be  right.   Both  can
benefit  from  the  operating  experience with  similar  units.  The  operating
characteristics of a limestone  FGD  scrubber can be established  during the
initial startup period, which  is also a time for  finalizing operating pro-
cedures and staff training.
     Operation  at steady-state  conditions is  the goal  of every scrubber
designer  and  operator.   The  system  must be  monitored and controlled  to
ensure proper performance, even at steady state.   During  periods of changing
load  or  variation  of  any  system  parameter,   additional  monitoring  is
required.  The  roles of variables and of components in system operation are
discussed in the following sections.
Varying Inlet S02 and Boiler Load
     The number of scrubber  modules  in service is  governed by  the flue gas
flow  rate.   As  boiler load  is increased, additional modules are  placed in
service and, conversely,  modules are removed from  service  when boiler load
is reduced.  With  each change  in load, the operator must check the scrubber
to verify that all in-service modules are operating in a  balanced condition.
     Although the  changes are  not as  rapid as boiler load changes,  system
operating parameters may  vary with time.   For example,  the  S02 concentration
in the  inlet flue  gas  may  change  because of variations in the  coal.   The
scrubber  system  should   be  able  to  accommodate  and compensate for  such
changes.   Operator surveillance of system performance is  needed, however, to
verify proper system response.   In suitably designed systems,  pumps  can be
added  and  removed from   service  as  the S02  concentration   increases  or
decreases.
     Effective control  of the  FGD system requires communication between FGD
operators  and  boiler  operators.   It  is  important  that   the FGD  system
operator be aware of impending load changes.   The limestone feed rate should

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OPERATION/MAINTENANCE:   Standard Operations	        5-4
be reset according to these changes, and the operator should not rely soUiy
on the pH controller, which responds slowly.
Verification of Flow Rates
     The operating  staff  should routinely monitor and record  readings  from
all  instruments  used  to  measure  flow  of the  different  process  streams.
Deviations  from  anticipated values can  indicate potential  problems,  either
in the  scrubbing  system or with specific instruments.   As  the  only means of
obtaining performance  data,  the routine monitoring of instrumentation  is a
principal task  of the  operator.   On the basis of previous  experience,  the
operator may  interpret the  data  or may recognize the need  for unscheduled
maintenance,  the operator  should  keep in mind that  steady-state conditions
may well  fluctuate  with time in a manner that does  not affect the overall
performance of the system.
     The easiest method of  verifying liquid flow rates or  evaluating pump/
nozzle erosion is for an operator to determine the discharge pressure in the
scrubber  slurry  recirculation  header  with  a  hand-held  pressure  gauge.
(Permanently  mounted pressure  gauges frequently  plug  in  slurry  service.)
The  discharge pressure of  the  recirculation pumps should be  determined
manually on a periodic basis.   An  increase  in  discharge  pressure usually
indicates plugging of  some  spray  nozzles.  A decrease in discharge pressure
indicates wear of either the spray nozzle orifices or the pump  impellers, or
both, in which case maintenance work should be scheduled.
     Flow  in  slurry piping  can be  checked  by touching the pipe.   If  the
piping  is cold to  the  touch at the  normal  operating temperature of 125° to
130°F, then the line is plugged.  It is difficult to  verify flow in individ-
ual slurry  pumps  discharging to a common  manifold.  Where  slurry  pumps are
piped  in parallel,   an  operating  pump can be used  to  backflush  a plugged
pump.   With the  plugged pump shut down,  and  the  pump discharge valve open,
the pump suction valve should  be slowly opened.  Pressure  from  the common
discharge  manifold   then  forces slurry  backwards  through  the  plugged  pump
until it spins free  in the backward direction.  The shaft torque will be the
same as in  the driving mode so that threaded impellers should not unscrew if
this procedure is followed correctly.

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OPERATION/MAINTENANCE:   Standard Operations	5-5
     Unplugging long runs of slurry lines is difficult.   Overnight pressuri-
zation of  lines  from  one end is sometimes  effective when  lines  are plugged
with  slurry but  not  with  scale.   This  method  is  especially useful  when
someone is  available throughout  each  shift to rap on the  pipe or otherwise
provide the vibration  needed to help reslurry thixotropic material.   Water
from  a high-pressure  fire  protection  system can  also be  used to  unplug
slurry lines.  Dismantling or replacing the piping is  usually a last resort.
Surveillance of Scrubber Operations
     Visual  inspection of the  scrubber section and hold tanks can  identify
scaling, corrosion, or erosion  before they impact the generation  output  of
the unit.   Visual  observation  can  identify leaks, accumulation of liquid  or
scale  around  process  piping,  or  discoloration  on  the  ductwork  surface
resulting  from  inadequate  or  deteriorated  lining material.   When  these
checks  are  coupled with timely  maintenance, many  costly  repairs may  be
avoided.   Thus,  routine  surveillance  of  the scrubber  system is needed  to
spot potential  problems in time to implement corrective  action.
     The scrubber  module  must  be designed to withstand  the  demands imposed1
on  it,  chiefly high temperatures  and erosive/corrosive environments.  The
site-specific combination of the  projected demands governs the selection  of
materials  of construction.   Carbon steel,  alloys, and all  types  of coatings
or  liners  are  subject to deterioration or scale  formation.   System opera-
tions  must  be  conducted in accordance  with the  design.   For example,  to
equalize the effects of erosion, all modules should be  subjected  to an equal
number  of  operating hours.   Combining such operating practices with timely
maintenance will maximize the service life of the overall limestone scrubber
system.  Station  outages should be used as  an  opportunity  for  maintenance
personnel to enter the scrubber modules for a visual  inspection.
Mist Eliminators
     In  limestone  FGD   scrubbers,  mist eliminators  have been  subject  to
buildup of slurry  solids and  chemical  scale in the narrow  passages  with a
resulting restriction of gas flow.   When scaling occurs  in  an FGD system,  it
is  usually  noticed first in the mist eliminator  as an  increase  in pressure

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OPERATION/MAINTENANCE:  Standard Operations	5-6
drop.  Continued  uncontrolled  growth  could lead to shutdown of the scrubber
module.  Scaling  may  be tolerated for as  long  at-  6 months at one installa-
tion, whereas at another it may cause problems after 1 week.
     Many  techniques   have been  employed  to  improve  mist  collection  and
minimize operational  problems.   The  mist eliminator can  be washed with  a
spray  of  process makeup water or a  mixture of makeup water  and thickener
overflow water.   Most  limestone  FGD  systems are designed with  a two-stage
mist eliminator,  which  allows  more washing on the  first  stage.   Washing of
both sides  is recommended; washing  may  be  continuous  or intermittent,  or
both.  Intermittent washing with  a  high  liquid  flow rate  may  remove hard
scale;  continuous  washing  is  necessary  to limit scale buildup.   An inter-
mittent wash sequence  may be based on a number of factors:
     1.   Time interval and unit load maintained since the last wash.
     2.   Pressure drop across the mist eliminators.
     3.   System  liquid inventory level  (too  much washing  can  cause  water
          balance problems).
     4.   Experience with a specific mist eliminator design.
     In addition to the mist eliminator design features discussed in Section
3,  several  specific  operating procedures  affect  the  performance  of mist
eliminators.  The range of flue gas flow rates over which a module operates
is one example.   If a scrubber is operated below, or even above, the design
L/G  range,  problems  are  likely.   For  these  reasons, operations  must  be
conducted within the design ranges.
     Successful,  long-term  operation   without   mist  eliminator  plugging
generally  requires  continuous  operator  surveillance,  both  to  check  the
differential  pressure  across  the  mist eliminator  section and  to visually
inspect the appearance of blade surfaces.
Flue Gas  Reheat
     In-line reheaters  are  frequently used for flue gas reheat;  these units
can be subject to corrosion by chlorides and sulfates.  Plugging and deposi-
tion can  also occur, but are more rare.   Usually, proper use of soot blowers
prevents  these problems.   Soot  blowing may not remove hard scale, but if it

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OPERATION/MAINTENANCE:   Standard Operations	  5-7
is done frequently  enough  the blowing may prevent deposition of hard scale.
Generally,  soot  blowing once every 4  hours  with air or steam  is  adequate.
Fans. Ductwork, and Chimney
     Fly ash erosion and  deposition  on the fan  blades  are the  major opera-
tional problems with dry  and wet fans, respectively.   These  conditions can
cause vibration and  high  torque,  leading to excessive  noise, bearing fail-
ure, and rotor cracking.
     Extensive use has  been made of ductwork and chimneys lined  with refrac-
tory  that  is  coated  with  various  materials  or  fabricated from  alloys.
Essentially all materials have been subject to acid attack when  the scrubber
was operated outside of the design range.
     Severe problems have  occurred  in  operation of both louver  and guillo-
tine dampers.   Typical  problems are corrosion, erosion,  fly ash  buildup that
prevents  opening  and  closing,  and  mechanical   failure.   The most serious
deficiency  is  that  all  dampers  leak,  and zero  leakage  can be achieved only
by using  double dampers with a barrier of higher-pressure air between them.
Even with  this design,  however, dampers may still  leak in a dirty environ-
ment.  When leakage  is  severe,  it may  be  necessary to  shut down the boiler
to allow entry into the FGD system for maintenance.
Limestone Receiving, Storage, and Slurry Preparation
     Operational  procedures  associated with  handling  and  storage  of lime-
stone are  similar  to those of  coal handling.   At some  plants,  the capacity
for  fugitive dust  collection in conveyor enclosures has been undersized, so
that escaping  dust  presents  a potential safety hazard for operating person-
nel.  The  ball mill  used for limestone grinding and slurry preparation must
be vented.  Operation of pumps, valves, and piping in the slurry preparation
equipment is similar to that in other slurry service.
Limestone Slurry Feed Control
     Slurry feed requirements  are generally determined from the pH level of
the  EHT,  slurry  recycle line, or scrubber blowdown  stream.   Frequent back-
flushing  of sensor  lines  where flow-through pH elements  are used and fre-
quent  calibration   with   buffer   solutions  are  routine  requirements for

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OPERATION/MAINTENANCE:  Standard Operations	5-8
reliable operation.   One method for ascertaining  instrument  operability is
to  cross-check measured  values with readings  on redundant  instruments or
with values  determined  from analysis of grab samples.   These readings must
be made  immediately,  with a temperature-compensating pH meter.  Delay would
allow the  system  pH to rise because of the presence of unreacted limestone.
Another method  is  to correlate slurry pH, reagent feed rate,  and outlet S02
concentration during the initial system tests.   Then, in routine operations,
the three  variables can be compared to detect  a  malfunction  in any instru-
ment.
     A recent study at the Shawnee test facility was undertaken to develop a
reliable field  laboratory method  for  determining the pH  of  slurry liquor.
The method was to be applied in a series of short-term tests in which condi-
tions were changed every  6 to 8  hours.   In these  tests the pH was  to be
controlled to  within ±0.2  pH unit by  laboratory measurement.  The  study
report (Bechtel 1976) recommends the following:
     1.    Use  of  easily  cleanable  electrodes  with glass-to-glass  seals.
     2.    Use of commercially  available  certified buffers for standardizing
          the pH meters.   Buffer  pH should be within 0.5 unit of system pH.
     3.    Storage of  the  glass electrodes in hot (50°C) buffer (pH 4) satu-
          rated with KC1 when not in use.
     4.    Air  conditioning  of the  field  laboratory to  safeguard the pH
          meters.
     5.    Changing of electrodes at the beginning of each shift.
     6.    Frequent  monitoring  of  the agreement  between values obtained with
          the laboratory and the in-line pH meters, with action to be initi-
          ated to correct any disagreement greater than 0.2 pH unit.
Additional  methods  of  improving the reliability of pH sensors are indicated
in Table 5-1.
Pumps, Pipes, and Valves
     Operating  experience  has   shown that pumps, pipes,  and  valves  can be
significant sources of trouble in the abrasive and corrosive environments of
a  limestone  FGD   system.   The  flow  streams of  greatest concern are the

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         TABLE 5-1.   METHODS OF IMPROVING pH SENSOR RELIABILITY3
         Dip-type sensor
     Flow-through sensor
 Provide enough agitation in
  the tank to prevent accumulation
  of solids on the electrode
 Locate probe away from quiescent
  zones but provide mechanical
  support

 Provide an external tank for easy
  access
 Feed tank by a slipstream from the
  recycle line
Provide extremely short sample
 lines (1 to 2 ft) of at least
 1 in. diameter in slipstream of
 the recycle line

Avoid installing sample taps at
 the bottom of horizontal slurry
 lines

Provide backflushing capability
 (also can be used for cali-
 brating)

Install upstream deflector bar
 to prevent erosion of the pH
 cell
                               Both types:

                     Provide redundant sensors (100
                      percent redundancy)

                     Conduct frequent calibration
                      (once every shift)

                     Ensure proper mechanical and
                      electrical hookups during
                      installation
Jones et al.  1978.
                                   5-9

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OPERATION/MAINTENANCE: Standard Operations	5-10
limestone reagent  feed  slurry,  the EHT/spray slurry recirculatlon loop, and
the scrubber bleed stream.
     When equipment  Is  temporarily removed from slurry service,  it  must be
thoroughly flushed.   Failure  to  flush will result  in plugging  by suspended
solids.   In  colder  climates, protection  against freezing may  be required.
     Because pumps, pipes, and valves are vulnerable to a comparatively high
failure  rate when  used  in  either abrasive  or corrosive process  streams,
redundancy is  needed for  successful  operation.  With  redundant equipment,
the staff can continue operation as repairs are made.   Where  capital  invest-
ment is  of concern,  an  alternative to two full-capacity components  is three
50  percent  capacity  components.   Although  such considerations  are mostly
applicable to the  design  and procurement phase of  a  project,  the operating
staff must be fully  familiar with the design  intent  so as to maintain high
availability for the life of the plant.
Thickener
     The solids content of the  thickener underflow may vary  between 20 and
40 percent,  depending upon thickener design and the  composition of solids in
the'slurry.   Considerable  operator surveillance can be required to  minimize
the suspended  solids in  the  thickener  overflow so  that this  liquid can be
recycled  to  the  scrubber system  as supplementary pump  seal  water,  mist
eliminator wash water, or limestone slurry preparation water.
     If  the  thickener feed  rate  and slurry  properties are  constant,  then
proper control  of flocculant dosage rates (if any) and thickener bed density
can generally  be  achieved.   Because  of  load-foil owing system  inputs,  how-
ever, these parameters  are rarely constant.   Therefore, for  optimum results
the operator  must  maintain   surveillance  of  such  parameters  as underflow
slurry  density,   flocculant   feed  rate,  feed  slurry characteristics,  and
turbidity of the overflow.
     Thickener overflow is returned  to  the scrubber, either  by mixing with
the fresh water used to  wash the mist  eliminator  or to the  hold  tank for
level control.   A thickener upset condition that causes excessive amounts of
suspended solids  to be carried  into the  overflow  can seriously upset the
makeup water system.

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OPERATION/MAINTENANCE:   Standard Operations	      5-11

Sludge Disposal
     As  it leaves  the thickener, the  scrubber sludge  may  either be  dis-
charged to a  pond  or subjected to a  second  stage  of dewatering in prepara-
tion for  landfill  disposal.   Each of these options  entails  a  different set
of operational  considerations.   For  slurry  disposal, enough  lines must be
installed  to  accommodate the  anticipated  range of  boiler loads.   When any
line is  shut  down,  it  must be flushed and drained  so that the  slurry  solids
do not  settle and  plug  the pipe.   In  severe  climates,  protection against
freezing  is  needed.   A  second  pipeline network is  required  to  return the
supernatant pond  water to the  unit  for reuse.   Operation of  both  the  dis-
charge to  the  pond and the return water equipment  requires attention  of the
operating  staff.   In addition  to normal  operations, the pond  site must be
monitored  periodically  for  proper  water  level,   embankment  damage,   and
security  for  protection  of the  public.   Even after  the unit  has  ceased
operations, some  type  of  continued  care  is  usually mandated  for pond  dis-
posal sites.
     Landfill  disposal  involves the operation of secondary dewatering  equip-
ment  such as  vacuum  filters,  centrifuges,  or settling/evaporation  ponds.
Operators  are  required to adjust the process equipment  to maintain optimum
conditions over  a broad  spectrum  of  boiler  loads.   Again, when  any  of the
process  equipment  is temporarily  removed  from service,  it  must  be flushed
and cleaned to prevent deposition of sludge solids.
     Depending upon the composition of the solids,  the dewatered slurry  will
contain  50 to 80  percent solids.   For  stabilization, fly ash  may  be mixed
with the  scrubber  sludge in pug mills or muller type mixers.  Regardless of
whether the sludge  disposal  system incorporates dewatering only,  stabiliza-
tion with  fly  ash,  fixation with lime,  or one of the proprietary processes,
personnel  are  required to operate the equipment and to maintain  the  proper
process chemistry.
     Transport of  the  wastes between process units  and  from those units to
the  landfill  disposal  site necessitates additional operating staff.  Proper
placement  of  the wastes at  the  landfill,  pile  compaction, and  shaping
require both personnel  and special equipment.

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OPERATION/MAINTENANCE:  Standard Operations	5-12

Process Instrumentation and Controls
     Operation  of  a  scrubber  system requires more  of the operating  staff
than  surveillance  of automated control  loops  and  attention to  indicator
readouts on a control panel.  Manual control and operator response to manual
data indication  are  often more reliable than automatic  control  systems and
are often needed to prevent failure of the scrubber control system.
     Typical problems include  mist eliminator plugging,  severe scale forma-
tion  caused by  pH sensor  malfunctions,  solids  contamination in  thickener
overflow recycle water,  and pump damage caused  by false  level  or flowmeter
indications.   Many  of  these  problems  can  be  prevented  when  a  scrubber
operator can integrate manual control techniques effectively.
     For units  of  more than 500 MW  capacity, maintenance  of  pH sensors can
be a full-time  task for one or more instrument  technicians.   Frequent acid
washing, cleaning, and calibration with buffer solutions have been required
to ensure reliable operation.   In  addition, frequent backflushing of all pH
sensor  lines  has  been  needed  at  some  installations  having flow-through pH
sensor elements.
     Malfunction of  pH  sensors,  typically resulting  in a constant output
signal  despite  changing  level  of  slurry pH,  can usually  be  discovered by
cross-checking  procedures.   A good  cross-checking  technique is  to monitor
several redundant pH  sensors or to determine the slurry pH periodically with
grab samples and portable pH meters.  The sample pH should be checked imme-
diately.
     In many scrubbing systems, especially in high-sulfur coal applications,
the continuous  gas analyzers  that determine outlet  S02  concentration have
malfunctioned.   The malfunctions have been caused by plugging of sensor taps
and lines and by corrosion attack inside the analyzer.  The outlet S02 level
can  be estimated  on  the basis of a  sulfur balance around  the  scrubbing
system.  The  sulfur  balance can also be checked by  reference  to  a calcium
balance of  reagent feed rate,  fly ash collected, and sludge produced.  Such
methods are not recommended as primary control techniques, but are useful as
cross-checks of  various  instruments.  Usually,  the outlet S02 concentration
determined by grab samples with wet chemical analysis can be correlated with

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OPERATION/MAINTENANCE:   Initial  Operations	5-13
scrubber slurry pH when  the coal  sulfur content Is  known  and when the unit
load and slurry recirculation rate are both fixed.   Thus,  pH of the scrubber
slurry  can  be used  as  an  indication  of the  outlet S02  concentration  for
known operating conditions.

INITIAL OPERATIONS
     Very seldom does a  system  perform properly when it  is first placed in
service.  Even though stringent quality control may be  exercized during the
construction phase,  it  is  usually  necessary to optimize the  control  func-
tions and to correct minor problems.   Although the  individual components may
be  completely  checked out during  construction  tests, the  integrated system
performance can  be evaluated only when the system  is placed  in operation.
Initial Operational Tests
     Any problems  in the design,  performance, or operation  of  a limestone
FGD system  should  be  identified through complete and comprehensive testing.
Such  testing   is  normally  accomplished  immediately after  initial  startup.
Where  possible,  single  tests may  be conducted for  multiple purposes.   For
example, tests conducted to ascertain  system  operability  can  also demon-
strate  compliance with emission  limits.   Tests conducted  under the proposed
normal  operating  procedures can  verify the procedures and familiarize the
station  staff  with  the   installed  system.   The  station maintenance  staff
should  make any  required  repairs,  execute field  modifications,  calibrate
instruments, and  optimize  the  control  system.  A  log  of  all  activities,
including details of aborted or failed tests, will  provide clues to specific
solutions for  various  problems and  will serve as  the basis  for any  field
changes.
     Extensive documentation of startup tests  may seem more bothersome than
useful  during  initial operations.  These data, however,  constitute the base
against which future performance trends are measured.  Test data may also be
useful  in   routine  operational  decisions  (e.g., the maximum rate  of load
changes  may be  influenced  by  response of  the scrubber  to  load changes).
Correlations between  measured  system parameters at various stages in the
process  may be  useful   in  developing  techniques  for equipment operation.

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OPERATION/MAINTENANCE:  Startup	5-14
                                                               *
Together with the operating log, the test data complete the detailed history
of the system.
     After  the  initial  startup tests  have established  a norm  for  system
operation, additional testing is conducted for two purposes:   to verify per-
formance guarantees and to demonstrate compliance with regulations.   Testing
for fulfillment of supplier guarantees is discussed in Section 4.
     At each  installation, the  applicable  standards for  emissions  of S02,
NO , and  particulate will  be  established by the required permits.   At new
coal-fired  generating  stations  the pollutant  emissions  must be monitored
continuously.
     Continuous source monitors  were  not originally intended to demonstrate
compliance with emission standards.   Several states, however, are developing
implementation plans that  utilize  continuous monitoring data.  The  EPA has
issued the following reference methods* for sampling and analyzing regulated
emissions:  selection of  sampling  location, Method 1; S02 emissions, Method
6; particulate emissions, Method 5;  and NO  emissions, Method 7.

STARTUP, SHUTDOWN, STANDBY, AND OUTAGE
     Startup  and  shutdown are two  nonsteady-state  scrubber  operating modes
that occur frequently.   Furthermore, two nonoperating conditions that neces-
sitate  action by  the  operating  staff  are  scrubber  system  standby  and
extended station outage.   Each of these situations is of special  interest to
the limestone FGD system operating staff.
Scrubber Startup
     Before flue  gas is  introduced into a  scrubber module, proper  prepa-
rations must be made.  Generally, limestone slurry is added to the system as
a "lean" stream (low slurry solids content), and reaction product solids are
permitted  to  build  up to  a  specified control  level.    Specifically,  the
solids content and the  pH of the limestone  reagent in the scrubber must be
brought to  predetermined  levels.  The slurry recycle lines  and  sprays also
* Described in Appendix A of Part 60, Title 40, Code of Federal Regulations.

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OPERATION/MAINTENANCE:   Shutdown.  Standby.  Outage	     5-15
must be placed in operation.   All  of these  actions must be accomplished with
enough  lead time  to allow  the  parameters  to come  into the design  range
before the scrubbing operation begins.
     A prerequisite to the startup activities is initiation of the limestone
grinding process to ensure the availability of limestone slurry feed.   Since
the duties  of the  operating  staff are  greatest during  transient  scrubber
conditions,  such as  startup  and  shutdown,   the  inventory  of slurry  feed
should be increased to maximum levels in preparation for startup.
     After scrubber  operation begins,  the  operating staff must be  ready  to
process  the scrubber  bleed  stream.   Operation  of  the  thickener  must  be
monitored.   If sludge  disposal  is to a pond  and  if the pipelines have been
drained, they must  be  filled  before pumping can begin.   If disposal is to a
landfill,  the  secondary dewatering  equipment must be  ready for  operation.
     When  the  scrubber is placed in service,  the operating  staff must  be
available  to monitor  system  response  as  boiler load  is  increased.   The
equipment lineup may need  to  be altered accordingly.   As the flue  gas flow
rate  through  the   scrubber  modules approaches  the  maximum  design  value,
additional modules are prepared and brought into service.
Scrubber Shutdown
     As boiler load  is reduced in preparation for unit shutdown,  the startup
sequence is  executed in reverse.   Even with automated controls,  the operat-
ing staff  must  closely monitor the scrubber response to the changing condi-
tions.  When  the boiler load  is reduced and  the  unit is not shut down, the
scrubber  operators  must ensure that the system  reaches  equilibrium  at the
new conditions.  Associated activities  such  as limestone slurry preparation
and  sludge  processing must  be  adjusted  to accommodate  such   changes  in
scrubber status.
System Standby
     A  scrubber  module that  is ready to process  flue gas is  said  to be  on
standby.  The module may  have been removed from service because of a reduc-
tion  in  station  load and  now prepared for  service because of boiler startup
or  an anticipated  increase in  boiler  load.   When a  module  is removed from

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OPERATION/MAINTENANCE: System Upsets	           5-16
service  because of  a power  reduction,  the scrubber  bleed stream must  be
terminated and  the  bleed line flushed.   Failure to  stop  the scrubber bleed
flow  would  waste the  limestone  additive.   Failure to flush the  line would
lead  to plugging  as  the  solids  settle.   When  a module  is  brought  into
service,  the operator  must prepare  the  blowdown line  to accept flow  by
checking valve position and backfilling the line if necessary.
Extended Outage
     Additional  operations  are  necessary when a scrubber module  is  removed
from service for an extended period.   The limestone slurry recycle pumps and
the recycle  lines  should be drained and flushed.   If the hold  tank agitator
is  not  left  in service, the hold  tank  must also be drained and  flushed  of
solids.
     During  an  extended outage,  the operating staff should conduct  inspec-
tions of equipment  that is normally inaccessible.   By entering the shutdown
scrubber  module, an  operator can  check  the  conditions  of the  structural
materials and  linings  for  evidence of corrosion.   He can also  inspect tower
internals,  spray  nozzles,  and  the  mist  eliminator  for  scale  deposits,
abrasive  wear,   or  evidence of  other  developing  problems.   All  auxiliary
equipment should also be  inspected.   With proper operation and  servicing,
the limestone FGD system will  provide high reliability and availability when
returned to service.

SYSTEM UPSETS
     The scrubber  is the  last  stage in  a series of  systems  in  which any
malfunction  can impact scrubber operations.   Upsets are  usually  associated
with the boiler, the scrubber system, or the sludge disposal system.
     A boiler trip will terminate the flow of flue gas through  the scrubber.
Except for the possible discharge of unreacted limestone slurry to the waste
processing equipment,  there should  be  no  adverse  impact on  the  scrubber.
Transient  conditions  causing  an  increase  in  flue gas  flow may  produce
scaling of the  mist eliminator  or excess  liquid  carryover.  The  effects of
greater-than-design  inflow  of particulate  to the scrubber would depend upon
participate  composition.   Although   there  would  usually  be  no  adverse

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OPERATION/MAINTENANCE:   Operating Staff	;	5-17
effects, the solids content of the scrubber bleed stream would increase.   If
the fly ash is alkaline, S02 removal may be higher than anticipated.
     Inability of  the  scrubber to  process  flue gas could lead to a boiler
upset  and  removal  of the  unit  from service.   Failure of a  single  scrubber
module  could  lead to  a reduction  of  station output.   A reduction  in  S02
removal  efficiency can  be caused  by  an  inoperable  slurry recycle  pump,
plugging of  spray nozzles, or  an  insufficient  supply of limestone slurry
feed.   Scaling  of the mist eliminator could  cause an increase in flue  gas
velocity and a  reduction in mist eliminator performance.   The high  pressure
differential resulting from mist  eliminator scaling could cause an  unneces-
sary  increase  in  fan operating costs,  even with no imminent threat to  the
station output.
     The  inability to   process  scrubber bleed  for  sludge  disposal  could
impair  S02  removal efficiency  and  station output.  Since waste  processing
systems usually  incorporate some  spare capacity, station output should only
be reduced, at worst.   The solids processing system should incorporate some
type  of surge  capacity.   Operating flexibility  provided by redundancy or
installed surge  capacity will  allow the staff to  work around a malfunction
of sludge processing equipment without a reduction in station output.

OPERATING STAFF AND TRAINING
     The  size,   experience level,  responsibilities,  and training   of  the
operating staff  are significant factors in FGD system performance.   A con-
servative approach is to assign highly qualified personnel  to the operating
staff  until  the  system response to  the range  of operating conditions is
understood  and   reactions  to  changes  in performance  become routine.   The
number of personnel assigned to each operating shift will vary with  the type
of equipment and with the normal operating mode of the unit,  i.e., base-load
versus load-foil owing.
     In staffing, the scrubber operations and waste disposal  operations must
be considered  separately.   The permanent  assignment of key  personnel to
specific work areas will allow them to become  completely familiar  with the
process  equipment  and   its   chemistry.   As  the  operating personnel gain

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OPERATION/MAINTENANCE:  Operating Staff	   5-18
understanding of the system, they will be able to anticipate problems before
unit output becomes impaired.
     In addition to  the normal  complement of equipment operators and super-
visory personnel  on the  operating  crew of each  shift,  certain specialists
should be  available  to assist them.   For example, a chemical engineer would
be  a valuable  resource during atypical  operating conditions.  A  chemical
laboratory  technician  should also  be  available  to analyze the  process
chemistry in the event of suspected trouble.   During normal operations, this
technician  can  monitor  routine  system  performance  through  sampling  and
laboratory analyses.   Except for  the period following initial operation and
testing of a  newly installed system, the chemical  engineer  and the labora-
tory  technician need  not  be  dedicated full  time to scrubber operations.
     The time period following initial startup and operation of the scrubber
system  presents an  excellent  opportunity for  training  of  the  operating
staff.  When  a  scrubber system is first placed in operation, vendor person-
nel are usually available on site to ensure that the equipment is operating
properly.   During  this period all  equipment should  be  operated  by utility
staff personnel, under the guidance of the vendor representatives.   Whenever
possible,  written  procedures should  be  followed  so  that any  error can be
identified and corrected.
     The following estimate of the operating staff personnel  requirements is
based on experience at full-scale, limestone FGD systems  at coal-fired power
plants:
                                             Number of personnel
          Operating responsibility	    per shift per unit
          FGD scrubber                       2 to 4
          Limestone receiving and storage    1/3 to 1/2
          Slurry preparation                 1/3 to 1/2
          Waste processing and disposal       2 to 10
     Although some  reduction in  crew size may  be possible  where  multiple
units are  operated  at the same site,  the reduction will  depend  on what
equipment  is  common to all units.    Requirements  for waste  processing  and

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OPERATION/MAINTENANCE:   Preventive Maintenance	   5-19
disposal personnel  vary with operating  schedules  and method of final  dis-
posal.   It  should be kept in mind  that  the limestone FGO scrubber  is  only
one  system  in series with the rest  of  the operating units in a powerplant
and  that the  size and  experience  level  of  the  operating  staff must  be
correlated with the overall  utility requirements.

PREVENTIVE MAINTENANCE  PROGRAMS
     Preventive maintenance is the practice of  maintaining system components
in such  a way as to prevent malfunctions during periods  of operation and to
extend the  life  of the  equipment.  The  goal of preventive maintenance is to
increase availability of the FGD system by eliminating the  need  for emer-
gency repair.
     The  term preventive  maintenance is  almost  synonymous  with  periodic
maintenance.  The  manufacturer specifies  how often each  component  should be
serviced, usually  depending  upon  the type of  duty it  sees.  The manufac-
turer's  operating  instructions  for  each  component should specify both the
schedule  and procedures  to  be  used  in  preventive maintenance activities.
Such procedures  may  be  as simple as lubrication of  a pump or as complex as
complete disassembly for  inspection  and  overhaul.   The upkeep of pumps, for
example, normally  involves  lubrication   and inspection of bearing wear and
liner conditions.  Replacement of valve  seats  and packing may be  necessary
after  a specified  number of  hours of  operation.   Routine  inspection  of
linings  in  chimneys, ducts,  fans, and the  scrubber permits early  detection
of  damage.   When  corrective actions are  taken before  the  damage  becomes
severe,  repairs  are usually  less complex, less costly,  and  less  time con-
suming.
     The operating log  also  can  be useful  in  establishing  appropriate pre-
ventive  maintenance  schedules.   The operating log and  the equipment main-
tenance  records  together  should  indicate trends on which to  base  a program!
of systematic upkeep.
Scrubber Modules
     The scrubber  module is  typically  a  passive  component with  no  moving
parts except  for agitators  in the EHT.   Of primary concern in the scrubber

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OPERATION/MAINTENANCE:   Preventive Maintenance	_L_	5-20
module is the  integrity of the structural materials,  including  the lining,
and any  scale  buildup  that may be occurring.  Maintenance personnel  should
enter and inspect  the  scrubber module at least annually.  Where there is a
recurring problem  with either  scaling  or corrosion,  more frequent inspec-
tions are in order.
     The EHT must also be checked for buildup of settled sludge.   The struc-
tural integrity of pump suction strainers must be checked.  Agitators should
be  inspected  for not  only  corrosion  and erosion but  also for bearing wear
and seal deterioration at the tank wall.
Mist Eliminators
     Scale  deposits  typically  are  the  chief  maintenance  factor with mist
eliminators.   The  mist eliminator may  be subject  to nonuniform flow or a
faulty wash system.  Plugged or worn spray nozzles,  solids  deposits in the
supply piping,  or  other malfunctions  may cause a  loss of  wash pressure and
lead to  localized  scaling.   Wash spray pressure should  be monitored.   Mist
eliminators should  be   inspected  periodically  and, again, if a  problem  is
recurring, inspections should be made more often.
Reheat System
     Procedures  for  maintaining  the  reheater system  vary  with  heating
method.    In-line  reheaters  are  subject  to both scaling and corrosion.   In
addition  to visual  inspection,  pressure  testing  and measurement  of heat
transfer  efficiency  are useful  in  quantifying the magnitude of a  reheater
problem.
     In  an  indirect reheat  system,  the  mixing chamber and  the  air heating
equipment must  be  checked  routinely.   Scale and corrosion  are the principal
concerns.
Dampers, Fans, Ductwork, and Chimneys
     The  location of the  fans and the ductwork governs  the  types of poten-
tial problems  that could  occur.   Components  located  in the  wet portion  of
the system, downstream of  the scrubber modules, are  subject to  scaling and
corrosion.  Upstream fans  and ductwork could be subject to erosion, depend-
ing on the particulate removal efficiency.   All points in the system must be

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OPERATION/MAINTENANCE:  Preventive Maintenance	5-21
checked  for integrity  of lining  materials and  for  damage resulting  from
collection  of condensation products  in low-lying pockets.   Such pockets nay
occur  at expansion joints or  instrument taps  into  the duct,  or  at  points
where  the  sloping  of  straight  duct runs may be inadequate.   Chimneys  should
be inspected for effects of acid condensation or thermal  cycling.
Limestone Slurry Preparation
     Preparation of  limestone  slurry subjects  the  ball  mill to  abrasive
wear.   This equipment  must  be  lubricated  often during operating  periods.
Because  the equipment  sees  intermittent  service,  it  should  be  inspected
visually each  time it  is placed in service.  Periodic disassembly is  also
needed to  check  for  excessive  wear on the  milling surfaces.   The frequency
of disassembly  depends on the  composition of the limestone.   Conveyor and
feeders  should be  inspected  and maintained at the same  time as the grinder
or mill.
Limestone Slurry Feed
     In  addition to  the pumps,  piping, and  valves, the  slurry storage tank
and  associated  equipment must  be  maintained.   Even  though   the  prepared
slurry  is   agitated,  some settling  can  occur.   If the  settling  of  solids
becomes  excessive, the  transfer pump suctions and drain lines  could  become
clogged.  Tank agitators must be inspected for impeller abrasion, corrosion,
and  bearing wear.  Maintenance  of the slurry feed system is critcal because
failure of  this equipment strongly impacts the FGD system operation.
Pumps. Pipes, and Valves
     Slurry pumps  are  normally  disassembled at least  annually.   The purpose
of  the  inspection  is  to verify  lining integrity and  to detect  wear and
corrosion  or other  signs of  potential  failure.   Bearings  and  seals  are
checked  but not necessarily replaced.   The  frequency of  these inspections
depends  on  the  slurry solids content and also on the size and shape  of the
limestone particles.
     Pipelines  must  be  periodically disassembled or  tested in  other  ways
both for solids  deposition and for wear.  Measurements of flow capacity and
pressure drop,  and possibly  ultrasonic inspection, could be substituted for

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OPERATION/MAINTENANCE:  Preventive Maintenance   	5-22
disassembly.  Slurry  lines  are  subject to solids deposition and to abrasive
or corrosive wear.   Deposition  is more prevalent in straight runs of piping
where flow patterns are undisturbed.   Wear may predominate at points of flow
perturbation,  such  as  elbows,   restricting  orifices,  instrument taps,  or
other sidestream  connections.   Appropriate component  design,  e.g.,  flanged
pipe connections, can simplify maintenance procedures.
     Valves  must  be serviced routinely, especially  control  valves.   Valves
that are  operated rarely must  be exercised  to ensure that they are func-
tional.   Valve operators, both motor and pneumatic, must also be functional-
ly  tested.   Seats  must be  checked  to verify  leakage rates.   Valve stem
packing must be  checked to  ensure  that  leak-tightness  does not  prevent
proper valve operation.
     Lined valves, such  as  those used  in slurry  service,  require more fre-
quent maintenance than unlined valves.  They should be disassembled at least
yearly and their liners inspected.  Valves with replaceable seats are easier
to maintain and therefore are preferable.
Thickeners
     Thickeners are  usually  constructed of  a concrete base slab with coated
carbon steel walls.  The coating should be inspected periodically to prevent
massive corrosion, which could  cause delamination of  the  coating or damage
to the  underlying materials.   Drag  rakes,  torque arms,  and  support cables
must also  be inspected for wear.  The drag  rake should be  lifted when the
system is shut down.   Drive motors must be lubricated frequently.
Sludge Disposal Equipment
     Secondary dewatering  devices,  mixing components,  and transport equip-
ment also  must  have periodic  maintenance  to  check for abrasive wear and
solids deposition.  Motors  and device couplings require periodic checking of
lubricant level and lubrication as needed.
     Vacuum  filters,  both  drum  and belt type,  require periodic replacement
of the filter media.   Frequency is determined by the media materials, filter
cake removal method, hours  of operation, and abrasiveness of the sludge.  In
a centrifuge, both  the  scroll  coating  and the  bowl  surfaces are subject to
wear.  Materials  of construction usually  preclude  corrosion  problems, but

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OPERATION/MAINTENANCE:   Preventive Maintenance	5-23
this must  be  verified  during maintenance.  Similar considerations  apply to
mixing and handling equipment.
Process Instruments and Controls
     All electronic  equipment  must be  calibrated periodically.   The  fre-
quency is  governed by  the type of service and the accuracy to which a vari-
able must be controlled.
     Numerous installation and maintenance techniques  have proved beneficial
in ensuring the  reliability  of pH sensors (Table 5-1).   A consistent diffi-
culty with pH measurement is that electrodes and amplifiers are often poorly
installed or badly located.   Experience  has shown that where maintenance is
inconvenient,  the operators  often neglect it to the point that pH measure-
ment becomes  unreliable.   Thus,  ease  of access  to  the  electrodes is  very
important.   The  electrodes should be cleaned and standardized at least once
every  shift.   The best practice  is to install dual  pH  metering systems so
that calibration can be cross-checked continuously.
     Wiring between the electrodes  and the preamplifier  should  be  as short
as possible; some vendors mount the preamplifier in the electrode housing to
prevent  short-circuiting.   This arrangement,  however,  has the disadvantage
of placing the electronic component in a wet atmosphere, which could lead to
failure of  the  preamplifier  if the housing fails.   All vendors offer either
voltage  or current output  signals, most of  which are  field-adjustable for
both range and span.
     The electrode station  should contain a workbench and a cabinet to hold
spare  parts, small tools,  and standardizing  solutions.   At  least two iden-
tical  electrode  assemblies  are desirable, with valves and switches arranged
for simple crossover to a set of standby electrodes when a set is checked or
serviced.   All  amplifiers and  calibration  controls  should  be installed at
the electrode  station  so  that one  person can perform  the necessary adjust-
ments.    This   arrangement eliminates  the  need  for  communication between
control room and maintenance personnel during calibration.
     Flow devices, pressure sensors, temperature monitors, and level instru-
ments  are  susceptible  to similar problems.   A sound  preventive maintenance
program will minimize adverse conditions resulting from an inadequate design
that cannot be changed by modifications.

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OPERATION/MAINTENANCE:  Unscheduled Maintenance	5-24
     Experience with  process instrumentation and controls  in  limestone FGD
system  applications   has  shown  that a  good preventive maintenance  program
begins  with daily  operating procedures.   Proper  use  of  instruments  will
include  daily  flushing  of  most  instrument  lines  in  slurry  service  just
before  monitoring  of  process variables.  Routine comparison of  the  instru-
ments in  a  process stream with similar  instruments in  parallel  streams can
point out incipient  failures.   Operating data, especially  from  the  startup
test program, can also indicate potential problem areas.
Housekeeping
     Because of the nature of  the materials handled and  the  process equip-
ment used in a  scrubber and its  support system, the  work area can become
dirty.   Leaks and even routine use of equipment will lead to accumulation of
sludge  on the  floors.    In  the limestone  receiving  and sludge processing
areas,   dust  can  become  a significant problem.   The resulting dirty environ-
ment not  only  affects  the operating and  maintenance   staff adversely,  but
also reduces overall  system reliability by accelerating the  rate of wear and
other malfunctions.   In  such an environment, both operating and maintenance
personnel  must  follow   a continuous  and  stringent  housekeeping  program.

UNSCHEDULED MAINTENANCE
     Even the most rigorous  preventive  maintenance program  will  not prevent
random  failures,  to  which  the  maintenance  staff must  respond.   Redundancy
achieved through excess capacity and the use of multiple fractional-capacity
process streams will  enable  the station to  continue power  production while
repairs are effected.
     Most malfunctions are  correctable  by unscheduled maintenance.   In some
situations,  usually during  initial  system startup,  design modifications may
be  required  to bring the system into  compliance with  operating standards.
As  system operation continues,  means of improving system performance may be
identified.    Improvements   are  normally   incorporated  during  scheduled
outages.

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OPERATION/MAINTENANCE:   Unscheduled Maintenance	5-25
     Each component  of a  limestone  FGD system  Is  subject to  malfunctions
from a variety  of  causes.   Most such malfunctions are typical  of those that
occur in  other systems, regardless  of the  application;  examples are  pump
motor  failures and  malfunctions  of  bearings  and  valve  operators.   Some
malfunctions, however,  are unique to  the components of a  scrubber  system.
The  discussion  that  follows  deals  with these  problems  and  the probable
responses.
Scrubber Modules
     Degradation of the coating  or lining of the scrubber vessel and subse-
quent damage to  the  base  material  are  normally identified and  corrected
during preventive maintenance.   Structural  failure  of spray tower trays and
recycle pump  suction  screens  have occurred as a  result  of excessive vibra-
tion, uncorrected  corrosion damage,  or high  pressure  differentials.   These
malfunctions  must  be  repaired   immediately  before  operations  are  resumed.
Repairs may  include  patching  or repairing the lining in the vicinity of the
failure.
Mist Eliminator
     Failure of the  mist eliminator  in an operating system is typically due
to scaling  and plugging,  as  indicated by  excessive pressure  differential.
The  scale  may be  removed  either  by, thorough  washing  or  by  mechanical
methods,  in which maintenance personnel enter the scrubber and manually chip
away  the  scale  deposits.   The  cause  of the  scaling should be determined
(e.g., a  clogged  spray nozzle,  insufficient wash water flow or pressure, or
improper  process  chemistry).   If  the  cause  is  not identified,  scaling or
plugging is likely to recur.
Reheat System
     Reheater  malfunctions could be  caused  by  tube  failures in  in-line
reheaters,  by  damper problems  in bypass reheat, or by  nonuniform  flows in
indirect  reheaters.   The  lack  of sufficient  reheat capability could cause
condensation and corrosion  in the stack.

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 OPERATION/MAINTENANCE:   Unscheduled Maintenance	__^.	5-26

 Fans
      Fans can develop vibrations resulting  from deposition  of scale  in  wet
 service  or  from erosion  of blades  in  dry service.   Before operation  can
 resume, the cause of the  vibration must be eliminated and  the  fan repaired
 and rebalanced.                                                  ,
iliitwork                   ..."  -I""' ''"••••.-  i-'
"*     Most problems associated with ducts  develop  over  a  long  period.   Dete-
 rioration of the  lining and damage  to the  base  material are  usually detected
 and corrected during preventive  maintenance.  Correction  may include modifi-
 cations  to  the  system design.   Sudden  or gross  failures,  such as  a  major
 leak, call for immediate repair.   Temporary  repair or patching may suffice
 until the  next  scheduled  outage.    Again,  the  source  of  the malfunction
 should be identified;  for  example,  if the problem is traceable to improper
 process  chemistry,  system operation should be adjusted  accordingly.
      Acid condensation in  a chimney can  cause   lining deterioration  and
 subsequent damage to the base metal.  These problems are usually  identified
 during preventive  maintenance  inspections  and probably require  long-term
 solutions.   Temporary  repairs  could  become necessary,  in which  case  the
 chimney  should be routinely monitored to  check the extent of  any  additional
 damage.
      Scale  can  also  accumulate  on  dampers.   Unscheduled  maintenance  is
 limited  to  removing the scale  and investigating  the cause.  Whenever pos-
 sible,  the  cause  is eliminated; e.g.,  by adjusting the process  chemistry.
 Where the scale accumulation is  attributed to a design  deficiency,  equipment
 modifications may be  necessary.  Flue  gas  leakage is one  such  deficiency
 that usually requires  equipment  modifications.
 Limestone Slurry  Preparation
      Malfunctioning  components  such  as  ball   mills  must  be  repaired  in
 accordance  with   the   manufacturer's  instructions.   Some  facilities   have
 experienced  trouble with plugging  of  the  limestone feeder due to  intrusion
 of  moisture.  Excessive dust has  also  been a problem  at  some facilities.

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OPERATION/MAINTENANCE:  Unscheduled Maintenance	. :•;.' :'	5-27
Correction of these  problems  will  probably necessitate changes In equipment
design.
Pumps. Piping. Valves
     Excessive wear  of the Impeller  or separation  of the  lining  from the
pump  casing  Is  a  common  problem.   Pieces  of the lining can  plug  the pump
discharge or  flow downstream,  Impeding  the operation bf-Sther components,
such  as  valves  and  Instrument  sensing lines.  These  malfunctions  are cor-
rected by repair of the pumps.
     Operation of  slurry  pipeline  with insufficient flow velocity can cause
clogging.  High  flow velocity  or  extended service  can  cause  erosion.   If
pipes cannot be unclogged, they must be replaced.
     Malfunction and  binding  of a valve operator  are typically caused  by
wear-induced misalignment.  It  may be possible to  continue  operations with
manual actuation  of the  valve  until   repairs  can be  made.   During steady-
state  conditions,  it may  be  possible to  replace a valve operator without
interrupting process operations.  Lined valves are subject to the same types
of  failures  as  lined pumps.   Use of valves constructed with replaceable
seats and/or other internal  parts  will facilitate  repairs.   Leakage  of the
valve stem packing can often  be corrected by tightening enough to terminate
the  leak but still  prevent  binding.   With  some  types of stem leakage the
packing must be replaced.
Thickeners and Sludge Disposal Equipment
     The thickener  underflow  can  become plugged because of excessive solids
in the slurry.   A  plugged underflow or rapidly settling  sludge solids will
produce  a  blanket in  the bottom  of  the thickener.  The rake  must then  be
raised so that  the torque remains within acceptable limits.   If the torque
cannot be kept within limits,  operation of the thickener must be terminated.
The  thickener  must  be drained  and  the  sludge  blanket removed  manually.
     Where secondary  dewatering equipment is used  in  preparation  for land-
filling, water  removal may  be inadequate.   An excessive concentration  of
sulfate  solids  in  the sludge  can lead to cracking of the filter cake on the
vacuum filter.   Cracking of the filter cake is not usually serious, however.

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OPERATION/MAINTENANCE:  Unscheduled Maintenance	        5-28

Process Instrumentation and Controls
     Slurry  service  is probably the most severe  application  for instrumen-
tation.  Most malfunctions  result from plugged sensing Tines  caused  by low
velocities or flow restrictions.  Fouling of probes also has been a problem.
Although temporary measures such as routine flushing of the sensing lines or
frequent cleaning of the probes will allow operation to continue, the system
should be  modified to  eliminate  the  cause  of plugging/fouling.   The  best
approach is to install instrumentation that minimizes the amount of required
maintenance.   Table  5-2  lists  types  of  instruments preferred  for various
applications in a limestone FGD system.
Troubleshooting Techniques
     Troubleshooting of an  FGD  scrubber and ancillary equipment calls for a
multiphase program along the following lines.
     Phase 1:  Problem Identification.  This phase  begins with  a  detailed
inspection of  the  system.   All  observations  (positive  and  negative)  are
listed, interpretations are  developed (why  things were the way they were),
and  finally,  methods  and  items  that will  improve performance  are  recom-
mended.  Recommendations  may call for design  modifications,  replacement of
components or accessories, or fabrication of new equipment.
     Phase 2:  Implementation.   After  thorough  analysis,  the  Phase 1 recom-
mendations should be  implemented  by repair and by replacement with procured
and  fabricated  components.   The   system  is then  started up  and debugged.
     Phase 3:  Testing and Sampling.   A  performance test  must be conducted
to evaluate  the  effects of the work on system operation.   Testing may be by
stack  sampling and/or  measurements with the system in continuous operation.
     Phase 4:  Operational Troubleshooting.   Certain symptoms  are attribut-
able to  more than one cause.   Table  5-3 lists  typical  symptoms,  probable
causes, and suggested remedies.   This list should not be regarded as exhaus-
tive of  all  possibilities;  no checklist, maintenance  protocol, or operator
instruction  manual  can take the  place of a well-trained maintenance staff
familiar with the equipment and its operating history.

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              TABLE 5-2.   PREFERRED FGD SYSTEM INSTRUMENTATION
  Process variables
             Type of instruments
Slurry level


Slurry density

Slurry flow

pH level



Flue gas pressure


S02 concentration

Slurry pressure



Flue gas temperature
Ultrasonic sensors, admittance probe sensors,
 and flexible diaphragms with purge flushing

Nuclear density meter

Magnetic flow meters; sonic (Doppler) detectors

Dip-type pH sensor located in auxiliary
 measuring vessel that can be isolated;
 equipped with ultrasonic cleaning device

Capacitance cell electronic differential
 pressure transmitters with water purging

Extractive or "in-situ"

Capacitance cell electronic pressure trans-
 mitters with diaphragm seals and water
 purging

Thermocouples with stainless steel protector
 tubes
                                     5-29

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              TABLE 5-3.  OPERATIONAL TROUBLESHOOTING CHECKLIST
      Symptom
   Potential cause
    Recommended action
Low pressure drop
 (scrubber section)
High pressure drop
 (scrubber section)


Low pressure drop
 (mist eliminator)


High pressure drop
 (mist eliminator)
High temperature
 in stack
Pump leaks
Increase in pump
 pressures
Reduction of pump
 flow rate/pressure
Pump noise/heat



Corrosion




Erosion



Scaling

Pipe plugging
Low flue gas flow rate
Low liquid flow rate
Eroded or dislocated
 scrubber internals
Meters plugged

High flue gas flow rate
Plugging in ducts or
 scrubber

Low flue gas flow rate
Low liquid flow rate
Media dislocated

High flue gas flow rate
High liquid flow rate
Clogging
Flooding

Too much reheat
Liquid temperature too
 high

Packing or seals
Nozzle plugging
Valves closed
Impeller wear
Nozzle abraded
Speed too low
Defective packing
Obstruction in piping


Misalignment
Bearing damage
Cavitation

Inadequate
 neutralization
High Cl  concentration


Incompatible materials
High recycled solids
 content

Improper chemistry

High solids content
Abrupt expansion/
 contracti on/bends
Check  fan
Check  pump/nozzles
Inspect

Clean  lines

Check  fan
Inspect


Check  fan
Check  pump/nozzles
Inspect

Check  fan
Check  pump/nozzles
Inspect/clean
Inspect/drain

Check  flue gas temperatures
 upstream and downstream of
 reheater
Check  sump temperature


Replace
Replace
Open valves
Replace
Replace
Check  motor
Replace
Check  pipes, strainer, and
 impeller

Check
Replace
Check

Check  pH control

Check  Cl  content of recir-
 culation slurry

Replace
Wastewater system


Change chemistry variables

Cleaning
Change pipe fittings
                                     5-30

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OPERATION/MAINTENANCE:   Staffing	5-31

Spare Parts
     The  spare  parts inventory  maintained  on site must be  able  to support
the required maintenance activities.   Initial inventories should be based on
recommendations of  the  equipment vendor.   Lead times  required  for delivery
of parts  from the  supplier's  warehouses must also be considered.   Initial
spare parts  inventories  may  have to be adjusted  depending  upon system per-
formance and the degree of redundancy in the installed system.
     The quantity of spare  parts can be reduced  by  standardizing sizes and
types  of  equipment.   Standardized parts  also reduce  the  time  needed  for
maintenance activities.
MAINTENANCE STAFF REQUIREMENTS
     The maintenance staff for a limestone FGD system must include personnel
from a  number  of disciplines.   Mechanics are needed  for  component repairs.
Electricians  are also  needed,  as well  as  instrument  technicians familiar
with  the system.   These specialists  are assisted by  laborers and  by the
operating staff.   Because different  policies govern  the  tasks  performed by
each type  of craftsman,  an  optimum maintenance  staffing scheme  cannot be
outlined firmly.
     Assignment  of maintenance  personnel  to shift coverage also varies with
individual facilities.   Where maintenance on the back shift is performed by
"on-call" personnel,  the standard day-shift maintenance crew will be large.
Where operating  personnel  can  perform basic maintenance,  such as instrument
flushing, requirements for the maintenance staff can be reduced.  The poten-
tial number  of unscheduled maintenance activities  (i.e.,  because of equip-
ment malfunctions) must  also  be considered in sizing the maintenance staff.

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OPERATION/MAINTENANCE:  References                        	5-32
                          REFERENCES FOR SECTION 5
Bechtel Corporation.   1976.   pH Study at the  Shawnee Test Facility.   Phase
II.  EPA Contract 68-02-1814.

Jones, D.  G., A. V. Slack, and K. S. Campbell.  1978.  Lime/Limestone Scrub-
ber Operation and Control Study.  EPRI-RP 630-2.

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                                 APPENDIX A
                      CHEMISTRY OF LIMESTONE SCRUBBING

     The primary objective of  any FGD system is to  enable  the powerplant to
operate  in  compliance  with  the  S02  emission  regulations.   In  order  to
achieve this objective  in  a  cost-effective and reliable manner,  a  limestone
FGD system must  maximize  both  the S02 removal and  limestone  utilization! and
must precipitate  the calcium  sulfite/sulfate  solids in a  controlled  scale-
free manner.   Numerous  interrelated  chemical  variables and  mechanical  fac-
tors must  be controlled.   In an effort to  simplify  this subject  matter,, the
process chemistry  is  considered separately from the  key operational  factors
that affect  the  FGD  system performance;  the latter  are  discussed  in Appendix
B.   The  separation  cannot  be maintained  absolutely  because  the  chemical1
reactions are  an  integral  part of process operation.  Therefore,  wherever ft
is  possible  the  significant interrelationships are  identified  and  related
material in other parts of the manual is cited.
     The  intent  here is to  present  a brief, theoretical  description  of the
basic  process  chemistry involved  in wet limestone scrubbing.  Although; the
subject  has  been  treated in  the  FGD literature  by  various  authors„  the
treatments  are  often very  rigorous, or  are empirical  and  site-specific.
This appendix  is  intended  as an introduction to scrubber process chemistry;
more detailed  treatments of  these topics are given  in  the  references  cited.

CHEMISTRY DESIGN OBJECTIVES
     The chemistry of a simple limestone slurry scrubber should  be designed
to maximize  S02  removal  and  to avoid  scaling of calcium sulfite  (CaS03) and
calcium sulfate  (CaS04)  in the scrubber.   This  is achieved  by separating the
functions  of  the  scrubber  and  the  effluent  hold  tank  (EHT)  so that S02
removal takes  place in  the  scrubber while solids precipitation  occurs  pri-
marily in the  hold tank.
                                     A-l

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CHEMISTRY:  Design Objectives	    A-2
     In  the   :rubber, S02 is  removed  from the flue gas and,  ideally,  about
half  of the  limestone  is  dissolved.   Usually  the  dissolved alkalinity  is
insufficient  to  provide for  S02  absorption as bisulfite  (HS03~);  therefore
either  CaC03  or CaS03  solids must dissolve  in the scrubber.   Ideally,  one
mole of CaC03 should dissolve for two moles of S02 absorbed:
                  CaC03(s) + 2S02 + H20 -> Ca++ + 2HS03~ + C02
If  excessive  CaC03 is  present,  CaC03  dissolution can  lead  to precipitation
of CaS03 and possibly to scaling in the absorber:
                         CaC03(s) + S02 -» CaS03 -»  + C02
If insufficient CaC03 is present, CaS03 will dissolve in the scrubber:
                     CaS03(s) + S02 + H20 -> Ca++ + 2HS03"
     The pH  levels at  the  scrubber inlet  and outlet are indicative of  the
solids  reactions  that will  occur.   A high  pH level (5.8 to  6.5)  indicates
excessive dissolution  and precipitation  of  CaS03 in the absorber.   The  S02
removal by a  given scrubber system is generally  higher with excessive  CaC03
at high pH and  lower with little excess  CaC03 at low pH.   Hence there can be
tradeoffs between  S02  removal  and CaS03 scaling and between S02 removal  and
limestone utilization.
     Ideally, all  of the crystallization  of  CaS03 and CaS04  and about half
of  the  dissolution of CaC03  occur  in  the EHT.  The stoichiometry  for  CaS03
crystallization is given by:
               CaC03(s) + 2HS03" + Ca++ -» 2CaS03(s) + C02 + H20
Completion of these  reactions with minimum supersaturation requires adequate
EHT volume and a high level  of suspended solids.
     Because  the   flue  gas  contains 3  to 10  percent  oxygen, a substantial
fraction of  sulfite  will oxidize  to  sulfate in the scrubber.   Depending  on
S02 and 02 gas  concentrations, 10 to 100  percent  of the S02 absorbed may be
crystallized as calcium  sulfate.   With an adequate EHT  design,  the solution
entering  the  scrubber  will   be  slightly  supersaturated to  CaS04.   In  the
scrubber, sulfite  oxidation  and  calcium solids dissolution will  increase  the
concentration  of  dissolved   CaS04   and  tend to cause CaS04 scaling.  The

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CHEMISTRY:  Design Objectives	A-3
critical concentration at which  CaS04  precipitates  uncontrollably  is  avoided
by maintaining a  liquid  circulation  rate high enough to minimize  the change
in  concentration  of  CaS04  solution  across  the  scrubber  and  sufficient
nucleation sites for desupersaturation.
     If  the  amount of  sulfate in the  waste solids  is  less than  15 to  20
percent  of  the total sulfate  plus sulfite,  CaS04 will  crystallize in  solid
solution with  CaS03,  rather  than  crystallizing as gypsum  (CaS04-2H20),  its
usual form (Jones  et  al.   1976).   As a result,  the  constraints  on  EHT design
and liquid circulation rate can be relaxed without gypsum scaling.
     Composition of the  solution  in  CaC03 slurry scrubbing  can  be predicted
approximately  from solid/liquid  equilibria.   Most  scrubber inlet  solutions
are  nearly  in equilibrium  (saturated)  with  CaS03 and CaS04 solids.   Depen-
ding  on the  amount  of  excess  limestone,  the  solutions  are  at  0.1 to  50
percent  saturation with  respect  to CaC03 with a  C02  partial pressure of 0.1
to 1.0  atm.   In  the absence of soluble salt  impurities the  scrubber solution
                                                             ™    ++        +
is primarily dissolved  CaS04.   Soluble impurities such as Cl ,  Mg   ,  and Na
accumulate in  the scrubber  loop  and  frequently constitute  the  primary  con-
stituents.
     Radian  Corporation  (Lowell  et   al.   1970)  developed  an equilibrium
program  for this  system,  which has been modified by  Bechtel (Epstein 1975).
Table A-l  lists  the  calculated concentrations of important  solution  species
in  solutions  dominated  by  CaCl2 and  by MgS04.  Variation of  excess  CaC03
affects  primarily  the solution pH and the concentrations  of the  ion pairs:
CaS03°, CaS04°, MgS03°,  and MgS04°.
     A  number  of  specific  rate  processes contribute to overall system  per-
formance.  The effects  of these  processes must  be integrated in the scrubber
and the  EHT to project  the relationships of  S02  removal, limestone utiliza-
tion, pH, oxidation,  scaling,  and other system variables.   In  the remainder
of this  appendix  we  describe in detail  the four  most  important   rate  pro-
cesses:
     1.   S02 gas/liquid mass transfer
     2.   Limestone dissolution

-------
TABLE A-l.   TYPICAL SOLUTION COMPOSITIONS

CaC03 saturation
CaS03 saturation
CaS04 saturation
p
C02, atm
p
S02, atm
PH
C02, mmol/liter
HC03, mmol /liter
HS03, mmol /liter
S03~, mmol /liter
CaS03°, mmol /liter
S04~, mmol /liter
CaS04°, mmol/liter
Ca , mmol/liter
Cl , mmol/liter
^^
Mg , mmol/liter
MgS04°, mmol/liter
MgS03°, mmol/liter
CaCl2
0.10
1.0
1.0
0.1
0.9 x 10"
5.64
1.8
0.57
2.2
0.1
1.5
6.4
6.0
49.0
100.0
8.7
1.2
0.1
MgS04
0. 10
1.0
0.3
0.1
0.9 x 10"6
6.1
1.8
1.8
6.9
1.1
1.4
20.3
1.8
6.0
100.0
69.5
24.6
5.5
                  A-4

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CHEMISTRY:  Mass Transfer	A-5
     3.   Oxidation
     4.   CaS03/CaS04 crystallization
Other  rate processes  that  are not discussed specifically include C02  desorp-
tion and CaS03 dissolution.

S02 GAS/LIQUID MASS TRANSFER
     The  overall   process  of  S02  absorption in  a  CaC03  slurry  typically
requires three  rate  processes  in series:   S02 diffusion  through a gas film,
S02 diffusion through a liquid film, and  CaC03 dissolution.   In general, any
one of these  processes can limit the S02  absorption,  though if CaC03 disso-
lution  is  important, S02  liquid-phase diffusion  is  also  important.   The
following discussion  deals with  the effects of gas  and solution composition
on S02  mass  transfer through the gas and  liquid  films.  This is followed by
a  discussion  of CaC03 dissolution  in the  scrubber and the  EHT.   Appendix 6
describes the effects of physical parameters on gas and liquid films.
     The  two-film model  of gas/liquid  mass  transfer assumes  that  concen-
trations of S02 are in equilibrium at the gas/liquid interface:
                              PS02i = H CS02i                    (A-l)

     where          PSQ   = partial pressure of S02 (atm)
                    CSQ   = liquid S02 concentration (mol/liter)

                        H = Henry's constant (atm/mol-liter)

The  total  flux of S02 (N,  gmol/cm2-sec)  must be  the same  in the  gas and
liquid films, as given by:
               gas film:  N = k_ (P$0  - PSQ  )                  (A-2)

            liquid film:  N = k,° (Ccn   - Ccn )                (A-3)
                                 I    oU2 •     oU2
P         C
 S02 and  S02  represent the concentrations of S02 in the bulk gas and liquid
phases  respectively.   The  mass transfer coefficients,  k  and  k,°, vary with

-------
CHEMISTRY:  Mass Transfer	  A-6
agitation and  species  diffusivities  in the respective gas and liquid phases,
but are  independent  of compositions.   The liquid-film mass  transfer coeffi-
cient, k,°,  must be corrected by  the enhancement factor, <|>, to  account for
chemical   reactions  that  permit  S02  to  diffuse through  the liquid  film  as
bisulfite or sulfite species rather than as undissociated S02.
                                                                   r
     Equations  A-l,  A-2,  and A-3  can  be combined  to  eliminate   S02.  and
p                                                                       '
 S02. and give flux in terms of the overall gas-phase coefficient, K :

                         N = Kg 
                         T     T      H
     where               IT"  = IT  + *k~
                          0     Q      I
In CaC03  slurry  scrubbing,  the equilibrium S02 partial  pressure  of the bulk
          HC                                                 P
solution,   S02, is usually negligible compared with that of  S02.
     If the  ratio  of the first and second terms,  k,°/Hk ,  is much less than
1, S02 mass  transfer is controlled by liquid film resistance.  If it is much
greater  than  1,  gas  film  resistance  is  controlling.   Typical  values  of
k-|°/Hk  in CaC03  slurry scrubbing are 0.05 to  0.20 (Rochelle 1977).   There-
fore, gas-  and liquid-film  resistances  are equally  important  with  enhance-
ment  factors  ranging  from 5  to  20,  whereas gas-film resistance  should tend
to dominate with enhancement factors greater than 20.
     The   enhancement  factor,  <|>,  is  a  strong function  of  gas and  solution
composition  (Chang  and  Rochelle 1980).   S02  diffusion through  the  liquid
film is enhanced  by  the following instantaneous  reversible  reactions,  which
convert it to bisulfite in the liquid film:
                              S02 + H20 2 H+ + HS03"
                             S02  + S03= + H20 +• 2HS03"
The  extent  to  which  these  reactions  occur depends  on the bulk  solution
composition  and  the  partial  pressure  of S02  at the gas-liquid  interface.
The hydrolysis  of  S02 to H  and  HS03"  is depressed by  higher  HS03   concen-
                                                                        p
trations   and  is relatively less  important than S02 diffusion  at  high   S02.
The reaction of  S02  with sulfite is  usually  limited  by  diffusion of S03~ to
the gas-liquid  interface and  by supply  of  S03~ in the  bulk liquid.   Thus,

-------
CHEMISTRY:  Mass Transfer	A-7
higher concentrations of  S03~  or sulfite species, such as  CaS03°  or MgS03°,
give greater enhancement factors.
                                                                      p
     Figure A-l  gives typical enhancement factors as  a function of  S02  at
the gas-liquid  interface  and pH in 0.1  M CaCl2 solution saturated  to  CaS03
(Rochelie 1980g).
Effect of SO? Gas Concentration
     The enhancement  factors are a strong function of  S02  gas  concentration.
At  gas  concentrations  lower than  100  to  500  ppm  S02,  most CaC03  slurry
scrubbers  can be  assumed to  be  limited by  gas-film resistance.    In  this
range, the  S02   flux  and  the amount  of  S02 to be  removed are both propor-
tional to  S02 gas  concentration.   As S02 gas  concentration increases  above
500 ppm,  the enhancement  factor decreases substantially  and  S02 flux  does
not increase  as fast as  the amount  of  S02 to be  removed.   Therefore,  the
percentage of S02  removal  decreases at higher  S02 gas concentrations.   As a
result,  achieving  a  given percentage  of  S02  removal  is usually easier at low
gas concentration  than  at high  gas concentration.   Because of changing  gas
concentration across  a  scrubber,  the  mass transfer of  S02  can be  limited by
liquid-film resistance  at the gas  inlet and  by gas-film  resistance at  the
gas outlet.
Effects  of pH and Excess Limestone
     The pH  level  of the scrubber solution  is often  a direct indicator of
concentrations  of  both  HS03"  and  S03"  in  the bulk  solution and  is  often
correlated with  S02  removal.   Because of the  tendency  of  the  solution  to be
in  equilibrium  with CaS03 solids,  either by dissolution or by crystalliza-
tion,  the  HS03   concentration is  roughly correlated with pH  by the equili-
brium:
                    CaS03 (s) + H+ 1 Ca++ + HS03"
                    [HS03~] = K [H+]/[Ca++]
Hence lower  pH  at any point in  the system always gives higher HS03 ,  which
inhibits the  hydrolysis  reaction and  thereby  reduces  the  enhancement factor
and S02  removal.

-------
Figure A-l.  Effect of pH and p$0o on SC^ absorption.
                            A-8

-------
CHEMISTRY:  Mass Transfer _ A-9
     The  pH  level  at the scrubber Inlet  is  also an indicator of the  amount
of  excess limestone;  higher pH  indicates more  excess  CaC03.   As  solution
passes  through  the scrubber, the drop in  pH  depends on the extent  to which
limestone dissolves  and  replenishes  the  alkalinity of the  solution.   With a
large excess  of CaC03, high pH  and  low  HS03" are maintained throughout  the
scrubber, whereas  with  little  excess CaC03,  the pH drops and HS03   increases
as  S02  is absorbed.   In  the absence of  CaC03,  CaS03 will dissolve with an
even greater increase in HS03 :
                       CaS03 + S02 + H20  -»• Ca"^ + 2HS03"
     In  general  the S03~ concentration  is not  a function  of  pH,   since it
tends to be controlled by the equilibrium:
                           CaS03 (s) *• Ca++ + S03~
At  high pH,  however,  CaS03  tends  to  crystallize  in  the scrubber,  giving
somewhat  higher S03;  at  low pH,  CaS03  tends  to dissolve, requiring lower
S03.
Effect of Alkali Additives
     If  a  soluble alkali salt  of hydroxide, carbonate, sulfate, or sulfite
is  added to a CaC03 slurry  scrubbing  system, the alkali will  accumulate in
solution primarily as  the sulfate salt.   For example, with Na2C03  addition:
     Na2C03 + 2HS03 + 2CaS04 (s) -> 2Na+ + S04 + 2CaS03 (s) + H20 + C02
This  reaction  results  in higher  levels  of dissolved sulfite because  of  the
equilibrium:
                       CaS03(s) + S04 *• CaS04(s) + S03
The total sulfite  concentration  will be  proportional to the  sulfate concen-
tration  and inversely  proportional  to  the  relative  saturation level  with
respect to CaS04 solid (gypsum):
                         [S03] = K      a [So]
                                    CaS04
The higher  level  of dissolved sulfite enhances diffusion  of S02 through the
liquid  film by reaction with  S02  to  produce bisulfite.   It  can also reduce

-------
CHEMISTRY:  Mass Transfer	A-10
the  pH drop across  the scrubber  and  minimize the  dissolution of  CaC03  or
CaS03  In the scrubber.
     The most  significant  alkali  additives are salts of sodium or magnesium.
Magnesium  generates  more  sulfite  species because  of its  interactions  with
sulfite and sulfate, giving the equilibrium:
                    MgS04° + CaS03 (s) ^ CaS04 (s) + MgS03°
The  MgS03° ion  pair  diffuses  at  a rate  about 45  percent  less than  free
sulfite  ion  generated  in  sodium  solution.  Therefore, magnesium  and  sodium
additives  are  probably  equivalent  in  their effects  on  S02  mass  transfer.
Figure  A-2 shows the effect of total  dissolved  sulfite (S03  + HS03)  on the
enhancement  factor  at  pH  4.5  in  solutions  containing  Na2S04  or  MgS04
(Rochelle 1980c).
     Calculated  concentrations  of sulfite and bicarbonate  species  generated
by  addition  of  MgO  and Na2C03 are  given in  Figures A-3  and  A-4  (Rochelle
1977).    These  calculations  assume  CaS03  saturation  of  1.0, with  variable
levels  of  CaS04  saturation;  they  also assume that  the solutions  contain 0.1
M  Cl  .   The effect  of CaS04  saturation is  significant.   In systems  where
oxidation  is less  than 15  to 20 percent, gypsum  does not  crystallize and the
CaS04  saturation can  be as low as 20  to 30  percent.   With 30 percent gypsum
saturation the  effectiveness of  alkali  additives  is  increased by  a  factor
of 3.
     The simplest  alkali  additives  are  Na2C03  and MgO.  Dolomitic  lime can
be used to provide MgO; however,  the magnesium content of most dolomitic or
high-magnesium  limestones  is  not expected  to be  reactive.   Cooling  tower
blowdown,  makeup water, or alkaline fly ash can  be a  significant source  of
MgS04 or Na2S04.
     Chloride  accumulation in  a   scrubber  system  suppresses  the desirable
effects  of alkali  additives by  permitting  their  accumulation as  chloride
salts  rather than sulfate salts.   The  HC1  in flue gas is absorbed  in  dis-
solved  CaCl2  in  the  absence of  alkali  additives.    The  first increment  of
alkali  additive ends up as  dissolved chloride by  the reaction:
          2Ca++ + 2Cl" + MgO  + 2HS03 -> Mg++ + 2Cl" + 2CaS03(s) + H20

-------
                       [S03=]T/PSO , mmol/liter
Figure A-2.   Effect of total  dissolved sulfite concentration
on mass transfer enhancement  at pH 4.5, 55°C,  with 1000 ppm
                     S02> 0.3 Molar ionic strength.
                              A-ll

-------
   0.20,
   0.10
2 0.05
£ 0.02
   0.01
3 0.005
  0.002
  0.001
              T
            i   i
            ' '1
          CT » 0.10 mol/l1ter
          CaSO, SATURATION =1.0
1  I     i   i   i  i i 11  i
   1    CaS04  SATURATION  -  0.3_
— P
           C02
          T - 50 "C
0.10 atm
    CaC03
- SATURATION
- 1.0
           I
       I     I   I   I  I  I I I I    I   I     I   I   I  I M I I
      0.01    0.02
                0.05   0.10     0.2       0.5      1.0
                 Mg CONCENTRATION,  mol/liter
      Figure A-3.  Dissolved alkalinity generated  by addition  of MgO.
                                  A-12

-------
    0.05
£  0.02
o

0>
    0.01
   0.005
   0.002
    0.00,
                    CaC03 SATURATION =0.1
                                    CaC03 SATURATION =0.01
  CaS04
SATURATION
  = 0.3
  CaS04
SATURATION
  = 1.0
                0.1       0.2          0.51.0      2.0
                TOTAL Na CONCENTRATION, g-mol/liter
     Figure  A-4.   Dissolved alkalinity generated by addition of
                                A-13

-------
CHEMISTRY:  Mass Transfer	A-14
By the  same  reasoning,  makeup water contain  >g NaCI  or MgCl2 has a negative
effect.   The  actual  amount  of dissolved  sjlfate  should be  approximately
equal to the "liquid goodness factor" (LGF) defined by:
                    [SOj] : LGF = [Mg++] + JsCNa""] - ^CCl"]
Work at  Shawnee  has  shown that an  LGF of 0.2 to 0.4 gmol/liter is sufficient
to get  significant enhancement of  S02 absorption (Burbank and Wang 1980s).
Effect of Buffer Additives
     Any  additive with  buffer capacity  between  the pH  of  the gas-liquid
interface (3 to  4)  and the pH  of  the bulk liquid (4.5 to  5.5)  will  enhance
diffusion  of  S02  through  the liquid  film  by  converting  it to  bisulfite
(Roche!le 1977; Chang and Rochelle  1980):
                         S02 + A" + H20 +• HSOg + HA
Such buffer  additives also reduce  pH drop across the  scrubber  and minimize
dissolution  of CaC03  and  CaS03 in  the  scrubber.   Buffer  concentrations as
Iwo as  500  to  1000  ppm provide significant enhancement  of S02 absorption and
can also  permit  satisfactory  S02 removal  at lower pH with improved limestone
utilization.    Unlike  alkali  additives,  buffer  additives  are unaffected by
chloride accumulation.
     Table A-2 gives  the  concentration  of several alternative  buffer addi-
tives  required to get  an enhancement factor of 20  in a  typical  scrubbing
solution (Chang  and Rochelle  1980).  Relative costs  of additive makeup have
been  calculated, on  the  assumption  that makeup  rate  is proportional  to
additive  concentration.   Acetic formic  acid appear  to be most  attractive,
but would  have problems  with volatility.   Sulfosuccinic  and sulfopropionic
are made  in situ by  adding maleic anhydride or acrylic  acid,  respectively,
to the scrubber loop.
     Waste streams  of carboxylic  acids  from the manufacture  of  adipic acid
or cyclohexanone  are  not listed in Table A-2, but should  be more cost-effec-
tive than  the  additives  shown there.   Figure A-5 shows  enhancement  factors
for several  different buffers  at  typical  scrubber conditions  (Weems 1981).

-------
               TABLE A-2.   CONCENTRATIONS OF BUFFER ADDITIVES
                REQUIRED TO ACHIEVE ENHANCEMENT FACTOR OF 20


Basis:  $ = 20, 55°C, pH 5.0, [Ca++] = 0.1 M, [S02]i = 0.5 mmol/liter,

        [S03=]T = 10 mmol/liter
Acid
Formic
Acetic
Adipic
Sulfosuccinic
Sulfoprop ionic
Hydroxypropionic
Phthalic
Succinic
Benzoic
Glycolic
Lactic
Acid,
mmol/liter
17.7
14.3
7.0
7.8
16.1
14.5
6.4
7.3
15.8
22.4
24.7
Cost,3
$/lb mol
12.9
11.4
63.5
44. lb
28. 8C
28. 8C
58. ld
106. Oe
36.7
30.4
73.8
Relative
cost
0.51
0.37
1.0
0.77
1.04
0.94
0.84
1.74
1.30
1.53
4.10
.  Chemical Marketing Reporter, July 2, 1979.
  Maleic anhydride
 . Acrylic acid
  Phthalic anhydride
  Succinic anhydride

Source:  Chang and Rochelle 1980.
                                     A-15

-------
     36
     34
     32
     30
     28
     26
     24
     22
     18
     16
     14
     12
     10
      8
               HYDROXPROPIONIC
        ADIPIC

SULFOSUCCINIC
                                           SULFOPROPIONIC
                    I
                      I
I
                    5           10           15
                      BUFFER  CONCENTRATION, nrool/liter
                                              20
Figure A-5.   Effect of organic  acids on the enhancement factor,
  pH 5, 0.3  M CaCl2, 55°C,  1000 ppm S02, 3 mM total sulfite.
                               A-16

-------
CHEMISTRY:  Limestone Dissolution _ A-17
     Adi pic acid  has  been tested  extensively  by EPA  at Research  Triangle
Park and  Shawnee.   Figure  A-6 shows the  effect  of adipic  acid  on  S02 removal
in the  TCA scrubber  at  the Shawnee  test facility (Burbank and Wang  1979).
EPA  is  supporting  a demonstration  of adipic  acid  at the  Southwest No.  1
station, City of Springfield,  Missouri.
     Under some  conditions organic  additives such as adipic acid will  oxi-
dize in the scrubber system.   Shawnee operation has  demonstrated  that  adipic
acid losses are minimized  by  operating at a scrubber  inlet  pH less  than 5.0
with 10  to 50 ppm  of  dissolved  manganese (Burbank and Wang 1980b;  Rochelle
1980f).    Under these  conditions  the adipic acid makeup is equal to that lost
with entrained solution in the waste solids.
Effect of Forced Oxidation
     Complete  oxidation  of sulfite  by  air injection  in the EHT  or  as  a
result of  low  S02/02 ratios in the flue  gas  will  usually  improve  S02 absorp-
tion (Borgwardt 1978).   Oxidation  reduces the bisulfite concentration in the
solution  and thereby  permits  increased enhancement of  S02 diffusion through
the  liquid film  by means  of  the hydrolysis  reaction (Chang and  Rochelle
1980):
                            S02 + H20 -> H
This  is  especially true  at  lower pH  levels  (4 to 5), where  operation  with
CaS03 solids would give a high concentration of HSQ~3.
     Complete  oxidation  in  the  scrubber  loop  is undesirable  with  alkali
additives  because  it eliminates  dissolved sulfite,  which  is  required  to
enhance mass transfer.   Alkali  additives are  effective with forced oxidation
of a  slurry  bleed stream.   Complete  oxidation does not reduce the  effective-
ness of buffer  additives,  but with operation  above pH 5 it greatly increases
oxidative degradation of organic acids (Burbank and Wang 1980b).

LIMESTONE DISSOLUTION
     Limestone dissolution occurs  in both the scrubber and the EHT.   Ideally
about  half  of the limestone would dissolve in the scrubber  to  maximize S02
removal  and  pH   and  prevent  CaS03  scaling.   In   practice  the fraction

-------
oo
                100
                 90
                 80
               u
               t-
               J 70
                 60
                 50
                 40
 APIDIC ACID
CONCENTRATION
 O 2060 ppm
 a 1360 ppm
 o 1000 ppm
 o 650 ppm
                                         •     t
                  4.2 4.3  4.4  4.5.  4.6  4.7  4.8  4.9 5.0  5.1  5.2  5.3  5.4  5.5  5.6  5.7  5.8  5.9  6.0  6.1  6.2
                                                     INLET LIQUOR pH
           Figure A-6.   Effect of  scrubber  inlet pH  and adipic acid  concentration on SOo removal
                           Source:   Burbank and Wang 1979.   3rd Shawnee Report.

-------
CHEMISTRY:   Limestone Dissolution _ A- 19
dissolved  in  the  scrubber varies  with the  amount  of  unreacted  limestone
slurry.  With  a large excess of  limestone,  practically  all  of the dissolu-
tion will  occur in the scrubber.  With  little  excess limestone,  most of  it
will dissolve the the EHT.
Equilibrium
     With  a  large  excess  of  limestone or a  large  EHT,  the solution in the
EHT will approach equilibrium with CaC03 as given by:
                    CaC03 (s) + 2H* * Ca^  + C02 + H20
                          H       Prn 0.5
                                   LU2
The equilibrium  pH depends on  the dissolved  calcium  concentration and  the
equilibrium partial  pressure  of C02  over  the solution.   Hence an  accumula-
tion of  CaCl2  tends to give' lower pH,  whereas an accumulation of  sodium or
magnesium sulfate  should  tend  to give higher  pH because  dissolved  calcium is
reduced by the equilibrium:
                         CaS04  (s) 2 Ca++ + SO^
     Carbon dioxide generated  by limestone  dissolution in the  EHT or  the
tank is  usually  stripped  out  of the  solution  by the  flue gas in  the  scrub-
ber.  Very little  C02  should  desorb from the  EHT  unless the  C02 vapor pres-
sure exceeds  1 atm.   Solution  entering the  EHT is probably  saturated with
S02 at the conditions  of  the flue gas,  about  0.1 atm.  As CaC03 dissolves in
the EHT,  C02  accumulates in  the EHT  solution.  Thus  the equilibrium  C02
partial  pressure  in the  EHT depends  on the fraction of  limestone  dissolved
in  the  EHT,  the  amount  of S02  absorbed,  and  limestone  dissolved per pass
through the scrubber.  Therefore,  equilibrium pH out of  the  EHT tends  to be
lower with less  excess CaC03 (giving a larger fraction dissolved in the EHT)
and with a lower liquid  circulation  rate or  higher  S02 gas concentration
(giving  higher make-per-pass).    The  combined  effects  of  Ca concentration  and
C02 partial pressure can  give  an equilibrium  pH  of  5.5  to 6.5 in  the  EHT.

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CHEMISTRY:  Limestone Dissolution _ A- 20
Mass Transfer
     In  the scrubber  and  in  the  EHT  with  low excess  limestone or  short
residence  time,  the  solution  usually  is not  in equilibrium  with  CaC03.
Dissolution of  limestone in the  system must  occur  at the same rate  as  S02
absorption.  Composition  of the  system  solution will adjust until the  rate
of  dissolution  is equal  to the rate  of  S02  absorption.   Generally a  low pH
level reduces the rate  of S02  absorption  while  increasing  the  rate of CaC03
dissolution.
     The  rate   of CaC03  dissolution  is  usually  controlled by diffusion  of
acid/base  species  in  the  liquid film  surrounding  a  limestone particle
(Rochelle  1979; 1980b).   In the simplest  case,  dissolution  is  controlled by
H  diffusion from the bulk solution to participate in the reaction:
                         CaC03  + H+ +• Ca++ + HC03
Therefore,  limestone  dissolves  about 10 times  faster  at pH 4.5  than  at pH
5.5.  In  this   case the  rate of  CaC03  dissolution  (R.,  gmol/sec) is  given
approximately by the mass transfer expression:
                   R  = k  • X       •  V  n      '       ~
                   Kd   kd   *CaC03   v
where:
              k. = mass transfer coefficient, m/sec
          Xp CQ3 = volume fraction CaC03 solids
               V = slurry holdup, m3
            d    = mean particle diameter, m
             a V 6
             pH* = equilibrium pH at the limestone surface
     The dissolution  rate is  a strong function  of particle  size  distribu-
tion.  The  mass transfer  coefficient  is constant  with large  particles;  it
varies  inversely  with diameter with smaller  particles.   The  total  surface
area available  for mass  transfer varies directly with the volume fraction of
CaC03  solids  in  the  slurry  and with  the  specific  surface  area,  which  is
inversely   proportional   to   particle  diameter.   Therefore   the   rate   of

-------
CHEMISTRY: Limestone Dissolution	A-21
dissolution always tends  to  be greater with finer grinds  of limestone.   With
reasonably pure  limestones (>  90 percent CaC03),  the reactivity depends only
on  the  particle size  distribution  and  not on the  source  of  the  stone
(Rochelle  1980a,b).    Available  stones  differ  primarily  in  grindability.
Thus a marble will be reactive if it is ground sufficiently fine.
     The  absolute  rate of limestone  dissolution varies with  liquid  holdup.
Typical  liquid residence  time  in the  scrubber is  1 to 10  seconds,  whereas  in
the EHT  it  is  hundreds or thousands of seconds.   The pH,  however,  is  as much
as  one  unit  lower   in  the  scrubber,  and  the EHT frequently  operates  much
closer to the  equilibrium pH.   Therefore,  absolute  rates  of  dissolution are
about the same in the scrubber and the EHT.
     Effect of Sulfite.   In   the  presence  of  normal  levels  of  dissolved
sulfite,  CaC03  dissolution  is  2 to 4  times faster  than  in  solutions  with
greater  amounts  of dissolved sulfite  (Rochelle 1980e).  The mass transfer  of
acid/base species  is enhanced  by the diffusion  of S03/HS03  buffer  species
participating in the reaction:
                      CaC03 + HS03  *  Ca++ + S03  + HC03
The concentrations  of Ca    and  S03 are  greater  at the surface  of limestone
than in  the  bulk solution,  so  there  is  a  tendency for CaS03  to precipitate
at the limestone surface.
     At  high levels  of dissolved CaS03 in the bulk solution,  CaC03 dissolu-
tion can  be  significantly inhibited or even  stopped  by formation of  a  CaS03
layer  on the limestone  (Rochelle 1980e).   Such  high  CaS03 supersaturations
typically occur  in  slurries with no  CaS03  solids but high  levels of  dis-
solved sulfite.  This  condition can be encountered  in unsteady-state opera-
tion or  in  steady-state operation where the oxidation rate is  high enough  to
deplete  solid sulfite  but not  to eliminate dissolved sulfite.   The condition
has occurred  frequently at  the Shawnee test facility with forced oxidation
in the EHT (Burbank and Wang  1980b).
     Limestone Utilization.  In  a simple  limestone slurry scrubbing system
the  limestone  utilization is  an independent variable that varies  directly
with the amount of limestone  fed to  the system.   $62 removal  requires some

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CHEMISTRY: Limestone Dissolution	A-22
excess limestone  in the  scrubber,  which usually leads to excess  limestone  in
the solid waste,  or  utilization less than 100 percent.   For a given level  of
S02,  limestone  utilization  can be improved up  to  a point by using  a  larger
EHT  or several  EHT's  in series  (Borgwardt 1975;  Burbank  and Wang 1980a).
Utilization can  also  be improved  by finer grinding of the  limestone.   With
an increase in  slurry  concentration  the concentration of CaC03 solids  in the
slurry  can   be  held   constant  while the  fraction of the total  solids  is
reduced,   with a  resulting  increase  in  limestone  utilization.    Typically,
scrubber  systems  are  designed for use  of 5 to 50 percent  excess  limestone.
     Countercurrent  and  crossflow  scrubbers  can  be  useful  in  improving
limestone utilization.   With  a double-loop countercurrent  scrubber, the top
scrubber is operated .at  high  pH with poor limestone utilization  but good S02
removal.   Slurry  from  the  top scrubber  is  fed to the bottom scrubber  (gas
inlet), which operates  at  low  pH  with  high  limestone utilization.    With
crossflow scrubbing, the slurry bleed from several parallel  scrubbers  oper-
ating  with  excess  limestone   at  high-pH  is  fed to  an additional  parallel
scrubber operating with  little excess limestone at low-pH.   The  reduced S02
removal in  the low-pH scrubber is more than offset by  improved  performance
of the high-pH scrubbers.
     Limestone utilization  can  also  be improved by separating and recycling
unreacted limestone from the  scrubber bleed.   When coarse  limestone is  used
and rather fine  CaS03  crystals are produced,  separation can be achieved  by a
cyclonic separator.  This arrangement permits operation of  the scrubber  with
excess limestone  even  though  the  system undergoes little  loss of unreacted
limestone.
     Buffer and Alkali  Additives.   Additives  used  to enhance S02  gas/liquid
mass  transfer have both good  and  bad effects on limestone  dissolution.   In
general  any  additive  or scrubber design  that improves  S02 removal  at  a
constant  limestone  utilization rate can  be  used  to  achieve the same S02
removal with improved limestone utilization.
     Buffer additives such  as  adipic acid also provide  specific  enhancement

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CHEMISTRY:  Oxidation	A-23
of  limestone  dissolution.   The  buffer species contributes  to diffusion  of
acid/base species by the reaction:
                         CaC03 + HA * Ca++ + A" + HC03
Thus  diffusion  of the  buffer  acid  can substitute  for diffusion  of  H .
Figure A-7  shows the  effect  of a simple  buffer  such as acetic acid on  the
dissolution rate of CaC03  (Rochelle  1980d).   The effects of  adipic acid  and
other useful buffers are similar.
     Alkali  additives  reduce the  level   of  dissolved  calcium and  thereby
increase  the  equilibrium  pH  for  limestone  dissolution.    Increasing  the
concentrations of  the S03/HS03 buffer (without increasing CaS03  saturation)
may also increase the CaC03 dissolution rate.
     There  is reason to  believe,  however,  that Mg  additives may  have  a
negative effect  on  CaC03  dissolution.   There is  evidence that Mg    inhibits
CaC03 crystallization and thereby  increases  CaS03 saturation  (Jones et  al.
1976).   An  increase  in CaS03 saturation  is  likely  in  turn  to cause  CaS03
blinding of limestone,  which  will  reduce  limestone utilization.   This  nega-
tive feature  of  Mg additives  may be offset by  the added capability to  oper-
ate at lower pH and still  achieve acceptable S02 removal.
     Inhibitors.    Several  substances  are  known to be potent  inhibitors  of
CaC03  dissolution.    Heavy metals  such  as  Fe  and  Mg  can   form  insoluble
carbonates  that  adsorb  on and  blind the  CaC03  surface.  Phosphate,  poly-
acrylic  acid,  and  perhaps  other polyelectrolytes used  in water treatment or
as  flocculating  agents  inhibit  CaC03   dissolution  by  surface  adsorption
(Rochelle 1977).

OXIDATION
     Because  flue  gas contains  3  to  10  percent 02,  S02  absorbed  as  sulfite
and bisulfite can be  irreversibly oxidized to sulfate:
                             HS03 + 3s02 -»• S04 + H+
As discussed  with  regard  to CaS03/CaS04 crystallization, it  is usually best
to oxidize  either  less than 15  to 20  percent of the absorbed  S02  or all of
it.  The  most difficult operating regime is in the range of  20 to 50 percent
oxidation.  To enable operation out of this  regime and generation of easily

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csj
                                      	 CALCULATED
      10
                                  5                     10

                             TOTAL ACETATE, mmol/liter
       Figure A-7.  Dissolution rate of CaC03 in 0.1M CaCl2, 25°C.
                     Source:  Rochelle 1980d.
                                 A-24

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CHEMISTRY:  Oxidation	A-25
dewatered  solids,  operators  can   achieve   complete   forced   oxidation  by
sparging  air in  the EHT  or by  oxidizing  the  scrubber bleed slurry in  a
separate  sparged  tank.   Hence, the  objectives of controlling  oxidation  are
either to inhibit it or to effect it completely.
Mass Transfer
     The  reaction rate  of SOs/HSOa  with dissolved 02  is  reasonably  high,
especially with dissolved Mn and Fe,  which  are  usually present in  scrubber
systems  (Hudson   1980).   The solubility  of  oxygen  in  aqueous solutions  is
very  limited.   Therefore,  the degree  of  oxidation is frequently limited by
the  diffusion of  02  through the liquid film.  As  soon as 02  penetrates  the
liquid film  it  is consumed by reaction with  SOa/HSOg.   Under this condition,
the  apparent oxidation  rate  is  unaffected  by  catalysts,  inhibitors,  and
other  variables  that  would  normally  affect inherent  oxidation  kinetics.
Rather, the  degree of  oxidation  depends on gas/liquid  contacting efficiency,
moles of S02 removed, and the concentration of oxygen in the flue gas.
     The  liquid-phase  mass transfer  coefficient  of 02 absorption  is  essen-
tially the same as  that which applies  for physical  absorption of S02  (k?a).
Therefore,  additional  oxidation  should  be  expected if  S02  removal  is  im-
proved by adding  mass  transfer capability in  the  form  of higher liquid flow
rate  or  increased pressure drop,  or in some  other  form.   Hence, if  oxidation
is  to be minimized,  a scrubber  should not be overdesigned for S02  removal.
The  use  of alkali or buffer  additives could reduce the  degree of  oxidation
by  permitting equivalent  S02  removal with reduced mass transfer capability.
     The  02/S02   mole  ratio  in  the gas  can directly  affect  the  degree  of
oxidation.   The  02 absorption rate  and oxidation rate are directly propor-
tional to 02  concentration.   If  the percent  S02  removal  is  constant, the  S02
absorption rate  will be  directly  proportional to  S02  inlet  gas concentra-
tion.  Therefore, the percentage of oxidation of the product solids  should
be  directly  proportional  to the  02/S02 ratio. This factor  is  evident  in  the
observed  trend  from  15 to 30 percent  oxidation with high-sulfur coal  and 50
to  100 percent oxidation with low-sulfur coal.
Reaction Kinetics
     With very  high  concentrations  of catalyst and  bisulfite  it is possible
to  consume  the oxygen  even before  it gets  through the  liquid film.   Since

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CHEMISTRY:  Oxidation	;	;	A-26
the effective film  thickness  is  smaller, a very fast  chemical  reaction will
enhance the mass  transfer  of  02.   The most important  catalysts in  limestone
scrubbing are probably  Mn  and Fe.   As much as  50 ppm Mn  has been accumulated
in testing of forced oxidation at Shawnee.  As little  as 0.5  ppm Mn can have
a  significant  effect  on the  oxidation  kinetics  (Hudson,  1980).   There  is
also evidence that N02  can have  a catalytic  effect on  oxidation (Rosenberg
and Grotta 1980).   High levels of dissolved SOa/HSO^  also appear to enhance
oxidation  kinetics.   Generally higher levels  of  oxidation are observed  in
operation at  lower pH  levels  because of the  increase in HS03  concentration
and in concentrations of metal catalyst, both  of which appear to be solubil-
ity limited.  At Shawnee,  oxidation  levels with  high-sulfur coal   have  in-
creased  from  20  to 30  percent   at  pH  5.5 to 50  to  60 percent at pH  4.5
(Burbank and Wang 1980b).
     In the presence of potent inhibitors or in the absence of  catalysts  and
dissolved sulfite,  it is sometimes possible to achieve oxidation rates  lower
than those predicted from  mass transfer  theory.  Sodium  thiosulfate has been
identified and  tested  as a potent inhibitor  in lime  scrubbing  (Hoicomb  and
Luke. 1978).  At  concentrations as low as 25 ppm it essentially  stops sulfite
oxidation at pH  5.0 (Hudson  1980).  This substance  should also be  an effec-
tive  inhibitor   in   limestone  scrubbing.   Hudson   (1980)  also  showed that
glycolic acid was a moderate  inhibitor.
Forced Oxidation
     Forced oxidation, described  in Section 1  among the process  options,  is
considered because  it permits more effective  sludge  dewatering,  which  re-
duces the quantity of  sludge  produced  and makes  it  easier  to dispose  of.
Forced oxidation can  be accomplished by sparging  air  into  the EHT  or  by
oxidizing the slurry bleed in a  separate sparged  reactor.  Forced  oxidation
in the low-pH EHT of a double-loop scrubber is relatively easier because of
the sustained low pH.    In  both  bleed stream  and double-loop  forced oxida-
tion,  the dissolution  of  CaS03  solids   can  limit  the  oxidation  rate.   In
single-loop oxidation,  CaS03  solids  are never crystallized  and do  not need
to be dissolved; however, the  blinding of limestone can be severe.

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CHEMISTRY:  Oxidation	:	A-27
     Testing of single-loop  forced  oxidation  has shown that complete oxida-
tion can  be  achieved  over the normal pH range,  5.0 to 6.2  (Borgwardt 1978).
C02 is stripped by  air sparging of the EHT,  so  that  pH  values as  high as  6
to 6.5  can be achieved  with high  air  stoichiometries.   High inlet pH com-
bined with very low dissolved bisulfite gives somewhat better  S02 removal in
the  scrubber.   Insufficient  air stoichiometry  or agitation  can  result in
reasonably complete solids oxidation  (95 to 98 percent) without oxidation of
dissolved  sulfite.    Because there  are  no CaS03 seed  crystals,   excessive
CaS03 supersaturation  can  build  up  in the  scrubber and result  in blinding of
CaC03.   This  problem  can  be avoided by using more  air  or agitation and by
reducing  the moles  of S02 absorbed  per pass through the scrubber (increasing
the liquid rate)  (Burbank and Wang 1980b).
     In bleed stream  or  double-loop oxidation, the pH level  usually must be
lower than  5.5  to  permit rapid dissolution  of  CaS03 solids (Head and Wang
1979).   Hudson (1980)  studied oxidation  kinetics of CaS03  slurries  at pH 4.3
to 6.0  and concluded  that the oxidation rate declines at higher pH because
the  reduced  solubility of the CaS03  reduces  it  dissolution rate.   A low pH
level  in  bleed  stream  oxidation  requires either the  addition  of sulfuric
acid to  neutralize  excess CaC03 or scrubber-loop operation to achieve very
high limestone utilization.   High  limestone utilization can be achieved with
satisfactory  S02  removal  by addition  of adipic acid  (Burbank  and Wang
1980b).    Alkali  additives permit the use  of  bleed stream oxidation at pH  6
to 7.5  by increasing  the concentration  of dissolved sulfite  in equilibrium
with CaS03 solids (Head and Wang 1979).
     Low-pH Operation.   Complete  S03/HS03 oxidation in  the  scrubber can
permit satisfactory S02  removal  at pH  values as low as  3.5 to 4.0.  With
low-sulfur  coals  having  alkaline  fly  ash  it   is   sometimes  desirable to
operate at pH levels  in  this low range to leach Ca,  Mg,  and  Na alkali from
the fly ash  and  thereby  reduce  limestone makeup.   The combination of  low pH,
high metals  concentrations,  and  high 02/S02 results  in complete oxidation in
the scrubber and  gives sufficient S02 removal.   Complete  scrubber  oxidation
at  low  pH  has  also  been achieved by  a  special scrubber design  in  which
additional  air  is  sparged  in  the  scrubber  vessel   (Morasky  et  al. 1980).

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CHEMISTRY: Crystallization _ _ A-28
CaS03/CaS04 CRYSTALLIZATION
     S02  absorbed  by  CaC03  slurry  scrubbing  is  removed  from the  system
continuously as  CaS03  and CaS04 solids.  Normally little  sulfite  or sulfate
is  lost  in solution purged  from the system.  Therefore CaS04  must crystal-
lize at  the  rate of sulfite oxidation and CaS03  must crystallize at the rate
of  S02 absorption  minus oxidation.   In a given system the  driving forces for
crystallization  and  supersaturation will  adjust until these rates  are bal-
anced.
     The  crystallization  processes  are  important  because  they  are directly
responsible for  the  quality  of solids, ease of dewatering, and  for the level
of  supersaturation.  High  levels of supersaturation can lead to scaling and
plugging  of  the scrubber  and can  inhibit  limestone dissolution.   Ideally,
all crystallization  of CaS03  and  CaS04  should  occur in the EHT,  but prac-
tically,  some crystallization in the scrubber is acceptable.
Crystallization Kinetics
     The  solution  driving force  for  crystallization is defined in  terms  of
relative saturation (RS).   For example, the RS for gypsum is given by:

                    K             . aCa++ ' aSOl
                            -  2H20   K<.p
                                      ;>KCaS04-2H20

The activities of  Ca  +  and SO^,  aCA++ and aS04,  vary roughly with concentra-
tions of  dissolved calcium.   The  solubility product,  Ksp,  is  derived  from
solubility data  such  that  RS  is equal to  1  when the solution is in equilib-
rium with gypsum solids.   If  RS is less  than 1,  the solids tend to dissolve;
if greater than 1, they crystallize.
     Figure A-8  shows the  general  dependence of crystallization  or precip-
itation on  RS for  both  CaS03 and CaS04.  At low values of RS  only crystal
growth  occurs and  little   nucleation  or  formation of  new  crystals is  ob-
served.    In  this region the  crystallization rate  is proportional  to super-
saturation.    Above  a  certain  critical  level of  saturation, 1.3  to  1.4  for
gypsum  (CaS04) and  6  to 8  for CaS03  (Ottmers et  al. 1974), nucleation of new
crystals  occurs   at   an   increasing   rate   and at higher  saturations,  and

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o
 CM

CM
O
(/)

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CHEMISTRY:  Crystal 1 ization	    A-30
dominates  the  crystallization.    Excessive nucleation  results  in  smaller
crystals,  which  are  more  difficult to  dewater.    High  saturations  giving
excessive  nucleation  also  result  in crystallization of  CaS03 and  CaS04  on
foreign surfaces,  i.e.,  in  scaling of scrubber equipment.  One  objective  of
EHT design is to minimize the RS of solution going to the scrubber.
     At moderate RS levels the crystallization rate is given by:

                              Rc = kCPVRWS(RS~1)
where:
          RC = net crystallization rate,  gmol/sec
          kc = crystallization rate constant, gmol/m2-sec
          p  = specific surface area of solids, m2/g solids
          Vj, = volume of slurry, m3
          W$ = slurry solids content, grams solids/m3 slurry
Since RC  is  a constant related to the rate of S02 absorption and oxidation,
an increase  in  the volume,  VR, of the EHT or  of the slurry  solids  content,
Ws, generally reduces RS of solution leaving  the hold tank.   The  specific
surface area, p,  varies  inversely with the mean particle  diameter.   Since  an
increase  in  nucleation gives  smaller particles and  larger specific surface
area,  it  also increases  the crystallization rate.  Nucleation  is usually a
stronger function  of  saturation  than crystal growth, so there can be inter-
acting effects  of an  increase in RS.   In general, operation at larger EHT
volume and  higher slurry solids  content  reduces  RS and yields  larger par-
ticles.   Typically,  EHT residence times of 5  to 30 minutes  and  solids con-
centrations of 8  to  15 percent are adequate to desupersaturate scrubber feed
solutions.
CaS03 Scaling
     The  relative saturation  of  the sulfite  solid product,   CaSOs'^O,  is
strongly dependent on  pH  because its solubility is dominated  by  the  equilib-
rium:
                                       •  k    ^ ^      —
                              CaS03 + H  «- Ca   + HS03

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CHEMISTRY:  Crystallization	A-31
Solution  entering  the scrubber  should  be slightly supersaturated to  CaS03.
As  the solution  passes  through  the scrubber,  the  HS03  concentration  In-
                                                 A A,
creases  because  of  S02  absorption  and  the  Ca    concentration Increases
because of CaC03  or CaS03  dissolution,  but the pH will  decrease  because  S02
absorption as  HS03  adds  H*  to  the  solution.  Therefore  the  RS of  CaS03
leaving the  scrubber  depends  on the extent to which  the pH drop  is  neutral-
ized by CaC03.  With  little CaC03 dissolution, the RS of CaS03  at the  scrub-
ber exit  can be less  than 1.   If  there  is a large excess of CaC03  with  re-
sulting CaC03 dissolution,  the  RS of CaS03 will be excessive and  will  result
in  CaS03  crystallization  and nucleation  in the  scrubber.   Depending  on  the
concentration of  CaS03 solids  and design of the scrubber, these  conditions
may or may not cause scaling.
     Shawnee  operation has   shown  that  reliable  performance  of  the mist
eliminator requires avoidance of excess  CaC03  (Head 1976).  The  presence of
excess CaC03 causes CaS03  to crystallize in the mist  eliminator  and results
in  a  "sticky"  mud  deposit.   At  Shawnee  the mist  eliminator was  kept clean
with  limestone  utilization  greater than  85 percent.   This  result  was  ob-
tained in operation with  Fredonia fine  limestone.   Less reactive  or coarser
stones should  give reliable  operation  at lower utilization.   One potential
problem  of  double-loop  scrubbing  is  the  presence  of  excessive, unreacted
limestone in the high-pH loop.
CaS04 Scaling
     CaS04 can  crystallize  gypsum,  CaS04-2H20, and as a  hemihydrate  in solid
solution  with  CaS03,  (CaS03)1_x'(CaS04)x'%H20.    Relative  saturation   is
usually defined in  terms  of gypsum.   If  the sulfate  concentration (or  oxida-
tion) of  the solid solution is  less than  15 to  20 mole percent,  the  gypsum
saturation is less  than  1.0,  so gypsum  does not crystallize  (Borgwardt 1973;
Jones  et  al.  1976; Setoyami  and  Takahashi  1978).   Under these  conditions,
crystallization rates are  the  same as  those for CaS03.   Furthermore,  since
gypsum saturation  is  less  than  1.0, there  is  usually no problem  with  gypsum
crystallization or scaling in the scrubber.
     With oxidation levels  greater than  15 to 20 percent,  CaS04  is  crystal-
lized  both  as   gypsum   and  as  solid solution containing  15  to 20 percent

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CHEMISTRY:  Crystallization _ ; _ A-32
CaS04.   Gypsum saturation  can be estimated  at 50°C  by  the equation  (Head
1977):
          RS • cca" cso; <    + 47)
where:
             = ionic strength in solution containing only Ca  ,  Mg  ,  $64,
               and Cl"
                               jpA    • «         _
           C = molarities of Ca  , Mg  , and 504
     Solution  entering the  scrubber should  be  slightly  supersaturated  to
gypsum.  As  the  solution  passes through the scrubber,  the  sulfate concentra-
tion  increases because of  sulfite oxidation  and the calcium  concentration
increases because  of  CaC03  and CaS03 dissolution.   For a given  percentage of
oxidation, the absolute concentration of  changes of 504 and Ca    vary  pro-
portionately with  the moles  of S02 absorbed  per liter of  solution  passing
through the  scrubber  (the  S02 make-per-pass).   Therefore,  the gypsum  satura-
tion  leaving the scrubber is  greater than that entering,  to an  extent  pro-
portional to the make-per-pass.   The make-per-pass varies  inversely with the
ratio of  liquid-to-gas  flow  rates (L/G)  through the scrubber.   If the L/G is
too  low (and  the  make-per-pass  is too  large),  the  gypsum RS  leaving  the
scrubber can exceed the  critical  level  of  1.3  to 1.4 and  can lead to severe
scaling.
     Chloride  accumulation of  alkali additives can influence gypsum scaling.
With  the  accumulation of CaCl2  in the  scrubber inlet solution,  the  concen-
tration of. dissolved  sulfate is reduced.   Under these conditions  a change of
Ca    concentration across the scrubber  has less  effect  on  the  outlet gypsum
saturation than a  change  in  sulfate concentration.  Therefore, oxidation in
the  scrubber  is more troublesome  than  solids dissolution.   On  the  other
hand, the accumulation  of  sulfate salts  from alkali  additives depress the Ca
concentration  in the  scrubber inlet.   Therefore oxidation and  the change of
sulfate concentration  are unimportant,  and .the dissolution of CaC03 or CaS03
solids  and  increase of dissolved  calcium are critical to gypsum scaling.

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CHEMISTRY:  Summary	A-33
Forced Oxidation
     The  use of  forced  oxidation in the scrubber loop  can  eliminate  problems
with both CaS03  scaling  and  gypsum scaling.   If care is  taken to  oxidize
completely  both  solid  and dissolved  sulfite and  to  avoid the  use of  ex-
cessive limestone, crystallization  and  scaling of CaS03 are avoided.  Gypsua
scaling is  prevented by  the  presence of  excess gypsum  surface area in  the
                            ^^        ^
scrubber.    Increases in  Ca   and  SO^  across the  scrubber are depleted  by
controlled  crystallization on   the  gypsum  solids  in the scrubber before
excessive saturation can  develop.   Operation of forced oxidation  systems  has
shown no evidence of high saturations of gypsum (Head and Wang  1979).

SUMMARY
     The  chemical performance of a  limestone slurry scrubbing  system can be
represented by the degree of  S02 removal  and the extent of scale-free opera-
tion.  A  given  system must be designed  to operate over  a  specific  range of
02  and S02  gas  concentrations  and with  a specific  degree  of chloride  or
sulfate accumulation from  impurities in  the  flue  gas,  fly ash, and makeup
water.   The important  chemical  design  variables  are limestone grind,  EKT
volume, percentage   of  slurry solids,  and  optional  process  configurations
such as forced oxidation  and  double-loop  scrubbing.   Other design variables
that may  be manipulated  to control  or  optimize an operating  system  include
liquid to gas ratio  (L/G), limestone utilization (or pH),  and  concentrations
of  additives  such  as  soluble  alkalis,  buffers,  and  oxidation  inhibitors/
catalysts.
S02 Removal
     S02  removal  is directly   related  to  limestone  utilization,   particle
size,  and solids concentration  in  the  scrubber.   Relatively  lower  utiliza-
tion,  finer grind,   and  higher  solids concentration facilitate S02  removal.
To  a lesser extent,  greater  volume of  the EHT  also  improves S02  removal.
High  L/G  not  only  increases  mass  transfer  by physical   effects  but  also
enhances  S02  removal  by  reducing  the  need  for  CaC03  dissolution  and  by
reducing  bisulfite concentration in the scrubber.  These variables  all  tend
to  give  higher   pH and  lower  bisulfite  concentration  in the scrubber,  which

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CHEMISTRY:  Summary _      A-34
promote the  liquid-phase diffusion  of  S02 as bisulfite  through  enhancement
of the hydrolysis reaction:
                                           +
                              S02 + H20   H
     Alkali and buffer  additives  improve S02 removal without  reducing  lime-
stone  utilization.   Alkali  additives generate  high  concentrations of  dis-
solved  sulfate,  which induce  higher  sulfite (SOs) concentrations  by  solid/
liquid equilibria of the form:            ,
                         CaS03 (s) + S0« t CaS04 (s) + SOj
Both sulfite  and  basic  buffer species (A~) enhance liquid-phase diffusion by
reacting with S02 to allow its diffusion as bisulfite:
                            A" + S02 + H20 + HA + SOl
The  maximum  effect  of  these  additives  is achieved  when  S02 absorption  is
controlled by liquid-film diffusion rather than gas-film diffusion.
     Double- loop  scrubbing  and  other  process  options  can  lead  to  lower
limestone utilization in  the scrubber,  and thereby improve  S02  removal  at a
given rate of limestone utilization in the system.
     Forced oxidation in  the scrubber loop improves  S02  removal  by removing
dissolved bisulfite  from  the scrubber feed.  This  increases  the  enhancement
of mass transfer by the hydrolysis reaction.
     The percentage  of  S02  removal  is usually greater at lower S02  inlet gas
concentrations.    The enhancement  of liquid-film diffusion by  the hydrolysis
reaction and  by  reaction  with sulfite is greater at lower S02 concentration.
In the  range of  100 to 500 ppm  S02,  S02  removal   is controlled  by gas-film
diffusion.
     For  soluble  salts  in  the flue  gas,  flyash,  makeup  water, and  alkali
additives there  is  a strong interaction  of  chloride and sulfate  accumula-
tion.   With  the  soluble  ions, Na+, Mg++,  and Cl", the  sulfate  accumulation
is given by the "liquid goodness factor" (LGF):
                         LGF = Mg++ +  2Na+ - 2Cl"
Therefore  in the range  of  positive  LGF, higher  chloride   levels tend  to
reduce  the  accumulation of  sulfate in  solution and  its positive effects  on
S02 removal.

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CHEMISTRY:  Summary	A-35
Scale-free Operation
     For scale-free  operation  the  EHT  must  be designed  and controlled  so
that there is  no  excessive  supersaturation  of  CaS03  or  CaS04  in  the  solution
returning to the  scrubber or in solution leaving the  scrubber.
     Relatively higher  EHT  volume  and  solids concentration  reduce  super-
saturation at  the hold tank exit,  with a corresponding reduction of  super-
saturation leaving  the  scrubber.   An  increase in solids concentration  pro-
vides an additional  secondary  reduction of  scrubber  supersaturation  by means
of controlled crystallization in the scrubber.
     Low  limestone  utilization  or  blinding of finely  ground limestone  can
cause CaS03  scaling  in  the  scrubber,  which in  turn  gives  CaS03  crystalliza-
tion by the stoichiometry:
                       CaC03(s) + S02  •» CaS03(s) + C02
Moderate  levels  of  CaC03   dissolution  in the  scrubber give the  acceptable
stoichiometry:
                    CaC03 + 2S02 + H20 •» Ca++  + 2HS03 +  S02
     A  high   L/G   ratio   is  needed   to   reduce  the   increase  in  gypsum
(CaS04*2H20)  saturation across  the  scrubber.   An   increase in  L/G  ratio
reduces the  S02 make-per-pass  and therefore reduces  the moles/liter  of CaC03
dissolution and sulfate formation.
     Low 02/S02 in  the  flue gas or the use of a potent oxidation inhibitor
(e.g., sodium  thiosulfate)  can prevent gypsum  crystallization and  scaling by
reducing solids oxidation   below  15 to  20  percent.   Under these  conditions
calcium sulfate  is  crytallized  as  a   solid solution With the CaS03 solids,
and the gypsum saturation can be substantially less than 1.
     Forced  oxidation  in  the  scrubber  loop  prevents  both  CaS03  and  CaS04
scaling.    It eliminates CaS03 in the  solids  and solution.  The presence of
high  CaS04  solids concentrations permits desupersaturation  in  the  scrubber
by controlled crystallization on gypsum surfaces.

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CHEMISTRY:  References                                        	A-36
                          REFERENCES FOR APPENDIX A


Borgwardt,  R.  H.   1974.    Symposium  on  Flue Gas  Desulfurization,  Atlanta,
Georgia, November, 1974.  EPA-650/2-74-126a.  NTIS No. PB-242572.

Borgwardt,  R.  H.   1975.   Increasing Limestone utilization  in  FGD Scrubbers.
Paper presented  at the AICHE 68th Annual Symposium, Los Angeles, California,
November 16-20, 1975.

Borgwardt,  R.  H.   1978.   Effect  of  Forced  Oxidation   on  Limestone  SO
Scrubber Performance.   In:   Proceedings  of the  Symposium on  Flue Gas Desul-
furization, Hollywood,  Florida.   November 1977.   Vol. I.   EPA-600/7-78-058a.
NTIS No. PB-282 090.

Borgwardt,  R.  H.   1979.    Significant  EPA/IERL-RTP  Pilot  Plant  Results In
Proceedings  Industry.   Briefing  on EPA Lime/Limestone Wet   Scrubbing  Test
Programs, August 1978.  EPA-600/7-79-092.  NTIS No. PB-296 517.

Burbank, D. A.,  and S. C. Wang.   1979.   Test Results on Adipic Acid-Enhanced
Lime/Limestone Scrubbing  at the  EPA Shawnee Test Facility.   Presented at the
Industry Briefing  on EPA  Lime/Limestone Wet Scrubbing Test Program, Raleigh,
North Carolina, December 5.

Burbank, D. A.,  and S. C. Wang.   1980a.   EPA Alkali Scrubbing Test Facility:
Advanced Program—Final  Report (October  1974 to June  1978).   EPA 600/7-80-
115.  NTIS No. PB80-204 241.

Burbank,  D.  A.,  and  S.  C.  Wang.   1980b.   Test  Results  on  Adipic  Acid -
Enhanced Limestone Scrubbing at  the  EPA Shawnee  Test Facility  -  Third Re-
port.   Presented  at  the  Symposium on  Flue  Gas   Desulfurization,  Houston,
Texas, October 28-31, 1980.

Chang,  C.  S., and G.  T.  Rochelle.   1980.  Effect  of Organic  Acid Additives
on  S02  Absorption into CaO/CaC03 Slurries.  Proceedings of the  Second Con-
ference on  Air Quality Management in the Electric Power Industry, University
of Texas at Austin, Austin, Texas.

Epstein, M.   1975.   EPA Alkali  Scrubbing Test Facility:   Summary of Testing
Through October 1974.  EPA-650/2-75-047.   NTIS No.  PB-244 901.

Head, H.  N.   1976.   EPA  Alkali  Scrubbing  Test  Facility:   Advanced Program,
Second Progress Report.  EPA-600/7-76-008.  NTIS No. PB-258 783.

-------
CHEMISTRY:  References
                                                             A-37
Head,  H.  N.   1977.   EPA Alkali Scrubbing Test  Facility:   Advanced Program,
Third Progress Report.  EPA-600/7-77-105.  NTIS No. PB-274 544.

Head,  H.  N., and  S.  C.  Wang.   1979.    EPA  Alkali Scrubbing  Test Facility:
Advanced  Program,  Fourth  Progress Report.   EPA-600/7-79-244a.   NTIS  No.
PB80-117 906.

Hoi comb,  L.,  and  K.  W.  Luke.   1978.    Characterization  of Carbide  Lime to
Identify  Sulfite  Oxidation Inhibitors.   EPA-600/7-78-176.   NTIS  No.  PB-286
646.

Hudson,  J.  L.   1980.   Sulfur  Dioxide  Oxidation  in  Scrubber  Systems, EPA-
600/7-80-083.  NTIS No. PB80-187 842.

Jones,  B.  F., P.  S.  Lowell, and  F. B.  Meserole.  1976.   Experimental  and
Theoretical  Studies  of Solid  Solution  Formation  in  Lime  and Limestone S02
Scrubbers.  Vol.  1.  EPA 600/2-76-273a.  NTIS No.  PB-264 953.

Lowell,  P.  S.,  et al.   1970.   A  Theoretical  Description  of  the Limestone
Injection - Wet  Scrubbing Process.  Vol.  I.   NAPCA Report.  NTIS No. PB-193
029.

Morasky,  T.  M.,  D.  P.  Burford, and 0.  W. Hargrove.   1980.   Results of the
Chiyoda  Thoroughbred  -  121  Prototype  Evaluation.   Presented  at  the  EPA
Symposium on  Flue  Gas Desulfurization,  Houston, Texas,  October 28-31, 1980.

Ottmers,  D.  M. ,  Jr.,  et al.  1974.   A Theoretical and Experimental Study of
the  Lime/Limestone  Wet  Scrubbing  Process.    EPA-650/2-75-006.   NTIS  No.
PB-243 399.

Rochelle, G.  T.   1977.   Process Synthesis and  Innovation in Flue Gas Desul-
furization.  EPRI  FP-463-SR.

Rochelle, G. T.   1979.  Monthly Progress Report  for EPA Grant  R806251.
Rochelle,
R806251.

Rochelle,
R806251.

Rochelle,
R806743.

Rochelle,
R806251.
Rochelle,  G.   T.
R806251.
G.  T.    1980a.   Monthly  Progress  Report  (April)  for  EPA Grant


G.  T.   1980b.   Monthly  Progress  Report  (May)  for  EPA Grant


G.  T.    1980c.   Monthly  Progress  Report  (June)  for  EPA Grant


G.  T.    1980d.   Monthly  Progress  Report  (July)  for  EPA Grant


        1980e.   Monthly Progress  Report (October)  for  EPA Grant

-------
CHEMISTRY:  References	A-38

Rochelle,  G.  T.   1980f.   Monthly Progress  Report (October)  for  EPA  Grant
R806743.

Rochelle,  G.  T.  1980g.   Monthly Progress  Report  (December)  for  EPA  Grant
R806743.

Rosenberg, H. S.,  and H. M. Grotta.   1980.   NO  Influence on  Sulfite Oxida-
tion  and  Scaling  in  Lime/Limestone  FGD  Systems.   Env.  Sci.  and Tech.,
14(4):470-472.

Setoyamak, K.,  and  S.  Takahashi.  1978.  Solid  Solution  of Calcium Sulfite
Hemihydrate and Calcium Sulfate.  Yogyo-Kyokai-Ski, 86[5].

Weems,  W.  T.   1981.   Enhanced Absorption  of Sulfur  Dioxide by Sulfite and
Other Buffers.  M.S. Thesis, University of Texas  at Austin.

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                                 APPENDIX B
                             OPERATIONAL FACTORS

     The theoretical material  in  Appendix A, which deals with  the  important
process chemistry parameters,  also  provides  background for  understanding the
effects of  some operational factors  of a limestone FGD system.  Many  chem-
ical  and  operational  factors  are  interrelated.   For  example,  the  deter-
mination of  liquid-to-gas  (L/G)  ratio on the basis of liquid phase alkalin-
ity  (LPA)  in  the  scrubbing  liquor  is  a  chemical procedure,  whereas  the
relation  of  L/G ratio to  liquid/gas contact  area is a physical  considera-
tion.  Similarly, any  discussion  of scale formation and means  of preventing
it must consider both chemical  and physical aspects.
     Because of  such interrelationships,  this  appendix is  complementary to
Appendix A.  It  emphasizes on-line  operation of a  limestone FGD system, and
thus many  of the references cited  pertain to experience gained  with  opera-
tional systems.

LIQUID-TO-GAS RATIO
     The proper  ratio  of slurry flow rate in  the  scrubber to  flue  gas flow
rate is very important for effective removal of S02.   The  primary effect of
increasing  liquid  circulation  flow rate  (or a  higher  L/G ratio  for a  given
gas  flow  rate)  is  to increase the  rate of mass transfer from gas to liquid,
which in turn increases S02 removal  efficiency.
Effect on S02 Removal
     According  to  Corbett  et  al.  (1977),  the differential S02  absorption
rate can be calculated by the following equation:
                            GdY = KG (adV) P (Y-Y*)                (Eq.  B-l)
                                     B-l

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OPERATIONAL FACTORS:  L/G Ratio _    B-2
where      G = molar flow rate of the gas, Ib-mol/h
           Y = mole fraction of S02, Ib-mol S02/lb-mol flue gas
          KG = over-all mass transfer coefficient, Ib-mol/ft2-h-atm
           a = gas-liquid interfacial mass transfer area, ft2/ft3 slurry
          dV = volume of the slurry holdup in a small differential, ft3
           P = total pressure of the system, atm
          Y* = bulk gas-phase mole fraction of S02 in equilibrium with the
               bulk absorbing liquor, Ib-mol S02/lb-mol gas

If  we  assume that  the equilibrium  mole  fraction of S02  (Y*)  is  negligible
compared with the  actual  mole fraction (Y)  in  the  gas phase and that values
of  the over-all  mass  transfer  coefficient (Kg) and the gas-liquid  inter-
facial area  (a)  are constant throughout the scrubber, the above equation can
be  integrated to yield the following (Corbett et al. 1977):
               1 =  (Yin ' Yout)/Yin = 1 - exp C- KG aP (V/G)]     (Eq. B-2)

where      q = S02 removal efficiency, percent
         Y.  =. mole fraction of S02 at the inlet of the scrubber, Ib-mol
          in   S02/lb-mol flue gas
        Y  .  = mole fraction of S02 at the outlet of the scrubber, Ib-mol
         OUL   S02/lb-mol flue gas
           V = volume of slurry holdup in the scrubber, ft3

Equation B-2  shows  that for a given molar gas flow rate (G),  the S02 removal
efficiency can  be  increased by increasing the over-all mass transfer coeffi-
cient  (Kg),  the gas-liquid  interfacial  area (a),  the total  pressure  of the
gas phase  (P),  or  the liquid holdup  (V).   In limestone scrubbing,  the pres-
sure is seldom  increased and is close to 1 atmosphere.  The effect of liquid
flow rate on other variables is discussed later.

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OPERATIONAL FACTORS:  L/G Ratio	B-3
     Corbett  et  al.  (1977)  have  also  shown  the  following  relationship
between  the  overall  mass  transfer  coefficient  (KQ)   and  individual  mass
transfer coefficients:
                    I/Kg = (l/kg) + (H/k£)                         (Eq.  B-3)

where     k  = gas-side mass transfer coefficient, lb-mol/ft?-h-atm
          k. = liquid-side mass transfer coefficient, ft/h
           H = Henry's Law constant, atm-ft3/lb-mol
              *
Because the  individual  mass  transfer coefficients (k  and  k.)  increase with
                                                     9       *
the gas and  liquid flow rates (Wen  et  al.  1975), an increase  in  the  liquid
flow rate increases the overall mass transfer coefficient (Kg).
     The gas-liquid  interfacial  area (a) available for  mass transfer depends
on  the  type of  the  scrubber, but  may also  be  affected by the  liquid flow
rate.   If a  venturi  or spray scrubber is used and if the droplets are spher-
ical and uniform, the area is given by:
                                    a = 6/d                        (Eq. B-4)

where      a = interfacial area, ft2/ft3 slurry
           d = diameter of the droplet, ft

For  a given  number  of  nozzles, an  increase in  liquid  flow rate  produces
finer  droplets  and thereby  increases  the  interfacial  area in a  venturi  or
spray scrubber.
     If  a mobile-bed  scrubber  is  used,  the area  is  usually expressed  in
square feet per  cubic foot of packing.   If the  packing is a  bed  of uniform
spheres, the area is given by:

                                   a' = 6/d1                       (Eq. B-5)

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OPERATIONAL FACTORS:  L/G Ratio	B-4
where     a1 = interfacial area, ft2/ft3 of packing
          d1 = diameter of the sphere, ft

In this  case,  an increase in liquid  flow rate does not increase  the  inter-
facial area.
     The  liquid  holdup (V)  in  a scrubber,  which affects  the degree  of  S02
removal,  is  given  by  Equation  B-6  for  a venturi  or spray  scrubber  and
Equation B-7 for a mobile-bed scrubber:
                               V = Vs (1 - E)                      (Eq.  B-6)

                               V = Vp (1 - E1)                     (Eq.  B-7)

where     V  = volume of the scrubber, ft3
          V  = volume of the packing, ft3
          E  = voidage, ft3/ft3 of scrubber volume
          E1 = voidage, ft3/ft3 of packed volume

For  a  given scrubber,  the  volume (V )  is  fixed.   For a venturi,  spray,  or
mobile-bed  scrubber,  the voidage  decreases  with an increase in  liquid  flow
rate, which  increases  the  holdup of liquid  in  the  scrubber (Treybal  1968).
Minimum L/G Ratio and Liquid Phase Alkalinity
     The minimum  liquid  flow rate required for  a given amount of S02  removal
is  determined  by  an  overall  material   balance.   Because  each  mole of  S02
reacts with 1 mole of alkalinity in the liquid phase,
     Molar rate     Molar rate of alkalinity   molar rate of       (Eq.  B-8)
       of S02    £     fed to the scrubber   +  alkalinity
       removal                                 formation by
                                               dissolution

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OPERATIONAL FACTORS:  L/G Ratio	;	B-5
If  there  is no  dissolution  of solids  in  the scrubber,  gas and  liquid  flow
rates are related as follows:
                     G (Yin " Yout) - L [(LPA)R ' (LPA)s]          (Eq'  B"9)

where      L = volumetric flow rate of the liquor phase of the recycle
               slurry, ft3/h
      (LPA)R = liquid phase alkalinity of the recycle slurry, lb-mol/ft3
      (LPA)  = liquid phase alkalinity of the spent slurry, lb-mol/ft3

Equation B-9 can be rearranged to give

                       min     in    out        R        s

Note  that  in  Equation  B-10  the  L/G  ratio  is expressed  in cubic  feet  of
liquid per pound-mole  of gas because G  is  a  molar flow rate of the flue gas
(Ib-mol/h).   The  significance  of  (LPA)R  and  the need  for dissolution  of
CaC03 in the  scrubber are best illustrated by an example.   For  a high-sulfur
coal application,  the following values are typical:

         Y.  = 3 x 10"3 Ib-mol S02/lb-mol flue gas (equivalent to 3000 ppm
          ln   so2)
        Y  .  = 3 x 10"4 Ib-mol S02/lb-mol flue gas (equivalent to 300 ppm
               S02)
      (LPA)R = 1.25 x 10"4 lb-mol/ft3 (equivalent to 2 x 10"3 g-mol/liter)
      (LPA)s = 1.25 x 10"5 Ib-mol/ft3 (equivalent to 2 x 10"4 g-mol/liter)

These values  would correspond to  a  minimum L/G value of  24.0  ft3/lb-mol  or
500  gal/1000  scf.   Because  this L/G value  is economically  and technically
infeasible, the  value of  (LPA)R must be  increased by  a  factor of  6  to  8.
This  increase can  be achieved  by  the  use of  alkali  or  buffer  additives.
Alternatively, the  system  must be designed to  achieve  dissolution  of solids

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OPERATIONAL FACTORS:  L/G Ratio	B-6
in  the  scrubber.   The  differential  rate of  solids dissolution can  be  cal-
culated as follows:
                      Rd = (kd)(Ap)(dV) (C%lk-Calk)            (Eq.  B-ll)
where     R. = rate of solids dissolution, Ib-mol/h
          k. = mass transfer coefficient for solid dissolution, ft/h
          A  = surface area of the particles, ft2/ft3 of slurry
          dV = differential of the volume of the slurry holdup, ft3
       C* ,.  = solubility or equilibrium concentration of the alkaline
         31 K   species, lb-mol/ft3
        C , .  = actual concentration of the alkaline species,  lb-mol/ft3

For simplicity,  let  us assume that the average  rate  throughout the scrubber
is given by
                      Rd = (kd)(Ap)(dV) (C*a]k - Calk)            (Eq.  B-12)

where  the  bars  indicate an  average value  across  the scrubber.   Combining
Equations B-8, B-9, and B-12, we obtain
               (kd)(Ap)(dV) (C«a,k - Ca,k) > G (Y.n - Y)       (E,.  B-13)
or
               min  (LPA)R - (LPA)s + kd Apd(f) (C*alR - Calk)    (Eq.  B-14)

     Thus,  the  minimum  L/G ratio  needed to  produce  a  given S02  gradient
(Y.  - Y  .) can  be decreased  by  increasing the  alkalinity of the  recycle
slurry, the  dissolution rate  constant  (kd), the  surface area of  particles
(AD)» and the liquid  holdup (V) or the  residence  time of the slurry  in  the

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OPERATIONAL FACTORS:  Gas/Liquid Distribution	B-7
scrubber (V/L).  To bring  the minimum (L/G)  value  between  3  and  4  ftVlb-mol
(62.5 to 83.3  gal/1000  scf) for the LPA values  used  above  would  require that
the  dissolution  term (k.)  in Equation B-14  be between 7.8 and 5.6 x 10 4
lb-mol/ft3 (12.5 and 8.9 x 10"3 g-mol/liter).
     In the  foregoing example,  some  typical  LPA  values  are used to  illus-
trate that limestone  scrubbing without additives requires most  of the  alka-
linity  to  be  provided  by  dissolution  of  limestone  in  the scrubber.   The
effect  of  S02 make-per-pass,  as described  in  Appendix A,  can  reduce  this
dissolution  requirement.   Certain minimum values  of  LPA  are obtainable  in
both  the  recycle  and the  spent  slurry,  based  on  driving  force  considera-
tions.  With  a countercurrent  scrubber,  these  minimum values correspond  to
equilibrium  S02  mole  fractions of  less than  5 times the  actual  S02  mole
fraction in the  flue  gas  at the scrubber outlet (Y  .) and inlet (Y^).   For
a specific case, these  minimum values of (LPA)R and  (LPA)S should  be  used in
Equation B-14 to determine the minimum L/G ratio.
Actual L/G Ratio
     In a  tray or  mobile-bed scrubber, the  maximum permissible  liquid and
gas  flow rates are determined by the  flooding  characteristics of  the scrub-
ber  (see  the later  discussion of gas  velocity in this appendix).   Another
consideration in determining  the actual L/G  ratio is  the  control  of  scaling
and  plugging (also discussed  later).   The actual  L/G  design ratio should be
higher than the minimum L/G ratio.

GAS/LIQUID DISTRIBUTION
     In limestone   scrubbing,  proper  distribution  of  gases and liquids  is
critical for maintaining the  design S02 removal efficiency.  Poor distribu-
tion  will  reduce  both the time  of  gas-liquid  contact   and  the  effective
interfacial mass  transfer   area.   Plexiglas  models have been used to  verify
and  support  the  gas/liquid  distribution  design  of  commercially available
scrubbers.   This  model  information should be considered  in the design  of a
full-scale unit.

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OPERATIONAL FACTORS:  Gas/Liquid Distribution	        B-8

Gas Distribution
     Uniformity of  gas  distribution  across  the scrubber should be  a primary
design consideration  in the  selection of a  scrubber.   Analysis of Plexiglass
models have shown  that  even  gas distribution can  be maintained by the use of
aids  such  as   ladder  vanes  or  perforated  trays.   Spray  tower  scrubbers
require no  aids  because the  energy expended by the sprays  against the rising
flue  gas  is  sufficient to  redistribute the gas  uniformly.   Enough  spray
nozzles must be  used  to cover the scrubber  cross-sectional  area with a spray
pattern providing  considerable  overlap and  uniform dense spray zones through
which the gas must pass (Saleem 1980).
Gas Inlet Design
     Gas  inlet  design  should  also be optimized  in the scrubber because of
its effect  on  gas  distribution.   Long uninterrupted sections  of  inlet duct-
work  should  be provided to  minimize  any disturbances  caused by bends.   If
long  runs  of  ductwork  cannot be used and  if the direction of the  gas  flow
changes at  the  scrubber inlet,  the  use  of  turning  vanes is recommended.
     Additionally,   the  gas  inlet  design must keep liquid from entering  and
drying on the  hot  surface  and thus prevent  the creation of a  wet/dry inter-
face  that will  allow  buildup  of deposits.   These  deposits  can  seriously
interfere with  gas flow.  Diversion  plates  should  be  installed around  the
inlet duct  opening to  prevent  liquid entry.   Spray nozzles in the  vicinity
of the duct must be carefully angled to avoid spraying  into the duct (Saleem
1980).
Liquid Distribution
     In a venturi  scrubber,  the gas  pressure drop across the variable throat
atomizes  the  liquid  drops into  finer droplets,  and control of the  pressure
drop  can  be  effected  by  positioning  the  variable throat.   The degree  of
atomization is  thus  limited  by  the  allowable power consumption of  the  fan.
     In a  spray scrubber, the  droplet  size  is  controlled primarily  by  the
nozzle  type,   size of  the  nozzle  opening,   and  pressure  drop  across  the
nozzle.   Typical  nozzle pressure  drop is 10  to  20 lb/in.2   Although finer
droplets are desirable  because  they  increase the  interfacial  area,  the lower

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OPERATIONAL FACTORS:   Gas Velocity and Pressure Drop	B-9
limit on  the  droplet size is set by  the  allowable power consumption of  the
pumps and  the entrainment of  the droplets  by the gas.   The spray  nozzles
should  produce  droplets  with  an average diameter of  roughly 2500  microns
(Saleem 1980).   The  angle of  the spray  pattern  is another  important  vari-
able.   The  angle should  be  large enough  to effect maximum  gas-liquid con-
tact, but  small  enough  to  minimize the  coalescence of  droplets from  two
adjacent  nozzles and   the  impingement  of  droplets  on  the  walls.    In  a
vertical spray  scrubber,  it is essential  that spray  headers be situated  at
various  levels  to prevent  channeling  (segregation of  the  gas  and  liquid
flows) caused by coalescence of the  droplets as they flow downwards.
     In a mobile-bed or packed-bed  scrubber, fine  droplets  of slurry  are  not
necessary.   At  least five points of liquid introduction should be provided
per  square  foot  of  tower  cross  section  (Treybal 1968).   In a  mobile-bed
scrubber, movement of the packing on each stage ensures  proper liquid redis-
tribution.    In  the  only packed-bed  scrubber  used  commercially  (Research-
Cottrell/Munters) the packing  has built-in liquid  redistribution.   In a tray
scrubber, the design of inlet  nozzles,  downcomers, and  trays is critical  to
proper liquid distribution.

GAS VELOCITY AND PRESSURE DROP
     For a  given superficial  molar  gas  flow  rate (G), the  cross-sectional
area of the  scrubber is  determined on  the basis  of an operating  gas  veloc-
ity,  which   may  be   expressed  for  purposes  of  calculation   in   molar
(lb-mol/h-ft2),  mass (lb/h-ft2),  or  volumetric (ft3/h-ft2)  units.   Designing
for  operation  at the  highest possible   gas  velocity  minimizes  the  cross
section  and  capital  cost  of  the  scrubber.   The upper  limit on  the  gas
velocity  is  set by  the  flooding   potential   (or the  pressure  drop)  and
entrainment potential,  which are discussed below.
Flooding Potential
     In  a  mobile-bed   or packed-bed  scrubber, the  pressure  drop  of  gas
through  the  system  is   influenced  by  the gas and liquid flow  rates in  a
manner  shown  in Figure  B-l  (Treybal 1968).   In the  region below Line A  of
the  figure, the  pressure  drop  increases with  gas  velocity  at a given liquid

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                            FLOODING
               t
00

o
 SUPERFICIAL LIQUID FLOW RATE,

 L' = Ib/h-ft*

 SUPERFICIAL GAS FLOW RATE,

 G' = lb/h-ft2


^-  = PRESSURE DROP PER UNIT
c     DEPTH OF PACKING, 1n.H0
                        ~
                                                     LOG
                    Figure B-l.  Effect of gas and liquid flow  rates  on gas  pressure  drop  in  a
                                        packed-bed scrubber (Treybal  1968).

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OPERATIONAL FACTORS:   Gas Velocity and Pressure Drop	B-ll
flow  rate.    The   liquid  holdup  is reasonably  constant with  changing  gas
velocity,  although  it  increases  with  liquid  flow  rate.   In  the  region
between  Lines  A and  B,  the liquid  holdup increases  rapidly  with gas  flow
rate,  the free  area for  gas flow  becomes smaller,  and  the pressure  drop
rises  rapidly.   This  condition  is  known  as  loading.   As the  gas rate  is
increased  to  Line B  at a  fixed  liquid  rate,  there is  a  change  from  a
liquid-dispersed to  a gas-dispersed  state (inversion).  A  layer of  liquid
may  appear  at  the top  of  the  packing,  and  entrainment of  liquid by  the
effluent  gas  may increase  rapidly.   This condition is  known as  flooding.   At
a  given  gas  flow,  flooding will  occur  if  the  liquid flow rate  is  increased
beyond a maximum value (see Figure B-l).   It  is not practical to  operate a
scrubber  in  a flooded condition;  most are  operated  in the  lower part  of the
loading zone.
     In  a tray scrubber,  as the gas  velocity increases at  a fixed  liquid
flow rate, the  gas is dispersed  thoroughly into the liquid, which in turn is
agitated  into a froth.   This action provides  a large interfacial surface
area.  At high gas  velocities,  however, the entrainment of  liquid droplets
above  the upper tray  increases,  and the  absorption efficiency  is  reduced.
In  addition,  a  rapid increase  in  pressure  drop  forces  the  level of  the
liquid in the downcomer  to rise.   Ultimately the  liquid  level  may reach the
tray  above.    Further increases  in  gas  velocity  cause flooding  when  the
liquid fills the  entire space  between the  trays  and the  flow of gas  is
erratic.
Entrainment Potential
     Entrainment of  liquid droplets by  the gas is another factor  in  deter-
mining the  maximum permissible gas  velocity and should be considered  along
with the potential for  flooding  of tray,  mobile-bed,  and packed-bed  scrub-
bers.  In venturi  or spray scrubbers,  which  do  not  undergo flooding,  en-
trainment potential is the primary consideration.
     A droplet  falling into a gas  flow under  the influence of  gravity will
accelerate until drag force  balances  the gravitational force.   After  that,
it  falls  at  a constant velocity  known as the terminal  settling velocity.   If
the  gas   velocity  exceeds  the terminal  settling  velocity  of the  droplets,

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OPERATIONAL FACTORS:  Turndown	B-12
heavy entrainment will  occur.   Perry and Chi 1 ton (1973) have given relation-
ships for calculating  the terminal  settling velocities of liquid droplets in
gas streams.  The terminal  settling velocity decreases with the droplet size
and thus increases the entrainment potential at a given gas velocity.
Pressure Drop
     The operating  gas  velocity is  determined from the maximum gas velocity,
which  is dependent  on the  aforementioned factors.   The gas  pressure, drop
through  the  scrubber depends  on  the gas  velocity and the  type  of scrubber
internals.   The total  gas pressure  drop consists  of  losses  in the bends and
losses  by   contraction and  expansion  in  the  inlet  and  outlet  ductwork,
scrubber, mist  eliminator,  reheater, and  stack.  The  pressure drop  in the
scrubber and  the  mist  eliminator usually  constitutes  the major  portion of
the total pressure drop.
     As  described in  Section  3 (see discussion  of process  control), changes
in pressure drop  across the scrubber, mist eliminators, or reheater can be a
symptom  of  internal plugging.  Normal  pressure drop  ranges from  5  in.  H20
(in spray towers)  to 15 in.  H20 (in  mobile-bed scrubbers).

TURNDOWN CAPABILITY
     In  a limestone  FGD system, it  is  important that  the design level of S02
removal  provide a safety  margin when the flow of flue gas is reduced because
of reduced  boiler  load.  The  ratio of  maximum to minimum  gas  flow that a
scrubber can  handle  without reducing S02 removal  or  causing unstable opera-
tion is called turndown capability.
     Equation B-2   indicates  that  any  reduction  in  the  gas  flow  rate  (G)
tends  to increase  the S02  removal efficiency.   A  reduction  of  gas  flow,
however, decreases  the  overall  mass transfer coefficient (Kg)  by decreasing
the  individual  coefficients  (k ,  k.).   A  decrease   in  the  gas  flow  also
reduces  the   interfacial  area  (a)  by  decreasing  the  gas   dispersion  (tray
scrubber),   liquid  agitation  (mobile-bed and  packed-bed scrubbers), or  the
pressure drop (venturi  or rod-deck  scrubbers).  Thus,  the  effect of reduced
gas flow rate on  S02 removal  depends primarily  on the type of scrubber.   In
a spray  scrubber, the  interfacial area  is  not dependent on the gas flow rate

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OPERATIONAL FACTORS:   Scale Prevention	B-13
or  the  pressure  drop.   Thus,  the  S02  removal efficiency  increases with  a
reduction in gas flow.
     Turndown capability  is affected by  some mechanical limitations.  At  a
given  liquid flow  rate in a  tray scrubber,  the  liquid may  start to drip
through  the  tray  openings as  the gas  flow  is  reduced.   This  phenomenon,
known as weeping,  is  caused by reduced  gas  pressure.   At a  given  liquid flow
rate in  a mobile-bed  or packed-bed scrubber, reduction  of  gas  flow may lead
to channeling.
     In  general,  the  turndown  capability  of a spray  scrubber (3  to 4)  is
superior to  those of  tray scrubbers  (2  to 3), mobile-bed scrubbers  (about
2), and venturi  scrubbers (less than 2).
     An  FGD system  incorporating  parallel  scrubber  modules  renders good
overall  turndown  capability,   regardless  of the type  of scrubber,  provided
that the minimum  load  is limited to the minimum capacity of a  single module.
Such an  arrangement allows a  stepwise turndown capability,  but  requires good
control and  distribution  of flue gas  through the parallel modules.   Gas flow
distribution problems must be  carefully considered  in  design of a new power-
plant, when  the flue  gas is to  be  distributed  to parallel  modules  by use of
dampers  in  a common  duct rather than by use  of  an individual fan  for each
module.   Many operators  believe  that  use  of individual  fans provides better
control of gas  distribution.

PREVENTION OF SCALE FORMATION
     In  limestone  FGD systems  the  major  operating  problem is formation of
calcium  sulfite and calcium sulfate  scale.  The causes of scale  formation
are discussed  in  Appendix A;   the  effects of scale  formation and methods of
controlling  it are  considered  here.   Every  effort must be made  to prevent or
control scaling because  it can lead to plugging by  accumulation  of scale or
other solids  such as  fly ash and  recycle  slurry  solids,  which  in  turn  can
necessitate  a  scrubber  shutdown.    When screens,  piping,  nozzles,  packing
material, mist  eliminator  blades,  or  liquid  distribution  internals  become
plugged with scale, the pressure differential  increases  and flow  rate capac-
ities are reduced.  Scale formation can also occur  in  instruments and sensor

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OPERATIONAL FACTORS:  Scale Prevention	B-14
lines  such as  pH  sample  taps, pressure  differential  sensors, level  indi-
cators, pressure  gauges, and gas  sampling taps.   When this type  of  scaling
is severe, the system cannot be reliably controlled.
     Plugging can  result from  sudden scale formation during an upset condi-
tion or  from scale buildup over a long interval.   Calcium sulfite scale  is
soft,  whereas  calcium sulfate  scale  is very  hard.   The soft scale hardens,
however, when scrubbers  are  shut down  for more than  a few hours and  accumu-
lations  are not  immediately washed  away; under those  conditions the  soft
sulfite  scale  begins  to oxidize  and  forms the much  harder sulfate  scale.
Scale  accelerates  the  effect  of  corrosion  either by  concentrating  elec-
trochemical attack  beneath a  layer of  scale deposited on  a metallic  surface
or by  damaging  protective  coatings when a chunk of  scale  is dislodged.   Even
stainless  steel  can   be severely  damaged by stress-corrosion attack  and
pitting underneath  scale deposits, especially  if  the slurry contains  a high
concentration of chloride in solution.
     Scale formation  also  can  significantly influence  gas  flow  distribution,
especially in the  mist eliminator  area, where  uniform  distribution is criti-
cal for preventing  high  local  velocities and  subsequent carryover of solids
and liquids.
Calcium Sulfite Scaling
     In Appendix  A, the scaling phenomenon was explained in terms of  rela-
tive  saturation  (RS).   The  critical  RS  value  at  which  nucleation  of  a
species begins  to  occur  should  not be exceeded because nucleation  would lead
to uncontrolled  precipitation  or  scaling.  The rate  of  precipitation  of  a
species is as follows (Corbett et al.  1977):

                          Rp = (Kp)O)(VR)(Ws)(RS-l)              (Eq.  B-15)

where     R  = net precipitation rate, Ib-mol/h
          K  = precipitation rate constant, Ib-mol/ft2-h
           P = specific surface area  of solids, ftVlb of  solids
          V  = volume of slurry in  the reaction tank,  ft3

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OPERATIONAL FACTORS:   Scale Prevention	B-15
          VL = slurry solids content, Ib solids/ft3 slurry
          RS = relative saturation (unitless)

The molar precipitation  rate  (R ) is fixed for a system,  and is  equal  to the
molar rate  of  S02  removal.   Thus, the RS  value can be  minimized by increas-
ing  the  slurry  solids  content  (W<.) or  the recycle tank  volume  (VR).   By
expressing  precipitation rate  in  the form  of reaction  kinetics,  Borgwardt
(1975) has  shown that the volume  required for a given amount of precipita-
tion  is  considerably less with a  series  of mixed tanks than with  one mixed
tank.  Basically,  an  FGD system  should  be operated  in such  a way as  to
confine  calcium sulfite  precipitation  to the  scrubber effluent hold  tank.
This  is  achieved by  controlling pH to keep  the limestone feed  stoichiometry
below 1.4.
     Control of pH.  Wen  et al.  (1975)  report that calcium sulfite scale can
be minimized by  keeping  pH at the scrubber inlet below  6.2.   They found that
at a  pH  of  6 or less  the rate of scale  formation was only 5 percent of that
at pH greater than  6.2.   The  factors  determining the pH  of slurry  in the
hold  tank are  S02  content of  the  flue gas,  residence time  in the  tank, L/G
ratio, and  stoichiometric ratio.   Achieving a  high rate  of  S02  removal  from
combustion  of  high-sulfur coals requires a  relatively  longer residence  time
and higher  L/G and stoichiometric ratios to maintain a given pH  level.
     Operating  at  reduced  pH  can  lead  to  reduced S02  removal,  especially
with  high  inlet S02 concentrations.  When this occurs, a part  of  the lime-
stone slurry  may  be  fed to  the  top of  the scrubber.  The  horizontal  spray
scrubber is especially suitable for this arrangement.
     The soft  calcium  sulfite scale dissolves  as  the pH  is  reduced, because
of its increased solubility.   Thus, when  formation of  calcium  sulfite scale
is noticed, the  pH of the recycle  liquor  should be reduced  for a short time
by reducing limestone feed rate.
     Limestone Utilization.   Appendix A  discusses  the  effect   of  stoichi-
ometric  ratio  on  calcium sulfite  scaling.  Operations  at  the  EPA Shawnee
Test  Facility  have shown that maintaining a low stoichiometric  ratio or high
limestone utilization  can aid in keeping the mist eliminator free of sulfite

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OPERATIONAL FACTORS:  Scale Prevention	B-16
scale  (Head  et al.  1977).   Where  limestone  utilization is greater  than  85
percent, an  intermittent  bottom wash keeps the mist eliminator  free  of soft
(calcium  sulfite  or carbonate)  scale deposits.  Where  utilization  is  less
than 85 percent,  a continuous  bottom wash is needed to limit the  accumula-
tion of soft solids to less than 10 percent of the open area.
     Within  economic  and design  constraints,  the system should be operated
at the  highest  possible  L/G ratio.   A high liquid flow rate would  reduce the
concentration  and  thus  the  RS of  the   calcium  sulfite.   It  also  reduces
stagnation of the slurry and thus reduces plugging.
Calcium Sulfate Scaling
     Two basic  modes have  been used to  prevent  calcium sulfate scaling  in
limestone  FGD  systems:   coprecipitation  and control  of  supersaturation
(Devitt,  Laseke,  and Kaplan  1980).   These techniques  are discussed  below.
     When a  system is operated so that  the  maximum oxidation level  in the
slurry  is  less than 16  percent, the  scrubbing liquor  remains  subsaturated
with calicum sulfate (gypsum),  which is  removed from the  system as a copre-
cipitate or  solid  solution with calcium  sulfite.   In this  case, the  discus-
sion of calcium sulfite scale control is  applicable.
     When the  oxidation  level  exceeds 16 percent,  the  liquor is  supersatu-
rated with gypsum  (RS  >  1).   In this case, a  part of calcium  sulfate  must  be
removed from the  system  as gypsum.  The  critical  level  of gypsum RS  is 1.3
to 1.4.  Thus,  the supersaturation must  be kept below  1.3  to prevent gypsum
scaling.
     A means of controlling  calcium sulfate supersaturation is to  circulate
a minimum  amount  of calcium  sulfate  seed crystals,  which  act as  nucleation
sites that enhance  the  homogenous precipitation of calcium sulfate.   Wen  et
al.  (1975) found  that  circulation of 1 percent gypsum  seed crystals  reduced
the  rate  of  scaling to  about  40  percent of that  in saturated  solutions
without gypsum  solids.   Concentrations of gypsum  greater than 1 percent did
not  reduce  the  scaling  rate  further.   Concentration of  the seed crystals
should  be  maintained by  keeping the solids  content of the slurry not  less
than 8  percent.   It should be higher if  fly ash is also  present.   Concentra-
tion of the  seed  crystals can  also  be  maintained  at  a stable level,  with

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OPERATIONAL FACTORS:   Scale Prevention	B-17
relative saturation maintained at  about 1.1,  by  operating  In  a  forced  oxida-
tion mode.   An EPA pilot-plant  study  (Borgwardt  1978)  indicates that when
the  forced  oxidation  was  transferred  from  the  scrubber  to  an  external
oxidizer  tank,  gypsum   scaling   was  minimized,  operating  stability  was
improved, and  the  need  for  constant monitoring to maintain  supersaturation
at less  than 130 to 140  percent was  eliminated.  Later EPA testing has shown
that forced  oxidation  by sparging air  directly  into  the effluent hold tank
causes carbon  dioxide  to be  stripped from the solid slurry; carbon dioxide
stripping enhances  limestone  dissolution  and  nullifies  any  adverse effects
on S02 removal.
     The  RS  of  gypsum  in  the recycle  liquor,  similar to  that of calcium
sulfite, can be reduced  by  providing  a  hold tank large  enough to allow  a
minimum  of  eight (8)   minutes  residence time  for  complete precipitation of
gypsum in the tank.
Mechanical Considerations
     In  spite  of efforts to  prevent  scaling, some scale formation can occur
in an  operating limestone FGD  system.   The following mechanical considera-
tions, therefore,  should be addressed  in  the  design  phase so as to facili-
tate occasional cleaning  of the FGD system.
     Depending  on the  type  of scrubber, the scale deposits  occur at various
locations in the scrubber internals.   Manholes  should  be  installed at each
stage of the scrubber  for easy access by maintenance  personnel.   View  plates
can allow observation  of deposits  or mechanical  problems when  the system is
shut down.   Also,  the scrubber  effluent  hold tank  should be  equipped with
side  doors  to  allow  entry of maintenance personnel  for  removal of  solids
deposits during shutdown  periods.
     It  is  essential   to prevent plugging of pump suction  lines and  spray
nozzles.   Thus, strainers  should be  installed in pump suction lines, and the
spray  nozzles   and  headers  should be  checked  during  regular  maintenance.
     The transformation  of soft  sulfite deposits into hard sulfate scale has
been  observed   in  the  wet/dry  regions of  separated flow,  (including the
venturi  section  of   a   presaturator  or  an adjustable venturi) and  in the

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OPERATIONAL FACTORS:  Chloride Control	          B-18
sudden cross-sectional expansion  at  the entrance to the  scrubber.   In  addi-
tion to the  chemical  transformation  of soft deposits  to  hard scales,  deposi-
tion of  slurry solids and  fly ash  can occur  in  the wet/dry regions.   The
design therefore  should  incorporate  into  the quiescent  zone some means  of
washing away deposits with intermittent  sprays or blowing them away  with
sootblowers while they are still soft.

CHLORIDE CONTROL
     Chlorides  enter  an  FGD system  from  chlorine  in the  coal  and in  the
makeup  water.    Chlorine  in  the  makeup water  is generally a  significant
source  of chloride only  if cooling  tower blowdown or  other wastewater  is
used for  scrubber  makeup.   In  any case, most chlorides come from chlorine in
the coal.   When  the coal  is combusted, the chlorine  is converted to  hydrogen
chloride  (HC1)  gas.  The hydrogen chloride  in  the  flue gas is  captured  by
the scrubber  and at steady state  leaves the  system  as chloride  ions in the
interstitial  water  of the  waste  sludge.   When  the  chlorine content of the
coal  is  high  and  is  bound to organic  compounds,  so  that no  significant
reduction  is  achieved by washing, the steady-state chloride ion concentra-
tions  in  the  scrubbing  liquor and  the dewatered sludge  are  significantly
high.   High  concentrations  of  chloride ions in the scrubbing liquor  adverse-
ly affect  the  process  chemistry and  promote corrosion  and subsequent failure
of construction materials (see Section 3).
     Chloride ions  are among  the effluents from the FGD  system  that must be
discharged in  an  environmentally acceptable  manner.   The RCRA  provisions
limit the  trace  element  and major anion content of  leachate from a  disposal
site to  100  times  the drinking water criteria specified under  the  National
Interim Primary  Drinking  Water Regulations.  The  water-recycle  requirements
are critical  for  FGD  systems in water-short areas, where  salinites and  river
volume control the  withdrawals  as well as  the discharges  (Dascher and Lepper
1977).
     In view  of these constraints,  options  should  be considered .to  reduce
high concentrations of chloride  ion  in scrubbing liquor by external means.
Various methods are available  for extraction of chlorides from  the scrubbing
liquor.  Vapor-compression  (V-C)  evaporation  is presented as an  example.   A

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OPERATIONAL FACTORS:  Energy Demand	B-19
slipstream from  the quencher loop (if separate from the  scrubber loop)  or a
thickener  overflow stream  can be  treated  for  chloride  ion removal.   The
treated water may  then  be reused  as FGD  system makeup  water (Borgwardt 1980)
or as  makeup  to  boiler  feed water demineralizers.   A chloride removal  system
can  effectively  reduce  the  total  dissolved solids (TDS) level  from  30,000
ppm  to  less  than 50 ppm (Weimer 1977).   Such  a  system,  however,  generates a
concentrated chloride brine  (about  300,000 ppm or  30 percent),  which must be
disposed of.
     Simply  stated, V-C  evaporation concentrates  the decanted  feed  liquor
and  returns the  condensate  as purified water.   The concentrate  of dissolved
salts from the evaporator bottom is sent to disposal.
     The  methodology  involves  initial   pH  adjustment,  heat  exchange  to
recover the heat from the product water,  vacuum  deaeration to remove noncon-
densible gases,  and evaporation of water from  the  feed liquor.   The ratio of
the  concentration  of TDS achieved  in the  concentrate to  that  in  the  feed
liquor  is  approximately  14,  if one assumes  a  TDS  level  of 30,000 ppm  in the
feed liquor.
     A  V-C evaporation  system handling  150  gpm  of  feed  liquor has  been
successfully operated to  purify wastewater from  the Wellman-Lord  FGD  system
at the San Juan  Generating Station, which  is jointly  owned by  the  Public
Service Co.  of  New Mexico and the  Tucson Gas  and Electric Co.  (Dascher and
Lepper  1977).   This  chloride treatment  system  is  simpler  and  requires  a
lower  capital  investment  than   other  chloride  control  options,  although
operating  costs  are somewhat  higher because  of  higher  energy  consumption.

ENERGY DEMAND
     A  limestone FGD system  consumes  energy  to  force the flue  gas through
the  system, to grind  the limestone, to handle various liquid/solid streams,
and  sometimes  to  reheat  the exiting gas.   On the  gas side, a flue gas fan
(forced draft or induced draft)  uses energy to offset the gas  pressure  drop
produced by  various portions of the  FGD  system,  such  as the ductwork,  pre-
saturator, scrubber,  and  mist eliminator.  On  the liquid  side,  energy  is
expended to recirculate  the slurry to the scrubber, to pump water and  slurry

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OPERATIONAL FACTORS:  Energy Demand	B-20
streams  to the  various parts  of the  system,  and  to  prepare the  effluent
sludge for final disposal.
     There is  an  increasing emphasis on dewatering  and  stabilization  of FGD
wastes for managed  landfill disposal and a corresponding decrease in ponding
of  the  wastes.  Formerly,  a pond was  expected to  fulfill  three functions:
clarification,  dewatering,  and  temporary  or  final  sludge  storage.   The
increased  emphasis  on  disposal   in  landfill and  structural  fills,  and  on
attaining  closed-loop  operation,  has   stimulated   the  use  of  clarifiers,
vacuum filters, and centrifuges,  all  of which consume energy.
     Reheating  the  saturated flue gas  consumes more energy than any other
part  of  the   scrubber  system.   The  reheat may  be  required  for several
reasons.    One  is to  provide buoyancy to  the flue  gas  and thus  reduce the
nearby ground-level concentrations of pollutants.  Another reason for  reheat
is  to prevent condensation of  acid-containing,  wet,   cool  gas  from  the
absorber  in  the induced draft fan,   exit  duct, or  stack;  such  condensation
can accelerate  corrosion of downstream equipment.  Further, reheat minimizes
the settling of mist  droplets as localized fallout and  the formation in cold
weather of a heavy steam plume with resultant high opacity.
     Most of the  energy consumed by the FGD  system  is  attributed to reheat,
the flue  gas   fan,  and  slurry recirculation  pumps.   The energy  used  by the
fan  depends  upon  the  pressure  drop   across  the  scrubber,  which  varies
according  to   the  type  of  scrubber.  Spray tower  scrubbers,  for  example,
require lower  fan power because  of low pressure drop; however,  a high  slurry
recirculation  rate  and  the  nozzle pressure drop required  for efficient S02
removal add to the pump head and power requirements.
     A TVA  study (McGlamery,  Tarkington,  and Tomlinson 1979) has  estimated
that total  energy consumption of a  typical limestone FGD  system,  including
reheat, on a  500-MW boiler with  a gross  heat rate of 9000 Btu/kWh for  gener-
ation of  electricity is 3.3 percent of  the input energy.   The  computations
were based  on 3.5  percent  sulfur in the  coal  and  an allowable  emission  of
1.2 Ib S02/million  Btu  heat input.   The  system  employs  a mobile-bed  scrubber
and a  settling pond  for  sludge  disposal.   Flue  gas reheat of 50°F is pro-
vided.   The indirect  in-line reheat  steam consumption is  1.8  percent  of the
input energy.

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OPERATIONAL FACTORS:   Energy Demand	B-21
     The  total  energy  consumption of  typical   limestone  FGD  systems  on  a
500-MW boiler firing  high-sulfur  coal  with reheat and firing low-sulfur coal
without  reheat  is  shown  in  Figure B-2.   The  values  were calculated  fron
energy requirements for  the base  case  as  described  in  Section 2.   The  elec-
tricity  consumption  is higher for  the  low-sulfur coal  because the  flue  gas
flow  rate  is higher  than  with  the high-sulfur coal.  The  total  electricity
consumption, therefore,  is  determined  primarily by the gas  volume  (the fan)
and  is  influenced only marginally  by  total  S02 removal.   Thus,  excess  com-
bustion  air  and total air inleakage can have a significant effect  on energy
consumption  in the FGD system.  The total  energy consumptions  with  the  high-
and  low-sulfur  coals  are  2.7  and 1.5  percent of  the  total   input  energy,
respectively.

-------
                           ELECTRICITY
                                                STEAM
                150
CO

ro
PO
                125
             3

             -o
              X* 100
                 75
S
o
i
a
                 50
                 25
                 a.
                       BASIS: NEW 500-MH (GROSS) BOILER.
                       MAXIMUM LOAD. HEAT RATE OF
                       9366 Btu/k Hh
                      BOILER FIRING HIGH-
                      SULFUR (3.7S SULFUR)
                      COAL; 90S SO, REMOVAL
                      WITH REHEAT *
                              BOILER FIRING LOW-
                              SULFUR (0.7S SULFUR)
                              COAL; 70S SO? REMOVAL
                              WITHOUT REHEAT
         As  steam  and electricity  used
         by  the system.
                                                                                     3.0
                                                                    £
                                                                    C  2.5

                                                                    S
                                                                                  .   2.0
                                                                                     1.5
S
i
                                                                                     1.0
                                                                                     0.5
                                                                              BASIS: NEW 500-MW (GROSS) BOILER.
                                                                              MAXIMUM LOAD. HEAT RATE OF
                                                                              9366 Btu/k Wh
         BOILER FIRING HIGH-      BOILER FIRING LOW-
         SULFUR (3.7S SULFUR)     SULFUR (0.7S SULFUR)
         COAL; 90S SO, REMOVAL    COAL; 70S SO? REMOVAL
         WITH REHEAT '           WITHOUT REHEAT

    b.   As portion  of  total  coal fired
         in the  boiler.
                                Figure  B-2.   Total  energy  consumption by typical  limestone
                                                               FGD systems.

-------
OPERATIONAL FACTORS:  References                             	B-23
                          REFERENCES FOR APPENDIX B
Borgwardt, R. H.   1975.   Increasing Limestone Utilization  in  FGD Scrubbers.
Presented at the 68th AlChe Annual Meeting, Los Angeles, California.

Borgwardt, R. H.   1978.   Effect of Forced  Oxidation  on Limestone/SO  Scrub-
ber  Performance.   In:   Proceedings  of the Symposium on  Flue  Gas Desulfuri-
zation,  Hollywood,  Florida,  November 1977.  Vol.  I  EPA-600/7-78-058a.   NTIS
No. PB-282 090.

Borgwardt, R. H.   1980.   Combined Flue Gas Desulfurization and Water Treat-
ment  in  Coal-Fired Power  Plants.   Environmental  Science and  Technology,
14(3):294-298.

Corbett, W.  E.,  et al.   1977.  A  Summary  of  the Effects of Important Chemi-
cal Variables Upon  the  Performance of Lime/Limestone Wet Scrubbing Systems.
Electric  Power   Research  Institute,  Palo  Alto,  California.   EPRI  FP-639.

Dascher, R.  E.  and R.  Lepper.  1977.  Meeting  Water-Recycle Requirements at
a Western Zero-Discharge Plant.  Power, 121(8):23-28.

Devitt,  T. W.,  B.  A.  Laseke,  and  N.  Kaplan.   1980.   Utility Flue Gas Desul-
furization in the U.S.   Chem.  Eng. Prog., 76(5):45-57.

Head,  H.  N., et  al.   1977.    EPA Alkali Scrubbing  Test Facility:   Advanced
Program,  Third  Progress  Report.    EPA-600/7-77-105.   NTIS  No.  PB-274  544.

McGlamery, G.  G.,  T. W.  Tarkington,  and S. V.  Tomlinson.   1979.  Economics
and  Energy  Requirements  of  Sulfur  Oxides  Control   Processes,  TVA.   In:
Proceedings  of  the Symposium  on Flue  Gas Desulfurization,  in  Las Vegas,
Nevada,  March  1979.  Vol.  I.   EPA-600/7-79-167a.   NTIS No.  PB  80-133 168.

Perry,  R.  H., and G.  H. Chilton,  eds.   1973.   Chemical Engineers' Handbook.
5th ed.  McGraw-Hill Book Co., New York.

Saleem,  Abdus.  1980.   Spray Tower:  The Workhorse  of  Flue-Gas Desulfuriza-
tion.   Power, 124(10):73-77.

Treybal,  R.  E.   1968.  Mass Transfer Operations.  2d  ed.   McGraw-Hill Book
Co., New York.

Wen,  C. Y.  and L. S.  Fan.    1975.   Absorption of S02  in  Spray  Column and
Turbulent  Contacting  Absorbers.    EPA-600/2-75-023.   NTIS  No.  PB-247  334.

Weimer,  L.  D.  1977.   Effective  Control  of  Secondary  Water  Pollution From
Flue Gas Desulfurization Systems.   EPA-600/7-77-106.

-------
OPERATIONAL FACTORS:   Bibliography
                  B-24
                         BIBLIOGRAPHY FOR APPENDIX  B
Head,  H.  N.   EPA  Alkali Scrubbing Test  Facility:   Advanced Program,  Second
     Progress  Report.  EPA-600/7-76-008.  NTIS No.  PB-258  783.

Head,  H.  N.   EPA  Alkali Scrubbing  Test Facility:   Advanced Program,  Third
     Progress  Report.  EPA-600/7-77-105.  NTIS No.  PB-274  544.

Head,  N.  N.   and S. C.  Wang.   EPA Alkali Scrubbing Test  Facility:  Advanced
     Program,  Fourth  Progress  Report.   Vol.  1,  Basic Report;  and Vol.  2,
     Appendices.   EPA-600/7-79-244a  and  EPA-600/7-79-244b.   NTIS  Nos.   PB
     80-117906 and PB  80-117914.

Jones, D.  G.,  A. V. Slack, and K. S. Campbell.  1978.  Lime/Limestone  Scrub-
     ber  Operation  and  Control  Study.   Electric  Power  Research  Institute,
     Palo Alto,  California.  RP 630-2.
Slack,  A.  V.   1978.   Lime-Limestone  Scrubbing:
     Chem. Eng. Prog., February 1978.  74(2):71-75.
Design  Considerations.
Uch.ida, S.,  C.  Y.  Wen, and W.  J.  McMichael.   1976.  Role of Holding Tank  in
     Lime  and  Limestone  Slurry Sulfur Dioxide  Scrubbing.   Ind. Eng.  Chem.,
     Proc. Des. Dev., 15(1):18-27.

Wen, C. Y.,  and C. S. Chang.   1978.  Absorption of S02 in Lime  and  Limestone
     Slurry:   Pressure Drop  Effect  on  Turbulent  Contact  Absorber Perform-
     ance.  Environmental Science and Technology,  12(6):703-707.

-------
                                 APPENDIX C
                             COMPUTER PROGRAMS

     Two computerized  systems  are available  for use by the  utility  Project
Manager  during selection and  evaluation of  a  proposed limestone  scrubbing
system.  A  third  computer  program has resulted  in the development of  simpli-
fied  equations with  which to  calculate  relative  gypsum saturation  levels
that  will  reduce scaling  potential  during system operation.  This appendix
describes these three programs and their use.
     The  Lime/Limestone  Economics Computer Model was  sponsored  by the  EPA
and developed  by  the TVA and Bechtel  National,  Inc.   This  program can assist
the  Project  Manager  in  selecting  the most  cost-effective  options  for  a
scrubbing system, in  developing  initial plans,  and in evaluating  trade-offs.
It is  also  useful  for determining the  size of  individual  components  and the
effect of S02 removal efficiency on energy demand.
     The  FGD  Information  System  (FGDIS),  also  sponsored by  the EPA,  was
developed by  PEDCo  Environmental,  Inc.   This  program can  supply  the  Project
Manager  with  needed  experience  information  and  can  serve  as  a tool  for
initial  design costing and  feasibility screening.   The Project  Manager can
use  the  FGDIS  to   determine  predominant  scrubber  design  configurations,
average  L/G  ratios,  prevailing types of reheat  systems, situations requiring
no reheat,  and materials  of  construction used  in specific  components.   The
design  data  can  be  coupled  with   performance  information  to  develop  a
design-versus-reliability  analysis.    Additionally,   the   FGDIS  enables  the
Project  Manager  to  identify  problem  areas  and to  alter  proposed  design
features to prevent these problems.
     The  Bechtel-Modified  Radian  Equilibrium  Program  is based  on  EPA/TVA
experience  at Shawnee Station.   This program  has  been simplified into two
convenient  equations,  which  can  be  used  to maintain  the   relative  gypsum
saturation  level  of an operating limestone scrubbing system  below 130 to 140
                                     C-l

-------
COMPUTER PROGRAMS:  TVA/Bechtel
percent.  Thus,  these  equations constitute an important tool in reducing the
scrubbing unit's scaling potential.

LIME/LIMESTONE ECONOMICS COMPUTER MODEL
Background
     Under  EPA  sponsorship,  TVA and  Bechtel  developed  the  Lime/Limestone
Economics Computer Model,  which can be used  to  estimate  the capital  invest-
ment  and  annual  lifetime  revenue  requirements  for  a  full-scale  lime/
limestone FGD  system.   The  process and material balance  relationships  used
in the  model  are  based on data obtained  from  the  EPA test  facility  at the
TVA Shawnee station in Paducah, Kentucky.
     The test  facility  is  incorporated into the  flue  gas  ductwork of Boiler
10 at the Shawnee  plant.   The  facility originally  consisted of three paral-
lel  scrubber  units,  each  capable  of  processing about 30,000  acfm of  flue
gas.   The original scrubbers  were  a venturi/spray  tower combination  unit,  a
turbulent  contact  scrubber,  and   a  marble-bed  scrubber.   The  marble-bed
scrubber was  shut  down in  1973 and  converted to a  cocurrent scrubber  in
1978.   Tests  conducted   on   venturi/spray  towers  and   turbulent  contact
scrubbers provided the basic data for the computer model.
     Development of the FGD  computer model began in 1975.   The  current model
is a combination of  two separate models:   one predicting  process parameters,
equipment sizes, and  capital  costs, and the  other  projecting the  annual and
lifetime revenue requirements  based on the parameter,  size, and cost  predic-
tions.
     Bechtel developed  the material  balance  relationships,  flow  rates, and
stream  compositions.   On  the  basis of Bechtel 's  input,  the TVA  determined
the  equipment  size relationships,  developed  cost  bases,  and  performed the
system analysis.   These efforts yielded a model  capable of estimating capi-
tal  investment costs.  The  TVA  then  developed  procedures  for  using  this
output  to   project  the annual and  lifetime  revenue  requirements.   Recent
enhancements of  the model  allow automatic calculation of  revised  NSPS emis-
sion  limits based  on  coal  composition;  also  included  are an option  for
partial  scrubbing  with bypass  and  revised TVA/EPA premises  for comparative
economic evaluations  of emission control processes.

-------
COMPUTER PROGRAMS:   TVA/Bechtel	:	C-3
     The TVA has published  several  papers  (McGlamery  et al.  1975;  Stephenson
and  Torstrick  1978;  Torstrick  1976;  and  Torstrick,  Henson,  and  Tomlirison
1978) relating to  the development of the model and its use.   Stephenson  and
Torstrick  (1979) provide user's  guidelines  in a  concise  form.   An  updated
user's  manual  describing  recent  developments is  to  be  available early  in
1981.   Additionally  the TVA staff will assist a  user either by running  the
program with  data  or by providing  a  copy  of the program.   Further informa-
tion  may  be  obtained  from  Mr.  Robert   L.  Torstrick,   Tennessee  Valley
Authority,  Energy  Design  Operations,  501  Chemical  Engineering  Building
(CEB), Muscle Shoals, Alabama  35660 [telephone (205)  386-2814].
     Using  the  Lime/Limestone  Economics  Computer Model,  TVA staff  members
have  evaluated  the economics of  flue gas  desulfurization and of byproduct
handling and waste  disposal.  The evaluation report  (McGlamery et al.  1980)
summarizes  capital  investments  and  annual  revenue requirements  based on  a
three-phase analysis  of  FGD processes and  of sludge disposal practices.   The
report  includes  projections of the potential  1985 market for  FGD byproduct
sulfur/sulfuric acid.  The  authors  suggest new premises that will  be used to
evaluate  FGD  processes  in  the  early 1980's  and indicate  some  effects  of
these premises on  assessments of  limestone scrubbing  economics.   The report
also presents the  results  of a  recent evaluation  of  limestone scrubbing in a
spray tower;  forced  oxidation,  use of adipic acid,  and  gypsum disposal  by
stacking are considered.
     An updated  version of  the model  was  recently published by EPA (Anders
and  Torstrick  1981).   This  version supplements  Torstrick's 1979  report  and
extends the  number of scrubber options  that can be  evaluated.  It includes
spray tower and  venturi/spray  tower  scrubbers,   forced   oxidation  systems,
systems with  scrubber loop  additives (MgO or adipic acid),  revised design
and  economic premises,  and  other  changes reflecting process  improvements  and
variations.
Model Description
     The TVA/Bechtel  model   is  intended  for use  by   Project  Managers during
the  preliminary  engineering phase  of  an FGD project to analyze the process
and  system  options   and   their   effects   on  system costs.   The model  can

-------
COMPUTER PROGRAMS:  TVA/Bechtel	   C-4
generate  a preliminary  conceptual  design  and  cost  package  for  a lime  or
limestone  FGD  system.   Since the costs  estimated  by  the model are  based  on
conceptual input  parameters,  the  cost values predicted by  the model must  be
considered  as  preliminary.   These  values  should be   useful,  however,  in
weighing the impact of various process parameters on final costs.
     The model  calculates  the flow rates and compositions of various streams
in the  FGD system;  sizes  of the  major equipment items  are  then  determined
from  the   flow  rates.   The  costs  of  materials and  installation labor are
determined for  the  equipment items, and capital investment  is calculated  by
adding indirect costs to the installed costs of the system equipment.
     Annual operation  and  maintenance  costs are based upon  the annual  con-
sumption of  raw materials  and  utilities.   Annual  capital charges are  added
to these costs  to arrive at the total  annual  cost of the system.   The  life-
time  annual  revenue requirements for  the  system are  generated from data  on
the input capacity and projected useful life of the system.
     The input  to the  program  consists of  the major parameters of the FGD
system.   The  input  parameters needed  to run the program  are listed  in  Table
C-l.   The  ranges  of major  input parameters  used in development of the  model
are shown  in  Table C-2.   Values beyond the indicated ranges  are  not  neces-
sarily invalid, but  the potential  for error is greater  when these  ranges are
exceeded.
     The user can  select program  options by following the input instructions
outlined in  the user's manual.   The  model  options  are  summarized  in  Table
C-3.
     The program output consists of several reports:
          1.   .Input data
          2.    Process parameters
          3.    Pond size parameter and costs
          4.    System equipment sizes  and costs
          5.    Capital investment
          6.    First-year annual revenue requirement
          7.    Lifetime annual revenue requirement
Samples of these reports are presented in Tables C-4 through C-10.

-------
         TABLE C-l.  REQUIRED INPUT PARAMETERS, TVA/BECHTEL PROGRAM
Plant and Site Data

     Plant rating
     Annual rainfall
     Seepage rate
     Annual evaporation
     Land area available for disposal pond
     Expected pond capacity
     Plant remaining life
     Plant operating pattern for the remaining life

Boiler and Fuel Data

     Boiler heat rate
     Excess air
     Temperature of flue gas to scrubber
     Temperature of flue gas to stack
     Heating value of coal
     Coal composition (C, H, 0, N, S, Cl,  ash, and H20)

Process Parameters

     Temperature of reheater steam
     Heat of vaporization of reheater steam
     Scrubber L/G ratio
     Superficial gas velocity in the scrubber
     Face velocity of gas through reheater
     Amount of S02 to be removed
     Effluent hold tank residence time
     Value for stoichiometry
     Soluble MgO in limestone
     Soluble MgO added to the system
     Insolubles in limestone
     Moisture content of limestone
     Soluble CaO in particulates
     Soluble MgO in particulates
     Percent solids in scrubber slurry
     Percent solids in discharged sludge
     Percent solids in clarifier underflow
     Percent oxidation of sulfite in scrubber
     Percent solids in filter cake
     Filtration rate
     Limestone hardness index
     Size of ground limestone
     Entrainment level as percentage of wet gas

(continued)
                                     C-5

-------
TABLE C-l (continued)
System Options

     Solids settling rate in clarifier
     Number of turbulent contact stages in the scrubber
     Number of turbulent contact scrubber grids
     Height of spheres in each stage
     Number of spare limestone preparation units
     Number of operating scrubber modules
     Number of spare scrubber modules
     Sludge disposal option
     Depth of finished pond
     Maximum excavation depth
     Distance between pond and scrubber area
     Pond lining option

Cost Parameters

     Maintenance cost factor expressed as percent of capital  investment
       exclusive of pond
     Pond maintenance factor
     Plant overheads
     Administrative research and services rate
     Annual capital charge basis
     Insurance and interim replacement cost factor
     Limestone unit rate
     Operating labor unit rate
     Supervision labor unit rate
     Steam rate
     Process water rate
     Electricity rate
     Chemical engineering material  cost index
     Chemical engineering labor cost index
     Capital investment base year
     Revenue requirement base year
     Operating profile
     Chemical engineering plant index
                                     C-6

-------
    TABLE C-2.   BASIS FOR MAJOR VARIABLES,  TVA/BECHTEL PROGRAM
               Variable
       Basis/range
Type of plant
Plant size, MW
Fuel sulfur, %
Scrubber gas velocity, ft/s
L/G ratio, gal/1000 acf
Effluent hold tank residence time,  min
Number of operating scrubber modules
Number of spare scrubber modules
Sulfur converted to SOa, %
System pressure drop

Base year for capital investment
Base year for revenue requirement
          New
        100-1300
          2-5
          8-12.5
         28-75
          2-25
          1-10
          0-10
          0-100
3 in. H20 maximum per
 turbulent contact scrubber
 stage
Midpoint of project duration
First year of FGD operation
                                C-7

-------
               TABLE C-3.  MODEL OPTIONS, TVA/BECHTEL PROGRAM
Process Options

     Limestone FGD
     Lime FGD

System Options

     Number of scrubbers
     Redundancy
     Sludge processing
     Disposal pond
     Pond liner
     Pond capacity
     Operating profile
     Fixation/stabilization
     Landfi11
     Additions in 1981

Cost Factor Options

     User capability to select percentages for individual indirect cost
      components

Output Options

     User capability to select any or all available output reports
                                     C-8

-------
                              TABLE  C-4.    INPUT  DATA REPORT
                          •Ml CAM IIAWLl MO tut


                                             ••• INPUTS •••


                          •OILIR CHARACTERISTICS
                          •*•»•• »%••••»•»•••••»

                          MEGAWATTS •   ICO.

                          •OKI* MEAT MTE •  «eoo.  ITU/KWM

                          EXCESS AIR •  ». mcENT,  INCLUDING

                          NOT GAI TEMPERATURE • 100.  DEC P

                          (Oil ANALYSIS' MT • AS HMD  I

                           C      ri      e      N      S      Cl    ASM    M20
                          ST. »*   ».i»   t.eo   i.t*   i. it   o.ie  i». oo  io.T*
(continued)
                          ASM OVEKMEtD •  10,0 MRCENT

                          HEATING  VALUE 0' CO»L • IOIOO. ITU/ll

                                                 EFFICIENCY,    |MtSS|ON<
                                KEHDVAL               I         IIS/M ITU
                          UPSTREAM OF SCRUUEft        »».!          O.U

                          WITHIN SCRJIIER             so.o          e.e«

                          ALKALI


                          LIMESTONE  I

                                 CAC01       •  tT.iS NT  t  DRY MtlS
                                 SOLUIlE K60 •   0.0
                                 HERTS      •   I.IJ
                                 HOISTURE CONTENT •   j.oo  n Mto/iee us DRV LIMESTONE
                                 LINESTONI HARDNESS MORK  INDEX FACTOR • 10.00
                                 LIMESTONE DECREE OF MIND  FACTO* •  i.IS
                          FLY ASM
                                 SOLDUE CAO •   0.0  NT t
                                 SOlUllE M60 •   0.0
                                 1NERTS      • 100.00
                          KAN NATfRlAL NANDLINC AREA



                          HUMIIR OF  REDUNDANT ALKALI MEDIATION UNITS •    I
                                                C-9
Reproduced from
best available  copy.

-------
TABLE C-4  (continued)
                            •CftUMtt  SYSTEM VARIAILSS

                            HUNIER  Or OPSMTIN6 SCRUM INC TRAINS •   *
                            NUNIER  OF MOUNOANT SCRUIMNC TRAINS •   I
                            NUNIER  Of 110$ •   >
                            NUMIER  OF MIDI •   4
                            NEI6MT  99 SPHERES »ER 110 •  »,0 INCHES
                            LIOUID-TO-CAS RATIO •  IS. CAL/1000 »CF
                            SCRUIIE*  cts VELOCITY • ja.j rt/tEc
                            SDI REMOVAL •  II. PERCINT
                            STOJCMjtHtm RATIO     TO IE CALCULATED
                            INTRAIHHENT LEVEL • e.io MT s
                            IMT RESIDENCE TIME •  u.o HIN
                            soi OXIOIZEO  IN SYSTEM •  »o.o PERCENT
                            SOLIDS  IN »ECI»CULATE6 lLU«RV •  II.0 NT S

                            SOLIDS  DISPOSAL S»STf*

                            COST Of LAND  •  »00.00 DOLLARS/ACRE
                            SOLIOS  IN SYSTEM  SLUDGE DISCHARGE •  «o.o HT s
                            MAXIMUM PO^D  AR|A •   100. ACRES
                            MAXIMUM EXCAVATION •  ts.oo FT
                            DISTANCE  TO POND  •  StIO. PT
                            POND LINED WITH  U.O  INCHES CLAY

                            STEAM REHEATfR  (IN.LINE)
                            SATURATED STEAM TEMPERATURE  •   »TC, DEC P
                            HEAT OP VAPORItATION OP STEAM  •  Til. ITU/LI
                            OUTLET FLUE 6AS TEMPERATURE  f  17». DEC P
                            SUPERFICIAL CAS VELOCITY (PACE VELOCITY) •  tl.O FT/SEC
                               IT     SR       SROLD
                                i     i.»i     i.ie
                                i     i.*i
                                                    C-10
Reproduced from
best available  copy.

-------
                       TABLE C-5.   PROCESS PARAMETER REPORTS
                      FLUE c«s TO STACK
                              NOLI PERCENT
          LB-HOLE/HR
        IB/HR
coz
S02
02
N2
H20
U.67J
0.033
4.472
»8.BbS
14.933
Ot2069E»09
0,39436*02
0,8000E»04
0.1232E»06
0.2676E»OS
.9193E*06
.I807E*04
.23606*06
.1*J2E«07
.4B20E»0*
                      SPECIFIED  802 REMOVAL EFFICIENCY •   19,0 ft

                      CALCULATED S02 EMISSION •    O.BS POUNDS  PER  MILLION |TU

                      CALCULATED 502 CONCENTRATION IN STACK CAS •      192,  PPM
                      FLVASH EMISSION •  0.09 LBS/M1LLION ITU
                                      •  0.042 GRAINS/SCF (WET)
                            OR
                 *11.  IB/HR
                      STACK CAS FLOW RATE •  ,1130E»07 SCFM (»0 DEC F/  1  ATM)
                                          •  ,1380E»07 ACFM (175,  DEC ft  \  ATM)
                      STEAM REHEATER (IN-LINE)


                      SUPERFICIAL CAS VELOCITY  (FACE VELOCITY)  •  25,0 FT/SEC

                      SQUARE PIPE PITCH • 2 TIMES ACTUAL PIPE O.D.

                      SATURATED STEAM TEMPERATURE •  470, DEC F

                      OUTLET FLUE CAS TEMPERATURE • 17J. DEC F

                      REQUIRED HEAT INPUT TO REHEATER • o.6e82E+oe BTU/HR

                      STEAM CONSJMPTION • 0.9163E*05 LBS/HR
                      OUTSIDE PIPE
                      DIAMETER/ IN.
                        i.OO
PRESSURE DROP/
IN. H20
    0.7*
HEAT TRANSFER
COEFFICIENT/
BTu/HR FT2 DEC F
  0,20B2E»02
(continued)

INCONEL
CORTEN
TOTAL
REHEATER
OUTSIDE PIPE
AREA/ SQ FT
PER TRAIN
0>1284E»04
0.1313E«0*
0.2397E*0*
NUMBER OF
PIPES PER
BAHK PER
TRAIN
B7
B7
B7
NUMBER. OF
BANKS (ROMS)
PER TRAIN
»
*
7
                                          C-ll
                                                  Reproduced from
                                                  best available copy.

-------
 TABLE C-5  (continued)
  WATER BALANCE INPUTS
    KAINFAIKIN/YEAR)
    POND SEEPAGE(CM/SEC>*10»*B
    POND EVAPORATION!IN/YEAR)
  13,
  10.
  10,
  MATER BALANCE OUTPUTS
  MATER AVAILABLE

    RAINFALL
    ALKALI
        TOTAL
 562.  0PM
   5.  GPM
 967.  CPH
211059. LB/HR
  2*53. LB/HR
211512. LB/HR
  MATER REQUIRED

    HUMIDIFICATIQN
   • ENTRAPMENT
    DISPOSAL MATER
    HYDRATION WATER
    CLARIFJER EVAPORATION
    POND EVAPORATION
    SEEPAGE

    TOTAL  MATIR REQUIRED
 42B.  GPM
  10.  GPM
 167.  GPM
  11.  GPM
   Ot  GPM
 922.  GPM
  10.  GPM

im,  GPM
        LB/HR
  5102. LB/HR
 91329. LB/HR
  572*. LB/HR
     0. LB/HR
260605. LB/HR
  9970. LB/HR

56f075. LB/HR
  NET  MATER  REQUIRED
 611
                                                                J03563.  LB/HR
(continued)
                                       C-12
                                              Reproduced from
                                              best available copy.

-------
TABLE  C-5 (continued)
         SCRUBBER SYSTEM

         TOTAL NUMBER OF SCRUBBING TRAINS  (OPERATINC»REOUNOANT>  •   5
         S02 REMOVAL «  es.o PERCENT
         PARTICULATE REMOVAL IN  SCRUBBER SYSTEM  •   30.0  PERCENT
         TCA PRESSURE DROP ACROSS   3  BEDS  •   a.6 IN.  MZO
         TOTAL SYSTEM PRESSURE  DROP « i«.e IN. Hto
         OVERRIDE TOTAL  SYSTEM  PRESSURE  DROP  • 20.0 IN.  HZO
         SPECIFIED   LlQUID-TO-GAS-RATIO     • 55.  GAL/1000  ACF
         LIMESTONE ADDITION « 0.4906E+05 LB/HR DRY  LIMESTONE
         CALCULATED LIMESTONE STOICHIOMETRY   •   l.*l  MOLE  CAC03  ADDED AS  LIMESTONE
                                                    PER  MOLE  S02 ABSORBED
         SOLUBLE CAD FROM FLY ASH  •  0.0   MOLE PER  MOLE  S02  ABSORBED
         TOTAL SOLUBLE MGO        -  o.o   MOLE PER  MOLE  soz  ABSORBED
         TOTAL STJICHIOMETRY
   MOLE SOLUBLE    5.6*

         MAKE UP rfATER .  611.  CPM

         CROSS-SECTIONAL AREA  PER SCRUBBER  •    *25.  SO  FT
         SYSTEM SLUDGE  DISCHARGE
SPECIES
CAS03 .1/2 H20
CASU4 .2123
CAC03
H20
CA4*
MO**
SU3—
SU4--
CL-
LB-MOLE/HR
0.2356E+03
0.9996E*02
0,1333E*03
o.5i60£*04
0,7219E*01
0,0
0.15J9E*00
0,1066E*01
0,1199E*02
LB/HR

0.30*2E*05
0.1720E*05
0.133«E*05
0.1810E*0*
0.9333E*OS
0.2893E*OI
0.0
0.1233EO2
0.102*E*03
0.*250E*03
                                                    SOLID    LIQUID
                                                    CUMP/    COMP,
                                                    WT  X     PPM
                                                    27.41
                                                    21.26
                                                     2.BB

                                                             3073.
                                                                0.
                                                              131.
                                                             10B8.
                                                             4313.
         TOTAL DISCHARGE  FLOW RATE  •  0.1569£*06  LB/HR
                                   •    237.      GPM

         TOTAL DISSOLVED  SOLIDS  IN  DISCHARGE  LIQUID  m   6805.  PPM

         DISCHARGE LIQUID PH •  T.32
(continued)
                                           C-13

-------
 TABLE C-5  (continued)
                         SCRUBBER SLURRY BLEED
SPECIES

CAS03 .1/2 NIO
CAS04 .2H20
CACOJ
                                          tl-MOLI/HR   IB/MR
                         M20
                                                       O.IO«tE*09
                                                       0.0
P16**
SOI—
$04—
CL-
                         TOTAL FLOW RAT§ • Q.41ISE+06
                                         •    760.    CPM
                          55   0.|9{*E«0*
                  ,2727E*02
                  .0
                  ,3816E»00
                  ,4028E*01   O.IITOE»0>
                 0.452»E*02   0.1»05E*04
                         TOTAL SUPERNATE RETURN
                         SPECIES

                         H20
                         503—
                         SO*—
                         CL-
                 ll-MOLE/HR   LB/MR
                 0,U96E*05
                 0,208*E*02
                 0.0
                 0.4443E+00
                 0,3076E*01
                 0,3461E»02
                              0.0
                              0.935IE»02
                              0.1227E*0*
                         TOTAL PLOW RATE • 0.271BE*06
                                         •    344.     CPH
                         SUPERNATE TO MET BALL HILL
                         SPECIES

                         H20
                         CA*4
                 LB-MDLE/HR
                         $03—
                         $04—
                         CL-
                 0,2354i*01
                 0.0
                 0.30216-01
                 0.947|E*00
                 0(>9IOE»01
                              LB/HR

                              O.I044E*03
                              0.0
                              0.4020E»01
                              0.1386E»0>

TOTAL PLOW RATE • O.I071E*05 LB/KR
(continued)
                                       C-14
                                             Reproduced from
                                             best available copy.

-------
TABLE C-5  (continued)
                         LIMESTONE SLUMy  'IfD
                         SPECIES

                         CAC09
                         SOLUBLE NCO
                         INSOLUHES
                         M20
                         CA**
                         NC*«
                         801—
                         804—
                         CL-
                 ll-NOLI/MR   It/Ml.
                 0,47»1E*01
                 0,0
                         TOTAl
                 0,0
                 0,59411.01
                 0, 97041*00
                 0,41»4f«01
0.0
0.1>ME»0«
0.»242E»0>
0.10011*01
0.0
0.41111*01
0.»»M«OI
0.14T»I»0»
           ftAT| •  O.B177E*05  LB/HR
                •     109.     CPH
                         SU>tRNATi HITURN TO SCftUIBER OK |M7
SPfCIES
H20

MC**
$09—
$04—
Cl-
LB-HOLE/HR
0,19271*05
0,1BA»I*02
0,0
0,9*4IE*00
0,27911*01
0,90701*02
IB/HR
0.29*OE*0*
0.740*E*09
0.0
0.91941*01
0.2bZ9t*09
0.10BBEO*
                         TOTAL
           •ATf  .  0.24111*06  LB/MR
                •     4B2.     CPH
                         RECVCLl $LJ«»Y TO SCMUIIEK
SPfCIES

CAS09 ,1/2 M20
CAS04 ,2H20
CAC09
                                          LI-HOIE/HH    IB/MR
M20
CA»*
HO**
$09—
$04—
CL«

TOTAL PLO« RATE •
                                          0,2172E*05
                                          0,»21BE«04
                                          0,122*1*05

                                          0,19051*07
                                          0,2515C*04
                                          0,0
                                          0,5949E*02
                                          0,9719C*09
                                          0,417*1*04
0.2B05EO7
0.15141*07
0.12901*07
0.92911*01
O.l00li*04
0.0
0.42*41*04
0.99411*09
0.14BOI*04
                                           0.9I9«E*08
                                            700«7.     CPH
(continued)
                                          C-15
                         Reproduced from
                         best available copy.

-------
TABLE C-5 (continued)
                          FLUE CAS COOL INC SLURRY
SPECIES

CAS09 .1/2 N20
CAJO* ,IH20
CAC09
                                           LI-NOLI/HR   ll/HR
                          HZO
$09—
SO*—
CL-

TOTAL FLOW RATE •
                                            ,1I2»E»OJ
                                            .0
                                            ,19001*01
                                            .27021*02
                                            .10)71*09
0.11I*E*0*
O.M*tE*09

o!29»9E*07
O.T990E*0*
0.0
0.9129E*09
0.2999E*0*
0.10T7E*09
                                            0.2I06E*07 LB/MR
                                              5097.    CPH
                                        C-16
                                               Reproduced from
                                               best available copy.

-------
      TABLE C-6.  POND  SIZE AND COST  PARAMETERS
                   POND  DESIGN


 OPTIMIZED  TO  MINIMIZE TOTAL  COST  FLUS  OVERHEAD
POND DIMENSIONS
DEPTH Of POND
DEPTH OF EXCAVATION
LENGTH Of PERIMETER
LENGTH OF DIVIDER
AREA Of
AREA OF
AREA OF
AREA OF
        INSIDE WALLS
        OUTSIDE WALLS
        POND
AREA OF POND SJTE
AREA OF POND SITE
                                 21.94 FT
                                  1,1* FT
                              1*103.   FT
                               2703.   FT
VOLUME OF EXCAVATION
VOLUME OF SLUDGE TO BE
DISPOSED OVER LIFE OF PLANT
 1909,
 1677,
  1*6.

 1926,
10269.
 6369.
THOUSAND YD2
THOUSAND YD2
THOUSAND YD2
THOUSAND YD2
THOUSAND YD2
ACRES

THOUSAND YD3
THOUSAND YD3
ACRE FT
POND COSTS (THOUSANDS OF DOLLARS)
                                LABOR
                                         MATERIAL   TOTAL
CLEARING LAND
EXCAVATION
DIKE CONSTRUCTION
LINING! 12. IN. CLAY)
SODDING DIKE WALLS
ROAD CONSTRUCTION
                                 917.
                                2«*6.
                                101*.
                                1261,
                                  61,
                                   9.
                       117.
POND CONSTRUCTION
LAND COST
                                973*.
     93.


     72.
                      1034.
                      1263.
                       lit,
                        27.
                      9806,
                      1213.
POND SITE
OVERHEAD
                                                    7019.
                                                    1948.
TOTAL
                                                   10967,
                          C-17
                                Reproduced from
                                best available copy.

-------
TABLE C-7.  EQUIPMENT SIZES AND COST REPORT
RAW MATERIAL HANDLING AND PREPARATION
INCLUDING 2 OPERATING AND 1 SPARE PREPARATION UNITS
ITfM
CAR SHAKER AND HOIST
CAR PULLER
UNLOADING HOPPER
UNLOADING VIBRATING FEEDER
UNLOADING BELT CONVEYOR
UNLOADING INCLINE BELT
CONVEYOR
UNLOADING PIT OUST COLLECTOR
UNLOADING PIT SUKP PjMp
STORAGE BELT CONVEYOR
STORAGE CONVEYOR TRIPPER
MOBILE EQUIPMENT
RECLAIM HOPPER
RECLAIM VIBRATING FEEDER
RECLAIM BELT CONVEYOR
RECLAIM INCLINE SElT CONVEYOR
RECLAIM PIT OUST COLLECTOR
RECLAIM PIT SUMP PJMP
RECLAIM BUCKET ELEVATOR
FEED BELT CONVEYOR
PEED CONVEYOR TRIPPER
(continued)
DESCRIPTION
20HP SHAKER T.3HP HOIST
2SHP PULLER' IMP RETURN
16FT OIA, 10FT STRAIGHT
SIDE HT, CS
).5HP
20FT HORIZONTAL, 3HP
IIOFT, 30HP
POLYPROPYLENE BAGTYP|,
2200 CFM,7.3HP
60GPM, 70FT MEAD, 5HP
200FT, 3HP
)OFPM, 1HP
SCRAPPER TRACTOR
TFT WIDE/ 4.23FT HT, 2FT
HIDE BOTTOM' CS
).5HP
200FT, 3MP
19)FT, 40HP
POLYPROPYLENE BAG TYPE
60GPM, 70FT HEAD, 3HP
90FT HIGH, 73HP
60. FT HORIZONTAL 7.3HP
10 FPM, IHP

NO.
1
1
1
1
1
1
1
1
1
1
1
2
2
1
I
I
1
1
1
1

MATERIAL
18582.
49145.
4180.
121)4.
17527.
60670.
3258.
)371.
17974.
1)482.
1)6171.
1079.
14268.
40447.
17730.
3258.
1)71.
•0894.
1022).'
1)482.

LABOR
1866.
1866.
7711.
1866.
0.
24875.
124)8.
746.
16169.
2*88.
0.
1741.
1711.
8706.
1)9)0.
124)8.
7*6.
1617.
1)68.
2488.

[Reproduced from
best available copy. ^K?

-------
TABLE C-7 (continued)
FEED UN
•IN HEIGH FEEDER
SYRATOHY CRUSHERS
•ALL MILL DUST COLLECTORS
•ALL HILL
HILLS PRODUCT TANK
MILLS PRODUCT TANK AGITATOR
MILLS PRODUCT TANK SLURRY
JIFT OIA, flFT STRAIGHT
SIDE HT, COVERED/ CS
1*FT PULLEY CENTERS/ IHP
7IHP
POLYPROPYLENE lAG TYPE
2200 CFM/ T.3HP
12.3TPH, 166. HP
5500 GAL 10FT OIA/ 1QFT
HT/ FLAKEGLASS LINED cs
10HP
52,0PM/ 60PT HEAD/
1
I
1
3
I
9
»
3
I»179.
f*»0>.
11*7*5.
If 774,
475996.
14561.
14673.
7610.

-------
TABLE C-7  (continued)
                                         SCRUBBING
                INCLUDING  4 OPERATING AND  1 SPARE SCRUBBING TRAINS
              ITEM
     DESCRIPTION
NO. MATERIAL    LABOR
      MECHANICAL ASH COLLECTOR
      F.D. PANS

      SHELL
      RUBBER LINING
      MIST ELIMINATOR
      SLURRY HEADER AND NOZZLES
      GRIDS
      SPHERES
         TOTAL TCA SCRJBBER COSTS
      REHEATERS
      SOOTBLOWERS
      EFFLUENT HOLD TANK

      EFFLUENT HOLD TANK A&lTATOR
      COOLING SPRAY
      ABSORBER RECYCLE PJMPS
      MAKEUP WATER PUMPS
      TOTAL EQUIPMENT COST
33x PARTICULATE REMOVAL
 20.0IN H20' KITH 1615.
HP MOTOR AND DRIVE
231267.GAL*  34.OFT OlA/
 34.OFT HT, FLAKEGLASS-
LINED CS
  63.HP
127*.GPM 100FT HEAD/
  59.HP/ 4 OPERATING
AND  6 SPARE
 8761.GPM/ 100FT HEAD/
 406.HP/  B OPERATING
AND  7 SPARE
1
5

5
5
60
5
5
10
4245 15.
1173839.
812263.
, 1199962.
368717.
313679.
471794.
175683.
3342115.
1046932.
404466.
181225.
345651.
117520.
70325.
113566.

276548.
43280.
298505.
354208.
127622.
17352.
15   656657.
 2549.GPM/  200.FT HEAD/  2
 215,HP/  1 OPERATING
AND  1 SPARE
      19790.
51730.
 1626,
                             8414911. ,1364980.
  (continued)
                                          C-20

-------
TABLE C-7  (continued)
                                  WASTE DISPOSAL
           HIM
     DESCRIPTION
NO. MATERIAL    LABOR
   ABSORBER BLEED RECEIVING
   TANK
   ABSORBER BLEED TANK AGITATOR
   POND FEID SLURRY PUMPS


   POND SUPERNATE PUMPS



   TOTAL EQUIPMENT COST
 J7760.GAL, 17,OFT DJA/   1
 14.OFT HT, PLAKGLASS-
L1NEO CS
  760.GPM,  110.FT HEAD
  46.HP*  I OPERATING
AND  1 SPARE
  544.GPM*  192.FT HEAD*  2
  44,HP/  1 OPERATING
AND  1 SPARE
      10467.

      1*772.



       1512.
11221.



 1511.

 2B67.



  785,
                               96331.    36411.
                                        C-21
                                              Reproduced from
                                              best available copy.

-------
TABLE C-8.  CAPITAL INVESTMENT REPORT
LIMESTONE SLURRY PROCESS -- BASIS) 500 MM UNIT, 1980 STARTUP
PROJECTED CAPITAL INVESTMENT REQUIREMENTS - BASE CASE EXAMPLE 500 MW
INVESTMENT, THOUSANDS OF 1979 DOLLARS

EQUIPMENT
MATERIAL
LABOR
PIPING
MATERIAL
LABOR
DUCTWORK
MATERIAL
LABOR
FOUNDATIONS
MATERIAL
LABOR
POND CONSTRUCTION
STRUCTURAL
MATERIAL
LABUR
ELECTRICAL
MATERIAL
LABOR
INSTRUMENTATION
MATERIAL
LABOR
BUILDINGS
MATERIAL
LABOR
SERVICES AND MISCELLANEOUS
SUBTOTAL DIRECT INVESTMENT
ENGINEERING DESIGN AND SUPERVISION
CONSTRUCTION EXPENSES
CONTRACTOR FEES
CONTINGENCY
SUBTOTAL FIXED INVESTMENT
ALLOWANCE FOR STARTjP AND MODIFICATIONS
INTEREST DURING CONSTRUCTION
SUBTOTAL CAPITAL INVESTMENT
LAND
WORKING CAPITAL
TOTAL CAPITAL INVESTMENT
RAM MATERIAL
HANDLING AND
PREPARATION

U52.
109.

212.
91.

0.
0.

126.
)2S.
0.

270.
100.

172.
1*2.

105.
2*.

16.
57.
130.
1969.
157.
6)5.
198.
197.
5)57.
44).
667.
6669.
7.
150.
6826.
SCRUBBING

7990.
1287.

2)29,
741.

1982.
11*9,

92,
276.
0.

171.
180.

1*2.
875.

74).
122.

0.
0.
646.
19725.
1775.
1156.
986.
1972.
27615.
2209.
1)1*.
11118.
1.
7«*.
1168*.
WASTE
DISPOSAL

51.
36.

9)6.
1*1.

0.
0.

11.
16.
1806.

1.
6.

100.
226.

1.
1.

0.
0.
2)6.
7826.
706.
1252.
191.
78).
10956.
877.
111).
1)1*1.
1221.
29).
14666.
TOTAL

9500.
1628.

1696.
1176.

19(2.
1169.

229.
816.
5806.

661.
686.

616.


816.
168.

16.
IT.
1012.
11)20.
2817.
10*1.
1)76.
11)2.
46128.
1)10.
929).
52954.
1211.
1189.
5)17*.
CASE 00|
DISTRIBUTION
PERCENT
OF DIRECT
INVESTMENT

10.1
5.2

11. T
l.T






1













.1
.1

.7
.T
.4

.4
,)

.6
.6

.T
.5

.1
.2
.1
100.0
9.0
16.0
1.0
10.0
160.0
ll.<
16.8
168.0
3.9
1.8
1T).T

-------
                       TABLE C-9.   FIRST YEAR ANNUAL  REVENUE REQUIREMENT REPORT
o
 i
rs>
co
 ID a
   "
 er
 a ~

 "1
 O 3
 •o
LIMESTUNE SLURRY PROCESS -- BASISl 500 M* UNIT, 1«BO STARTUP
PROJECTED REVENUE REQUIREMENTS - BASE CASE EXAMPLE 500 MM
DISPLAY SHEET FOR YEAR* J
ANNUAL OPERATION KU-MR/KW • O|Z
11.)* TONS PER HOUR DRY
TOTAL FIXED INVESTMENT ssmooo
ANMUJL.OUAHim UM1I-COS1«»
Dimi.CaSIS
lAtf.HtlEtliL
LIMESTONE 110. T K TONS S.OO/TON
(.(ME 0.0 K TONS *0.00/TON
SUBTOTAL RAM MATERIAL
cosmsioM-tom
OPERATING LASOR AND
SUPERVISION 10»60.0 MAN-HR 12.00/NAN.HR
UTILITIES
STE*M »1J**0.0 K IB l.TQ/K IB
PR3CESS HATER 16)510.0 K GAL 0.12/K CAL
tlECIRICITV HS19UO.O KMM 0.010/KMH
MAINTENANCE
LABOR AND MATERIAL
ANALYSES 2*20.0 HR IT.OO/HR
SJSfOTAi CONVERSION COSTS
SJITOTAL DIRECT COSTS
IMOllECl.COSlS
OEMtCUTIQN
COST 0* CAPITAL AND TAMES, 17.201 OF UNDEPREC IATEO INVESTMENT
INSURANCE t INTERIM REPLACEMENTS, i.i?t OF TOTAL CAPITAL INVESTMENT
OVERHEAD
PLANT, 50.01 OF CONVERSION COSTS LESS UTILITIES
ADMINISTRATIVE, RESEARCH, AND SERVICE,
10. Ot QF PPERATINC LABOR AND SUPERVISION
SJBTOTAl INDIRECT COSTS
TOTAL ANNUAL REVENUE REQUIREMENT
E3UIVALENT UNIT REVENUE REQUIREMENT, MILLS/KHH
HEAT RATE 9000, STU/KWM . HEAT VALUE UF COAL 10500 BTU/LB
CASE 001
SLUDGE
TOTAL
ANNUAL
COSI*»
(15*00
-.— ....0
»B3*00
2)0*00
T02900
19900
116*600
1?U*00
.. . 41200
11*1*00
*TTItOO
msioo
952*100
«*T900
icoiooo
.... 23000
129*5100
. .1221*100
T.IT
CCAl RATE 966900 TOMS/T*

-------
                 TABLE C-10.  LIFETIME  ANNUAL REVENUE REQUIREMENT REPORT
LlMESTONC SLURRY PROCESS — (AStll  500 t
-------
COMPUTER PROGRAMS:  FGDIS     	:	C-25

Model Usefulness
     The  TVA/Bechtel  computer  model  can  be  highly  useful  to the  utility
Project  Manager  in  evaluating  various  limestone  FGD scenarios.   It  can
significantly reduce the  time  and effort expended in the  planning stages to
analyze the effects of  various  operating variables on process  parameters  and
final costs.  For example,  an  economic  evaluation of sludge disposal  options
can  be  performed very rapidly and will  delineate the  effect of each sludge
disposal option on final costs.
     The  project  engineer can  apply  the model  by analyzing the  effects of
various  input variables  in  greater  detail.   The process   parameter  reports
indicate  the  effects  of  input parameters on system  flow rates and on  sizes
of  individual  equipment  items.   For  example,  the model's  output  display of
the  effects  of various L/G  ratios on  system performance will  aid  in selec-
tion of the proper L/G ratio for optimum efficiency of  S02  removal.

FLUE GAS DESULFURIZATION INFORMATION SYSTEM
Background
     Since July 1974,  PEDCo  Environmental,  Inc., has conducted for the EPA a
program called  the Utility  Flue  Gas  Desulfurization Survey.   The  object of
the  program  has  been  to provide  the   EPA  with  information   and  technical
assistance concerning U.S.  utility  FGD  technology.  The program has recently
been  expanded  to include  Japanese  utility FGD  applications  and  domestic
particulate  scrubbers.    The  major  product  of this program is a  quarterly
status  report, which  is  distributed worldwide to recipients who are directly
or indirectly involved in development of FGD technology.
     The  quarterly survey   reports  cover  all  aspects of operational  and
planned FGD  systems  and  the units  to  which they are   (or  will  be)  applied.
The  plant  data  range  from simple  identification of unit and location to more
specific  information  regarding  such   matters  as  applicable  environmental
regulations, boiler type  and flow rates, and  average fuel  analysis.   Infor-
mation  on  emission controls  identifies  the  primary means  of particulate and
S02  control.  Wherever  possible the  report presents  information on component

-------
COMPUTER PROGRAMS:  FGDIS	;	C-26
designs  and operating  parameters, waste  disposal  strategy,  reheater  data,
and other  technical  data,  as well as both the  reported and adjusted capital
costs and annual revenue requirements.
     The major feature of  the report is  the identification  and description
of operational  FGD systems.   The performance of these  systems  is  described
in depth,  with  emphasis upon mechanical  reliability,  removal  efficiency,
problems  encountered,  and  solutions  implemented.   The  importance  of  an
accurate description of  the  operational  FGD  systems   is  twofold:   (1)  it
provides the owner/operator  utilities, system  suppliers,  and  design  engi-
neering firms  with  information upon which to base  present  and future design
strategies,  and (2)  it provides  the  EPA staff  with  information needed  to
develop enforcement  strategies and to plan and  implement research,  develop-
ment, and demonstration projects.
     At  first  the  summary  report was  prepared and  published  in  a  semi-
automated  manner.   A series  of computer  files  were developed  and  manually
updated.   This  method of  information update and  retrieval  was  adequate  at
the time;  only  19  FGD systems were in  operation at the end of 1974, and the
total  committed capacity was less than  40,000 MW.  The  report has  become
progressively more  comprehensive and voluminous as the number  of  committed
FGD  systems  continues  to  grow.   Figure  C-l  illustrates by  year the  rapid
increase in  planned  and operational  systems.  This  growth was caused primar-
ily  by  the  promulgation  and  enforcement of  more stringent S02  emission
standards.    As the  number  of operational FGD systems  increased   and  more
performance  data became  available,  the  need for  a  more efficient system for
information storage and retrieval became apparent.
     In mid-1976 a  fully automated information  storage  and  retrieval  system
replaced the manually  updated  files  and  significantly improved the  effi-
ciency  of  report preparation.   From 1976 to 1978  the number  of  operating
systems  increased  to 46, and the total committed capacity rose  to  approxi-
mately  63,000  MW.   A  corresponding  sharp rise  in  the  amounts  of  available
data  made  it  evident that  the  report should  be  produced  more rapidly  to
reflect  current  technology.   Preparing  meaningful  analyses  of  the  data
became  increasingly difficult,  and therefore  a  decision was  made to convert
to a data-base system.

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CO
 o
 ft.
 a.
 s
 o
 o
100

 90

 80

 70


 60

 50

 40


 30

 20

 10
                  OPERATIONAL
                  UNDER CONSTRUCTION
                   PLANNED
                 i
1
                 1974        1976        1978         1980
               NOVEMBER    DECEMBER    NOVEMBER       JUNE
     Figure C-l.  Growth in the number of FGD systems which are
             planned, under construction and operating.
                                  C-27

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COMPUTER PROGRAMS:  FGDIS	C-28

System Description
     A  data-base is  a collection  of information  files  consisting of  data
elements  linked  in  a  logical  manner.    The data  elements are  stored  in
groups, or  blocks,  of similar data that can  be  repeated  as the need arises.
With  this  flexibility  such  a  system  can  easily  accommodate  the  ever-
increasing amounts of data.
     An important feature of the data-base system  is that  it provides users
with on-line access  to  the  data files, with  comprehensive  data manipulation
capabilities.    The  data files  are  updated continuously  to ensure  that the
information is  as complete  and current  as possible.   Interested  users can
access  the  data  files  in the  interim between  published reports;  they can
thus examine and  analyze  the more specific information  no longer included in
the  quarterly  report.  This  data-base system responds  to  standard  and  pre-
dictable  information  needs  and  to  unexpected,  ad hoc requests  arising  from
unique situations.
     The  Flue  Gas  Desulfunzation  Information System  (FGDIS)  files  are
stored  at the  National  Computer Center  (NCC),  an  EPA  facility  located  in
Research  Triangle Park,  North  Carolina.   The  system  is  easily  accessible
through a nationwide  telephone  communications network (COMNET)  that current-
ly  offers local  telephone   numbers  in 21  cities,  as well  as   through  WATS
service to locations for which local numbers are not available.*
     The data in  the FGDIS  files are  acquired from FGD system  operators and
suppliers, as well  as from  publications  and  other  technical  documents.   The
data  files  are  updated continually  on  the  basis  of  information  supplied
monthly by  FGD  system operators, and  computer printouts of  the  compiled data
are  submitted  periodically  to  system operators and suppliers for review.
     Figure  C-2  illustrates  the  data-base  structure,  showing  the  major
categories of information and the related subcategories.    Within  each block
(or  information  area)  of  the FGDIS is a series  of  elements (or components).
Table  C-ll  presents  a sample  of  information   in  the  FGDIS.   Each element
* Information about access to the FGDIS can be obtained from Mr. Walter L.
  Finch, Product Manager, National Technical Information Service, 5285 Port
  Royal Road, Springfield, Virginia 22161 [telephone (703) 487-4807].

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1
MCtMOO
Mil* CMAfUCT
OUUIT CMAKACT


1
MTIWMyfMAl
LOCATMM
Stt CAPACITY
Figure C-2.  Computerized data base structure, FGDIS program.

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                    TABLE  C-ll.   MAJOR DATA  FIELDS,  FGDIS  PROGRAM
                   Field
                    No.                Description

                   1                   IDENTIFICATION NUMBER
                   3                   UTILITY  -  INDUSTRIAL
                   6                   PLANT NAME
                   8                   PLANT ADDRESS
                   17                  S02  EMISSION LIMITATION - LB/MM BTU
                   18                  NET  PLANT  GENERATING CAPACITY - MW
                   102                 FURNACE  TYPE
                   103                 BOILER SERVICE LOAD
                   105                 MAXIMUM  BOILER FLUE GAS FLOW - ACFM
                   106                 FLUE GAS TEMPERATURE - F
                   107                 STACK HEIGHT - FT
                   109                 STACK FLUE LINER
                   112                 STACK GAS  INLET TEMPERATURE - F
                   200                 FUEL DATA
                   201                 FUEL TYPE
                   202                 FUEL GRADE
                   207                 AVERAGE  HEAT CONTENT - BTU/LB
                   209                 AVERAGE  ASH CONTENT - X
                   211                 AVERAGE  MOISTURE CONTENT - X
                   213                 AVERAGE  SULFUR CONTENT - X
                   215                 AVERAGE  CHLORIDE CONTENT - X
                   217.                 FUEL FIRING RATE - TPM
                   301                 SALEABLE PRODUCT/THROWAWAY PRODUCT
                   302                 GENERAL  PROCESS TYPE - WET/DRY/ETC
                   303                 PROCESS  TYPE
                   304                 PROCESS  ADDITIVES
                   305                 SYSTEM SUPPLIER
                   309                 NEW/RETROFIT
                   311                 TOTAL UNIT S02 DESIGN REMOVAL EFFICIENCY
                   312                 CURRENT  STATUS
                   400                 PILOT PLANT
                   410                 CURRENT  LEVEL OF DEVELOPMENT
                   506                 TOTAL CAPITAL COST - $
                   507                 TOTAL $/KW
                   512                 TOTAL ANNUAL COST - $
                   513                 TOTAL MILLS/KWH
                   600                 FGD  SPARE  CAPACITY INDEX - X
                   700                 FGD  SPARE  COMPONENT INDEX
                   800                 FGD  DESIGN & PERFORMANCE DATA
                   901                 ABSORBER NUMBER
                   902                 ABSORBER TYPE
                   916                 ABSORBER GAS FLOW - ACFM
                   917                 ABSORBER GAS TEMPERATURE - F
                   919                 ABSORBER L/G RATIO - GAL/1000ACF
                   920                 ABSORBER PRESSURE DROP • IN HZ0
                   921                 ABSORBER SUPERFICIAL GAS VELOCITY - FT/SEC
(continued)


                                               C-30

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TABLE  C-ll  (continued)
                   Field
                    Mo.

                   932
                   1000
                   1100
                   1101
                   1104
                   1107
                   1110
                   1111
                   1200
                   1300
                   1301
                   1304
                   1314
                   1315
                   1400
                   1500
                   1600
                   1700
                   1701
                   1704
                   1712
                   1713
                   1800
                   1900
                   2000
                   2001
                   2100
                   2200
                   2400
                   2500
                   2501
                   2600
                   2602
                   2603
                   2607
                   2700
                   2800
                   2900
                   3100
                   3200
                   3300
                   3307
                   3309
                   3312
                   3313
                   3400
                   3402
                   3403
                   3405
                   3447
                   344B
                   3449
                   3450
Description

ABSORBER NUMBER OF SPARES
CENTRIFUGE
FANS
FAN NUMBER
FAN TYPE
FAN LOCATION
FAN GAS CAPACITY - ACFM
FAN GAS TEMPERATURE - F
VACUUM FILTER
MIST ELIMINATOR
MIST ELIMINATOR NUMBER
MIST ELIMINATOR TYPE
MIST ELIMINATOR SUPERFICIAL GAS VELOCITY - FT/SEC
MIST ELIMINATOR PRESSURE DROP - IN H20
PROCESS CHEMISTRY CONTROL
PUMPS
TANKS
REHEATER
REHEATER NUMBER
REHEATER TYPE
REHEATER TEMPERATURE BOOST - F
REHEATER ENERGY REQUIRED
THICKENER
DUCTWORK
WATER BALANCE
WATER LOOP TYPE (CLOSED/OPEN)
REAGENT PREPARATION EQUIPMENT
END PRODUCT
SLUDGE
TREATMENT
TREATMENT TYPE
DISPOSAL
DISPOSAL TYPE
DISPOSAL LOCATION
DISPOSAL CAPACITY - ACRE-FT
PARTICULATE CONTROL
FABRIC FILTER
ESP
PARTICULATE SCRUBBER
LITERAL
EMISSION CONTROL SYSTEM REMOVAL PERFORMANCE
S02 INLET CONCENTRATION
S02 OUTLET CONCENTRATION
X S02 REMOVAL
SOj ANALYSIS METHOD
SYSTEM DEPENDABILITY PERFORMANCE
PERIOD - HR
BOILER - HR
UNIT AVAILABILITY - X
SYSTEM AVAILABILITY - X
SYSTEM OPERABILITY - X
SYSTEM RELIABILITY - X
SYSTEM UTILIZATION - X
                                                C-31

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COMPUTER PROGRAMS:  FGDIS	C-32
represents  a single  piece of  data stored  within that  block  that Is  con-
sistent  with the  particular  information area  in which  that  piece of  data
resides.   Each  element  consists of  an  element  number  (for  identification
purposes)  and  an element  name,  which  identifies  the type  of  data that  is
stored  (or  can  be stored) within the  element.   Because of continuing changes
in  FGD  technology, the  FGD structure  diagram and definition are  subject  to
change.
     Access  to  the  FGDIS  files and  data   elements  is  provided  through  a
user-oriented  language,   by  which  information  in  the  data  files  can  be
examined  without  the necessity  of  prior programming experience.   The  data
can be  tabulated  in  a manner  consistent with the  specific information  needs
of  the  user, and functions  are provided  for statistical  analyses of the
numerical  data.   A  detailed  description  of the  use of  the  FGDIS and the
individual  commands   available   for  data  manipulation  is  provided  in the
user's  manual  (PEDCo  Environmental,   Inc.  1979); the  data base  management
system  is  described  more  fully  in  the system reference  manual  (MRI  Systems
Corporation 1974).
System Usefulness
     The  extensive design and performance  data stored within the  FGDIS and
the  virtually  unlimited  data  manipulation  capabilities  of System 2000,  a
general   data base management  system,   make  the FGDIS  a  very useful design
tool based  on  actual and  current FGD  system information.   The  designer can
determine, for example,  the predominant limestone scrubber design  configura-
tions,   average  L/G ratios, or  prevailing types  of  reheat  systems.  He can
couple  the  design data  with performance information to  develop an  analysis
of  design  features   versus reliability.    In  addition,  the  designer can
identify  and examine  the actual problems  associated  with  a specific  com-
ponent  design or  application and thus possibly  avoid the  problems  encoun-
tered at  current  installations.   The  design information  can also  be used  in
conjunction  with  the reported and adjusted  cost  data for an economic  eval-
uation  of  various systems.  Such an   evaluation  could be based not only  on
process  type, but  also on such parameters as fuel sulfur content, geograph-
ical location,  and boiler size.

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COMPUTER PROGRAMS:   FGDIS	C-33
      Although  the number  of ways of  compiling FGDIS information is virtually
limitless, the following examples  provide a sample of  the output resulting
from  commands of  the following four general categories:
           1.    Data  listing
           2.    Data  tally
           3.    Statistical analysis
           4.    Tabular report generation
      Data Listing.

                 >ftIHT FUEL NT* WERE COMPMV MME Ct TCMCBSEE VALLEY AUTHORITY MB
                 >PLANT NAME CO MINUS CREEK MB UNIT NUNIER EO 81
                     FUEL TYPE* COAL
                     AVERAGE HEAT CONTENT - ITU/LI*  10000
                     AVERAGE ASH CONTENT - X* 23.00
                     AVERAGE MOISTURE CONTENT - X* 10.00
                     AVERAGE IULFUR CONTENT -X* 1.70
                     FUEL FIRING RATE • 1PM 231

The data listing  commands  available  for  use  with  the  FGDIS provide a simple
means  of retrieving data  from the  system in a sequential list.   This example
shows  output  from  a request for  a listing of  fuel data  about  TVA's Widows
Creek  unit 8.   Through the  use  of commands similar  to the  one  above, any
portion  of the FGDIS can  be  examined;  the  data  listed will  be  only those
that meet the qualifications specified in the "Where" clause.
     Data Tally.
                              >TALLY F1MT CfA KlIONl
                                CLENENT-    PLANT EPA REGION
                               •••••*••••••••*•**••*••
                               FRE8UENCY  VALUE
2
3
26
49
39
35
IS
40
23
9
1
2
3
4
S
4
7
1
9
UNIQUE VALUES
                                    232 OCCURRENCES

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COMPUTER  PROGRAMS:   FGDIS	:	C-34
     The  tally commands  available  for use with  the FGDIS  provide a means  of
obtaining statistical  information  about  unique values  of elements  stored  in
the  system.   In  this example,  a tally of  the element "Plant EPA Region" was
requested.   The  output  shows  two  frequencies  for 1  (indicating  the  FGDIS
contains  information  for  two  units  located  in  EPA Region 1), three fre-
quencies  for  2, and  so  on.    The  output  also  indicates that the  nine EPA
regions  in  the  system  include  a  total  of  232  plants far which  data are
recorded  in  the FGDIS.
     Statistical  Analysis.
                      >ftINT AVO C19 WHERE C303 CO LIHESTONEl
                      «V8 OROSS UNIT GENERATING CAPACITY - UK* 474.2*1
     System  functions  can be  used in  conjunction  with  the  FGDIS  to perform
the  following  statistical  analyses  of  numerical  data:   average,  standard
deviation,  summation,  maximum  value,  minimum value,  and  number  of occur-
rences.   In  the above  example, the request was  for  the  average  value  of
"C19"  (average  gross  unit  generating  capacity)  for  systems  that  have   a
process   type   "C303"  (limestone  systems);   the  output  appears  below the
request  as 474.291 MW.   The  system  functions can  be helpful  in determining
such things  as maximum  or  minimum pressure  drops  for  a specific  component,
or average L/G  ratio for a specific process type.
     Tabular Report Generation.
                        WHICH AM TOTAL m Of Ml SYSTEM Ml fARTICLE KIUIIEIS
                      STATUS            WITS        TCC-HU           ESC-HU
               OPERATIONAL
               UNICR CONSTRUCTION
               PLANNING
                 CONTRACT AIMRIEI
                 LETTER OF INTENT
                 REOUESTINi-EVALUATIM HIS
                 CONSIIERIN6 ONLT
                  UITOTAL
               PARTICLE SCRUIIERS
                  SUITOTAL-IOflESTIC
               JAPANESE FSI
                  IOIAL
73
39
2V
7
IS
40
2C3
IS
211
4
224
27133
17853
13761
35VO
0424
24200
9*993
2998
99991
1433
10144*
24744
UI34
I29«9
3590
•424
23980
92533
2077
94410
1455
9A045

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COMPUTER PROGRAMS:  Equilibrium Model	C-35
     A  feature  designated  "Report Writer"  enables  the user  to define  and
generate formatted  reports  of data contained in  the  system.   In the  above
example, a  report  program  was  defined by  a set  of  commands  to generate  a
tabular  listing  of the  number,  total  controlled  capacity  (TCC), and  equi-
valent  scrubbed  capacity   (ESC)  of domestic and  foreign  FGD  systems  and
domestic particle  scrubbers by status category.   The TCC is  the sum of  the
gross generating capacities of  units  brought into compliance with S02  regu-
lations  by  FGD systems, and  the  ESC  is  the product  of the  TCC times  the
average  fraction  of  flue   gas  scrubbed.    The  report  writer  capability  is
helpful when a tabular listing of FGDIS data is  required.

BECHTEL-MODIFIED RADIAN EQUILIBRIUM PROGRAM
Background
     In 1970, Radian  Corporation  developed a computer program  under  sponsor-
ship  of an  EPA  predecessor  agency.   This program provided  a  theoretical
description of the  limestone  injection, wet scrubbing process  for removal of
S02  from  power plant  flue  gas (Lowell et al.  1970).   The Radian  computer
program  calculated  numerous  chemical  equilibria  for  the   aqueous  lime/
limestone scrubbing system.
     The  original   information  and  guidelines  of  the  Radian  program  were
modified by  Bechtel Corporation  under contract to  the EPA as  part of  the
ongoing research and  development  effort  at the Shawnee test  facility.   This
Bechtel-Modified Radian  Equilibrium Program  is  used extensively  to support
the Shawnee effort (Burbank and Wang 1980).
     Examples  of  the  program  output are presented  in  progress   reports
prepared  by  the  Bechtel  Corporation  about  the  Shawnee  work.    Especially
useful  is  the "Third  Progress  Report" (Head et  al. 1977), which describes
the  input  requirements and output capabilities of the program.   The report
also  presents parametric  plots   and  nomographs based  on a  semitheoretical
mathematical model  that  predicts  S02  removal by limestone wet  scrubbing as a
function  of  operating variables.   The  equations  used to  construct  these
plots and nomographs  were  based on and verified by Shawnee  data; the results
compared  favorably with  results  predicted by  the  Bechtel-Modified  Radian
Equilibrium Program.

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  COMPUTER PROGRAMS:   Equilibrium Model _       . _ C-36
       Recent work  sponsored by EPA  Involves  the use  of adipic acid  as an
  effective additive in limestone scrubbing systems  (Head  et al. 1979).  this
  work has  resulted  in updating  and  expansion of the  equilibrium program to
  include  a  version applicable  to use  of adipic acid.  This  version gives
  detailed chemical  process information for use of adipic acid to achieve high
  S02  removal  efficiency  (Burbank et al.
;  ^r'   Information  on how  to use this ^pfogfaW can  be  obtained by contacting
  Mr.  Robert H. Borgwardt, U.S. EPA, Industrial Environmental Research Labora-
  tory,  Research Triangle Park~,~North Carolina 27711.
  Model  Usefulness
       The  Bechtel -Modified Radian Equilibrium Program  is  useful  for evaluat-
  ing  limestone scrubber performance, especially for monitoring scaling poten-
  tial  by  calculation of  the gypsum  relative  saturation  levels.   The moni-
  toring of  gypsum is important  because the Shawnee  testing  has  shown that
  scaling  usually  occurs  when the saturation level exceeds 130 percent.  Other
  user  benefits are  the  S02 removal  equations,  parametric plots,  and nomo-
  graphs presented  in  the "Third Progress Report" cited  earlier.
       Simplified  equations for calculation of gypsum  saturation  in limestone
  wet  scrubbing liquors at 25° and  50°C  were  fitted to the predictions of the
  modified  Radian  program by use of liquor data from the Shawnee long-term wet
  scrubbing reliability tests.   Results obtained with these equations differed
  little  from  those  generated by  the  Radian  program.    The equations  are
  accurate  for concentrations  of  total dissolved  magnesium and chloride ions
  up to 15,000  ppm.
       Persons  not  having access to the  modified  Radian program can use these
  simplified equations  for  simple, accurate,  and  convenient prediction  of
  gypsum saturation levels:
            Fraction CaS04  saturation at 25°C = (Ca)(S04)[(300/I)+76]
            Fraction CaS04  saturation at 50°C = (Ca)(S04)[(263/I)+47]
  where I = ionic strength  of the liquor, g-mol/liter =  3[(Ca)+(Mg)]+(S04)
    (Ca) = measured concentration of total dissolved calcium,  g-mol/liter

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COMPUTER PROGRAMS:   Equilibrium Model	     C-37
   (Mg) = measured concentration of total  dissolved magnesium,  g-mol/liter
  (S04) = measured concentration of total  dissolved sulfate,  g-mol/liter

The  calculation  of  ionic strength  assumes that  the  liquor  contains  only
calcium,  magnesium,   sulfate,  and  chloride  ions   in  solution.   The  ionic
balance is as follows:
                         (Cl) = 2 [(Ca)+(Mg)-(S04)]
Therefore,
                            I = 1/2 ZM^2
                              = 1/2 [4(Ca)+(Mg)+4(S04)+(Cl)]
                              = 3 [(Ca)+(Mg)]+S04
where M. = molarity of component i
      Z. = unit charge of component i
     Concentrations  of  potassium  and sodium  ions  have  been  small  at  the
Shawnee test facility and thus are not included in  the ionic  strength  equa-
tion.   Preliminary  evaluation of  liquor  from other  limestone and lime  wet
scrubbing systems indicates  that substantial  amounts of dissolved sodium can
be accounted  for by  adding  the  concentration  of dissolved sodium  (in  gram-
moles per liter) to the ionic strength, I.

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COMPUTER PROGRAMS:  References                                           C-38
                          REFERENCES FOR APPENDIX C                       .


Anders, W.  L.,  and R. L. Torstrick.   1981 (in press).  Computerized Shawnee
Lime/Limestone Scrubbing Model:  User's Manual.  EPA 600/8-81-081.

Burbank, D.  A.,  and S.  C. Wang.   1980.   EPA Alkali Scrubbing Test Facility:
Advanced  Program Final  Report (October 1974  to June  1978).   EPA-600/7-80-
115.  NTIS No. PB 80-204241.

Burbank, D.  A., et  al.   1980.  Test  Results on Adipic  Acid-Enhanced Lime-
stone  Scrubbing  at the  EPA Shawnee Test  Facility—Third Report.   Presented
at the  Symposium on Flue Gas Oesulfurization, Houston, Texas, October 28-31,
1980.

Head,  H.  N.   1977.   EPA Alkali Scrubbing Test  Facility:   Advanced Program,
Third Progress Report.  EPA-600/7-77-105.  NTIS No.  PB-274 544.

Head,  H.   N.,  et  al.    1979.   Recent Results From EPA's  Lime/Limestone
Scrubbing  Programs—Adipic  Acid as  a  Scrubber  Additive.   In:   Proceedings:
Symposium  on  Flue  Gas Desulfurization, Las Vegas,  Nevada,  March 1979.  Vol.
1.  EPA-600/7-79-167a.  NTIS No. PB80-133168.

Lowell, P. S.,  et al.    1970.   A Theoretical  Description of  the Limestone
Injection  -  Wet Scrubbing  Process.   Vol.  1,  Final Report  for the National
Air Pollution Control Administration under Contract No. CPA-22-69-138.  NTIS
No. PB-193 029.

McGlamery,  G.   G.,  et  al.   1975.   Detailed  Cost Estimates   for Advanced
Effluent Desulfurization  Processes.  EPA-600/2-75-006.  NTIS No. PB-242 541.

McGlamery, G.  G., et  al.   1980.   FGD Economics in 1980.   Presented at the
Symposium  on  Flue  Gas Desulfurization, Houston, Texas, October 28-31, 1980.

MRI  Systems  Corporation.   1974.   System  2000,  Reference Manual.   Austin,
Texas.

PEDCo  Environmental,  Inc.   1979.   Flue  Gas  Desulfurization  Information
System Data Base User's Manual.  Cincinnati, Ohio.

Stephenson,  C.  D., and  R.  L.  Torstrick.  1978.  Current Status of Develop-
ment  of  the Shawnee  Lime/Limestone  Computer  Program.    In:   Proceedings,
Industry Briefing  on  EPA Lime/Limestone Wet Scrubbing Test Programs, August
1978.  EPA-600/7-79-092.   NTIS No.  PB-296 517.

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COMPUTER PROGRAMS: References	C-39

Stephenson,  C.   D.,   and  R.  L.  Torstrick.    1979.   Shawnee  Lime/Limestone
Scrubbing  Computerized Design/Cost-Estimate  Model  Users Manual.   EPA-600/
7-79-210.  NTIS No. PB80-123037.

Torstrick, R.  L.   1976.   Shawnee Limestone/Lime  Scrubbing  Process Computer-
ized  Design  Cost Estimates Program:  Summary  Description Report.   Presented
at the  Industry  Briefing  Conference, Raleigh, North Carolina, October 19-21,
1976.

Torstrick, R. L.,  L.  J.  Henson, and  S.  V.  Tomlinson.   1978.  Economic Eval-
uation  Techniques,  Results, and  Computer Modeling  for Flue  Gas  Desulfuri-
zation.   In:   Proceedings  of  the  Symposium on  Flue  Gas  Desulfurization,
Hollywood, Florida,  November 1977.   Volume I.   EPA-600/7-78-058a.   NTIS No.
PB-282 090.

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                                 APPENDIX D
                     INNOVATIONS IN LIMESTONE SCRUBBING

     This appendix discusses new  types  of scrubbers and new process modifi-
cations that are  commercially  available for use with limestone scrubbing or
that  have  strong commercial  potential.    It  also  includes  a  preliminary
economic  evaluation  of  current and future  limestone  systems.   The  informa-
tion presented is  not  intended to be exhaustive;  further  details  are given
in the references indicated.

NEW TYPES OF SCRUBBERS
     Jet  bubbling and  cocurrent  scrubbers  are  two  innovative   kinds  of
scrubbers that can be  used with limestone scrubbing systems.   Additionally,
a  charged  particulate  separator  (CPS)  can  be  added  to  a  conventional
scrubber.   This subsection examines these two types of scrubbers and the CPS
option.
Jet Bubbling Scrubber
     Chiyoda Chemical Engineering  and  Construction Company, Ltd.,  has built
and operated a limestone  scrubbing system that produces gypsum as  a byprod-
uct.  The central  feature of this Chiyoda  Thoroughbred  121 system  is a jet
bubbling  reactor  (JBR), in which S02 scrubbing, forced oxidation,  neutrali-
zation, and crystallization all occur.
     As shown  in Figure  D-l,  flue gas  enters a  relatively  shallow liquid
layer through vertical  spargers.   The  velocity of the flue gas causes it to
entrain the  surrounding  liquid,  so  that  a jet bubbling  or  froth  layer is
created.   The  resulting   gas-liquid  interface  is large  and  enhances  S02
removal.
     Liquid in the JBR is  moderately agitated by air bubbling and mechanical
stirring.   Oxidation air  is  introduced into the reactor by air spargers at
                                     D-l

-------
                                                            CLEAN GAS
 WATER
FLUE GAS
                                                       LIMESTONE  SLURRY
           AIR
                          GYPSUM SLURRY
           Figure D-l.   Jet bubbling reactor (Laseke 1979).
                                  D-2

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INNOVATIONS:  Scrubber Types	',	D-3
rates 200  to  300  percent of stoichiometric requirements (Laseke 1979).   Ex-
cess oxidation air, adequate residence time, and sufficient suspended solids
promote gypsum crystal growth.   Gypsum settles to the bottom of the JBR, and
a bleed stream of gypsum is continuously drawn off.
     Compared with conventional limestone scrubbers,  a jet bubbling scrubber
offers  several  design and  operating  features that  can  improve operability
and reduce costs:
          No large slurry recirculation pumps
          No nozzles or screens
          High limestone utilization
          Reduced  effect  of limestone type and grind  on  operation (because
          of low operating pH)
          Enhanced mist  eliminator performance  (because  of low  slurry en-
          trainment in the gas)
          Minimal  scale deposition over a wide range  of operating conditions
     With  funding from  the Electric  Power  Research  Institute  (EPRI) and
Southern Company  Services,  Chiyoda constructed and  operated a  20-MW proto-
type  jet  bubbling  scrubber at  Gulf Power  Company's Scholz  Station.   The
prototype scrubber was  tested  for 9 months and was  shown to function reli-
ably at various conditions  with flue gas  from  a  coal-fired boiler.  Sulfur
dioxide removal efficiency  was 95 percent when the inlet flue  gas contained
3500 ppm  of S02,  and gypsum  produced  by the  JBR  settled quickly  and was
dewatered easily  (Radian Corporation 1980).  Average  limestone utilization
exceeded 98  percent,  and  scale  deposition was minimal  (Radian Corporation
1980).
Cocurrent Scrubber
     A  cocurrent   scrubber  offers  several  advantages  over a  conventional
countercurrent scrubber.   The  equipment  configuration  better  fits ductwork
and  fan arrangements  in most powerplants.  Because  flue  gas  enters  the
scrubber at a high elevation and leaves near ground level, the  mist elimina-
tor  and  reheat system, which  tend to require the most maintenance,  can be

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INNOVATIONS:  Scrubber Types	D-4
near  ground  level.   The  scrubber  system  fans  can be  on ~ne  ground,  and
ductwork  to the stack can usually  be shorter and  less  complex.   Also,  the
change In  flue  gas direction at the  bottom  of  a cocurrent scrubber and the
vertical  orientation  of the  mist  eliminator enhance  liquid  separation and
drainage.   Another  benefit of a cocurrent scrubber is that flooding is less
likely to  occur.   Because  of the enhanced liquid separation and the reduced
tendency  to  flood, gas  velocity   is increased  and  scrubber  size can  be
smaller.
     With  funding  from  EPRI,  TVA tested cocurrent  scrubbing  at  the Colbert
1-MW pilot plant.   These tests provided design  data used  to  modify a 10-MW
prototype  unit  at  the  Shawnee test facility for cocurrent scrubbing.   From
August 1978 to  July 1979,  TVA tested the  10-MW  prototype  scrubber.  Sulfur
dioxide removal  efficiency during initial tests of the prototype consistent-
ly  exceeded 90  percent  at inlet flue  gas S02  concentrations  ranging from
1500 to 3000 ppm (Jackson 1980).
     In  tests conducted at  the  Shawnee test  facility from  August 1979  to
July 1980,  the  cocurrent  scrubber  was  altered  for forced  oxidation with a
single effluent hold tank (Figure D-2) and with multiple tanks (Figure D-3).
During  these more  recent  tests, TVA identified operating conditions  that
consistently  removed  more  than 90  percent of  S02 from  the flue  gas  and
oxidized more than  95  percent of calcium  sulfite  in  the scrubber slurry to
gypsum (Jackson  1980).
Charged Particulate Separator
     A recent commercial development is the use  of  a CPS after a conven-
tional  limestone  scrubber  (e.g., a venturi-spray  tower combination).   The
CPS has  been designed  as a wet  electrostatic precipitator and,  in combina-
tion with  a  conventional  scrubber,  offers several  advantages  over  other
scrubbing  systems.  If  a dry precollector (e.g.,  an  electrostatic precipi-
tator)  is  used  ahead  of  an S02  scrubber,  fly  ash is  collected and removed.
A scrubber  with a  CPS,  however, allows  the  use  of alkaline material in the
fly ash for S02  scrubbing.
     A scrubbing system  with  a dry precollector  followed  by  a wet scrubber
usually requires two waste disposal systems:   one  for dry waste, the other

-------
                                                 FLUE GAS INLET
                                          MIST
                                       ELIMINATOR
o
en
               TO INDUCED
               DRAFT FAN
                                 IN-LINE
                                 REHEATER
IR
COCU1
SCRU
X
RENT
BBER

""^i

                                                                 PRESATURATION PUMP
                                           RIVER WATER
                       LIMESTONE
                        SLURRY
                      PREPARATION
                         TANK
                   MIST ELIMINATOR
                   CIRCULATION PUMP
                    RIVER WATER
                                                                                                  "0 LIMESTONE SLURRY
                                                                                                        FEED PUMP
    SCRUBBER
CIRCULATION PUMPS
                                                            AIR
                         MIST ELIMINATOR
                           CIRCULATION
                              TANK
                                                   DISPOSAL
  CAKE
                                           TO POND
                                           DISPOSAL^   13 - L!
             BELT OR DRUM   THICKENER
                FILTER    UNDERFLOW PUMP
                                                          FILTER CAKE
                                                           RESLURRY
                                                             TANK
              FILTRATE
                PUMP
 RECYCLE
  LIQUOR
SURGE  TANK
RECYCLE LIQUOR
 RETURN PUMP
                                     Figure D-2.   Cocurrent scrubber used for forced  oxidation
                                      tests with a  single effluent hold tank (Jackson 1980).

-------
                                              FLUE GAS INLET
             TO INDUCED
             DRAFT FAN
o
                                       MIST
                                     ELIMINATOR
                               IN-LINE
                              REHEATER
COCU RENT
SCRUBBER
                                                                         EFFLUENT
                                                                        CIRCULATION
                                                                        {HOLD TANK
                                                                                      CIRCULATION
                                                                                       ,PUMP
                                                              BACKMIX-'  PRESATURATOR PUMP-*
                                                               PUMP
                           BELT OR  DRUM    THICKENER
                              FILTER    UNDERFLOW PUMP
                             FILTRATE
                              PUMP
 RECYCLE
  LIQUOR
SURGE  TANK
                  RIVER WATER


                   LIMESTONE
                    SLURRY
                  PREPARATION
                     TANK

           ULIMESTONE SLURRY
                 FEED PUMP
                 MIST ELIMINATOR
                 CIRCULATION PUMP


                  RIVER WATER
                       MIST ELIMINATOR
                         CIRCULATION
                           TANK
                                        TO POND
                                        DISPOSAL
                                                        FILTER CAKE
                                                         RESLURRY
                                                           TANK
RECYCLE LIQUOR
 RETURN PUMP
                               Figure D-3.   Cocurrent scrubber used for forced  oxidation
                                tests with multiple effluent  hold  tanks (Jackson 1980).

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INNOVATIONS:  Process Modifications	     D-7
for sludge.   In contrast, a  scrubber  with  a CPS allows discharge of  waste
into a  common disposal  system,  which has  fewer components to  operate and
maintain.
     In  a  conventional  venturi-spray  tower  combination,  the  pressure drop
across the  venturi  unit  is  often set  at 15 to  20 in. H20 to  remove most
particulates.  When a CPS is  used after  a venturi-spray tower combination,
however, the venturi serves only as a precollecting device  and operates at a
pressure drop  of 3 to  5  in.  H20 (Martin,   Malki,  and Graves  1979).   This
smaller pressure drop can permit a significant reduction in operating costs.
     A scrubber  with  a  CPS  can  remove  S02  to any required  emission  level
regardless  of  the sulfur content of the  coal.   Further, limestone  comsump-
tion by  a  scrubber  with a CPS is only slightly greater than the theoretical
requirement, whereas consumption  by  a conventional limestone scrubbing sys-
tem is substantially greater.
     Combustion  Engineering   has  conducted  pilot  tests   at  the  Sherburne
Station  of  Northern States Power Company to develop  the CPS  option with a
venturi-spray  tower combination.   According to  Martin,  Malki, and Graves
(1979),  this  pilot  system (known as  the Two Stage Plus) not  only  can meet
the particulate and S02 standards set in June 1979, but also costs  less than
most wet scrubbers.   Also,   Peabody  Process Systems  has  developed  a  CPS
option for  use with a conventional scrubbing system.

NEW PROCESS MODIFICATIONS
     Conventional limestone scrubbing  can be modified in several ways.  For
example, limestone can be used with aluminum sulfate as in  the Dowa process,
or it can  serve as  a regenerant in dual-alkali  systems.  Alterations in the
type and grind of limestone can significantly affect S02 removal efficiency.
Also, forced oxidation, gypsum stacking, adipic acid addition, and magnesium
addition can  improve  a conventional limestone  scrubbing system.   This sub-
section discusses these process modifications.
Dowa Process
     The Dowa  process  (Figure  D-4)  is  a dual-alkali  S02  scrubbing process
that uses a solution of basic aluminum sulfate [A12(S04)3]  to absorb S02 and

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                     REHEATER
                                                                                              LIMESTONE
                                                                                                SILO
                                                                                                WEIGH
                                                                                                FEEDER
     MIST
  ELIMINATOR
GAS FROM
 BOILER —r*
                                                                                          RECLAIMED
                                                                                          ABSORBENT
                                                                                            TANK
                                                                                    TO POND
                                                                      WASTE SLURRY
                                                                         TANK
                    Figure D-4.   Typical  flow  diagram  of the  Dowa process
                              (Crowe, Hoi linden, and Morasky 1978).

-------
INNOVATIONS:   Process Modifications	D-9
a slurry of  limestone  to regenerate the absorbent.   The Dowa Mining Company
of Tokyo, Japan,  developed  this  process, which the Air  Correction  Division
of UOP,  Inc.,  will market  in the  United States.   In Japan, the process  is
commercially  used with  smelters,  sulfuric acid  plants,  and  utility  and
industrial  oil-fired boilers.  At  the  Shawnee  test facility, the Dowa  pro-
cess has been tested with flue gas from a coal-fired boiler.
     Several  potential advantages  of the Dowa process prompted the Shawnee
tests.    For  example, use of a  clear  solution (rather  than a slurry)  for
absorption eliminates erosion of equipment and buildup of  slurry solids  on
internals of the  mist  eliminator and scrubber.  Also, the  Dowa process re-
quires  lower  limestone  stoichiometry  and produces  byproduct  gypsum  with
dewatering characteristics  better  than  those  of unoxidized limestone scrub-
bing sludge.
     A disadvantage  of the  Dowa  process is that it  requires more  equipment
and  is  more  complex than  conventional, single-loop,  limestone  scrubbing.
Further, the pH of the scrubbing solution  is  approximately  3,  whereas  that
of limestone slurry is 5 to  6.   The lower pH requires materials  of  construc-
tion that  are more  acid resistant  (e.g.,  316L or 317L  stainless  steel  or
lined carbon steel).
     Initial   Shawnee tests  of   the  Dowa  process  with a  packed  mobile-bed
scrubber (known  as  a  Turbulent  Contact Absorber  and  supplied  by  the  Air
Correction Division  of Universal  Oil Products) showed a maximum S02 removal
efficiency of  85  to  90 percent (Jackson, Dene, and Smith 1980).   Because of
problems with  gas flow  distribution,  rigid packing was  used in  later  fac-
torial  absorption  tests,  in which the  operating conditions  were  identified
for  consistent achievement  of   an  S02  removal  efficiency greater  than  90
percent (Jackson,  Dene, and  Smith 1980).  Neutralization and gypsum  dewater-
ing  were generally  satisfactory,  and  no problems  with  reliability  were
encountered.
Limestone Regeneration in Dual-Alkali Systems
     Limestone can be  used  as a regenerant in  dual-alkali  systems.  Avail-
able near  most  industrial   sites,  limestone  is less expensive than  lime,
which typically  is  used  for absorbent  regeneration  in  present  dual-alkali

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INNOVATIONS:  Process Modifications	D-10
systems.   Studies  indicate,  however, that  impurities  in  limestone,  partic-
ularly  magnesium,   can  impair  the  settling  and  dewatering properties  of
byproduct  solids  (LaMantia et  al.  1977).   Limestone  is  less  reactive than
lime and  thus  requires a longer reaction time.   In addition, calcium utili-
zation  rates tend  to be lower when absorbent is regenerated with limestone.
     Raising temperatures  to increase reaction rates  might reduce settling
and  dewatering  problems with solids.   Also,  magnesium can be  precipitated
from  a slipstream.   To effect such  improvements, however, would probably
eliminate  the  economic advantage  of using limestone  rather than  lime as a
reagent (LaMantia 1977).
     Oberholtzer et al. (1977) suggest that within certain constraints lime-
stone  can  be  used  as a regenerant without inordinately complicating a dual-
alkali  system.  Tests  of a four-reactor system with a 2-hour residence time
yielded calcium  utilizations between  78 and 92  percent  over  a reasonable
range  of  solution  concentrations    As is typical  of actual  operating condi-
tions, the solution was not heated above 122°F.   Fredonia  limestone contain-
ing  1.1 percent magesiurn  as  magnesium  carbonate  was used, and no efforts
were  made to  control  magnesium  solubility.    Under   these  conditions,  the
system  produced solids that  settled  well  and   could   be vacuum-filtered
easily.
     To increase the  use of  dual-alkali systems for S02  control, the EPA is
sponsoring  a  program  in which  limestone will serve  as the regenerant.   In
Japan,  limestone has  already been successfully used to regenerate absorbent
in dual-alkali systems.   Louisville Gas & Electric Company will attempt to
duplicate the Japanese success in tests at the Cane Run Station and will use
the  results of  these tests to design  future  commercial dual-alkali  systems
with limestone regeneration.
Effect of Limestone Type and Grind
     The type and  grind of limestone used  in a single-loop, packed mobile-
bed  scrubber  can  significantly  affect S02  removal  efficiency  (Borgwardt et
al. 1979).  Tests of three types of high-calcium limestone at the Industrial
Environmental  Research  Laboratory  (IERL),  Research  Triangle   Park  (RTP),
North Carolina, showed that Fredonia limestone was more efficient than Stone

-------
INNOVATIONS:  Process Modifications	D-ll
Man  limestone,  which in  turn  was  more efficient than Georgia  marble.   The
ranking was the same for fine and coarse grinds.  Borgwardt et  al.  (1979)
report that  at  a stoichiometric ratio of 1.5, S02 removal efficiencies were
88  percent with  fine Fredonia  limestone,  74 percent with fine Stone  Man
limestone, and 70 percent with fine Georgia marble.
     The  IERL-RTP  tests   also  indicated  that  fine  grinding  enhances  S02
removal and  that sludge  quality is affected  by the  type of limestone used.
According  to Borgwardt  et al.  (1979),  the  limestone feed rate needed  to
maintain a  given S02 removal  efficiency with coarse grinds (70 percent -200
mesh) was  roughly  50 percent  greater than that needed to maintain the same
efficiency with fine grinds (84 percent -325 mesh).   Also, the  settling rate
and  filterability of  scrubber  slurry  increased  as  S02 reactivity  of  the
limestone decreased.
     Under  current  EPA   sponsorship,  Dr. Gary  T.   Rochelle has  developed
additional  data  showing   that   limestone  type and  grind  can  enhance  S02
removal.   The reader is  referred to references in Appendix A for details of
Dr. Roche!le's work.
Forced Oxidation
     Tests in 10-MW prototype units at the Shawnee test facility have demon-
strated that  forced  oxidation  of waste sludge material  into calcium sulfate
(gypsum) reduces  its volume and increases its settling speed by an order of
magnitude  (Head,  Wang,  and Keen 1977).   Further,  oxidized calcium sulfate
(gypsum) can  be  filtered  to more than  80 weight  percent solids and handled
like moist soil, whereas  unoxidized calcium sulfite byproduct  can be fil-
tered  to  only   50  to 60 weight  percent solids  and displays  thixotropic
properties (Head, Wang, and Keen 1977).
     Forced  oxidation in  a single  scrubber  loop  was  successfully demon-
strated  with limestone  slurry  in  the  packed mobile-bed  scrubber  at  the
Shawnee test  facility (Figure  D-5).   Contact between the air and slurry was
achieved in  the  scrubber  loop by pumping slurry from a small downcomer hold
tank through  an  air eductor to a larger oxidation tank,  where limestone was
added.   Slurry  from the  oxidation tank was returned to the scrubber.  Typi-
cally, the  eductor pH was 5.15, the  oxidation  tank pH was 5.5, and the air

-------
    MAKEUP WATER-
FLUE GAS
LIMESTONE
CLARIFIED
LIQUOR
FROM SOLIDS
DEWATERING
SYSTEM


    BLEED TO SOLIDS
   DEWATERING SYSTEM
                                 FLUE  GAS
                                 u
                                  o o o o
                                 o o o o o
                                  o o oo
                                 00000
                                  o o o o
                                 0.0 0.0.0
          PACKED
        MOBILE-BED
         SCRUBBER


— O-


•M M •
1
c

0
COMPR
A
WATER
P


-------
INNOVATIONS:  Process Modifications	D-13
stoichiometry was 2.5 pound-atoms  of oxygen per pound-mole of  S02  removed.
Effective oxidation  required  maintenance  of air/slurry contact in  the  dis-
charge plume from the eductor (Head,  Wang, and Keen 1977).
     Tests  at  the  IERL-RTP pilot  plant  also  indicate  the feasibility  of
oxidizing calcium sulfite slurry within the scrubbing loop  of  a single-stage
limestone scrubber at normal operating pH  (Borwardt 1977).   Forced oxidation
improved  the  dewatering properties  of  sludge and did not  adversely  affect
S02  removal efficiency,  limestone  utilization,  and  scrubber  feed  super-
saturation.   Good performance  can  be anticipated with 98  percent oxidation
and  at  chloride concentrations  at  least  as great as  20,000  ppm  (Borgwardt
1977).
     Tests  at  a 140-MW  unit  have  shown that forced  oxidation  of limestone
scrubber  sludge to  gypsum  is  a viable technique for water  disposal.  At air
stoichiometries of 1.75  to  2.0 pound-atoms of oxygen  per  pound-mole  of S02
removed,  approximately 95 percent  of the  slurry was oxidized  (Massey et al.
1980).   As  a result  of  these tests,  TVA will  use forced  oxidation  as the
preferred method for  disposing of  scrubber sludge from Widows Creek Units 7
and  8 and  is  designing  scrubber trains  for  Paradise Units 1  and  2  with a
forced oxidation option  to  produce a calcium sulfate  waste product (Massey
et al. 1980).
     Many commercial  scrubber  suppliers  now offer  forced oxidation as  an
effective means of reducing disposal  problems associated with  handling waste
sludge materials.  The  reader  is referred to Appendix F for  information on
commercial systems using forced oxidation.
     In  Japan  and  Germany, forced  oxidation  systems convert  waste  sludge
into  gypsum pure enough for  wall board manufacture.   Research-Cottrell has
designed  such  a forced  oxidation  system  for  Tampa Electric Company's Big
Bend  Unit 4.   This  is  expected to  be  the first of many  U.S.  systems  that
will produce salable byproduct gypsum.
Gypsum Stacking
     The  U.S.  phosphate  fertilizer  industry  has  used  gypsum stacking  to
dispose  of  waste gypsum for  more  than 20 years.   Typically,  gypsum stacks
are  structurally stable  stockpiles  that cover 50 to 300 acres and can reach

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INNOVATIONS:  Process Modifications	     D-14
heights  of 150  feet  (Morasky  et  al.  1980).   Unoxidized calcium  sulfite
sludges from  conventional  limestone scrubbing have not been stacked because
of  their  poor  handing and  dewatering characteristics.   Forced  oxidation,
however, can  be  used to produce calcium sulfate (gypsum) instead of calcium
sulfite waste.   This  gypsum  dewaters easily and can  support  a considerable
load (Radian Corporation 1980).
     Construction  of an  FGD gypsum  stack (Figure  D-6) is  fairly  simple.
Slurry  is  fed to an area bounded by a starter dike and having a decant pipe
or pond  for removal  of supernatant liquor.  When  this  inner  area is filled
with  solids,  a  dragline   is  used  to  dredge  gypsum onto  the side  of  the
starter dike.  Thus, a cast gypsum perimeter dike is created,  and the height
of  the  structure  is  raised.   As more  slurry is  added, the process  is
repeated.
     Operation of  a  gypsum stack is generally much  easier  and simpler than
operation  of a  landfill  for conventional  unoxidized FGD  wastes.   Because
gypsum can be pumped to the disposal area in a slurry, the problems of daily
handling and  transportation  of  wastes  to a landfill  are avoided.   Further,
the stacking  method  dewaters  gypsum by gravity and thus eliminates the need
for  mechanical  dewatering.    Within  the  stacking  area,  gypsum  quickly
dewaters and  forms a  stable  material; thus,  additional compaction  is  un-
necessary.
     Compared  with conventional  unoxidized FGD wastes, FGD  gypsum offers
several advantages.   For  example,  it can be stacked and stored in a smaller
area than  that  required  for landfilling of calcium sulfite sludge.  If pure
enough, FGD  gypsum can also be used in agriculture  and for the manufacture
of wallboard  and portland cement.   Research-Cottrell and others are design-
ing forced oxidation systems that will  produce this pure  gypsum  for sale.
     Contamination of  nearby  surface  and  ground waters  by  FGD wastes (both
unoxidized  sludges and gypsum stacks)  is possible.   Process  water can con-
tain  concentrations  of  sulfate,   calcium,  chloride,  and  magnesium  several
orders of  magnitude  greater  than those usually found in natural surface and
ground  waters and those  allowed  by drinking  water standards.  Also,  such
trace elements as  arsenic,  chromium,  and selenium  can be present in process
water at  levels greater  than those permitted by  drinking water standards.

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STARTER DIKE
                                          :  SEDIMENTED FGD GYPSUM  I
                                       CAST GYPSUM PERIMETER
                                         DIKE, FIRST Lin.
                                               CAST GYPSUM PERIMETER
                                                 DIKE,  SECOND LIFT
                                                     CAST GYPSUM PERIMETER
                                                       DIKE, THIRD LIFT


      Figure D-6.   Construction  of  an  FGD gypsum  stack  (Golden 1980).
                                    D-15

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INNOVATIONS:  Process Modifications	   D-16
Thus,  seepage  from FGD  gypsum stacks must be controlled  or  prevented,  and
surface and ground waters near stacks must be monitored.
     Although gypsum from the phosphate fertilizer industry has been stacked
for  many  years,  the geotechnical and environmental  feasibility  of stacking
FGD  gypsum has  been  investigated  only recently.   A prototype  FGD  gypsum
stack  was constructed  and operated  at the  Scholz  Station  of  Gulf  Power
Company from October  1978 to  June 1979.  Gypsum  was produced by the proto-
type Chiyoda Thoroughbred 121 scrubber evaluated at the same time.  Analysis
indicates  that  FGD gypsum has settling, dewatering,  and  structural  charac-
teristics  similar  to,  and in  some ways more  favorable  than,  those of phos-
phate  gypsum (Radian  Corporation 1980).   Limited monitoring of ground water
showed  no increase  in  concentrations  of  trace  elements, but   did  reveal
increases  in concentrations of calcium,  sulfate,  and total dissolved solids
(Radian Corporation 1980).
Adipic Acid Addition
     Tests at  the 0.1-MW  IERL-RTP  pilot plant  and at the 10-MW prototype
units  of  the Shawnee  test  facility have indicated  that addition of adipic
acid to a limestone  wet scrubbing system significantly enhances  S02 removal
efficiency (Head  et  al.  1979).   Adipic acid  is a dicarboxylic organic acid
[HOOC(CH2)4COOH] in powder form; it is available commercially  and is used as
a raw  material  in the nylon manufacturing and food processing  industries.
When adipic acid  concentrations  in limestone slurry  ranged between  700  and
1500 ppm,  the  IERL-RTP  and  Shawnee tests  consistently showed  S02  removal
efficiencies greater than 90 percent.   Head et al. (1979)  report that adipic
acid effectively  enhanced S02  removal  efficiency even with chloride concen-
trations  as  great  as 10,000  ppm at Shawnee and 17,000  ppm at  the  pilot
plant.   Addition of adipic acid improved S02 removal efficiencies equally in
systems with and  without forced oxidation,  caused only minor  differences in
the  dewatering  and  handling  properties  of  oxidized sludge, and did  not
produce scaling.
     One  problem  has  been  the  decomposition of adipic  acid  at  ordinary
scrubber  operating conditions,  especially  in  systems with forced oxidation.
In earlier Shawnee tests, 8 to 9 pounds  of adipic acid were consumed per ton

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INNOVATIONS:  Process Modifications	D-17
of limestone to maintain adipic acid concentration in the slurry at 1500 ppm
(Head et  al.  1979).   Recent Shawnee tests suggest that maintenance of inlet
slurry  pH below  5.0 minimizes  adipic acid  decomposition (Burbank  et  al.
1980).
     In the  spring of  1980,  the EPA contracted with  Radian  Corporation to
evaluate  adipic acid  enhancement of limestone scrubbing by a full-scale FGD
system.    This  program is  being  conducted at the Southwest Station  of City
Utilities  near  Springfield, Missouri.  In  the late  summer of  1980,  tests
began at  Southwest Unit 1, a 194-MW unit firing high-sulfur bituminous coal
(Hicks,  Hargrove,  and Col ley 1980).
     Initial  results  at  Southwest  Unit  1  have  been  encouraging.   Before
adipic acid  addition, S02  removal efficiency averaged roughly 65 percent at
the  normal  operating pH  of 5.5.  When 800  to 1000 ppm of adipic acid was
added to the scrubbing liquor, S02 removal efficiency increased to more than
90  percent;  and  at full  load,  S02  removal  efficiency reached  95  percent
(Hicks,  Hargrove,  and Colley 1980).   Limestone utilization also improved.  A
test at an operating  pH of 5.0 and an adipic acid concentration of 1500 ppm
yielded an  S02  removal efficiency of  more  than  90 percent  and limestone
utilization of 99 percent (Hicks, Hargrove, and Colley 1980).
     Also,  the  EPA has  sponsored tests at  Rickenbacker Air  National Guard
Base near  Columbus, Ohio,  to show that adipic acid enhances the S02 removal
efficiency  of  an   industrial-size limestone  scrubbing  system.   Adipic acid
addition  increased S02  removal  efficiency from a marginally effective level
to a high removal  efficiency.
Magnesium Addition
     The  addition  of  modest amounts of magnesium can  improve  the operation
of  a limestone   scrubbing  system,  especially the  S02 removal  efficiency
(Josephs 1980).   Depending on pH and ionic effects,  magnesium is 100 to 1000
times as  soluble   as  calcium in  the  scrubbing liquor.  Magnesium addition
thereby  improves   liquid phase  alkalinity  and  S02  absorption  and  often
reduces  overall slurry  pumping rates.  Enrichment with magnesium also pro-
vides more soluble alkali for chloride to combine with and prevents pH drop,
although  chloride  interference   requires  greater  magnesium   addition.   If

-------
INNOVATIONS:  Economic Evaluation	.	D-18
enough  magnesium is added  to absorb most  S02 in  the flue gas,  limestone
dissolution is limited, and scaling potential is greatly reduced.
     In 1976,  the effects  of magnesium oxide addition were studied  at the
Shawnee test facility.   Limestone  slurry was used  in  the packed  mobile-bed
scrubber to scrub flue gas containing fly ash.   Under typical  operating con-
ditions, S02 removal  efficiencies  were  77,  84, and  94 percent at effective
magnesium  ion  concentrations of 0,  5000, and 9000  ppm,  respectively (Head
1977).  Head  found that  magnesium oxide addition at  the levels  tested did
not always  produce  gypsum-subsaturated  operation,  but that  lower  saturation
levels  tended  to  increase  liquor  sulfite  concentration  and S02  removal
efficiency.
     Pullman Kellogg  (now a  division of Wheelabrator-Frye)  offers a commer-
cially  proven  process that  uses  magnesium  addition to  enhance the  S02
removal efficiency  of limestone scrubbers.   A Pullman Kellogg system will
soon be operational at Associated Electric Cooperative's Thomas Hill Station
in  Moberly, Missouri.   The reader  is  referred to  Appendix  F for further
details on other Pullman Kellogg magnesium-enhanced systems.

ECONOMIC EVALUATION
     New  types  of  scrubbers  and  new process  modifications offer economic
advantages over conventional limestone scrubbing.   Often,  innovations  can be
combined  to  increase  potential  savings.   Under  EPA  sponsorship,  TVA  is
evaluating  the  economics  of  current and future limestone scrubbing systems.
Complete results of the evaluation will  be published in 1981.
     A  preliminary  economic  comparison  has  been made  of  conventional,
improved,  and advanced limestone  scrubbing  systems (McGlamery et  al.  1980).
The conventional  system  is  defined  as  a  packed mobile-bed  scrubber with
onsite ponding  of calcium sulfate  sludge; the improved system is  defined as
a  spray tower  with forced  oxidation  and  landfilling  of  gypsum; and the
advanced  system is defined  as  identical to  the  improved  system, but with
adipic acid addition.  Table  D-l presents design conditions for  these sys-
tems,   Table  D-2  shows  capital  investments,  and  Table D-3  lists  annual
revenue requirements (McGlamery  et al.  1980).   Although capital  investments
for the  improved and  advanced  systems   are  slightly greater  in  most areas

-------
                        TABLE D-l.   PROCESS DESIGN CONDITIONS FOR LIMESTONE  SYSTEMS
                                                                                   a
                                  Conventional
                                     system
                                                Improved system
                                                  Advanced system
o
to
Type of scrubber
Forced oxidation
Adipic acid addition
Waste disposal
Scrubber gas velocity, ft/s
L/G ratio, gal/103ft3
Limestone stoichiometry
Air stoichiometry
Sulfite oxidation, %
Type of fan
Spare scrubber
Filter cake solids, wt. %
Pond settled solids, wt.  %
Spare ball mill
Reheat
Bypass available
Mobile-bed
No
No
Pond
12.5
58
1.3
0
30
Induced draft
Yes

40
Yes
In-line steam
50% emergency
Spray tower
Yes
No
Thickener, filter, landfill
10
90
1.3
2.5
95
Induced draft
Yes
80

Yes
In-line steam
50% emergency
Spray tower •
Yes
Yes (1000 to 2000 ppm)
Thickener, filter, landfill
10
80
1.2
2.5
95
Induced draft
Yes
80

Yes
In-line steam
50% emergency
      Source:   McGlamery et al.  1980.

-------
 TABLE D-2.   CAPITAL  INVESTMENTS FOR LIMESTONE SYSTEMS
                        ON  500-MW PLANTS3
                   ($103 except as  indicated)

Direct investment
Hater ial handling
Feed preparation
Gas handling
S02 absorption
Reheat
Solids disposal
Total
Services, utilities, and miscellaneous
Total
Landfill or pond construction
Landfill equipment
Total
Indirect investment
Engineering design and supervision
Architect and engineering contractor
Construction expense
Contractor, fees
Contingency
Total fixed investment
Other capital investment
Allowance for startup and modifications
Interest during construction
Land
Working capital
Total capital investment
Total capital investment, $/kl
Conventional
system

3,498
3,485
9,600
19,830
2,851
2.063
41,327
2.480
43.807
13.960
57,767
3,346
1,016
8,126
2,888
7.315
80,458
5,012
12.551
1.905
3.104
103,030
' 206
Improved
system

3,497
3,484
11,129
22,988
3,304
2.868
47,270
2.836
50,106
2,076
500
52,682
3,663
1,028
8,378
2,608
7.158
75.517
5,732
11.781
641
3.161
96,832
194
Advanced
system

3,503
3,490
10,821
22.351
3.213
2.850
46,228
2.774
49,002
1,983
495
51,480
3,579
1,005
8,187
2,549
6.990
73,790
5,606
11,511
611
3.090
94,608
189
Source:   McGlamery et al.  1980.
Basis:   Plant is in upper  midwest.  Project begins mid-1980 and ends mid-
1983.   Average cost basis  is mid-1982.   Spare pumps, one spare scrubbing
train,  and one spare ball  mill are included.  Disposal pond and landfill
are located 1 mile from plant.  Investment includes FGD feed plenum, but
excludes stack plenum and  stack.

The conventional system is a mobile-bed scrubber with onsite ponding of
calcium sulfite sludge.
The improved system is a spray tower with forced oxidation and landfill ing
of gypsum.

The advanced system is identical to the improved system, but with adipic
acid addition.
                                D-20

-------
  TABLE  D-3.   ANNUAL  REVENUE REQUIREMENTS FOR  LIMESTONE  SYSTEMS'
                                ($103 except  as  indicated)

Pint-year direct costs
Raw materials
Limestone
Adlplc *c1d
Total raw MteHals cost
Conversion costs
Operating labor and supervision
FGD
Solids disposal
Utilities
Process water
Electricity
Steam
Fuel
Maintenance
Labor and material
Analyses
Total conversion costs
Total direct costs
First-year indirect costs
Overheads
Conventional
system

1.126
_____
1.128

460


35
1,732
1.273


3,923
104
7.527
8,655


Plant and administrative (BOX of conversion costs l«»t utilities) 2.692
Total f1r«t-y«ar operating and Maintenance (0ra capital ctiarg»» (14.7k of total capital InveitMrtt)
Total f1r»t~y»ar annual r*v*m»t requirement*
Levelized first-year 0AM costs (1.886 x first-year 04* costs)
Level ized capital charges (14. 7* of total capital Investment)
Levelized annual revenue requirements
Total first-year annual revenue requirements, mills/kWh
Levelized annual revenue requirements. eills/kWh
11.3*7
15.145
26.492
21.401
15.145
36.545
9.63
13.29
Improved
system

1.128
_______
1.128

658
529

26
2,018
1.365
199

4,025
104
8,924
10,052


3.057
13.109 '
14.234
27,343
24,724
14.234
38.958
9.94
14.17
Advanced
iystem"

1.041
216
1.257

658
517

26
1,874
1,367
189

3,937
104
8,672
9,929


2.998
12.927
13.907
26 ,834
24,381
13.907
38,288
9.76
13.92
Source:  HcGlamery et al. 1980.*
Basis:  Upper nidwest plant location, 1984 revenue requirements
       New plant life - 30 yr
       Power unit time on stream - 5,500 h/yr
       Coal burned - 1,116,500 tons/yr
       Boiler heat rate - 9,500 Btu/kWn
       Total capital investment:
           Conventional - $103.030.000
           Improved    - $ 96,832,000
           Advanced    - $ 94,608,000
The conventional system Is a mobile-bed scrubber with onsite ponding of calcium sulfate sludge.
The Improved system Is a spray tower with forced oxidation and landfilllng of gypsum.
The advanced system 1s Identical to the Improved system, but with adipic acid addition.
                                      D-21

-------
INNOVATIONS:  Economic Evaluation	.	D-22
than those for the conventional system, the conventional system has disposal
site  (pond)  construction costs  much  greater than the  disposal  site (land-
fill)  construction  costs  of  the  other  systems.   Thus,  overall  capital
investments  for the  improved  and advanced  processes  are  smaller.   Annual
revenue  requirements for the  improved and  advanced systems,  however,  are
slightly greater  than those  for the conventional system,  primarily because
of the  lower costs  of  labor,  supervision, and electricity for the conven-
tional system.

-------
INNOVATIONS:  References                                                D-23
                          REFERENCES FOR APPENDIX D


Borgwardt, R. H.   1977.   Effect of Forced Oxidation on Limestone/SO  Scrub-
ber Performance.   In:   Proceedings:   Symposium on Flue Gas Desulfurfzation,
Hollywood, Florida, November  8-11,  1977.   Vol. I.  EPA-600/7-78-058a.   NTIS
No. PB-282 090.

Borgwardt,  R.  H.,  et  al.   1979.   Limestone  Type-and-Grind Tests  at  EPA/
IERL-RTP.   Presented  at  the  5th Shawnee  Industry  Briefing  Conference,
Raleigh, North Carolina, December 5, 1979.

Burbank,  D.  A.,  et al.   1980.   Test Results  on  Adipic  Acid-Enhanced  Lime-
stone Scrubbing  at the EPA Shawnee Test  Facility—Third Report.   Presented
at the U.S. EPA Sixth Symposium on Flue Gas Desulfurization, Houston, Texas,
October 28-31, 1980.

Burbank, D. A.,  and S.  C. Wang.  1980.   EPA Alkali Scrubbing Test Facility:
Advanced Program -  Final  Report (October 1974 -  June  1978).   EPA-600/7-80-
115.   NTIS No. PB 80-204 241.

Crowe,  J.  L., G.  A.  Hollinden, and  T.  Morasky.   1978.  Status  Report of
Shawnee Cocurrent  and  Dowa Scrubber Projects and Widows Creek Forced Oxida-
tion.   In:  Proceedings  of the Industry Briefing  on EPA Lime/Limestone Wet
Scrubbing  Test  Programs, August  29,  1978.   EPA-600/7-79-092.   NTIS  No.
PB-296 517.

Dauerman,  L., and  K.  Rao.  1979.  Double Alkali Process for Flue Gas Desul-
furization Optimizing  for the Regeneration of Sodium Sulfite; Part I:   Lime
as Regenerant, and Part II:   Limestone as Regenerant.   Presented at the 72d
Annual Meeting  of  the Air Pollution  Control Association,  Cincinnati,  Ohio,
June 24-29, 1979.

Golden, D. M.   1980.   EPRI FGD Sludge Disposal Demonstration and Site Moni-
toring  Projects.    Presented  at the  U.S.  EPA Sixth  Symposium on  Flue  Gas
Desulfurization, Houston, Texas, October 28-31, 1980.

Head,  H.  N.   1977.   EPA Alkali Scrubbing Test Facility:   Advanced Program,
Third Progress Report.  EPA-600/7-77-105.

Head,  H.  N.,  et al.  1979.  Recent Results From EPA's Lime/Limestone Scrub-
bing  Programs—Adipic  Acid  as  a Scrubber  Additive.    In:   Proceedings:
Symposium  on  Flue  Gas Desulfurization,  Las Vegas, Nevada, March 1979.   Vol.
1.  EPA-600/7-79-167a.  NTIS No. PB80-133168.

-------
INNOVATIONS:  References	      D-24

Head, H.  N.,  S.  C.  Wang, and  R.  T.  Keen.   1977.  Results of Lime and Lime-
stone Testing With  Forced Oxidation at the EPA Alkali Scrubbing Test Facil-
ity.  In:   Proceedings:   Symposium on  Flue  Gas Desulfurization, Hollywood,
Florida,  November 8-11,  1977.   Vol.  I.  EPA-600/7-78-058a.  NTIS No. PB-282
090.

Hicks,  N.  D., 0. W.  Hargrove,  and  J. D. Colley.   1980.   FGD Experiences,
Southwest  Unit  1.   Presented  at the  U.S.  EPA Sixth  Symposium on Flue Gas
Desulfurization, Houston, Texas, October 28-31, 1980.

Jackson,  S.  B.   1980.   Cocurrent Scrubber  Tests:   Shawnee  Test Facility.
Presented  at  the  U.S.   EPA  Sixth Symposium on  Flue Gas Desulfurization,
Houston, Texas, October 28-31, 1980.

Jackson,  S.  B.,  C.  E.  Dene,  and  D.  B. Smith.   1980.   Dowa Process Tests,
Shawnee Test  Facility.   Presented at  the U.S. EPA  Sixth  Symposium on Flue
Gas Desulfurization, Houston, Texas, October 28-31, 1980.

Josephs,  D.  X.   1980.   Magnesium Enrichment  Improves  Flue  Gas Scrubbing.
Power Engineering, 84(9):71-72.

LaMantia,  C.  R.,  et al.   1977.  Dual Alkali Test and Evaluation Program.  3
vols.   EPA-600/7-77-050a-c.   NTIS No.  PB-269  904,  PB-272 770, PB-272 109.

Laseke,   B. A.,  Jr.,  et  al.   1979.    Electric Utility  Steam  Generating
Units—Flue Gas  Desulfurization Capabilities as  of  October 1978.   EPA-450/
3-79-001.   NTIS No.  PB-298 509.

Martin,  J.  R.,  K.  W.  Malki,  and  N.  Graves.   1979.   The  Results  of a Two-
Stage Scrubber/Charged   Particulate  Separator  Pilot Program.   In:   Second
Symposium  on  the  Transfer and Utilization of  Particulate  Technology.   Vol.
I.  Control of Emissions from Coal Fired Boilers.  EPA-600/9-80-039a.

Massey,  C.  L.,  et  al.   1980.  Forced Oxidation of Limestone Scrubber Sludge
at TVA's  Widows  Creek  Unit 8 Steam Plant.   Presented at the U.S. EPA Sixth
Symposium  on  Flue Gas  Desulfurization, Houston, Texas, October 28-31, 1980.

McGlamery, G.  G.,  et al.   1980.   FGD Economics  in  1980.   Presented at the
U.S.  EPA  Sixth  Symposium  on  Flue  Gas  Desulfurization,  Houston,  Texas,
October 28-31, 1980.

Morasky, T. M., et al.   1980.  Evaluation of Gypsum Waste Disposal by Stack-
ing.  Prepared  for, but  not  presented at, the U.S.  EPA Sixth Symposium on
Flue Gas Desulfurization, Houston, Texas, October 28-31, 1980.

Oberholtzer,  J.  E.,  et  al.   1977.    Laboratory  Study  of  Limestone Regen-
eration  in Dual  Alkali  Systems.  EPA-600/7-77-074.   NTIS No.  PB-272 111.

Radian  Corporation.   1980.    Evaluation  of  Chiyoda  Thoroughbred  121  FGD
Process and Gypsum  Stacking.   Volume 1:  Chiyoda Evaluation.   EPRI CS-1579.

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                                 APPENDIX E
                        MATERIAL AND ENERGY BALANCES

     This  appendix  exemplifies  the  procedures  for performing  a  material
balance  and  estimating the  energy  requirements for a limestone FGD  system.
An understanding of  the  material  and energy balances will  enable the Project
Manager  to verify  limestone usage,  makeup water  requirements,  and  energy
demands of the system.
     The process selected  for  this  illustration and the process assumptions
are described  in the following  section.   The process chemistry is  simplified
for the  purpose of illustration.   A set of  conditions  is  specified,  and the
material  balance  is developed  in a  stepwise  fashion.   Sample calculations
for the  material  balance  are  followed by  an  estimation of energy  consump-
tion.    Two sets of  calculations are  performed:   the  first  for  flue  gases
resulting  from combustion  of  high-sulfur  eastern  coal  (3.7% S),  and  the
second, for low-sulfur western  coal  (0.7% S).
     For  both  coals, the  S02  removal for  the flue gas actually treated in
the scrubbers  is 90  percent.  For the low-sulfur  coal,  however, the overall
system  requirement  is  70 percent  S02  removal.   Therefore,  22  percent by
volume of  the  flue gas can be  allowed to bypass the FGD system untreated and
be used  for  reheat.   This untreated gas provides  an energy savings  of about
50 percent for a  500-MW plant, as  compared to the high-sulfur  coal  case.

PROCESS DESCRIPTION
     A  flow diagram  for  the  limestone  scrubbing   FGD  process  is  shown in
Figure E-l, along  with stream  characteristics.   The process depicted and the
values  shown  in this  figure are those  of  the  high-sulfur coal  model,  for
which the  material  balance calculations  are performed later in this  section.
In this  example, 210 ton/h of  coal  is fired to generate approximately 500 MW
                                     E-l

-------
                                                     •j  REHEATER }-
AIR

BOILER FURNACE
SYSTEM
s
FRESH
MAKEUP HATER
FLUE
GAS
	 ^ . fSP
WITH


®


iiro
1 PARTICIPATES f
FLY ASH
MAKEUP
BOnOM ASH HATER
1©
LIMESTONE
•
„ . SLURRY
PREPARATION





SCRUBBER
T-^
If
EFFLUENT
HOLD TANK





-------
MATERIAL BALANCE:  Process Description	E-3
(gross)  of  electricity.   Although  an  FGD system  of this size  usually  con-
sists  of several modules,  it is assumed  in these  stream calculations  that
all the modules are combined.
     The  major components  of coal  are  carbon,  oxygen, nitrogen,  hydrogen,
sulfur,  free  moisture,  and  ash.   Chloride,  a minor component,  is  important
because  of  its  corrosion  effect and its impact  on  process chemistry.   Chlo-
ride  in  the   coal  forms  hydrochloric  acid  during combustion.   At  steady
state,  this  acid  is assumed to  be   completely  absorbed by  the  scrubbing
solution  and  removed with  interstitial  water  in the waste sludge.  In  some
coals, alkalinity in  the  ash reacts with sulfur dioxide  or  sulfur  trioxide;
this  effect is neglected  here.   A typical  analysis of high-sulfur coal  is
presented  in  Table  E-l.    The  heating  value  (HV)  of this  coal  is  11,150
Btu/lb.
     The  fly  ash in  the  flue gas  (Stream 1)  is removed in a cold-side  ESP
ahead  of the  scrubber  .   The maximum particulate  emission  rate must be  in
compliance  with  the  NSPS  promulgated  by EPA  in September 1979, which  is  0.03
ID/million  Btu  heat  input.   After passing  through the  ESP the  flue  gas
(Stream  2)  enters the scrubber at 290°F, and the S02 is removed  by  limestone
scrubbing.  The current  NSPS requirement  for  S02  removal   is   90 percent,
which  translates to  0.66  Ib S02/million Btu heat  input at the outlet  of the
scrubber.
     The temperature  of the saturated  flue gas  from the  scrubber (Stream 3)
is increased  by  40°F  in a  reheater.   It is  assumed that there  is  no  carry-
over of  mist  droplets in  the reheated  gas.   (For  a well-designed mist elim-
inator,  the  accepted carryover  rate   is 0.1 gr/scf.)   The  cleaned and  re-
heated flue gas  (Stream 4) is discharged to the  atmosphere  through a  stack.
In calculation of gas flow rates the ideal  gas law is assumed.
     Typical pressure drop data are shown in  Table E-2.
     A 60  percent solids  limestone slurry  is  prepared in  a ball  mill  from
water  and  limestone.   A   typical  limestone  analysis  is  presented in  Table
E-3.    In this example,  it  is assumed  that the MgC03 available  for reaction
with  S02 is  1.5 percent  and that the unavailable portion  is   confined  to
inerts.  The  actual  percentage of  MgC03 in  the limestone supplied could  be
higher, but only a portion is available for reaction.

-------
    TABLE  E-l.  DESIGN PREMISES:   HIGH-SULFUR COAL  CASE
Plant capacity (gross)
Boiler:
Coal:











Limestone:

Scrubber:


Sulfite-to-sulfate
oxidation:
Solids: •



S02 inlet loading:
Max. emission:
(NSPS)
Reheat:
Flue gas treated:
S02 removal:
Flue gas components
(inlet to scrubber)
Particulates
Carbon dioxide
Hydrogen chloride
Nitrogen
Oxygen
Sulfur dioxide
Moisture
Total

Type
Type
Source
Consumption
Heating value
Sulfur content
Oxygen content
Hydrogen content
Nitrogen content
Carbon content
Chloride content
Moisture content
Ash content
Stoichiometric ratio (SR)a
Utilization (^)
Liquid-to-gas ratio

Inlet gas temperature


Limestone slurry feed tank
Effluent hold tank
Thickener underflow
Dewatered sludge

S02
Particulates
Indirect In-line
No bypass for reheat
Treated gas
Flow rate,
Ib/h
138
942,500
432
3,745,500
324,500
31,080
293.500
5,337,650
500 MW
Pulverlzed-coal-flred
Bituminous
Pennsylvania
210 tons/h
11,150 Btu/lb
3.7%
7.3X
4.3%
1.2%
61.2%
0.1X
8.5%
13.7%
1.10
90.9%
75 ga 1/1000 acf
(saturated)
290°F
20%

- 35%
14%
30%
60%
6.6 lb/106 Btu input
0.66 lb/106 Btu input
0.03 lb/106 Btu input
40°F
100%
90%
Composition,
wt. X «ol %

17.66 11.76
0.01 <0.01
70.17 73.45
6.08 5.57
0.58 0.27
5.50 8.94
100.00 100.00
Defined as moles of CaC03 and MgC03 (if available) fed per nol  of S02
and HC1 (if present) absorbed.
                               E-4

-------
MATERIAL BALANCE:  Process Description	       	   E-5
                   TABLE E-2.   TYPICAL PRESSURE DROP DATA
          SO2 scrubber
            (mobile bed)
            (spray type)
          Mist eliminator
          Reheater
          Duct work
                                        Pressure drop,  in.  H20
    6-8
    2-3
  0.3-1'
     1
     2
a
     a Depends on generic type and whether it includes a wash tray.   Pressure
       drop through the Shawnee mist eliminator is typically 0.3 in.  H20.
                       TABLE E-3.   LIMESTONE ANALYSIS
                             (percent by weight)
                         CaCO
                             3
                         MgC03
                         Inerts
94.0
 1.5
 4.5
     This  60  percent solids slurry  (specific  gravity = 1.50) is  diluted  to
20 percent solids (specific gravity = 1.12)  with makeup water in  a  slurry
feed tank  (not shown),  then  pumped to  the effluent hold tank  (EHT),  which
maintains a 14 percent solids concentration (specific gravity  1.09).
     Slurry from  the  EHT is recirculated through the scrubber for  removal  of
S02, and a portion  of the slurry (to  be  determined by the material  balance)
is bled  off to the  thickener.   The thickener underflow,  containing 30 per-
cent solids  (Stream 13), is dewatered  to 60 percent solids  in  a  dewatering
device such  as vacuum  filter  or centrifuge.   The dewatered  sludge (Stream
15) is sent  to the  disposal area,  and  the filtrate is returned to  the  thick-
ener.   A portion of thickener overflow is sent to the mist eliminator.
     The liquid-to-gas  (L/G)  ratio  in the scrubber normally  ranges from  40
to 100 gallons of slurry per 1000 acf  of gas, depending on  the sulfur con-
tent of the  coal, type of scrubber,  S02 removal  efficiency,  and  water  avail-
ability.    In these  examples,  the L/G ratios are  55 gal/1000 acf (saturated)

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MATERIAL BALANCE:  Process Description      _ E-6
for the  low-sulfur coal and 75  gal/1000  acf (saturated) for the high-sulfur
coal.  The  spent slurry from the  scrubber  downcomer Is collected in the EHT
along with spent mist eliminator wash water.
     The chemistry of  limestone scrubbing is discussed in detail in Appendix
A.   For  the  sake  of simplicity, only overall  reactions  are presented here.
     In  the  scrubber,   the S02  is  absorbed by the  reaction  with CaC03.   The
overall reaction is:

          CaC03 +  S02 + H20 -> CaS03-*sH20 +.  JsH20 +  C02                   (1)

Available MgC03  also reacts with  S02  in a similar  fashion  and gives magne-
sium sulfite (hexahydrate or tri hydrate).
     Some  of  the  calcium  sulfite formed  in  reaction (1)  is  oxidized  to
sulfate  with the  oxygen  in  the flue  gas.   The degree of  oxidation in this
example is 20 percent.   The reaction is expressed by:
          CaS03-*sH20 + %02 + ]>5H20 -» CaS04-2H20*                         (2)

     All  the  hydrogen chloride  (HC1)  is absorbed  and  subsequently neutral-
ized by  the  alkaline species to give  magnesium  chloride.   The solids in the
waste  stream 15  are mainly CaS03'*sH20,  unreacted CaC03,  inerts,  coprecip-
itates (CaS04-CaS03-J5H20), and CaS04-2H20.
     The  fresh  makeup  water, which includes pump seal water, mist eliminator
wash water,  and slurry feed preparation water,  is  supplied through Stream 6
and Stream 8.
     The  following  summarizes   the  assumptions  on  which calculations  are
based:
     1.   Flue  gas  flow  and composition  remain  constant for  a particular
          case.
     2.   The composition  of the limestone supplied is 94 percent CaC03, 1.5
          percent available MgC03, and 4.5 percent inerts.
* At  20% oxidation,  16 mol %  of calcium  sulfate  will  be in the "form of a
  solid solution CaS04-CaS03*%H20.

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MATERIAL BALANCE:   High-sulfur Coal	;	E-7
     3.   Conversion of sulfur  In  the coal to S02  in  flue gas  is  assumed to
          be 100 percent;  however,  95 percent is a typical  value.
     4.   Any contribution due to S03 in the flue gas is neglected.
     5.   Actual alkalinity supplied  in  the form of limestone for  absorption
          of S02 and HC1 is 1.10 times the stoichiometric amount.
     6.   Any HC1  generated by  chlorine  in the coal is completely  captured
          in the FGD system.
     7.   Degrees of sulfite-to-sulfate  oxidation  are  20 and 50 percent  for
          the  high-sulfur  coal and  low-sulfur  coal   cases,  respectively.
     8.   About  16  mol percent of  total  S02  removed  to the  sludge  forms
          coprecipitates of calcium  sulfate with calcium sulfite hemihydrate
          when rate of  oxidation (sulfite-to-sulfate)  is 16 percent  or more.
     9.   Although a large  FGD  system usually consists  of  several scrubbing
          modules, it  is  assumed in  stream calculations that all modules  are
          combined.
    10.   The fly  ash  in  the  fuel gas  is removed primarily by a cold-side
          ESP and  has  no  effect on   S02  absorption.   There  is no secondary
          particulate removal  in the  scrubbing module(s).
    11.   The L/G  ratio applied in  the scrubbing module(s)  are 55  gal/1000
          acf (saturated)  for the  low-sulfur coal  and  75 gal/1000  acf (satu-
          rated) for the high-sulfur  coal.
    12.   The pressure drop  across   the  entire  FGD  system  is  11  in.  H20.
    13.   There  is  no  mist carryover  in  the  gas  leaving the FGD  system.

MATERIAL BALANCE CALCULATIONS:   HIGH-SULFUR COAL CASE
     The basis  for material balance  calculations  is  depicted  schematically
in  Figure  E~2.   Inputs to the FGD   system include flue  gas from  the  ESP,
limestone slurry,  and  makeup water,  as  shown  in Figure  E-2a.  The outputs
are cleaned  flue  gas  and  dewatered sludge.   Figure  E-2b depicts the S02  and
fly  ash particulate   balance  with  the   ESP  and  the  FGD  system.   The  ESP
reduces  the  fly ash particulates* from  the  flue gas  to  below the  maximum
* Based on 46,000 Ib/h particulates entering the ESP, the particulate removal
  efficiency is 99.7%.

-------
      FLUE GAS
      FROM ESP"
     LIMESTONE.
      SLURRY
                 CLEANED
                 FLUE GAS
                   FGD
                  SYSTEM
                 DEWATERED
                  SLUDGE

               THE FGD SYSTEM.
MAKEUP
WATER
                                                               CLEANED FLUE GAS
                                                              (S02 AND PARTICULATE)
                     __—I—
         DIRTY FLUE GAS
         (SO? AND FLYASH
          PARTICULATE)





ES



p






1
FGD
SYSTEM
1
                          FLY
                          ASH
FLUE GAS CLEANING WASTES
(SLUDGE AND DISSOLVED SOLIDS)
          b.  S02 AND FLYASH PARTICULATE:  ESP AND FGD SYSTEM.
oo
                   COAL-
                    AIR-

1
1
T-»
1
4— •»
1
1
1
t_ 	


BOILER
FURNACE
SYSTEM
~f-
BOTTOM
ASH




	


ESP

-4-
FLY
ASH




	
CLEANED
FLUE GAS
	 •„..
1
•
FGD
SYSTEM
-4-
SLUDGE

1
*T-
••--
__j


-LIMESTONE
-WATER

                      c.  OVERALL INPUTS AND OUTPUTS:  BOILER/FURNACE,
                                  ESP, AND FGD SYSTEMS.
               Figure E-2.  Schematics of basis for material  balance calculations.

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MATERIAL BALANCE:   High-sulfur Coal _ E-9
allowable  participate  emission (0.03  Ib/million Btu  heat  input).   The  FGO
system  removes  the required  amount  of S02.   Figure E-2c shows the  overall
inputs  and outputs to the boiler- furnace  system,  the ESP,  and the FGD  sys-
tem.
     Beginning with the  known amount and composition of  the  flue  gas enter-
ing the FGD system and  the  emission  regulation,  material  balance calcula-
tions are performed in five steps, as follows:
     0    S02 removal  requirement
     0    Limestone requirement/slurry preparation
     0    Humidifi cation of flue gas
     0    Recirculation loop and sludge production
     0    Makeup water requirement
     Once  the makeup water requirement is known, the  overall  water utiliza-
tion is  established on the basis  of the mist eliminator (ME)  wash  procedure.
The important interplay  of ME wash requirements and balance  of water in the
limestone scrubbing system is discussed later.
S02 Removal Requirement (Step 1)
     Under current NSPS  regulation,  the FGD system must remove 90  percent of
the inlet  S02 for this  high-sulfur coal  case.   The  allowable  S02  emission
from the plant is:
     31,080 Ib/h S02 x (10%) S02 emission allowed = 3,108 Ib/h S02
     S02 removed by scrubber = S02 input - maximum allowable S02 emission
                             = 31,080 - 3,108
                             = 27,972 Ib/h S02
                               1 Ib-mol S0g
                    0
                               64.06 Ib S02
                             = 436.65 Ib-mol/h S02
Limestone Requirement/Slurry Preparation (Step 2)
     The theoretical  limestone  requirement depends on the amounts of S02 and
HC1 to  be  removed from the FGD  system.   All  of the HC1 from the flue gas is

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MATERIAL BALANCE:  High-sulfur Coal
                                                        E-10
removed  and  leaves the  system  with the Interstitial water of  the dewatered
sludge:
Ib-mol HC1  removed/h = 432    x
                           h
                                     = 11.84
                                                      h
                                                     ID
Since 1  mol S02  requires  1 mol alkalinity  (as  CaC03 and MgC03), and  1 mol
HC1  requires h  mol alkalinity,  the  theoretical  alkalinity requirement  is
436.65 + h(ll. 84) = 442.57 Ib-mol/h.   The  actual alkalinity supplied  is 110
percent of theoretical  value.  Thus:
          Actual alkalinity = 442.57

                            = 486.83
                                     Ib-mol
It  is  supplied  from  limestone  containing 94  percent  CaC03,  1.5  percent
MgC03, and 4.5 percent inerts.
     The composition  of  the  available  limestone for S02 absorption  is  sum-
marized in Table E-4.
               TABLE E-4.   COMPOSITION OF AVAILABLE LIMESTONE
                             FOR S02 ABSORPTION
                     (Based on 100 Ib limestone supplied)


CaC03
MgC03
Total
Molecular
weight
100.09
84.33

Weight,
Ib
94.0
1.5
95.5

Ib-mol
0.9392
0.0178
0.9570

Mol percent
98.15
1.85
100.00

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MATERIAL BALANCE: _ : _ E-ll
The  amount  of CaC03 in the  limestone  is  the amount that satisfies the  fol-
lowing equation:
                    Actual alkalinity  x „,
                                      98.15 Ib-mol
                QI        v
               . O J   .     X
                     h      100 Ib-mol available alkalinity
          =  477.82  Ib-mol/h  CaC03  and   x 1?^0911b,C,?nCOa  to  convert  to
pounds                                        lb"mo1  CaC°3
          = 47,825 Ib/h CaC03

Similarly, the amount  of  MgC03  available for S02 absorption in  the limestone
supplied is:
      Avanable MgC03 = Actual Salinity x
                                            100
                               Ib-mol      1.85 Ib-mol MgC03
                                       x     lb.mol
                      - Q m 1b"mo1 Mnrn  v 84.33 1b MgCOa
                      - 9.01   E    MgC03 x  lb.mol Mgg0aa

                      = 760    MgC03
Thus:
     Total alkalinity = CaC03 + MgC03
                      =47,825 Ib/h + 760 Ib/h
                      = 48,585 Ib/h
which is 95.5 percent of total limestone supplied.   Therefore:
     Total limestone  _ x.+.-i a-n,3i ,•„,-*    total limestone supplied
        supplied      " total alkalinlty x   available alkalinity
                      • 48,585 ,b
                      = 50,875 Ib/h limestone, or 25.45 tons/h
which contains 2290 Ib/h inerts.

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MATERIAL BALANCE:  High-sulfur Coal	    E-12
Limestone requirement summary (Stream 5, Ib/h):
                         CaC03         47,825
                         MgC03            760
                         Inerts         2.290
                              Total    50,875
The amount  of  excess  makeup water used to prepare limestone solids slurry Is
established  as  the difference  between that required by the mist  eliminator
wash procedure and the total makeup water required by the system.
          Water remaining for slurry preparation = 197 gpm
                     or 197 gpm x 500 &&
                      = 98,500 Ib/h of water for limestone slurry
          And the flow rate of slurry (Stream 7) = 149,400 Ib/h
Humidification of Flue Gas (Step 3)
     Use of a psychrometric  chart permits rapid estimation of the humidity
and  temperature of  the  wet flue  gas  leaving  a  scrubbing  system.   These
values  are  then used to  determine  the amount of water  required to saturate
the flue gas.
     When unsaturated,  hot  flue  gas  is introduced into a  scrubbing system,
water  evaporates  into the  flue gas  under  adiabatic conditions at constant
pressure  and  cools   the  gas.   The  wet-bulb  temperature  remains  constant
throughout the period of vaporization.
     If  evaporation   continues  until   the  flue gas  is  saturated with  water
vapor,  the   final  temperature of  the gas will  be the  same  as its  initial
wet-bulb temperature  (dew point).   For air-water  vapor mixtures,  the  wet-
bulb temperature and adiabatic cooling lines are practically the same.
     The humidity  of  the  inlet flue gas is 0.0582 Ib water  per Ib  dry gas at
290°F.   As  vaporization takes  place, the humidity  of  the gas is  increased
and  the dry-bulb  temperature  must correspondingly  decrease  along  the  wet-
bulb temperature line (adiabatic  cooling line).  The psychrometric chart in
Figure  E-3  is  prepared  for an "air-water" system.

-------
A - INLET GAS TO SCRUBBER
B - OUTLET GAS FROM SCRUBBER
C - GAS AFTER REHEATING
           ADIABATIC
         COOLING LINE
                                                               t.
                                                               
-------
MATERIAL BALANCE:  High-sulfur Coal _ E-14
     The humidity of flue gas is defined as follows:
          humiditv  W  -  ,  (18.02 1b)/(lb-mol  H,0)
                 y> wx    (molecular wt of dry gas)/(lb-mol  dry gas)

     The mols of  S02  removed from the flue gas  are  replaced with  equal mo Is
Of  C02  per  equation  (1)  on page E-7.   Based  on the  design  premises,  the
molecular weights of  dry flue gas at the  inlet  and  outlet  of  the  FGD  system
are calculated as 30.51 and 30.38 respectively.
     For use  with the  psychrometric  chart,  the  humidity  of inlet flue  gas
must be corrected for air:
 hnmiHitv/ nf air - n nw?   lb H20      molecular wt.  of dry gas (inlet)
 humidity of air - 0.0582 lb dry?'gas  x - molecular wt. of dry air

                      = 0.0582
                      = 0.0613 lb H20/lb dry air

     In Figure  E-3, point  A corresponds  to humidity  0.0613  lb  H20/lb dry  air
at  290°F.   Point  B is  the  intersection  of  the adiabatic  cooling  line with
the 100 percent saturation  line,  which is 0.104 lb H20/lb dry  air  at  128°F.
The corrected value for saturation humidity of flue gas  is
                      =n 104   lb H20        molecular  wt.  of  dry  air
                      ~       lb dry air    molecular wt. of dry gas (outlet)
                      = 0.0992 lb H20/lb dry gas @ 128°F

Note that  the molecular weight  of outlet  dry  flue  gas is lower because  90
percent of the S02 removed is replaced by C02.
     The amount  of water vapor in the  outlet  gas is 0.0992 lb water/lb  dry
gas x 5,034,980 Ib/h dry gas = 499,500 Ib/h water or  999 gpm.
     The total mass flow rate of the  gas at the outlet is
                  5,034,980 Ib/h dry  gas + 499,500 Ib/h water
                = 5,534,400 Ib/h gas

-------
MATERIAL BALANCE:  High-sulfur Coal
E-15
     The amount of water required for humidification is
                   Saturation - Inlet
                 499,500 Ib/h - 293,500 Ib/h
               = 206,000 Ib/h water or 412 gpm
     On a molar  flow rate basis, the outlet  gas  contains 165,760 Ib-mols of
dry gas per hour and 27,710 Ib-mols H20 per hour.
     The volumetric  flow  rate  of outlet gas from  the scrubber at 128°F and 2
in. H20 is

                                                               359 ft3
                                                               Ib-mol
7,710]
407
>lb-mol
h x
in. HzO*
(460 H
(460 H
i- 128)°F
i- 32)&F
h
x 60 min
                       (407 + 2) in.  H20
                    = 1,376,700 acfm at 128°F
          * Atmospheric pressure.

     The composition  of the  cleaned  flue gas  leaving the scrubber  is  sum-.
marized in Table E-5.
           TABLE E-5.   COMPOSITION OF CLEANED FLUE GAS (STREAM 3)

Parti cul ate
C02
N2
02
S02
H20
Total
Mass flow rate,
Ib/h
138
961,654
3,746,000
324,000
3,108
499,500
5,534,400
Composition,
wt. %
0.0
17,38
67.68
5.85
0.06
9.03
100.00

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MATERIAL BALANCE:  High-sulfur Coal _ E-16
     Flow rate of gas leaving the reheater (Stream 4) at 168°F
                    - 1 376 700 acfm x (46° * 168)°F
                    - 1,376,700 acfm x (460 + i28)°F
                    = 1,470,350 acfm
     When  the gas  is  heated,  the  humidity of  the  gas  remains  constant.
Therefore,  the gas  property moves  along the  dotted  line from  B to C  in
Figure E-3.   Point  C  represents the gas  leaving  the reheater.   The relative
humidity (HR) at this point, as read from the figure, is 26 percent.
               Before reheat                 After reheat
               1,376,700 acfm                1,470,350 acfm
                  at 128°F                      at 168°F
                 HR = 100%                      HR = 26%
Recirculation Loop and Sludge Production (Step 4)
     The overall  effect of  the scrubbing system is that 90 percent  of  the
incoming S02  is  removed  from the flue  gas  and  transferred to  the effluent
sludge.  The  mols  of  S02 removed from the flue gas equal  the  mols of sulfur
in the sludge.   The hydrogen chloride is assumed to be removed by MgC03   as
MgCl2.    The   remainder    of   the    MgC03    reacts   with   S02   to   form
MgS03'3H20.    Some  of  the  MgS03 may oxidize  to  MgS04,  but  for the  sake  of
simplicity the formation of MgS04 is  neglected here.

          Excess  CaC03 supplied = supply - use by system
                                = [477.82 - (442.57 - 9.01)] 1
                                = 44.26 Ib-mol/h CaC03 or 4,430 Ib/h
This excess CaC03 leaves the system with the waste sludge.
In determination of  sludge  composition, the sulfite  formation  is  determined
as follows:
          CaS03 formed by the precipitation reaction = 433.56 Ib-mol/h CaS03
          Likewise, the MgS03 formed =3.09 Ib-mol/h MgS03.

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MATERIAL BALANCE:  High-sulfur Coal
                                   E-17
At 20 percent oxidation of sulfite to sulfate:

CaS04 formed by precipitation = 433.56 1b^mo1 x 0.20 = 86.71 Ib-moJ/h CaS04*2H20

     16 mol percent CaS04 is in the form of CaS04-CaS03'JjH20
       (solid solution)

That  is,  CaS04   coprecipitates  with  CaS03'*5H20  (16  mol  percent of  total

sulfur in the sludge).

          CaS04-2H20 crystals formed = 86.71 Ib-mol    20-16
                                               h    x   20
= 17.34 1b"l"°1 = 2986
          n
                                                      2986    CaS04-2H20
                                                           n      *   *
          CaS03-%H20 left in the product = (433.56 - 86.71) Ib-mol/h

               = 346.85 Ib-mol/h CaS03^H20 = 44,744

          MgS03-3H20 crystals formed =3.09 Ib-mol/h MgS03-3H20 = 490 Ib/h
          CaS04-CaS03-%H20 formed = 69.37 IU "   CaS04-CaS03-J5H20
               = 18,403 Ib/h                n

          CaS03-?sH20 crystals formed = (346.85 - 69.37) = 277.48 Ib-mol/h
               = 35,837 Ib/h CaS03-JsH20

     Formation of C02 is as follows per equation (1):

          By the precipitation reaction = 433.56 Ib-mol/h = 19,081 Ib/h
          By MgC03                      = 3.09 Ib-mol/h = 136 Ib/h
          Total C02 formed              = 19,217 Ib/h

     The composition of sludge solids is summarized in Table E-6.
              TABLE E-6.  WASTE SLUDGE SOLIDS FOR HIGH-SULFUR COAL

CaC03
CaS03-%H20
CaS04'CaS03%H20
CaS04-2H20
Inerts
Total
Mass flow rate,
Ib/h
4,430
35,837
18,403
2,986
2,290
63,946
Composition, wt. %
(dry basis)
6.93
56.04
28.78
4.67
3.58
100.00

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MATERIAL BALANCE:  High-sulfur Coal _        E-18
     The amount  of water  In the  60  percent solids  sludge  (Stream 15)  Is:
               63.946 Ib/h solids x «
                    = 42,630 Ib/h or 85 gpm.
     Since the  solids content  in  waste stream  13 from the thickener  is  30
percent, the total slurry flow rate is:
                    63,946 Ib/h M11d. x
                                   = 213,153 Ib/h or 358 gpm
                                     at 1.19 sp.  gravity
and the water content is 149,207 Ib/h by difference or 298 gpm.
     The amount of  filtrate  returned from  dewatering device to  the  thickener
is:
                       Inlet - Outlet
                    (149,207 - 42,630) Ib/h
                         = 106,577 Ib/h water or 213 gpm.
     Effluent slurry  to  thickener  (Stream  11)  from the recycle  loop contains
14 percent solids and the total slurry flow rate is
                    63.946 ,b/h „,«. x
                         = 456,757 Ib/h or 838 gpm
                           at 1.09 sp.  gravity
and water content is 392,811 Ib/h by balance or 786 gpm.
     The  amount  of return of  thickener  overflow (Stream 12) to the  recycle
tank is
                    (392,811 + 106,577 -  149,207) Ib/h
                         * 350,500 Ib/h or 701 gpm.
Figure E-4 depicts  the  material  balance  around the thickener  and  dewatering
device.
     Slurry  flow  to the  scrubber (Stream 10)  is shown in Figure  E-5.   The
L/G ratio for  the scrubber in this process  is  given  as  75 gallons of slurry
(14 percent  solids) per 1000 ft3 of gas.  Since  the  gas flow rate from  the

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                                 OVERFLOW
                              (RETURNED TO EHT)
                         350,500  Ib/h H20 or 701 gpm
                             FILTRATE
                    106,500 Ib/h H20 or 213 gpm
      (14% SOLIDS)
    63,946  Ib/h  SOLIDS
393,000 Ib/h H20 or 786 gpm
                                 THICKENER
    UNDERFLOW
   (30% SOLIDS)
63,946 Ib/h SOLIDS
                        DEWATERING DEVICE
                                             149,207 Ib/h H20 or 298 gpm
                                                                            WASTE SLUDGE
                                                                            (60% SOLIDS)
                                                                         63,946 Ib/h SOLIDS
                                                                      42,500 Ib/h H20 or 85 gpm
            Figure E-4.  Material balance around thickener and solids dewaterlng system.

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  *FRESH MAKEUP WATER
150,000 Ib/h or 300 gpm
       FLUE GAS IN
        31,080 Ib/h S02
       942,500 Ib/h C02
       293,500 Ib/h H20
          or 587 gpm
                                               CLEAN
                                              FLUE GAS
                                          3,108 Ib/h  S02
                                          961,700 Ib/h C02
                                          499,500 Ib/h H2o
                                            or 999 gpm
                               SCRUBBING
                                MODULES
•FROM EFFLUENT HOLD TANK
 7,944,665 Ib/h
 48,802,960  Ib/h  H20
   or 97,606 gpm
                                  TO
                              EFFLUENT HOLD
                                 TANK
                          7,953,277 Tb/h-SOLIDS
                    48,747,000 Ib/h H20 or 97,494 gpm
 *FOR  MIST  ELIMINATOR WASH REQUIREMENT
          Figure  E-5.  Material balance around scrubbers,
                                    E-20

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MATERIAL BALANCE: Hiah-sulfur Coal	E-21
scrubber is  1,376,700  acfm,  the slurry flow rate  at  1.09  sp.  gravity  or 9.16
Ib/gal is:
                75 gal     1.376.700 ft3 gas   60 min    9.16 1b
               1000 ft3 x        min        x   h        gal
               = 56,747,625 Ib/h (104,124 gal/min) of slurry

which  contains  14  percent  solids  (7,944,665  Ib/h)  and 86  percent  water
(48,802,960 Ib/h) or 97,606 gpm of water.
     The amount  of  S02 transferred to the  liquid  phase  from  the  flue gas  is
27,972 Ib/h.   The amount  of C02  transferred from the liquid  phase  to the
flue  gas  is 19,235 Ib/h.  This is based on a  transfer of 1  mol  of  C02 for
every mol  of  S02 absorbed in the scrubbing slurry.   The amount  of water lost
to saturate the flue gas is 205,820 Ib/h or 412  gpm.
Amount of solids in = 7,944,665 Ib/h (Stream 10)
                    + (27,972 - 19,360) Ib/h
                    = 7,953,277 Ib/h
Amount of water in  = 293,500 Ib/h (Stream 2) or 587  gpm
                    + 105,000 Ib/h (Stream 8) or 300  gpm
                    + 48,802,960 Ib/h (Stream 10) or  97,606 gpm
                    £ 49,246,500 Ib/h or 98,493  gpm of water
Amount of water out = (49,246,500 - 499,320)(Stream 3) Ib/h
 (Stream 9)         = 48,747,180 Ib/h or 97,494  gpm
Amount of solids out = 7,953,277 Ib/h.
 (Stream 9)
Total Makeup Water Required (Step 5)
     The amount  of water  leaving with  the  saturated flue gas and with the
waste sludge  is  the amount that must  be  made up.  The makeup water consists
of the  amount of water required  for  humidification plus the  water  leaving
with the sludge, i.e.,  412 + 85 = 497 gpm.

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MATERIAL BALANCE:  Hiah-sulfur Coal _       E-22
Mist Eliminator Wash
     After calculation of the  total  makeup  water  requirement  of 497 gpm, the
most important  item  is  calculation  of the fresh water  requirements for mist
eliminator wash.   On the basis  of  work  at Shawnee, the  EPA has developed
recommended guidelines  (Burbank  and Wang  1980) for  a  mist  eliminator wash
procedure that  satisfies the  needs  of keeping a  closed- loop water balance
and maximizing  limestone utilization while  maintaining  scale-free operation.
     At  limestone  utilization above  85  percent,  the  guidelines recommend
intermittent fresh water wash  on both the top  and bottom  of  the mist elimi-
nator.    The top  should be  washed   sequentially  with  fresh  water  at 0.54
gpm/ft2  for  3  min/h  at  13  psig  using six  nozzles per  50 ft2 of area.  The
bottom  wash  should  be   intermittent  and  full  face  with  1.5  gpm/ft2  for 4
min/h at 45 psig using ten  nozzles per 50  ft2.
     At  limestone   utilizations  below  85  percent,   a  full-face continuous
bottom  wash  should  be used  with  clarified  thickener  overflow at  a  33 to 50
percent  blend with  fresh water.   The wash  rate should  be 0.4 gpm/ft2 at 12
psig using four nozzles  per  50 ft2.
     Scrubber operation  at  limestone  utilization below  70  percent  is not
recommended.
     For this high-sulfur coal  case  it follows  that:
     Volumetric flow             ^ Gas  velocity through  _ Cross-sectional
     rate of scrubber outlet gas  ' eliminator             area of  eliminator

     1-376-700  5TS   + TiT  ' 2295
     then fresh makeup water is on a continuous  hourly basis:
          Top wash required
               2295 ft* x 0.54 ft  x i^S x ^ . 62 gpm
          Bottom wash required
               2295 ft* x 1.50 f£  x i*n x ^ = 230 gpm
          Total fresh water for ME:   292 gpm

-------
MATERIAL BALANCE:  High-sulfur Coal	E-23
     This fresh  water  is  used intermittently, but the  calculation  gives  292
gpm  on  a continuous hourly  basis.  Therefore,  an even 300 gpm  is  adequate
for scale-free operation at greater than 85 percent utilization.
     Since the total system  makeup requirement is  497 gpm,  supplying 300  gpm
for  ME  wash leaves  an excess of  197  gpm.   Normal  practice is to  use this
water to  slurry  the  limestone.   The amount of water remaining will  establish
the  solids  content of the limestone slurry feed in weight  percent.   Slurry
solids  can  range from as  low as 20 weight percent  to as high as 50  weight
percent,  as practiced  at Shawnee.   In  this case:   50,900 Ib/h  limestone
required  is  mixed with 197 gpm  (98,500  Ib/h) water to give 50,900  +  98,500
or 149,400 Ib/h total solution and J^pp = 34 weight percent solids.
     The overall  water balance  that provides  for a good mist eliminator wash
procedure is shown in Figure E-6.

ESTIMATION OF ENERGY CONSUMPTION:  HIGH-SULFUR COAL CASE
     Energy  is consumed by  the  scrubber fans  that  pass the flue  gas through
the  scrubbing system,  by  slurry recirculation pumps and  other  pumps,  and by
the  reheater.   Comparatively small  amounts  of  energy  are  also  consumed by
the  thickener,   dewatering  device,  agitators,  conveyors, bucket  elevators,
and  other components.  The  energy demand of flue gas fans,  slurry recircula-
tion pumps,  and  reheater are calculated  as  a basis for estimates  of total
energy  consumption.  Energy  consumption  in the other areas  of  the system is
assumed  to   be  20 percent  of  that used  by   fans  and recirculation  pumps.
Table E-7  gives  equations for  use in  determining  the  energy requirement of
fans, slurry recirculation pumps, and reheater.
Flue Gas Fans
     The  gas entering  the scrubbing system  undergoes  pressure drops  across
the  scrubber,  mist  eliminator,   reheater,  and  ductwork.   In this  example,
assume  a  pressure drop of  7 in.  H20  across  the scrubber, which  could vary
depending  on  the  scrubber  type  (Table E-2).   For  instance,   spray-type
scrubbers require a high slurry recirculation  rate  and the nozzles  add to
the  pump  head and power  requirement, but gas  pressure drop  is  lower than  for
the  other types.   For spray-type  scrubbers  there is  a  variable trade  off

-------
ro
                           FRESH MAKEUP  (8
                            WATER FOR
          TOTAL
       FRESH MAKEUP
          WATER	
         497 gpm
                          MIST ELIMINATOR
                              300 gpm
                     MAKEUP
                     WATER
                     197igpm

                        1®
    LIMESTONE
    24.45 T/h
   SLURRY
PREPARATION
                                                                             OVERFLOW
                                                                             701 »gpm
                                                                         786
                                                                   BLEED  I9P1"*
                                                  THICKENER
      PEWATERING
298   I  DEVICE
gpm*
        SLUDGE
      TO DISPOSAL
        85 gpm*
     *gpm for water content only.
                                      Figure E-6.   Overall  water  balance.

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            TABLE E-7.   ENERGY REQUIREMENT CALCULATIONS
 C  = Specific heat, Btu/(lb)(°F)
  P = Energy required, Btu/h
  E = Head energy, Btu/h
 Hs = Head, ft.
L/G = Ratio of liquor flow to flue gas rate, gpm/1000 acf at the outlet
  o
  m = Air flow rate at the inlet of reheat section, Ib/h
 AP = Pressure drop through FGD system, in.  H20
  Q = Gas flow rate at the outlet of scrubber, acfm
 AT = Degree of reheat, °F
Flue gas fans (70% fan efficiency assumed)
     P = 0.573 x AP (in. H20) x Q (acfm)
Slurry recirculation pumps (70% pump efficiency assumed)
     P = 0.918 x HS (ft) x (L/G) (gal/1000 acf)
     H  = (L/G) (sp.gr) Q x (9.18 x 10"4)
or
     P = Hs (ft) x L (gpm) x (sp.gr.) x (0.918)
Reheat of scrubber flue gas
     E = m (Jfe) x Cp () x AT (°F)
                                E-25

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MATERIAL BALANCE:	  E-26
between  lower fan  power  (low  pressure  drop)  and  higher pump power  (more
L/G).  No  clear  advantage is apparent for the  spray-type scrubber in regard
to power requirement.
     The total pressure drop accross the FGD system is  11 in. H20.
          Power required by fans,  Px = 0.573 x AP (in. H20) x Q (acfm)
            (70% efficiency)       Pj = 10.18 x 106 Btu/h
Rapid  estimates  of  fan  energy  requirement  can  be made for various  plant
capacities, as shown in Figure  E-7.   As  the  figure shows, the power required
for high pressure drop (AP) at a 500-MW plant is high.
Slurry Recirculation Pumps
     The pumping of  slurry from  the EHT to the  top of  scrubber  requires
considerable  energy.   This energy  consumption  depends  on L/G ratio,  satu-
rated gas  flow rate, slurry density, and pump  delivery  head.   At  an assumed
delivery head of  90  ft, the power required  by  slurry  recirculation pumps at
70 percent efficiency is
     P2 = Hs (ft) x L (gpm) x (sp.gr.) x 0.918
     P2 = 90 x 104,962 x 1.09 x 0.918
     P2 = 9.45 x 106 Btu/h.
Similarly,   the  power  required  by  other pumps  can be  determined  from  the
above equation.   The graph in Figure E-8 is useful  for a  quick estimate of
the recirculation pump  energy  requirement.   Comparison with Figure E-7 shows
that it  is  less  energy-intensive to increase L/G  than  to increase  the system
gas pressure drop.
The total power consumed by fans and recirculation pumps is
          (Pi + P2)
          = 10.18 x 10 6 Btu/h + 9.45 x 106  Btu/h
          = 19.63 x 106 Btu/h
The power consumed in other areas is assumed  to be
          = 0.20 x (Pj + P2)
          = 0.20 x 19.63 x 106
          = 3.93 x 106 Btu/h

-------
        20,000 -
        15,000 -
     *  10,000
     o
         5,000 -
I       111       I       I



                PLANT CAPACITY = 1000 MW








                                 500 MW
                                    I
                                                    100 MW
                             I
I
                     10     20      30     40     50     60


                                   AP, 1n. H,0
    Figure E-7.   Fan energy requirement  (to obtain energy consumed as

coal fired to produce this electricity,  divide by 35 percent-efficiency),
                                    E-27

-------
      8000-
      6000-
    C/l
    §4000
      2000-
                      40     60    80     100
                        L/G, gal/1000 acf
  Figure E-8.  Recirculation pump energy requirement (to obtain
actual energy consumed as coal  fired to produce this electricity,
                divided by 35 percent efficiency).
                                E-28

-------
MATERIAL BALANCE:  High-sulfur Coal	E-29
Thus the total power consumption except reheat as electrical power
          = 23.56 x 106 Btu/h
Then 23.56 x  106 Btu/h divided by 35  percent is equivalent to the amount of
energy  that  the coal  must supply to  the plant to generate  this  electrical
power.
          23.56x10* Btu/h  =67.31xl06Btu/h
                 0.35
Therefore,  67.31  x 106  Btu/h must be  supplied as coal to the  power plant.
Reheat of Scrubber Flue Gas
     The  heat  input  needed  to  reheat  the  saturated  flue  gas from  128° F
(Stream 3) to 168°F (Stream 4) is given by
     E = m () x Cp jp x AT °F  (Table 7)
In this case
     E = 5534.3 x 103 Ib/h x 0.25 ^f x 40°F
     E = 53.13 x 106 Btu/h
Then  55.35 x  106 Btu/h  is  used  in  a 90  percent thermally  efficient  unit
(55.35 x Tpjj  = 61.5 x 106 Btu/h).  The total reheat energy required is 61.5
x 106 Btu/h, which must be supplied as coal input to the powerplant.
Total Energy for FGD System as Percent of Plant Input
     The percentage  of  energy input to the power plant on a coal-fired basis
that is consumed by the FGD system is calculated as follows:
          Total power consumption  =  67.31 x 106 Btu/h
          plus total reheat energy =  61.50 x 106 Btu/h
               Total FGD energy    = 128.81 x 106 Btu/h
The total energy input to the plant is given by
     210 T/h x 23 x 106 Btu/T = 4830 x 106 Btu/h
     (from Table E-l)
and percentage consumed by the FGD system is
       Total FGD energy (128.81) x 106 Btu/h
     Total plant consumption (4830) x 106 Btu/h
x 100 = 2.70%

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MATERIAL BALANCE:  Low- sulfur Coal _ _ E-30
For this  500-MW plant  firing  a  3.7  percent sulfur  Pennsylvania  bituminous
coal, the energy demand of the FGD system is 2.7 percent  of the total  energy
input (as  coal) to  the power plant.   The energy consumed by the  reheater
alone is 1.3  percent of the total heat input to the  power plant, or approxi-
mately half of the total consumption by the FGD system.
     The energy  input  to the power plant to  produce  this amount of electric
power (assuming 35 percent overall efficiency) is
MATERIAL BALANCE CALCULATIONS:   LOW- SULFUR COAL CASE
     Design premises for  an  FGD system at a 500-MW plant firing 0.7  percent
sulfur coal  are presented in table  E-8.   According to the current NSPS  for
utility power plants,  the S02  removal requirement  for the  boiler firing  0.7
percent sulfur  coal  is 70 percent  (outlet emissions  to  be less than 0.6  Ib
S02/106 Btu  heat input)*.   In  such  a  case,  it  is possible to  reheat with
bypass gas instead  of  using  an indirect  in-line  reheater.   A portion of  the
flue  gas   is  bypassed  around  the  scrubbing  module(s)  and  mixed  with  the
cleaned gas.   The degree  of  reheat is limited  by the inlet S02  concentration
of the gas and the S02 removal  requirement.
     Figure  E-9 shows  the basic  flow  diagram for  the  model  limestone  FGD
system on  a  500-MW  plant burning  low-sulfur  coal.  The flow  is  similar  to
that  in  Figure  E-l except  that  the flue gas  leaving  the FGD  system  is
reheated with untreated flue gas.   Basic  process chemistry remains the  same.
The  following  discussion  describes  a procedure for determining  the  portion
of inlet flue gas that bypasses the system and summarizes the steps involved
in  calculations  of stream  characteristics.    All  the  process  assumptions
listed earlier are applicable here.
     As depicted  in  Figure E-9, the ESP collects particulatesT  from the flue
gas  to  a   level  below  the maximum allowable  particulate emission  (0.03  Ib/
million Btu heat input) under the current  NSPS  regulation.
* Federal Register, June 11, 1979.
* Based on 30,800 Ib/h particulates entering the ESP, the particulate
  removal efficiency is 99.55 percent.

-------
    TABLE E-8.   DESIGN PREMISES:   LOW-SULFUR COAL CASE
Plant capacity (gross)
Boiler:
Coal:











Limestone:

Scrubber:


Sulfite-to-sulfate
oxidation
Solids:



S02 inlet loading:
Max. emission:
(NSPS)
Reheat:
Flue gas treated:
SO 2 removal:
Flue gas components
(inlet to scrubber)
Particulates
Carbon dioxide
Hydrogen chloride
Nitrogen
Oxygen
Sulfur dioxide
Moisture
Total

Type
Type
Source
Consumption
Heating value
Sulfur content
Oxygen content
Hydrogen content
Nitrogen content
Carbon content
Chloride content
Moisture content
Ash content
Stoichiometric ratio (SR)a
Utilization (^)
Liquid-to-gas ratio

Inlet gas temperature


Limestone slurry feed tank
Effluent hold tank
Thickener underflow
Oewatered sludge

S02
Particulates
Flue gas bypassed
Flue gas not bypassed
Required for treated flue |
Flow rate,
Ib/h
138
1,228,440
280
4,760,000
423,000
7,495
409,700
6,829,053
500 MW
Pulverlzed-coal-flred
Sub-bituminous
Wyoming
267.6 tons/h
8,750 Btu/lb
0.7X
10.75X
4.5%
1.2X
62.6%
0.05%
13.0%
7.2%
1.10
90.9%
55 gal/1000 acf
(saturated)
285°F
50%

- 33%
14%
30%
60%
1.6 lb/106 Btu input
0.48 lb/106 Btu input
0.03 lb/106 Btu input
34°F
78%
as 90%
Composition,
wt. X mol X

18.00 11.93
0.00 0.00
69.70 72.65
6.19 5.65
0.11 0.05
6.00 9.72
100.00 100.00
Defined as moles of CaC03 and MgC03 (if available) fed per mol of S02
and HC1 (If present) absorbed.
                               E-31

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                              FRESH
                           MAKEUP HATER
•OILER FURNACE
SYSTEM
FlUE
GAS
U1TH
S02 AND
ESP
1
                      PARTICULATES
                               FLY ASH
           BOTTOM ASH
MAKEUP
1 MATER
1®
ONE
•
SLURRY
PREPARATION


                                                                                  SLUDGE
                                                                                TO DISPOSAL
                             Stream characteristics
Gas stream No.
Flow. 1000 acfm
Flow, 1000 Ib/h
Temp. , »F
S02, Ib/h
HC1, Ib/h
C02, 1000 Ib/h
H20, 1000 Ib/h
Participates, Ib/h
1
1653.6
5313.0
295
5830
220
955.76
318.75
107
2
465.5
1516.0
285
1665
60
272.68
90.95
31
3
1372.65
5507.0
128
583
0
959.42
514.40
107
4
1852.2
7023.0
162
2248
60
1232. 10
608. 35
138
Liquid/solid
stream No.
Flow, gal/nrin
Flow, 1000 Ib/h
Temp. , °F
CaC03, Ib/h
H20 (free),
1000 Ib/h
CaSOg-1/2
H20, Ib/h
CaSO«-H20, Ib/h
CaSO«-CaS03-l/2
H20, Ib/h
Inerts, Ib/h
5
_
9.76
70
9,175.5

0

-
-

-
439
6
39
19.5
70
0

19.5

0
0

0
0
7
66
29.3
70
9175. 5

19.5

0
0

0
439
8
370
185.0
70
0

185.0

0
0

0
0
9
91,182
41,484
128


35,673






10
91.285
41,493
128


35,684






11
171
94.0
128
850

80.8

3,597
4,795

3,475
439
12
100
50.1
100
0

50.1

0
0

0
0
13
74
43.8
100
850

30.7

3,597
4.795

3.475
439
14
44
21.9
100
0

21.9

0
0

0
0
15
29
21.9
100
850

8.8

3,359
4,795

3,475
439
A blank Indicates an unknown value; a dash shows that an item does not apply.


       Figure E-9.   Model  limestone  FGD system on 500-MW  plant,
                             0.7 percent sulfur coal.
                                           E-32

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MATERIAL BALANCE:  Low-sulfur Coal _ E-33
Determination of S0g Removal Requirement and Bypass Gas Fraction (Step 1)
     The over-all S02  removal  required is 70 percent.   Because 22^ percent by
volume  of  the inlet  flue  gas bypasses  the system, higher  S02 removal  (90
percent) from  the  remainder of the gas  is  needed to achieve  an overall  S02
removal efficiency of 70 percent.

          Allowable S02 emission = 7495 Ib/h S02 x 0.30
                                 = 2248.5 Ib/h S02

By difference  the total  S02 removed = 5246.5  Ib/h S02 or 81.9 Ib  mol/h S02

Let  x  be  the fraction  of  inlet gas  treated.   Equating  the  amount  of  S02
removed
               (x)(0.90) =
                    x  =  jp^j  = 0.778 or 77.8 percent
Therefore, 100  -  77.8 = 22.2 percent  of  the incoming flue gas  bypasses the
system and is used for reheat purposes.

The amount of flue gas treated (dry basis) is

          6,419,350 Ib/h x 0.778 = 4,994,300 Ib/h dry flue gas
Limestone Requirement/Slurry Preparation (Step 2)
     The theoretical  limestone  requirement depends on the amounts of S02 and
HC1 to be  removed from the flue gas.   The amount of HC1  in flue gas Stream 1
(100 percent removal assumed) is
          (280 x 0.78) lb/h/(36.46 Ib per Ib mol) =6.0 Ib-mol HCl/h.
The theoretical alkalinity requirement is based on the amount of S02 and HC1
present in the gas stream:
          81.90 + 5s(6. 0) = 84.90 lb-mol/h.
Actual alkalinity required = 84.92 x 1.10 = ,93.40 lb-mol/h

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MATERIAL BALANCE:  Low-sulfur Coal _    E-34
Table E-4 gives the required composition of the limestone.
     The limestone requirement summary (Stream 5) is as follows:
                         CaC03           9175.5 Ib/h
                         MgC03            146.5
                         Inerts           439.0
                              Total      9761 Ib/h limestone or
                                          4.88 tons/h
     The amount  of  excess  makeup water used  to  prepare the limestone solids
slurry  is  established by  the  mist  eliminator  wash procedure  and  the total
makeup water required:
               = 39 gpm

               = 39   m * 50°
               = 19,500 Ib/h of water
and the flow rate of slurry (Stream 7) = 29,261 Ib/h
Humidifi cation of Flue Gas (Step 3)
     The procedure  followed  is  similar to that in the high-sulfur coal  case.
The flow rate  of water in the flue gas entering the scrubbing modules (after
bypass) at  285°F is 318,750 Ib/h or  ~  638 gpm.   The dry flue  gas  flow rate
(into scrubber)  is 4,994,300 Ib/h.
     The molecular  weights of  dry flue  gas  at  the inlet and  outlet of the
scrubbing modules are 30.40 and 30.38, respectively.

Humiriitv finiotl -    318.750 Ib/h H,0      30.40 (molecular wt of dry gas)
numiaiiy unieij - 4,994,300 Ib/h dry gas X 28.97 (molecular wt of air)
                      = 0.067 Ib H20/lb dry air
This conversion  facilitates the use  of a psychrometric chart  based  on air-
water.   The saturation  temperature of  the  gas is  128°F.   The  humidity  of
saturated gas is calculated as follows.
          Saturation humidity = 0.108 Ib H20/lb dry air at 128°F

-------
MATERIAL BALANCE:   Low-sulfur Coal	:	 E-35
To correct for flue gas multiply by ratio  of  molecular weights
          °-108 JSiir  x ioi  = °-103  lb  H2°/1b dry 9as
The amount of water vapor in the  outlet  gas  is
          0.103 lb H20/lb dry gas x 4,994,300 Ib/h dry gas
          = 514,400 Ib/h of water or 1029  gpm.
The amount of water required for  100 percent saturation
                 Saturation - Inlet
               (514,400 - 318,750) Ib/h
               = 195,650 Ib/h or  391 gpm.
On a molar  flow rate basis,  the  outlet  gas  contains 164,340 Ib-mol/h dry gas
and 28,560 Ib-mol/h H20.
     The  volumetric  flow rate of  outlet  gas  (before mixing  with untreated
gas) at 128°F and 2 in.  H20 is

          (164 340 + 28 560) 1b"mo1 x (460 + 128)°F   	h_ x 359 ft3
          (164,340 + 28,560)   h     x £460 + §2)6p  X 6Q  m1n x lb_mol

                 407 in. H,0*
               x  409 in. H20

               = 1,372,650 acfm at 128°F
          * Atmospheric pressure.
The amount  of cleaned  flue  gas  entering the mixing  chamber (Stream  3) is
5,507,050 Ib/h,  which accounts for  the S02 replaced by the  evolved C02 in
the gas.  The  flow rate of untreated gas (Stream 2) is  1,516,000 Ib/h.  Let
T(°F) be the temperature of the mixed gas.   The  energy balance gives
5,507,050    x 0.25      x (T -  128)°F = 1,516,000  Ib/h x 0.25      x  (285 - 1)°F
T  is  computed  from this  relationship  to  be 162°F.   Hence the  degree of
reheat  is  162°  -  128°  = 34°F.   This  amount  of  reheat  is assumed  to be
adequate.

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MATERIAL BALANCE:  Low-sulfur Coal _         E-36
     The flow rate of mixed gas leaving the system (Stream 4) is
                   860 1b"mo1 x $22°F x    *      359
                  .BDU        X -s  X
                    = 1,852,200 acfm at 162°F
Recirculation Loop and Sludge Production (Step 4)
     The mols of  S02  removed from the flue  gas  equal  the mols of  sulfur  in
the  sludge.   The hydrogen  chloride  is assumed  to be  removed  by MgC03  and
CaC03 as MgCl2 and CaCl2.
The excess CaC03 supplied = [91.67 - (84.92 - 1.74)]
                                = 8.49 Ib-mol/h or 850 Ib/h CaC03
This excess CaC03 leaves the system with the waste sludge.
     In determination of  sludge  composition, the CaS03 precipitation =  81.9
Ib-mol/h  CaS03.   At  50 percent oxidation  of sulfite to  sulfate the CaS04
formed by precipitation is
               81.9 luh"    x 0.5 = 40.95 Ib-mol/h CaS04.
Sixteen mol  percent CaS04  is  in the  form of CaS04*CaS03-%H20  (solid  solu-
tion) and the remainder is CaS04-2H20
          CaS04-2H20 crystals formed   = 40.95 1b"mo1 (^ETT^)
                                                 n      ou
                                       = 27.85 !Sj22l
                                       = 4795 Ib/h CaS04-2H20
          CaS03 left in the product = 40.95 Ib-mol/h CaS03
          CaS04-CaS03-%H20 formed =13.1 Ib-mol/h
                                      = 3475 Ib/h CaS04-CaS03-Vi20
          CaS03-3$H20 crystals formed = (40.95 - 13.1) Ib-mol/h
                                       = 3597 Ib/h CaS03-J5H20
          83.2 Ib-mol/h of C02 is formed in the scrubbing modules or
            3661 Ib/h.

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MATERIAL BALANCE:  Low-sulfur Coal
E-37
Composition of the sludge solids is summarized in Table E-9.
                  TABLE E-9.  WASTE SLUDGE SOLIDS FOR LOW-SULFUR COAL

CaC03
CaS03-%H20
CaS04'CaS03-J5H20
CaS04-2H20
Inerts
Total
Mass flow rate,
Ib/h
850
3597
3475
4795
439
13,156
Composition, wt. %
(dry basis)
6.46
27.34
26.41
36.45
3.34
100.00
     The amount of water in the 60 percent solids sludge (Stream 15) is
               13.156 Ib/h ..lid. x
               = 8771 Ib/h of water or ~ 18 gpm
     Figure  E-10 .  depicts  the  material  balance  around  the  thickener  and
solids dewatering device.
     Slurry flow to the scrubber (Stream 10) is shown in Figure E-ll.
     The L/G  ratio  for  the scrubbing modules  in  this  process  is  given as 55
gallons of  slurry  (14 percent solids) per  1000  ft3  of saturated  gas.   Since
the saturated gas  flow  rate  is  1,372,650 acfm at 128°F,  the slurry flow rate
at 1.09 specific gravity entering the scrubbing modules is
           55 gal    1.372.650 ft3 gas   60 min   9.16 Ib
          1000 ft3 x        min        x ~~h    x    gaT
               = 41,492,455 Ib/h (75,440 gpm) of slurry
which  contains   14 percent  solids  (5,808,944  Ib/h)  and  86  percent  water
(35,683,511) or 71,367 gpm of water.
     The amount  of S02 transferred to  the  liquid phase is 5,247  Ib/h.   The
amount of  C02 transferred from the  liquid  phase to  the  flue gas is 3.661
Ib/h.   The  amount  of  water lost to saturate  the flue gas  is 195,650  Ib/h or
319 gpm.

-------
                                      OVERFLOW
                                (RETURNED TO PROCESS)
                            -71,000  Ib/h H20 or 142 gpm
                                             FILTRATE

                                     22,000  Ib/h H20 or 44 gpm
to
00
           (14* SOLIDS)
         13,156 Ib/h SOLIDS
     -80,000  Ib/h H20 or 160 gpm
THICKENER
                     UNDERFLOW
                   (30%  SOLIDS)
SOLIDS DEWATERING
 SYSTEM DEVICE
             -31,000 Ib/h H20 or 62 gpm
                                                                                 WASTE  SLUDGE
                                                                                 (60% SOLIDS)
                                                                              13,156  Ib/h  SOLIDS
                                                                           -9,000 Ib/h  H20 or  18  gpm
       Figure E-10.   Material  balance,  required thickener, and dewaterinq device solids dewatering system.

-------
       FRESH MAKEUP
      WATER (FOR ME)-
185,000 Ib/h H20 or 370  gpm
        FLUE GAS IN
        5830 Ib/h S02
        955,760 Ib/h CO?
        318,750 Ib/h H~0
          or 638 gpm
                                               CLEAN
                                               FLUE GAS
                                           583 Ib/h  SO?
                                           959,420 Ib/h  C02
                                           514,400 Ib/h  H20
                                            or 1,029 gpm
                                SCRUBBING
                                 MODULES
  FROM EFFLUENT  HOLD  TANK
'5,808,944  Ib/h  OF  SOLIDS
   35,683,511  Ib/h  H20
    or 71,367 gpm
                                   TO
                              EFFLUENT HOLD
                                  TANK
                         5,810,530 Ib/h OF SOLIDS
                    35,673,000 Ib/h H20 or 71,346 gpm
           Figure E-ll.   Material  balance around scrubbing modules,
                                    E-39

-------
MATERIAL BALANCE: Low-sulfur Coal	E-40

     As Figure E-ll illustrates:

          input streams:  2, 8, 10
          output streams: 3, 9

     Amount of solids in = 5,808,944 Ib/h (Stream 10)
                         + (5247 - 3661) Ib/h
                         = 5,810,530 Ib/h

     Amount of water in  = 318,750 Ib/h (Stream 2)
                         + 185,000 Ib/h (Stream 8)
                         * 35.683.511 Ib/h (Stream 10)
                         = 36,187,301 Ib/h

     Amount of water out
       (Stream 9)        = [36,187,301 - 514,400 (Stream 3)] Ib/h
                         = 35,672,901 or 71,346 gpm

     Amount of solids out
       (Stream 9)        = 5,810,530 Ib/h

Total Makeup Water Required (Step 5)

     The amount  of water  leaving with the  saturated flue gas and with  the

waste sludge  is  the  amount that must be  made  up.   The makeup water consists

of  the  amount of  water required  for  humidification plus the water  leaving

with the sludge, i.e., 391 + 18 = 409 gpm.

Mist Eliminator Wash

     As  in  the high-sulfur coal  case,  the  409 gpm of makeup  water required

by  the system must be split between that needed for the mist eliminator wash

and the excess that is used to prepare the limestone slurry feed.

     For this low-sulfur coal case:
      Volumetric flow rate   ^ Gas velocity through _ Cross-sectional
     of scrubber outlet gas  '    mist eliminator           area


                  1'372>650     <-> *      = 286°ft2-
                                           sec

Then, according to  the Shawnee guidelines (Burbank and Wang  1980) the fresh

makeup water requirement is calculated as follows:

     Top wash required:

          2860 ft* x 0.54 ^ x L*2 (_!!_) = 77 gpm

-------
MATERIAL BALANCE:   Low-sulfur Coal	E-41
     Bottom wash required:
          2860 ft* x 1. SO f! X i|!B {gfo.) = 286 gp.
          Total fresh water for ME:   363 gpcn
As before,  it is  noted  that this  fresh water  is  used intermittently  at  a
higher  flow rate  but is calculated  on a  continuous  basis.   Therefore, an
even 370 gpm is adequate for scale-free operation.
     Since this low-sulfur  coal  system requires 409 gpm as makeup water and
370 gpm  is  directed to the mist  eliminator wash,  an excess of 39 gpm can be
used to slurry the limestone:
          9,761 Ib/h of limestone required is mixed with the 39 gpm
          = 9,761 + 39 (500) =
          = 9,761 + 19,500 = 29,261 Ib/h of limestone slurry
              9 761
          and OQ oci = 33 weight percent solids.
              £3, £01
The  overall  water  balance  that  provides  for a  good mist eliminator  wash
procedure is shown in Figure E-12.

ESTIMATION OF ENERGY CONSUMPTION:  LOW-SULFUR COAL CASE
     Energy  is consumed by the  scrubber  system fans, recirculation  pumps,
and other pumps.   As before, comparatively small amounts  of  energy are also
consumed by  other  components;  the total of this consumption is assumed to be
20 percent of that by fans and recirculation pumps.
Flue Gas Fans
     In  this  example  the  assumed  pressure  drops  across   the  scrubbing
modules, mist  eliminator,   ductwork,  and mixing chamber  are  7, 1,  2,  and  1
in. H20, respectively.
     Total  pressure drop across  the  FGD system is 11 in. H20.   Assuming 70
percent fan efficiency
         P!  = 0.573 x AP (in. H20) x Q  (acfm)
             = 0.573 x 11 x 2,119,300
          Pj = 13.36 x 106 Btu/h

-------
J,
ro
     TOTAL FRESH
     MAKEUP WATER-
      409 gpm
     LIMESTONE
     4.88 T/h
                    MAKEUP
                    WATER
                    39,gpm
                      I
                            FRESH
                            MAKEUP
       WATER
       370 gpm


        FLUE GASQ
           IN  —
        638 gpm
                                                       CLEAN
                                                   •W FLUE GAS
                                                      1,029 gpm
                                         SCRUBBER
                                                  *-
           39 gpfti
            6)
   SLURRY
PREPARATION
EFFLUENT
  HOLD
  TANK
    *gpm for water content only.
                                                        71,367 gpm*
                                                                OVERFLOW
                                                                142 gpm
                                                         160
                                                         BLEED
                                         l_T
                     THICKENER
                                                      FILTRATE
                                 44
                                62
                                gpm*
DEMATERING
  DEVICE

                                      SLUDGE
                                    TO DISPOSAL
                                      18 gpm*
                                Figure E-12.  Overall water balance.

-------
MATERIAL BALANCE:  Low-sulfur Coal _ ; _ E-43
Slurry Reclrculatlon Pumps
     The pumping of  slurry  from the recycle tank to  the  top  of the  scrubber
requires considerable amounts of  energy.  This  energy consumption  depends  on
L/G ratio, saturated  gas  flow rate, slurry density,  and  pump delivery head.
Assuming a  delivery head of  90  ft,  the  power consumption by recirculation
pumps is
          P2 = 0.918 Hs (ft) x L (gpm) x (sp.gr) (Table E-7)
             = 0.918 x 90 x 75,440 x 1.09
             P2 = 6.79 x 106 Btu/h
          The assumed power consumed in the  other areas
                = 0.20 (Rj + P2)
                = 0.20 (13.36 + 6.79) x 106  Btu/h
                = 4.03 x 106 Btu/h
Therefore the total power consumption by the FGD system is 24.18 x 106 Btu/h.
     The energy  .input  as  coal  fired  to  the  power  plant  to produce  this
electric power (assuming 35 percent overall  efficiency) is

                              = 69.09 x 10"  Btu/h.
Hence the percentage  of the energy input to the power plant that is consumed
by the  FGD  system  is 69.09 x 106/4683 x  106*  x 100 or 1.50 percent.   Note
that  no  energy is  consumed for  reheat  of flue gas in this  low-sulfur coal
case because of the heat provided by untreated flue gas.
     Table E-10 summarizes  the energy demands for both low-sulfur  and high-
sulfur coals.
* 267.6 T/h coal-fired x 8750 1^ x 2 x j°8 lb = 4683 x 106 Btu/h as the
                              ID        i
  total heat input of powerplant.

-------
MATERIAL BALANCE:  Low-sulfur Coal
E-44
            TABLE E-10.  ENERGY DEMAND FOR FGD ON A 500-MW PLANT

Low-sulfur coal
High-sulfur coal
Electrical
energy,
103 kWh
7.1
6.9
Electrical
power,
106 Btu/h
24.2
23.6
Power
input,
106 Btu/h
69.1
67.4
Reheat
input,
106 Btu/h
Nil
61.5
Total
energy demand,
percent of
coal fired
1.5
2.7
     The table  shows that  use  of the  untreated  flue gas for reheat  in the
low-sulfur coal  case yields  about  a 50  percent  energy savings  relative to
the high-sulfur coal case.

-------
MATERIAL BALANCE:  Reference	E-45
                                  REFERENCE
Burbank, D.  A.  and S.  C.  Wang.   1980.  EPA Alkali  Scrubbing Test Facility:
Advanced Program  Final  Report (October 1974 to  June 1978) EPA-600/7-80-115.
NTIS No. PB 80-204 241.

-------
                                 APPENDIX F
                            LIMESTONE UTILITY FGD
                         SYSTEMS IN THE UNITED STATES
     Table F-l presents  current  information about wet limestone utility FGD
systems in the United  States.   The main source of  this  information was the
Flue  Gas  Desulfurization  Information System  (FGDIS),  a computerized  data
base  sponsored by the U.S.  EPA and  developed by  PEDCo  Environmental  (see
Appendix  C).*  Additional  information came from Black & Veatch Consulting
Engineers and from  some  scrubber suppliers.  When discrepancies were found,
FGDIS  data  were  generally  assumed  to be accurate.  The other  sources,
however,  were  contacted  and were  sometimes able  to correct  or supplement
FGDIS data.
     The high S02 removal efficiencies in Table F-l for the  flue gas treated
indicate that limestone scrubbers can meet the stringent requirements of the
1979  New  Source  Performance Standards.   Also, the  large number of entries
suggests that limestone scrubbing is a very reliable and economical  means of
S02 control.   Limestone  scrubbers  are thus expected to remain the main type
of S02 control  system in the United States.
* Information about  access  to the FGDIS can  be  obtained from Mr.  Walter L.
  Finch, Product Manager, National  Technical  Information Service,  5285 Port
  Royal Road, Springfield, Virginia 22161 [telephone (703) 487-4807].
                                     F-l

-------
                      TABLE F-1.  WET LIMESTONE UTILITY FGD SYSTEMS IN THE UNITED STATES



Utility and plant
Alabama
Electric Coop
Tomblgbee 2

Alabama
Electric Coop
Tombtgbee 3

Arizona
Electric
Power Coop
Apache 2


Arizona
Electric
Power Coop
Apache 3


Arizona Public
Service
Choi la 1


Arizona Public
Service
Choi la 2


Arizona Public
Service
Choi la 4



f*
sr\
* X
If
Ul

179



179




98





98




119




264




126



£
**
i

New



New




New





New




Retro
fit



New




New




S
3
B
i/i

9/78



6/79




8/78





6/79




10/73




4/78




6/81


S
o
o
+*
c
S
c
o
o
a
3
1/1

1.15



1.15




0.55





0.55




0.50




0.50




0.50


i.
«f
*rl
f_
|
3

Peabody



Peabody




Retearch-
Cottretl




Research-
Cottrell



Retearch-
Cottrell



Research-
Cottrell



Research-
Cottrell

£. i.
U V
u c
« o>
& «
k. k
XI c
b?

Burns 4
McDonnell


Burnt A
McOonnel 1



Burnt &
McDonnell




Burnt &
McDonnell



Ebatco




Ebatco







|
U
CM
S
o

Spray tower



Spray tower




Combination
(spray/
packed)



Combination
(tpray/
packed)


Combination
(spray/
packed)


Combination
(tpray/
packed)


Combination
(spray/
packed)

M
|
o
o

2



2




2





2




2




4







M
6
o
•ji

«. *-
1!

ESP



ESP




ESP





ESP




Venturl




ESP




ESP




fc
u





















X




X







a
I
i
I





















SP




SP








1
*>
g



































§
*
M
f
M



































S
*<
f
VI



































3§
*>"
*H
v •.
S
M M

Pipeline



Pipeline




Pipeline





Pipeline




Pipeline




Pipeline




Pipeline




M
I
,
Pond



DMM!
rin HI




Pond





0MPU|
fvnu




m^ 	 *
fond




fond




fond


I
ro
      {continued)

-------
        TABLE F-l (continued)








Utility and plant
Associated
Electric Coop
Thoews Hill 3

Basin Electric
Power Coop
Laramie River 1


Basin Electric
Power Coop
Laramie River 2


Big Rivers
Electric
O.B. Wilson 1

Big River*
Electric
O.B. Wilson 2

Central Illinois
Light
Duck Creek 1



Central Illinois
light
Duck Creek 2

Colorado Ute
Electric Assn.
Craig 1





f
ii
c
y
S£
** u
£&
ts
lAl


670



570




570




440



440



416





450



447






*;

?
o
i


New



New


.

New




New



New



New





New



New








S
3
a
vt


1/82



7/80




6/81




7/84



1/86



7/76





1/86



10/80

M
_"
S
U
4^
O
*J
C
V
«J
c
o
u
L.
3
I/I


4.80



0.81




0.81












3.66





3.30



0.45








1
!


Pultun
Kellogg


Re scare h-
Cottrell



Research-
Cottrell



PullMn
Kellogg


PullMn
Kellogg


Rlley
Stoker




Not
selected


Peabody

i
4-*
2 k.
o *t
•B Oi
s|

M
U U
II
E^>
u c
 41


Burns &
McDonnell


Burns &
McDonnell



Burns ft
McDonnell











Gilbert/
CoMaon-
wealth
Assn.






Steams-
Roger


^
^
J3
e

s~
o
1


Spray tower
(Weir)


Combination
(spray/
packed)


Combination
(spray/
packed)


Spray tower
(Weir)


Spray tower
(Weir)


Packed
tower (rod
deck)







Spray tower






VI
•1
i
•s
o
z


4



5




5












4









4

„
as

•i
"^
«»_
•j
2
S~


91.5



90




90












85









85


o
*>

-------
TABLE F-l (continued)







Utility and plant
Colorado Ute
Electric Assn.
Craig 2

Coeawnwealth
Edison
Powerton 51



Oeseret Generation
i Transmission
Coop
Noon Lake 1


Oeseret Generation
& Transmission
Coop
Noon Late 2

Hoosler Energy
Division
Nero* 1

Hoosler Energy
Division
Nerom 2

Houston Lighting
ft Power
Limestone 1


Houston Lighting
» Power
Limestone 2





JB
e*
*".
> **
«J U
il
Ul


400



450






410





410



441



441



750




750






%.
£
?
u
e
i


New



Retro-
fit





New





New



New



New



New




New







43
§
a
vt


12/79



4/80






9/84





0/88



5/82



9/81



12/84




12/85


-
S
u
*-
4-»
c
1
o
u
V. .
•3
H-
l/t


0.45



3.53






0.50





0.50



3.50



3.50



1.08




1.08





k
X
I
j{
1


Peabody



UOP






Combus-
tion En-
gineering



Not
selected


Mitsu-
bishi


Nitsu-
blshl


Combus-
tlon En-
gineering


Combus-
tion En-
gineering
|
J= U
U «l
!s§,
is
4-* -O
ui
>» i.
vt O
i. L.
.X tu
If
i/i «


Steams-
Roger


Sargent &
Lundy





Burns &
McDonnell
















Ebasco




Ebasco



t

5
*/i
i/t
o
1


Spray tower



Packed
tower
(mobile
bed)



Spray tower









Packed
tower


Packed
tower


Spray tower




Spray tower






M
|
^.
O
s


4



3






3

















5




S


X
u
c

•5
H-
41


U
sf


as



74






95





95



90







90




90


k
o
%
c
~e
*•>
+•
M
1
O
1


Chevron



Chevron






Chevron

















Chevron




Chevron






«
£
e
o
!


In-line



Indl-
direct
hot air




a*- — —
ffOnv









Bypass



Bypass



Bypass/
ambient
air


Bypass/
ambient
air



a
3
U !>
l!
"o"5
II


ESP



ESP






Fabric
filter




Fabric
filter


ESP



ESP



ESP




ESP







,.
\
u


X



X






X









X



X



X




X








Truck



Truck
















Truck



Truck















_
m
5
c
*•
Ik


Landfill4



Landfill*






landfill





Landfill



Landfill



Landfill



Pond




Pond


 (continued^

-------
       TABLE F-l  (continued)





Utility and plant
Houston lighting
A Power
H.A. Parrlsh 8
Indianapolis Power
A Light
Patriot 1

Indianapolis Power
A light
Patriot 2

Indianapolis Power
A Light
Patriot 3

Indianapolis Power
A Light
Petersburg 3


Indianapolis Power
A light
Petersburg 4


Iowa Electric
Light A Power
Guthrie County 1


Jacksonville
Electric Authority
New Project 1

Jacksonville
Electric Authority
New Project I

(continued


1
r\ TI
1*
u«
Effective i
capacity.


492


650



650



650



532




530




720




600



600






e
£
i


New


New



New



New



New




New




New




New



New






s
9
a
1
S
tft


11/82


0/87











12/77




10/84




11/84




12/85



6/1?


-.







0.60


3.50



3.50



3.50



3.25




3.50




0.40




3.00



3.00




t

"a.
ex
-«
Scrubber si


Cheaiico


Not
selected


Not
selected


Not
selected


UOP




Research-
Cottrell



Coefcus-
tion En-
gineering


Not
selected


Not
selected

|
£ ft.
U V
t. c
a o>
EM
w
*• *o
*» f_
Scrubber $;
engineer 01

















Gibbs A
Hill



Stone A
Webster



Black A
Veatch











i.
a

o
•s
1

















Packed
tower



Combination
(spray/
packed)


Spray tower















M
€>
3
i
O
O
1C

















4




3




4











-
U
C
41
U

H-

ss
o*v


Bypass














ESP









ESP




ESP



ESP







I Thickener

















X




X




X












a
c
u
S
€1
*.
O
1

















WF




OVf


















c
o

i
s.



























\












s
^J
tuj
3
5
l/t


X








































^
___
1
4/1

















IUCS




IOCS




















51
•• M
Means of M
transporta

















Pipeline




Pipeline




















a
M
•5
*
c
•Z


Landfill














Pond




Pond




Landfill











I
in

-------
TABLE F-l (continued)




Utility and plant
Kansas City Power
A Light
La Cyne 1



Kansas Power A
Light
Jeffrey 1


Kansas Power A
Light
Jeffrey 2


Kansas Power A
Light
Jeffrey 3


Kansas Power A
Light
Lawrence 4


Kansas Power A
Light
Lawrence S


Lakeland Utilities
Nclntosh 3

Louisville Gas A
Electric
Hill Creek 1



i
u*
M
si
« 0
Ul


820





540




490




490




125




420



364



358




retrofit

i


New





New




New




New




Retro-
fit



Retro-
fit


New



Retro-
fit



**
9
a
3
r
s
in


2/73





8/78




4/80




0/83




1/76




11/71



8/81



1/81


S
«•-
0
content
L.
I/I


5.39





0.32




0.30




0.5




0.55




0.55



2.56



3.75



fc
I
V*
U
|
e


Bibcock A
Wilcox




Coriws-
tlon En-
gineering


Coafcus-
tton En-
gineering


Combus-
tion Eng-
gineerlng


Coatus-
tion En-
gineering


Coabus-
tlon En-
gineering

Bibcock A
Wilcox


CoatNis-
tlon En-
gineering
f. t.
U 01
u c
•a 
M
O
*


Sieve
tray
and
chevron


Chevron




Chevron




Chevron




Chevron




Chevron



Chevron



Chevron




i
e
o
'I


Indi-
rect
hot air



Bypass




Bypass




Bypass




In-line




In-line



Bypass



In-line



j
Is
i. >
H- *-
O •*
II


Venturl





ESP




ESP




ESP




Venturl
(rod)



Venturl
(rod)


ESP



ESP


A


\
U

-
X





X




X




X




X








X



R


a
•?
I
*s
s
































VF



vr




i
8

U.







































§

sUbllU

^
«/l









































fixation
^
o»
TJ






























'

IOCS



IOCS




2j

-------
  TABLE F-1  (continued)





Utility and plant
Louisville Gas t
Electric
Mill Creek 2


Michigan South
Central Power
Agency
Project 1

Middle South
Utilities
Arkansas
Lignite S


Middle South
Utilities
Arkansas
Lignite 6


Middle South
Utilities
Unassigned 1


Middle South
Utilities
Unassigned 2


Middle South
Utilities
Wilton 1






1*
U*J
V»
Effective
capacity,


350





55




890





890




890




890




890






4-*
O
L.
?
L.
O
1


Retro-
fit




New




New





New




New




New




New






S
Startup di


12/81





6/82




0/90





0/92




0/89




0/93




0/88


ft*







3.75





2.25




0.50





0.50




0.50




0.50




0.50





L.
«
|
i/l
1
.0
3
vt


CoBbus-
tion En-
gineering



Babcock &
Wilcox



CoBbus-
tion En-
gineering



CoBbus-
tion En-
gineering


CoBbus-
tion En-
gineering


CoBbus-
tion En-
gineering


CpBbus-
tlon En-
gineering
*>
^

U
t_
g
*>
M
X
Ml
Scrubber i
engineer i


Fluor
Pioneer









Sargent
& Lundy




Sargent
& Lundy



Sargent
ft Lundy



Sargent
& Lundy



Sargent
S Lundy




|
t
u
Wl
ca
•t-
0
t-


Spray tower





Spray tower




Spray tower





Spray tower




Spray tower




Spray tower




Spray tower






a
a
o
o
X


2










6





6




6




6




6


M

>,
U
S
u
•1
tg
i
sT


85










92





92




92




92




92



L.
O
•O
C
*E

1
»»-
O
1


Chevron










Chevron





Chevron




Chevron




Chevron




Chevron






i
u
o
1


In-line





In-line




Bypass





Bypass




Bypass




Bypass




Bypass





S
U >
O 49
*l


ESP





ESP




ESP





ESP




ESP




ESP




ESP







1 Thickener


X





































°?
ewaterl
•o
*-
o
*


VF































-...






Idatlon
M
O
I
U.








































g
•O
•si
3
•
«>>
M
f
VI









































1
««
*.
I
VI


IOCS






































51
ts
•5|
M M
C C
15


Track










Pipeline





Pipeline




Pipeline




Pipeline




Pipeline






S
a
•*
1
u.


Ltndflll










Pond





Pond




rOHQ




Pond




Pond


(continued)

-------
       TABLE F-1 (continued)
,




Utility and plant
Middle South
Utilities
Wilton 2


Muscat ine Power A
Mater
Muscat ine 9


New York State
Electric A Gas
Soecrset 1
Northern States
Power
Sherburne 1


Northern States
Power
Sherburne 2



Northern State
Power
Sherburne 3


Pacific Ga* A
Electric
MontezuM 1

Pacific Ga* A
Electric
MontezuM 2




i1
M
S£
t|
u


890




166



625


740




740





860




800



800




«*-
e
s
*
i


New




New



New


New




New





New




New



New





S
•3
Startup


0/91




9/82



6/84


3/76




3/77





5/84




6/89



6/90

M
~"
S
«*-
o
«J
c
1
s
1,


0.50




3.00



2.20


0.80




0.80










0.80



0.80




fc
ex
3
tfl
J>


Coabus-
tlon En-
gineering


Research-
Cottrell


Peabody


Coafcus-
tlon En-
gineering


Coatous-
tton En-
gineering



Coefcus-
tlon En-
gineering


Not
selected


Not
selected
^-
JS
!i
6 £
»/i
>» u
w» O
i- k.
fl


Sargent
A Lundy







Ebasco


Black A
Veatch



Black A
Veatch

















]j
1
U
w»
S~
•5
•J


Spray tower




Conbl nation
(spray/
packed)

Spray
tower


Packed
tower
( Barbie-


Packed
tower
(Mrble-
bed
















M
|
*•-
o
o
JC


6











12




12
















M
O
C
«l
u
*!
u
1
CM
S


92




94



90


50




50

















S



1


Chevron








Chef roit


Chevron




Chevron



















«
1
0
1


Bypass




In-line



Indirect
hot air


In-line




In-line


















S
«
Is
*» ~-
h. >
m «i
o.^
*». *—
o •


ESP




ESP



ESP


Venturl
(rod)



Venturl
(rod)









Fabric
filter


Fabric
filter




fc
£











X


X




X


















•«»,
c
s
1
*











WF



























§
i
s
b.







X






X




X


















1
N
3
M
f











X



























S
S
1







































ss
15
o o
S|
It •*


Pipeline








Track


Pipeline




Pipeline



















^
i
M
•5


Pond




Landfill



Landfill


Pond




afc— j
rono
















I
CD
      (continued!

-------
        TABLE F-1 (continued)





Utility and plant
ruins Electric
G*T Coop
Plains
Escalante 1


Public Service of
Indiana
Gibson 5

Salt River Project
Coronado 1

Salt River Project
Coronado 2

Salt River Project
Coronado 3

San Miguel
Electric Coop
San Miguel 1

Sealnole Electric
SMlnole 1

Swlnole Electric
SeBinole 1

Slkeston Board
of Municipal
Utilities
Slkeston 1



•s
X
M *
* as
** u
tl
taJ



233




650


280


280


280



400


620


620




235




**
rttrofl
o
i



New




New


New


New


Nsw



New


New


New




New





S
9
Startup



12/83




0/82


11/79


7/80


0/89



11/80


3/83


3/85




3/81

-

S
u
0
c
tl
8
i.



0.80




3.30


1.00


1.00


0.60



1.70


2.75


2.75




2.80




jj>
o.
a.
3
M
L.
1



Caabus-
tion En-
gineering


Pultswn
Kellogg

Pullswn
Kellogg

PullMn
Kellogg

Not
selected


Babcock 1
Wilcox

Peabody


Peabody




Babcock &
wilcox
£

U tl
L. c
•«-
Is
VI
If



Burns &
McDonnell






Bechtel


Bechtel






Tippet i
Gee

Burns A
Roe

Burns •
Roe







1
n
e
u
. o~
t/1
o



Spray tower




Spray tower


Spray tower


Spray tower






Packed
tower

Spray tower


Spray tower




Tray tower
(sieve)




M
t>
•5
o
JE



3




4


2


2






4


S


5




3

-
X
0
c
*>
"
V
o
f
a1



95







82.5


82.5






86


90


90






ft.
O
aa
c
E
M
E
•s
I



Chevron







Chevron


Chevron






Chevron


Chevron


Chevron




Chevron





C
<*-
O
1



None







Bypass


Bypass


Bypass



In-line


None


None








•
• j
.1
*!



ESP




ESP


ESP


ESP






ESP


ESP


ESP




ESP/
venturt




^
O
£



X




X


X


X






X


X


X








o
T!
S
»*-
O
1



c




w












OVF


BVF


BVT









§
«j
m
•o
M
o
1




































S
1
VI
I





















X
















^
**
«c
1
V)








IUCS















ncs


no










|!/
II











Pipeline


Pipeline






Truck


Truck


Track




Pipeline

,



a
&
¥t
•o
ik



Landfill




Landfill


Pond


Pond






Landfill


Landfill


Landfill




Pond

I
to
         (continued)

-------
       TABLE F-l  (continued)





Utility and plant
South Carolina
Public Service
Cross 1
'
South Carolina
Public Service
Cross 2
South Carolina
Public Service
Ntnyah 2

South Carolina
Public Service
Winy ah 3

South Carolina
Public Service
Minyah 4


Southern Illinois
Power Coop
Marion 4


Southern
Mississippi
Electric Power
R.D. Morrow 1

Southern
Mississippi
Electric Power
R.D. Morrow 2

Southwestern
Electric Power
Dolet Hills 1



1*
if"
*
Effective
capacity


500


500


140



280



280




173





124



124



720



-
O
fc.
?
fe
1


New


New


New



New



New




New





New



New



New




S
I


3/85


11/83


7/77



5/80



7/81




5/79





8/78



6/79



0/86
-
,_
S
o
o
*J
c
c
o
u
u
3


1.80


1.80


1.70



1.70



1.70




3.75





1.30



1.30



0.70



V
|
J


Peabody


Peabody


Bibcock A
Milcox


Babcock A
Nilcox


Ajwrican
Air
Filter


Babcock A
Wilcox




Riley
Stoker


Riley
Stoker


UOP
*•
£j
JT f-
£ *»
2«
irt
>» U
M 0
it
li


Burns
A Roe

Burns
ft Roe


Burns A
Roe


Burns A
Roe


Burns A
Roe



Burns A
Roe




Burns A
Roe


Burns A
Roe





|
1
u
01
• -s
1


Spray tower


Spray tower


Tray tower
(sieve)


Tray tower
(sieve)


Spray tower




Spray tower





'Packed
tower


Packed
tower






M
•I
1
i
<*-
o
s


3


3


1



2



2




2





1



1




«
J*
U
C
•1
It-
1)
*•».
I
s1


90


90


90



90



90




89.4





85



85





o
«•
C
^
M
(S
•s
1


Chevron


Chevron


Chevron



Chevron



Chevron




Chevron





Chevron

/

Chevron
i






m
:' £
w
t*.
O
I


Indirect
air and
bypass
Indirect
air end
bypass

Bypass



Bypass



Indi-
rect
hot air


None

f



Bypass



Bypass






S
3.
•»- U
**,•»-
fl








ESP/ .
venturl


ESP/
venturl
•
\'
ESP 'iij
'I
.'If
';!;!
.'ir
ESP ]





ESP

•

ESP



ESP





u
f.


X


X
r
,j

X



X



X




X





X



X






a
!
j
^ .
i
. j ;
"i
OVF


twr


. ,[
crf

"V





..•fi-
";'
';;-"','
'1^
'^
"'J
''



w



BVF




V .
, *

g
M
4
3
•.

1
•'





•j
-,










*
,-
















|
M
•
i





















X





X



X







5
*J
,c
1
Wl


X


X














'' f








....






' ' '<': • \ . -1

. .;
2§
w
-------
TABLE F-1 (continued)






Utility and plant
Southwestern
Electric Power
Oolet Hills 2

Southwestern
Electric Power
Henry W.
Plrkey 1
Springfield City
Utilities
Southwest 1



Springfield Water.
Light t Power
Dal loan 3


Taapa Electric
Big Bend 4


Tennessee Valley
Authority
Paradise 1

Tennessee Valley
Authority
Paradise 2

Tennessee Valley
Authority
Shawneo 10A





1
•S j

iM
•> >
t U
Ul


720




720


194





205



475




704


704


10






+*
H-
e
V
L.
k.
O
1


New




New


New





New



New




Retro-
fit

Retro-
fit

Retro-
fit







S
Startup


0/8B




12/84


4/77





10/80



12/84




3/82


6/82


4/72



O
U
o
«•»
c
s
0
u
t.
'a
4/1


0.70




0.80


3.50





3.30



2.35




4.20


4.20


2.90





«

"a.
a.
3
L.
o
£


Not
selected



UOP


UOP





Research-
Cottrel 1


Research-
Cottrell



ChMlco


Cheoico


UOP



**
j= i-
U 01
1- C
S£
4-* *O
in
in o
tfc
II
U O»
u c
I/I «l










Burns A
McDonnell




Burns A
McDonnell


Stone A
Webster









Bechtel





P
i.
0
Vt
OJ
s
H-
0
•s







Spray tower


Packed
tower
(•obile-
bed


Coabination
(spray/
packed)

CoMbination
(spray/
packed)


Spray tower


Spray tower


Packed
tower
(•oblle-
bed)



M
01

H.
O
O







4


2





2



4




6


6


1



-
Ol

H-
<*-
41

[







99*


80





95



90




84.2


84.2


85*



o
c
jj
"i

•n
'E
•s
I







Chevron


Chevron





Chevron



Chevron










Chevron






„

f
0







None


e,«nni.
mntV





None



Indi-
rect
hot air


In-line


In-line


Direct
coebus-
tlon


t>

3
U «

II
•s-s
si


ESP







ESP





ESP



ESP




ESP/
venturi

ESP/
venturi










u
u







X


X









X




X


X


X





jf
i.
«
«-*
1
o
S







W


DVT





CY



DVF




OVF


DVF


C








•o
K
O
U.
















X



X




X


X






c
o
*»
«


a
m
VI
f










I



























|

*>
f







IUCS






























Se
o
•i^
«s
%- t.
o o
it










Truck





Truck



Truck




Truck


Truck


Pipeline






^_


•5
c










Landfill





Landfill



Walt board
Manufacture



Landfill


Landfill


Pond




-------
     TABLE  F-1  (continued)






Utility and plant
Tennessee Valley
Authority
Shewn* 10B


Tennessee Valley
Authority
Widows Creek 7


Tennessee Valley
Authority
Widows Creek 8


Texas Municipal
Power Agency
Gibbons Creek 1


Texas Power
t Light
Sandow 4


Taxas Power
4 Light
Twin Oaks 1
\
Texas Power
4 Light
Twin Oaks 2

Taxes Utilities"
Martin Lake 1





f*
a*.
> «?
** u
aa


10




575




550




400




382




750



750


595





44
?

i


Retro-
fit



Retro-
fit



Retro-
fit



Maw




New




New



New


New






S
3
a
3
I



4/72




9/81




5/77




1/82




11/80




8/84



8/85


4/77


M
1
<«.
O
**
«J
c
o
o
L.
S


2.90




3.70




3.70




1.06




1.60




0.70



0.70


0.90




u

t
J{
£


ChaaHco




Coabus-
tion En-
gineering


TVA




Coabus-
tion En-
gineering


Coafcus-
tton En-
gineering


Chaailco



Cheaiico


Research-
Cottrell

I
.c L.
1_ c

E*A
*
X t-
 •!


Bechtel




TVA




TVA




Tippet 4
Gee



Brown 4
Root



United
Engineers


United
Engineers

C.T. Main



1

U
o~
to
0
*

,
Spray tower




Spray tower




Packed
(grid)



Spray tower




Spray tower




Spray tower



Spray tower


Combination
(spray/
packed)



8

•>-
O
S


1




4




4




3




3




3



3


6


X
u
c
€1

«l


fc


85*




84




80




90




92




92



92


98e


fe
**
c
i
"i
¥
v.
o
*


Chevron




Chevron




Chevron




Chevron




Chevron




Chevron



Chevron


Chevron





~
I
•5
H-


Direct
co«bus-
tion


In-line




Indi-
rect
hot air


In-line




Bypass




Bypass



Bypass


Bypass



€>
IV
Is
«J f
u >
ev-o
o*-»
!l


Venturl




ESP/
venturl
(rod)


ESP/'
venturl



ESP




ESP




ESP



CSP


ESP






fe
£
u







X









X
















X



a
o»
c
I
!
"S
*


Off




VF




VF




VF
















C





jj
^
3
8
•o
u
k


X




X



|
I1









X




X



X






1
**
a
3
M
•y
f


































X





i
|
**-
«v
f
VI

















IUCS






















5§
M •*-
S- k.
°s
M M
ii


Pipeline




Pipeline




Pipeline




Truck




Pipeline




Pipeline



Pipeline


Rail






a
M
T»
S


Pond




Pond




Pond




Landfill




•MM!
rwNi




Pond



Pond


Landfill


I
PO

-------
       TABLE  F-1   (continued)


_ .



Utility and plant
Texas Utilities'
Martin late 2


Texas Utilities"
Martin late 3


Texas Utilities
Martin Late 4


Texas Utilities
Montlcello 3


Utah Power 1 Light
Hunter 3

Utah Power i Light
Hunter 4



1
?{
*» as
> *•»
** *o
£3

595



595



750



800



400


400




JJ
«^
o
i.
f
u
O
i

New



New



New



New



New


New





3
a
3
t:

5/78



2/79



0/85



5/78



6/83


6/85

-
•9
O
U
H-
o
44
c
c
0
u
i.
3
f/l

0.90



0.90



0.90



1.50



0.55


0.55



fe

"o.
a.
3
M
i.
^
1

Research-
Cottrell


Research-
Cottretl


Research-
Cottrell


Chemlco



Chemlco


Chemlco

*j
5 t
i. C
E S
«> -o
M
i. 1.
5 S
J3 C
U C
in tl

C.T. Main



C.T. Main



C.T. Main



C.T. Main



Brown A
Root

Brown t
Root

t.
*
S~

"5
1

Combination
(spray/
packed)

Combination
(spray/
packed)

Combination
(spray/
packed)

Spray tower



Spray tower


Spray tower




w
i

o
d

6



6



B



3



3


3

•*
X
u
c
u
V-
1
i
L.
sT

98e



94h



94h



92



90


90

fe '
c
J
•J
M
*E
•s
1

Chevron



Chevron



Chevron



Chevron



Chevron


Chevron
_|



M
|
0


Bypass



Bypass



Bypass



Indi-
rect
hot air

Bypass


Bypass


m
•
u «
1!
•*. f—
o •>
tl

ESP



ESP



ESP



ESP



Fabric
filter

Fabric
filter




u
g
u

X



X



X







X


X


o
Oi
c
t
I
e
S

C



C



c














B
o
3
S
V
S
S.























|
*>
^f
3
*y
f

X



X



X



X



X


X




s
3
^
f
«y*

























• e
•"5
•»- i.
o o
M S
C fZ
H

Rail



Rail



Rail



Pipeline












!
'w
c

Landfill



Landfill



landfill



Pond



Landfill


landfill

 I
CO
        * Effective scrubbed capacity Is  defined as the gross generating capacity tlaes the average fraction of the flue gas that Is scrubbed.
          Abbreviations are as follows:
             BVF - bait vacuuB filter
             C   - centrifuge
             CY  - Cyclone
             DVT - dn» vacuua filter
             SP  - settling pond
             VF  - vacmai filter
        c Supplier Information about actual performance.
        4 within a year, line disposal Kill be used.
        * Ponding Is currently used because Insufficient sludge Is produced for landfill Ing.
          Dry HM It used as a flecculant.
        9 Two lore scrubber andules per unit are being added at Martin Lake Units 1. 2, and 3.  These aodutes will  Increase
          SO, removal efficiency to 95 percent.  Also, an ambient air reheat system and forced oxidation are being  added.
        h Supplier Information about guaranteed performance.
        ' Only 25 percent of sludge It treated by  forced oxidation then vacuum filtered.

-------
                                 APPENDIX G
                          MATERIALS OF CONSTRUCTION

     The  descriptions  of  scrubber equipment  in  Section  3 of  this manua-1
refer to  materials of  construction within the context of equipment  function
and  operation.   A brief analysis  of  the major materials categories is also
given at  the end of that  section.   This appendix gives additional  informa-
tion regarding the major categories of materials, as  follows:
     1.    Base metals  -  composition  of  materials (carbon steels, stainless
               steels,  and  high-grade alloys);  details of  major test pro-
               grams.
     2.    Protective  linings,  plastics,  and  ceramics  -  properties  and per-
               formance characteristics  of organic and inorganic  nonmetallic
               materials.
This appendix  also  includes  experience  histories of the metals  and protec-
tive lining materials  used in limestone FGD systems.

BASE METALS                                                                 '
     In a manner  consistent  with the common  or  trade  names  given in Section
3,  Table  G-l provides the  nominal  chemical  composition of base  metals that
have been used,  are planned  for  use, or have been  tested in limestone  FGD
systems.
Test Programs
     To date, four major  test programs have  been performed  for evaluation of
base  metals  with respect  to performance  and  corrosion-resistance in  wet
scrubbing systems.  These programs,  sponsored by International  Nickel Com-
pany  (Inco), Stellite,  EPA,  and  Combustion Engineering,   have  focused  on
performance  in  wet  lime/limestone  FGD systems.   Results of these test pro-
grams are summarized in the paragraphs that follow.
                                     G-l

-------
                         TABLE G-l.  NOMINAL CHEMICAL COMPOSITION OF BASE METALS
                                       (wt % except as indicated)

Carbon U*e1
«-?85
»ISI 1010
HSl*
Stainless steel (wrought)
00
t-Srlte 2«-l
30*
3041
316
3161
317
3171
Stainless steel (east>
CO-4 MCu
HtoVorade alloy (wrought]
Carpenter t
Carpenter 20
Uddenolm 9041
C1i*ia» 18-2
Incoloy 8?$
Hastelloy G

Haynes 68
Maynes 20

Jessup JS 700
Allegheny Al-61
Allegheny 29-4
NUronfc SO
Hastelloy C
Haste) toy C-276
Mattel loy C-4
Inconel 625
High-grade alloy (cast)
IN-B62
Other «etals
Titanium
Zirconium 702

Nt



0.44



9.5
10.0
13.0
13.0
14.0
14.0

5. 5
4.25
34.0
25.0
0.4
42.0
4S.O

2.0
26.0

25.0
24.0
O.I
12.0
54.0
54.0
Ja lance
60.0

24.0




le

Balance
Balance
Balance

Balance
Balance
Balance
Balance
Balance
fla lance
Riiliincr
Balance

61.0
66.0
39.0
47.0
72.0
30.0
20.0

2.0
42.0

46.0
46.0
66.0
54.0
5.0
5.0

5.0

44.0

0.13
0.05

Cr



0.9?

16-1R
26.00
ID. 5
in. 5
17.0
17. 0
19.0
19.0

26.0
27.0
20.0
20.0
10.5
21.0
22.0

29.75
22.0

21.0
20.0
29.0
22.0
15.5
15.5
15.7
21.5

21.0


0.05

MO






1.00


2.25
?.?5
3 ?5
1 ?5

2.0
1.40
2.50
4.50
2.0
3.0
6.5

1.0
5.0

4.5
6.5
4.0
2.5
16.0
16.0
14. A
9.0

5.0




(11

•0.15

0.41










3.0
0.10
3.30
1.50
O.?0
i.no
2. no


















(.

0.35
0.1
O.OB

0.12
0.0?
•fl.OH
•0.01
•O.OH
•0.03
O.Ofl
0.03

•0.04
0.05
'0.07
-0.02
0.016
0.03
0.03

•1.0
'0.05

0.03
'O.P25
0.004
0.03
• o.on
•0.0?
O.OOR
•0.1

•0.04

0.02?
0.015

'» i



0.46

1.00
0.?5
• .0
• .0
• .0
• .0
• .0
.0

•1.0
0.38
0.60
0.35
0.40
0.35
0.35

0.36
•1.0

0.5
•0.5
0.01
1.00
•1.0
•0.05
0.04
'0.5

O.H




Mn

•0.3

0. II

1.00
0. IS
1.5
1.3
1.7
I.B
•2.0
•2.0

•1.0
0.50
0.30
1.70
0.4
0.65
1.30

1.4
'2.5

1.7
«l.5
0.10
5.0
• 1.0
. 0.12; Tl. balance
*. 0.05; "'. <0.l; co^lned
Ir and Hf, -99.2
o
IS}

-------
MATERIALS OF CONSTRUCTION:   Base Metals	G-3
     Inco Test Program (Inco 1980).     Inco   has   undertaken   an  extensive
program  to  test  the  performance  of  the  iron-nickel-chromium-molybdenum
family of alloys  in  lime/limestone FGD systems.  Some important  trends dis-
covered in  this work are illustrated in Figure  G-l, which  shows correlations
between the degree of  localized attack on 316L  and 3171  stainless steels  and
the  pH and chloride  levels  of  the  scrubbing  liquor.   Localized  attack
includes  both  pitting  and  crevice  corrosion.   The  data   indicate that
localized corrosion  increases  as  the chloride level  increases and scrubbing
slurry  pH  decreases.    Moreover,  at  a  given  slurry  pH,  the  environment
becomes more aggressive  as  chloride level increases,  or at a  given  chloride
level  as  pH decreases.   Thus,  low-pH/high-chloride environments are  the most
aggressive  with  respect  to  corrosion.    Although   the  data do  indicate
definite  correlations,   the  scatter minimizes  their  usefulness   in  specific
design.
     Inco also conducted a  series of intensive  corrosion studies  on  Module A
of  the Cholla  1  FGD  system  of  Arizona Public  Service.   In these  studies
various alloys  were  tested in  six  exposures  at six locations  in  the  FGD
system.  The test results  showed that susceptibility to pitting  and crevice
corrosion is the  principal  criterion for judging the  suitability of a metal
to  resist the corrosive  action of these  environments.   Pitting was  observed
in  316 stainless steel, Carpenter  20,  and Incoloy 825.   Deep   pitting  was
observed  in  304  stainless  steel.    All  the remaining  materials displayed
complete resistance  to corrosion.
     Stellite Test Program (Leonard 1978).   The Stellite  Division  of  Cabot
Corporation has  performed  a series  of spool tests under  conditions similar
to  those  in the  tests  performed by  Inco.   On the basis  of the test  results,
Stellite developed a  set of guidelines for  service application of the mate-
rials tested.   These guidelines are summarized in Table G-2.
     EPA Test Program (Crow 1978).    Two   10-MW   lime/limestone  scrubbing
systems in  service at  the  Shawnee test facility have  been used in the evalu-
ation  of  the materials  of  construction.   Specimens of  alloys  and  nonmetallic
materials have been  exposed at test locations in the  scrubber systems having
different degrees of  severity  of exposure.   In  the most  recent tests, speci-
mens were  exposed to modified slurries, and alloys of  different molybdenum
contents were evaluated  for resistance to pitting.

-------
10C.OOC
10.00C
 i.oo:
&
t
  IOC
   10
         SEVtRt PITTING OR
         CREVICE CORROSION,


                    /C
              oB*'p*
        SOHriWS SEVERE CX  rf
        piniHi o» CREVICE.
              CORROSION
                     X
                  /
  '   NOT SEVERE
/   OR CREVICE COR»:SION
    1  I    I    I    I    I    I
          1   3   «    S   6   7   t t.S
      a.   316L  stainless steel
                                               O  PENETRATION RATE. « 10 »1U/yr


                                               £  PENETRATION RATE. 10 to 2S nlU'jrr


                                               O  PENETRATION RATE. > 2$ •HW/r


                                               C  ESTIMATE


                                               I  pM VARIED CONSIOERABLT


                                               NOTE:  DASHED LINES INDICATE THAT
                                                     ZONES ARE NOT ClEARlT QEMNEO
                                                     10.000
                                                     1.000
                                                        10
                                                         1
                                                           SEVERE PITTING OR
                                                           CREVICE CORROSIOly
                                                                             00
                                                                    srvtRt
                                                                    IHG o»
                                                                 CREVICE
^
                                                                                   o   -

                                                                       W SEVERE PITTING
                                                                      OR CREVICE CO«R:SION
                                                                   1    1    1    1     1
                                                            1    2    3   4   &    (   ?   8

                                                                          P"



                                                           b.   317L  stainless steel
  Figure  G-l.   Effects of pH  and  chlorides  on  stainless steel  alloys.


 Source:   International   Nickel Company,  Inc.
                                            G-4

-------
TABLE G-2.  SERVICE APPLICATION GUIDELINES BASED ON
               STELLITE TEST RESULTS
Service
condition
Maximum
temperature, °F
PH
Chloride
content, ppn
316L
stainless
steel
150
Over 5.5
Under 1500
Haynes 20
150
Over 4.5
Under 3000
Haste! loy
G
110-160
3.5-6.0
100-5000
Haynes 625
110-160
3.5-6.0
1000-5000
Has tell oy
C-276
>160
Under 3.5
Over 5000
                        G-5

-------
MATERIALS OF CONSTRUCTION:  Base Metals	'	G-6
     The test conditions  are  given in Table G-3,  and the test data are  shown
in Table G-4.
     The alloy  test  specimens  were least attacked by corrosion  in the  inlet
flue gas duct,  where the gas temperature was maintained above the dewpoint.
The corrosion rates  of carbon steel and Cor-Ten  were only 3 mils  per  year.
                                             * „- S  ,--~~~    =A.~
Cooled and humidified  flue  gas was much more corrosive.   In an  earlier test
in  which  humidification  sprays were  used, the  corrosion  rate  for carbon
steel  was more than 330 mils per year.
     A section  of  316L stainless steek inlet duct^which extends about  7 in.
inside  the  chamber  immediately above  the  venturi,  has-(undergone  chloride
stress-corrosion cracking.  The  cracks  originated on the outside  surface  of
the duct  that  extends into the  scrubber  chamber,  and  some penetrated  the
wall.   The  residual  stresses  produced  by  cold-forming  and  welding and the
cyclic  stresses produced  by   vibrating  equipment could have added to  the
severity of  the cracking.  The outside surface  of  the  section  that  failed
contained a  tightly  adhering  scale approximately 51 mils  thick.   Also, pits
to depths of  39 mils were found under the  scale,  which  contained 0.85 weight
percent of chloride  as calcium chloride.   Splashing  of  the process limestone
slurry occurred  at the adjustable plug, causing deposition  of wet solids  on
the surface  of   the  duct.   The  slurry  contained  400 to  2440  ppm chlorides,
and the  high temperature of  the  duct  wall  (260° to  330°F) caused concen-
tration of chlorides  in the duct.
     Below the  venturi throat,  the  greatest attack  on the  specimen was  by
erosion-corrosion.   The  specimens  of  mild  steel and Cor-Ten  were completely
destroyed,  with  penetration rates  greater  than  1850  mils per year.  The most
promising alloys in  the order of  decreasing resistance  to erosion-corrosion
were Zirconium  702,  Haynes 68,  317L  stainless  steel, 316L  stainless  steel,
Allegheny 29-4,  Allegheny AL-6X, and Climax 18-2.
     In general,  the  attack on  carbon  steel and Cor-Ten has  varied greatly
in the scrubber tower  during  the test program.   In earlier tests, the corro-
sion of specimens  in the top  of the tower  was greater than that  of specimens
near the middle and  bottom.   During the fourth  series,  however,  corrosion  of
the specimen  near  the  mist  eliminator diminished,  very likely  because the

-------
  TABLE G-3.  OPERATING CONDITIONS DURING CORROSION TESTS AT THE SHAWNEE
                               TEST FACILITY  .
Location
Inlet duct
Venturi throat
Bottom of scrubber
Top of scrubber
Downstream of
mist eliminator
Downstream of
reheater
Physical parameters
Temperature, °F
275-300
80-170
125-130
125-130
125-130
125-130
Velocity, ft/s
32-67
40-100
4.5-9.4
4.5-9.4
4.5-9.4
32-67
Gas flow rate,
103 acfm at 330°F
17-35
15-30
15-30
15-30
15-30
«$v»-
--- 17-35
         Chemical composition



Constituent
S02
co2
02
H2°
HC1
N2
Fly ashb

Content
at inlet
duct, vol '»
0.2-0.4
10-18
5-15
8-15
0.01
74
2-7
Content
downstream of
reheater,
vol 'K
0.02-0.10
11-19
6-16
9-16

69
0.01-0.04



Species
S03"
co3"
S04"
Ca++
Mg++
Na+(K+)
ci"

Scrubber
effluent
slurry,3 ppm
40-3900
5-150
400-1400
540-3000
. 100-5500
60-110
3300-5700
Ionic composition
Slurry pH ranged from 4.3 to 6.5, temperature ranged from 90°  to 130°F,
suspended solids content ranged from 8 to 18 wt %,  and dissolved solids
content ranged from 0.5 to 8.4 wt %.

Measured in gr/scf.
                                    G-7

-------
    TABLE G-4.  DATA FROM CORROSION TESTS AT THE SHAWNEE TEST FACILITY





Hastelloy C-276
Inconel 625
Hastelloy G
Haynes 6B
Multimet
Nitronic 50
AL 29-4
Jessop 700
317L
Nitronic 50M
Climax 18-2
316L (2.3f, Mo)
Zirconium 702
AL 6X
31 6L (2.8C.- Mo)



No. Of
tests
12
12
12
14
10
9
14
12
9
18
12
18
15
13
14


No. of
pitted
specimens
0
0
0
1
1
1
3
3
4
6
8
10
0
3
9

Maximum
depth
of pits,*
mil



9
4
b
2
7
b
4
14
8

17
2
No. of
specimens
with
crevice
corrosion
0 .
0
1
2
0
3
2
2
2
5
5
8
0
3
8
No. Of
specimens with
pitting and/or
crevice
corrosion
0
0
1
3
1
4
5
5
6
11
13
18
0
6
17
Values show the actual  depth of penetration during the test period.
Minute pit.
                                   G-8

-------
MATERIALS OF CONSTRUCTION:   Base Metals	G-9
automatic spray system  for  mist eliminator washing also washed the spool of
test specimens located immediately below it.
     The scrubbed  gas,  which was  reheated to  about  250°F,  corroded carbon
steel and Cor-Ten  at  a  rate of 4 mils  per year.   Minute pitting and crevice
corrosion occurred on  some of  the stainless  steels.   The  316L stainless
steel  stacks  were corroded  at  higher  rates  than  specimens  of  the   same
material  suspended inside  the  stack.   This  may be  attributable  to  the
operating conditions when the  temperature of the  specimens is a few degrees
higher  than  the stack walls, even  though the stack is insulated.  Further,
during  any  outage  of the  reheater,  corrosive  condensate drains  down the
stack walls  without flowing over  the   specimens  suspended in the center of
the stack.
     Combustion Engineering Test Program (Lewis  1978).    The   test  program
initiated by  Combustion  Engineering consisted of  corrosion tests of numerous
ferritic and  austenitic iron-  and  nickel-base  alloys  in  both the field and
laboratory.    In  the  field,  these  materials  were exposed  as  coupons at
several   locations  in  commercial  FGD systems.  In  addition, full-scale opera-
tional  scrubber  components  made  of carbon steel,  304  stainless steel, 316
stainless steel, and  316L  stainless steel were  inspected  for corrosion.  In
the  laboratory,  loop  and  bench-scale  tests  were  conducted  to  characterize
the  influence of  high-chloride  slurries  on the  same materials  that  were
tested as spools in the field.
     Various  components  of  eight operating commercial-scale FGD systems and
one demonstration  system were  systematically inspected  for materials  perfor-
mance.   The  inspection  included  removal  of selected component  sections and
subsequent  metaHographic  analyses  for  susceptibility  to pitting,  stress-
corrosion cracking, crevice corrosion, and general corrosion.   The  scrubber
slurries typically contained 600 ppm  chloride  ions  and   10 percent  solids,
with a  pH range of 5  to 6.5 at 130°F.   The  inspections revealed that  carbon
steel  had   poor general  corrosion  resistance   except  in reheaters,  where
resistance was  good.   Some  general corrosion  and often   severe pitting and
stress-corrosion cracking  were found  in components fabricated  of  stainless
steel.   Components of  316  and 316L  stainless steel  were  free of  stress-
corrosion cracking and  had  only minor  pitting.   The period of exposure of

-------
MATERIALS OF CONSTRUCTION:   Base Metals	G-10
these  materials  in  a  commercial system  was  2-1/2 years.   The spool  tests
indicated  that 309,  18-8-2,  and  304L stainless  steels  were attacked more
severely  than 316L  stainless  steel,  317L  stainless steel,  18 Cr-2Mo-Ti,
Alloy 20 Cb3, and Alloy 825.
Performance, Economic,  and Fabrication Considerations
     In  view  of  the test  results just  described, together  with industrial
experience, 316L stainless  steel  is  finding widespread use  as  a material  of
construction;  however,  under  certain  conditions of  scrubber  liquor pH,
temperature, and chloride  content, this  alloy can undergo  localized  attack.
Under  these  more  stringent  conditions,  nickel-based  alloys  with  higher
molybdenum  and chromium content  are  superior to  316L  stainless steel.   It
has been  shown that  resistance  to stress-corrosion cracking is  achieved with
higher  nickel  contents  because nickel  accelerates  repassivation  of the
metallic  surface (Rhodin and  Carson  1959).  Although  more expensive  initial-
ly, these  high-grade alloys may be economically  justified for use in  certain
severe scrubber environments.
     The  beneficial  effect of  molybdenum  content is shown in Figure G-2,
which  indicates that the  resistance  of alloys to  pitting and crevice corro-
sion  generally increases  as  the molybdenum  content  increases  (Crow  1978).
Moreover,  molybdenum  content  alone  does  not  ensure  resistance  to  localized
attack.   Chromium content  is  also a  consideration, as indicated in  data from
the  Inco  test program  (Hoxie  and Michaels  1978), shown  in Figure G-3 and
confirmed  by  the  Shawnee  test  program  results  plotted   in  Figure G-4  (Crow
1978).  These  data  seem to indicate  that the  resistance  of various  alloys  to
localized  corrosion  is a  function  of the  combined  molybdenum  and  chromium
content.
     The  high  nickel,   molybdenum, and chromium  contents of the high-grade
alloys generally  make  them more  expensive than  316L  stainless steel.   As
shown  in  Table G-5, most  of  these  alloys  are stronger  than  316L  stainless
steel and  thus can  be  used in  thinner plates  with resultant cost reductions
that  reduce the  price differential  (Leonard 1978).   Figure G-5 plots the
weighted  price ratio  against  the  Shawnee test  results,  demonstrating the
potential  advantage  of Nitronic  50  over 316L  stainless steel  (Crow 1978).

-------
   100
    90
QC
O

«   8°
CJ
O

«X

C5
70


60
=  50
40


30
ft
            2*
             1
                                   10
                                                       11
                          316L  STAINLESS  STEEL  (2.3*  Mo)
                          CLIMAX  18-2
                          316L  STAINLESS  STEEL  (2.8°«  Mo)
                          216 STAINLESS STEEL
                          317L  STAINLESS  STEEL  (3.2'«  Mo)
                          NITRONIC  50
                          AL 6X
                          AL 29-4
                          HASTELLOY G
                          INCONEL 625
                          HASTELLOY C-276
                                8
                                  10
                                         12
14    16
                       MOLYBDENUM CONTENT,  wt.
            Figure G-2.  Effect of molybdenum content
         on resistance to pitting and crevice corrosion.
                               G-ll

-------
    §
    s

    a
    S
    I
JO


29

28


27


26


2S

24


23

22


21


20

19
                                      I      I       I
                                      1  3161 STAINLESS STEEL
                                      2  INCOLOT 82S
                                      J  UOOEHOLM 904L
                                      4  CO-4MCU
                                      S  IN-B62
SPECIMENS Of HASTELLOV c-«, NASTELLOY c-276.
AND  INCONEL 62S bERE ALSO TESTED.  NO PITTING
OR CORROSION OCCURRED.  THE COMBINED MOL»8-
OENi* AND CMROMII* CONTENT or EACH SPECIMEN
CiCEEDEO 30 ml. t.                    ,
                  I
                         I
                               I
                                      I
                                            I
                 10     20     10     40     SO     60
                   GENERAL  CORROSION PLUS NAUMUM PITTING
                      AND  CREVICE CORROSION, ntls/yr
                                                70
     Figure G-3.   Effect  of  molybdenum  and  chromium
     content on corrosion resistance (Inco  tests).
                                3161 STAINLESS STEEL (2.31
                                316L STAINLESS STEEL (2.8*. Mo)
                                31?L STAINLESS STEEL
                                N1TRON1C SO
                                AL 61
                                HASTULOY G
                                INCONU 62S
                                HASTELLOT C-276
            10   20    30    40   SO    M    70   80    90

               SPECIMENS H1TH PITTING AND CREVICE CORROSION. S
                                                            100
Figure  G-4.   Effect  of molybdenum  and  chromium  content
          on corrosion resistance (Shawnee  tests).
                                 G-12

-------
       TABLE G-5.   PRICES OF HIGH-GRADE ALLOYS AND STAINLESS STEELS


Hastelloy C-276
Haynes 625
Inconel 625
Incoloy 825
Hastelloy G
Haynes 20
Nitronic 50
JS-777
Uddeholm 904L
317L stainless steel
316L stainless steel
Price of
1/4- in.
plate,
$/ft2
82.42
59.95
70.95
46.2
46.71
30.36
22.28
31.80
32.50
20.54
16.50

Price ratio
(316L SS = 1.00)
5.00
3.63
4.30
2.80
2.71
1.84
1.35
1.93
1.97
1.25
1.00

Design
stress
ratio3
1.60
1.60
1.76
1.76
1.60
1.20
1.60
1.20
1.20
1.20
1.00

Weighted
priceb
ratio
3.12
2.27
2.44
1.59
1.69
1.53
0.84
1.60
1.64
1.04
1.00

Weighted
price,
$/ft2
51.48
37.45
40.26
26.24
27.88
25.24
13.86
26.40
27.06
17.16
16.50
Design stress is expressed in pounds per square inch at 100°F and represents
the maximum operating stress recommended by the American Society for Testing
and Materials.   Type 316L SS = 1.00.

Weighted price ratio represents the price ratio divided by the design stress
ratio.

Weighted price represents the price times the weighted price ratio.
                                    G-13

-------
(X

UJ
CJ

oc.
D.

O
X
cs
                                       316L  STAINLESS  STEEL  (2.31  Mo)
                                       316L  STAINLESS  STEEL  (2.8X  Mo)
                                       317L  STAINLESS  STEEL
                                       NITRON1C  50
                                       HASTELLOY G
                                       INCONEL 625
                                       HASTELLOY C-276
     0.8
        0    10     20    30     40    50     60    70     80    90    TOO

      SPECIMENS WITH LOCALIZED ATTACK (PITTING AND CREVICE CORROSION),*


                 Figure G-5.  Weighted price ratio and
                         corrosion resistance.
                                  G-14

-------
MATERIALS OF CONSTRUCTION:   Protective Linings	G-15
     The  use  of  high-grade alloys demands careful  fabrication.   Specifical-
ly,  the welding  recommendations of  the alloy producer  should be  followed
precisely.  Failures of  high-grade  alloys generally occur because of  faulty
welding  rather  than corrosive attack  by the flue gas being treated.   Fail-
ures  in  rotating  parts  can  be  traced  to  fatigue.   Some alloys  (Uddeholm
904L,  Carpenter  20) may suffer  pitting when welded.  Furthermore,  delivery
and availability  in suitable  form (sheet, plates,  tubes,  etc.)  on  a commer-
cial scale  may  present  serious limitations with some other alloys—Allegheny
6X, Allegheny 29-4 (Uddeholm Steel Corporation 1977).

PROTECTIVE LININGS
     The  utility  industry  uses  two major types  of  protective   linings:
organic  and  inorganic.   Organic  linings  include resin, rubber,  and  plastic.
Inorganic  linings  include  bricks,  ceramics,  and  concrete.   These  various
lining materials are briefly discussed in the following paragraphs.
Resin and Rubber Linings
     Polyester,  vinyl  ester,  and  epoxy are commonly used  in  utility  FGD
systems.   Polyester resins are  generally known  for their excellent  resis-
tance  to acid and  good  resistance  to heat and abrasion.   Vinyl  resins have
been improved  to  the point that properties of the vinyl  esters are  typically
better  than  those  of  polyesters  (Singleton  1978).  Epoxy   resin  coatings
generally have  less  resistance to acids than do  other resins, but  adhere to
metals  better  and have  higher tensile strength and good  elastic properties.
Bituminous  and furan  resins   are less  widely  used.  Table  G-6 shows some
physical characteristics of the resins.
     Among  several  types  of   rubber  liners, black  natural  rubber  and syn-
thetic  neoprene  rubber  liners are  most  commonly used in  limestone  FGD sys-
tems.   Natural  rubber  is softer and more  resilient and  has more tear resis-
tance  than  neoprene rubber.   Neoprene,  however,  provides   more  corrosion
resistance  and  can  withstand  higher  temperatures.   Table G-7 shows some of
the characteristics  of both materials  (Fontana and Greene  1967).

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TABLE G-6.   TYPICAL CHARACTERISTICS OF RESINS

Furan
Tensile strength, TO3 psi
Coefficient of expansion,
105 in. /in. per °F
Barcol hardness
Temperature resistance, °F
Flexural strength, 103 psi
Abrasion resistance, Taber
Wear Index
Epoxy
Tensile strength, 103 psi
Coefficient of expansion,
10s in. /in. per °F
Barcol hardness
Temperature resistance, °F
Flexural strength, 103 psi
Abrasion resistance, Taber
Wear Index

Vinyl ester.
Tensile strength, 103 psi
Coefficient of expansion,
10s in. /in. per °F
Barcol hardness
Temperature resistance, °F
Flexural strength, 103 psi
Abrasion resistance, Taber
Wear Index
Polyester
Tensile strength, 103 psi
Coefficient of expansion,
10s in. /in. per °F
Barcol hardness
Temperature resistance, °F
Flexural strength, 103 psi
Abrasion resistance, Taber
Wear Index
Resin alone

1.20
2.0

Not reported
350
3.80
Not reported


1.80
3.00

Not reported
175
3.80
Not reported



2.30
1.60

Not reported
180
4.20
Not reported


2.30
1.90

Not reported
225
4.80
Not reported

Resin with
glass flakes

1.25
1.40.

28
125
2.66
83


3.35
1.50

40
160
6.74
129



2.30
1.50

38
160
6.00
167


2.05
1.50

42
160
6.10
177

Resin with
fabric mat

8.15
1.50

20
125
19.85
57


3.40
1.90

45
180
9.50
140



6.70
1.50

50
160
10.50
185


6.60
1.50

52
160
12.20
187

                      G-16

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  TABLE G-7.   CHARACTERISTICS OF BLACK NATURAL RUBBER AND NEOPRENE RUBBER

Hardness range3
Tensile strength,3 103 psi
Maximum elongation, %
Abrasion resistance
Maximum ambient temperature
allowable, °F
Resilience
Aging resistance
Flame resistance
Tear resistance
Natural rubber
40-100
4.50
900
Excellent
160
Excellent
Good
Poor
Excellent
Neoprene rubber
30-90
3.50
1000
Very good
225
Very good
Excellent
Good
Good
Indicates values for soft rubber;  values run higher1 for hard rubber.
                                   G-17

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MATERIALS OF CONSTRUCTION:   Protective Linings	G-18
     Tests of black  natural  rubber and neoprene  rubber  liners  at  the  Shawnee
test facility show that  natural  rubber Is  superior  (Crow  1978).   The  natural
liner withstood  the  design  scrubber  environment,  with no  signs of  general
corrosion or erosion.  Neoprene  rubber liners did show wear  from erosion  in
the area where  the  flue  gases entered the  scrubber.   The  neoprene liner also
formed some blisters  after 3 years of operation.
     Rubber liners do  have  disadvantages.   They  are susceptible  to adhesion
losses, mechanical damage,  abrasion  wear,  and fire.  Overheating can  cause
adhesion losses  and  exposure  of  the substrate to the corrosive environment.
Rubber  liners  can be  torn  or cut  by material  in  the  flue gases or during
operation, installation,  or  removal  of other equipment.   Natural rubber can
withstand  abrasion  better  than  neoprene  rubber,  but  neither material can
withstand the abrasion in  the venturi throat.  Because  rubber  liners  are not
flame-resistant,  extreme care must  be exercised  when  welding  near  them.
     The  most   important considerations  in  selection  and use  of  organic
coating materials include testing,  surface preparation, coating application,
and engineering  design.   As  with  the metallic materials, a series of  tests
should  be  conducted  in  both  the  laboratory  and  the field.   The  three  vari-
ables  to  be  considered  in  testing  are  type of coating,  thickness  of the
coating, and degree  of surface preparation.   Surface preparation  is probably
the most  crtical factor  in  performance of  protective  coatings.   Two  prime
sources of  information regarding  the preparation of  steel for  application  of
a  coating  are  the   National  Association  of  Corrosion  Engineers   (NACE,
Houston,  Texas)  and  the   Steel   Structures  Painting  Council  (Pittsburgh,
Pennsylvania).     Detailed    specifications   for   surface  preparation  are
summarized  in  published  literature.    The  advantages  and  disadvantages  of
various  techniques   for  application  of  the  coating  after  the  surface  is
prepared are summarized  in  Table  G-8.   The key to the life  and effectiveness
of  an  organic  lining  surface  under  corrosive  conditions  is design.  The
following  elements  of good  design  can extend  the  performance  of  coating
materials.  Flat surfaces  are preferred,  and round  or  curved surfaces are
preferred  to  sharp  angles.    Crevices should  be  avoided, and  surfaces made
smooth.   For  example,  welds  should be ground smooth, and  splatter  removed.
Similarly,  scaffolding brackets  and  other items should  be removed,  and the

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                                   TABLE G-8.  COATING APPLICATION TECHNIQUES
                                  Positive features
                                                       Negative features
    Brush
    Roller
    Conventional
o
vo
    Airless
Maximum paint saving; no overspray or
fogging; minimum masking; minimum capital
investment
Greater area of coverage than brush;
minimum masking
Safe; minimum film thickness available;
equipment is strong; minimum solvent en-
trapment; pattern can be changed by adjust-
ment of air flow; mist coats and feathering
possible, parts less expensive than airless

Maximum production; reasonable thickness
control; cleaner air, less fogging; simple
pattern control; simple rigging; generally
applicable without thinning
Questionable film control; requires human
effort and skill; very costly on large
areas (1000 square feet of coating per
day can be applied)

Questionable film control; requires human
effort and skill; costly (2000 square
feet of coating per day can be applied)

Maximum material loss; maximum power
loss; maximum heat loss and cleanup time;
pressure head loss (lose 0.6 psi per foot
vertical rise); control in hands of
operator; compressor and pots are heavy

Inherent problem called spitting; hand
trigger control; pressure variations

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MATERIALS OF CONSTRUCTION:  Protective Linings	G-20
surfaces from which  they  came should be ground smooth.  Welding is  preferred
to  riveting,  and continuous  welding is preferred  to  skip or  spot  welding.
Lapped  edges should be  welded.   Where  rivets  are  used,  they  should be
countersunk.  Faying surfaces  (surfaces  that rub  against  one another)  should
be  avoided.   Flexing surfaces should be  avoided  or should be  given special
coatings that will not  crack or  spall.   All  surfaces should be  well  drained.
     Although difficulties  with  organic  lining materials are  sometimes  due
to   improper  application  and  poor  design,  selection  of   inappropriate
materials is a  principal  cause of failures.   Experience  in recent  years  has
enabled designers to categorize  the basic types of  failures and to  recommend
selection  of materials  on  the  basis  of  specific properties.   Table  G-9
summarizes  the  types and causes of  failure and the  basis  for  appropriate
selections.
Plastic Linings  (Kensington 1978; Furman 1977)
     Plastic  linings can be  made of tetrafluoroethylene (TFE, or  Teflon),
fiber-reinforced  plastic  (FRP),   and Armalon.   The most  chemical-resistant
plastic commercially  available  is  TFE,  which is unaffected  by  all alkalis
and  acids   except  fluorine  and  chlorine gas  at  elevated temperatures.   It
retains its  properties  up to 500°F.   Fluorinated  ethylene-propylene (FEP),  a
copolymer of TFE  and hexafluoropropylene,  has properties  similar to those of
TFE  except  that  it  is  not recommended  for continuous  exposures above  400°F.
An  advantage  of FEP  is that  it can be extruded on conventional  equipment,
whereas TFE parts must  be made  by complicated powder-metallurgy  techniques.
Chlorotrifluoroethylene  (CFE),  another  derivative  of TFE,  also  possesses
excellent corrosion resistance to all acids and alkalis up to  690°F.
     Plastics are reinforced  by  addition of  fiberglass, which  is supplied as
roving  or  reinforcing  mat.   Roving  fiberglass  is  a rope-like  bundle  of
continuous  strands,  and  the  reinforcing  mat consists  of chopped  strands.
Complete  FRP construction   is   being  used  in  FGO  systems,  especially  for
piping, tanks,  and scrubber  internals (liquid distributor, mist eliminators,
spray headers,  etc.).   The  most  commonly used resins are  epoxy, vinyl  ester,
furan,  polyester,  and  chlorinated  polyester.  During  pilot-plant  tests  at
Shawnee (Crow 1978), 12  specimens of each of these five  types were tested.

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                       TABLE  G-9.    CAUSES OF  ORGANIC  LINING FAILURES  AND
                             PROCEDURES RECOMMENDED TO  PREVENT  FAILURE
Type Of failure
Causes of failure
Procedures  recommended to prevent failure
Chalking
Checking tnd
 cracking
All ignoring or
 mud crocking
Blistering


Peeling




Undercutting
lr.tfco.il
 de laT.i nation
Abrasion Or
Chemical e«posure
fungal or
 bacterial  attack
Disco)ora:ion
Action of actinic rays from tun on
basic resin;  improper pigmentation

Gradual coating shrinkage caused by
withering, heating. and cooling;
continued polymerization or oiidation

Application of hard coating over •
softer coating; continued polymeriia-
tion and shrinkage of a coating from
the surface toward the interior

Moisture vapor transmission (MVT),
osmosis, electroendosmosis

Poor inherent adhesion, reaction
• ith subs-trate. high HvT. dirty or
contaminated  substrate
Poor inherent  adhesion;-high permea-
bility to nater, oiygen, and salts;
smooth, nonporouS Substrate
very rapid and  complete curing o?
coating,  strong cross linking to
insolubility, contamination of Surface
Ir'.pdCt, continued  rubbing, or abrasion
fror. vibration;  rubber-tired vehicles
on floors;  movement or handling of
equipment

Reaction or solution of coating caused
tj fumes from or contact «ith acids,
alkalis, solvents, etc.

Coating formulated with biodegradable
oil, plasticiiers, or resins (bacte-
ria and fungus organisms feed on these
ingredients)

Sunlignt, ultraviolet light, and
•eatner; coating resins «ith reactive
parts that  break do«n or change in
Select • coating composed of weather-resistant rtsins such •»
acrylics and high-hiding, noncatalytlc ptgntntt

Select coatings fornultttd with reinforcing pigments in
addition to colored pigments
Select • coating with high adhesion; never apply hard, tough
coating such as an epoiy on a softer primer or undercoat
such as • petroleum resin
Select coatings  such as epoiies and vinyls  with high
adhesion and Ion HVT

Select a coating with strong adhesion and Ion MVT  that is
inert to Substrate; assure clean surface for  coating
application; apply over primer Kith strong  adhesion to
Substrate such as  inorganic zinc priners

Select a coating »ith lov permeability to moisture.
oiygen, and chemicals; assure maiimum adhesion by  clean.
sandblasted, or  otherwise abraded surface;  apply over
primer »ith strong adhesion Such as inorganic line primer

Select coating that  is soluble in its Own solvents such
as vinyl lacquer; apply second coat before  first coat  is
cured to insolubility, or before eiterior contamination
occurs

Select a coating «ith high abrasion resistance  such as
pol/uretnane, »Hh very strong adhesion such at  an epo*/,
or >ith resilience and impact resistance sucn es a vin/1
Select a coating for  the  specific chemcal eiposure;  all
ingredients should be resistant to the chemical  involved.
a coating test is suggested  for positive results

Select coatings that  are  completely inert to biological
degradation or coatings that contain permanent bacten-
acidat or funguioal  additives
                                                          Select coatings  »Hh  light-resistant resins Sucn  as
                                                          acrylics, or those containing opaque pigmentation or
                                                          ultraviolet  absorbers
                                                        G-21
                                                                  Reproduced  from
                                                                  best  available  copy.

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MATERIALS OF CONSTRUCTION:  Protective Linings	    G-22
All  the  specimens made  with  epoxy and  vinyl esters were in  good  condition
after the  test.   Of  the specimens made with furan,  11  were in good  condition
and  1  in  fair condition; of those made with polyester, 8 were in good condi-
tion and  4  in fair condition; and of  those made with  chlorinated polyester,
2 were in fair condition and 10 in poor condition.
     The  basic causes  of  FRP  failure  include   lack  of  specifications  and
inspection,  poor  workmanship,   incorrect   resin  selection,  mishandling  of
equipment during  shipment  or  installation,  and use  of  equipment beyond rated
conditions  (Kennington  1978).   It  is  recommended that Standard  PS  15-69  of
the  National  Bureau  of  Standards  (a product of the  joint effort of  the resin
suppliers,  the fabricators, and  the Society of Plastic Industry) be  set  as
the  minimum  specification.  This  standard may be modified to satisfy partic-
ular  requirements for  obtaining  quality FRP equipment.  The bids must  be
evaluated on the basis of  a  thorough  review of  the supplied  design.   Plant
inspection of equipment before shipment is  advisable.
     The purchase  cost  of  FRP is less than  that  of metal.   In addition,  FRP
weighs  less than  metal alloys  and thus  reduces  freight  and  installation
costs.   Unlike steel  structures,  FRP  constructions  are  not  usually  degraded
by  exterior  corrosion.   The  maintenance   cost  for exterior  protection  is
therefore reduced.   In  spite  of  these  advantages,  FRP  has  some limitations.
The  temperature  restriction,  based  on  currently  available resins,  is  250°F.
Under  current American  Society   of  Mechanical  Engineers  (ASME) Codes,  FRP
tanks are  not available  for  use at pressure above 1  atmosphere.   Although
FRP  products have good  chemical  resistance,  FRP is not recommended  for  use
in the presence  of nitric acid,   hydrofluoric  acid,  sodium hydroxide (30% or
more),  and sulfuric acid (50% or  more).
     Despite outstanding physical properties  that make them  desirable corro-
sion barriers, TFE and  FEP cannot be bonded to a metallic tank wall  readily,
cannot be  used  under  vacuum  or in conditions   of  agitation,  and   are  very
expensive.    A  recent  composite material, designated ArmaIon, is being devel-
oped for  production  of  equipment with an inner  surface of TFE  and  an outer
surface of   FRP  (Furman 1977).    The essential features  of  Armalon are  its
integral  TFE-fabric  bond and  its ability   to  form  a tight  bond  with  either
FRP  or metal.  Armalon  is  claimed to allow  service  under vacuum or  agitation

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MATERIALS OF CONSTRUCTION:   Protective Linings	G-23
and under  extreme  thermal  cycling without linear delamination.  Armalon/FRP
is reported to  have  withstood  combinations of  hydrochloric acid, hydrobromic
acid,  and  traces of  hydrofluoric  acid at  194°F for over  1 year with no signs
of degradation.   It  also  handled  sodium hydroxide  (20%)  and sulfuric acid
(10%)  at 176°F  for 6 months  with  no  apparent deterioration.   The material  is
claimed to  be  inert  at temperatures  up to 300°F,  in wide variations of pH,
and in  the presence  of  chlorides.    Also, the initial price of Armalon/FRP
equipment  is claimed  to  be very competitive with that of Hastelloy G equip-
ment (Furman 1977).
Bricks (Brova 1977; Mellan 1976)
     Inorganic  linings  can be  made  of bricks, ceramics, and concrete.  The
types  of bricks most  commonly  used in FGD systems are red shale,  fire clay,
and silicon  carbide.   Each  type  of  brick has  limitations that restrict its
use.   Red  shale should  be used where  minimum permeation of  liquor through
the brick  is  required and thermal shock  is not a factor.   Fire clay should
be used  where  thermal  shock is a factor and  minimum absorption  is not re-
quired.   Silicon carbide  brick should be  used  where  high abrasion-resistance
is required.
     In the venturi  throat,  silicon  carbide  brick in  conjunction  with  furan
resin  mortar  has  proved  to  be  a  suitable construction  material.   It can
withstand abrasion caused  by fly  ash in the flue gases.   Fire clay brick can
be used above  the  mist eliminator and at the  inlet  to the scrubber.   In the
scrubber  inlet,  fire  clay brick with  a  furan  resin  mortar  is  recommended
because slurry  from  the sprays does  not  contact the gases and because mist
in the  flue  gases  is minimal above the mist eliminator.   Red  shale brick can
be  used in  the main  body  of the  scrubber.   This  section  is  normally  in
contact with  the  slurry,  and  the  temperature  of the flue gases  is  reduced.
A furan resin should be used as the mortar lining.
     Brick  alone  will  not  prevent  the  scrubber  shell  from corroding.   An
impervious  membrane   must  be  applied  between  the  brick and the  scrubber
shell.  The  purpose  of  the  brick is to protect the membrane from  abrasion
and excessive  heat.   The  membranes  are  made  from vinyl  resins,  natural and
synthetic rubbers,  or asphaltic materials.

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MATERIALS OF CONSTRUCTION:  Protective Linings	G-24

Ceramics (Gleekman 1978)
     Selection  of ceramics for  construction is governed  by their  physical
properties, which result  in  objects of relatively  thick cross section,  heavy
weight,  and  lack of  resistance  to  impact.   Because ceramic  shapes  are
usually made  by  extrusion or casting, the shapes  that  are symmetrical  (such
as nozzles, cylindrical scrubbers,  ductwork, and piping)  are most  suitable,
particularly  on  a production  basis.   Because  vitrification  is required  to
achieve maximum  resistance,  the  limitation  on size  is that of the furnace
used to vitrify  the  object.   It  is  virtually impossible to make adaptations,
changes, or repairs  in  the field that will  provide  resistance  equivalent to
that of the as-fired material.
     Because  ceramic is   composed  of  siliceous  material,  the  shapes  are
susceptible  to  corrosion or  deterioration  in hot  alkaline  media and  in
certain inorganic  acids;  particularly concentrated  hydrochloric and  hydro-
fluoric acid.   Fluorine compounds  rapidly attack the glass bond  in ceramics
and  break  down   the  body  structure.   At  high  temperatures,  concentrated
sulfuric acid  and  steam attack ceramics.   Though ceramics  are fired at  2200°
to  2400°F, the  operating temperature to which   they  are subjected   is  of
utmost  importance  to service  life  because  of  their low  heat  transfer rate
and  resultant  poor resistance to  thermal shock.   The  temperature  differen-
tial across the  thickness of  a ceramic item  should not  be  greater  than  90°F.
     Armorizing  is used with  ceramics as well  as  with  many other  materials,
to  improve the  resistance of  the  material  to  impact.   Armorizing  generally
consists of applying polyester or  epoxy  resin reinforced with glass cloth or
mat  to the exterior of  the  nonmetallic  material.   In tower  construction,
ceramic shapes  are  joined with  bell and spigot  joints  as  well   as  bolted
flange joints.   Because of the fragility of ceramic  materials,  care must be
taken  to ensure  the  proper use of  torque wrenches  with  flanged  joints and to
observe the pressure limitations  of belland  spigot joints.
Concrete (Gleekman 1978)
     Concrete   is made  up  of a fused and ground clay-limestone  product with
an  aggregate.    Concrete  need  not  be fired as a  formed shape to achieve
strength and  resistance,  which result from  hydration,  usually  as  a function

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MATERIALS OF CONSTRUCTION:  Operating Experience	G-25
of time and  temperature.   Concrete  is  usually  reinforced with  steel and more
recently with fibrous materials  to  meet  strength  requirements.   In  limestone
FGD systems, concrete is used in presaturators, tanks,  and  piping.
     Concrete has  obvious  disadvantages of porosity  and  susceptibility to
attack by  alkaline and certain  acidic media.   For this reason, much atten-
tion is given to  protective coatings,  including thin films of  liquid such as
silicones  and penetrating  oils.  As  with ceramics, weight, poor resistance
to thermal  shock,  and joining problems  must be  considered.
     Some  concretes  have  excellent thermal resistance and can  be used at
relatively  high  temperatures  without  danger  of  spall ing.   An additional
advantage  is that  concrete can be gun-applied  and  does not  have to be  cast.
In  the  broad category of  Gunite, selection  of the cement and  the  aggregate
can produce  variations  in  properties  of  the material.   Often,  Gunite  is used
for temporary repairs of  structures such as packed towers  because  operators
cannot afford  the long cure cycles needed  with  ordinary materials of con-
struction.

OPERATING EXPERIENCE (PEDCo Environmental 1981; Rosenberg  1980)
     The balance  of  this  appendix reviews currently available  information on
operating  experience  with  construction materials  for  limestone  FGD systems.
This  review  first discusses  major  equipment  items   in  the  gas circuit,
including  prescrubbers, scrubbers,  mist  eliminators,  reheaters, fans,  ducts,
dampers,  expansion joints,  and stacks.   It then examines  items in.the  slurry
and  solids  circuit,  including  pumps;   storage   silos;  ball  mills;   spray
nozzles;  piping;  spray  headers;  valves;  tanks;  thickeners, vacuum filters,
and centrifuges; agitators and rakes;  and pond  linings.
Prescrubbers
     Included in  the  category of prescrubbers  are  quenchers,  presaturators,
and venturi  scrubbers.   All these  items  of equipment  wet and  cool the gas.
In  addition,  venturi  scrubbers  effect  particulate  removal  (primary  or
secondary) and some S02 removal.

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MATERIALS OF CONSTRUCTION: Operating Experience	       G-26
     Because of  hot and  possibly  erosive operating conditions  in  quenchers
and presaturators,  organic  linings  have been used with a concrete protective
layer  (Marion  4).   Other  materials  used  include  hydraulically  bondec.
concrete-lined  carbon  steel  (Petersburg  3,  Winyah  2, and  Southwest  1),
Carpenter 20 (Tombigbee  2 and 3),  and Uddeholm 904L (Oallman 3).  Petersburg
3,  Southwest  1,  and Dallman  3, all  of  which  use  high-sulfur coal,  have
reported  materials problems.   Petersburg  3 has  experienced erosion of  the
concrete  layer  after less  than  1 year  of operation.    At  Southwest 1,  the
concrete-lined  carbon  steel  has  been  replaced  with   Uddeholm  904L,  which
performed adequately  for  only  a  limited period of time.   After approximately
2 years,  patching  with  Hastelloy G was  necessary.   Following  these repairs,
further  deterioration of  the  Uddeholm 904L was observed. Additional  correc-
tive  actions  have  not yet  been  determined.   At Dallman  3,  the  presaturator
is  also  constructed of Uddeholm 904L.   Pitting caused by attack from  chlo-
ride  was  observed  shortly after  startup.  Chloride levels of 15,000 ppm were
measured at the time the pitting  was first noticed.
      The  hot,  wet,  erosive  environments  of  venturi scrubbers  require
erosion-  and  corrosion-resistant materials.   Mat-reinforced epoxy  phenolics
covered  with hydraulically  bonded  concretes are used at Will County 1 and La
Cygne  1.  Organic  linings covered  with prefired ceramic bricks or blocks are
used  in  the venturi  throats  at Cholla  2, Will  County  1,   La Cygne 1,  and
Widows Creek 8.   No major problems have been  reported with  these materials,
which have operated up to 8 years at Will County 1.
     Organic  linings used  without  protective  coatings (Cholla   1  and  2,
Sherburne  1 and   2,  Widows  Creek  8)  have  failed because of disbonding.
Concrete  linings   without organic  linings underneath  have   worked well  at
Winyah   2   (hydraulically  bonded)   and  Widows  Creek  8  (silicon  carbide
castable).
     Unlined 316L  stainless steel  has been used at the modified  scrubbers on
Lawrence 4  and  5,  Cholla 1, Sherburne  1 and  2 (throat), and Winyah  2  (wear
plate).   The chloride level  (2000  ppm) in  the Cholla  1 system has  caused
rapid  attack  of  the 316L  stainless  steel  and  prompted a switch  to  mat-
reinforced,  epoxy-lined mild  steel  (vessel) and Hastelloy C (externals) for
Cholla 2.   The  chloride  attack  problem at  low pH levels is  leading  to the

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MATERIALS OF CONSTRUCTION:   Operating Experience	G-27
selection of alloys  more  resistant to corrosion (e.g.,  317L stainless steel,
Uddeholm 904L,  and Incoloy 825) for scrubbers now in the design stage.
Scrubbers
     Various  designs  and  many  materials  have  been  used   in  scrubbers.
Virtually  every  scrubber  incorporates a  combination  of  materials.   Major
types  of material have  included 316L  stainless  steel, rubber-lined carbon
steel, organic-lined carbon steel, and ceramic-lined carbon steel.
     The scrubbers at  Choi la  1, Ouck Creek  1,  and  La Cygne 1 and the modi-
fied  scrubbers at  Lawrence  4  and 5  are constructed primarily of 316L stain-
less  steel.   This material  typically requires  a  higher  initial  investment
than  lined carbon  steel,   but  may  require  less  maintenance and downtime.
Weld  patching  and replacement  of new plates  can  typically  be  accomplished
with  more  ease  than  lining repair or reapplication.   Components  made of  316L
stainless  steel  (e.g.,  trays,   plates,  supports,   and  fasteners) have  been
used  in many  other scrubbers.   High  chloride concentration,  however,  can
present  a  corrosion  problem,  and abrasion by scrubber slurries can sometimes
cause  wear failure.   The   operating experience thus  far  ranges  from  1-1/2
years (Lawrence 5) to 6-1/2 years (La Cygne 1).
     The only  major problems  reported to date occurred at  Cholla 1,  where
corrosion  and  pitting  of  the 316L  stainless  steel occurred  because of the
relatively high chloride  content (2000 ppm)  of  the  scrubbing liquor.   Also,
some  patches  have  been made in  the stainless steel  sidewalls of  the new
scrubbers at Lawrence 4 and 5.
     Rubber-lined  carbon  steel  costs  less  than  stainless  steel, but  more
than  carbon  steel  protected  with  organic  linings.   Both natural and  syn-
thetic  rubbers  (neoprene  and  chlorobutyl) have been used.   Natural rubber is
typically  superior to  neoprene  in both abrasion and chemical  resistance, but
synthetic  rubber  is  less  flammable.  The  rubber is  applied  in sheets,  which
are  bonded to  the  steel  with  adhesive.   Care must be  taken  in lapping the
rubber  because  the  laps  are the  areas most  subject to failure by debonding.
Thus,  it is extremely  important that  the rubber  be lapped  so  that  liquids
flowing  over the  surface  do not get under the lap.   Disadvantages in  the use
of  rubber  linings  are  the difficulty  of  repair and the possibility  of fires

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MATERIALS OF CONSTRUCTION:  Operating Experience	G-28
caused by  welders'  torches.   Besides  having excellent resistance to  chemi-
cals  in  the gas  and  slurry,  rubber  is outstanding in abrasion  resistance.
Because rubber provides  such a high degree of  abrasion resistance,  it might
be  used  to advantage  in localized,  high-abrasion areas in scrubbers.  This
has been accomplished  at Tombigbee 2  and*3,  where a natural rubber lining  is
used in the spray zone.
     The scrubbers  at  Petersburg 3,  Southwest  1,  and Widows  Creek 8 are
constructed primarily  of  neoprene-lined carbon  steel;  the  scrubbers  at R.  D.
Morrow 1 and 2  are  constructed of chlorobutyl-lined  carbon steel; and the
scrubbers  at Will  County   1  and  Winyah 2  are  constructed  primarily   of
natural-rubber-lined steel.   After 7000  hours  of operation at Widows  Creek
8,  the neoprene  lining in the tapered hopper bottom portion of  the scrubbers
disbonded and was replaced  with 316L  stainless  steel cladding.   Also,  sparks
from  a  welder's  torch  caused the neoprene  lining to catch  fire at  Widows
Creek 8.  The longest  operating experience thus far has been  roughly 8 years
at Will County 1, where the original linings are still  in  service.
     Organic-lined carbon steel  has the lowest  initial cost of  construction.
The performance  of  lined carbon steel has varied  from  satisfactory  to poor.
Reasons  for  the variation   in  performance  include  selection  of  marginal
linings and  improper application of. linings, or  inadequate surface  prepara-
tion.   Linings selected  for  some first-generation limestone  FGD systems were
poor.    Little experience was available regarding  linings in S02  scrubbers
when  these  first  systems were  constructed.    Wear resulting from  abrasive
slurries was  a  common  cause of  failure.   In  some cases,  the problem was
solved by  relining  with  a  more abrasion-resistant  material  such as  a mat-
reinforced  epoxy lining  in  place  of  a glass-flake-filled polyester lining.
Blistering  and  disbonding of  the  linings  have  also occurred in  some  cases.
     Several  scrubbers  are  presently  in  operation  with  their  original
organic linings,  which  include  mica-  and glass-flake-filled polyesters  and
mat-reinforced  linings.   The  only problems  reported were  at Sherburne  1,
where some  patching of the mica-flake-filled lining was required.
     Some  organic  linings  in  scrubbers  have required  replacement.   The
original  scrubbers  at Lawrence  4 and  5 were relined  several  times with
various  linings, none  of which provided both  the abrasion resistance  and

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MATERIALS OF CONSTRUCTION:   Operating Experience	G-29
chemical resistance needed.  The  modified  scrubbers were  constructed  of 316L
stainless steel.   A mica-flake-filled polyester  lining at Sherburne 2  dis-
bonded  between  layers,  resulting  in  bubbles and  blisters.   The lining was
replaced with an  inert-flake-filled  epoxy.   At Hawthorn 3 and  4, the glass-
flake-filled polyester  lining failed  because  of  temperature  and pH excur-
sions,  and because  of damage from welding.  The scrubbers were relined with
316L  stainless   steel  cladding.   At  R.  D.  Morrow  1  and 2,  the original
glass-flake-filled  polyester lining  failed  because  of an  accumulation  of
spot  failures and pinhole  leaks.   The scrubbers  were  relined with  chloro-
butyl rubber, which has  held up well  since application.
     Although  linings  have  been  used  in  many   scrubbers,  their  time  in
service  in  most  instances  has  been  too  short to  obtain  an  effective
appraisal of their  performance.   From the information available  at present,
the  following conclusions  are  possible regarding  the  use  of organic linings:
          High-quality  lining  materials   should  be   selected.   The  minimum
          requirements   are   trowel-applied,   glass-flake-filled   polyester
          linings  of  80-mil  nominal   thickness  in areas  subject to normal
          abrasion   and    heavy-duty,   fiber-mat-reinforced   materials   of
          1/8-inch  nominal  thickness  with   abrasion-resistant  fillers  in
          areas  . subject  to  high abrasion  (wherever  slurry  is  projected
          against linings).
          Lining  materials must  be  applied by skilled,  experienced appli-
          cators who will  stand by their work.
          Applicators must understand metal  surface  preparation  procedures
          and use them properly.
          Careful quality control procedures  must be  used.
     Although  very  few  scrubbers  have  been  constructed  predominantly  of
ceramic-1ined carbon  steel,  many  have used  some  type  of ceramic  lining  at
specific high-abrasion areas  such as venturi throats  and  sumps.   The use  of
ceramics has been  limited  because of high cost,  high weight,  and occasional
problems  caused  by  brittleness.   The  bottom  of  the  scrubber  sumps  at
La Cygne  1  and Winyah  1  are  lined with  hydraulically  bonded  concrete.
Acid-resistant brick is used to  protect the bottom section of the  scrubbers
at  Huntington  3 (interval  effluent  hold tanks).  Although good performance
has  generally been  reported  for  the ceramic linings, the  experience  to date

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MATERIALS OF CONSTRUCTION:  Operating Experience      	  G-30
is  too limited  in most  instances  to  draw  reliable conclusions  regarding
long-term performance and maintenance requirements.
Mist Eliminators
     Practically all mist eliminators  in  use are of  the  chevron type with  a
variety of  vane shapes.   The vanes  are most often constructed of  some  form
of  plastic  with or without fiberglass reinforcement, but alloys  have  also
been  used.   In  general,  mist  eliminators have been satisfactory from the
materials  standpoint.     Because   of  plugging   problems,   mist   eliminators
require frequent cleaning plus  spray washing on a  continuous  or  intermittent
basis.  Personnel  often break  FRP mist eliminators  during cleaning by strik-
ing them with  hammers  or  walking  on  them.   This breakage  can  be  prevented by
care  in  cleaning  and  use  of  sufficiently  thick  FRP.   Also,  unreinforced
plastic mist eliminators  are  subject to warping, sagging,  and melting during
temperature excursions.
     Base metal mist eliminators  are used at Duck  Creek  1 (Hastelloy G) and
Widows Creek  8  (316L  stainless  steel).    The  only  problem  reported is at
Widows Creek  8,  where  mud  deposits  lower  pH and thus cause  chloride corro-
sion.   An FRP  mist eliminator,  however, would not  have survived  the cleaning
that has  been necessary.
Reheaters
     If flue gas  is  not  reheated  before leaving the scrubber, acid condensa-
tion can occur  in  the  downstream  ductwork, fan  (if  present),  and stack.  The
main types of reheat are   in-line,  indirect, and  bypass.
     If an  in-line  reheater is  used, it is  installed either  in  the scrubber
or  in the  discharge  duct  after   the  mist eliminator.    The  heat  medium is
either steam  or hot water.  The  materials of construction for  the  tubes or
coils  range from carbon  steel  to   Inconel   625.  Acid  corrosion from sulfuric
acid condensation  is a problem  with  carbon  steel,  and stress corrosion  from
chlorides  is  a  problem   with  stainless  steels  and alloys.  Therefore, the
severity  of  corrosion  problems with carbon  steel  is related to  the  concen-
tration of  S02 in the scrubbed flue gas,  whereas  the severity  of corrosion
problems  with  stainless steels  and alloys  depends  on the  chloride content of
scrubbing liquor  or wash  water that may   be carried  over  from  the  mist

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MATERIALS OF CONSTRUCTION:   Operating Experience     	G-31
                                 **•
eliminator.  Plugging  of the tube banks  is  also a problem, particularly  if
finned  tubes  are used.  Soot  blowers are  required to maintain  performance
with in-line reheaters.
     Circumferential-finned  carbon  steel  tubes containing  hot water  under
pressure have been  used  for heat at  Lawrence 4 and 5,  Sherburne 1 and 2,  and
Hawthorn 3  and  4.   Some  tube failures occurred  at Lawrence 4 and 5  after 6
years  of  operation,   but  there  have  been  no  serious problems  since  the
station  switched  to firing  low-sulfur  coal, presumably because of  less  S02
in the  scrubbed flue  gas.   At Sherburne 1  and 2,  which also burn low-sulfur
coal,  four  weld failures occurred early  in the operation because of exces-
sive  stress.    Only  infrequent   leaks  have  occurred  during  the past  3-1/2
years,  since  the  stress  was removed.  The  tubes at Hawthorn 3 and  4,  which
burn  a  blend of  low-  and  high-sulfur  coal, have  not  experienced  corrosion
problems,  although  plugging with deposits  caused  them  to be  replaced with
smooth carbon steel  tubes to facilitate cleaning.
     At  Will  County 1,  steam  in smooth  tubes is  used for  reheat.   The  top
tube  banks  were originally  Cor-Ten,  and the bottom tube banks  were origi-
nally  304L  stainless   steel.  Leaks  developed in 6 months,  and  the  original
tube banks  were .replaced with  carbon steel  banks  at  the top and 316L stain-
less  steel  banks  at the  bottom.   Pinhole attack on the carbon steel  requires
tube  replacement  approximately  every year,  and  stress-corrosion  cracking  of
the  stainless  steel   requires   tube  replacement  approximately every  1-1/2
years.   No  remedial   measures   are  planned  because  the  flue  gas  cleaning
system,  which is  now  used primarily  for particulate control, may be replaced
by an electrostatic precipitator.
     Smooth  316L  stainless  steel tubes  containing  steam  are also  used  for
reheat  at  Cholla  1  and La Cygne  1.   At  Choi la 1,  the  tubes  needed  replace-
ment  after  6 years of operation because  of corrosion.   At La  Cygne 1,  the
tubes  are  a replacement for the original 304  stainless steel,  which failed
because  of  corrosion.  The 316L tubes have a service life of at least 2 to 3
years at La Cygne 1 before replacement is required.
     Inconel 625  tubes with Uddeholm 904L baffles  are  used at Cholla 2.   No
corrosion problems  have  occurred after 1-1/2 years  of  operating experience.

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MATERIALS OF CONSTRUCTION:  Operating Experience	G-32
     In  the indirect  method of  reheat,  air is  heated  separately  from  the
scrubber  system.   Thus,  corrosion  and  fouling  problems  are  circumvented.
Carbon  steel  steam coils have been  used  at Petersburg 3 and Widows  Creek  8
(where  tubes have  copper fins) without any  materials  problems.   The longest
operating experience  thus far is 2-1/2 years at Widows Creek 8, where opera-
tion is continuing.
     When the  entire  flue  gas output does  not have to be  scrubbed  to  meet
emission  regulations,  partial bypass can  be used as a  reheat  method.   This
approach  has been  used at Tombigbee 2 and 3, Jeffery 1 and 2, Coronado 1 and
2,  Winyah 2,  and  R. D.  Morrow 1 and 2.   Materials  problems  associated  with
bypass  reheat are discussed in the subsection on ducts.
Fans
     The  majority  of  the fans used  in limestone  FGD systems are located up-
steam  of  the  scrubber.   These forced draft fans operate on hot  flue gases
and can be  constructed of ordinary  carbon  steel.   Cor-Ten is used at Choi la
1,  Hawthorn 3  and  4, and Lawrence 4;  and  stainless steel  blades are used at
Choi la  2.   In  some cases these hot-side  fans have been eroded by the fly ash
in  the  flue gas  stream, particularly if the electrostatic precipitators are
not operating' at maximum efficiency.  This has occurred at  Widows  Creek 8,
where  erosion  has  required  rotor   replacement  every  . 10  weeks even  though
there are chromium carbide wear plates.
     Several limestone  FGD  systems  have  the fans installed downstream of the
scrubber.    Corrosion-resistant alloys  are  required  if  these  induced  draft
fans are  exposed  to wet  flue  gas  at the  scrubber exit  temperature.   If  the
fans are  located after  the reheater, however,  less-resistant  materials  are
used,  particularly  if  the exit S02 concentrations are  low.   Fans are located
after  reheaters  at Will  County  1,  La Cygne 1,  and Sherburne  1 and  2.   The
fans at Will  County  1 are constructed  of  Cor-Ten, and the other  fans  are
constructed of  carbon  steel.   Some  polymer coatings and  Inconel  625 rotors
have been used  at  La Cygne  1, where corrosion and erosion problems have been
encountered since  startup.    These  fans  are  also  unique  in that  they  are
washed  every 4  days  to remove deposits caused  by particulate carryover from
the scrubbers.

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MATERIALS OF CONSTRUCTION:   Operating Experience	G-33

Ducts
     Inlet  and  bypass  ducts  are  generally  not  major  problem  areas for
scrubbers.  The  outlet duct,  however,  has  been a major problem area, partic-
ularly  for  units  with  duct  sections that  handle  both hot  and wet gases.
These sections  are for the  most part beyond  the bypass  junction on  units
without reheat.  Acidic conditions  become  more severe in systems with bypass
reheat  as  the  temperature  is  raised and other  corrosive  species  in the
unscrubbed flue gas (chlorine and fluorine) are introduced.
     Carbon steel  or  Cor-Ten  is  used as the material of construction at the
inlet duct  of  all  installations.   Hawthorn 3  and 4 were originally  designed
for  boiler  injection  of limestone  with  simultaneous  removal  of S02 and fly
ash  in  a  marble-bed absorber.  The  inlet ducts have a Gunite-applied refrac-
tory  concrete  lining  to  protect  the Cor-Ten  steel  from  potential erosion
caused by the high fly ash loading in the flue gas.
     Outlet  duct materials  include unlined  carbon steel  (in the  reheated
zone  of  Sherburne 1  and  2 and  the breeching  of  the modified  Lawrence 5),
Haste! loy G (at  Duck Creek  1), and  Inconel  625  (at  Marion 4).    Material
selection depends  on  such factors  as  the  location  and  use of reheaters and
the  type  of  coal  burned.   At most  stations, outlet ducts  are exposed  to
different flue  gas conditions;  i.e.,  before  and  after  the  reheater and/or
before and after the bypass junction.
     Systems with  wet outlet  duct sections predominantly use  organic linings
to  protect  the carbon steel or  Cor-Ten  structure.   An  exception  is the use
of  Hastelloy  G  in  the  outlet duct at Duck Creek  1,  where pitting  has been
severe  enough  to  cause  penetration of  the duct walls.   Deluge  sprays are
used  in  the ductwork  before the  stack to  protect  the organic lining on the
flue, and condensate  accumulates  in  the  outlet  duct,  which  has  a low pH.
There have  been  some  lining failures.   At R.  D. Morrow  1  and 2,  one glass-
flake-fined polyester  lining was  replaced with another  glass-flake-filled
polyester  lining  from  a  different manufacturer.   The second  lining  also
failed and  was  replaced with Hastelloy G  cladding, which  was applied to the
outlet  duct,  the  scrubbed-gas/bypass-gas   mix zone  (partial scrubbing and
bypass reheat  are  used),  and the breeching.  Failure  of  the Hastelloy G also

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MATERIALS OF CONSTRUCTION:  Operating Experience	G-34
occurred  in  some spots  in  the gas  mix zone,  resulting in  replacement with
Hasten oy  C-276  (applied over  the  top of  the Haste Hoy  G).   Subsequent
operation with  the  Hastelloy  C-276 indicates  that  it  too  is  failing (because
of  pitting  from  chloride attack).   Southern Mississippi  Electric Power  is
presently considering  the use  of an  organic  or inorganic  lining at  R.  0.
Morrow 1  and  2.   A glass-flake-filled polyester at Southwest  1 was replaced
by  inert-flake-filled vinyl ester.   For the most part, however, organic  lin-
ings  are  providing several years  of service  regardless  of whether high-  or
low-sulfur coal  is fired.  The longest operating  life  of an  organic  lining
has been  8  years  (for  a mat-reinforced  epoxy  phenolic at  Will County  1).
Fluoroelastomer  linings  are  used  at Apache 1  and  2  and  Tombigbee 2 and  3.
     Stations with  outlet duct sections  exposed only to  reheated gas  have
had a good  record of  materials  performance.    Problems  have  been  reported
only at Module  B of Choi la 1, where  flue  gas  is not  treated for S02  removal
(limestone slurry  is  not circulated through Module B)  and water has a  high
chloride  content.   Module B  is  operated for only particulate removal  because
there   is  no  electrostatic precipitator and the S02 emission  regulation  can
be  met with  one module  on   line.   No  problems have been  reported with  a
similar mica-flake-filled polyester  lining on  the 'Module  A  where  S02  is
scrubbed.   Because  the  service  lives of unlined carbon  steel  and Cor-Ten  are
6-1/2   to  8 years at units firing  high-sulfur and  low-sulfur coals  (La  Cygne
1  and Will  County 1),  organic  or  inorganic  linings  may  not be  required
(except as a  precautionary measure)  in outlet  duct sections  exposed  only  to
reheated gas.
     Although outlet duct sections handling both reheated gas  and  hot  gases
have fewer problems than those  handling both wet and hot  gases, a few  prob-
lems  have occurred.   Severe  corrosion  problems were  experienced with  the
carbon  steel  outlet  ducts  of  the  original  Lawrence 4 and 5  FGD  systems.
Cor-Ten,  however,  has  been used  successfully for  more  than 7  years  at Will
County  1.   The  failure  of a  glass-flake-filled polyester lining  at  Peters-
burg 3 may be related  to the large  size  of the unit.  Other organic  linings
that  have been used with no  problems  in outlet duct sections  handling  both
reheated  and  hot gases  include  fluoroelastomers (for 1-1/2  years  at  Tombig-
bee 2  and 3  and 1 year  at Apache  1 and 2) and an inert-flake-filled  vinyl

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MATERIALS OF CONSTRUCTION:   Operating Experience	G-35
ester  (for  1-1/2  years  at  Cholla  2).   Inorganic  linings  (hydraulically
bonded concretes) have  thus  been used successfully  for  7 years at  Hawthorn 3
and 4 and for 1-1/2 years at Huntington 3.
Dampers
     Mechanical problems with dampers, caused  by deposition of solids  from
the flue gas,  have  outweighed any materials problems.  Dampers  are  utilized
in  three  locations:   the  inlet  duct to  the scrubber,  the.outlet duct  from
the scrubber,  and  the  bypass duct from the  air preheater  or dry particulate
collector to the stack.   Some installations  have no  bypass  duct,  and all of
the flue gas goes through the scrubber system.
     The  inlet  dampers used  in  the  limestone systems  are  about evenly
divided between guillotine  and  louver types, with a few butterfly  dampers in
use.  Because  inlet dampers  are subjected  to  the  hot, relatively  dry  flue
gases  ahead of  the scrubber,  such  dampers can be  constructed of unlined
carbon steel.   This material  is used at  several sites,   particularly  those
where  S02   concentrations  are relatively  low.    Damper seals,  however,  are
usually made  of base metals, such  as 316L stainless steel, Hastelloy G, or
Inconel 625.   Corrosion of  carbon steel  inlet  dampers has  been reported  only
in  the  original systems at  Lawrence 4  and  5.    The  corrosion  was  probably
caused  by   acid  condensation resulting  from the high  concentration of  S02
(high-sulfur coal was  burned at  the time).   The modified systems at  Lawrence
4  and 5  use Cor-Ten  inlet  dampers  (low-sulfur  coal  is   presently  burned).
     The 316L  stainless steel damper seals  used in  conjunction with carbon
steel or Cor-Ten suffered  corrosion damage  at  Widows Creek  8 as a result of
high  chloride  concentrations,  and the seals were successfully  replaced  with
Inconel 625.   In spite of  hot,  relatively dry  conditions  at the inlet,  most
systems  have  inlet  dampers  constructed  of  Cor-Ten  or clad with stainless
steel  to  avoid  possible corrosion.   Inlet  dampers  made  of  316L stainless
steel  have operated  without major  problems at  Cholla 1  for 6 years,  and'
inlet dampers  made of  Hastelloy G  have  operated without major problems at
Duck Creek  1 for 3-1/2 years.
     Dampers on the scrubber discharge are  subject to  the wet  flue gas  and
thus  are  often made  of stainless  steels,  either as a cladding or for  the

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MATERIALS OF CONSTRUCTION:   Operating Experience	;	G-36
entire construction.  Duck  Creek  1 has  Haste Hoy G outlet dampers,  and R.  0.
Morrow 1 and  2  have Hastelloy G cladding on the outlet  side of the scrubber
discharge dampers.   The original  carbon  steel  outlet dampers at Lawrence 4
and  5  corroded and  were  replaced with  Cor-Ten  equipment.   Carbon  steel
outlet  dampers  at  La  Cygne  1 also  corroded  and were  replaced with  316L
stainless steel  equipment.   As a  result  of high chloride  concentrations,
316L  stainless  steel  dampers at Southwest  1 corroded badly in 3 months  and
were  replaced with  an Uddeholm 904L frame,  Inconel 625  seals,  and  Hastelloy
G  fasteners.   To date,  the replacement has  been reported  as  satisfactory.
     Where  bypass  dampers  are  used,  they  are  generally constructed  of  the
same  materials  as  the outlet damper,  because they  can be exposed to the  wet
flue  gas.   An exception is at Apache 1 and 2,  where  Inconel 625  cladding is
used  on  the wet side of the  bypass damper, as compared with 317L  stainless
steel  on  the  outlet  damper.    No  serious corrosion  problems  have  been
reported with  bypass  dampers;  the  chief  difficulties have  been  clogging of
seals and problems of mechanical operation.
Expansion Joints
      Expansion joints are  generally U-shaped and constructed of an  elastomer
with  fabric  (fiberglass  or  asbestos)  reinforcement.    Some  metal  bellows
expansion joints  are  also  used.   The major problem  with  wet-side  expansion
joints  has  been to choose  the  proper  metal for attachment, rather  than  the
fabric.    The  kind of   expansion  joint selected  depends  upon  the  highest
temperature  to  be  encountered.   Suppliers  suggest fabric-reinforced neoprene
at  250°F  or below, fabric-reinforced  chlorobutyl  rubber at 300°F  or below,
fabric-reinforced  fluoroelastomer   at  400°F or  below,  and  layered  asbestos
above 400°F.
     Metal   expansion   joints  have  been  used  successfully  in  some  cases,
especially  under  dry  conditions.    Even stainless steels,  however,  can pose
problems  if condensate  contains   high  concentrations of acid or  chloride.
Several  FGO  installations  have  replaced metal expansion  joints with elasto-
mer joints because of corrosion problems.

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MATERIALS OF CONSTRUCTION:   Operating Experience	G-37
                                 **•
Stacks
     The performance  of a stack  lining  depends on whether the scrubbed gas
is delivered  to the  stack  wet or  reheated,  and whether  the stack is also
used  for  hot bypassed  gas.  These  factors  appear to affect  the performance
of lining  material  more strongly  than differences  in  coal  sulfur content.
lining  application   techniques   (e.g.,   surface  preparation or  priming).
operating  procedures  (e.g., thermal  shock), design  aspects  (e.g., annulus
pressurization,  stack height,  and  stack gas velocity),  and  other factors.
Almost all  units with bypass capability have a  junction  in the duct leading
to the  stack where bypass  and scrubbed gases are combined.  The  exceptions
are Winyah 2 and Southwest 1, which bypass gas directly  to the stack.
     At Duck  Creek  1, there is no  reheat,  and  the bypass  gas is quenched  to
minimize stack  temperatures.   Problems  with the mica-flaked-filled polyester
lining have  occurred  in the lower portion  of  the wet stack.  No problems,
however,   have  been  encountered  in the  upper   part  during  3-1/2 years  of
service;  for  1 year,  high-sulfur  coal  was  fired.  Quenching the  bypass gas
may  have  contributed  to good  lining performance  in the  upper  part  of the
stack.
     La Cygne  1 is the only  operating  limestone scrubber that fires high-
sulfur  coal  and reheats  all  gas  (no bypass capability  is  available).    An
inert-flake-filled vinyl ester  stack  lining is   beginning  to  disbond after 2
years  of  service,  as  did  a  previous  flake-filled polyester  lining.    At
Sherburne  1  and 2, which  fire low-sulfur  coal and reheat all gas, unlined
Cor-Ten flues  have given  good service  for  3-1/2 years.    No stack failures
have been reported.
     Like most  stations, Petersburg 3 reheats scrubbed  gas and has emergency
bypass  capability.    This   station,  which  fires  high-sulfur coal  and has
organic stack  linings,  has  experienced lining  failure during bypass  opera-
tion.  Two  trowel-applied  coats  of a glass-flake-filled polyester are being
tried; results  are  not  yet available.   Stations burning high-sulfur coal and
not  experiencing stack  problems  have stacks lined with  either acid-resistant
brick and  mortar or inorganic  concrete (normally Gunite).   The hydraulically
bonded concrete mixes  that  have  been used contain calicum aluminate  cement

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MATERIALS OF CONSTRUCTION:  Operating Experience	G-38
as  the bonding  agent.   This  type  of cement  can withstand  mildly  acidic
conditions  (pH  >  4)  and  is  commonly used  in  refractory  concrete  mixes
exposed to  temperatures  of  500°F and greater (which cause  portland cement to
dehydrate).
     Stacks  lined  with either  acid-resistant  brick and mortar or inorganic
concretes (including a chemically bonded concrete) have experienced no major
problems  at  stations  burning  low-sulfur  coal   and  providing  reheat  and
bypass.  Although  the  experience time is short,  at least four  organic  lining
materials  are being  used  by  such  stations.   Two  of  these  materials  are
fluoroelastomers (at Apache  2  and 3), one is  a  glass-flake-filled polyester
(at Winyah  2), and one  is  an  inert-flake-filled vinyl ester  (at  Cholla  2).
The lining  at Cholla  2,  however, has  never been  exposed to  bypass  condi-
tions.   At  Winyah  2,  where  one-half of the flue gas is  always bypassed to
the stack,  some  spot  failures  have occurred  in the lining  below the scrubbed
gas  breeching  and  above  the  bypassed   gas  breeching.   These  spots  are
scheduled for repair with the same inert-flake-filled  vinyl ester.
     Acid-resistant brick linings  have  exhibited good performance  regardless
of operating  conditions.  In spite of this  success,  however,  their applica-
tion is  limited  because  acid  can penetrate  brick,  mortar, or  brick-mortar
interface under wet  conditions.   A possible  remedy to this limitation  is  the
pressurization of the stack  annulus.
     Hydraulically  bonded concretes  have  been  successfully  used  under  dry
conditions  for  up  to  8  years  by stations  burning high-sulfur  coal.   Chemi-
cally   bonded  mixes containing  silicate  binders are  generally  considered to
be  more acid-resistant  than  hydraulic  cement;  yet   none  is   in  use  where
high-sulfur   coal   is   fired.    Also,   the   weight   of   relatively   thick
(1-1/2-inch)  inorganic  concrete  linings could make them  unsuitable for  use
on  steel   flues  designed  for  organic  linings.    The  major  limitation of
concrete materials  is  the risk  of  substrate corrosion in  the event of acid
condensation.   Although  even  cracked  material  will  provide  a  physical
barrier to  minimize  acid transport  to the  substrate,  membrane  backing or
some other  means  of providing secondary substrate protection  is  desirable.

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MATERIALS OF CONSTRUCTION:   Operating Experience	G-39
     Organic  linings  have  been primarily  based on polyester or vinyl  ester
resin  binders,  both of  which have  deteriorated  sometimes when high-sulfur
coal  is  fired and commonly when bypass  reheat  is used.   Resins more  resis-
tant to  the  300°F  bypass temperatures and hot  acid  conditions  appear  neces-
sary  to  lengthen the  service life of organic  linings.  The  fluoroelastomer
linings  used  in newer  stacks  nay  significantly  improve  the performance
record  of  organic  linings.   Whether  the  new materials  will be  able  to
provide  reasonable  service  lives   in  hot and  wet  environments  (which  are
typical  of  stack  environments at  many  stations  burning high-sulfur  coal)
remains to be seen.
     Although  high-grade  nickel   alloys  have  not yet  been used  as  stack
liners,  they  are being  seriously  considered for  wet stack  applications  by
several  utilities to  avoid loss of generating  capacity  from  unplanned stack
failures.   These alloys  are  already  being used in the  stack breeching area
of  outlet ducts, and  in the  bypass  duct downstream of the exit, damper.   As
yet,  however,  they are  unproven  at  stacks  that alternately handle wet  gas
and hot gas.
Pumps
     Pumps are  used for  a  variety of  services  in FGD systems such  as  slurry
feed to  the  scrubber  spray headers, slurry  transfer,  and  clear water  trans-
fer.   Rubber-lined centrifugal  pumps are commonly used for slurry  recircula-
tion  with  generally satisfactory  results.   Ordinary carbon  steel  pumps  are
usually  used for  clear water transfer.   Stainless steel  pumps   also find
substantial  usage, especially where small capacity pumps  are  required.   The
experience  with pumps  has  varied widely  in  limestone  FGD systems.   Some
utilities report that  pumps have given  little  problem;  others  cite problems
with  slurry  pumps  as  the  greatest difficulty  in  keeping  systems  operating.
Although  the reasons  for  these differences are not  obvious  in all  cases,
some causes of rubber lining  failure are:
          Poor-quality lining
          Foreign objects  in  slurry (which cause mechanical damage)
          Cavitation (because of dry operation)

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MATERIALS OF CONSTRUCTION:  Operating Experiencing^	G-40
          Overly abrasive slurry (containing large particles)
          Overworked pumps (operating toe near maximum capacity)
          Undersized pumps
Foreign  objects  have damaged  the  rubber  linings  in scrubber  feed  pumps  at
Huntington  3.   Frequent  loosening of the  pump throat  liners  occurs at  La
Cygne  1  because ..of  cayitation when  the  tank level  is  not maintained.   At
Duck  Creek  1,  the  original  natural  rubber  puip^^njngs were  replaced  with
neoprene after numerous failures.                1
                                                 ,"/i-'-~
Storage Silos                               .rj«s^s-
     Because limestone  storage  silos  are subjected  to  alkaline conditions,
carbon  steel  is  :a::!;slatisfactory  material  of=  construction.   All  limestone
scrubber installations  have  carbon steel  silos except Widows Creek  8, which
has a  concrete  structure.   Southwest 1 uses  a  10-gauge  lining  of 304 stain-
less  steel   in  the  bottom  cone to  provide a  low coefficient  of  friction.
There have been no problems with the storage silos.
Ball Mills
     Most limestone  FGD  systems have  ball  mills to  grind  limestone  feed  to
the dssired  particle size.   Ground limestone is purchased  for  Cholla 1, but
Cholla 2  has a mill.   All ball  mills have carbon  steel  shells.   Although a
few are  unlined,  most have rubber  linings.   La Cygne 1 is unique in  that the
mill  has  a  Ni-Hard  lining.   About  two-thirds  of  the  lining  has  required
replacement, but  the reason  for the  failure  of the  Ni-Hard  is  not known.
The rubber-lined  ball  mill  at Widows Creek  8 required  relining after  4000
hours.
Spray Nozzles
     A wide variety  of  materials,  ranging  from  plastic to extremely  hard
alumina and  silicon carbide,  has  been used for spray nozzles.   Wear, plug-
ging,   and  installation problems  are the  only difficulties  reported  with
nozzles.   Wear problems have occurred only with plastic  nozzles (Duck Creek
1, Hawthorn  3  and 4, and the original Lawrence 4 and 5 systems) and metallic
nozzles  (Cholla  1, Hawthorn  3 and 4,  La Cygne 1, Southwest  1, and Widows
Creek  8).   Almost 8 years of  service,  however, has  been obtained from  316L

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MATERIALS  OF  CONSTRUCTION:  Operating Experience^!/    : '^
                                                :• /*«>-"                 	 r
 stainless  steel  nozzles at  Will  County 1.   Wear   problems  have not  been
 reported  with any  ceramic  materials;  and in spite  of  greater care required
 for installation, ceramic  nozzles  are commonly  used for slurry  service in
 the newer  scrubber  systems.   Both  silicon carbide and alumina materials
 generally  provide good  erosion resistance.   Cost,  availability,  and design
 considerations  will probably  influence  the selection  of specific materials
 more  than  any difference in wear resistance.  Stainless steel nozzles appear
 to  be preferred  for mist  elimihatbr  service,  but  ceramic  nozzles are7'also
 used.
 P i p i n g
      The piping  used in limestone FGD systems is required to handle alkaline
 makeup  slurry,  recycle and discharge slurry, and reclaimed  water.  The type
 of  material  selected  depends to  a  large  extent  on the  type  of  service
 encountered.   The   piping  that  handles  alkaline  makeup  slurry does  not
 require  the  acid resistance needed  for  recycle  and  discharge slurry piping,
 which  is  subjected to  the most  severe  service condition.   The reclaimed
 water  piping is  not subjected to  the  highly  erosive conditions  encountered
 by  slurry  piping.
      Rubber-lined carbon  steel  piping  is  most commonly  used  to  deliver
 alkaline makeup  slurry and has provided  generally good service.   Lining wear
 has occurred in  high-velocity  regions  at La Cygne  1 and  Winyah 2, and syn-
 thetic  rubber linings appear to be giving better service than natural  rubber
 linings  in delivering makeup slurry.  The longest operating life  has been at
 Will  County  1,   where  the  reducers eroded  and were  replaced after  6 or  7
 years.   At a few sites  FRP  has been used,  but has  suffered from erosion at
 Choi la  1 and  Duck Creek  1.
      The  slurry  recycle and discharge  lines are  made of either rubber-lined
 carbon  steel  or  FRP.   Problems  similar  to those encountered in the  makeup
 slurry  lines  have   occurred.   Disbonding of  the rubber  lining occurred at
 Sherburne  1  and  2,  and pieces of  rubber plugged spray headers and nozzles.
 When  the lining was intentionally  removed,  the carbon steel eroded badly at
 reducers   and elbows.   The   recycle   slurry   is particularly   abrasive  at
 Sherburne  1  and  2   because of  the  fly  ash  present  in the scrubbing  slurry

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MATERIALS OF CONSTRUCTION:  Operating Experience	;	G-42
(alkalinity from  the  collected fly ash is used as scrubbing reagent).   Thus,
the rubber-lined  carbon  steel  piping has been  replaced  with FRP piping.   At
Lawrence  4 and  5,  FRP  piping has also  replaced rubber-lined carbon  steel
piping, which  had eroded.   At Hawthorn 3 and 4, however, rubber-lined carbon
steel  piping  was used  to replace  FRP  recycle  slurry  piping  because  the
latter was too difficult to maintain and replace.
     The  reclaimed water piping,  which carries less  solid material,  has been
made of  FRP at  the  majority  of  locations.   A  few  cases of failure  of  FRP
piping at  joints  have been reported, but results  in  general have been good.
Flanged  or shop-fabricated  joints are  preferable  to field-cemented  joints
because  pipefitters  are not skilled  in making FRP joints.  Carbon  steel  is
also used as  a  material of construction  for reclaimed water piping;  it  is
unlined  at roughly  one-half  of  the  stations  where  it  is  used  and  rubber
lined  at  the  others.  No  problems have been reported with  these  materials.
Spray Headers
     The  materials  chosen  for  spray  headers,  which  are  located  inside
scrubbers,  are primarily  FRP  and 316L  stainless  steel.   At Choi la  2,  317L
stainless  steel   was  selected  to obtain  the  greater  corrosion  resistance
resulting  from its additional  molybdenum content.   No difficulties have been
reported  with  FRP  spray  headers,  but  316L stainless  steel  spray  headers
eroded  in the venturi   scrubber  at  Widows  Creek  8.   Rubber-lined  and  clad
carbon steel  has been  used  successfully at six  systems,  and  mat-reinforced
polyester  applied to  rubber-lined carbon  steel  is  used  at  Huntington  3.
Valves
     Valves  are   used  in  FGD  systems  for  isolation  and  control  functions.
Valve  problems  are   typically related  not to  materials  failures,  but  to
plugging  and mechanical  problems; and there seems to  be  a consensus  that the
number of  valves  in  the system should be minimized.   Rubber-lined valves are
the most  common,  although many stainless steel valves are used.   Knifegate,
plug,  pinch, and butterfly valves are  the  four kinds of  valves used  in  FGO
systems.

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MATERIALS OF CONSTRUCTION:   Operating Experience	G-43
                                 .^-
     Knifegate valves made  of  316L stainless  steel  have  been  used  for  isola-
tion  functions,  especially  where high  pressures  require  metal.   For  low-
pressure  applications,  FRP  has  been used.   There  have been  no reports of
serious corrosion of  stainless  steel  valves except at La Cygne  1, where the
average valve  lifetime  has  been only 1 year.  Trials of polyethylene-lined
butterfly valves  and valves with  polyethylene  plugs  and seats at this site
have yielded promising results.
     Rubber-lined plug  valves have  also been  used for isolation functions
and  have  encountered erosive wear at six systems.   They have been  success-
fully  replaced with  pinch  valves at  Duck Creek 1.   Butterfly valves  with
rubber  linings have  been  used  for  isolation  applications  at only  a  few
sites, and no problems have occurred with them.
     When  used  for  control  functions,   rubber-lined  plug  valves  have  had
erosion  problems.   Rubber  pinch valves  have been  successful, but  periodic
liner replacement is  required  when they  are subjected to high-velocity flow.
Rubber-lined butterfly  valves appear  to be  giving satisfactory performance
where  they  have  been  used  for  flow  control.    At  Duck  Creek  1,  eroded
rubber-lined plug  valves  have  been  successfully replaced  with  rubber pinch
valves.   At  Will  County  1,  however, the  original rubber pinch valves  on the
spent slurry  line  lasted  only 250 hours  and  were successfully replaced with
butterfly valves.
     In general,  the performance  of  valves is  site specific  and  depends on
the  amount  of  throttling,  the  chemical  and  physical   nature  of  slurry
particles,  and the  pH  of  the  liquid.   As indicated, the  trend in  scrubber
design is to reduce the number of valves to the minimum.
Tanks
     Tanks  have  generally  been  constructed  of  carbon  steel, and  many are
protected  with some  kind  of lining.   The  organic  linings  used  in  tanks
include  rubber,  flake-filled  polyester,  mat-reinforced  epoxy,   coal  tar
epoxy, and  bituminous resin.   Where pH  is high,  the use  of unlined  carbon
steel  is  common.   Concrete  and FRP  tanks have also  found usage.    In a few
instances  (e.g.,  at R. D.  Morrow 1  and 2 and  Duck  Creek 1),  FRP  has been
used for mist eliminator wash tanks; no problems have been  reported.

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MATERIALS OF CONSTRUCTION: ^Operating Experience	  "'	G-44
                                                         1 .-1 \-
     An  effluent hold  tank may  be either  a separate  external  tank or  an
integrated  internal  tank  situated  at  the bottom  of the  scrubber.   Carbon
steel  with some  form  of  glass-flake-filled  polyester  lining  is  the  most
common  construction  material.   Tank linings have been generally  successful,
although some  failures  have been reported.  At Hawthorn  3  and 4,  the organic
                                           •S-iggiS..:--.-
lining  failed  and was  replaced with 316L stainless steel.  At Sherburne  2,
the  mica-flake-filled  polyester  lining   disbonded  between  layers and  was
                         _T   '     .   •       '^••SF..'?'
replaced  by an  epoxy  lining.   No  failures  were  reported  in  rubber-lined
recycle  tanks,  nor  were any  reported  in tanks  with  mat-reinforced  epoxy
linings.   Unlined carbon  steel  or  Cor-Ten  is  used  for  the effluent  hold
tanks  at  Lawrence  4 and  5^  No  problems were  reported.   Concrete with a
g'fass-f lake-filled polyester ^fining  has been "used  successfully at Southwest
1.   It  appears that  the presence of fly ash in the slurry,  which  can be more
abrasive  than  limestone, does  not create any  additional  problems  with the
tank 1inings.
Thickeners and Vacuum Filters
     Thickeners  are  constructed   either  with a  concrete  bottom  and  steel
sides  or entirely of  carbon steel.   Various linings  (including  epoxy)  have
been used without difficulty.
     Drum  vacuum  filters  have  been used by all  systems  except Will County 1
and  R.  D.  Morrow  1 and  2, which use horizontal  belt  filters.    The  metal
components  are  carbon  steel (with  or  without an  organic  lining),  and the
filter  cloth  is  polypropylene  or nylon.   All problems have been  mechanical,
rather  than related to construction materials.
Agitators and  Rakes
     Agitators  for  slurry  tanks  usually have  rubber-lined  carbon  steel
blades  and shafts.   The  rubber provides  excellent abrasion  resistance,  as
well as  protection  against  corrosion.   Stainless steel and bare carbon steel
agitators  are  also used  in some  locations.   Rakes  for thickeners are also
commonly clad  with  rubber for water and corrosion resistance.   The materials
problems that  have occurred with these components are minimal.

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MATERIALS OF CONSTRUCTION:  Operating Experience   	G-45

Pond Linings
     No  pond problems  were  reported  by FGD  system users.  Where  disposal
ponds are  used,  the preferred lining material  is clay where the permeability
of the soil is low.

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MATERIALS OF CONSTRUCTION:  References                                  G-46
                          REFERENCES FOR APPENDIX G


Brova,  A.   A.   1978.   Chemical  Resistant  Masonary,  Flake,  and  Reinforced
Linings  for Pollution Control Equipment.  Proceedings -of  Corrosion Problems
in Air Pollution Control Equipment Symposium, Atlanta, Georgia.

Crow, G. L.   1978.   Corrosion Tests Conducted  in  Prototype Scrubber System.
Proceedings  of the  Corrosion  Problems  in  Air Pollution  Control  Equipment
Symposium,  Atlanta, Georgia.

Fontana, M.  G. ,  and  N.  0. Greene.   1978.   Corrosion  Engineering.   McGraw
Hill Book Co., New York.  pp. 157-159.

Furman, H.  N.  1977.  Chemical Engineering Progress.  72(ll):92-94.

Gleekman,  L.  W.   1978.   Old Materials  for Air  Pollution  Control  Equipment.
Proceedings  of  Corrosion   Problems  in  Air  Pollution  Control   Equipment
Symposium,  Atlanta, Georgia.

Hoxie,  E.   C. , and   A.  T.   Michaels.    1978.   How  to  Rate  Alloys  For  SOo
Scrubbers.   Chemical  Engineering,  pp.  161-165.

Inco.   1980.   The  Corrosion  Resistance  of Nickel-Containing Alloys  in Flue
Gas  Desulfurization  and  Other  Scrubbing  Processes.   Corrosion  Engineering
Bulletin No. 7.

Kensington,  K.  L.   1978.   FRP Applications and Specifications  in Pollution
Control  Equipment.    Proceedings  of  Corrosion Problems   in  Air  Pollution
Control Equipment Symposium, Atlanta, Georgia.

Leonard, R.  B.   1978.  Application of Nickel Chromical  Alloys  in Air Pollu-
tion  Control  Equipment.  Proceedings  of the Corrosion Problems  in Air Pol-
lution Control Equipment Symposium, Atlanta, Georgia.

Lewis,  E.  C. ,  et al.   1978.   Performance  of TP-316L SS and  Other Materials
in Utility  FGD Systems.  Proceedings of  the Corrosion Problems  in Air Pollu-
tion Control Equipment  Symposium, Atlanta, Georgia.

Mellan,  I.    1976.    Corrosion   Resistant  Materials  Handbook,  3rd Edition.
Noyes Data  Corporation, Park Ridge, New Jersey.

PEDCo  Environmental,   Inc.    1981.   Flue  Gas  Desulfurization  Information
System.   Maintained for the U.S. Environmental  Protection  Agency  under Con-
tract No. 68-02-2603, Task Order No. 6.

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MATERIALS OF CONSTRUCTION:  References	G-47

Rhodin,  T.   N. ,  and  H.   R.  Carson.    1959.   Physical  Metallurgy of  Stress
Corrosion Fracture.  Interscience Publishers, New York.  pp. 451-456.

Rosenberg,  H.   S., et  al.    1980.   Operating  Experience  with  Construction
Materials  for  Wet Flue  Gas  Scrubbers.   Presented at  the 7th  Energy Tech-
nology Conference  and Exposition, Washington, D.C.

Singleton,  W.  T.,  Jr.   1978.   Protective Coatings from Vinyl  Esters.  Pro-
ceedings  of   Corrosion   Problems  in  Air  Pollution  Equipment  Symposium,
Atlanta, Georgia.

Uddeholm  Steel Corporation.   1977.   Report on  the Corrosion  Properties  of
UHB Alloy 904L.  Totowa, New Jersey.

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