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
-------
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
-------
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
-------
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
-------
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?
-------
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
-------
Figure 1-5. Limestone FGD process: external presaturator.
1-13
-------
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
-------
CHARGED PARTICIPATE SEPARATOR
Figure 1-7. Limestone FGD process: venturi spray-tower scrubber
with a-charged participate separator.
1-16
-------
FLUE GAS
VENTURI SCRUBBER
MIST ELIMINA
UASHUATER
Figure 1-8. Limestone FGD process: EPA two-loop scrubbing.
1-17
-------
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
-------
Figure 1-10. Limestone FGD process: hydrocyclone used to
provide mist elimination wash liquor.
1-20
-------
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.
-------
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
-------
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
-------
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
-------
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
-------
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.
-------
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.
-------
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.
-------
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.
-------
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.
-------
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
-------
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.
-------
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.
-------
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
-------
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
-------
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."
-------
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
-------
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,
-------
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
-------
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
-------
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.
-------
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
-------
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
-------
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
-------
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.
-------
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.
-------
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.
-------
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
-------
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.
-------
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
-------
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
-------
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
-------
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
-------
FIXED THROAT
OPENING
Figure 3-1. Basic venturi scrubber and design configurations.
3-7
-------
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
-------
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
-------
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
-------
MIST ELIMINATOR
TRAY
MIST PRECOLLECTOR
SUMP
SIEVE
TRAYS
_7
Figure 3-3. Sieve tray scrubber and detail of tray.
3-11
-------
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
-------
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
-------
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.
-------
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.
-------
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
-------
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
-------
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
-------
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).
-------
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.
-------
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.
-------
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.
-------
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.
-------
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
-------
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,
-------
\\\\\\\\
\\\\\\\\
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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.
-------
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)
-------
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.
-------
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.
-------
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
-------
co
REHEATER
BOILER
1
AIR
PREHEATER
ESP
SCRUBBER
STEAM
^
1
*
i
*
STACK
OR
HOT WATER
Figure 3-12. In-line reheat system.
-------
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.
-------
GJ
i
REHEATER
AMBIENT
AIR
EXCHANGER
Figure 3-14. Indirect hot air reheat system.
-------
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.
-------
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.
-------
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.
-------
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
-------
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
-------
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.
-------
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.
-------
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
-------
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
-------
FLUE GAS
MIST
ELIMINATOR
WET ID
FAN
Figure 3-24. Wet ID fan installation.
3-71
-------
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
-------
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.
-------
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.
-------
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.
-------
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).
-------
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).
-------
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
-------
2.0
CO
Q
o
CO
CD
Q
_l
CO
1.5
o
o
o
OJ
0)
0)
o
re
1.0
CD
O
co
0.5
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
-------
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. *
-------
LIFT
INDICATOR
LAUNDER
co
00
CENTER DRIVE
UNIT AND LIFTING
DEVICE
DRIVE MOTOR AND
•GEAR ASSEMBLY
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.
-------
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
-------
OVERFLOW
FEED
FLOW DISTRIBUTION
'ORIFICES
PICKET-FENCE-
TYPE SLUDGE
SCRAPER
MOTOR
LAMELLA
PLATE PACK
Figure 3-27. The Lamella gravity settler thickener.
3-83
-------
CLOTH CAUtKINQ
STRIPS
AUTOMATIC VALVE
AIR AND FILTRATE
LINE
CO
i
00
.£»
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.
-------
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
-------
COVER
DIFFERENTIAL SPEED
GEAR BOX
CO
00
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.
-------
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.
-------
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.
-------
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
-------
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
-------
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.
-------
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.
-------
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.
-------
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.
-------
CO
I
to
H"ss%"|-n
commucTio
Ui "•«• LI
JTIco««»mucTio«Jj
IMHIKHfO Ij
CO»»»TIIUCtieM|
I octw ottroiu. I
Figure 3-31. FGD sludge disposal Alternatives (Knight et al. 1980),
-------
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
-------
GO
I
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.
-------
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
-------
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.
-------
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.
-------
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.
-------
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.
-------
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
-------
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.
-------
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.
-------
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.
-------
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
-------
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.
-------
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.
-------
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.
-------
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
-------
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
-------
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
-------
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
-------
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.
-------
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
-------
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.
-------
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.
-------
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.
-------
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.
-------
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.
-------
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.
-------
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
-------
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
-------
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
-------
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.
-------
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
-------
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
-------
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.
-------
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
-------
PROCESS
CONTROL
FUNCTION
FROM LIMESTONE
FEED SLURRY TANK
MAJOR
PROCESS
EQUIPMENT
SPRAY PUMP
*-TO THICKENER
Figure 3-36. Limestone feed control loop.
3-158
-------
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
-------
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.
-------
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
-------
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
-------
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
-------
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
-------
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.
-------
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.
-------
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.
-------
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.
-------
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.
-------
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
-------
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:
-------
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
-------
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.
-------
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
-------
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.
-------
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.
-------
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)
-------
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.
-------
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
-------
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
-------
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.
-------
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.
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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.
-------
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).
-------
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
-------
o
CM
CM
O
(/)
-------
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
-------
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
-------
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
-------
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.
-------
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.
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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).
-------
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,
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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.
-------
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.
-------
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
-------
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].
-------
1
MCtMOO
Mil* CMAfUCT
OUUIT CMAKACT
1
MTIWMyfMAl
LOCATMM
Stt CAPACITY
Figure C-2. Computerized data base structure, FGDIS program.
-------
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
-------
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
-------
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.
-------
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
-------
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
-------
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.
-------
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
-------
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.
-------
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.
-------
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.
-------
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
-------
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
-------
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).
-------
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
-------
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
-------
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
-------
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.
-------
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
-------
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
-------
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.
-------
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)
-------
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.
-------
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.
-------
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
-------
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
-------
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.
-------
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
-------
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.
-------
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
-------
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
-------
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.
-------
*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
-------
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.
-------
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.
-------
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
-------
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%
-------
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
-------
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
-------
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
-------
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.
-------
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.
-------
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).
-------
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
-------
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
-------
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.
-------
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.
-------
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
-------
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.
-------
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
-------
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.
-------
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
-------
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
-------
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
-------
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
-------
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.
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