United States
Environmental
Protection Agency
&EPA
Office of Solid Waste
and Emergency Response
Washington, DC 20460
Office of Air
and Radiation
Washington, DC 20460
Off ice of Research
and Development
Washington, DC 20460
EPAOOO-SW-00-000
June 1987
Municipal Waste
Combustion Study

Combustion Control of
Organic Emissions
                  DRAFT

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      MUNICIPAL WASTE COMBUSTION STUDY:
   COMBUSTION CONTROL OF MSW COMBUSTORS TO
     MINIMIZE EMISSION OF TRACE ORGANICS
                 Final Report
                      by
  W.  R. Seeker, W. S. Lamer and M. P. Heap
Energy and Environmental Research Corporation
                   18 Mason
            Irvine, CA 92718-4190
           EPA Contract 68-02-4247
       Project Officer:  James Kilgroe
          Combustion Research Branch
     Air and Energy Engineering Research
     U.S. Environmental Protection Agency
       Research Triangle Park, NC 27711
                Prepared for:

     U.S. Environmental Protection Agency
      Office of Research and Development
            Washington, D.C. 20460
                 May 1987

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This document  has  been approved for publication by the Office  of  Research and
Development,  U.S.  Environmental Protection Agency.  Approval  does  not signify
that  the  contents  necessarily  reflect  the  views and policies  of the
Environmental  Protection  Agency, nor does  the  mention of trade names or
commercial  products constitute endorsement or  recommendation  for  use.
                                     n

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                             ACKNOWLEDGMENTS

     The  authors wish to acknowledge the significant contribution  of a number
of individuals  and organizations.   Mr-  Walter Niessen of Camp, Dresser,
McKee, Inc.  actively participated in data gathering and aided  significantly
to the ideas  for combustion control  strategy.  We also wish to thank the
representatives  from the  manufacturers with which we had  detailed
discussions.  Without their willingness to share data and discuss combustion
control  strategy, this study would  not  have  been possible.  Finally, the
authors  want  to acknowledge Jim  Kilgroe (EPA-ORD)  and Steve Greene  (EPA-OSW)
both for  their financial support  and for their technical guidance.
                                 PREFACE

     This  document was  prepared  by  Energy and Environmental  Research
Corporation  as a task report  under EPA's Fundamental Combustion Research
Program  III  (Contract Number  68-02-4247).   The FCR  III program contract
Project  Officer was Mr. Jim Mulholland while  Mr.  James A. Kilgroe has served
as Task Officer-

     The  work  described in this  document is part  of a comprehensive Municipal
Waste  Combustion  Study  being  carried  out by EPA.   This  comprehensive
evaluation  is the result  of a combined  effort involving EPA's Office of
Research  and  Development,  Office of Solid Waste and Emergency  Response and
Office  of Air  and  Radiation.
                                   n i

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                             TABLE OF CONTENTS

Section                                                                  Page

  ACKNOWLEDGEMENTS AND PREFACE  	  ii

  1.0   EXECUTIVE SUMMARY	1-1
        1.1  Scope of the Study	1-1
        1.2  Combustion Control of Trace Organic Emissions  	 1-2
        1.3  A Combustion Control Strategy  	 1-4
        1-4  Recommended Research	1-6

  2.0   INTRODUCTION	2-1
        2.1  Background	2-1
        2.2  Program Overview	2-5
        2.3  References	2-8

  3.0   MUNICIPAL WASTE COMBUSTION PROCESSES  	  .  . 3-1
        3.1  Municipal Waste Characteristics  	 3-2
        3.2  Combustion Control of Pollutant  Emissions from
               Municipal  Waste  Combustion Facilities  	 3-5
             3.2.1  Basic Combustion Concepts	3-6
             3.2.2  Criteria Pollutants	3-12
             3.2.3  Emission of Organic Compounds	3-12
        3.3  Combustion Control of Starved  Air Systems 	 3-17
        3.4  RDF Systems	3-19
        3.5  References	3-20

  4.0   POTENTIAL FOR AIR EMISSIONS	4-1
        4.1  Organic Emissions  	 4-1
        4.2  NOX Formation and  Control	4-8
        4.3  Particulate and Trace Metals	4-14
        4.4  Acid Gases	4-18
        4.5  References	4-18
                                      IV

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                       TABLE OF CONTENTS (CONTINUED)

Section                                                                  Page

  5.0   CURRENT PRACTICES IN MASS BURN TECHNOLOGY	5-1
        5.1  Mass Burn Technologies	5-1
        5.2  Deutsche Babcock Anlagen	5-5
        5.3  Steinmueller	5-10
        5.4  Von Roll	5-17
        5.5  W+E Environmental Systems Ltd	5-23
        5.6  Martin  GmbH	5-27
        5.7  Volund	5-34
        5.8  Riley  Takuma	5-37
        5.9  Detroit  Stoker	5-42
       5.10  Combustion Engineering  - De Bartolmeis	5-46
       5.11  Westinghouse O'Connor  	 5-50
       5.12  Basic  Environmental Engineering  Inc 	 5-54
       5.13  Enercon/Vicon	5-58
       5.14  References	5-61

  6.0   CURRENT PRACTICES IN RDF COMBUSTORS	6-1
        6.1  Types  of RDF	6-1
             6.1.1   RDF-1 or MSW	6-1
             6.1.2   RDF-2 or Coarse  RDF  (c-RDF)	6-3
             6.1.3   RDF-3 or Fluff  RDF (f-RDF)	6-7
             6.1.4   RDF-4 or Powdered RDF (p-RDF)	6-11
             6.1.5   RDF-5 or Densified RDF  (d-RDF)	6-11
        6.2  Current RDF Projects	-	6-12
        6.3  Firing  Systems in Current RDF  Projects	6-12
             6.3.1   Detroit Stoker  RDF Firing Systems	6-12
             6.3.2   Combustion Engineering  RDF  Firing System  	 6-19
        6.4  References	6-22

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                       TABLE OF CONTENTS  (CONTINUED)

Section                                                                  Page

  7.0   CURRENT PRACTICES IN STARVED AIR  (TWO-STAGE) COMBUSTORS	 7-1
        7.1  Starved Air Technologies	7-1
        7.2  Consumat Systems	7-4
        7.3  Synergy	7-10
        7.4  References	7-12

  8.0   COMBUSTION CONTROL OF ORGANIC EMISSIONS FROM MUNICIPAL WASTE
        COMBUSTORS (MWCs)	8-1
        8.1  Design and Operating Problems - Failure Modes 	 8-1
             8.1.1  Mass Burn Waterwall Failure Modes	8-2
             8.1.2  Refuse Derived Fuel Spreader Stoker Failure Modes. . 8-8
             8.1.3  Small, Multi-Staged Modular Unit Failure Modes . . . 8-11
        8.2  Combustion Strategy for Minimizing Emissions of Air
               Pollutants	8-13
        8.3  Temperature	8-15
        8.4  Combustion Air	8-21
             8.4.1  Total Air Requirements	8-21
             8.4.2  Primary Air Requirements	8-23
             8.4.3  Distribution of Overfire or Secondary Air	8-24
             8.4.4  Verification of Appropriate Air Distribution  .... 8-30
        8.5  Combustion Monitoring Requirements	8-32
        8.6  System Control	8-34
        8.7  Minimization of Hydrocarbon  Species and Other Pollutants. . 8-35
        8.8  References	8-37

  9.0   HYDROCARBON CONTROL STRATEGY - SUMMARY 	 9-1
        9.1  Combustion Practices for Trace Hydrocarbon Emission
               Control  of municipal waste combustors 	 9-2
        9.2  Research Recommendations	9-7
             9.2.1  Combustion Control  Guideline Definition and
                      Verification 	 9-8
             9.2.2  Mechanisms of PCDD/PCDF Formation and Destruction.  . 9-11
             9.2.3  Tradeoffs in Other Pollutants	9-11

                                      vi

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                                    LIST OF FIGURES

Figure                                                                   Page

  2-1   Program approach	2-6

  3-1   The LNC Steinmueller mass burn design features	3-7
  3-2   Estimate of fuel bed thickness and accumulated inert layer
           (Run  18)	3-9
  3-3   Gas compositions from  fuel  bed probe  2  (Run  18)	3-10
  3-4   Relationships of CO and 02  for appropriate operating regions.  .  3-13
  3-5   Adiabatic  equilibrium  species distribution	3-16
  3-6   Components of typical  starved-air modular combustor  	  3-18

  4-1   Hypothetical mechanisms of  PCDD/PCDF  formation chemistry. .  .  .  4-3
  4-2   Thermal decomposition  characteristics of selected organics
           under dilute  (nonflame) conditions  for 1 second
           (Bellinger et  al. 1977)	4-6
  4-3   Impact  of  temperature  and fuel nitrogen on NOX emissions for
           excess air conditions (calculated using EER  kinetic set).  .  .  4-10
  4-4   Application of  reburning and de-NOx schemes  for  NOX  control
           of mass  burn  MSW furnaces	4-13
  4-5   Transformation  of mineral matter during combustion of metal
           containing waste	4-17

  5-1   Mass burning waste power plant at Widmer and Ernst
           at Bielefeld-Hertford, Germany	5-2
  5-2   Deutsche Babcock Anlagen mass burn furnace design features.  .  .  5-6
  5-3   Deutsche Babcock furnace geometry selected based on  refuse
           characteristics  (DBA, 1986) 	  5-9
  5-4   Relationship of CO and 02 for appropriate operating  regions
           (DBA, 1986)	5-11
  5-5   The l&C Steinmueller mass burn design features	5-13
  5-6   Steinmueller in-furnace testing and cold-flow  modeling	5-16
  5-7   Refuse combustion plant with von Roll two-pass boiler and
           flue gas scrubber	5-19
  5-8   Details of Von Roll grate and feeding devices	5-22

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                        LIST OF FIGURES (CONTINUED)

Figure                                                                   Page

  5-9   W+E combustion system and overthrust grate design 	  5-24
  5-10  Design features of Martin Refuse Combustors 	  5-29
  5-11  Adaptation of secondary air injection to changed refuse
          conditions in Martin Systems	5-32
  5-12  Martin Combustion Control System for mass burn combustors . .  .  5-33
  5-13  Volund System mass burn design features  	  5-35
  5-14  Cross sectional schematic of combustion  zone on a
          Riley-Takuma mass burn plant	5-39
  5-15  Cross-section of boiler with current design Detroit Stoker
          firing system 	  5-44
  5-16  Design features of DeBartolomeis grate	5-48
  5-17  Boiler cross-section of CE design using  db grate and
          EVT boiler	5-49
  5-18  Typical  plant configuration using Westinghouse/O'Connor
          combustor	5-51
  5-19  Cross-section of Westinghouse/O'Connor combustor	5-52
  5-20  Process flow diagram of Basic Environmental Engineering
          modular mass burn technology	5-55

  6-1   Albany, New York, Solid-Waste Energy-Recovery System
          (ANSWERS)	6-4
  6-2   Cross-section of Albany, New York boiler plant firing c-RDF .  .  6-5
  6-3   Ames, Iowa, Resource Recovery System for production of
          fluff RDF (f-RDF)	6-8
  6-4   Fluff RDF production system.  Illustration is for system
          developed by Combustion Engineering 	  6-9
  6-5   Trommel  Screen for RDF size segregation	6-10
  6-6   Side sectional view of Detroit Rotograte Stoker equipped
          with Detroit air swept refuse distributor spouts	6-15
  6-7   Detroit Stoker Hydrograte^M with water cooled, vibrating,
          continuous-discharge ash-discharge grates 	  6-17
  6-8   Detroit air swept refuse fuel distributor spout arranged
          with motorized rotary air damper	6-18
  6-9   Combustion Engineering continuous ash discharge type RC
          Stoker for RDF	6-20
  6-10  Combustion Engineering pneumatic RDF distributor	6-21
                                    vii'i

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                        LIST OF FIGURES  (CONTINUED)

Figure                                                                   Page


  7-1   Evolution of U.S. vendor companies supplying starved-air
          waste-to-energy systems,  including persons notably
          influencing system designs	7-3

  7-2   The standard Consumat module for energy-from-waste	7-5

  7-3   Internal transfer rams  in primary chamber of typical
          Consumat facility  	  7-6

  7-4   Theoretical temperature of  the products of combustion,
          calculated from typical MSW properties, as a function of
          refuse moisture and excess air or oxygen (Hasselriis, 1986)  .  7-7

  7-5   Synergy two-stage municipal  waste combustion process	7-11

  7-6   Synergy two-stage combustion saturate  steam system	7-13


  8-1   Mass  burn municipal waste combustor failure modes  	  8-3

  8-2   Boiler  sectional side of NASA/Hampton  mass fired waste-
          to-energy facility	8-7

  8-3   Failure modes of RDF spreader-stoker systems	8-9

  8-4   Temperature distributions in an  operating mass burn
          municipal waste combustor 	  8-16

  8-5   Required temperature for destruction of intermediate
          organics	8-18

  8-6   Thermal decomposition characteristics  of selected
          hydrocarbons	8-20

  8-7   Theoretical temperature of  the products of combustion,
          calculated from typical MSW properties, as a function of
          of  refuse moisture and excess  air or oxygen  (8,9)  	  8-22

  8-8   Typical designs of  overfire air  systems  	  8-27


  9-1   Research program to establish design guidelines  based  upon
          "Good Combustion  Practice"	9-9

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                                     LIST OF TABLES

Table
                                                                          Page
  1-1  Good Combustion Practices for Minimizing Trace Organic
         Emissions From Mass Burn Municipal Waste Combustors  (MWCs).  .  .  1-6
  1-2  Good Combustion Practices for Minimizing Trace Organic
         Emissions from RDF Combustors	1-7
  1-3  Good Combustion Practices for Minimizing Trace Organics
         Emissions from Starved-Air Municipal  Waste  Combustors  (MWCs).  .  1-8
  2-1  Summary of Organic  Emission Ranges  from Full-Scale  Municipal
         Waste Combustor Facilities	2-4
  2-2  Manufacturers  Fact  Finding	2-7

  3-1  Current and Forecast Composition  of  Disposed  Residential  and
         Commercial Solid  Waste  (Weight  Percent)  	  3-3
  3-2  Ultimate Analysis of Typical MSW  as  Presented by Hickman  et
         al.  (1984)	3-4

  4-1  Status of NOX  Control Options for Municipal  Waste Combustors
         (MWCs)	4-12
  4-2  Metals Present in MSW	4-16
  5-1  Design Features of Deutsche  Babcock Anlagen  Systems  	  5-7
  5-2  Design Features of Steinmueller Mass Burn Systems  	  5-14
  5-3  Design Features of Von Roll  Mass Burn Systems	5-20
  5-4  Design Features of W+E Mass  Fired System	5-25
  5-5  Design Features of Martin  Refuse Municipal Waste Combustors
         (MWCs)	5-30
  5-6  Design Features of Volund  Mass Burn Systems  .	5-36
  5-7  Design and Operating Features of Riley-Takuma  Technology	5-40
  5-8  Design Features of Detroit Stoker Mass Burn  Municipal  Waste
         Combustors	5-45
  5-9  Design Features of Basic Environmental Engineering Small
         Modular Mass Burn Technologies	5-56

  6-1  ASTM Classification of Refuse Derived Fuels  	  6-2
  6-2  Active RDF Projects	6-13
                                      x

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                          LIST  OF  TABLES  (CONTINUED)


Table                                                                     Page


  7-1  Selected Data on  Small-Scale  U.S.  Systems  Using  the
         Starved-Air Design	7-2


  8-1  Overfire Air Jet  Configurations  for Large  Mass  Burn
         Municipal Waste Combustors  (MWCs) 	  8-26


  9-1  Good  Combustion  Practices  for Minimizing Trace  Organic
         Emissions from Mass Burn Municipal  Waste Combustors (MWCs).  .  .  9-4

  9-2  Good  Combustion  Practices  for Minimizing Trace  Organic
         Emissions from RDF Municipal  Waste Combustors (MWCs)	9-5

  9-3  Good  Combustion  Practices  for Minimizing Trace  Organic
         Emissions from Starved-Air  Municipal  Waste  Combustors (MWCs).  .  9-6

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1.0
EXECUTIVE SUMMARY
1.1
Scope of the Study
     This  report is an assessment of combustion  control  of organic emissions
from  municipal  waste combustors (MWCs).   The  information presented in this
report  was developed during  a comprehensive,  integrated study of municipal
waste combustion.  An overview of the findings  of  this  study may be found in
the Report to  Congress on  Municipal  Waste Combustion.   Other  technical
volumes  issued as part of the Municipal  Waste Combustion Study  include:
     •    Municipal Waste Combustion Study:
          Report to Congress
                                          EPA/530-SW-87-021A
     •    Municipal Waste Combustion Study:
          Emissions Data Base for Municipal
          Waste Combustors
                                          EPA/530-SW-87-021B
          Municipal Waste Combustion Study:
          Combustion Control of Organic Emissions   EPA/530-SW-87-021C
          Municipal Waste Combustion Study:
          Flue Gas Cleaning Technology
                                          EPA/530-SW-87-021D
     •    Municipal Waste Combustion Study:
          Costs of Flue Gas Cleaning Technologies   EPA/530-SW-87-021E
          Municipal Waste Combustion Study:
          Sampling and Analysis
                                          EPA/530-SW-87-021F
          Municipal Waste Combustion Study:
          Assessment of Health Risks Associated
          with Exposure to Municipal Waste
          Combustion Emissions
                                          EPA/530-SW-87-021G
                                      1-1

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     •    Municipal Waste Combustion  Study:
          Characterization of the  Municipal
          Waste  Combustion Industry                 EPA/530-SW-87-021H

     •    Municipal Waste Combustion  Study:
          Recycling of Solid Waste                 EPA/530-SW-87-021I

     The objectives of this study  were:

     1.   To determine the current state of combustion  control  of municipal
          solid  waste combustion technology
                                                                       4
     2.   To formulate  a  combustion control strategy  based upon "best
          engineering practice"  that will minimize  the emission  of trace
          organics from waste-to-energy plants

     3.   To define the  research which  is necessary to develop  and verify
          this combustion control  strategy.

Although  the focus of  this  study  was concerned  with  the  best  combustion
practices  which will  minimize  the emissions  of organics,  including
polych 1 orinated dibenzo(pjdioxin and  furans  (PCDDs/PCDFs),  the inter-
relationship with other pollutants  such as particulate matter, metals,  NOX.
other  organics, and carbon monoxide was also considered.  The study focused
on the  design  of new units  and  the operation  and monitoring  of new and
existing  units  from the  viewpoint of the combustor/boiler subsystem.  No
consideration  was given  to  the  impact  of the  design and operation of  air
pollution control devices upon  the  emission of trace organic species because
it was covered  in  the  volume  entitled "Flue Gas  Cleaning Technology"
(EPA/530-SW-87-021E).

1.2       Combustion Control  of  Trace Organic Emissions

     Combustion control  of organic species  from three types of municipal
solid waste  combustors, was considered  in this study:

                                     1-2

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     1)    Mass  burning excess air (mass burning)
     2)    Starved  air two-stage (modular)
     3)    Refuse-derived fuel firing (RDF)

RDF can  be burned  in fluidized bed combustors  but  they represent only a  small
fraction  of the waste-to-energy systems either  in  service or planned.   Thus,
this technology was not considered.

     Municipal waste,  unlike fossil  fuels, is  extremely heterogeneous.   Its
composition varies with  the seasons  and  is affected by weather  thus,  its
calorific value  can vary widely and many  systems are therefore designed to
handle  wastes  with heating values ranging from  3,800 to 6,000 Btu/lb.   The
size of MSW also varies  and this affects the feed to the combustion device.
The three incinerator types listed above have  different design philosophies
which  enable them  to take  account of variation in  fuel input properties  which
is imposed by the  characteristics of MSW.

     Mass  burn excess  air  municipal  waste combustors (MWCs) burn  minimally
treated MSW in a thick  bed supported by a pusher grate.  Sufficient air is
provided  in the  bed region to burn  the fuel, although  combustion of  the
volatile  gases is completed above the bed.   Variation in fuel  properties is
counteracted by  controlling the feed  rate, grate speed and the amount  and
distribution of air which is supplied through and above the grate.   In  a
starved-air two stage system the unit is divided  into two distinct  sections.
The first section receives  the waste and is operated with only 40 percent of
the air needed for combustion, thus, it acts  as  a gasifier.  The  fuel  rich
effluent  is burned in  the second section which may contain heat exchange
surfaces.  Heat  extraction is minimized  in the first, fuel  rich section.
Pre-processing to produce a refuse  derived fuel (RDF) is the third method
used to take account of  the heterogeneous  nature  of MSW.  RDF can  be burned
alone,  on a spreader stoker, in a fluidized  bed  or in combination  with coal
in stoker, pulverized coal or cyclone boilers or in combination with  other
fuels  such as wood chips.
                                      1-3

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     Thermodynamic equilibrium considerations  indicate-that under  excess air
conditions  and the characteristic temperatures typical of municipal waste
combustors  (MWCs), emissions of organic  species should be so low that they
can be  considered to be zero.  However, measurements have been made  showing
that some plants  have significant emissions  of trace organic species  some of
which are toxic.  The basis for combustion  control  of emissions of organics
from municipal  waste combustors (MWCs)  is to  provide conditions whereby the
combustion  products can  approach equilibrium.  This  requires that all
combustion  products are affectively mixed  with oxygen at a temperature which
is sufficiently  high to allow the rapid destruction of all organic  species.

     Hydrocarbons,  some of which may  be toxic or which may be precursors to
the formation of toxic hydrocarbons,  can  be formed during the combustion of
MSW.  The fuel  is not completely mixed with  air  because of the heterogeneous
nature  of  the fuel.  Thus,  fuel-rich pockets  will  exist  and under these
conditions  hydrocarbons  can be formed.   However,  kinetic  considerations
indicate  that they can  be destroyed  rapidly in the presence of oxygen at
elevated  temperatures.   The goal  of  combustion control  is  the complete
destruction  of  all hydrocarbon species in  the  combustion system of municipal
waste combustors (MWCs).   Approaching this  goal will minimize emission of
potentially toxic species as well as other species which may be  precursors
and capable of forming  toxic compounds downstream in cooler regions of the
boiler or the air pollution control  devices.

1.3       A  Combustion Control Strategy

     Strategies  for good combustion can minimize the emission of hydrocarbon
pollutants  from  the combustor/boiler  subsystem  of a waste-to-energy  system.
These practices  will also contribute to the  operability of the plant  because
they will  reduce furnace  corrosion  rates  and  improve  efficiency  levels.
Thus, while  it may not  be possible  to entirely eliminate trace  organic
emissions  it should be  possible to operate cost-effective, waste-to-energy
systems  with  minimal emissions of potentially toxic organics  (fractional  part
per trillion  emission concentration range).

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     This study  synthesized  "best combustion  practices" from:  theories  on
the basic  mechanisms  of  PCDD/PCDF formation  and destruction in combustion
systems;  information  provided by manufacturers on the design of waste-to-
energy  systems,  and operating/emissions  data from specific plants.  The
practices are designed  to:

     1.    Limit the  formation  of hydrocarbons
     2.    Maximize  the  destruction of these  same compounds prior to the  exit
          of the  combustor/boiler should they be formed

As such,  these practices restrict conditions which promote the formation  of
hydrocarbons.  In  addition,  they ensure that the environment experienced  by
these hydrocarbons will  promote the destruction of these compounds.   Thus,
the conditions within the  combustor environment  that satisfy these goals  are:

     •    Mixing of fuel  and air  to  minimize  the existence of long-lived,
          fuel-rich  pockets  of combustion products

     •    Attainment of sufficiently  high temperatures in the presence  of
          oxygen  for the destruction of hydrocarbon species

     •    Prevention of quench zones or low  temperature  pathways that  will
          allow partially reacted  fuel  (solid or gaseous) from exiting the
          combustion chamber.

     The development of "best combustion practices" to minimize emissions  of
trace organics from municipal waste combustors (MWCs) involves all  of these
elements:

     t    Design.   Design  the  system to satisfy  several  criteria which  will
          ensure  that temperatures  and the degree of mixing within the
          combustor  are consistent with the  minimization  of formation and the
          maximization  of  destruction of the species of concern.
                                      1-5

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     •     Operation  Control .   Operate  the  system  in a  manner  which  is
          consistent with the  design  goal and provide  facility controls  which
          prevent operation outside on established operating envelope.

     t     Aerification.   Monitor to ensure  that the system is continually
          operated in accordance  with the design goals.

It  is  suggested that  if all of  these elements are  satisfied,  then the
emission  of  hydrocarbons from  the combustor of a municipal  waste  combustor
(MWC) will  be minimized.

     This project  has  identified the  components of  each of these elements
which make  up  the best combustion  practices  and that  are  expected  to  be
important in  the control of  PCDDs  and PCDFs  trace  organic emissions  from
municipal  waste combustors (MWCs).   In  addition, preliminary recommendations
have been made  on the values  of the individual components.  Identification  of
the elements was based upon the current  design practices that have been  shown
to  restrict  successfully trace organic emissions.   The combustion control
components  are summarized in Table 1-1 for mass burn systems.  It must  be
explained  that the  values  presented in  Tables  1-1,  1-2 and 1-3 are
preliminary  targets and  require  considerable verification to ensure  their
appropriateness.   It is possible, for example, that the goals of the strategy
can  be  satisfied by focusing  solely on the verification elements.  In  this
manner,  an  innovative  scheme would be allowed provided that it could  be
demonstrated  that the  system  satisfies the  goals  of flue  gas CO, excess
oxygen,  furnace temperature and in-furnace CO concentration uniformity-

1.4       Recommended Research

     The "best  combustion practices"  defined above for mass burn MSW, RDF and
starved-air modular combustors  were derived from an analysis of the available
information  which  includes  little  direct evidence relating to the
appropriateness  of the preliminary target values recommended in Tables  1-1,
1-2  and  1-3.   Further work is  needed  to better define and verify these
recommendations.  Further work  is required in three major  areas:

                                    1.-6-

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     TABLE  1-1.  GOOD  COMBUSTION  PRACTICES  FOR MINIMIZING TRACE  ORGANIC
                  EMISSIONS FROM  MASS BURN MUNICIPAL  WASTE COMBUSTORS
Element
 Design
 Operation/
  Control
 Verification
           Component
Temperature at fully mixed
height

Underfire air control
                Overfire air capacity (not
                an operating requirement)

                Overfire air injector
                design

                Auxiliary fuel  capacity
Excess air


Turndown restrictions


Start-up procedures


Use of auxiliary fuel


Oxygen in flue gas

CO in flue gas


Furnace temperature
                Adequate air
                distribution
         Recommmendati ons
1800°F at fully mixed  height
At least 4 separately  adjustable
plenums.  One each under  the  drying
and burnout zones  and  at  least two
separately adjustable  plenums under
the burning zone

40% of total  air
That required for penetration  and
coverage of furnace cross-section

That required to meet start-up
temperature and 1800°F criteria  under
part-load operations

6-12% oxygen in flue gas (dry  basis)
80-110% of design - lower limit may
be extended with verification  tests

On auxiliary fuel to design
temperature

On prolonged high CO or low furnace
temperature

6-12% dry basis

50 ppm on 4 hour average - corrected
to 12% C02

Minimum of 1800°F (mean) at fully
mixed height across furnace

Verification Tests  (see text Chapter
8 and 9)
                                       1-7

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      TABLE 1-2.   GOOD COMBUSTION  PRACTICES FOR  MINIMIZING  TRACE ORGANIC
                   EMISSIONS  FROM RDF  COMBUSTORS
Element
 Design
 Operation/
  Control
 Verification
           Component
Temperature at fully mixed
height

Underfire air control
                Overfire air capacity
                (not necessary operation)

                Overfire air injector
                design

                Auxiliary fuel  capacity
Excess air


Turndown restrictions


Start-up procedures


Use of auxiliary fuel


Oxygen in flue gas

CO in flue gas


Furnace temperature
                Adequate  air
                distribution
                                                      Recommmendations
1800°F at fully mixed height
As required to provide  uniform  bed
burning stoichiometry (see  text)

40% of total  air
That required for penetration  and
coverage of furnace cross-section

That required to meet start-up
temperature and ISOOpF criteria  under
part-load operations

3-9% oxygen in flue gas (dry basis)
80-110X of design - lower limit  may
be extended with verification  tests

On-auxiliary fuel to design
temperature

On prolonged high CO or low  furnace
temperature

3-9$ dry basis

50 ppm on 4 hour average - corrected
to 121 C02

Minimum of 1800°F (mean) at  fully
mixed height

Verification Tests (see text Chapter
8 and 9)
                                       1-8

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    TABLE 1-3.   GOOD COMBUSTION  PRACTICES FOR  MINIMIZING  TRACE ORGANIC
                 EMISSIONS FROM STARVED-AIR  COMBUSTORS
     Element
Design
Operation/
 Control
Verification
         Component
Temperature at fully mixed
height

Overfire air capacity

Overfire air injector design


Auxiliary fuel capacity



Excess air


Turndown restrictions



Start-up procedures


Use of auxiliary fuel


Oxygen in flue gas

CO in flue gas


Furnace temperature


Adequate air distribution
     Recommmendations
1800°F at fully  mixed  height
80 percent of total  air

That required for penetration  and
coverage of furnace  cross-section

That required to  meet  start-up
temperature and 1800°F criteria
under part-load conditions

6-12% oxygen in flue gas  (dry
basis)

80-110% of design -  lower limit
may be extended with verification
tests

On auxiliary fuel to design
temperature

On prolonged high CO or  low
furnace temperature

6-12% dry basis

50 ppm on 4 hour average  -
corrected to 12% C02

Minimum of 1800°F at fully mixed
plane (in secondary chamber)

Verification Tests (see  text  Chapter
8 and 9)
                                      1-9

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1.    Guideline definition and verification
2.    Mechanisms of PCDD/PCDF formation from MSW
3.    Tradeoffs in other pollutants
                                 1-'

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2.0
INTRODUCTION
2.1
Background
     This  report is an assessment of combustion  control  of  organic emissions
from  municipal  waste combustors (MWCs).   The information  presented  in this
report  was developed during  a comprehensive, integrated study  of municipal
waste  combustion.  An overview of the findings of this  study may be found in
the Report to  Congress on  Municipal  Waste Combustion.   Other technical
volumes issued as part of the Municipal  Waste Combustion Study  include:
     •    Municipal Waste Combustion Study:
          Report to Congress

     •    Municipal Waste Combustion Study:
          Emissions Data Base for Municipal
          Waste Combustors
                                          EPA/530-SW-87-021A
                                          EPA/530-SW-87-021B
     •    Municipal Waste Combustion Study:
          Combustion Control of Organic Emissions   EPA/530-SW-87-021C
          Municipal Waste Combustion Study:
          Flue Gas Cleaning Technology
                                          EPA/530-SW-87-021D
          Municipal Waste Combustion Study:
          Costs of Flue Gas Cleaning Technologies   EPA/530-SW-87-021E
          Municipal Waste Combustion Study:
          Sampling and Analysis
                                          EPA/530-SW-87-021F
          Municipal Waste Combustion Study:
          Assessment of Health Risks Associated
          with Exposure to Municipal Waste
          Combustion Emissions
                                          EPA/530-SW-87-021G
                                       2-1

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     •    Municipal Waste Combustion Study:
          Characterization of the Municipal
          Waste  Combustion Industry                EPA/530-SW-87-021H

     •    Municipal Waste Combustion Study:
          Recycling of Solid Waste                 EPA/530-SW-87-021I

     It is expected  that the  number of waste-to-energy plants  within the
United  States will dramatically increase  in  the next decade.   Some estimates
indicate  that the amount of waste being  burned will  increase  by a factor of
three  by  1990.   "Characterization of the  Municipal Waste Combustor Industry"
volume  (EPA/530-SW-87-021H)  provides a detailed analysis  of current and
projected  MSW-to-energy capacity in the U.S.

     One major  concern that stems from proliferation of waste-to-energy
plants is  trace emissions  of potentially toxic organic compounds.   Of
particular  interest are  the  cogeners of PCDD and PCDF species,  with major
concern centering on  2,3,7,8 Tetrachlorodibenzo-p-dioxin (TCDD).

     As part of  EPA's comprehensive evaluation  of  municipal waste combustion,
an emissions data base  has  been  established and documented in  the volume
entitled  "Emission Data Base for Municipal Waste  Combustors."   Table 2-1 has
been extracted from that data base to illustrate the wide variability in PCDD
and  PCDF  emissions  from  various  facilities  from which field  test data is
available.   As  shown, the emission rate  for  total   PCDDs and total  PCDFs vary
by  four to  five  orders  of magnitude.   Data forming the  high  end  of
theseaission range come  from field tests performed on the  facility  at
Hampton,  Virginia (Haile,  1984); Scott  Environmental  Services, 1985; Nunn,
1983;  Howes,  1982) and at Philadelphia,  Northwest  (NeuTicht,  1985).  Both of
these  facilities represent  older system designs where the  primary design
concern was  waste volume reduction.  Data  forming  the low end of the emission
range  come  from field tests  performed  on new facilities at Tulsa, Oklahoma
(Seelinger,  1986), at Marion County, Oregon (Ogden  Projects, 1986)  and at the
Westchester  facility at  Peekskill,  New York (New York State Department of
Environmental Conservation, 1986).  Each  of  these  new facilities'  designs is

                                    2-2

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       TABLE 2-1.  SUMMARY OF ORGANIC EMISSION RANGES FROM
                   FULL  SCALE MUNICIPAL  WASTE  COMBUSTION
                   FACILITIESa
Dr\ 1 T 1 1 +• a n-f-
rO 1 1 U uail u
2,3,7,8 TCDD, ng/Nm3
2,3,7,8 TCDF, ng/NM3
TCDD, ng/Nm3
TCDF, ng/Nm3
PCDD, ng/Nm3
PCDF, ng/Nm3
Emission
Mass Burn
0.018-62.5
0.168-448
0.195-1,160
0.322-4,560
1.13-10,700
0.423-14,800
Concentration F
Starved Air
<0. 278-1. 54
58. 5C
1.24-43.7
15.0-345
77.2-1,550
118-1,760
vangeb
RDF-Fired
0.522-14.6
2.69C
3.47-258
31.7-679
64.4-2,840
164-9,110
a  Results from commercial-scale  facilities  only-


b  All concentrations are  reported  in  units  corrected  to  12  percent

   C02.


c  Data available for only one  test.
                                 2--3

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based on  extensive research  and development efforts by  the manufacturer to
produce municipal  waste  combustors  (MWCs) with high thermal efficiency and
low trace  organic emission rates.

     The broad  range  in emission rate  data illustrated  in  Table 2-1 and the
success of modern, wel1-operated facilities in significantly reducing trace
organic emission rates  strongly suggests that  municipal waste combustor
design and operation are critical  components in  developing an  overall organic
emission  control  strategy.   More direct evidence on the importance of
facility design  and operation is available  through studies being conducted at
the municipal waste combustor facility  in  Quebec,  Canada.  In those studies,
an  older  facility was  modified to reflect  current low emission  design
philosophy.   Exhaust emission measurements were obtained  before (1984 tests)
and after the facility  modification  (1986 tests).  Preliminary results
(Finkelstein,  1986) are  indicated below:

                        1984 Tests              1986 Tests

     Total PCDD      800-3980 ng/Nm3         12-205 ng/Nm3

     Total PCDF      100-1100 ng/Nm3       49.3-336 ng/Nm3

All of  these  data are corrected to 12 percent C02-  In the  modified facility
tests (1986  tests) the  indicated data  range reflects operation  of the
facility  in a "fine tuned"  versus a  "poor combustion" mode.   Thus,  the
combination of combustion  system design  and operational tuning was  sufficient
to  reduce total PCDD/PCDF  from a high of nearly 4000 ng/Nm3 to  12 ng/Nm3-
For this facility, combustion control  was sufficient to reduce the  PCDD/PCDF/
emission  rate from near  the  top end  to  near  the  bottom end of the range
indicated in  Table 2-1.   The goal of the current study is to  gather and
evaluate  available  information to help identify those components of
engineering design, operation, and monitoring practices which minimize trace
organic  emissions from municipal waste combustor systems.
                                     2-4

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2-2       Program Overview

     This  volume of the  comprehensive report  series  on municipal waste
combustion is  the culmination of a program having three major  objectives:

     •    To determine the current state  of combustion control of municipal
          waste  combustion technology;

     •    To recommend design and operating approaches which will  minimize
          emissions of trace organics;  and

     •    To  define the  research which is necessary to  develop and verify
          these  design and operating approaches.

The  focus  of this  volume is to present information on the combustion
practices which will minimize the emissions of organic compounds including
PCDDs  and PCDFs.  However, the interrelationship with other  pollutants  such
as  particulate matter,  metals, NOX> and carbon  monoxide must  also  be
established  so  that conditions are not followed that minimize emissions  of
PCDD/PCDF  while increasing  the emissions of  other potentially  harmful
compounds.   The focus of this volume is on the design  of new units and the
operation and  monitoring of both new and  existing units.

     In Figure  2-1, the approach followed in the study that resulted in  this
volume  is shown.  The approach was designed in an attempt  to  make use of the
knowledge base, which resides with those engineers who  design and construct
waste-to-energy facilities.  Detailed,  face-to-face meetings were held  with
fifteen of the major organizations associated with waste-to-energy operations
in the  United States and Europe.  Four other U.S. manufacturing firms  were
contacted by  telephone.  Table 2-2 provides a listing of those organizations
who contributed to this study with their technical expertise in a series of
individual  fact-finding meetings.  Archival  combustion  literature and
literature  on PCDD/PCDF  formation mechanisms made up  the remainder of the
input data base.
                                      -2-5

-------
MANUFACTURER DESIGN
APPROACH AND
KNOWLEDGE BASE
ENGINEERING ANALYSIS
 -  SYSTEMS
 -  FAILURES
 -  CASE STUDY
DESIGN/OPERATING
   GUIDELINES
 -  REVIEW PROPOSALS
 -  RECOMMENDATIONS
 RESEARCH NEEDS
  -  UNCERTAINTIES/
      ISSUES
  -  EXPERIMENTAL
      APPROACH
                                     LITERATURE DATA
COMBUSTION
 CONTROL
  FINAL
 REPORT
                Figure 2-1.  Program  approach.
                             2-6

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TABLE 2-2.  MANUFACTURERS  FACT FINDING
•  EUROPEAN MANUFACTURERS
        VOLUND (DENMARK)
        DEUTSCHE BABCOCK (GERMANY)
        STEINMULLER (GERMANY)
        VON ROLL (SWITZERLAND)
        WIDMER AND ERNST (SWITZERLAND)
        MARTIN (GERMANY)

•  OTHERS
        DANISH EPA
        PROF.  HUTZINGER (UNIV. OF BAYREUTH)
        ANRED  (FRENCH AGENCY FOR WASTE
               RECOVERY AND DISPOSAL)
        AMERICAN BOILER MANUFACTURERS ASSN.
                       AMERICAN MANUFACTURERS
                            RILEY STOKER *
                           .COMBUSTION ENGINEERING
                            WESTINGHOUSE/O'CONNOR
                            DETROIT STOKER
                            FOSTER WHEELER
                            BABCOCK & WILCOX *
                            CONSUMAT INCINERATION  SYSTEMS
                            JOHN BASIC*
                            ENERCON  *
                           CONTACTED BY  TELEPHONE ONLY

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     The second  phase of the program was the analysis  and  interpretation  of
the available data on design  and .operating of  municipal waste combustion
equipment.   This  phase included an  analysis of current practices in different
types of municipal waste combustor systems and potential  failures that might
occur in both design and operation  that could lead to  the  emission of trace
organics.   The engineering  analysis  phase also  included  an examination  of
case  studies of  incinerator facilities of older design and operating
practices  which  were  found to have higher PCDD/PCDF emissions than is common
for modern  units.

     From the  engineering analysis  phase, recommendations  were made on design
and operating approaches  that would reduce PCDD/PCDF/furan emissions and
prevent system failure.  This program phase included a review and critique  on
the  draft guidance prepared by PEER  Consultants  (1986).   Finally
recommendations  were made on  research  approaches to  address  those
uncertainties and  issues that cannot be addressed  with  the  present knowledge
base.

     The remaining  chapters cover overviews of the  municipal waste combustion
process  and potential for air emissions (Chapters 3 and  4; current practice
in different  types of combustion systems (Chapter  5, 6 and  7; and approaches
to combustion  control  to minimize PCDD/PCDF emissions (Chapter 8 and 9).

2.3       References

     Finkel stein,  A.,  et al .   Presentations  by Environment Canada  at
     Municipal Solid Waste  Incineration Research and Planning Meeting.
     Durham,  North  Carolina.  December  9-11, 1986.

     Haile, C. L.,  et  al.  Assessment of Emissions of Specific Compounds from
     a Resource  Recovery Recovery  Municipal  Refuse  Incinerator (Hampton,
     Virginia).   EPA-560/5-84-002.   June 1984.

     Howes, J. E., et  al.  Characterization of Stack Emissions from Municipal
     Refuse-to-Energy Systems (Hampton, Virginia; Dyersburg, Tennessee; and

                                      2-8

-------
Akron,  Ohio).   Prepared by  Battelle Columbus Laboratories  for U.S.
Environmental  Protection Agency/Environmental Research  Laboratory.
1982.

Neulicht,  R.   Emission Test  Report:   City of Philadelphia Northwest and
East Central  Municipal Combustors.   Prepared for  U.S. Environmental
Protection  Agency/Region III  by  Midwest Research  Institute.   October
1985.

New  York  State Department  of  Environmental Conservation.   Emission
Source  Test Report - Preliminary Test Report on Westchester RESCO.
January 8,  1986.

Nunn,  A.  B.,  III.  Evaluation of HC1 and Chlorinated  Organic  Compound
Emissions  from  Refuse Fired  Waste-to-Energy Systems  (Hampton, Virginia;
and Wright-Patterson Air Force Base, Ohio).  Prepared  for  U.S. EPA/HWERL
by Scott Environmental Services.  1983.

Ogden  Projects.   Tulsa Waste-to-Energy Tests Show  Low Dioxin.  Coal and
Synfuels Technology.   September 1,  1986, p. 6.

PEER Consultants,  Inc.   Design  and Operating Guidance to Minimize
Dioxins and Other Emissions from Municipal Waste  Combustors.  Draft
Report  to  EPA Office of Solid Waste Under EPA Contract  68-02-6940.
May 19, 1986.

Scott  Environmental  Services.  Sampling and  Analysis  of Chlorinated
Organic Emissions  from the Hampton  Waste-to-Energy  System.   Prepared for
The Bionetics  Corporation.  May 1985.

Seelinger, R., et  al.   Environmental  Test Report (Walter B.  Hall,
Resource  Recovery  Facility, Tulsa,  Oklahoma)-    Prepared by  Ogden
Projects,  Inc. for Tulsa City County Health Department.   September 9,
1986.
                                  2-9

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3.0       MUNICIPAL  WASTE COMBUSTION PROCESSES

     The current  report considers combustion control of organic species  from
three types  of  municipal waste combustors,  namely:

     •    Mass-burning  excess air (mass burning)
     •    Starved  air two-stage  (modular)
     •    Refuse derived fuel firing (RDF),

The  mass-burning  excess air and starved-air two  stage categories include the
combustion systems which burn raw municipal solid waste.  Descriptions of th'e
hardware  and design features offered by various  mass-burn and modular system
suppliers  are presented  in Chapters  5 and 7 respectively-  Processes  have
been  developed which  convert  raw municipal solid waste into a more uniform
fuel  termed refuse derived fuel (RDF).  RDF may be burned in a wide variety
of furnaces including furnaces designed  to fire coal or other fossil fuels
such  as wood or  bagasse.   RDF may also be co-fired with traditional  fossil
fuels.  The processes  to produce RDF and hardware configurations being built
for  combustion of  RDF are presented in Chapter  6.  Fluidized bed combustion
systems are being  built to burn  RDF; however, as  illustrated elsewhere in the
comprehensive municipal  waste combustion  study, fluidized bed systems
represent only a  small portion  of the current and projected refuse-to-energy
market.  Accordingly, this technology is not included in this report.

     This  chapter  provides a  brief  review of the physical  and chemical
processes  which  occur during  municipal  waste combustion.  The discussion
illustrates the  relationship  between combustion process design and  the
emission rate of major  pollutants of concern:

     •    Polycyclic organic matter  (including  PCDDs and PCDFs)
     •    Particulate matter
     t    Nitrogen oxides
                                    3-t

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Throughout  the discussion,  any reference  to  emissions rate  or  exhaust
concentration  will  refer to conditions at  the  boiler exhaust,  upstream of any
flue gas air  pollution control  device (APCD).

3-1       Municipal Waste Characteristics

     An  understanding of  municipal waste combustion must begin  with  an
appreciation for the non-uniformity of MSW and the implication  of  that
variability  on  combustion system design.  Typical wastes  might  include paper
products,  food  scraps, tin and aluminum cans, glass bottles,  a  wide  range of
plastic items,  cloth items, floor sweepings, etc.  During the  summer months
there  will  be plastic bags .of yard clippings, and in the fall,  dead leaves
might  replace grass clippings.  Thus,  a municipal waste combustor burning
minimally treated waste must be able to handle a heterogeneous feed.

     Characterizations of the constituency of MSW represent the average over
large  volumes of waste  and may also include temporal averaging.  Table 3-1
presents a characterization  of MSW  in  the  United States  as reported  by
Franklin Associates (1986).  Table 3-2 presents an ultimate  analysis of MSW
as  reported  by  Hickman, et al., (1984).   The  higher heating value of the MSW
described by both Franklin Associates  and  by  Hickman, et  al., was on the
order  of 4500 Btu/lb.  This heating value is  typically used to  estimate mean
operating conditions for  mass burning  municipal waste  combustion  facility
designs, Riley  Stoker (1986) and  Detroit Stoker (1986); note  that system
performance  guarantees  are typically based  on burning 4500  Btu/lb  MSW, but
that systems are designed  to accommodate waste with average heating values
which range from 3800 to 6000 Btu/lb.  During  rainy periods the  heating value
of  MSW  will  decrease while dry periods or holiday seasons tend to result in
MSW with increased  heating value.

     The municipal waste delivered to a resource recovery facility will have
items of all  sizes  and shapes.  Many municipal waste combustors  remove bulky-
oversize  items such  as  refrigerators but  the characteristic  size  is
sufficiently large that unprocessed MSW  must be burned on a  grate.  Feeding
the MSW at a  controllable rate and achieving  uniform distribution across the
                                    3-2

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TABLE 3-1.  CURRENT AND FORECAST COMPOSITION OF DISPOSED RESIDENTIAL
            AND COMMERCIAL SOLID WASTE  (WEIGHT PERCENT)
Component
Paper and Paperboard
Yard Wastes
Food Wastes
Glass
Metals
Plastics
Wood
Textiles
Rubber and Leather
Miscellaneous
TOTAL
Year
1980
33.6
18.2
9.2
11.3
10.3
6.0
3.9
2.3
3.3
1.9
100.0
1990
38.3
17.0
7.7
8.8
9.4
8.3
3.7
2.2
2.5
2.1
100.0
2000
41.0
15.3
6.8
7.6
9.0
9.8
3.8
2.2
2.4
2.1
100.0
                                 3-3

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TABLE 3-2.  ULTIMATE ANALYSIS OF TYPICAL MSW AS
            PRESENTED BY HICKMAN ET AL. (1984)
     Ultimate Analysis

       Moisture

       Carbon

       Hydrogen

       Oxygen

       Nitrogen

       Chlorine

       Sulfur

       Inorganics  (ash)
 25.2

 25.6

  3.4

 20.3

  0.5

  0.5

  0.2

 24.4
100.0
                       3-4

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grate  is difficult  but necessary  for proper operation.   Mass  burning
facilities  use cranes or  front-end loaders to fill  a  loading chute.
Hydraulic rams  or pusher grate  sections are  used to load the MSW  into the
municipal waste combustor.  Municipal waste combustion facilities use a thick
fuel bed which  tends to damp  out  effects due to the  variability in fuel
heating value  and the inability  to feed waste at a  precise rate.

     Once the  MSW is in the furnace  it is slowly moved by grate action toward
the ash  dump.  For the MSW to  burn it must be  exposed to combustion air and
heated.  Combustion air is  introduced both through the grate (underfire or
undergrate  air)  and through air jets located  above the bed (overfire air).
To  achieve  fuel  burnout the grate must be designed to agitate or  stir the
thick  fuel  bed as it moves  the  MSW  from entrance to exit.  Each of the grate
manufacturers have developed  their designs to accomplish this objective as
discussed in Chapter 5.

3.2      Combustion Control  of Pollutant Emissions from  Municipal Waste
         Combustion Facilities
     Chapter 4 provides  a discussion of  the  potential  air emissions from
municipal  waste  combustors.   The  "emissions  data base for municipal waste
combustion"  study provides  a  summary of available field  test data from
municipal  waste  combustion  facilities including  emissions data  on  criteria
pollutants,  acid gases,  trace  metals,  and various hydrocarbons  including
toxic  organic  pollutants.   An  evaluation of PCDD and PCDF emissions from
resource  recovery facilities  is  available in a  recent report provided  to EPA
by Camp Dresser and McKee (1986).

     Details  of the combustion process will impact  directly the emission rate
of several criteria pollutants  (CO,  NOX and  total particulate  matter) and
unburned  hydrocarbons.   In  addition,  they may impact the emission rate of
trace  metals.   The following  subsections will   review a limited  number of
basic  combustion  concepts and  then  discuss  the relationship between
combustion processes and emissions.   The focus  of this discussion will be
municipal  waste combustion in  mass burning, excess  air facilities.
                                   3-5

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3-2.1     Basic  Combustion Concepts

     Figure 3-1  is  a schematic of the  grate region of a mass  burning excess
air municipal  waste combustor-   The waste-fuel  enters the  grate from the
charging  hopper (1) on  the  left and is moved  toward the ash  dump on the
right.  Table  3-2 presented an  analysis  for an MSW containing  24.4 weight
percent ash.   Ideally, the combustible  constituents would be  burned and the
ash dumped  from  the  system for landfilling.  Air must be supplied to sustain
the burning process.   To  completely burn  the  waste the theoretical  air
requirement is  determined by the formula
                                 »
     Wa =  11.5  C  +  34.5  (H-0/8) + 4.32S

where Wa is  the  mass  of  air per mass of  fuel  at stoichiometric  conditions and
C, H,  and 0 and S are  the  weight fractions of carbon, hydrogen, oxygen and
sulfur in  the  fuel, respectively.   For one pound of MSW defined in Table 3-2,
3.25  pounds of air  (approximately 42.8 cubic feet at 70°F and 1  atmosphere)
are  required  to convert all  of the carbon  to  C02, all of  the  hydrogen to
water,  and  all  of the  sulfur to S02-   If  this quantity of air and MSW were
mixed  and burned to  completion there would  be a zero oxygen concentration in
the exhaust.   For a variety of reasons,  including the variability of fuel
characteristics, municipal  waste combustion  facilities are operated with
significantly more  than  the theoretical  air  requirements, typically  70 to 100
percent excess  air at full load.   If a  facility were burning  the  MSW defined
in Table 3-2 with 100 percent excess excess  air, the extra 3.25 pounds of air
per pound of  fuel  would serve as a diluent  to the combustion  products.  The
pound  of  MSW  burned would  still release only 4500 Btu of heat and thus the
temperature of the  combustion gas would  be  decreased relative to that at
lower excess air  conditions.

     The  MSW  defined  in  Table 3-2 contained approximately  25 percent
moisture.   As  illustrated  in Figure 3-1,  many municipal waste combustors
provide an  arch over the  grate  region where  the MSW enters the  municipal
waste combustor.  Heat from the hot arch  wall  is transferred to the  fuel bed,
raising the MSW temperature enough to  begin driving off the  moisture.  This
                                    3-6

-------
Figure 3-1.  The L&C Steinmueller mass burn design features,
                            3-7

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initial  grate  section is  referred to as the drying  grate.  Undergrate air
provided to  this  section will  help  to  drive off the moisture, particularly if
the air  is  preheated slightly.   The important  point  is that the volatile
matter  released by the MSW in the  initial grate section is mainly water and
thus limited net  heat release occurs in this region of  the furnace.

     The second  section  of grate  shown  in Figure 3-1 is referred to as the
burning  grate.  Because much of  the MSW moisture will  be driven off in the
drying  grate  region, the  fuel  entering the  burning grate will  have  a
significantly  increased heating  value.  The fuel is  still in a solid state
and the  bed may be upwards  of a  meter  thick.   Williams,  et al .,  (1974)
describe a  series of pilot scale  studies designed to  define the controlling
phenomena  in thick burning  beds  of solid waste.  That report also reviews
several attempts  to model the process. As described in that report  (also see
Niessen) et al . ,  (1972))  the  top layer of  the  fuel bed  is ignited  by
radiation  received from the hot combustion gas and the  refractory lining of
the lower furnace.  This is followed by slow propagation  of the ignition wave
down  into  the  fuel bed.  The experimental data indicated that the underfire
air flow  rate  and the  fuel consumption  rate were  essentially  in
stoichiometric  proportion but that  the gas composition  at the top of the fuel
bed generally  corresponded  to the equilibration  of the water-gas  shift
reaction:

     C02 + H2 = CO  + H20

Figure 3-2  is  taken from the Williams et al., report  and shows the temporal
variation  in ignition front location, bed thickness and  depth of accumulated
inert.   These  data were obtained using a  stationary bed  of waste fuel and do
not include the effect of fuel bed  agitations by motion of the grate.  The
data  do  illustrate, however, many of the basic features of bed burning.
Figure 3-3, compiled from data taken  during the same experiment, illustrates
gas composition  measured by a sampling probe  located within the fuel bed, 12
inches above the grate.   As  shown,  the  ignition front reaches the sampling
location in approximately 2000 sec. Prior  to that time the  sampling  probe
only detects underfire air-  As the ignition front moves closer  to  the  grate,
                                   3-8

-------
I
10
         LJ
         K
         <
         cr
         CD

         LJ
         >
         o
         CD
         Ld
         U
         Ul
         5
                                        Location  of  Top  of Bad
     Location  of
      Ignition Front
Depth  of  Accumulated
       Inert           —
         Active  Burning

             Depth
0.8 -
               0
                0
                100O         2000         3OOO

                          TIME  (SEC)
                                                                        4000
                   Figure 3-2.  Estimate of  fuel  bed thickness and accumulated inert
                              layer (Run 18).

-------
CO
I
O
-
00
            -    90
                 20
            CD
            cc
            Q
            O

            to
            O
            o.
            2
            O
            u
*• Time Ignition Front
       4 ,-
      O
Ignition Fron t  s

 at Grate
                                                                             to
                                                                             CD
                    2000
                             3OOO
                                        4000
                                    ELAPSED   TIME(SEC)
                      Figure 3-3.  Gas compositions from fuel bed probe 2 (Run 18).

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the local  oxygen  concentration falls with  a  commensurate increase in  C02, CO,
H2 and  CH4.   After  approximately 2500 sec,  the  local 02 concentration falls
to zero  indicating  that  the active  burning  region is below  the  sampling
location.

     The data  trends shown in Figure 3-3  are consistent with viewing the MSW
fuel bed  as  a gasifier.   The process begins with reactions producing char,
C02 and  H20  plus the vaporization of any  free moisture.  This is followed by
endothermic reactions  (reactions for which heat must  be added) consisting of:

     C + H20   ^  CO + H2  and
     C + C02   ^  CO + CO

Note in  Figure  3-3  the continual  increase  in CO and H2 concentration until
approximately 3300 sec.  while the local C02  concentration is seen to  fall.

     In an actual  MSW  municipal waste combustor, propagation of  the  ignition
front  and gas concentration profiles will be far more complex than described
above.   The  bed material  in the experiments by Williams,  et al.  (1974) was
either simulated  or real RDF and not MSW.   Further, in actual municipal waste
combustors the grate  action is designed to mix and aerorate the MSW bed which
results  in dispersion of  the ignition  front.  Regardless, the pilot-scale
results  illustrate  several  of the  critical  features of  municipal  waste
combustion  in  the  burning  grate  region.   One of the critical  features
illustrated  is  the  need  for overfire air.  The underfire air  added to the
burning  grate coupled with  grate  agitation combined to define the rate at
which  the MSW is consumed.  The gases leaving the fuel bed, however, contain
a significant concentration of ^2* CO, and unburned hydrocarbons.  Additional
air  must  be mixed with  that  effluent  to complete the conversion of
combustibles to C02 and  H20.  The  complete  conversion  is essential  for
maximizing combustion  efficiency and for minimizing pollutant emissions.   A
portion  of that air will  be provided by underfire  air which short  circuits
the fuel  bed by channeling.   The majority,  however, must be provided by the
overfire air  supply.
                                   3-11

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3-2.2     Criteria Pollutants

     Combustion process details  can  have a significant  impact on the emission
of several  criteria pollutants  including CO, NOX and  total  particulate.
Discussion  of how combustion  conditions impact  on emission and control of
NOX,  particulate matter and trace  metals is contained in Chapter 4.

     As noted  in  the previous section, substantial  concentrations of CO will
be released from  the fuel bed.  Controlling emission of CO from the facility
exhaust  requires that  a  controlled quantity of air  be  mixed with  that
effluent.  Carbon monoxide is the most refractory species in the oxidation
chain  from  hydrocarbon  to  C0£ and H20.   The dominant  kinetic step for CO
oxidation is:

     CO + OH —»  C02 + H

Adding too  much  air to  the effluent  from the bed  will depress the local gas
temperature and  the concentration  of the OH radicals.   If too little air is
added,  the  probability  is  increased that pockets of gas from the burning
grate  will  exit  the hi gh. temperature regions without ever being mixed with
oxygen.   Figure  3-4 is  taken  from data provided  in a meeting with Deutsche
Babcock  (see Section 5.2)  and illustrates the existence of an appropriate
operating range for minimum CO emissions.

3.2.3     Emission of Organic Compounds

     One  of the major objectives  of the current report is to develop  a
framework  for defining municipal waste combustor  design and operating
practices which minimize  the  emission of trace  organic compounds.   The
organics  of particular  concern  are  the polychlorinated dibenzo-p-dioxins
(PCDDs)  and the polychlorinated dibenzofurans (PCDFs).   Available field test
data were outlined previously in  Table 2-1 and indicated  that PCDD emissions
ranged from 1.13 to 10,700 ng/Nm3  while PCDF emissions  ranged from 0.423 to
14,300 ng/Nm3.   In developing a  control strategy for  these compounds,  it is
important to recognize  that these emissions rates,  including those at the
                                   3-12

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 X i—I
 Oh-
 z-a:
 OCC
 OCJ
 co 2:
 ceo
 CJ
            3      6      9      12


           OXYGEN CONCENTRATION
      A -  INSUFFICIENT  AIR  C+|02-*CO

      B -  APPROPRIATE OPERATING REGION

      C -  "COLD BURNING"
Figure 3-4.  Relationships of  CO and CL for
            appropriate operating regions.
                  3-13

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high end  of the range,  represent trace  concentrations.  The measured PCDD
concentrations  range from approximately  1  ppb  (molar basis) to a  fraction of
a part  per trillion.

     Combustion  begins with a fuel which  is itself an organic and  proceeds by
destroying organics through a complex series of oxidation reactions.   In the
earlier  discussion of  basic combustion concepts, an equation was  presented
for calculating  the  amount of air theoretically required to completely  burn a
given  mass of hydrogen  fuel.   That  equation is based on  determining the
quantity  of air required to oxidize fuel carbon to C02, fuel  hydrogen  to h^O
and  fuel  sulfur to SC^.  The actual  burning process is propagated  through a
complex  series  of chemical reactions.  Exhaust emission of organic  compounds
represents a failure to complete the oxidation reactions.   Chapter 4.0 will
discuss  current theories/understanding of  how  PCDDs and PCDFs are formed and
destroyed during municipal waste combustion.  Generally, however,  emission of
organics  will occur because of a failure  to supply sufficient oxygen  to all
of the reacting gases or termination of the oxidation reaction through some
quenching process.

     As noted previously, definition of an acceptable PCDD or PCDF emission
rate does not currently exist.  Considering the fact that dioxin emission
concentrations  in  the  part per trillion range  may be of  concern,  it is
important to determine  if there is  a lower  limit on emission reduction by
combustion control.  That limit, if any, may be examined through  equilibrium
calculations which predict the concentration  of  reaction product species in
the  absence of mixing  or chemical kinetic  limitations.  Equilibrium
calculations depend on the elemental  composition  of the mixture (moles of C,
H, 0,  N,  S, Cl, etc.),  the reaction temperature, and the thermodynamic
properties of product species.

     Kramlich et al.,  (1984) studied the  equilibrium product  distribution  for
various  chlorinated benzene/air mixtures.   Since equilibrium calculations
depend on elemental composition of  the  mixture rather  than the chemical
structure of the reactants, results from the Kramlich study should reflect
limiting condition trends for municipal waste combustion.   Sample results  are
                                   3-14

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presented  in  Figure 3-5 as  a  plot  of species  concentration versus percent
theoretical  air  assuming  that the mixture is at  the  adiabatic  flame
temperature.   The  curve  labeled  THC represents  the  predicted  total
hydrocarbon  concentration  (sum of concentration  of all hydrocarbon species
present).   For mixtures with  at  least 55 percent of  the  theoretical air
requirements  (and no heat  loss)  the equil ibrium hydrocarbon concentration
will be  less  than 1 ppt.   As  the mixture ratio becomes more fuel-rich, the
presence  of  light hydrocarbons  such as CH4  (and later  Cz^Z'  etc.) are
predicted.   Part per trillion concentrations of  benzene and tolune are not
predicted  until the mixture contains  less than 30  percent of the theoretical
air requirements.   Prediction of ppt equilibrium concentrations of PCDD, PCDF
or precursors  such  as chlorobenzenes and chlorophenols are restricted to even
more fuel  rich  conditions.

     Equilibrium  calculations may also  be  used  to evaluate other limiting
case characteristics.  Consider, for  example, the  situation when a pocket of
organic  gas does exist in the municipal waste combustor and is mixed into an
oxidizing  gas stream.  That  situation may be  described  by the  overall
reaction:
          Kp  -
                PC02
               porganic
where  P  represents the partial  pressure of a given  constituent and Kp  is the
equilibrium constant.  The equilibrium constant is fundamentally related to a
measurable thermodynamic property called the Gibbs Free  Energy (AG) by:

     Kp = EXP (AG/RT)

where  T  is temperature and R is the Universal gas  constant.  If the organic
compound  is taken as a furan,  G would be approximately  492 Kcal/mole (Stull,
et al.,  1969).   For typical  municipal waste combustor exhaust gas C02, H20
and  02 concentrations and for  an  assumed reaction  temperature, the
equilibrium concentration  of  furan can be calculated.  For temperatures as
                                   3-15

-------
   10
                                 I       I
0

cr
   10
   10'
     -2  -
2  10~6
   10
     -8
  10
    -10
       0     20
                                                OVER ENTIRE RANGE
                                               I	|	|	I
60     80    100    120    140    160    180   200

PERCENT THEORETICAL AIR
            Figure 3-5.  Adiabatic equilibrium  species  distribution.
                                    3-T6

-------
low as  1000K,  that equilibrium furan concentration  is  less than ID'100 mole
fraction.

     The above  discussion illustrates two significant  aspects of combustion
control for  trace organics.   First,  at reasonable temperature levels and
under  excess  oxygen conditions,  there is no  thermodynamic barrier  to
achieving  essentially zero  emission  level of trace  organic.  (There may,
however,  be  mixing or kinetic barriers).  Second, semi-volatile organic
compounds  are not  thermodynamically stable under high temperature conditions,
unless  the mixture fuel/air ratio  is very fuel-rich. The fact that organics
such as PCDD and PCDF are emitted from municipal  waste combustors indicates
that  chemical  kinetic or mixing factors have  prevented the gases  from
reaching  the equilibrium condition.  The chemical  kinetic and mixing aspects
will be considered  in  greater detail  in later portions of this  report.

3.3       Combustion Control of Starved Air Systems

     Figure 3-6  illustrates the basic components of starved-air systems which
are typically  small  (less than 100 ton/day capacity per unit).   MSW  is
generally  fed  to the  system with a front-end loader and charged into the
primary chamber with a hydraulic  ram.  The typical  solids  residence time in
the first  stage  chamber is on  the order of 10 to 12 hours which helps to damp
out variations  in fuel heating value and the cyclic nature of the charging
process.

     The primary  zone functions as  a gasifier with  only about 40 percent of
the theoretical air  requirements.   The relatively  large volume  of  this
chamber coupled with  low  air flow  results in low  gas velocity and minimal
particulate entrainment.

     In mass burn,  excess  air systems, the effluent from the  fuel  bed
contains  significant  concentrations  of CO, H£  and unburned hydrocarbon.
There  is,  however, an overall  excess air condition in the lower portion of
the municipal waste combustor.  In modular, starved  air systems, the overall
primary zone effluent is fuel-rich.  Completion of the  combustion process is

                                  3-17

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CONTROLLED AIR
SECONDARY
SR >100%
       HYDRAULIC RAM
         CHARGING
                                    WASTE HEAT
                                    RECOVERY
                                             CONTROLLED AIR TO
                                             PRIMARY CHAMBER
                                             SR<100%
   Figure  3-6. Components of typical starved-air modular combustor.
                               3-18

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totally  dependent upon the efficiency of air  mixing in the secondary chamber.
The small  modular  systems have no  heat  removal from either the primary  or
secondary  chamber.   Energy recovery  is  accomplished in a  downstream waste
heat boiler.

3.4       RDF Systems

     As  noted  at the beginning  of  this chapter, there are two  basic
approaches to dealing with the problem of  variability in MSW.  The mass burn
and starved-air  systems  accept the  variability  of MSW  as  a fuel
characteristic  to  be controlled  in  the combustion system design.  The-
alternate  approach  is to  pre-process MSW  to  generate a more uniform fuel.
The objective  is to modify the fuel  characteristics sufficiently for the
refuse-derived fuel( RDF) to be burned in  boilers designed for fossil  fuels,
e-g. spreader stokers and suspension fired units.  Also,  RDF might be  burned
in  cyclone boilers,  pulverized coal-fired  boilers or fluidized bed furnaces.
Processed  fuel  might be  burned separately or it might be burned in a co-
firing  configuration using the RDF as a supplement to the baseline  fossil
fuel.

     To  function properly, the  RDF characteristics  should  be  matched  to
combustion system  requirements.   In  its simplest form, RDF is produced  by
passing the raw  waste through a  shredder  after first removing bulky items
such  as refrigerators.   This results in  a  fuel  which can be burned  in
spreader  stokers.   RDF production  may also  include metals removal which
produces  a salable  by-product.   Often that  process will be coupled with
screening  and  secondary shredding.  Such  a  fuel is better suited for firing
in  spreader stokers than simple "shred-and-burn" operations.  Production  of
refuse-derived fuel  is a relatively recent  engineering innovation; thus it  is
not surprising that  RDF systems have been  plagued with reliability problems.
Some  systems  have  produced a fuel  product with less than the desired
characteristics  which creates significant difficulty achieving uniform fuel
feed to  the boiler.
                                  3-19

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3-5       References

     Camp Dresser  &  McKee, Inc.,  "Dioxin  Emissions  from Resource Recovery
     Facilities  and  Summary of Health Effects,"   Draft  Report prepared  for
     U.S.  EPA  Office of Solid Waste,  November 19, 1986.

     Detroit  Stoker, 1986.   Discussions and data presented by Detroit Stoker
     personnel  (T.  A.  Giaier and D. C. Reschley)  to W.  S. Lanier  and
     W.  R.  Seeker in Monroe, Michigan,  September 9,  1986.

     Franklin  Associates, Ltd.,  "Characterization of Municipal Solid Waste in
     the United  States, 1960.to  2000," (prepared  for  the U.S. Environmental
     Protection Agency) Washington,  D.  C., July 11,  1986,  pp.  1-8.

     Kramlich,  J.  C.,  M.  P.  Heap,  W.  R. Seeker,  and G.  S. Samuelsen.
     20th Symposium  (International) on Combustion, p.  1991,  The Combustion
     Institute.

     Niessen,  W.  R., A. F. Sarofim,  C.  M. Mohr, and  R.  W.  Moore, "An Approach
     to Incinerator  Combustible Pollutant Control,"  Proceedings National
     Incinerator  Conference, ASME, New  York, pp. 248-259,  1972.

     Stull,  R.  D.,  E. F. Westrum, and G. C.  Sinke;   The  Chemical
     Thermodynamics of Organic Compounds, Wiley, 1969.

     Telecon.  Riley Stoker Corp.,  with Lanier, W.  S.,  EER Corp., August  17,
     1986.   Conversation about Riley Stoker's MSW System design practices.

     G.  C.  Williams, A. F.  Sarafim,  J.  B. Howard,  and J.  B. L.  Rogers.
     "Design  and  Control   of  Incinerators,"  Final  Report to Office  of
     Research  and  Monitoring,  U.S. Environmental Protection Agency under
     Grant  No.  EC-00330-03,  1974.
                                   3-20

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4.0       POTENTIAL FOR AIR EMISSIONS

     During the combustion of municipal waste, a number of  pollutant species
can be  produced.   The pollutants  include both criteria pollutants and other
pollutants  as  follows:

     •    Nitrogen oxides, NOX
     0    Carbon monoxide, CO
     •    Acid gases  (HC1, S02, HF, H2S04)
     •    Particulate matter
     t    Metals  (As, Be, Cd, Pb,  Hg,  Ni, etc.)
     •    Toxic  Organics

The focus of this  study is on the  combustion control of emissions of organics
such  as  chlorinated dibenzo-p-di oxi n (PCDD) and chlorinated dibenzofurans
(PCDF).   However, the control  of PCDD and  PCDF  emissions should  not  be
pursued  without consideration  of how control scenarios will influence the
emission  of other pollutants.  In  some  instances, the control schemes may in
fact  adversely impact the  system's ability  to  control  the emissions  of
another  pollutant.  The  following sections  will  provide information  on
formation and  control schemes of the variety of potential  air pollutants.

4.1       Organic  Emissions

     There  are  a  number of  organic  compounds that  have  been  measured in
effluents  from municipal  waste  combustors  (MWCs).  In   addition,  other
hydrocarbons  including  a  broad  range of aromatic and chlorinated organics
might  potentially be emitted.   The principle emphasis  at present is being
placed on  the chlorinated congeners  of  dibenzo-p-dioxi n  (PCDD)  and
dibenzofuran  (PCDF).  However, PCDD and PCDF comprise only  one of many types
of organics  emitted from municipal  waste combustion (MWC)  facilities that may
be eventually of concern.   For  this reason, control approaches to prevent
emissions  of  trace organics in general, not just PCDD and PCDF, are included
in the  subsequent chapters although the emphasis is clearly directed towards
PCDD and PCDF.
                                    4-1

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     As illustrated previously  in  Table  2-1, a wide range of PCDD and PCDF
emission  rates have been  measured from  various  refuse  fired  facilities
throughout the  world.  A comprehensive  review  of available emission data from
municipal  waste burning  facilities has  recently been compiled for EPA by
Midwest  Research Institute  and is included as a volume entitled "Emission
Data Base  for Municipal Waste Combustors."

     There are  many  different theories  concerning the formation of PCDD's and
PCDF's  from MSW combustion  systems  (Germanus, 1985, Niessen et al ., 1984,
1986).  The best-supported  theories are illustrated in Figure 4-1.  The first
theory  involves the breakthrough of  unreacted PCDD/PCDF present in  the raw
refuse.   A few measurements have indicated the presence of trace quantities
of  PCDD/PCDF  in the refuse  feed that,  if not burned in the  furnace, could
account  for the levels of emissions (Lustenhouwer et al.,  1980).  The trace
levels of PCDD/PCDF  found in the solid  waste feed would have to be completely
unreacted to  account for  the  emissions  levels  of these  species which is
unlikely in any combustion  environment  (Germanus, 1985).

     A more plausible  theory involves  the conversion of species  referred to
as precursors which  are of  similar structure.  For example,  relatively simple
abstraction  and  combination  reactions can  convert chlorophenols  and
polychl orinated biphenyls to PCDD/PCDFs.  These precursors can be in larger
abundance in  the refuse  and  can be produced by pyrolysis  in  oxygen-starved
zones.   There  is direct  evidence of  gas  phase yields of PCDD/PCDF from PCB
fires  (Axelrod, 1985) and lab and bench-scale studies on PCB,  chlorinated
benzene and chlorinated phenols (Olie et al.,  1983, Hutzinger et al.,  1985).

     The  third -mechanism  involves the  synthesis of PCDD/PCDF  from a  variety
of  organics and a  chlorine  donor.  Again,  the simplest mechanisms  involve
those  species  that  are structurally related to PCDD/PCDF but a full spectrum
of  plausible  combustion intermediate  chemistry could be proposed to  lead to
precursors and eventually to  PCDD/PCDFs.   For example,  the analysis for
intermediates  formed during the combustion of complex fuels such  as coal and
wood  indicate  yields of  (unchlorinated)  PCDD  and PCDF species (Hites and
Howard,  1978)  that could  be  chlorinated when a suitable chlorine donor is
                                   4-Z

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  I.
PRESENCE IN REFUSE
         Cl
                                                         Unreacted
                                                         PCDD/PCDF
 II.
Evidence:  Occasional PCDD/PCDF contamination  in  refuse

FORMATION FROM RELATED CHLORINATED PRECURSORS
III,
                                                         Furan
Evidence:  PCDD/PCDF on soot from PCB  fires
           Lab  and  bench  studies  of PCB,  Chlorinated  Benzene
           and Chlorinated Phenols yielded  PCDD/PCDF

FORMATION FROM ORGANICS AND CHLORINE  DONOR
           PVC     \
           Lignin  J
           +  Chlorine donor
              Nad , HC1,  Cl2
PCDD/PCDF
 IV.
          Evidence:   Lab  scale tests  of vegetable  matter, wood, lignin,
                     coal with chlorine source yielded PCDD/PCDF
SOLID PHASE FLY ASH REACTION

           Precursor

               +     Cl  Donor
                                         low
                                        temp
                                                              PCDD
          Evidence:   Lab  scale demonstrating potential  for ash  catalysis
                     reactions of PCDD's to other homologues.
    Figure 4-1.  Hypothetical mechanisms of PCDD/PCDF formation  chemistry.
                                    4-3

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available.   Complex fuels which  contain  lignin have been  shown to yield
higher levels  of PCDD/PCDF than  non-lignin fuels  (Niessen, 1984).

     The  final hypothetical  mechanism involves catalyzed reactions  on fly ash
particles  at  low  temperatures.   The  lab  scale  evidence indicates that the
chlorination  of  related-structure  precursors  on fly ash particles to form
PCDD/PCDF  can occur but that the  rates are relatively slow (Eiceman et al.,
1982;  Rhgei  and  Eiceman,  1982).  Recent work (Vogg, Metzger and  Stieglitz,
1987)  has  shown  the importance of this mechanism as well  as quantifying the
temperature dependence.

     In summary,  current theories on formation  of PCDD/PCDF involve either
unburned  PCDD/PCDF from the refuse  or  related-structure  precursors,  or
precursors that  are likely to be formed  in oxygen-starved  zones  of the
furnace.   The precursors  can   be  converted to PCDD/PCDF even  at low
temperatures  on  fly ash.  Vogg  et  al.  (1987)  indicate  that the  optimal
temperature  for  such catalytic  reactions  is approximately 600°F.  Hence it
is  important  to  destroy the PCDD/PCDF  and the precursors in the furnace.
Therefore,  the  destruction mechanisms are equally as  important  as the
formation  mechanisms.   Assuring complete destruction  will  likely be the
more  appropriate  combustion control  method rather than the prevention of
formation.

     The destruction of PCDD/PCDF  is expected to  be similar to the oxidation
of  other  aromatic chlorinated hydrocarbons  primarily involving free radicals
(such  as  OH)  which attack the structure or  unimolecular decomposition (Shaub
and Tsang,  1983).  The  details  of  the  mechanism are uncertain and  more
fundamental  research is required.  Direct experimental  measurements of the
thermal  decomposition characteristics  of non-chlorinated PCDD and  PCDF
species are  available from  data  at the University of  Dayton Research
Institute  (UDRI)  (Duvall and Rubey, 1977).  These studies using  the Thermal
Decomposition Analytical  System  (TDAS)  provide  a definition of  the thermal
requirements for destruction under  highly diluted reactant conditions.   These
conditions,  sometimes  referred  to  as  nonflame, provide  a  conservative
indication of the temperature to  which the  test  species must be subjected in
                                   4-4

-------
order  to  achieve destruction.   The  thermal decomposition  behavior of (non-
chlorinated)  PCDD/PCDF and hypothetical  precursors are shown in  Figure 4-2 at
a specific residence time.

     The thermal  decomposition data indicate  that relatively low temperatures
are required to destroy PCDD and PCDF  and that the temperature  requirements
are similar  to the decomposition temperature required for  non-substituted
biphenyl.   Chlorinated biphenyls have  higher  decomposition temperatures and a
similar increase in the temperature  requirements are expected  for PCDD/PCDF
due to the expected similarity of the  decomposition mechanisms of PCB and
PCDD/PCDF.  These data  indicate that 800°C  (~1500°F) is a characteristic
temperature  for high levels of destruction  (>99.9 percent)  of PCB and by
analogy  for destruction  of PCDD/PCDF.   This  temperature is in  general
agreement with the Dow  Chemical conclusions that TCDD decomposition occurs
above  800°C  (Bumb,  et  al . 1980, Stehl  et  al.  1973).  Other potential
precursors  to  the  formation  of PCDD/PCDF can  have  somewhat  higher
decomposition  temperatures  but are  still  low relative to combustion
temperatures.

     The  UDRI  decomposition  data  (Rubey et al ., 1983 and  Dellinger et al.,
1983)  also indicate that temperature  is  more important than  residence time.
The data for  the  thermal decomposition  temperature generally correlates with
the residence time t as
               E
                a
          T = —
               R
   -tA  \  /[02J

vln(fr)  /  \0.21
                                          -1
= Thermal  Decomposition
       Temperature
where  [03] is  the  oxygen mole fraction,  b  is the reaction order with respect
to oxygen, R is the universal gas constant,  fr  is the fraction  of  the species
remaining and Ea  and  A  are constants  which  depend on the test  species in
question.  Thus, the destruction of the  species depends predominantly on the
temperature and  effectively there exists a  threshold temperature  above which
the compound will be  rapidly destroyed.   Effective combustion control schemes
must  ensure that all  PCDD/PCDF and precursors  experience the required
                                    4-5

-------
     100
                                      HEXACHLORO-
                                         'BENZENE
 CD
      10
 CJ
 cc
      .1 _
      1000
                     DIQXIN
                     FURANS
 1100  1200   1300  1400   1500  1600

           TEMPERATURE  (°F)
Figure  4-2.
Thermal  decomposition  characteristics  of selected
organics under dilute  (nonflame) conditions for
1  second  (Dellinger  et al. 1977).
                             4-6

-------
threshold  temperature  if they are  to  be  destroyed.   For  PCDD/PCDF and
potential  precursors,  the  threshold temperature is  near  925°C (1700°F).

     Another  possible control scheme involves removal  of PCDD/PCDF from the
flue  gas.   PCDD/PCDF  are condensible  at  flue gas  temperatures and will
deposit on  fly ash  particles very efficiently.   Because  of the higher surface
areas  of  smaller  particles,  the condensation will  occur preferentially on
smaller particles.   Hence,  a removal  system  must efficiently remove the  finer
particles  in order  to  effectively remove PCDD/PCDF-   Data  on full scale  tests
have  clearly shown the effectiveness of a  high efficiency baghouse with dry
injection  flue gas treatment (Martin, 1986)  in removing PCDD/PCDF from the
flue gas.

     Thus,  in summary, the current theories  concerning  the emission of  PCDD/
PCDF  and  other  similar organics  include  any or all  of  the following
possibilities:

     •    Lack of  destruction of PCDD/PCDF originally  present in the feed
          refuse

     •    Conversion of precursors present in the feed stock or formed in the
          combustion  processes  to  PCDD/PCDF  and  lack  of destruction of the
          synthesized  PCDD/PCDF

     t    Lack of  destruction of precursors  in  the combustion system and
          conversion  of  the precursors to  PCDD/PCDF  on fly ash particles at
          low temperatures.

     Combustion  control   of the  emission of PCDD/PCDF  must ensure that both
PCDD/PCDF  present  in  the  feed and formed in  the combustion of precursors are
destroyed  in the  high temperature  combustion zone.   This implies ensuring
that  all  furnace  gases experience sufficiently high  temperatures to destroy
both PCDD/PCDF and  potential precursors.  Since even  trace emissions of PCDD/
PCDF  are  of concern, both  spatial and temporal  variations must be considered
in the  combustion  control scheme so that there are  no  pathways which at any
                                    4-7

-------
time are  not sufficiently  hot.   The presence of oxygen  lowers the required
temperature; ensuring complete  mixing at uniformly  high temperatures  will
ensure  low PCDD/PCDF emissions.

4.2       NOX Formation and Control

     Air emissions  of NOX from municipal waste combustors (MWCs) are highly
variable.   Measurements have been  made on a number of plants as summarized  by
O'Connell  et al  (1982), Rigo et al  (1982), Russel and  Roberts (1984) and MRI
(1986).   Discussions with  manufacturers have  further  indicated the  high
variability  of  NOX emissions.  The reported levels range  from less than  0.05
to  near  1.0  Ib/MMBTU with  average values for  all  facilities reported  at
around  0.25 Ib/MMBTU (130 ppm at 12 percent C02).  The  sources have indicated
that there  is no clear  trend in NOX emissions  from different types  of
municipal  waste  combustors (MWCs).

     For  most of the country, there are no  NOX  emissions  standards for
municipal  waste  combustors (MWCs).  Even  though there are  no national limits,
municipal waste  combustion (MWC)  facilities  will  generally have NOX
regulations  imposed  locally.   Manufacturers  of  facilities indicated  that
plants  under permitting  evaluation in  several locations around the country
were having  to  meet NOX emission  limits  imposed by either the state or local
environmental agencies.

     In the  south  coast air  basin  of  California  (Los  Angeles  area), for
example, all  refuse-to-energy facilities  exceeding 50 tons/day are subject  to
New  Source Review.  In addition the  district restricts NOX emissions to  less
than 225  ppm (-3 percent,  02 dry, 15 min average).   This  requirement  is
expected  to  dictate  special  consideration  to  ensure compliance.  The New
Source  Review requires a preconstruction review of the new facilities and  if
net  cumulative emissions  exceed 100 Ib/day then facilities must have Best
Available  Control  Technologies  (BACT), emissions offsets and not cause a
violation  or make measurably worse an existing  violation of a  national
ambient  standard.   Other  local  districts are also requiring NOX limits  on
their own.
                                    4-8

-------
     The NOX formed in municipal  waste  combustion (MWC)  facilities occurs by
two separate  pathways, thermal fixation  of  molecular nitrogen  ("thermal NOX")
and conversion of nitrogen  from the  fuel.  The thermal  fixation mechanism
involves  high  temperature reactions  of free radicals of nitrogen and oxide.
The controlling mechanism have been  determined to be Zeldovich  reactions of
the form:

          0 + N2  =  NO + N
          N + 02  =  NO + 0

These  reactions are strongly temperature  dependent as  shown  in Figure 4-3.
This figure represents a computer simulation of the NOX formed  for a specific
condition  and residence time  (  ) and clearly indicates the strong temperature
dependence of thermal NOX formation.

     A comparison  of  the actual  NOX levels  with those expected by thermal
mechanisms at  the maximum  temperature expected in the furnace indicates
another NOX  source must  be  operational.   For example,  less than 100 ppm of
thermal NOX  would be expected from thermal  processes  for conditions which
exist in  normal  furnace  designs.   This extra  source of NOX could be
attributed in  part to the  conversion  of  nitrogen bound in the  refuse feed.
This  mechanism was first  discovered  by  burning  coals and fuel  oils in
nitrogen-free  oxidizers  where the  only  nitrogen  source was  in the fuel.
These  studies  have indicated that the  conversion of the fuel-bound nitrogen
is highly  dependent on the local oxygen  availability to volatile  species, the
amount  of fuel bound nitrogen and the chemical  structure of the  nitrogen and
fuel  in the  refuse.  The conversion efficiency can vary  from  near 50 percent
for highly mixed  conditions to near  5 percent  for oxygen  starved-staged
combustion conditions for coal bound nitrogen (Chen et  al ,  1982).  As shown
in  Figure 4-3, the  conversion of fuel  nitrogen is  not expected  to be
temperature dependent.

     Traditionally, NOX emissions from  municipal  waste combustion (MWC) have
not been  controlled.  However, with  local air regulations,  there may be a
need  to control  NOX to attain standards.  Also,  the  general tendency to
                                   4-9

-------
 10,000
Q_
Q_
   1000
   100
o  10
    1.0
    0.1
              3140   2813
 T(°F)

2509    2310
2112    1941
               THERMAL  NO
                                           0.5% FUEL  N
          l

   MAX
   EXPECT^
   ED
   ADIA-
   BATIC
   TEMP.
           30% EXCESS AIR
           r= 0.5 SEC.
                                                 FUEL  N
      0.45    0.50    0.55   0.60    0.65    0.70

                            103/T(K~1)
   Figure 4-3.  Impact of temperature and fuel nitrogen on NOX
               emissions for excess air conditions  (calculated
               using EER kinetic set).

-------
produce higher  temperatures and better mixing  for  PCDD/PCDF control  will  lead
to higher  NOX  emissions.   Manufacturers  of MSW combustion equipment  have
indicated  that NOX emissions from modern-designed plants will  probably be
higher than those  from older plants.

     Only  a few  of the  potentially  applicable  control  schemes  have  been
demonstrated  at full-scale  for MSW  combustion  devices.   In  Table  4-1 is
provided  a summary of  the  status of  the  control schemes for NOX  and their
potential  impact  on control of organic emissions such as PCDD/PCDF.   Flue gas
recirculation  is  a  demonstrated technology for thermal NOX which involves the
dilution  of combustion air to lower the flame temperature.  This scheme has
been  demonstrated  by Volund as an effective  means of controlling NOX  in its
refractory-lined  high  temperature municipal  waste combustors.   Flue gas
recirculation  lowers flame and furnace temperatures which is expected to be
counter  to the control  requirements  for PCDD/PCDF.  The significance  of the
detrimental  impact on  PCDD/PCDF emissions due to reduced bulk  temperatures
has yet to be determined.

     Reburning with an  auxiliary fuel  such  as natural gas is currently being
developed  for  fossil fuel  fired  furnaces as a  retrofittable  combustion
control  scheme for NOX.   The process  could potentially be applied  to MSW
municipal  waste combustors  as shown  in  Figure  4-4.  The SR-j terms in  this
figure refer to  the stoichiometric ratio with  SR < 1.0 representing  fuel-rich
conditions.   Enough reburning fuel  should be injected at a location  low in
the furnace  to create a hot, slightly oxygen-starved zone.  The  overfire air
is injected above  the  reburning  zone  to  complete the combustion process.
Reburning  can  be  combined with urea or ammonia injection to optimize the NOX
reduction.  In'addition  to  NOX reduction,  reburning has the potential for
destruction of PCDD/PCDF due to the high temperatures and high concentration
of flame  radicals  that exist in  the  reburn zone (Overmoe  et  al, 1985).
Reburning  with natural gas  holds the promise of combined NOX and  PCDD/PCDF
control but currently is only a concept that has not been tested above  bench-
scale.
                                   4-1-1

-------
          TABLE 4-1.
 STATUS  OF  N0y CONTROL OPTIONS FOR MUNICIPAL WASTE  COMBUSTORS
             A
      APPROACH
COMPATIBILITY WITH

ORGANIC  CONTROL
        BENEFITS/

      DISADVANTAGES
          STATUS
FLUE GAS RECIRC.
  DETRIMENTAL
INEXPENSIVE
EFFECTIVE FOR
  THERMAL ONLY
DEMONSTRATED FOR SOME
SYSTEMS  (VOLUND, VICON)
I
—k
INJ
THERMAL deNO* BY
  NH3 INJECTION
   NO IMPACT
70-80% EFFECTIVE
NH3 SLIP
FURNACE INJECTION
FULL SCALE  INSTALLATION
  (COMMERCE A JAPAN)
TESTING UNDERWAY
REBURNING WITH
  NATURAL GAS
  BENEFICIAL
POTENTIAL PCDD/PCDF CONTROL
50% EFFECTIVE
CONDITIONS NOT WELL
  SUITED FOR REBURNING
MAY REQUIRE MODIFICATION
  OF AIR FLOWS
CONCEPTUAL ONLY

DEMONSTRATION UNDERWAY
  FOR FOSSIL FIRED BOILERS
SELECTIVE CATALYTIC
  REDUCTION
   NO IMPACT
CATALYSIS POISONING
EXPENSE
90% EFFECTIVE
FULL SCALE
  (JAPAN)
                                                                                         INSTALLATION

-------
STOICHIOMETRY RATIOS
       (SR)	
        SR3 =  1.9
        SR2 =  0.9
        SR1 =  1.1
                                            OVERFIRE AIR
                             REBURNING  FUEL
       Figure 4-4.
Application of reburning  and  de-NOx  schemes
for NOX control  of  mass burn municipal waste
combustors.
                                 4-T3

-------
     Other NOX  control schemes for municipal  waste combustors (MWCs)  involve
post-combustion zone  control.  For example,  thermal  DeNOx  involves the
injection  of  ammonia in the upper furnace,  to achieve selective reduction of
NOX.  There are  a  few examples of the application  of this technology in  Japan
and recently  in the U.S. for municipal  waste combustors (MWCs) (e.g.,  Hurst
and  White,  1986).    The  NH3/NO reactions  are extremely  sensitive to
temperature so that the injection location must  be carefully selected.   Also,
there  is  generally some slip of NH3 which does  not completely react that can
cause odors,  fouling and the production of visible plume.  New tests at  full-
scale  installations should indicate  the  viability of this technology for
municipal  waste  combustors  (MWCs) in the near future.

     Another  post-combustion  control  scheme  involves selective catalytic
reduction  (SCR) which enhances the reaction of  NO and  NH3 to form N£.  The
use of a  catalyst enables the reactions to take  place at lower temperatures
over  a broader  temperature window and there is  little NH3 slip.  The  process
achieves very  high NOX  reductions  (typically 80  percent).  There are numerous
full-scale installations of SCR on oil- and gas-fired boilers principally in
Japan  although there  is  too little experience with SCR with MSW combustion
effluents  to know  if catalyst poisoning is to be a factor.

     In summary,  the   levels of NOX emissions from MSW combustion facilities
are generally  on the  order of 100-300  ppm (12  percent C02).  However, the
emissions  vary widely due to differences in  nitrogen content in  the raw
refuse and the thermal  environment in different municipal waste combustors
(MWCs).   Trends to higher temperatures and more uniform  mixing for PCDD/PCDF
control are expected  to  increase  NOX  emissions in newly designed  plants.
State and local  regulatory agencies  will likely dictate NOX emission
standards  in the future which will require the implementation of separate NOX
control schemes.  Unfortunately, few control schemes are  available which have
been evaluated at  full-scale.
                                   4-14

-------
4'3       Partlculate and Trace Metals

     Fine  fly ash  particles are  produced as  a  natural  consequence of
combustion  of heterogenous  refuse.   The refuse feed  can contain 30 to 50
percent  noncombusti bles by mass.  Some of the finer ash material  can  become
entrained  in  the flow  and  be carried out of the  furnace.   Also certain
inorganic  compounds present  in  the  burning bed are volatile at combustion
temperature and  will leave the combustion zone  as a gas before recondensing
downstream.  Finally,  carbonaeous  particulate  matter or  soot is  always
present  during the combustion of  solid fuels and if it  is not burned  out it
will also escape  the furnace.

     A number of  measurements  have been undertaken  on  municipal  waste
combustion  (MWC) facilities and  several survey  programs are available which
summarize  the  results  on  particulate emissions (see  e.g., MRI,   1986;
O'Connell  et  al , 1983;  Rigo  et  al,  1982; and  ISWA,  1986).   Uncontrolled
particulate loadings have been found  to range up to 3 Ib/MMBTU depending on
the refuse  and combustor  characteristics.  There is  some  evidence that
indicates that starved air units have  the lowest  uncontrolled emissions while
spreader stoker  units are the highest.  Control  of  particulate is necessary
to  meet  NSPS.  The volume entitled "Flue Gas Cleaning Technology" provides  a
thorough  review of downstream air  pollution control devices.

     Of  particular  concern for particle emissions  is the emission of heavy
metals in  the  form of fine particulate.  In Table 4-2, a summary is provided
of  the typical metal concentrations  expected in municipal solid waste.  In
Figure 4-5  is  shown the transformation of these  metals during the combustion
process.   The  ash included in the  refuse can either  remain in the bed  and be
rejected together with  other residuals, be entrained into the flow or be
vaporized and  re-condense downstream.   The residual matter generally includes
the incombustible fraction of the refuse and such metals as iron, aluminum,
copper and  zinc  along with the other  inerts for  example, calcium and silica.
Roughly  about 1-3 percent of the  incombustible  fraction is entrained  as  fly
ash in the  1-20 micron range.  A smaller fraction is vaporizable.  Within  the
bed,  the  locally-reducing  high  temperature conditions  can lead  to  the
                                   4-T5

-------
  TABLE 4-2.  METALS PRESENT IN MSW
MUNICIPAL SOLID WASTE
ELEMENT
Ag
AT
Ba
Ca
Cd
Co
Cr
Cu
Fe
Hg
K
Li
Mg
Mn
Na
Ni
Pb
Sb
Sn
Zn

AVG.
3
9000
170
9800
9
3
55
350
2300
1.2
1300
2
1600
130
4500
22
330
45
20
780

CONCENTRATION (ppm)
RANGE
<3-7
5400-12000
47-450
5900-17000
2-22
<3-5
20-100
80-900
1000-3500
0.66-1.9
920-1900
<2-7
880-7400
50-240
1800-7400
9-90
110-1500
20-40
<20-40
200-2500

Data from Law and Gordon,  1979
                4^16

-------
                                      VOLATILE
                                      INORGANIC
                                   [Na, Zn, Ba, Hg]
        INTERNAL
        REDUCING
       ENVIRONMENT
                                     HETEROGENEOUS
                                     CONDENSATION OR
                                     ADSORPTION
  ASH
PARTICLE
I ,
^4
                                                                                 COAGULATION^.  •..';•
                                                                                            v> •••'!*
Hg, Pb, Fe,  Mg,  OXIDATION
CHLORINATION AND VAPORIZATION
       ...  NUCLEATION    .	TX
     (e.g.,  PbCl  )
              CHAR PARTICLE
              DURING COMBUSTION
           ENTRAINED
           PARTICLES
                                                                     RESIDUAL  FLY  ASH
                                                                        (1-20 urn)
                BURNING
                BED OF
                SOLID WASTE
                                                            FUME
                                                          (~0.05 /urn)
                                                                        RESIDUALS
Figure 4-5.   Transformation  of  mineral matter during  combustion of metal  containing waste,

-------
formation  of oxides and  chlorides of metals which can be volatile.  Also,
some metals  such as sodium,  mercury and to a lesser extent,  cadmium, have
high vapor pressures as the pure metal.  These materials can  volatilize and
re-condense either homogeneously as a fume or heterogeneously on the  fly ash.
The condensation mechanisms  favor the finer particles due to their higher
surface  area to mass ratio.   Thus, the finer  particles  that  can escape
particulate  control devices, i.e., submicron, are enriched in  these  volatile
species (sometimes more than 100-fold over the refuse incombustibles).

     The actual  partitioning of the metals among the residuals,  fly ash and
fume depends on  the waste composition  as well as the combustion environment.
The impact of the design and operating  variables on the partitioning can not
currently  be predicted  with  confidence.   Equilibrium metal s partitioning
analysis are currently being developed under EPA support which  will  lead to a
better understanding of the fate of metals.  However, it is clear that design
and  operating conditions  defined for PCDD/PCDF control will  influence
particulate and metals emissions.  For example, higher velocities through the
bed will increase the entrainment of particles.  Changes in bed stoichiometry
for proper air distribution  will  influence  the vaporization  of  volatile
metals.   Also, temperature increases  will favor vaporization  of the metals.
Therefore, the impact of design and operating  conditions  for PCDD/PCDF
control   on the  emission  of metal  enriched fume requires  further
investigation.

4.4       Acid Gases
     The acid  gases of interest  as pollutant  emissions from MSW combustion
facilities are-S02, HC1, ^04 and HF.   The  emissions of these species is a
direct  function of  the amount of elemental sulfur, chloride and fluoride in
the  feed  refuse.   For  example, municipal waste has been found to have 0.12
percent sulfur on  average and 30-60 percent is  converted to S02 (O'Connel et
al,  1983).  The balance of the sulfur is retained in the residual ash or is
absorbed  on fly ash.   In  the same manner,  roughly half of the chloride is
emitted as HC1 .   Most  state and local  environmental protection agencies are
requiring  acid gas  control.   The control technologies all involve flue gas
                                   4-T8

-------
scrubbing  either with dry, semi-dry or  wet alkaline sprays.   Combustor design
and operating  conditions  for  control of PCDD/PCDF are expected to have no
impact on  the emission of or the ability to control acid gas  emissions.

4.5       References

     Axel rod,  D.,.  MD,   "Lessons  Learned  from the Transformer Fire at the
     Binghampton  (NY) State  Office Building."   Chemosphere 14  (6/7)  p.
     775-778, 1985.

     Bumb,  R.  R.   "Trace  Chemistries of Fire:   A Source of Chlorinated
     Dioxins",  Science, 210, 385, 1980.

     Chen, S.  L.,  M.  P.  Heap,  D.  W. Pershing, and G. B. Martin.  "Influence
     of Coal Combustion  on the Fate  of  Volatile and  Char Nitrogen During
     Combustion."   Nineteenth  Symposium  (Int.) on Combustion/The Combustion
     Institute  Pittsburgh, p. 1271, 1982.

     Del linger, B.  et al., "Laboratory  Determination  of High Temperature
     Decomposition Behavior of Industrial Organic Materials".   Proceedings of
     75th  APCA  Annual meeting, New  Orleans, 1982.

     Duvall,   D.  S.  and  W. A. Rubey,  "Laboratory  Evaluation  of  High
     Temeprature  Destruction of Polychlorinated Biphenyls and  Related
     Compounds,"   EPA  600/2-77-228, 1977.

     Eichman, G. A. and H. 0. Rhgei. Chemosphere, 11, p. 833,  1982.

     Germanus,  D.   "Hypothesis Explaining the Origin of Chlorinated Dioxins
     and Furans in  Combustion  Effluents."   Presented at  the Symposium on
     Resource Recovery, Hofstra University, Long Island, New  York, 1985.

     Hites,  R.  A.  and J. B. Howard.  "Combustion Research on  Characterization
     of particulate Organic matter  from Flames".  EPA-600/7-78-167,  1978.
                                   4-T9

-------
Hurst,  B.  E.  and C. M. White.  "Thermal  DeNOx:   A  Commercial Selective
Noncatalytic  NOX  Reduction  Process for Waste to Energy  Applications".

Hutzinger,  0., M. J. Blumich, M. V-  D. Verg.  and  K.  Olie.  "Sources and
Fate  of  PCDD and PCDFs:   An Overview",  Chemosphere,  14  (6/7) p. 581,
1985.

Law,  S.  L.  and  G. E.  Gordon.   "Sources  of  Metals  in Municipal
Incinerator Emissions".   Environ. Sci. Tech.,  13,  p.  433, 1979.

Lustenhower,  J.  W.  A.,  K.  Olie,  and 0. Hutzinger.   "Chlorinated Dibenzo-
p-Dioxin  and Related  Compounds in Incinerator Effluents.  A Review of
Measurements  and  Mechanisms of Formation," Chemosphere,  9,  p. 501, 1980.

Martin, GMbH.   Data  presented to W. R. Seeker and  W.  R.  Niessen, Munich,
July  1986.

MRI 1986.  Emission  inventory for  EPA,  1986.  See  Appendix A of the
Comprehensive Report.

Niessen,  W.  R.   "Production of PCDD and PCDF  from  Resource Recovery
Facilities,  Part  II",  1984 National  Waste Processing Conference ASME
proceedings,  p.  358, 1984.

Niessen,  W.  R.   "Dioxin  Emissions from Resource Recovery Facilities and
Summary of Health Effects."  Report prepared for EPA OSW, 1986.

O'Connell; W. L.,  G.  C.  Statler and R. Clark.  "Emissions and Emission
Control  in  Modern Municipal  Incinerators".   1982  National  Waste
Processing Conference.   ASME  Proceedings, p. 285,  1982.

Olie, K., M.  V.  D.  Berg,  and 0. Hutzinger.  "Formation  and Fate of PCDD
and PCDF Combustion  Processes",  Chemosphere 12 (4/5) p.  627,  1983.
                               4-20

-------
Overmoe, B.  J., S.  L.  Chen,  D. W.  Pershing,  and  G.  B.  Martin.
"Influence of Coal Combustion  on the Fate of Volatile and Char  Nitrogen
During Combustion."  Nineteenth Symposium (Int.)  on Comb./The Combustion
Institute, Pittsburgh, p.  1271, 1982.

Rhgei , H. and  G. A.  Eiceman.   "Adsorption and Thermal Reactions of
1,2,3,4-TCDD  on Fly  Ash from Municipal  Incinerator."  Chemosphere, 11
(6)  p.  569,  1982.   See also  Chemosphere,  13:  p.  421,  1984 and
Chemosphere  14  (3/4) p. 259,  1985.

Rigo, H.  G., J.  Raschka, S.  Worster.  "Consolidated Data Base for  Waste-
to-Energy Plant  Emissions."   1982  National  Waste  Processing  Conference.
ASME Proceedings,  p. 305,  1982.

Rubey,  W.  A.,  J. Torres,  D.  Hall,  J.  L.  Graham,  B.  Dellinger.
"Determination of the Thermal  Decomposition Properties of  20  Selected
Hazardous Organic  Compounds",  EPA Cooperative Agreement Report,  1985.

Russel ,  S.  H.  and J. E.  Roberts.  "Oxides of Nitrogen:  Formation and
Control  in Resource  Recovery Facilities".  1984  National  Waste
Processing Conference.  ASME  Proceedings, p. 441,  1984.

Shaub, W. M. and  W. Tsang.   "Dioxin Formation in  Incinerators".   Envir.
Sci. Tech.,  17,  p. 721, 1983.

Stehl , R. H.,  R.  R. Patenfuss, R.  A. Bredeweg, and R. W. Roberts.   "The
Stability of Pentachlorophenol  and  Chlorinated Dioxins to Sunlight,  Heat
and  Combustion" in Chlorodioxin  - Origins and  Fate.  E. H. Blair, ed.
ACS Washington,  D. C., p.  119,  1973.

Vogg,  H.,  M.  Metzger,  and  L.  Stieglitz.   "Recent Findings on the
Formation and  Decomposition of PCDD/PCDF in  Solid Municipal  Waste
Incineration."   Proceedings  of Emissions of Trace Organics  From
Municipal  Solid  Waste  Incinerators,  Specialized Seminar,  Part 1,
Session  2, Copenhagen 20-22,  January 1987.
                              4-21

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World  Health  Organization,  Regional Office for Europe.   March  17-21,
1986.   Notes  from Meeting  on Working Group  on  Risks to Health from
Dioxins from Incineration of Sewage Sludge  and Municipal Waste.
                               4-2-2

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5.0
CURRENT PRACTICES IN MASS BURN TECHNOLOGY
5.1
Mass Burn Technologies
     In Figure  5-1 is  shown an example of a  large,  modern,  mass  burning
facility.   The facility consists of the  following basic components:

     •    Waste  storage, handling and  feeding  system  (1,2,3)
     •    Combustion Systems (4,5)
     •    Boiler and Generators  (6,11)
     •    Air Pollution Control  Equipment (7,9,10)
     •    Residuals Handling (12,13,14)

The major distinguishing  features of  mass burn technologies is the  lack of
virtually any  processing of the refuse and the large capacities (>100 TPD).
The combustion  system consists  generally of a  grate  on which the solid waste
burns  in  a layered bed that is progressively  moved  down the grate.  The air
required  for combustion of the  waste  is introduced both underneath the grate
and directed through the bed (referred to  as  primary air) and sbove  the bed
through secondary air jets.

     Most of the major organizations which manufacture mass burn technologies
were  contacted  as  part  of  compiling  information presented in this  volume.
These included the following:
          Manufacturer
                                    American  Licensee
     Volund (Denmark)
                                Waste Management
     Deutsche Babcock (Germany)
     Steinmueller (Germany)
                                Browning Ferris  & American  Ref-Fuel
                                Dravo Energy Resources,  Inc.
     Von Roll  (Switzerland)
                                Signal  Resco
                                    5-1-

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                                                        1 Waste bunker
                                                        2 Crane
                                                        3 Charging hopper
                                                        4 Grate
                                                        5 Combustion chamber
                                                        6 Steam boiler
                                                        7 Electrostatic precipitator
                                                        8 Flue gas fan
                                                        9 Wet scrubber
                                10 Stack
                                1 1 Turbine-generator
                                1 2 Fly ash conveying system
                                1 3 Residue discharging system
                                14 Residue bunker
                                1 5 Primary air system with
                                   prehealer
                                1 6 Secondary air system
un
ro
                                              Longitudinal section of the
                                              Waste power plant Bielefeld-Herford,
                                              Federal Republic of Germany
                                              Combustion capacity: 3 X 385 t/24h
                                              Steam production: 3 X 52.4 t/h
                               Figure  5-1.
Mass  burning  waste  power  plant  at  Widmer and  Frnst
at  Bielefeld-Hertford,  Germany.

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                                                 American  Licensee
                                         Blount
                                         Odgen Martin
                                         American-Japanese  Joint Technology
     Manufacturer

Widmer & Ernst (Switzerland'

Martin (Germany)

Riley/Takuma

Detroit Stoker
(Grate Supplier)

Combustion Engineering
(with de Bartolomeis)

Foster Wheeler
(Boiler Supplier)

Babcock and Wilcox
(Boiler Supplier)

Westinghouse/01Conner

Enercon/Vicon
The topics  discussed  with these manufacturers were  their perception on PCDD/
PCDF formation  mechanisms, design approaches to prevent PCDD/PCDF formation,
and design and operating  guidelines that would be useful for minimizing PCDD/
PCDF emissions.

     The manufacturers all indicated that modern mass  burn designs were both
capable  of  and were  achieving what  they considered to  be low  PCDD/PCDF
emission  levels.  The current design  philosophy  relied predominantly on
optimizing  the  combustion zone performance  by  attempting to maximize
combustion efficiency  and minimize furnace non-uniformities.   Flue  gas CO and
62 were  treated  as an adequate indicator of combustion efficiency.  However,
                                    5-3

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flue gas  concentration of  CO  was  not generally  considered to be directly
relatable  to  PCDD/PCDF emissions  but rather was used  as an indication  that
the systems,  once tuned and adjusted properly, were  being maintained in  the
appropriate operating range.

     The  key  techniques employed to minimize  non-uniformities were to
optimize the mixing across the furnace  generally using secondary air injected
at high  velocities over  the  grate region, to optimize the grate and waste
feed to  obtain uniform bed coverage, and to adjust  the combustion zone  air
distribution  to natch the air with the burning characteristics of the solid
waste.   There  has been much  emphasis on combustion uniformity and techniques
to minimize non-uniformities  due  to  the manufacturers desire to minimize
fire-side corrosion due  to  reducing zones.   To  a  lesser extent, some
manufacturers expressed  the  need  for sufficient time at temperature or at
least  sufficient temperature in  the upper  furnace  region.   Also, some
manufacturers  indicated a design philosophy of lowering  flue gas temperatures
combined with using baghouses to remove particulate  matter onto which PCDD/
PCDF and other non-volatile hydrocarbons may have condensed.

     Individual designer/manufacturers implement the  philosophy in different
ways.   The next sections will  highlight the approaches followed by some of
the major manufacturers  of large  mass burn waste-to-energy facilities.  It
should  be indicated that many of  the designs  originated  in  Europe where
waste-to-energy plants are much  more common and the  concern for PCDD/PCDF
emissions has been factored  into  the designs for some time.  The resulting
technologies  employ very sophisticated combustion systems and controls along
with utility  type boiler furnaces.  Compared  to other types of municipal
waste combustion technologies, modern mass burn waste-to-energy systems  can
be considered to be second  or even third generation  in terms of designs  and
operation to  minimize PCDD/PCDF  emissions.   The systems  described here
represent the most current practice  that has  been pursued for combustion
control  of PCDDs/PCDFs.   However, it must  be pointed  out that these
descriptions  are current design  practices  and are not indicative of all
systems  currently  operating  in the  United States.
                                   5-4-

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5.2       Deutsche Babcock Anlagen

     The modern  Deutsche Babcock  Anlagen  (DBA) mass burn  technology is the
result  of  a  significant  amount of research and testing  of different system
configurations.   The  design  features of the  DBA mass  burn system are
highlighted  in Figure 5-2  and  Table 5-1.  One  unique  feature of the DBA
system  is  the  use of a roller grate that carries the refuse down to the next
roller  grate.   Stirring actions occur at the transition  between the rollers
where the  burning  refuse can tumble  and expose new combustible material.

     DBA  did  not  have  any direct cause and  effect data for PCDD/PCDF
emissions  in their design  but  did have evidence of low emissions with the
current  design and operating  approach.  Measurements are available  on
seventeen  DBA plants primarily  in  Germany  for CDD/CDF analysis on ESP dust,
residuals,  scrubber  water and flue  gas  (DBA, 1986).  These data indicate flue
gas emissions of  less than 10~3 ng/m3 of  2,3,7,8 TCDD for  many of the plants.
The  average  total  CDD/CDF emission level was 22.5 ng/m3  for five completely
tested facilities.

     The DBA design philosophy was oriented towards optimizing combustion
efficiency and maximizing uniformity of  mixing.  The general design criteria
included  hot combustion temperature, exhaust CO levels of  less than 100 mg/
Mm3,  high  excess air  levels (9-10 percent 02) and prevention of a reducing
environment above  the secondary air  injection point.

     A key element  of  the DBA  approach is the use of overfire or secondary
air  injection  schemes which serve  to mix all  of the furnace gases above the
grate.    DBA has  performed a  large number  of  experiments on different
configurations in order to find  optimum  furnace configurations and injection
schemes for  overfire air.  Experiments  have included both  cold-flow furnace
modeling  (water and air)  and  field in-furnace  measurements.  The newest
design  has overfire air  injection at a nose  in the furnace in which the
effluent  from  the  grate  is  pinched to allow a smaller  mixing distance for
overfire  air.  The  DBA design uses  25 percent of the air  as overfire air and
injects  it  at 100 m/sec.  This results  in a  penetration depth of the overfire
                                    5-5

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            OLD  GEOMETRY
NEW GEOMETRY
              I
                    O
Figure  5-2.   Deutsche Babcock  Anlagen mass burn furnace design
             features.

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TABLE 5-1.  DESIGN FEATURES OF DEUTSCHE BIBCOCK
          ANLAGEN SYSTEMS
     •   ROLLER  GRATE (1)

     a   FURNACE NOSES (6) FOR MINIMAL
        SECONDARY JET PENETRATION

     8   SECONDARY AIR (3)
        -   20-25% OF TOTAL AIR
        -   100m/sec VELOCITY
        -   70%  PENETRATION ACROSS
           FURNACE

     I   SIDE  WALL AIR FLOW (4)
        (ASPIRATED WALL)
     I   TIME  AT TEMPERATURE ABOVE
        SECONDARY INJECTION
        -   TA LUFT STANDARD
        -   SIC  CLADDING OF LOWER
           FURNACE (5) END OF
           FLAME TIP.
                 5-7

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air jets to approximately  70 percent of the distance  across  the furnace nose.
DBA indicated that  the  overfire air requirements as  suggested  in previous EPA
recommendations  of  50 percent of total air flow was  felt  to  be inappropriate.
Such high  levels would  result in poor  burnout.   DBA expressed the opinion
that operation with 20  to 25 percent  design air  flow  capacity air flow as
overfire air was appropriate.

     DBA has  examined the  impact of the  orientation of the furnace throat
relative to the grate and has developed specialized  furnace geometries for
different  refuse characteristics.  Three of the configurations are portrayed
in  Figure  5-3.  The configuration names refer to  the relative direction of
gas  flow over  the  bed to the direction of movement  of the  solid waste.  For
example, in the parallel  flow configuration, the volatiles  released from the
thermal  decomposition of the solid move in the same direction as the solids
due to the  presence of  a  hood or arch over the early  region  of the grate.  In
the  contra flow configuration,  the  gas flow is  generally in the direction
opposite to the direction or movement of solids.  The  configuration which is
considered to  be  the  most flexible is the center  flow  arrangement which is
between the other  two extremes.  In this configuration  the zone of volatile
thermal decomposition  is arranged  just below the  furnace throat.   The
volatile flame  can  freely develop and overfire air  can be added to uniformly
mix  the material  as it  enters the  radiant furnace region.  DBA recommends
contra  flow configuration for refuse  of low  calorific  value i.e. refuse
containing high amounts  of water  and ash due to the  "high pre-drying effects
and  preparation of the  refuse for ignition by recycling hot gas across the
first  rollers".   Parallel  flow configurations are  reserved for special high
volatile MSW or installations that have stringent  space limitations.  Such
special  configurations  are currently under construction at sites in Germany
(e.g. Duesseldorf).

     In  the  DBA design the lower  portion of the municipal waste combustor is
refractory-clad. The insulation consists of silicon carbide that is  directly
attached onto the water tubes.  The SIC refractory extends up  beyond  the  nose
to  halfway up  the  furnace itself.   This ensures  that sufficient time at
temperature is  achieved, such that  the DBA  designs  can  meet the current
                                     5-8

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    I. PARALLEL  FLOW
                                          O
II. CONTRA FLOW
III.  CENTRE  FLOW
                                                                       I    SPECIAL  REFUSE
                                                                           (HIGH  V.M.)
                                                                       II   LOW CALORIFIC
                                                                           REFUSE  (HIGH
                                                                           WATER OR  ASH)
                                                                       III  HIGHEST FLEXIBILITY
Figure 5-3.  Deutsche Babcock furnace geometry selected based on refuse characteristics  (DBA, 1986)

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German  requirements of  temperature above  the  last air injection  point
(800°C).   DBA has  examined the  previous EPA  criteria suggested for
"qualifying maximum volumetric heat  release"  rate and has concluded that the
definition  of the lower  furnace volume  is  difficult and portions of the
volume do not  actively participate in the  combustion process.  The  DBA design
has a heat  absorption rate in the lower furnace region of approximately half
of the heat  release rate in the upper furnace  region.  The upper  furnace heat
release  absorption  is roughly 48 kcal/m2-

     To monitor  furnace operation,  DBA uses  flue gas measurements of carbon
monoxide  and  oxygen.  Carbon monoxide is  maintained below the German TA Luft
standard  of 100 mg/Nm3  (~80 ppm)and  oxygen is generally between  9-10 percent
(dry).    The  stack gas  oxygen concentration  range suggested by  the DBA
operating experience is  shown in Figure 5-4.  The DBA system,  as any other
mass  burn systems, has an optimum operating  envelope of excess  air as shown
in Figure 5-4 as the conditio'n  of  minimum  carbon monoxide.  Excess oxygen
must  be  maintained at  levels that  ensure that all  zones within the furnace
have  sufficient oxygen even with  fluctuations in the volatile content of the
refuse.   However, too  high of  excess  oxygen  will excessively cool  the
combustion  zone due to  dilution.   Finally,  it  is also a DBA  operating
practice  to only operate  at full  design load  with only slight  variations
above and below the design conditions.

5.3       Steinmueller

     The  L&C  Steinmueller Corporation  is providing the combustion system to
Dravo, Inc.  who is  constructing two systems in the United States  in Portland,
Maine (500 tpd)'and Long Beach, California (1380 tpd).  Two other systems are
currently in  the permit stage in  Montgomery County, Pennsylvania (1200 tpd),
and Huntsville, Alabama  (690 tpd).

     The  personnel  at Steinmueller did   not have additional  information on
direct  cause  and effect relationships between PCDD/PCDF formation  and system
design  and  operation.   However, their  current practice was  found to have
average  2,3,7,8 TCDD emissions of less  than 0.02 ng/m3 for several plants in
                                  5-10-

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 x>
 CD)
 Zi
 oct
 OC_>
 CD 2:
 ceo
 CCCJ
 CJ
            3      6      9


           OXYGEN CONCENTRATION
      A -  INSUFFICIENT  AIR C+i02-*CO

      B -  APPROPRIATE OPERATING REGION

      C -  "COLD BURNING"
Figure 5-4.
Relationships of CO  and 0^ for
appropriate operating regions
(DBA, 1986).
                   5-11

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Germany  (Stei nmuel 1er, 1986).   Total CDD levels were found  to be on average
161 ng/m3.   The  Steinmueller approach  primarily relied on an  optimized firing
design with  high combustion efficiency and uniform mixing condition.  The gas
cleanup  system is also utilized by Steinmueller  as a backup.  Either dry
scrubbers  with baghouses  or  wet scrubbers with  ESPs were used  for the
Steinmueller  installations  in Germany.  There was concern expressed about
whether baghouses  with municipal waste combustors was the best application of
the technology.  For their  U.S. projects, Steinmueller/Dravo is using dry
scrubbers with  either an ESP or a baghouse.

     The Steinmueller firing system is  portrayed schematically in Figure 5-5
and the  key features are provided in  Table 5-2.   The design  employs dual ram
feeders  onto   a  forward-push block grate.   The  grate blocks move  in  a
reciprocating  motion and  push the refuse  down  the inclined grate.   The
radiant  furnace was constrained with front and  rear wall noses  and was
centered over  the grate.   The lower furnace was clad with silicon carbide
refractory for  corrosion control and to  lower heat absorption rates.

     A key aspect  of the Steinmueller  design philosophy is to achieve uniform
high  temperatures within  the  combustion volume.   Their current design is
capable  of  meeting the German standard  of 800°C above the last air injection
location.   In  order to achieve the  temperature, the refractory cladding is
added to  lower  the  heat  absorption rate.   Even more importantly, the
Steinmueller systems use an optimized furnace configuration  and overfire air
injection  scheme to ensure  mixing of volatiles with air and to maintain a
high  temperature combustion process.  The front nose acts to redirect the
volatiles into  the  hot gases from the  burnout portion of the  grate.   Numerous
high  velocity  secondary  air jets at the furnace  throat are then used to
ensure  uniform  mixing before entering the radiant furnace.  The Steinmueller
design  employs  80 mm diameter secondary jets injected at a velocity of  80 m/
sec with a  pressure drop of 600 mm of water-  The distribution of air was as
follows:   60  percent primary,  40 percent secondary, with an operating  range
of 80  to 90 percent total  excess air.  This  ratio was maintained at all
loads.   Steinmueller identified as a key  problem as the  operation of the
                                   5-12

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             I I
Figure 5-5.
The L&C  Steinmueller mass burn
design features.
                     5-13

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TABLE 5-2.  DESIGN FEATURES OF STEINMUEILER MASS BURN SYSTEMS
      •   FORWARD PUSH BLOCK GRATE (2)

      I   CENTER FLOW FURNACE WITH THROAT

      I   SECONDARY AIR (A)
         -   80  mm DIA.
         -   VELOCITY 80m/sec
         -   PRESSURE DROP ~ 600 mm w.g.
         -   40% OF TOTAL AIR

      a   CLADDING REFACTORY ON LOWER FURNACE  (SIC)

      I   SEPARATE.PLENUM CONTROL OF PRIMARY AIR  (3)

      I   CONTROLLED AND UNIFORM FUEL BED  DEPTH

      •   FIVE ADJUSTABLE UNDERFIRE AIR' ZONES  AND
         GRATE  SECTIONS TO CONTROL BURNING  RATE
                           5-14

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system at  low loads where  much poorer  mixing was found.   Under  low  load
conditions  Steinmueller has measured higher  CO  and oxygen levels.

     Steinmueller has relied heavily on  extensive cold-flow modeling and  in-
furnace  field measurements  to optimize the  design and operation  of their
systems  in  order to ensure high temperatures  and uniform mixing  conditions.
In  Figure  5-6 are provided some  representative examples  of  in-furnace
measurements which illustrate the impact of design and operational  changes.
In  Figure  5-6a is shown the detrimental  impact of  lower load operation.  At
full load  the temperatures in the radiant furnace are between 800 and 1000°C
(1470-1830°F) and the plane of CO concentration at 0.1 percent is uniformly
spread across the  furnace.  The furnace CO will  continue to oxidize resulting
in  stack emissions of  less  than 80 ppm.   At  lower load operation the upper
furnace  mixing is much  less uniform and  the temperatures have dropped by
almost  100°C (180°F).  The surface of constant CO concentration  is  shown to
be  strongly skewed to the front wall of the  furnace  indicating poor mixing at
this reduced load condition.  Also, exhaust CO will increase to 196  ppm with
spikes  as  high as 660  ppm.   In Figure  5-6b  are  shown cold flow  modeling
results on the impact of different combustion air distributions on the mixing
patterns.   A mixing parameter ("mischparameter") of unity across  the furnace
indicates  a high degree of mixing since all parts of the flow field have an
equal concentration of the gas tracer.  Uith normal  air distributions through
the various underfire plenums and overfire air jets, the furnace uniformity
is  excellent.   In  the  extreme case  of  no  overfire air,  the mixing is
extremely  poor indicating the important role that the secondary air  plays in
mixing.

     Figure  5--6b also  indicates  the air  control capabilities of the
Steinmueller system.  The operator has  control of  the underfire  air to  five
separate underfire air  plenums and  separately to overfire air  jets on  the
front and rear walls.  In this manner the operator has the ability to put the
air where  combustion  is  occurring  depending on the particular refuse
characteristics.   The system must be "tuned" to the  refuse characteristics at
startup of the facility; Steinmueller relies on in-furnace profiles of carbon
monoxide as the indicator of the "mixedness" condition at unit startup.   High
                                   5-15

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              (a)  Impact of Load
(b)  Impact  of Overfire Air
                                 FULL

                                 LOAD
                           NORMAL

                           OVERFIRE

                           AIR
                                                                          • Y/a-o.23
                                                                          D Y/B-0.3
                                                                          X Y/B-0.73
                                                        I9X 33X  I6X  23X I9X
en
I
                                 60%

                                 LOAD
                                                    24. BX
                                                    ,-S -,
                                                    QH 3
                                                    5*2
                                                    55,
                                                     a _
™ I
*0
$
a
$2
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S1
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  s
     ^!fel^>
        i
  *;-«^
             •*+
                                                         rmi
                           NO OVERFIRE AIR

                           LESS FURNACE

                           CONSTRICTION
                        Y/B-O.23
                       D Y/B-O.5
                       X Y/B-O.73
                       O ARBITRARY
                                                        I I .4X23.7X17. IX 5.7X 2 . 9X
                   Figure 5-6.  Steinmueller in-furnace testing  and cold-flow modeling.

-------
CO peaks  in the furnace indicates  that insufficient air is  being put at that
certain point.   Thus,  Steinmuel1er operational  success relies  on  the
combustion  air  scheme as follows:

     •    High  velocity, multiple  jets of overfire air for furnace mixing

     0    Multiple underfire  plenums for air  control with independently
          controlled stoker sections  (speed)

     •    Tuning of air flows using CO profiling as indicator of unmixedness

     0    Even  distribution of air in  the fuel bed

Finally,  Steinmueller uses carbon monoxide continuous monitoring in the flue
gas  as  an  indicator that combustion efficiency is being maintained at a high
level.   Steinmueller reports  that their systems generally have no trouble
meeting  the  TA Luft (German)  CO  standard of 100 mg/Nm3 (11  percent 02) on  a
30 minute rolling  average.

5.4       Von Roll
     Von Roll  is one of the larger  manufacturers of municipal and industrial
combustion  plants in the world with more than 170 plants  either operating or
under  construction on  five  different continents.   Signal Resco  has  the
American  license for this  Swiss  (Zurich) technology and currently has a
number  of  plants in operation or  under construction in the United States.
Signal  Resco uses special  Babcock  and Wilcox boiler designs integrated with
the Von Roll  grate.

     The Von Roll  philosophy  to prevent  PCDD/PCDF and  other trace organic
emissions  is to optimize the refuse combustion system.  Von Roll has little
cause  and  effect data  available but they do  have performance data which
indicates  that the Von Roll  system can be operated with low PCDD/PCDF
emissions.   Test data  supplied by Von Roll  (1986) for the Neustadt MSW
                                   5-T7

-------
combustion  plant  (225 TPD) indicated total  PPDD/PCDF of 88 ng/Nm3 before  the
air pollution  control device and 13.8 ng/Nm3 in  the  exhaust.

     The Von  Roll personnel were concerned  about  not only the possibility of
in-furnace  formation mechanisms but  also downstream mechanisms that might
occur  as  a  result of catalytic reaction of  hydrocarbon precursors on fly  ash
particles.  They  indicated that some recent work  by Vogg (1986) demonstrated
that catalytic  transformation could occur in the  temperature range of 200 to
300°C  (390-570°F).  Yon  Roll expressed the concern that  such downstream
mechanisms  could  lead to  the  presence of PCDD/PCDF  on residual fly ash.  The
Von  Roll  approach  is to  design and operate the  combustion system such that
the  destruction of all  organics  i.s  achieved.   If all  hydrocarbons  are
destroyed  in  the  combustion zone  then  even  downstream  mechanisms  are
prevented due  to the lack  of precursors to be reacted.

     The Von Roll  criteria for a good combustion environment are as follows:

     1.   Uniform  bed layer on grate

     2.   Proper air distribution  through the  bed  including  very high
          pressure  drops across the grate and multiple plenums

     3.   Load following and control procedures

     4.   Avoid slag buildup on the  side  walls (they use an air aspirated
          side wall  design in  Europe although not in the United States)

     5.   High injection  velocity  for the secondary air with numerous  air
          jets

     6.   The  amount of secondary air is held constant  at  30 percent of  the
          total  combustion air

     The features  of the Von Roll design are provided in Figure 5-7 and Table
5-3. Von  Roll  believes that one of the most important features of the design
                                    5-18

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in
i
            Figure 5-7.  Refuse combustion plant with Von Roll two-pass  boiler  and flue  gas  scrubber

-------
TABLE 5-3.  DESIGN FEATURES OF VON ROLL MASS BURN SYSTEMS

               t  GRATE SYSTEM (5)
                 - HIGH PRESSURE DROP
                 - PUSH BLOCK
                 - SELF CLEANING SLOTS
               •  CENTER FLOW FURNACE
               I  PRIMARY AIR
                 -2x5 PLENUM
                 - SEPARATE CONTROL
               •  SECONDARY AIR
                 - 30% TOTAL AIR
                 - 50 m/sec
                 - 5 cm dia WITH 1 m
                   SEPARATION
               •  ASPIRATED AIR
                 SIDE WALL (8)
               I  SiC CLADDING IN
                 LOWER FURNACES
               •  ESP PARTICULATE
                 CONTROL
                         5-20

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is the  grate  (Figure 5-8).  The grate  is  designed to have a high  air  pressure
drop.   Von  Roll  feels that with this feature uniform air distribution  through
the grate  can be achieved  even with variable thicknesses of refuse on the
bed.  The  underfire air is divided into  four to six plenums along the grate
with separate air feed control  to each region.  Larger systems will have two
plenums  side  by  side.  The air passes  through air slots that are  self-cleaned
by  the  movement of  the  grate  blocks.   The  grates  are forward-push
reciprocating blocks with  a  steep inclination angle.  For lower calorific
refuse  a modified grate is utilized wherein steps are present along the grate
to ensure breakup of clumps of refuse.

     The second  feature of the  Von  Roll system  that is important is the
mixing  level  within the furnace.  Overfire air jets are used as  the primary
means to  ensure adequate  furnace mixing.  The penetration of the secondary
air jets  across  the furnace which is  dependent upon velocity and  size of the
jets, is  crucial to the successful performance of the system.  Von Roll  has
performed  full-scale tests in order  to  optimize the design and  operation of
the overfire jets.   The current Von Roll design is provided  in Table 5-3
and the  orientation of  the  jets relative to  the furnace nose  is shown in
Figure  5-7.   An array of  complex injection schemes has been developed with
multiple  rows  and nozzle  diameter to  achieve  adequate  penetration  and
coverage of the  flow.  In-furnace profiling of carbon monoxide concentrations
performed  during system start-up is used as  the indicator of proper air
distribution and mixing.  The various air flows are adjusted to  minimize CO
peaks.   The CO profiling and air adjustment is performed both at  unit  startup
and annually.  Von Roll relies on exhaust CO concentration measurements as an
indicator  that  the system,  once tuned, is  being maintained in the proper
operating  envelope.    However, Von Roll does  not use  exhaust CO as an
indicator  of trace organic  or PCDD/PCDF emissions.  According  to Von Roll,
the typical  operating range for CO is  in  the range  of 50 ppm.

     The final  important feature  of  the Von Roll system is the combustion
control  system.  All waste-to-energy systems must have combustion  controls
that respond to changes in  steam demand and account for the variability of
the fuel  characteristics  of  the refuse.  In the Von Roll system the steam
                                    5-21

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Von Roll refuse feeding device
Von Roll combustion grate system
 Figure  5-8.   Details  of Von  Roll  grate  and feeding devices,
                                5-22

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production  rate is monitored and control  of the ram  feeder  frequency  is
modulated  along with  the  primary air  to  the middle region  of the grate
(burning  region) to maintain the correct  steam  rate.  Von Roll  systems  also
include furnace  temperature monitoring using thermocouples in the roof of the
radiant  furnace  (previously corrected for  radiation by comparison to  suction
pyrometry  measurements).   Furnace  temperatures are used  to  control  the
secondary  air flow rates.  For example, if steam  production rate goes down,
then primary air  is  increased in the middle  grate  region; if temperature  goes
down,  then  secondary  air flow is decreased.   The  grate movement rate is not
automatically controlled  but is manually adjusted depending on the refuse
burnout  characteristics.   Preheat  air is also  started manually for wet
refuse.   Finally exhaust oxygen is measured but  is not used in the automatic
control system.

     The Von  Roll  system also uses automatic  control for auxiliary burners.
Secondary  burners  will  automatically fire when  CO levels exceed 80  mg/Nm^.
It  is  a  TA  Luft  (German) regulation that there is  capacity for 60 percent  of
the load as  auxiliary fuel with startup at high CO and during system startup.

5.5       W+E Environmental Systems Ltd.

     W+E Environmental Systems  Ltd. (Widmer and Ernst) is a wholly-owned
subsidiary  of Blount  Inc.  (Montgomery,  Alabama) and is located in  Zurich,
Switzerland.  The  W+E design is portrayed in Figure 5-9 and details of the
design are  provided in  Table 5-4.  The W+E design has a unique grate.  The
grate is horizontal  with the reciprocating blocks  pushing the refuse over the
next block  in what is termed an  "overthrust"  motion.  The  double motion
overthrust  tends to first drop ignited particles  as the block moves out  from
under  the  layer and then  pushes  the  ignited particles back underneath the
non-burning  waste layer.  The ignited material  is  constantly pushed downwards
by  intensely rotating and  stirring the waste layer and forcing ignition  to
start  at  the bottom of the bed.   The air slot design causes a high pressure
drop across the grate and ensures uniform  air flow.  High air velocities  at
the slots  also  prevents  blockage of  the  slots.  The relative movement  of
blocks  acts to  continually clean  the grates and  acts to keep them open.  The

-------
           r-6
1_
                                              Motion sequence
                                              of the grate bars
Figure 5-9.  W+E combustion system and overthrust grate
           design.
                     5-24

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TABLE 5-4.  DESIGN FEATURES OF  W+E MASS FIRED SYSTEM
    I  GRATE  (3)
       -   HORIZONTAL
       -   "OVERTHRUST" MOTION
       -   SLOT  AIR  -  SELF CLEANING

    I  CONTRA  FLOW  FURNACE WITH MINIMAL
       CONSTRICTION

    •  PRIMARY  AIR  (9)
       -   SEPARATE  PLENUM CONTROL
       -   HIGH  PRESSURE DROP
       -   80  -  90%  OF PRIMARY TO FIRST
           2  ZONES

    8  SECONDARY  AIR  INJECTORS (10)
       -   80  m/sec
       -   70  mm dia
       -   FRONT AND BACK WALLS INTERLACED
       -   30%  OF  TOTAL AIR
       -   OFFSET  VORTEX

    •  SIC CLADDING TO 10 m
                      5-25

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underfire  (primary) air  is  introduced through  four  to five separately
controlled  plenums along the grate.

     Another unique  design feature of  the  W+E system is the overfi re  air
injection  scheme.   As with most  of the other mass  burn  systems the overfire
air injection is primarily used for furnace mixing  and flame height control.
High velocity  air jets on  the front and rear walls  are employed to achieve
jet penetration across the furnace and coverage of the  entire furnace flow.
The front  and  rear wall  jets  are  not directly opposed but rather they  are
staggered  to achieve an "interlacing" of the air streams.   In addition,  some
configurations  include directing  the jets to a firing circle which creates  a
vortex  motion  in the region  adjacent to the  overfire  air jets.  W+E  has
suggested  that  this rolling  vortex  zone arrangement  lowers particulate
emission  apparently because it serves as a "centrifugal bottle" to particle
carryover-   W+E has also  examined  the impact  of furnace design such as
furnace  noses at the overfire  air injection point.   No enhancement in mixing
was  achieved  with these  noses  alone; however,  the noses provided less
penetration differences for the overfire air jets.

     Thirty percent  of the total combustion air was used in the overfire
injection  with  an 80 to  100  percent  overall excess air.  The overfire  air
velocity  has recently been increased from 50-60 m/sec to approximately 80 m/
sec  (at full  load conditions).   This change was  introduced to improve  low
load operation where overfire air  jet velocities will  decline.  W+E indicated
a  belief  that  such overfire  air design improvements have  direct impacts on
PCDD/PCDF  emissions.  W+E  quoted  test results which indicated a five-fold
decrease  in PCDD/PCDF emission  levels with  these  modifications with no
measurable change in CO emissions  (W+E,  1986).

     The proper  air distribution  into underfire plenums and overfire air jets
is established  at startup by  in-furnace CO profiling and  air adjustments to
minimize  CO  concentration  peaks.   Once  the  system  is  tuned  to  the
characteristics of the refuse,  then W+E relies on exhaust  measurements of CO
as an  indicator of continuing  performance.   Under optimal  conditions CO
levels as low as  20-30 ppm could be achieved.   Based on W+E experience, CO is
                                   5-26

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used only  as  an  indicator of  insufficient air or  bad combustion conditions.
For exhaust  concentration of  CO  less  than 250  ppm,  W+E  believes  that no
correlation exists between exhaust CO and PCDD/PCDF  emission.  However,  some
correlation  was  suggested to  exist if CO was greater  than 250 ppm,  i.e. if
the system was  clearly being  operated in a failure  mode.   W+E has some
experience with the use of total organic carbon  as an indicator of PCDD/PCDF
emission.  W+E  suggests  that  total  organic carbon is directly related to
PCDD/PCDF  emissions.

     The  current  W+E furnace configuration (see Figure 5-9) could be
classified as a  "contra flow"  arrangement (compare  to  Figure 5-3).  W+E are
currently performing detailed  studies of the  necessity of such design
features.   Specifically,  W+E are  investigating  all   three types of
configurations, parallel, center and contra, to determine whether such design
modifications  will  improve performance and the cost impacts.

     The W+E  facilities employ  an  automatic load control system.  The  current
systems have  both control and  upset loops.  The primary  control loop monitors
the steam production rate and controls the ram feeder, grate speed  and air
flows.   Oxygen is  monitored and  if  the value falls  below a set value
(typically 6 percent) alarms  will  sound and  feeding of  the grate  will be
stopped.   If furnace temperature  falls below 800°C then  auxiliary  burners
will be  started  automatically.   Finally if load falls below 60 percent of
design,  then  the system will  shut down.  Future control loops will focus on
the use  of furnace temperature monitoring and control  of primary air.   This
new emphasis  is to  allow extensions to lower load operation.

5.6       Martin  GmbH

     Martin GmbH  (Germany) with  its partners Ogden  Martin  (Paramus,  NJ) and
Mitsubishi Heavy  Industries  (Japan) have constructed 130 refuse burning and
energy recovery facilities.   This  includes  250  operating units with  a
combined  burning capacity of  over 75,000 tons/day.  Martin  Systems is one of
the largest  manufacturers of  mass  burn municipal  solid  waste-to-energy
plants.   Martin  systems have  been tested for PCDD  and PCDF  emissions in  both
                                   5-2-7

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Europe  (e.g.  Wurzburg,  Stockholm and Munich plants) and the  United States
(e.g.  Chicago, Marion County and Tulsa).   Some of these data are available in
the comprehensive  municipal  waste study  series.  The  total  PCDD/PCDF
emissions  vary from 50 to 150 ng/Nm3 for  systems with ESP particulate control
and less than  5 ng/Nm^  for the  system with  a dry lime dust  baghouse
combination  (Martin, 1986).

     The design features of Martin refuse combustors are shown  schematically
in Figure  5-10  and design  details are  provided in Table 5-5.  The Martin
design philosophy  for  the  minimization of  trace organic  emissions is
described  in  Martin technical  literature  (Martin and Schetter,  1986) and
relies  on  optimization  of  the combustion process.  The Martin  approach to
minimize  organic emissions  focuses on  the  prevention of any hydrocarbons
leaving  the combustion zone and in that  manner minimize downstream  formation
of PCDD/PCDF  by eliminating PCDD/PCDF  precursors.  The approach is  based not
on one feature alone but  rath'er to the combined aspects of  the Martin
combustion  system.

     One of the main  features of the  Martin system  is  the grate shown in
detail  in  Figure 5-10.  The grate is inclined downwards from the  feed end at
an angle  of 26  degrees.  The grate blocks are "reverse acting"  i.e. they are
pushing  counter to the  direction of overall refuse movement.  This action
forces  burning  refuse back underneath  freshly fed material and  promotes more
complete burnout.

     The grate  underfire air is  introduced through gaps at the sides of the
head  of  grate blocks which  are  2 mm  wide.  The gap area is small  enough so
that  the  grate-has a high  pressure drop and therefore there is  uniform air
distribution through the refuse layer regardless of the refuse bed thickness.
The underfire air is divided into 5 to  6 zones along the length of  the  grate
through  the  use of separately controlled plenums.  For larger systems, the
air plenums are  doubled with side-by-side plenums.  The underfire air to each
plenum is individually controlled by dampers and flow orifices for each  grate
section.
                                   5-Z8

-------
                                                   © Initial pnase of reluse drying and
                                                     volatiliuuton, by means of flame
                                                     radiauon
                                                   
-------
TABLE 5-5.  DESIGN FEATURES OF MARTIN REFUSE COMBUSTORS
      I  GRATE
         -  REVERSE ACTING
         -  AIR SLOT, SELF CLEANING
         -  STEEP INCLINATION

      •  CONTRA FLOW FURNACE WITH
         MINIMAL CONSTRICTION

      •  PRIMARY AIR
         -  INDIVIDUAL PLENUMS
         -  HIGH GRATE PRESSURE  DROP

      I  SECONDARY AIR
         -  20-40% OF TOTALAIR
         -  FRONT AND BACK WALLS
         -  2-4" (50-100mm) DIA
         -  450 mm w.g.

      •  SIC CLADDING OF LOWER FURNACE
                        5-50

-------
     An  important  aspect of the Martin approach is the  "penetration of air
into all volatilization products"  and the use of secondary air nozzles to
"increase the degree of turbulence  in  the flame area".  The  current secondary
air nozzle  design should actually show in Figure 5-10 two  rows of air nozzles
on the  front wall, below the  front  arch, for  refuse with higher volatile
content.   The  adaptation of  the  secondary air injection  scheme from older
designs burning lower volatile refuse  to designs for higher volatile matter
is shown in  Figure 5-11.   The amount of overfire air is  between 20 and 40
percent  of  the  total combustion air  (100 percent excess air).

     Another  important aspect of the Martin  approach designed  for low
emissions  are combustion  control  systems  which attempt to reduce the
disturbances  resulting from changes  in refuse characteristics and limits load
variations.  New Martin facilities  have automatic combustion control  systems
which consist  of two independent loops  (see Figure 5-12).  The first loop
monitors wet flue  gas 02 (Zirconium oxide probe) and controls the refuse ram
feeder  and grate speed  to  control  MSW feed  rate.  The  second control  loop
monitors steam production  rates  and controls underfire  to maintain desired
steam production rate.  Development  work is continuing  on the combustion
control system including monitoring on furnace temperature and controlling
secondary  air.  Finally the  Martin  combustion control approach is to limit
load variations  to  85-110 percent of design full load.

     The newest  Martin  design  also  employs  air pollution control  devices
(APCD)  as  a  removal technique for PCDD/PCDF species.  For example, Martin
plants  at  Wurzburg, Stockholm and Marion County, Oregon have dry scrubber/
baghouse combinations.  This APCD  technology has been operating successfully
for  three  years and available data  indicates significant  reduction in PCDD
and  PCDF  emissions.  For  example,  total PCDD/PCDF emissions data  from
Stockholm  at the baghouse inlet were  74 ng/Nm3 while after  the baghouse, the
emissions were  less than 5 ng/Nm3 (Martin,  1986).
                                  5-31

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                               Furnoct "QS found'
                                                  Configuration at lirrn o<
                                                Isl meosucment series
                                                 Configuration at limt of
                                              2nd measurement series
to
ro
                       Front  wall overfire air
                      Numb* of nozzki	It
                      Anglt of mdnaUjn lo horizontal	-70*
                      NozzU moulh oVyrxlif	(Omm
                                                                      of nonkj.
                                                            	7ond 6
                                          Anglt of nchnalcn to horizorid	-IS*
                                          Malik mautfi damtUf	45and SSrrvn
                                                                                                                 of nozzlfS
                                          Angle of ncfnofon U> horamtal-
                                          NoizJ« moulh 
-------
en
U)
                                                                                                        Feed woltr
                       {[J Conliol ol rilust throughput

                       (T) Conltol of pflrrxvy  air
                         Figure 5-12.   Martin  Combustion  Control System  for mass burn
                                          combustors.

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5.7       Volund

     Volund,  located  in Denmark, has an installed  capacity of over 29,000 T/d
in 129 operating plants in Scandanavia,  Europe, Japan and the orient,  and in
the United States.   In the United States, Waste  Management has the technology
license  and  has  a new operating plant in  Tampa using  the Volund system.
Volund provides a combustion system that is distinct  from other European mass
burn technology-  Two separate concepts are available  depending on the refuse
characteristics  as  shown  in  Figure  5-13.   For  difficult to burn materials
such  as  vegetables, coarse pieces of wood,  and high  water content refuse,  a
rotary  kiln is  added at  the exit  of  the  traditional  grate region.   In
addition, the lower furnace  is constructed of  high grade, low conductance
refractory not SiC  clad  water tubes  as  in other European designs.   The
combustion zone also  includes a refractory arch  which  tends to keep radiation
heat  losses  from the burning zone to a minimum.   Hence, the combustion zone
temperature  is somewhat elevated over waterwall systems.  For these reasons,
Volund Systems can achieve impressive solids  burnout  levels.

     The  features of  the Volund System are provided in Table  5-6.  The Volund
grate  is a forward-push,  tilted design with longitudinal fixed and movable
beams.   Underfire air is  introduced between the beams and  is controlled by
three  separate plenums.  The Volund System has  a  relatively  low air pressure
drop  across  the  grate and  relies on the even  distribution  of refuse on the
grate  to ensure  uniform  air  flow.   The  secondary  air  is  added into  the
primary zone  over the grate region using overfire  air  jets and aspirated side
walls.

     Volund  System designers indicated that  good  mixing in the various zones
is a  key factor  to  controlling the  emissions of trace organics.  Volund's
approach, after  extensive  water-table flow modeling studies, has been to
position the refractory  arch above  the  grate  which splits and guides the
buoyantly rising gases  such  as to stimulate mixing.  The Volund furnace is
oriented above the  discharge area of the grate.   Mixing is  augmented by the
introduction  of overfire air but not dependent upon it.
                                   5-34

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Figure 5-13. Volund System  mass burn design features,
                          5-35

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TABLE 5-6.  DESIGN FEATURES OF VOLUND MASS BURN SYSTEMS
  I    GRATE
      -  FORWARD PUSH
      -  GRATE BARS
      -  AIR SLOTS BETWEEN BEAMS

  •    PRIMARY AIR
      -  LOW PRESSURE DROP AC'ROSS GRATE
      -  MULTIPLE PLENUM CONTROL, GENERALLY 4

  I    SECONDARY AIR
      -  INTO PRIMARY COMBUSTION ZONE

  l    EXCESS AIR 80-120%

  I    FURNACE CONFIGURATION
      -  BLOCK REFRACTORY LINED LOWER  FURNACE
      -  REFRACTORY ARCH WALL
      -  ASPIRATED AIR SIDE WALL
      -  UPPER FURNACE WATER WALL

  i    APCD CONCEPT
      -  FGR FOR NOX
      -  ESP FOR PARTICULATE
      -  HC1 WET, SEMIWET, DRY SYSTEMS

  •    TIME AT TEMPERATURE
      2.2 SECONDS  > 1000°C
                      5-56

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     The higher temperatures achievable  in  the  Volund  System could
potentially  increase NOX formation.   Volund can incorporate  a  flue  gas
reci rcul ation  technique to moderate temperatures in the combustion  zone to
effectively reduce thermal  NOX.  For particulate and acid gas removal,  Volund
offers  a variety  of air pollution  control  device configurations.  These
configurations  include wet scrubbers  alone  with flue gas reheat, semi-dry
(spray  dryers)  with either ESP or  baghouses, and dry injection of  calcium
compounds again with either baghouses or ESP.

     Volund  has undertaken  a series of emission tests  at  one  of  the
representative  plants to examine the impact of startup and load  changes and
the relationships  between temperature and  carbon monoxide  to  PCDD/PCDF
emissions.   These  data clearly indicated lowering of 2,3,7,8 TCDD emissions
as  time progressed after startup; almost an order of magnitude lower  2,3,7,8
TCDD emissions  were achieved  in  10 hours after startup versus  1 hour even
though  the average temperature increased only slightly in the same  period.
For example, the emissions of  2,3,7,8 TCDD dropped from 2.5 ng/Nm3 at  1 hour
after startup  to  0.4 at  5 hours  and  0.33 ng/Nm3 at 10 hours after  startup
(Volund 1986).   However the  minimum temperature measured  did  increase
significantly  (~150°C, 270°F) in  the same  period indicating  the  minimum
temperature  pathway must be  considered in  PCDD/PCDF destruction.  With
decreasing  load to 70 percent,  the  2378 TCDD and total PCDD/PCDF increased by
a factor of 2 (from 0.17 ng/Nm3 to  0.39 ng/Nm3  for  2,3,7,8 TCDD).  These data
have also  indicated to Volund that CO in the flue gas of a well designed
plant can  be used as a measure for monitoring and control since it  closely
tracked PCDD/PCDF emissions.  For  example,  just after ignition, CO levels
were near  1000 ppm without auxiliary fuel  start-up while  after 10 hours
exhaust  CO  were-below 100 ppm  (Volund,  1986).

5.8      Riley/Takuma

     The Riley Stoker  Corporation is a major United States manufacturer of
boilers and  combustion hardware.  Their first major entry into the MSW area
was through  the  facility at Braintree, Mass,  which began operation in 1971.
That facility  incorporated a  Riley  boiler  as well as  a  Riley horizontal
                                  5-37

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traveling  grate.   That grate  design was a direct  adaptation of the Riley
stokers  for  coal  and wood-fired  boilers.  Turner (1982)  provides a thorough
review of the  Braintree facility's operating history.

     Riley Stoker  is  currently  involved  in  at least  five  (5) MSW projects
(Riley,  1986).   Two of these  projects involve use  of Riley Stoker boilers
coupled  with  grates from  Ogden/Martin.  The  general  design and operating
features  of  facilities using  Ogden/Martin grates  were discussed earlier in
this  chapter.  Another project  - Jackson County,  Mich.  - represent  the
combination  of  a  Riley boiler  and the  Takuma grate.   Riley  has licensed  the
Takuma grate  which  is a Japanese developed design.

     The Riley/Takuma  approach to minimization of  PCDD/PCDF and other trace
organics  is  to  optimize the  refuse combustion system.  Riley personnel
expressed  an  opinion that the published data  base is not sufficient  to
correlate  PCDD/PCDF emissions with  operating performance parameters (e.g.,
stack CO emissions  or maximum qualifying volumetric  heat release rate).  They
feel  that  the basic design  features of the Riley/Takuma  firing  system,
coupled  with  a sophisticated automatic combustion control (ACC) system, will
provide  burning  conditions  sufficient to minimize unburned hydrocarbon
species  emissions.  The system  is designed to maintain  at least a one-second
residence  time above 1800°F as  required by several  State emission regulatory
bodies.

     Basic features  of the Riley/Takuma  firing system are illustrated in
Figure 5-14 and Table 5-7.  As shown,  there are four grate sections including
a  feeder grate,  a drying  grate, a firing grate,  and a finishing  grate.
Underfire  air is  provided to  the drying, combustion and  finishing grates
through  plenums  located under each  grate section.   Each plenum is equipped
with dampers and venturi  sections to modulate  and measure flow to  the
particular plenum.  Normally,  the total underfire  air flow  will provide 125
to  145  percent  of the theoretical  air  requirements with the majority of the
underfire  air proportioned  through the combustion grate-  They  hope to
achieve  a  uniform  (spatial  and  temporal) release of volatile matter  from the
burning grate.
                                   5-38

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                      REFUSE CHARGING
                      HOPPER & CHUTE
                         FURNACE  WATER
                         TUBE  PANEL
  FEEDER

  HYDRAULIC
  OIL CYLINDER

  DRYING
  GRATE
RIDDLING HOPPER
AND GRATE
AIR PLENUM
   COMBUSTION
   GRATE
          DOUBLE DUMPER
           RIDDLING CONVEYOR
                     BURN-OUT GRATE
                         HYDRAULIC
                        OIL CYLINDER
                         ASH DISCHARGER
                                                 CAST IRON
                                                 BLOCK OVERLAY
                                                 REFRACTORIES
                                                 OVERLAY
                                             AUXILIARY
                                             FUEL BURNER
                                             MAIN ASH
                                             CHUTE
    Figure  5-14.
Cross sectional schematic of combustion  zone  on  a
Riley-Takuma mass burn plant.
                                 5-3S

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TABLE 5-7.  DESIGN AND OPERATING FEATURES  OF RILEY-TAKUMA
          TECHNOLOGY
    •   A  GRATE SYSTEM
       -   FEEDER GRATE
       -   DRYING GRATE    (  UNDERFIRE  AIR
          rnMRiKTTHM PDATC?  PROPORTIONED AND
       -   COMBUSTION GRATE (  MONITORED  BY ACC
       -   BURNOUT GRATE

    I   80-90 PERCENT EXCESS AIR AT MCR
       -   70  - 80% OF TOTAL AIR THROUGH
          GRATE

    I   REFRACTORY CLADDING 30' ABOVE GRATE

    •   ACC SENSES EXHAUST 02 AND FEED
       -   TRIMS 02 WITH OFA FLOW  f
       -   MAINTAINS T > 1800°F WITH
          AUXILIARY BURNER

    i   THREE  LEVELS OF OFA PORTS

    I .  CO  < 200 PPM

    i   WILL OPERATE AT 55% WASTE FEED RATE
       ON  3800 BTU/# WASTE
       -   SYSTEM DESIGNED AT 100% - A,500
                      5-40

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     The Riley/Takuma design provides  a  center flow furnace  configuration by
placing an  arch over both the drying  and finishing grates.   The  boiler is a
straight wall  configuration  above the combustion grate.   Silicon carbide
refractory  cladding is provided to a  height approximately 30 feet above the
grate.  Overfire air ports are provided at three locations  and are designed
to achieve  complete coverage of flow from the lower furnace.  Overfire air
quantities  usually amount  to approximately  20  to 30 percent of the total
combustion  air.   Overall,  the system is designed to operate at  80 to
90 percent  excess  air.

     As stated  above,  Riley/Takuma attempts to achieve PCDD/PCDF emission
control  by  maintaining at least one-second residence time above 1800°F-  This
is accomplished through use of a sophisticated automatic combustion control
system  in  conjunction with  auxiliary  burners.   Measurements  are made of
exhaust oxygen concentration and  the  furnace  exit gas temperature (i.e.
temperature  at  convective section inlet). The ACC will modulate the overfire"
air flow to  maintain constant 03 concentration.  This is primarily a trimming
operation  on the overfire  air.  Correlations of required  furnace exit gas
temperature to assure one-second residence time above 1800°F (as a function
of load) will be developed and incorporated  in the microprocessor  of the ACC.
That minimum condition will be maintained by modulation of the fuel-charging
rate,  grate speed and underfire air flow rate, as well as through the use of
an auxiliary burner -

     The auxiliary  burner is  located in  the lower furnace region near the
grate   in  order to maintain  the time-at-temperature  requirement.   This
required positioning has  generated  operational  problems  associated with
burner  openings getting covered with slag.  Riley is currently working on an
advanced auxiliary burner design to circumvent that problem.

     A potentially important aspect of MSW  units designed by Riley/Takuma is
the  provision for operation at lower than  full  load.   Riley personnel
indicated  that  their systems could be  operated at loads as low as 55 percent
of rated input  (TPD) burning waste with 3800 Btu/lb heating value.  It  should
                                   5-4-1

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be  recalled,  however,  that  the ACC  will  still  maintain the time  at
temperature  criteria  through use of the auxiliary burner.

     Further inquiry into operational control  at reduced load conditions
indicated that  the  overall excess air level  was allowed to  increase.  Exhaust
02  concentration  versus  load curves were  not provided.   There is obviously
concern  over maintenance of overfire  air jet penetration and mixing  at
reduced load.   Riley  personnel noted that there were three  levels of overfire
air ports and that  levels  could be  shut down at reduced load.   That procedure
should  maintain jet penetration.  However, shutting  down a row of overfire
air jets is  accomplished manually and is not automatically  controlled through
the ACC.

     It  should be  noted  that the  MSW facility being constructed at Olmstead
County,  Minnesota will be the first Riley/Takuma  system in operation  in the
United  States.  Even  though there  is  significant operating experience  on
Takuma  grates  in Japan, it is reasonable to expect that Riley will fine-tune
its design and  control  strategy through experience  gained at Olmsted.

5.9       Detroit Stoker

     Turner  (1982)  reported that in 1982 there were nine mass-burn, waterwall
refuse  combustion plants in operation in the United States and that four  of
the plants employed Detroit Stoker  grates.  The combination of  Detroit  Stoker
grates with  various boiler manufacturers (Foster Wheeler, Babcock and Wilcox,
and Keeler)  reflects  the historic contracting procedure in  the  United States.
That  procedure generally  involves  selection of an  engineering  firm to  design
the plant with  major  component selection based on competitive bids.

     The  preceding   sections of  this  chapter have  discussed design and
operating philosophy  of  various  firms marketing  complete resource recovery
systems.   Detroit Stoker is not  a  MSW system supplier.  Typically, they
supply  fuel  feed,  grates  and air supply hardware (underfire  and overfire) to
the boiler  manufacturer-  Detroit  Stoker then works with the boiler designer
and control  system  contractor (this could be a third party in the American
                                   5-42

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contractual  approach)  to integrate  the various components  into an overall
system.

     Significant  testing  has  been undertaken of the Hampton  plant in Virginia
over the  last several years.  The PCDD/PCDF emissions  from this plant which
employs  a  Detroit  Stoker  grate are significantly higher than  other nass burn
waterwall  systems  (see Table 4-1 and the data base volume entitled "Emission
Data Base  for Municipal  Waste Combustors."   The  reasons for these higher
emissions  are discussed  in some detail in Chapter 8 of  this report.  Current
Detroit  Stoker hardware  design is significantly different  from that used at
Hampton  (Detroit  Stoker, 1986).  Figure 5-15 and Table 5-8 illustrates the
current design.  Major changes  have occurred in the distribution of underfire
air, addition of  flow constrictions and modifications to the overfire air
system design.  Each of these areas are discussed below.

     The underfire air  system now consists of separate plenums under each
grate  section.  The  grates themselves are constructed  as standard width
modules.   Thus, depending upon  the unit size the grate may consist of a 3 x 1
or  3 x  2 module  configuration  with either 3 or 6 underfire  air plenums.  Air
flow to  each  is individually adjustable.  Air  pressure  in the underfire
plenums  is typically on  the order of 3 to 3-1/2 inches water gauge which is
low by European design standards.  Detroit Stoker is convinced, however, that
this pressure level  is  sufficient to achieve uniform air distribution across
the drying,  firing and  finishing grates.  Establishment of  the underfire air
flow distribution  is  based on  visual observation of the flame.  Normally, 70
to 75 percent of  the underfire  air is directed through the firing grate.

     As the combustion gases  leave the grate region they are directed through
a constricted flow  region  formed by "noses" on both the  front and rear boiler
walls.   Clearly,  construction of the throat region is  the  responsibility of
the  boiler  manufacturer.   This is  a critical  area  requiring  close
coordination between the boiler and grate manufacturer. Overfire air ports
are located on both the  front and rear walls.  The number,  location and angle
of the overfire  air ports are  adjusted for each unit design following design
                                   5-43

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                                                           Key
                                                            1. Refuse Charging Hopper-
                                                               Charging Throat
                                                               Charging Ram
                                                               Grates
                                                               Roller Bearings
                                                               Hydraulic Power Cylinders
                                                               Vertical Drop Off
                                                               Overfire Air Jets
                                                               Combustion Air
                                                               Automatic Sifting Removal
Figure 5-15.  Cross-section of boiler with  current  design  Detroit Stoker firing system.

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TABLE 5-8.  DESIGN FEATURES OF  DETROIT
          STOKER MASS BURN COMBUSTORS
I  FUEL FEED,  GRATE  AND OFA
   BY DSC

•  BOILER NOT  BY  DETROIT STOKER

I  MULTIPLE GRATE AIR  PLENUMS
   WITH INDIVIDUAL CONTROLS (9)

I  PUSHER TYPE  GRATE DESIGN

I  FLOW RESTRICTIONS TO LIMIT
   COLD REGION  BY-PASS

I  MULTIPLE ROWS  OF  OFA PORTS
   -  40-50% TOTAL AIR
   -  DESIGN CONTROLS  FLAME
      HEIGHT

I  REFRACTORY  CLADDING (SIC)
   15-20 FEET  ABOVE  GRATE

I  DESIGNED TO  PROVIDE 2:1
   TURNDOWN
              5-45

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criteria for  controlling flame  height.  Typically  30 to 40 percent of  the
total  combustion air is supplied  as  overfire air.

     The lower furnace region  -  to  an elevation above  the overfire air ports
- is covered  with approximately 1 inch of silicon  carbide refractory  to
prevent tube corrosion.   Detroit Stoker feels  that  the  combination  of
insulated  walls,  flow constriction and careful  design of the overfire  air
jets  will   lead  to  high combustion  efficiency.  As  in  the case with
essentially all  manufacturers,  "good combustion" is  equated with low PCDD/
PCDF emission.   They do  not,  however,  have emission data to establish  the
validity  of  that  belief.   Detroit  Stoker recommends  that the  boiler
manufacturer provide at least one second  residence time above  1800°F.

     Startup  and  shutdown rely  on  auxiliary  burners.   These  burners  are
either gas or oil-fired  and  are located well  above  the  grate.  Exact
location,  size,  fuel, and controT  sequence for the auxiliary burners is  the
responsibility of  the boiler manufacturer.

     Detroit  Stoker grates are designed to operate with a 2:1 turndown ratio
but  Detroit Stoker prefers to  have  base-loaded  systems.  The  lower limit  for
turndown is generally established by  furnace temperature.  This condition
will also be characterized by deterioration in overfire air mixing as well  as
increased  CO and unburned  hydrocarbon emissions. Detroit Stoker suggested
that a  row of overfire air ports  could be dropped from service at low load
but that is not an integral part of  their suggested control strategy-

     The Detroit  Stoker  design  for overfire air mixing strategy has evolved
through experience gained  in numerous  MSW projects.   They are currently
considering cold-flow modeling  studies to refine their current designs.

5.10     Combustion Engineering - De Bartolomeis

     Over  40  percent  of  the world's thermal  electric power is produced by
equipment of Combustion Engineering  (C-E) design.  They offer  complete system
designs and hardware for resource recovery  applications  and will supply

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either mass  burning or RDF configurations.  Further discussion of C-E's RDF
system offering will be presented  in Chapter 6.  Combustion  Engineering has
recently  been selected as the system supplier for a 600 ton/day MSW facility
to be built  in  Chattanooga, Tennessee.  This will  be the  first C-E designed
mass  burn  facility in the United States.

     The  grate  system  for C-E MSW systems is obtained through an exclusive
North  American license  with the  Italian  firm,  De Bartolomeis  (db).
Figure 5-16  illustrates  the db  grate design  which consists of alternate
moving and  stationary  steps in  an overlapping  configuration  (Combustion
Engineering, 1986).  The  three-part step design includes  a scraper which
serves the  dual function  of cleaning the lower surface plate and forms the
minimum free  area passage for underfire air.

     Each grate  section is equipped with its own plenum to allow control  of
underfire  air.   It  should be noted that various grate sections are shown  to
be configured in a  continuous path,  without steps between  grate sections.
The  C-E personnel   indicated that the grate  motion provided  sufficient
aeration  of  the  fuel and felt that  steps between grate sections could result
in uneven  burning.  Unlike most other grate designs, the  slope of db grates
can  vary  from horizontal  to as much as 21 degrees.  C-E sales literature
indicates  the slope of  the grate  is based on  waste composition.  For the
typical  ranges of heating values found  in the United States, the slope is 8°-
Very low heating values  (<4000 Btu/lb avg. HHV) would require  steeper slopes.

     Figure  5-17 illustrates the db  grate integrated into a complete resource
recovery  system.  For MSW, C-E uses a boiler based on a design developed by
Energie und Verfahrenstechnik  GmbH (EVT).   EVT is partially owned  by
Combustion  Engineering.   Detailed information on overfire  air  mixing
considerations,  splits between underfire and overfire air,  combustion control
strategy,  startup procedures, etc. were not made available.
                                   5-47

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en

ji'
CO
                     Supporting Bo*
Surface Plate
                      Supporting Roll
                                        -Operating
                                         Shan
             Hydraulic
             Cylinder
                                                                              Enlarged Area Showing Air Flow
                                  Figure 5-16.   Design  features of  DeBartolomeis  grate.

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en
-Cv
10
               Figure  5-17.  Boiler cross-section of CE design using db  grate  and  EVT  boiler,

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5.11      Westinghouse/0'Connor

     Westi nghouse  Corporation is  a  relatively recent entrant in the list of
companies competing in the  combustion portion  of  the resource  recovery
industry.  That entry was  accomplished  through a 100  percent purchase of the
O'Connor Combustor  Corporation  (Westinghouse, 1986).   Prior to  the
Westinghouse  buyout, O'Connor  had  built  a  MSW  facility in Sumner County
(Gallatin),  Tennessee  as well  as facilities  at five locations  in Japan.
Current  projects  being designed or  under construction include facilities in
York County,  Pennsylvania;  Bay County, Florida; Dutchess  County,  New York;
and in  Bloomington,  Indiana.

     The O'Connor combustion  system.represents a unique approach for burning
municipal solid waste.   A typical  plant  configuration is  illustrated in
Figdre  5-18  and shows that  the  heart of the system is a water-cooled rotary
combustor-   The rotary cylinder consists of  alternating watertubes  and
perforated steel plates.  MSW is metered into the combustor via a feed chute
and  ram  feeder.  Preheated  combustion air  is  divided into six  zones and
enters  the combustor through the perforated plates forming the walls of the
cylinder. The rotary combustor turns slowly (10-20 RPH) and is oriented with
a slight (approximately  6°)  downward tilt.  The rotary section terminates
within  a waterwalled boiler  allowing the residue to fall  into an ash removal
system.

     Combustion air is drawn from  the waste pit and passes through an air
preheater at the  boiler exhaust.  Typical  air preheat temperature  is 450°F.
In the  Gallatin facility a portion of this  air was drawn off by a second fan
and  directed  to three overfire air port elevations in the boiler -  one below
the  rotary combustor dump  and two elevations above the rotary combustor.  As
shown in Figure  5-19, the main combustion air  is distributed axially down the
rotary  combustor.   Each  axial  section is subdivided into two zones.  With
counterclockwise  rotation,  air passing through the right zone  is forced
through  the  burning material  bed.  Air admitted to the left zone will enter
the  rotary combustor in a  region which  is effectively above the burning bed.
                                  5-50

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                                                                                       Gas Clean-Up Equiprmvr
                                Combustion Air
Waterwall
Boiler
                        Forced
                        Draft Fan
                Westingnouse
                O'Connor
                Water-Cooled
                Rotary Combustor
Ram
Feeding
                                                                                     Fly Ash
                                                                                     Conveying System
                     Ash Conveying
                     System
                                               Ash Removal System
     Figure 5-18.   Typical  plant configuration  using Westinghouse/O'Connor  combustor.

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Figure 5-19.  Cross-section of Westinghouse/O'Connor combustor
                                5-52

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Westinghouse  personnel refer to those two zones as  underfire and overfire air
respectively.

     In comparison to the grate firing systems discussed in earlier sections
of this  chapter, there  are several  other unique  features of the O'Connor
combustor.   Typical grate firing systems operate at  80 to 100 percent excess
air  while  the  Westi nghouse/0'Connor system operates  at  approximately
50 percent  excess air  (both  at 100  percent load  operation).  Traditional
grate  systems for MSW  firing provide silicon carbide cladding to the lower
waterwalls to prevent  corrosion.  There is no refractory  on the waterwalls of
the O'Connor rotary combustor.

     As noted  previously, the Gal latin, Tennessee  facility was equipped with
"overfire air"  ports  at three elevations in the  boiler.  In that facility,
the  region  from the  rotary combustor to the ash  pit of  the boiler  was
equipped with a stationary grate  referred  to as  an "afterburning grate."
This  particular facility has been  subjected to  extensive field testing by
Cooper Engineers, Inc.  as  part of a study sponsored by  the California Waste
Management Board. A  1984 report  (Cooper Engineers, 1984) states that "it was
found  from  earlier Cooper Engineers testing that the boiler overfire air and
grate  air was ineffective, so these systems were no longer used and all of
the  combustion  air  is  now  introduced through the  combustor as underfire and
overfire air."  This  has been confirmed in discussions with Westinghouse
personnel .   The test  report by Cooper Engineers includes data on criteria
pollutants  and heavy  metals, but does not include  information on PCDD/PCDFs/
furans and other air  pollutants.  It is not known whether the boiler overfire
air system had an impact  on  those pollutant emissions.

     All  of  the  existing facilities using the O'Connor combustor system were
constructed  prior to  the  Westinghouse purchase of  O'Connor.  For a variety of
technical  and marketing reasons, Westinghouse has  entered into a long-term
contractual  arrangement with Sumner  County.  They have made a variety of
major  system repairs  including replacement of the  stacks which collapsed due
to corrosion.   It is  also reported that the facility has  a  new  superintendent
who is a Westinghouse  employee.  The significance  of this arrangement is  that
                                   5-53

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Westlnghouse  intends to  use  the Gal latin  facility as both  a  show place
facility  and  as a test bed  for evaluating  issues such as  PCDD  and PCDF
emissions.

5.12      Basic  Environmental  Engineering  Inc.

     Basic Environmental  Engineering located in Glen Ellyn,  Illinois  builds
multistaged  mass burn combustors for smaller scale applications (Basic
Environmental  Engineering,  1986).   Basic offers systems with sizes  ranging
from  4  MMBtu/hr to  56 MMBtu/hr (10 to  150  T/d).  The construction  of these
units  is  done in a  "unitized" modular  fashion as opposed to field  erected
which  is  common  for  larger mass burn systems.   The primary zone of  the Basic
system  is  not starved air so that the  technology can be classified  as small
modular mass burn  combustion.  There  is  currently little information
available  on  trace  organic  emissions from  Basic Environmental  Engineering
Systems and  hence it is  uncertain whether  current design  practices  are
sufficient;  however, the design and operating  approach employed  by  Basic is
worthy of  consideration because Basic is  one  of  the leaders in advanced small
unit  systems.   Also  many of the features  of  the Basic technology  will  likely
have  favorable impact on trace organic  emissions.  The process flow  diagram
of  a  typical  Basic system is provided in  Figure 5-20 and design features are
summarized in Table 5-9.   The combustion system consists of three  zones as
fol 1ows:

     •    Pulsed hearth primary operated at near stoichiometric conditions
     •    Fired  afterburner secondary with secondary air addition
     0    Final  air addition in tertiary

The primary  zone includes a ram feeder  to transfer the solid waste  onto the
first  of  two  pulse  hearths.   (Basic uses  one,  two or three pulse hearths
depending  on  model  size.)  Primary combustion  air is introduced through the
hearth  at  approximately  stoichiometric  conditions.  The primary chamber is
constructed  of membrane  water wall.  The  firing density in the primary is
designed  to  12,000 Btu/hr/ft3 and Basic  personnel believe that this is a key
parameter  that  should not be exceeded.   Higher  volumetric charging rates are
                                   5-5*

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Figure 5-20.  Process flow diagram of Basic Environmental
              Engineering modular mass burn technology.

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TABLE 5-9.   DESIGN FEATURES OF  BASIC ENVIRONMENTAL
            ENGINEERING SMALL MODULAR MASS BURN
            TECHNOLOGIES
     •   THREE  STAGE  COMBUSTION
        -   PULSED  HEARTH PRIMARY
        -   FIRED  AFTERBURNER SECONDARY
        -   "THERMAL  EXCITER" TERTIARY

     •   PRIMARY
        -   MEMBRANE  WATER WALL
        -   PULSED  HEARTH
        -   STOICHIOMETRIC AIR

     •   SECONDARY
        -   FIRED  AFTERBURNER - TEMPERATURE CONTROL
        -   AIR INTRODUCTION WITH "THERMAL EXCITER"
           CYCLONIC  OUTWARD INJECTION
        -   REFRACTORY LINED

     •   TERTIARY
        -   FINAL  AIR ADDITION WITH EXCITER
        -   REFRACTORY LINED

     I   EXCESS AIR
        -   TYPICALLY 80%

     •   MONITORING & CONTROL
        -   TEMPERATURES OF EACH ZONE
        -   STEAM  PRODUCTION
        -   CO  (20-30 PPM)

     •   TIME AT TEMPERATURE
        -   IN  REBURN ZONES 1 SEC ABOVE 1800°F
        NOX  ~ 35  PPM
                        5-56

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expected  to  lead to poor burnout and excessive carbon and volatile carryover
to the secondary chamber.   Higher charge  rates also lift larger particle
sizes which  require greater residence  time to destroy  the  carbonaceous
particles  before  leaving the high temperature furnace zone.

     The  secondary  or  "stage  two"  chamber  includes a fired afterburner  and
additional  air  injection.  The first afterburner  is positioned at the exit of
the transition duct between the chambers  and can  be fueled on oil  or natural
gas.  The  design capacity of the afterburner is  20 percent of the total  Btu
input of  the system for normal municipal  solid  waste and is increased  to 40
or even  50 percent when the refuse includes: a)  large amounts of chlorinated
plastics  or b) when the  furnace is  operated at lower furnace input firing
rates, but the waste  fuel  is primarily  long chain hydrocarbons such  as
plastics  and rubber.  The afterburner is fired before startup for heating  the
system  (to 1400°F) and  is  cycled  on  and off to maintain temperature.   For
refuse with higher moisture content (less than  3800 Btu/lb) the afterburner
is continually fired  but  varied and  adjusted   automatically to setpoint
temperatures.   Additional  air is  added  into the second  stage after  the
afterburner using a unique injection scheme  called a "thermal exciter".   The
air is injected outward  from a refractory-lined closed cylinder positioned
along the center  axis of the secondary chamber.   The air flows through small,
high-velocity  jets and are oriented to achieve,  vortex flow in the secondary
chamber.

     The  tertiary  chamber consists of a second  "thermal exciter" for  final
air addition to an overall excess air level of approximately 80 percent.   The
chamber exit temperature is monitored and is  used as the control variable  for
the fired  afterburner.   The temperature is maintained at between 1600  and
1800°F depending on refuse  characteristics.   The maintenance of the exit
temperature of  the final combustion chamber ensures that the gases experience
at least  one second above that temperature  in the combined second and  third
chambers.   In  other words,  typical  design conditions  would also ensure
greater than 1  second above 1800°F under excess air conditions.
                                   5-57

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     The Basic combustion monitoring and control  system is relatively
straightforward.  Temperatures  of  each chamber exit are used  as the principal
control  variables and the air flows  and  afterburners are used to maintain the
temperature.   Basic is currently investigating the use of oxygen and carbon
monoxide monitoring for system control.  The Basic technology is capable of
achieving  high turndowns (up  to  4:1) without requiring additional  auxiliary
fuel to maintain  the destruction  temperatures of the reburn zones that it
required at full design conditions.  Example: if at full  load the waste fuel
burned  did  not  require auxiliary fuel, then down to 4:1 it would not need it.
Basic engineers would not advise  lower  load operation without auxiliary fuel
being required.  If a client  desires to operate to 3:1 turndown, the design
supplied  to  the client  will  be  changed from the standard control  of
60 percent  turndown without use of auxiliary fuel to maintain temperature.

     The distributed air addition  and  moderated uniform temperatures likely
keeps NOX emissions from being  excessive.  Basic estimates NOX levels of less
than 45 ppm  compared to other modular systems  in  the 170 ppm range.  The
exhaust carbon monoxide level  is  also low, typically in the range of 20-30
ppm.  Although CO is not typically  measured unless requested by the client,
Basic  believes that this  is  an  indicator  of the maintenance of good
combustion conditions.  Most currently installed Basic systems do not have
air pollution control devices;  however if the waste stream has greater than 2
percent by weight heavy metals  or  1/2 percent of plastics  with halogens or
halogen salts, then baghouses, electrostatic precipitators  and/or scrubbers
are recommended.

5.13      Enercon/Vicon

     Enercon   Systems,  Inc.  is  a  small corporation specializing  in the
engineering, design and construction  of  industrial energy systems.  They have
developed  and  patented  a  multistage  controlled excess air mass  burning
technology.    Vicon Recovery Systems,  Inc.  specializes  in the  design,
construction  and operation  of  MSW facilities.  Vicon is the  exclusive
licensee of the Enercon technology  for  MSW.  The Enercon/Vicon technology is
used at the MSW facility in Pittsfield, MA which is the site of a continuing
                                   5-5$

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extensive emission performance  evaluation  (evaluation developed  under the
auspices of  ASME with project  sponsorship  from numerous state government
agencies, DOE,  Ontario Ministries of Energy  and Environment, the Vinyl
Institute, and the Association of  Plastics Manufacturers in Europe).

     The Enercon/Vicon  technology  utilizes modular construction but  extends
the "modular"  concept to  significantly larger  scale than  the John Basic
Technology  discussed in the previous  section.  The Pittsfield, MA  facility,
for example, consists of three municipal waste combustors (or modules), each
rated at  120 tons/day; two furnaces are typically on line, with one standby.
The Enercon/Vicon system  technology relies on  refractory  lined chambers
without heat extraction.   Burning occurs  in  the primary chamber which is
equipped with a  recuperative combustion air  liner to minimize heat  loss from
the zone.   Combustion gases exist the primary  chamber  through  a  passage
referred to  as  the "mixing throat" and enter the refractory lined  secondary
combustion  chamber.  This chamber simply provides high temperature  residence
time to  maximize  trace organic burnout  and oxidation of CO to
     The incinerator module referred to  as the tertiary chamber  is actually
 an insulated  transfer  duct carrying hot combustion gases from  one or more
 secondary  chamber(s) to  one or more waste heat boilers.  At the Pittsfield
 facility,  there are  three incinerator modules feeding into a common tertiary
 chamber.   The combustion products are fed to two waste heat boilers designed
 to operate  with  1400°F boiler inlet temperature.

     Typically,  a  front  end  loader is used to fill the charging hopper  of the
 incinerator.   When  filled, the hopper  door is closed, the fire  door  opened
 and a  hydraulic ram actuated to shove  the MSW into the primary  chamber.  A
 fresh  MSW  charge  is  added at approximately ten minute intervals.   -The  design
 volumetric heat release rate in the primary zone is 10,000 Btu/hr per cubic
 feet.   Water  cooled, hydraulic rams are used  to move  the MSW along the
 stepped series  of  refractory  hearths.   Stoking rams  are  actuated on
 approximately  a  five  minute  cycle.
                                   5-59

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     Oxidizer is added to the primary chamber  in three ways.  Undergrate  gas
can  be  a mixture of recirculated flue gas  (underfire FGR) and fresh air drawn
from the tipping  room.  Visalli  et al.  (1986)  report that in the Pittsfield
facility,  undergrate oxidizer is 100 percent  FGR.  As noted previously,  the
primary chamber is equipped with  a recuperative air passage.  This preheated
air is injected at the head end of the primary  chamber above the burning  bed
of MSW.  The  third source  of oxidizer  to  the primary zone is recirculated
flue gas,  added through high velocity, overfire jets.  The flow rate of each
oxidizer  gas  source  (and distribution) is  controllable.

     Oxygen and gas  temperature monitors  are located within the secondary
chamber  and  provide feedback  to  control primary zone  operation. 'The
temperature  sensor provides a control  signal to modulate the quantity of
overfire  FGR while the 02 monitor is used  to  modulate the quantity of primary
combustion air.   Typically, the temperature in the secondary chamber is
controlled  to  1800°F while the excess air  is  cut to approximately 50 percent.
FGR modulation controls gas temperature by controlling the amount of dilution
but has minimal impact on 03  level.  Auxiliary  burners are provided to assist
in maintaining temperature for particularly wet MSW.  Oxygen concentration is
impacted by  modulation  of  the preheated combustion air since that oxidizer
stream contains  21 percent 02  (versus  4 to 8 percent for the FGR streams).
It  should  be  noted that the excess air  level  maintained by Enercon/Vicon is
significantly  less  than  that provided in typical mass  burn, waterwall
systems.

     Additional recirculated  flue gas  is  added  to the tertiary chamber to
control  the  boiler inert temperature.  This  final  dillution occurs after  the
combustion products  have remained in the secondary chamber for at least  one
second of a temperature of 1800°F.

     As noted  previously, the Pittsfield,  MA  facility is undergoing extensive
performance  evaluation  testing.   Only  limited  data is publicly available.
However, based  on the reportable information,  this technology is capable of
operation  at very low CO  and NOX  levels.   Over the operating temperature
range  (secondary  zone)  of  approximately  1600 - 1800°F, CO emissions were
                                   5-60

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found to be less than 30 ppm.   Even  though  the Enercon/Vicon system does  not
remove heat  from either  the  primary,  secondary or tertiary chambers,  the
system operates at relatively low temperature (due to extensive use of FGR)
and thus  the NOX  emission levels were typically in the 100 ppm range.

     In  addition to  the Pittsfield facility,  Enercon/Vicon  is  providing
several  new MSW  facilities.  A 600 tpd facility  (5 units at 120 tpd each) is
in advanced  shakedown at Pigeon Point,  Delaware.  This facility will  burn a
50/50 mixture  of RDF and  MSW (not necessarily combined).  A 240 tpd  (2 @
120 tpd)  facility is an advanced construction  stage at Rutland, Vermont while
a  360 tpd  (3 @  120 tpd) facility is  under construction at Springfield, Mass.
Finally, a 420  tpd (3 @ 140 tpd) facility is  being built at Wallingford,  Ct.
With the exception of  the  Wallingford  facility, each of the Enercon/Vicon
systems  use  120 tpd modules.  Each  of  these 120 tpd units consists  of  six
hearths  rated  at 20 tpd each.  To increase the  rating to 140 tpd,  a seventh
hearth  has  been  added to the Wallingford facility.

5.14      References
     Basic Environmental Engineering, 1986.   Discussion and data supplied to
     W. R. Seeker  in telephone conversation  with A. J. Matlin on October 16,
     1986.

     Combustion  Engineering,  1986.   Discussion and data supplied to  W. R.
     Seeker and  W. S.  Lanier  by  Combustion  Engineering in Windsor,  CT on
     September 8,  1986.

     Cooper Engineers, 1984.  Air Emissions Tests of Solid Waste Combustion
     in a Rotary  Combustor/Boi 1 er System AT Gallaton, Tenn. Report to West
     County  Agency of  Contra Costa  County Waste Co.  Disposal - Energy
     Recovery  Project.

     DBA, 1986.   Discussion  and  data supplied  by  DBA to W. R. Seeker and
     W. R. Niessen by  Deutsche  Babcock  in  Keyersfeld, Germany on August  1,
     1986.
                                   5-6T

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Detroit Stoker, 1986.   Discussion and data  supplied to W.  R. Seeker and
W. S. Lanier  by  D.  Reschly and T. Gaerer in Monroe, MI on  September 9,
1986.

Martin,  1986.   Discussions  and data supplied by  Martin personnel
(D.  Kreuch, J.  Horn,  H.  Weiand, G. Schetter, J. Martin,  D. Sussman) to
W. R. Seeker and W. R.  Niessen  in Munich, August 6, 1986.

Martin & Schetter, 1986.   "Reduction of Pollutant Emissions from Refuse
Incinerators by Means of Optimized Combustion Conditions."  8th Members'
Conference  of the IFRF, Nordwijkerhaut,  The  Netherlands, May 28-30,
1986.

Riley, 1986.  Discussion  and data supplied  to W. S. Lanier  in telephone
conversation with Riley personnel on August  17,  1986.

Steinmueller, 1986.   Discussion and data supplied  by Steinmueller
personnel   (H.  Pollack,  P. Daimer, 0. Kaiser) to  W.  R. Seeker  and
W. R. Niessen in  Duesseldorf, August  1,  1986.   Also report "Feurung
stechnische Moglictikeiten  zur Schadstoffreduzierung  bei
Mullverbrennungsunlagen",  K. Leikert and H.  Pollack.

Turner,  1982.   "Mass Burning  in Large-Scale Combustors" Thermal
Conversion Systems for  Municipal  Solid Waste, Noyes  Publications,
Park Ridge,  NJ.

Visalli, Joseph  R.  et al., "Pittsfield  Incinerator Research Project;
Status  and Summary of Phase  1 Report;  Plant Characterization  and
Performance Testing," Paper  presented at  12th Biennial  National Waste
Processing  Conference,  ASME, Denver, Colorado, June 1-4, 1986.

Vogg and Steiglitz.    Presented  at  the  5th International  Symposium on
Chlorinated Dioxins and Related  Compounds.   September  16-19, 1985.
Bayreuth, West Germany.
                             5-62

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Volund,  1986.   Discussion and  data  supplied by Volund  personnel  to
W. R. Niessen,  July 30,  1986.   See  also "Emission Test  at  a Danish
Energy  from  Waste Plant."  M.  Rasmussen, Volund Report  No.  32V1-8,
October  1985

Von  Roll,  1986.   Discussion and  data supplied by  Von Roll personnel  (W.
Staub and  A.  Scharsach)  to W.  R.  Seeker and W.  R. Niessen in Zurich,
August  4,  1986.   Also  Von Roll Report  on  the  testing at Neustadt/
Ostohstein, 1984.

W + E  Umwel ttechnik  AG, 1986.   Discussions  and data supplied by  M.
Sudobsky and M. Zweifer of WE Umwel ttechni k, AG  to W. R. Seeker and  W.
R. Niessen  in Zurich, August 5, 1986.

Westinghouse, 1986.  Discussions and  data supplied to W. R. Seeker  and
W. S.  Lanier by Suh Lee,  D.  Bechler  and S. Winston in  Pittsburgh,  PA,
September 10, 1986.
                               5-63

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6.0       CURRENT PRACTICES IN RDF COMBUSTION

     The  chemical  and physical  characteristics of a  fuel  are important
parameters  in  the design  of any combustion  system.  The previous  chapter
described firing systems used for burning  raw  municipal  solid waste  in  water
wall boilers.   Due to the  wide  variability  in MSW characteristics,  those
combustion  systems are significantly different from systems designed  to burn
traditional  solid fuels such as coal,  wood, hogged fuel, etc.  An individual
charge  of MSW  may physically vary from  small  scraps of paper to discarded
refrigerators.  The volatility of an  MSW  charge may have contributions from
discarded propane tanks and bottles of solvents as well as contributions from
rain-soaked yard  waste  and discarded firewall insulation.  The  ash
constituents will  include easily melted items such as aluminum cans  as well
as a liberal  amount of  sand and glass.   The melted aluminum tends to  solidify
on  grate  openings sealing  air passages,  while sand and glass tend  to  erode
the  grate.   Mass-fired MSW systems are specifically designed to accommodate
those variations.  An alternate design  approach is to process the MSW to
produce  a less  difficult fuel.  The degree  of  processing can vary from simple
removal  of  bulky items and  shredding  to extensive processing to produce a
fuel suitable  for co-firing in pulverized coal-fired boilers.  Preprocessed
MSW, regardless of the degree of processing is broadly referred to as  refuse
derived  fuel  or RDF.

6.1       Types of RDF

     The  American  Society for Testing and Materials  (ASTM),  through its
Committee E-38.01 on Resource Recovery Energy  has established classifications
defining  different types of RDF-  The various classifications and resultant
fuel descriptions are listed  in Table 6-1 and  discussed below.

6.1.1     RDF-1 or MSW

     As  noted  in  Table 6-1,  removal  of bulky items such as discarded water
heaters, refrigerators, etc.  produces RDF-1.   Producing RDF-1  is  accomplished
by  hand  removal of the bulky items  on the  tipping floor or  by careful pit
                                    6-1.

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       TABLE  6-1.  ASTM  CLASSIFICATION OF REFUSE  DERIVED FUELS
 Type
of RDF
                Description
RDF-1
(MSW)

RDF-2
(c-RDF)
RDF-3
(f-RDF)
RDF-4
(p-RDF)

RDF-5
(d-RDF)

RDF-6
RDF-7
Municipal  solid waste used as a fuel  in as-discarded form,  without
oversize bulky waste (OBW).

 MSW processed to coarse particle size, with or without ferrous
metal separation, such that 95 wt % passes through a 6-in.-square
separation, such that 95 wt % passes through a 6-in.-square mesh
screen.

Shredded fuel derived from MSW and processed for the removal of
metal, glass, and other entrained inorganics.  The particle size
of this material is such that 95£ by weight (wt %) passes through
a 2-in.-square mesh screen.  Also called "fluff RDF."

Combustible-waste fraction processed into powdered form, 95 wt %
passing through a 10-mesh  (0.035-in.-square) screen.

Combustible waste fraction densified (compressed) into the form of
pellets, slugs, cubettes,  briquettes, or some similar form.

Combustible-waste fraction processed into a liquid fuel (no
standards developed).

Combustible-waste fraction processed into a gaseous fuel (no
standards developed)".
                                      6-2

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management by the crane operator.   This  type  of  fuel retains the majority  of
fuel variability characteristics found in  raw municipal waste.  Clearly, the
combustion  system requirements to burn RDF-1  are  the same as for raw MSW.

6.1.2    RDF-2 or Coarse RDF (C-RDF)

     The second type of  refuse derived  fuel  described  in Table 6-1  is
referred to as RDF-2, "Coarse RDF," or c-RDF-  A primary shredder is used  to
reduce  the  "particle" size such that  95 weight percent passes through a 6
inch square mesh screen.  Flail  mills, hammer mills or rotary cutters may  be
used for this  primary shredding operation.  Preparation of coarse RDF may
also include  removal of  ferrous metals.   Several types of magnetic devices
have been  developed for that purpose.  Since the extracted metals generally
contain  loose  paper or other materials,  an air separation device is  often
used to clean the ferrous metal.

     The decision  on including  ferrous  metal  separation in the waste
preprocessing  is largely  an economic  issue.   Metal removal  generates  a
potentially  saleable by-product  and  also  reduces the mass  of material
processed in the municipal  waste combustor.   In the Albany, N.Y., Solid-Waste
Energy-Recovery System (ANSWERS) waste is  processed in one facility and then
transported by truck to the boiler facility.   In this instance ferrous  metal
separation  also  reduces costs associated  with fuel transportation.  Another
important consideration is that ferrous metals can also be separated from the
boiler's bottom  ash.  That is a practice  employed in several MSW systems
(e.g.,  the  Signal Resco facility at Baltimore, MD).  Ferrous metals recovered
from the ash  will  have first passed through  the boiler leaving a cleaner
product for  sale".

     The ANSWERS operation  described in  Figures 6-1 and 6-2 may be used  to
illustrate  two  important  RDF considerations.  First, by preprocessing the
waste  it may be economical  to store and  transport the RDF.  Though long-term
storage  and/or long distance transport would  generally be restricted to more
highly  processed forms of RDF,  these factors are an important consideration
in developing RDF processing technology.  The second important consideration
                                    6-3

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en
i,
              Vlbrellng-
              Pan Feeder*

            Picking  ^
            Olatlona •


Magnetic       Refuee
Separators     8*iredder»
                                                               Tipping
                                                               Room
                                                                                            Untreated Refute
                                                                                            Cor proceeding)
                            Stationary
                            Compactor,    %£„.„
                                                                       Ferroua-Melal Removal
                                                                       (for recycling)
                           Fu«l Product
                           ((or delivery to
                           generating plant)
                           Figure 6-1.   Albany,  New York,  Solid-Waste  Energy-Recovery System
                                           (ANSWERS).

-------
01
, f
in
                          Figure 6-2.   Cross-section of Albany, New York boiler plant
                                       firing c-RDF.

-------
was that the  coarse  RDF  produced at  the  ANSWERS processing facility
(Figure 6-1) was  to be burned  in a boiler  (Figure 6-2)  equipped with a
spreader stoker  and traveling  grate  firing system.  The two RDF  boilers at
ANSWERS can  fire  100 percent RDF.   The  firing system design, however, is an
adaptation of  hardware designs  developed  for coal- and wood-firing  systems.
Thus,  preprocessing of the waste  to produce c-RDF allowed the use of  existing
American boiler technologies as  an extension of experience with  a  variety of
industrial waste fuel and wood residues.

     Initial  operation  of the  ANSWERS  facility  produced RDF with a much
smaller particle size distribution than anticipated  in the boiler design.  As
a result, a  significant portion of the  fuel was entrained into  the  gas flow
instead of falling to the grate.   Hasselriis (1983)  reports that  "too much of
the RDF carried  up and out of the furnace" and that the incompletely burned
material  collected in the boiler hopper as  char.  Further, bridging tended to
occur  in the hopper.  Subsequently, the  shredder grates have been  changed at
the processing facility to produce a larger mean size RDF.  Using  the larger
sized  RDF, excessive carry-over can be  avoided if the boiler firing rate is
held to less  than 80 percent of rated capacity-  Other factors such as boiler
fuel  feed design,  boiler  height and volume, overfire air distribution and
grate  heat release rate can help compensate for this problem.

     The above described initial  experience at  ANSWERS illustrates that
significant  operational  problems can  occur  if  c-RDF,  to  be  burned  in a
spreader stoker firing  system, has too high  percentage  of  small  sized
particles.   It should also be  recognized,  however, that  the normal size
distribution  for c-RDF (95 weight percent passes 6  inch square screen) allows
relatively large items to  enter the firing  system.  Note that  95 percent
smaller  than any given mesh size implies the possibility of 5 percent larger
than  that size.   Accordingly,  it may be  very difficult to actually achieve
uniform  fuel  feed rate in  boilers firing coarse  RDF.  One approach to this
problem is a  higher degree of waste pre-processing.
                                    6-6

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6.1.3     RDF-3 or Fluff RDF (f-RDF)

    Fluff  RDF or RDF-3  production  involves removal  of ferrous and non-
ferrous metals,  removal  of glass  and other entrained inorganics,  as well  as
size reduction  such that  95  weight  percent of the mass will  pass through a
2 inch square mesh  screen.  This  type of fuel is also referred  to as f-RDF.
Figure 6-3 illustrates the  RDF  production  facility at Ames,  Iowa  which
generates RDF-3.   Size reduction  is  accomplished with primary and secondary
stage  shredding.   Flow to  the  secondary shredder occurs after magnetic
separation of ferrous metals and  size  segregation in disk screens.  Underflow
from the first  disk screen  (less than  1.5 inch) is fed to  a  second screen.
Underflow from the second screening (less than 0.375 inch) consists primarily
of fine glass,  grit and finer  fibers which are disposed of  in a landfill.
The overflow from secondary screening  is  combined with the secondary shredder
output  and fed to  an air  classifier.  The  heavy fraction from  the air
classifier is discarded to landfill  (after a secondary magnetic separation)
while the lighter  fraction is pneumatically transported 600 feet  to a large
storage bin next to the Ames Municipal  Electric Co.

     In  the  above described Ames  system  the RDF yield is  approximately 70
percent of the  raw  MSW input and the resultant  fuel has an  ash  content
typically less  than 10 percent.   The fluff RDF product was  originally co-
fired with pulverized coal  in  a  modified Combustion Engineering  tangential
boiler.  The main  boiler modification was addition of a bottom  burning dump
grate to improve  burnout of bottom ash.  Current operation  co-fires the RDF
with pulverized  coal  (PC) in a B&W  boiler specifically designed for f-RDF co-
firing.  Note that  the CE  unit  was  originally designed to fire  pulverized
coal rather than the PC/RDF mixtures.

    Figure  6-4 illustrates a slightly different  procedure for producing
fluff-RDF.  This  procedure  uses  trommel screens as opposed to  disk screens
for size segregation of the  shredded material.   General  operation of the
trommel screen is illustrated in  Figure  6-5 and shows that the  single trommel
should accomplish  the same task as the  scalpering disk screen and fine disk
screen combination  employed  at  Ames.  The  RDF production design  shown in
                                    6-7

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Raw  Inleed
Conveyor .
                                           Primary
                                          /Shredder
                                                Magnetic
                                              / Separation
                        Vibrating
                        Pan
                                Scalping
                                Disk  Screen
 I
00
                                                                            Separation-
                                                                            Zone Air -  ""
                                                                            Classifier
                                                                   Fine  Glass, Gril, & Finer Fiber
                                                                                                 Magnetic
                                                                                                 Separation
                                                                        / Heavy Fraction
                                                                       Q	^ to Landfill
                                                              Iron Fraction to Storage
                               Figure 6-3.   Ames,  Iowa,  Resource Recovery System for  production
                                              of  fluff RDF  (f-RDF).

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                                    Ferrous Metal
 Municipal
Solid Waste
 Receiving
                                          Residue
                                                                      Fuel Storage
   Figure  6-4.   Fluff RDF  production  system.  Illustration is for  system
                 developed  by Combustion  Engineering.

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                                 Municipal
                                 Solid
                                 Waste with
                                 Ferrous Metal
                                 Removed
en
J
                                               Residue

                                            Size: Under 1/2"
                                            Non-Combustibles
                  Heavies
             Sire: 1/2" to 2"-4"
               Combustibles &
              Non-Combustibles
 Oversize

Combustibles
Landfill         Processing

                  *    *
              Landfill    Fuel
                       Storage
                                                                                  Secondary
                                                                                  Shredding


                                                                                 Fuel Storage
                               Figure  6-5.   Trommel  Screen for RDF  size segregation.

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Figure 6-4 is being employed in  four  new facilities with scheduled operation
to begin in  1987,  1988  and two  in 1989.   For each of those  planned
installations the fluff-RDF will  be the primary fuel and will be burned in a
spreader  stoker, travelling grate  boiler system.  That combustion system will
be discussed in Section 6.3.2.

6.1.4     RDF-4 or Powdered RDF (p-RDF)

     The  best known  example of  p-RDF is ECO-FUEL™; however, it is  reported
that Combustion  Equipment  Associates, the ECO-Fuel producer, is no longer
in business.   As defined  in Table  6-1, p-RDF involves processing the waste
into a powder form with 95 weight percent passing through a  10-mesh screen.
The mechanical requirements for  producing such small size RDF are similar to
those  for producing fluff-RDF but include  a  milling step after  air
classification.

     Milling operation will generally  require predrying of the waste.  In the
ECO-Fuel  process hot exhaust gas  from  a  process heater is substituted for air
in the "air classifier"  converting  that device to a dryer as well.  Drying
continues during the ball milling process.  The resultant fuel is designed to
have a moisture content on the order  of 2 to 3 percent and an ash content on
the order of 10 to 12 percent. The higher heating value of RDF-4 is  expected
to be 7500 Btu/lb as compared  to approximately 5800 to 6000 for fluff RDF.
The use  of p-RDF would be for co-firing in pulverized- or cyclone-fired coal
fired utility  boilers.

6.1.5     RDF-5 or Densified RDF  (d-RDF)

     There have  been a  number  of demonstration programs which  produced
pelletized or briquetted  RDF for use in spreader stoker boilers  with
travelling or vibrating grates.   As  noted by Hasselriis (1983), these short
test burns have  shown that  d-RDF can be successfully burned, with little
effect on the boiler performance.   The main impact observed was  a drop in
boiler efficiency due to  the higher moisture content of d-RDF relative to
coal.  The main  difficulty  with d-RDF is  the  cost of fuel production and
                                   6-f1

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transportation  relative to  coal.   At the  current time there are no  known
boiler  systems continuously operating on  d-RDF.

6.2       Current RDF Projects

     As of October,  1986 there were  31 active  resource recovery projects
which are sufficiently advanced to  have  an estimated construction  completion
date (Mcllvane 1986).  Table 6-2 provides  a listing of those active projects,
noting the  location, the  projected operational  date, facility size and  the
combustion  system supplier.  As noted  in  this table, three of these projects
will use  fluidized bed  combustor to burn  the RDF.  It is also  noted that
these  new RDF  projects  tend to be very  large scale operations.  With  the
exception of  the Franklin Project, each  project  is larger than 500 tons  per
day.  The  Detroit Project is planned for  4000 tons per day.

6.3       Firing  Systems in Current RDF Projects

     The majority  of  the current  RDF projects will utilize boiler systems
from Combustion Engineering or from Babcock  and Wilcox.   Each of  these
manufacturers  is involved  with 5 new  RDF  facilities.  Combined, these  two
manufacturers  represent 81.4 percent of the active RDF projects  (tonne  per
day basis).   For the projects using B&W boilers "at least three (San Marcos,
Palm Beach  and Biddeford) will employ  combustion  systems supplied  by Detroit
Stoker.   The  CE facilities  will  use firing systems designed by CE.  The
following  sections describe those firing  systems.  As will be shown there  are
major differences in the two firing system design  philosophies.

6.3.1     Detroit Stoker RDF Firing Systems

     Detroit  Stoker company has  manufactured hardware  for burning non-
traditional  fuels  such  as  wood,  sawdust, bagasse, etc. since early 1940's.
Firing systems  developed  for burning RDF result in semi-suspension burning
where  the fuel  is  injected  through wall ports  into the furnace.  The fuel
partially burns in the suspension phase  with larger material falling to  the
grate  for burnout on the fuel bed.   Figure 6-6  shows the Detroit Stoker air-
                                   6-12

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                 TABLE 6-2.   ACTIVE* RDF PROJECTS
Project Name and
Location
Bay Area Resource Recovery
Redwood City, California
San Marcos Resource Recovery
San Marcos (Oceanside). CA
Mid Connecticut Hartford Project
Harford, Connecticut
Wilmington-New Castle Project
Wilmington, Delaware
Collier County Resource Recovery
Naples, Florida

Palm Beach Solid Waste Authority
Palm Beach County, Florida
Honolulu Resource Recovery (H-Power)
Honolulu, Hawaii
Biddeford Resource Recovery
Biddeford, Maine
Detroit Resource Recovery
Detroit, Michigan
Mankato Project 1 and 2
Mankato, Minnesota
Redwing Resource Recovery 1 and 2
Red Wing, Minnesota
Penobscot Resource Recovery
Bangor, Orrington, Maine
Projected
Facility Startup
1990
1989
1987

1986
1989

1989
1989
1987
1989
1987
1987
1988
Size and
Boiler Mfgr-
3600 TPD
C-E(D
800-1600 TOP
2000 TPD
C-E
720 TPD(3)
Vicon/ENERCON
600-900 TPD
Westinghouse/
Goetaverkin in
circulating
fluid bed.
3000 TPD
B&W
1800-2000 TPD
C-E
600 TPD
B&W
3300 TPD
C-E
2@470 TPD each
B&W
20470 TPD each
B&W
721 TPD
KTI
Active implies that project  has  progressed  to the  point  of  having
projected facility  start-up  data.  It does not imply that construction
has  necessarily begun.
                               6-13

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             TABLE  6-2.  ACTIVE  RDF PROJECTS  (CONTINUED)
Project Name and
    Location
   Projected
Facility Startup
  Size and
Boiler Mfgr.
Erie Resource Recovery
Erie,  Pennsylvania
Lubbock Resource Recovery
Lubbock, Texas
Petersburg Resource Recovery
Petersburg, Virginia

Southeastern Tidewater Energy Project
Portsmouth, Virginia

Franklin Project
Franklin, Ohio
      1987
      1987



      1986


      1987


      1987
600 TPD
Uestinghouse/
Goetaverken in
circulating
fluid bed.

500 TPD
Westinghouse/
O'Connor

650 TPD
2000 TPD
C-E

150 TPD
110 TPD RDF to
be burned in
fluid bed
combustor
  '  Combustion Engineering
(2)  Babcock  and Wilcox

(3)  Combined MSW/RDF system

-------
CTl


tfl
                     Figure 6-6.
Side sectional view of Detroit Rotograte Stoker equipped
with Detroit air swept refuse distributor spouts.

-------
swept fuel distributor system  coupled with a travelling grate  (Detroit
Rotograte Stoker).   Figure 6-7 shows the same RDF  injection system coupled
with  the Detroit Hydrograte  system.

     The  traveling grate  illustrated  in  Figure  6-6  is based on the design
developed for  stoker coal  firing.  The grate forms  a  continuous loop driven
by sprockets at the  front and  rear of the boiler.  As  the grate travels  from
rear-to-front  the  ash layer  thickness increases.   Ash is dumped from the
grate at  the  front  end of  the boiler.  The Detroit  Hydrograte™ illustrated
in Figure 6-7  has  an inclined, water cooled grate which  is intermittently
vibrated  to  shift  the fuel bed (and ash) toward  the  discharge at the front
end of the grate.  This grate  design is still under  development for use  with
RDF systems  but  has  not be.en incorporated into actual operating systems.
With  both grate systems, there  is  a single plenum  for underfire  air.
Openings  in  the  grates are designed  to provide a nearly uniform spatial
distribution of underfire air.

     A critical component  of  the design approach  is that the fuel  injection
hardware  can provide  a thin bed  of RDF which is uniformly distributed on the
grate.  The  ash  layer thickness will increase from  the rear to the front of
the boiler but the burning  RDF layer thickness will  be essentially constant.
If that goal  is  achieved,  spatially  uniform underfire air addition would
result  in spatially uniform heat  release and excess  air levels  in  the
furnace.   To accomplish that objective,  Detroit Stoker  has developed a system
comprised of  the  "Detroit  Refuse  Feeder" and the  Detroit air swept, rotor
distributor  spout.   The Detroit air  swept, rotary  distributor  spout is
illustrated  in Figure 6-8. A stream of air impinges  on the RDF as it falls
from  the  refuse  feeder, injecting  the fuel into  the  boiler.  A rotating
damper is used to  modulate the flow  of distributor air-  With the damper
fully open,  RDF  is  blown  toward the  rear of the  grate.   With the damper
closed, RDF tends to fall  near  the front of the grate.

     A significant portion  of the RDF will burn in  the  suspension phase.   The
suspension burning  and fuel  injection scheme allows the  RDF to impact the
lower furnace walls.   To prevent slagging the lower  furnace walls in boilers
                                   6-16

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Figure  6-7.   Detroit  Stoker Hydrograte   with water cooled,  vibrating,
             continuous-discharge ash-discharge grates.
                                   6-T7

-------
                   Fuel
                   Outlet
                                              Inlet
                        Rotating
                        Damper
                                 Fuel Feed
Figure  6-8.
Detroit  air swept  refuse fuel  distributor  spout
arranged  with motorized rotary air damper.
                                 6-1 a

-------
designed for  RDF  firing are  bare-tubed, not refractory clad  as  in mass-burn
systems.

     As  shown in Figures  6-6 and 6-7,  Detroit Stoker incorporates several
elevations of  overfire air  ports  on both the  front and  rear  walls of the
boiler.  One  elevation of overfire air ports  is  provided below the fuel
injector level.   Unlike the  systems for MSW  firing the  furnace walls are
straight.  Thus, the overfire air must penetrate across the entire plan view
dimension of the furnace.   Design of the overfire air jet system has evolved
over the years.  Detroit Stoker  is currently considering development of flow
modeling  capabilities to optimize their designs.

6.3.2    Combustion Engineering  RDF  Firing System

     The  RDF firing  system employed by CE  is based  on a spreader stoker/
horizontal travelling  grate  design.  Figure  6-9  illustrates  the overall
firing system  configuration  while  Figure 6-10 illustrates  RDF distributor
hardware.  As  shown, RDF  is  injected into the furnace by impinging a high
pressure air  jet  on  the  fuel as it falls  from the  feed  shoot.   The
distribution  air nozzle is adjustable which provides control over the spatial
distribution  of RDF on the grate.

     The  grate itself  travels from  the  rear to the front of  the boiler with
ash dumping  at the front  of  the  boiler.  As  shown in Figure 6-9, the CE
design incorporates multiple undergrate air  compartments  with a siftings
screw conveyor in  each compartment.  The air  flow to each undergrate air
plenum is individually controllable.   Thus, the air flow distribution through
the grate can be adjusted to match the RDF flow pattern developed by the fuel
feed system.

     Overfire air addition  is accomplished through a tangential  entry system
characteristic of CE utility  boilers.  The tangential overfire  air ports are
located  well  above the fuel  injection elevation.  The design  is obviously
influenced by  CE's utility  boiler  design philosophy, and by  CB experience
with burning  other waste fuel  on  traveling grates but has also been optimized
                                   6-T9

-------
RDF
Distributors
Grate Surface
Drive Shan
Undergrate
Air Compartment
                                                                              Tangential
                                                                              Overfire Air
                                                                                   Water
                                                                                   Seal
                                                                               Idler Shaft
                                                                            Sifting Screw
                                                                            Conveyor
     Figure 6-9.   Combustion Engineering continuous  ash  discharge type
                    RC Stoker for  RDF.
                                         6-20

-------
                            Fuel  Flow
 Adjustable  Nozzle
 5% Total  Combustion
 Air  at  30" w.g.  Pressure
                                             Wear  Liner
                          Adjustable  Retention
                          Plate
Figure 6-10.  Combustion Engineering pneumatic RDF distributor.
                          6-2T

-------
through extensive cold flow modeling studies.  The first of the CE-designed
RDF  facilities is  scheduled for completion  in  1987.
6.4       References
     Hasselriis,  F.   "Burning Refuse-Derived  Fuels  in Boilers".  Part IV of
     Thermal Conversion Systems  for Municipal  Solid Waste, H. L. Hickman,
     Noyes  Publication, N. J. 1983.

     Mcllvane Company.   "Waste  Burning Projects  and People",  The Mcllvane
     Company,  Northbrook, Illinois, October 1986.
                                   6-22

-------
7.0       CURRENT PRACTICE  IN STARVED AIR (TWO-STAGE) COMBUSTORS

    The  literature on municipal  waste combustion  tends to use  the terms
"small   system,"  "modular  combustor"  and  "starved-air combustor"
interchangeably.   In  the  current study a  distinction is drawn between one-
and  two-stage combustion systems regardless of system size or modular nature
of the manufacture.   Single-stage  systems which operate under excess air
condition through  the entire combustor were discussed in Chapters  5 and 6.
The current  chapter  discusses two-stage systems where  the first stage
operates  under fuel-rich  (starved-air) conditions.

7.1       Starved Air Technologies

    The  term  "modular  combustor" implies that the  unit is constructed at the
manufacturer's facility and shipped as a module(s) to the installation site.
Initial   development of this type of MSW system came as an advancement to the
small  batch-fed municipal waste combustors used to  burn waste from hospitals,
stores,   restaurants,  etc.  The first  facility to combine a capability for
continuous operation  and  energy recovery  was installed in 1976 by  Consumat
Systems, Inc. in  North  Little Rock,  Arkansas.   Table 7-1 is taken from a
report by Hopper (1983) (with updates) providing a  select list of starved-air
systems  in operation  in  the United  States.  As  illustrated by the data in
this  table,  Consumat  is the dominant system vendor for starved-air  systems.
Other significant  suppliers included  in  this table are Synergy/Clear Air,
Environmental Control Products (ECP),  and Scientific Energy Engineering
(SEE).   The  Hopper  report  was  published  in  1983  and  lists sixteen
manufacturers of  mass-burning starved air  systems.  Figure 7-1 was  provided
in  the  Hopper" report  to describe  the  evolution of those U.S. vendor
companies.  A cursory examination indicates  that in 1986 the proliferation of
companies  listed  in  Figure 7-1 have  begun  to  contract.   The Mcllvaine
Company's Market  Research report on waste  burning projects (Mcllvane, 1986)
indicates  that there are eleven active  projects (in planning  or under
construction) involving starved air system greater  than 40 TPD.  Six of those
systems  are  Consumat  projects.  Clear Air, Inc. and Ecolair, Inc.  each have
two  active projects while Synergy is involved in one project.
                                   7-T

-------
                TABLE  7-1.   SELECTED DATA ON  SMALL-SCALE  U.S.  SYSTEMS USING THE STARVED-AIR DESIGN
Location
Si loam Springs, Ark.a
Blytheville, Ark.a
Groveton, N.H.
North Little Rock, Ark.
Salem, Va
Jacksonville, Fla.c
Osceola, Ark.
Genesee, Mich.
Durham, N.H.

Auburn, Maine
Dyersburg, Tenn.
Windham, Conn.
Crossville, Tenn.
Cassia County, Idaho
Batesville, Ark.

Park County, Mont.
Waxahachie, Texas^

Miami Airport, Fla.
Portsmouth, N.H.
Red Wing, Minn.
Cattaraugua County, N.Y.d
Miami , Okla.
Oswego County, N.Y.
Pasagoula, Miss.
Oneida County, N.Y.
Tuscaloosa, Ala.
Hampton County, S.C.
Carthage, Texas
Center, Texas
Barron County, Wisconsin

No. of
Modules
2
(2)
2
4
4
1
2
2
3

4
2
4
2
2
(2)

2
2

2
4
2
3
3
4
3
4
(4)
(3)
1
1
2

Capacity '
( ton/day) ,
Each
Module
10.5
(36)
12
25
25
48
25
50
36

50
50
25
30
25
50

36
25

30
50
36
37.5
36
50
50
50
(75)
75
40
40
40

Date of
Startup,
Past or
Projected
6/75
8/75
Unknown
8/77
9/78
1978
1/80
2/80
9/80

4/81
8/81
8/81
12/81
1982
1982

1982
1982

1982
1982
1983
1983
1983
1983
1983-84
1983-84
1984
11/85
2/86
10/86
10/86

Capital Cost
$106)
Unknown
Unknown
Unknown
1.45
1.9
Unknown
1.1
2
3.3

3.97
2
4.125
1.11
1.5
1.2

2.321
2.1

Unknown
6.25
Unknown
5.6
3.14
Unknown
6+
Unknown
13
Unknown
Unknown
Unknown
Unknown

System Vendor
Consumat
Consumat
ECP
Consumat
Consumat
SEE
Consumat
Consumat
Consumat

Consumat
Consumat
Consumat
Env. Services Corp.
Consumat
Consumat

Consumat
Synergy/Clear Air0*

Synergy/Clear Air^
Consumat
Consumat
Synergy/Clear Air**
Consumat
Consumat
Unknown
Unknown
Consumat
Consumat
Consumat
Consumat
Consumat

Energy Market
Allen Canning Co.
Chrome Plating Co.
Groveton Paper Mill
Koppers
Mohawk Rubber
Unknown
Crompton Mill sa
Unknown
University of New
Hampshi re
Pioneer Plastics
Colonia Rubber
Kendall Co.
Crossville Rubber
J. R. Simplot
General Tire &
Rubber
Yellowstone Park
International
Aluminum Co.
Miami Airport
Pease Air Force Base
S.B. Foote Tanning
Cuba Cheese
B.F. Goodrich
Armstrong Cork
Unknown
Griffis AFB
B.F Goodrich
Westinghouse
Tyson Foods
Holly Farms
Twin Tower Cheese Cn.
Northern States Power
ro
                 a  System now shut down and equipment removed.



                 c  System now shut down.



                 d  Synergy and Clear-Air are now separate companies,  each marketing its own system



                0  Updates as reported by Consumat.

-------
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Figure 7-1.  Evolution of U.S. vendor companies supplying starved-air
             waste-to-energy systems, including persons notably
             influencing system designs.

-------
    As part of the current study interviews were conducted with  Consumat and
Synergy.   The  Consumat  interview was conducted at their manufacturing
facilities in Richmond, Virginia.  Synergy was interviewed by telephone.  The
interview format and topics  of  discussion were similar to those  used  in the
excess air mass burn technology evaluation.

    Both Consumat  and  Synergy  are small manufacturing firms  without
extensive research capability.   Recent developments have primarily  involved
improvements to the mechanical and operational aspects of their  product line.
Both of these  companies build the  starved-air systems in a modular  fashion.
Standard designs have been  developed  for various size requirements and are
offered as multiple modules to meet  the  demands of  a particular  project.

7.2      Consumat Systems

    Consumat Systems,  Inc.  has developed  a wide  range of standardized,
starved air, municipal  waste combustor designs with continuous  ratings from
8.6 to 100 tons of municipal  solid waste per day-   Figure 7-2 illustrates the
standard Consumat module.  A front-end  loader is used to place a  batch  charge
of MSW into the automatic loader.  A hydraulic ram  and charging  gate  assembly
injects the MSW into the .lower chamber.  This lower chamber can  be thought of
as the first stage in the two-stage system design.  The fuel  is  slowly moved
from the front  to rear of the first stage by a series of hydraulic  transfer
rams which are shown in the photograph  in Figure 7-3.  Holes in  the center of
the transfer rams are used  to  provide a controlled quantity  of air  to the
primary  chamber.   Under normal  operating  conditions  it will  require
approximately 12 hours for the solid waste charge to traverse from the  first-
stage entry to ash dump at the end of the chamber.

    The  quantity  of  air introduced  to the first-stage defines  the rate at
which  the mass  burns as  well  as the quantity of gaseous effluent  from the
first  stage.   Figure 7-4 presents results  from a theoretical  calculation
showing the variation in adiabatic flame temperature with percent theoretical
air.  Note that  peak temperatures occur when  the fuel/air  ratio is near
stoichiometric  conditions.   Under excess air conditions  (more than
                                    7-4

-------
              ©       ©      ©    ©       ©
The above cutaway view of the stan-
dard CONSUMAT- energy-from-waste
module shows how material and hot
gas flows are controlled to provide
steam from solid waste A front end
loader (1) pushes the waste to the
automatic loader (2). The loader then
automatically injects the waste into
the gas production chamber (3) where
transfer rams [4) move the material
slowly through the system. The high
temperature environment in the gas
production chamber is provided with
a controlled quantity of air so that
gases from the process are not burned
in this chamber but fed to the upper
or pollution control chamber (5). Here
the gases are mixed with air and con-
trolled to maintain a proper air fuel
ratio and temperature for entrance into
the heat exchanger (6) where steam
is produced A steam separator (7) is
provided to ensure high quality steam.
In normal operation gases are dis-
charged through the energy stack (8)
When steam is not required or in the
event of a power failure, hot gases are
vented through the dump stack (9).
The inert material from the combus-
tion process is ejected from the ma-
chine in the form of ash  into the wet
sump (10) and conveyed (11) into a
closed bottom container (12) which
can then be hauled to the landfill for
final disposal.
         Figure  7-2.   The standard Consumat module  for  energy-from-waste,
                                                  7-5

-------
Figure 7-3.  Internal transfer rams in primary
             chamber of typical Consumat facility.
                      7-6-

-------
 4000
UJ
Q

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LL)


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LL

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LLJ
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o
 3000 -
2000 -
 1000 -
                                    ,0% Moislure

                                            15%
         % Excess Oxygen:      0
                                                      10
                                                                12     14
                50    % SA   100
                                            150
200
250
                         300
       % Excess Air:
                                            50
100
150
250
Figure  7-4.
                    Theoretical  temperature  of the products of combustion,
                    calculated from typical  MSW properties, as a function
                    of refuse  moisture and excess air or  oxygen
                    (Hasselriis,  1986).

-------
100 percent  of  the theoretical  air requirements) gas temperatures fall  with
increasing air addition due to dilution  effects.  Temperatures also falls  as
the air flow is  reduced  below stoichiometric conditions  since the  full
heating value  of the fuel  is not released.   In the Consumat System design,
first-stage  air  flow is  substoichi ometric  and is controlled to maintain a
first-stage  exhaust gas temperature set  point.  The temperature set point  is
typically  in the 1200 to  1400°F range which corresponds  to first-stage
operation at  approximately 40 percent theoretical air.

     The first stage essentially functions  as  a gasifier producing a hot  fuel
which is transferred to  the  second stage.   In the second stage, additional
air is added through a  series of wall jets.  These jets  are oriented  in
opposed pairs in  at least three axial  locations in the secondary chamber.  It
is important to  note that  the temperature of the fuel-rich gas leaving the
first  stage (1200-1400°F)  is above the  autoignition  point for the gas
composition.   Thus, completing the burnout  process is a matter of getting air
to the  first-stage effluent.

     There  is no  heat extraction from either the first or  second stage of the
combustor.   Heat recovery does not occur  until a location downstream of th.e
secondary chamber.  The quantity of air added  to the second stage is adjusted
to maintain a given combustor exit gas temperature.  The temperature level  is
typically  in the 1800°F  to  2200°F range.  In the absence of waterwall  heat
extraction,  this is equivalent to between 80 and 150 percent excess air  at
the  second  stage  exit.   Thus,  approximately 80 percent of the total
combustion  air is introduced as secondary air.

     As  a  result of state and local  regulatory requirements relative to  PCDD
and PCDF emissions, Consumat has recently  increased the physical size of the
secondary  combustion chamber.  The length and diameter  of the chamber  have
been adjusted to  provide a minimum of one second residence time  downstream  of
final air  addition at a temperature above  1800°F.  In addition, an auxiliary
burner  is provided in both the primary and  secondary chamber.  Control of the
secondary  chamber auxiliary  burner  is used to assure  that the  time-at-
temperature requirement is maintained.
                                    7-8

-------
    The  control  system incorporated  into  the Consumat System design is now
automated.   The main parameter being  controlled is the first-stage exit gas
temperature.   Three system  parameters  are  used to hold  that temperature
constant.  These include,  in  order of  priority, the primary zone air flow
rate,  the frequency of fuel  loading and,  finally, a water quench is available
if the temperature should reach excessive levels.  As noted earlier,  the gas
temperature  at the second  stage exit  is  used  to control  the quantity of
secondary air  addition  and  the operation  of  the secondary zone auxiliary
burner.

    With the exception of furnace exit gas  temperature control, operation of
the Consumat  combustion components is decoupled from heat recovery operation.
The overall  system is designed to operate  at full load.  If for some reason
steam  production  is not required, or if a power failure should occur,  exhaust
gases from the  secondary chamber are vented  through a dump stack upstream of
the boiler.   In some cases, Consumat  utilizes either a steam condenser or a
steam  vent for  excess steam rather than by-passing  the flue gas.

    Available pollutant  emission data from  Consumat  Systems indicates
performance  commensurate  with large,  modern  waterwall  MSW grate  systems
(Consumat 1986).  Consumat Systems achieve  total particulate emission levels
of 0.08  grains per dry  standard cubic  foot (corrected to 12 percent C02)
without  the  use  of any APCD.  The PCDD and  PCDF emissions have been measured
from  three different Consumat systems  and  the  average 2,3,7,8 TCDD toxic
equivalent emission rate (EPA Method)  from  these three facilities is  8.0 ng/
Nm3 (see the data base  volume entitled "Emission Data  Base for Municipal
Waste Combustors."  This compares with an emission average of 6.0 ng/Nm3 for
modern mass  burn/water  wall  systems.   Typical  CO levels  (corrected to 12
percent  C02)  are  found to be in the 20 to 50 ppm range.

    The probable reason for  the low particulate emission rate is low gas
velocities which occur in the first stage.   As noted earlier, only about 20
percent  of the  total combustion air is  added in the primary zone.  Due to the
large diameter  of this chamber, the resultant gas  velocity is not sufficient
to entrain particles from the bed for transport to  the secondary zone.
                                    7-9 -

-------
7.3       Synergy

     Information on the two-stage combustion system being  offered by Synergy
Systems,  Corporation was obtained through a  telephone interview  with
Mr.  William McMillen and Mr.  George Hotti (Synergy,  1986).  Figure  7-5
illustrates the  Synergy two-stage  combustion system which has an external
appearance similar  to the  Consumat system.   There  are, however,  many
differences in  the design and  control of the  Synergy and Consumat Systems.
One of the obvious differences is that the Synergy system  is equipped with a
reciprocating  grate instead  of transfer rams.   The grate was designed by
Mr. George Hotti  based on his  extensive prior experience with Von Roll in
Switzerland.  Air is provided  to the  primary zone through slots between grate
rows and air holes at the end  of grate  plates.  The distribution of underfire
(or primary) air  is achieved through a series of five undergrate plenums,
each of which is individually  adjustable.

     The  amount of  air flow  added  in the  primary zone  was reported to be
approximately 20 percent of the  total air.  The primary  zone temperature was
estimated to be  on the order of  1600 to 1700°F.   The primary zone grate
system provides  an extremely  long (8 to 12 hour) solid  residence time.   The
bed thickness  at  the end of  the  grate  was estimated to  be on the order of
18 inches to 2 feet.

     The  primary (and secondary) zone  is refractory-lined  and is designed to
minimize  wall  heat loss.   Effluent from the  primary zone flows  into a
cylindrical secondary zone with a  tangential  entry.  The secondary zone is
separated into two regions by  a refractory ring.  Secondary air is added in a
co-flowing  (tangential) direction  in the first portion of the secondary
chamber and in  the refractory  ring.  Additional  secondary air is added
through radial air inlets in the downstream half of the secondary stage.

     Auxiliary burners are provided in  both the primary and secondary portion
of the municipal waste combustor. The  primary zone burner  is used to preheat
that chamber during system start-up.  The auxiliary burner in the secondary
                                   7-10..

-------
                                           SECONDARY AIR
                                      !        I       I
SECONDARY BURNER
                                                 PRIMARY AIR
             Figure 7-5.   Synergy two-stage combustion  process.
                                      7'1

-------
chamber is  used  for preheating  as well but its main function  is  to assure
that  furnace exit gas temperature  is maintained at the required level.

     Synergy has recently developed  a  control  scheme  based on continuous
measurement of  the gas flow from the  primary zone exit.   These  measurements
will  be  used to control  primary zone air  flow rate.

     Synergy feels that it is important  to control  both the  stoichiometry in
the primary zone  and to  preclude temperature peaks in  the secondary zone.
The  control  scheme they  have developed is  designed to  accomplish  that
objective.

     Figure  7-6 illustrates a.Synergy two-stage combustion system with energy
recovery and  shows that  no  heat is extracted until the waste  heat boiler.
The size of the secondary chamber has  been set to provide a  full two seconds
residence time  above 1800°F  (after final air  addition).    This expensive
design  change has been  made to accommodate anticipated emission  regulations.
The municipal  waste combustion facility  currently being installed at Perham,
Minnesota will  be  the  first  Synergy system incorporating  all  of the above
features.   Accordingly,  there is not yet any operational  data on CO,  NOX,
trace metals PCDDs or PCDFs from this type system.

7.4      References

     Consumat, 1986.   Discussions  and  Data presented by Consumat personnel
     (C. Ziegler,  C. Stout, W.  Wiley)  to  VI.  S. Lanier  and  W. R. Niessen,
     October 14, 1986.

     Hasselriis, F.  "Minimizing Trace  Organic Emissions from Combustion of
     Municipal Solid Waste by the  Use  of  Carbon Monoxide  Monitors."  1986
     National Waste Processing Conference, ASME Proceedings,  p.  129, 1986.

     Hopper, R.   "Small  Scale Systems".   Part III  in  Thermal Conversion
     Systems  for  Municipal  Solid Waste, H.  L.  Hickman,  et al ., Noyes
     Publications,  Park  Ridge,  NJ,  1983.
                                   i-n-

-------
                                                                        CONDf N5ME
                                                             LEGEND

                                                        , -  TWO STAGE INCINERATOR
                                                        2 -  RESIDUE EXTRACTOR
                                                        3 -  WATER TUBE BOILER
                                                        4 -  ECONOMIZER
                                                        5 -  WATER SOFTENER
                                                        6 -  DEAERATOR-STORAGE TANK
                                                        7 .-  BOILER FEED PUMP
Figure 7-6.   Synergy two-stage combustion  saturate
               steam system.

-------
Me II vane.   "Waste  Burning Projects and People".   Market  Survey  by the
Mcllvane Company,  Northpark,  IN, October 1986.

Niessen,  W.  R.   "Dioxin  Emissions from Resource Recovery Facilities and
Summary of Health  Effects",  OSU  Report, 1986.

Synergy,  1986.   Discussions by  telephone between  G.  Hoth and W. E.
McMillen with W.  S.  Lanier,  October  17, 1986.
                               7-14 ..

-------
8.0       COMBUSTION CONTROL OF ORGANIC EMISSIONS FROM MUNICIPAL  WASTE
          COMBUSTORS
     Tests  of modern  mass  burn Municipal  Uaste  Combustors  (MWCs)  have
indicated  emission levels of chlorinated dibenzo-p-dioxin  (PCDD) and furans
(PCDF) on  the order of 1 ng/Nm3-  That is equivalent to  an  exhaust gas mass
fraction of  a part per  trillion.  However,  older designs  such  as the mass-
burn refractory system at Philadelphia and even the newer  mass-burn waterwall
facility at  Hampton, Va.  have been found to have emissions as  much as four
orders of  magnitude higher  than those  from modern mass-burn  systems.  At
least part of the high PCDD/PCDF emissions from the Hampton facility may be
traced to  system control and operation.   Thus,  even though  modern municipal
waste combustors can be  designed  and  operated with low emissions of trace
organics,  they can be designed and operated improperly giving  rise to higher
emission  levels.   The purpose of this chapter is  to document  these combustion
practices  which, when  adhered to,  are expected to minimize the emission of
trace organics from municipal waste combustors without undue impact upon the
emission  of other pollutants.

8.1      Design  and Operating Problems - Failure Modes

     The  design  or  operating conditions  which result in  higher  emissions of
hydrocarbons  (including species such as PCDDs and PCDFs)  will  be referred to
as  failure modes.  A clear  definition  of the  potential failure modes will
provide insight into how those conditions can be  prevented and help to define
"good combustion  practice".  Design goals can then be recommended which will
help avoid  the failure.   In this manner, organic  emissions can be minimized.

     The  establishment of the dominant failure modes must be accomplished by
careful  consideration of the cause and effects of system  parameters on PCDD/
PCDF emissions.   In this study, failure modes were established  in two ways.
First,  the  experience base associated with the designers and manufacturers of
municipal waste combustors was investigated.  Each manufacturer contacted was
asked to identify failure  modes for  their particular  system.  Few of the
manufacturers had specific  cause  and  effect data.  Most  relied upon their

                                    8-T

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understanding of how PCDD/PCDF and other orgam'cs  might be formed or failed
to be destroyed  and how their specific system operated.  The second approach
relied on  an  examination of combustors which  have been reported to have PCDD
and PCDF  emissions on the high end of the range for  the current data base.  A
comparison  of these cases  with current practices  revealed key differences
which likely  contributed  to the higher emissions, and which are therefore
considered  to be failure modes.  The following subsections will discuss the
potential  failure modes which have been identified  in this study.  It should
be noted  that  those potential failure modes do not necessarily apply to every
design within  a  given class of combustor.
                                •
8.1.1     Mass Burn Waterwall Failure Modes

     The  potential failure modes identified by the manufacturers of mass-burn
waterwall  combustors are summarized in Figure 8-1.  Starting with the input
of the refuse  onto the grate, the failure modes are  as follows:

     1.    Non-Uniform  Introduction  of Waste:   Improper introduction of the
          refuse onto the grate will  result in clumping or poor distribution
          on the  grate and hence improper burning.

     2.    Insufficent  Control  of  Primary  Air Distribution:   Combustion air
          requirements  along  the  bed  vary, thus it  is  important  that
          underfire air  be  supplied  to  several independently-controlled
          undergrate plenums.

     3.    Insufficient  Primary  Air  Pressure^   Proper  distribution of the
          underfire air requires that a pressure drop be taken across a known
          plane - the grate.  Consequently,  it is  desirable that the grate
          resistance be sufficiently high to distribute the air flow control.

     4.    Load Control  Systems Allow Closure of Underfire  Air:  Most modern
          furnace  load  control systems use primary air  to modulate the
          combustion intensity of the bed.  For example, a high heating  value
          fuel charge will cause a step change in the amount of heat  released

                                    8-2

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oo
i
CO
                     IMPROPER LOAD
                     CONTROL OF AIR
                     LOW LOAD
                     OPERATION
                     CO CONCENTRATION
                     PEAKS
POOR SECONDARY
AIR JET PENETRATION
OR COVERAGE
                     VOLATILES
                     NOT CONTROLLED
             UNSTEADY
             NON UNIFORM
             FEED
              LOAD  CONTROL  ALLOWS
              CLOSURE OF UFA DAMPER'
                                                        TOO LOW OR HIGH
                                                        02 LEVELS
                                                          HIGH  CO  LEVELS

                                                          TOC LEVELS  ABOVE
                                                          ZERO  (AMBIENT)
TOO MUCH LOWER
FURNACE HEAT REMOVAL

INSUFFICIENT OFA

NO CONTROL  OF PRIMARY
AIR DISTRIBUTION
                                                          LOW AIR PLENUM
                                                          PRESSURE
                            Figure 8-1.  Mass burn municipal waste combustor failure modes.

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     on  the grate.  The heat release  rate can be lowered by dropping the
     primary  air  flow.   If this  control  technique  is  used to the
     extreme,  then  the  control  system  can cause  oxygen-starved
     conditions to exist in the entire  lower furnace.

5.    Control of Volatile Heat Release:   Whether this  item is a failure
     mode is a point of controversy  with different manufacturers.  Some
     manufacturers believe strongly that the volatiles released from the
     bed must be redirected into  hotter combustion zones by the furnace
     configuration.  Others believe that this is not critical  and  merely
     a refinement.

6.    Insufficient Overfire  Air:   In almost all  mass  burn  designs,
     overfire air is used to control  flame height and to mix the furnace
     gases  before  they  enter the upper  furnace.   If  insufficient
     overfire air is employed, then  mixing of the  furnace gases and the
     combustion  air  may be inadequate allowing the existence of f uel -
     rich pockets of combustion products.

7.    Inadequate  Secondary  Air Jet Design:  All  manufacturers stressed
     the importance of furnace mixing to prevent PCDD/PCDF emissions and
     generally  the  designs  relied on overfire  air  jets to promote
     mixing.  Poor coverage of the furnace flow (e.g. improper locations
     or  insufficient jets) or low  jet penetration (e.g. insufficient jet
     momentum) will  result in poor mixing of the overfire air and the
     gases rising from the grate.

8.    Excessive Lower Furnace Heat  Removal:  After mixing with air, all
     furnace gases should be hot  enough to ensure  that all  hydrocarbons
     are destroyed.   If there is  too  much  heat removal  in the lower
     furnace, then the temperature at the fully mixed height will  be too
     low to ensure complete destruction  of the pollutants.

9.    Low Load Operation:   Mass burn systems  are  not tolerant of load
     changes.  At  very low load,  temperatures may  fall  to the point

                               8-4

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         where  efficient destruction  of organics cannot  be  maintained.
         However,  even at moderately low loads, conditions  can deteriorate
         due  to  other failures  such  as  insufficient  overfire air, poor
         overfire  jet  penetration and  lower temperatures.   Some
         manufacturers suggested  that loads less than  85 percent  of the
         design are inappropriate.

    10.   Low or Hi gn 03  Level s:  If exhaust  excess oxygen level is  too low,
         then  oxygen-starved zones may exist within the furnace or  a  charge
         of volatile refuse may momentarily  lower the entire furnace  volume
         to oxygen-starved conditions.   If the oxygen level  is too high,
         then the excess air  levels will excessively cool the furnace  giving
         low  temperatures which will affect the destruction of hydrocarbon
         species.

     The fractional  part per trillion PCDD/PCDF emission  concentration
measured for some modern mass-burn, waterwall  combustors is, very likely, the
result  of  the  strong emphasis placed upon the attainment of  uniform
combustion conditions for the mitigation of fireside corrosion problems.  In-
furnace testing  and system re-designs have been undertaken to minimize mal-
distribution  of  combustion gases particularly in Europe.  Because  fireside
corrosion rates  are dependent upon gas-phase stoichiometry this emphasis on
the attainment  and verification of  uniform conditions will most  probably
minimize the  emissions of trace organics by preventing low  temperatures or
long life-times for  fuel-rich  products.

     A  few  MSW systems  have  been found tc have noticeably higher PCDD/PCDF
and unburned  carbon emission levels  than those measured in other  systems.
These systems were generally designed before there was concern over PCDD/PCDF
emissions.   Their  design objectives  were generally  to  ensure refuse
throughput and  constant steam production, without consideration being given
to the  impacts  of  design on organic  emissions.   One such  system  is being
investigated currently  to determine how it might be modified and operated to
minimize emissions of these species.  The failures of this system to  minimize
                                   8-5

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emissions and the proposed modifications will serve as a specific  example of
potential  failure modes of mass burn systems.

     The NASA/Hampton  refuse-fired  steam plant in Hampton,  Virginia, began
operation  in September, 1981.  The plant consists of two combustors each with
a design  rating  of 100 T/d.   The combustors  (Figure 8-2)  employ  six-foot
wide, two-section  reciprocating grates with  a  vertical drop-off between
sections.  Underfire air is controlled separately to the two  sections of the
grate and overfire air is added under the front wall  nose, on the back wall
and on  the  side  walls.   The  boiler furnaces  are top-supported,  natural-
circulation, single-pass systems.  Silicon carbide refractory is used in the
lower furnace  to a height  of two feet above  the last air injection point.
The primary  combustion control system modulates ram feeder frequency and
inlet dampers  on the underfire air fan in order to maintain  steam pressure.
A secondary  loop modulates the grate reciprocating rate to maintain upper
furnace  gas temperature.

     Measurements  of PCDD  and PCDF emissions from the  Hampton facility
generally established the  high end of the  emission data base range shown
earlier as Table 2-1.  The manufacturer has made major changes in  its design
of more  recent mass-burn systems and is currently investigating modifications
to the  design  and  operation  of Hampton.   Several  failure  modes have been
identified  by  the  manufacturer of the grate  used at Hampton as  probable
causes  for  the  non-optimal  emission performance.   The primary problem
identified  to  date is associated with a design oversight in the automatic
control system.   A detailed  discussion of  the control system developed by
NASA for  the  Hampton facility is included  in a  report  by  Taylor, et al.
(1981).   Briefly, the control scheme is designed to maintain a constant steam
production rate.  Steam production is controlled by adjusting the  modulation
rate of the burning grate and by adjusting a  damper on the forced  draft (FD)
fan supplying  underfire  air.   With  the facility operating at a given steam
production  set  point, the  primary function of  the control  system is to
account  for the variability  in MSW heating value and volatility. The control
scheme does an excellent job of maintaining a  constant steaming rate.  When  a
pocket of hi_gh  heating value MSW fuel begins to burn, steam production will

                                   8-6

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          Charging
          Hopper
 Water
Chargi
         Municipal  Refuse-Fired
               Boiler
         Capacity:  100 Tons/Day
          Figure 8-2.
Boiler sectional  side of NASA/Hampton mass
fired waste-to-energy facility.
                                      8-7

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start to  rise.   The controller will compensate by closing down the  FD  fan
damper.   The control system design oversight  was  failure to establish a lower
limit  for closure  of that damper.   Failure occurred  when  the  damper
completely closed off the underfire  air  (for short time periods)  leaving  the
refuse  bed to  smolder under starved-air conditions.  The manufacturer reports
that plant data  shows the  FD fan damper continually modulating from fully
open to  fully  closed.

     Compounding the problems caused by  the  automatic controller  is  the fact
that  this facility  was  constructed  with an  insufficient  overfire  air
capacity.  Although the  original  design called for 30 percent of the total
combustion air  to be supplied  as overfire air, recent tests  indicate that
actual  overfire  air capability is  only 15  percent.  This construction (not
design)  flaw  exacerbated  the control  system problem and allowed fuel-rich
pockets  of gas  to  escape  the high temperature  region  of  the furnace.
Furthermore,  although stack gas Q£  level  was  monitored continuously,  the
temporary fuel-rich operation was not detected because of significant air  in-
leakage.  This in-leakage, driven by the induced draft fan,  masked the f uel -
rich operation of the main boiler.

     The Hampton  facility was  the  first MSW installation in  the U.S. to
incorporate a complete automatic control scheme (Taylor et al. 1980)  and it
should  be noted that the failure modes noted above are probably correctable.
A lower  limit can be set on closure  of the FD fan damper,  and the system  can
be  brought up to  its original  overfire air  design specifications.  Well -
documented tests before and after modification could provide a data base upon
which to build design guidelines.

8.1.2     Refuse Derived Fuel Spreader Stoker Failure Modes

     Refuse  Derived Fuel  (RDF) fired on  spreader stokers have some potential
failure  modes that are similar to  those of mass-burn units.  However,  the
unique  features of RDF spreader  stokers  can  also lead to different problems.
A summary of  potential  failure  modes that could occur is provided in  Figure
8-3.  These include the following:

                                    8-8

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                  LOW TEMPERATURE
                    PATHWAYS
00
OFA DISTRIBUTION
NOT COMPLETE
                                                             HIGH CO LEVELS
                                                                       INSUFFICIENT  0,
                                             -v-
                                                   CARRY  OVER OF
                                                   UNBURNED RDF
                                                                           RDF DISTRIBUTOR
                                                                           NOT UNIFORM
                                                                     •FUEL  RICH ZONES IN
                                                                      UPPER FURNACE

                                                                      RDF PROPERTIES NOT
                                                                      MATCHED TO SPREADER
                                               INCORRECT UNDERFIRE AIR DISTRIBUTION
                           Figure 8-3. Failure modes of RDF spreader-stoker systems.

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!•    RDF Properties Not  Matched to Spreader-   Spreader  designs  are
     generally  matched to the  characteristics of  the RDF in order to
     disperse the  fuel adequately  on the  traveling  grate.  Changes in
     refuse size  and  density  can  adversely  impact  the distribution of
     the RDF.

2.    RDF Distribution Not Uniform.  The performance of the spreader is
     crucial to  the  successful operation of  the  combustor.  The spreader
     must spread  the  fuel uniformly  on the grate  or there may exist
     zones  which are oxygen-starved.

3.    Incorrect Underfire Air Distribution.  In a fashion similar to mass
     burn systems,  if the underfire air is not distributed in the same
     region as  the  combustibles,  then oxygen-starved zones can occur-
     Incorrect air distribution  can result if there  is too  low  a
     pressure drop across the grate and no ability  to  control  the air to
     separate plenums.

4.    Incorrect Overfire Air Distribution.  If  overfire air is not  mixed
     efficiently with  the furnace gases, then oxygen-starved conditions
     and non-uniform temperatures  can exist  in  the furnace which can
     result in the escape of unburned organics.

5.    Excessive  Heat Removal.   Excessive lower furnace heat removal by
     waterwalls  will result in  low combustion  gas temperatures which may
     be too low  to ensure the destruction of organic  intermediates.

6.    Carryover of  Unburned RDF.  The unique  feature of spreader  stokers
     is that RDF  is burned in partial suspension.  This can lead to an
     additional  potential failure mode if there  is  significant carryover
     of unburned RDF which may  contain precursors to  toxic organics.

7.    Low-Load  Operation.   RDF systems  are susceptible  to  failure at
     lower loads due to lower temperatures  and  poorer mixing conditions
     similar to  mass burn systems.

                               8-TO-

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    9-   Low or High Oxygen  Levels.  Just as with mass  burn  systems,  oxygen
         levels can be either  too  high or too low.

    The  measurements  on  PCDD/PCDF emissions from RDF  waterwall systems are
limited.   However,  there is  some indication that emissions from RDF firing
are  higher than those from mass-burn systems (Camp, Dresser and  McKee, 1986).
It should be emphasized, however,  that the indicated variations  may be due to
data base limitations as well  as  variations in the application  of downstream
air pollution  control devices.   The key differences in  RDF spreader stoker
systems that could account for  potentially higher emissions are  as follows:

    •    Semi-suspension  firing  of  RDF allows  carryover of unburned
         materials.  This unburned material may be a precursor or allow the
         formation of precursors of toxic  organics  in the cooler regions
         downstream of the injection point.  Destruction  may not be possible
         because residence times  are less and temperatures are  lower.

    •    Some  overfire injection schemes do  not appear to be designed to
         achieve complete coverage and  penetration of  the flow.  Mixing and
         uniformity are achieved  by fuel dispersion  rather  than by stirring
         caused by the overfire air jets.

    •    Unclad steam tubes  in the lower furnace increase  heat extraction
         rates  and lower combustion  temperatures.  This  may prevent the
         destruction of trace  quantities of organic species.

8.1.3     Small, Multi-Staged Modular Unit Failure Modes

    Small,  modular  starved-air combustion  systems have design strategies
which  are significantly different  than their  large  mass-burn excess air
counterparts.   Many or all  of  the  failure  modes listed for mass-burn
waterwall systems  could also  apply  to starved-air systems.  However, the
equipment generally sold eliminates  or modifies many of these potential
failure modes.
                                   8-1-1

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     Non-steady,  non-uniform mass  feed  and overcharging were identified as
potential  failure modes  for  excess air  systems.  Typical  solids  residence
time  in  starved-air systems is  on the order of  10 to 12 hours.  Consequently,
the effects  associated with any non-uniformity  in the fuel feed are  damped to
a large  extent because of the mass of the  fuel  bed.

     A major concern  in mass-burn  and RDF systems is inadequate control of
both   the amount and distribution of  underfire  air.  In contrast,  starved-air
systems  are  designed to  generate a fuel-rich  gas in the primary chamber,
which is then combusted  further in the secondary chamber.  Thus modulation of
the  primary air flow is less important  in  a starved-air system than in  mass-
burn  or  RDF  system.  Primary air flow  rates will,  however,  affect  waste
throughput  and ultimately the composition  of the residue.

     Perhaps the  most critical aspect of  starved-air systems with respect to
the  emission  of trace  organic  compounds is  the design of the  overfire air
system.  Fuel-rich pockets of  gas  flow  from the primary to the  secondary
furnace  zone.   Failure  to  mix  that primary zone effluent with  a sufficient
quantity of  air could  allow  trace  organics or their precursors  to exit the
secondary  furnace, providing  the  opportunity  for an  increase  in their
emissions.

     It  is  possible to  quench second-stage  burnout  reactions through
excessively  fast  heat extraction.  Current design philosophy minimizes heat
loss  for approximately one second (mean  gas residence time) after  secondary
air injection.   Thus, excessive reactant quenching is avoided.

     PCDD and PCDF emission levels from starved-air systems generally fall in
the  low  end "of  the range  of  emissions  from  MSW systems.  Thus, it appears
that  the current  design  philosophy for  starved-air systems is capable of
maintaining  relatively  low emissions of  these compounds.  However, failure
modes can be envisioned which would impact  emission levels adversely.
                                   8-T2

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8-2      Combustion Strategy for  Minimizing Emissions  of  Air Pollutants

     Definitive cause and effect data on the relationships  between design and
operating procedures and  emissions  of organics  such  as PCDD/PCDF  from
municipal waste combustors do not  exist.  A significant body of data has been
developed by Environment Canada in their tests on  the  Quebec City combustor.
These tests clearly demonstrate that combustion control  significantly reduces
trace organic  emissions.  Further analysis of that  data  may provide some of
the required cause and effect information.

     It  is clear that  PCDDs/PCDFs or at least their precursors are organics
which form as  intermediates in  the  combustion  process.  The vast bulk of
these intermediates will be destroyed as a natural  consequence of combustion,
leaving  products which consist of  primarily C02 and h^O.   However, because of
the  potential  toxicity  of  some of  the combustion  intermediates,  it  is
essential that they be destroyed as completely as possible.  Conditions which
favor the formation  of  intermediates should be  avoided.   Combustion
strategies can  be  developed around  the simple principle of minimizing the
emission of  all  combustion intermediates.  The available  data  on  low
temperature,  catalytic  reactions  to  form PCDDs/PCDFs  rely on emission of
organics such  as  phenols from the radiant section of  the combustor (Vogg et
al., 1987).

     The strategy for minimizing  the emission of combustion intermediates is
based upon a  desire  to  provide an  environment which will  ensure  the
destruction of  effectively  all  gaseous organic  species.  This environment
requires the  presence  of oxygen  at a sufficiently  high  temperature.
Furthermore,  it  is  necessary that,  to the extent  possible, all combustion
gases experience the same environment.  Thus, mixing is important because it
will  ensure uniform temperatures  and composition.   Mixing  should be rapid as
well  as  complete  in order  to maximize residence  times  for destruction of
intermediates.   The attainment  of the appropriate  temperatures will depend
upon  the balance  between heat released and heat absorbed.  All  of the above
will  maximize  combustion  efficiency.   Exhaust  CO concentration is a good
                                  8-13

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measure of  combustion  efficiency because  high levels  of  CO are normally
associated with  poor  air/fuel mixing.

     The simplistic  view of optimization of  combustion by the "three Ts",
time, temperature  and turbulenci, is not directly valid  in this context.  For
example, as  will  be discussed further  in  the next section, the gas-phase
residence tide should  not be considered  solely as  a necessary reaction time
but  rather  as a mixing time.  Time is required for air and intermediates to
mix but, once mixed at sufficient temperature,  the  destruction  reactions take
place virtually  instantaneously.  There is no  need  to hold the  mixed gases at
this  temperature for a longer time  to  destroy trace  organics.  Further,
turbulence  on its own is not sufficient to ensure  the necessary mixing.  Two
separate highly-turbulent  stream tubes  in  the furnace will  not mix despite
their  high  turbulence  level unless they  come  into  contact.  Thus, mixing of
the  furnace  gases with  air requires that  the  turbulent air jets be dispersed
throughout  the  combustion  gases.  Finally, the definition of a  mean
temperature  is  inappropriate.  The trace  species that escape the furnace may
experience  temperature  pathways significantly  different from the  mean
temperature  level.   Concepts based strictly on  mean times above temperatures
(such  as qualifying maximum volumetric heat  release rates) do not treat the
low temperature  escape mechanisms.

     Specific  elements  of a general combustion  strategy can be formulated to
minimize the emission  of  trace organics from municipal waste combustors.
These  elements must first  consider the  nature of  the  combustion process in
municipal waste combustion facilities  and  the practical  implications of
current systems.  There are also several  restrictions that must be placed on
the operating requirements.  These must account for  impact on the following:

     •   Availability of equipment
     •    Emission of other species such as NOX and  metals
         Economics of waste-to-energy systems
     •
     The elements of a combustion control  strategy based upon good combustion
practices include the following:
                                   8-14 -

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    •    Temperature
    •    Combustion air (primary and  secondary)
    •    Combustion monitoring
    •    Automatic combustion control
    •    Limits on operating conditions
    •    Verification that operating  conditions were adhered to

These elements and the establishment of performance criteria are discussed in
the next sections.   It should  be noted that  the following  represents an
initial attempt to  develop  such a strategy-  It has not been verified and
must necessarily be modified as the data  base  is expanded.

8.3      Temperature

    There  is a broad  distribution of temperatures within a municipal waste
combustor that  make mean  conditions difficult to define.  In  Figure 8-4 is
shown  temperature  profiles  in the  Steinmueller mass  burn  combustor at
Stapefeld, for both full  load and partial (60 percent) load operation.  Mean
times  at temperature defined by  maximum  volumetric heat release rates do not
characterize  conditions  in  the furnace adequately.  Three-dimensional heat
transfer, flow and combustion models  are generally required to  predict, even
crudely,  the  temperatures experienced by  the combustion  gases.   The
Steinmueller  data indicate  that a greater  than 200°C (360°F)  temperature
variation exists  across  any  plane in the mid-portion of the  furnace.  The
typical temperature variation is from 1000°C  (1832°F) at near center-line to
800°C  (1472°F)  near the walls.   At low  loads the temperatures  are  generally
lower  (peak 900°C) and the distribution  is skewed due to changes  in  flow and
combustion conditions.

    Ensuring that organics such as PCDDs/PCDFs are minimized from  municipal
waste  combustors  requires a knowledge of minimum temperature pathways.  The
formation mechanisms of  PCDDs  and PCDFs are extremely difficult  to define
reflecting the  complexity of the reactions that occur during the combustion
of  refuse.   The critical  step  necessary to  prevent  emissions of the
intermediate  organics is  to ensure that the  minimum temperature experienced

                                   8-15

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00
I
0V
                                        FULL

                                        LOAD
                                                                                             60%

                                                                                             LOAD
             Figure 8-4.   Temperature distributions in  an  operating mass-burn combustor.

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by the organics,  after they have mixed  with oxygen, is sufficient for  their
rapid  destruction.  Thus,  it is not  the  mean temperature  that must be
defined,  but  the minimum temperature experienced  by the combustion gases.

     Combustion control  of organics  requires that the combustion gases are
mixed with oxygen.  Only in the presence  of oxygen are the reactions,  which
complete  the combustion of organics to harmless  species such as CC>2 and 1^0,
rapid.   The  requirements for ensuring  adequate mixing are therefore at  least
as important as  the temperature criteria.   The mixing  requirements for
combustion air  are  discussed in the subsequent  sections.

     Municipal  waste combustion systems  are generally  characterized by
defining  a mean temperature at some (often  arbitrary) location in the radiant
section.   Using that characterization  approach,  the mean temperature must be
high enough  to ensure that the minimum temperature is sufficient to destroy
organic  species.   The location chosen for  the characteristic temperature is
also important.   The current evaluation  has selected that location as the
"fully mixed  height" - the point beyond the  final  air addition location  where
complete  mixing  should  have  occurred.   In mass  burn, waterwall  designs and
RDF  fired systems, overfire air jets  are  used above the grate  region to mix
air  with  the gases leaving the burning refuse  bed.  For most  systems, with
good engineering  practice  for overfire  jet  design, the fully mixed height
should be on the  order  of  one meter  (or  less) above or  beyond the last
overfire  jet.   For  less traditional systems  such  as the Westinghouse/O'Connor
combustor the  fully mixed height might be  in  the radiant furnace just  above
the  rotary portion  interface with the  stationary furnace wall (assuming that
there  is  no  additional  overfire air  addition).  For the Volund system the
fully  mixed height would  be above  the  refractory arch  after final air
injection and  after the split flows have merged.  For other, non-traditional
designs,  the location selected for the characteristic temperature should be
based on  sound  engineering analysis.

     The  mean  temperature must  be defined which  ensures that the minimum
temperature  at the fully mixed height is  sufficient to destroy PCDDs/PCDFs
and  their precursors.  Figure 8-5 is  an illustration of the conditions that

                                   8-17 -

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00

00
 DISTANCE
TO MIX OFA
    MINIMUM
   1GAS   TEMP
    REQUIRED
  TO  DESTROY
° DIOXIN AND
  PRECURSORS
                                                                            FURNACE
                                                                            CROSS-SECTION
                                                                            ISOTHERMS
        Figure 8-5.  Required temperature for destruction of intermediate organics.

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exist  in  a  mass burn combustor.  At the fully  mixed height above the overfire
air jets,  there  is  a  distribution  of  temperatures,  as  shown by the
hypothetical  cross-sectional  isothermals.  With good grate and overfire air
jet design, the range of temperatures  will  be kept to a minimum.   However,  a
200°C temperature variation could still  exist at this plane (e.g.  see  Figure
8-4).   Thus,  a  mean temperature must  be  selected such  that the minimum
temperature is greater than the thermal decomposition temperature required  to
fully  destroy  intermediate hydrocarbons.

     The  thermal decomposition data obtained by  UDRI  (Dellinger, 1982)  can  be
used  as  the basis for  indicating the temperature which will  be required  to
destroy  organics.   In Figure 8-6 is provided some of  the reference data for a
range  of  organic intermediates including  PCDDs/PCDFs, benzene and chlorinated
hydrocarbons.  PCDD/PCDF  species are generally unstable above 1300°F although
potential  precursors  such  as chlorophenols  are  stable to  1500°F.
Hexachlorobenzene is one  of  the most  stable  species; it does  not totally
decompose  below  1650°F although such  species (totally chlorinated organics)
are unlikely  candidates  as  PCDD and PCDF precursors.   A temperature of
1800°F,  which  is  quite  often associated  with municipal waste  combustion
requirements is roughly  150°F above the thermal  decomposition temperatures  of
the most  stable hydrocarbon intermediates.   Thus a mean temperature of  1800°F
at the fully  mixed height,  should be  adequate  to ensure that  the minimum
temperature is  sufficient to destroy  the most  thermally stable hydrocarbons
at very  short  residence  times.  In  particular, such a design guideline  is
adequate  for PCDD and PCDF species along  with  their potential precursors  such
as chlorophenols.

     It  is  possible  to demonstrate  the adequacy of a particular design  by
estimating the  mean temperature at  the mixed height for the design  oxygen
levels,  the design refuse  bed but with clean  waterwalls.  The use of clean
walls is  recognized to  be  conservative since  the existence of an ash layer
will  increase  the resistance to heat transfer  and therefore raise furnace gas
temperatures;  however,  the  characteristics of the layer cannot be defined
accurately. Experimental data could be used to  establish appropriate values.
                                   8-t9-

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  UDRI  THERMAL  STABILITY
1000   1100     1200    1300     1400     1500   1600

                  TEMPERATURES (°F)
THEORETICAL  CALCULATIONS

 -   FORMATION/DESTRUCTION  FROM

    CHLOROPHENOLS (NBS)
                                                  10'
                                                  to"
                                                O
                                                Q
                                                         r
                                                        600
             i
            900
 I

1200
 I
1500
                                                                                      1600
                                                                     fEMPERMURE (°F>
Figure 8-6.  Thermal  decomposition  characteristics of selected hydrocarbons.

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8.4       Combustion Air

     An  excess of  combustion air is  essential for the complete  oxidation of
combustible  material.   Combustion will  not  be completed,  and harmful
intermediate  species may  form ar,d escape  the furnace if  oxygen is not
provided  in sufficient quantities at  the  appropriate location.  Four factors
concerning combustion air are important;  they  are as follows:

     •    The total amount of air
     •    The distribution of primary  air
     •    The distribution of overfire air
     •    The verification of appropriate air  distribution

Each of  these aspects associated with  combustion  air will be discussed in the
following  sub-sections.

8.4.1     Total Air Requirements

     The first aspect  involves  the  amount of total combustion  air which is
necessary  for complete combustion.  There can  be  both too much, or too little
air.   With  too  little  air,  the furnace  will be oxygen-starved either
throughout the furnace  or  in  localized  zones.  The total  air  flow must be
sufficient (1) to ensure that there are  no localized zones which are oxygen-
deficient and  (2) to dampen out surges  due  to  excessively volatile refuse.
In other  words,  there must be sufficient excess oxygen available to dampen
out  spatial  and  temporal  variations  in  fuel composition to avoid the
possibility  of oxygen-deficient zones.   On the  other hand,  too  much air can
excessively cool the furnace.  Adiabatic  flame temperatures (see Figure 8-7)
are hottest  at stoichiometric  (zero percent excess oxygen)  conditions and
fall  off with increasing excess air due to the dilution of furnace gases with
air that must be heated.

     The range of  excess air  that is appropriate can be established based
upon  current practice and experience.  Mass-burn waterwall systems have been
found to  operate  best with flue gas  oxygen  levels between 6 and 11 percent.

                                   8-21

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                       4000
CD
ro
PO
                 LLJ
                 Q

                 Q.
                 5
                 Q
                 UJ
                 <
                 O
                       3000 -
                       2000 .
                 o     1000 .
                                                          ,0% Moisture


                                                                  15%
                                                                     30%
50
                                            % SA   100
150
200
250
300
                             % Excess Air:
                                                             50
                                   100
                        150
                         250
                       Figure 8-7.   Theoretical  temperature of  the products  of  combustion,
                                     calculated  from typical MSW properties,  as  a function
                                     of refuse moisture and excess  air or oxygen (8.9).

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Operation  with less than 6 percent may  be  acceptable for certain situations
such as extremely well-  mixed mass burn  system.   It is reported that  the
Westinghouse/0'Connor system and the Enercon/Vicon systems operate at  lower
excess air  levels and still achieve relatively  low trace organic emissions.
Similar levels (6-11 percent) are  current  practice for small  modular two-
stage systems.  These excess oxygen levels correspond  to excess air levels of
between 40-100 percent.   RDF-fired systems  are typically designed to operate
at lower  excess  air levels,  taking advantage of  the reduced variability in
fuel  characteristics.

8.4.2     Primary  Air Requirements

     The second aspect relating to combustion air  is  the proper distribution
of primary or underfire  air.  For mass-burn  and RDF  systems, primary air is
introduced through the grate and into the bed of burning refuse.   For multi-
zone systems, the primary  air is  introduced  to the first zone.  The  key
requirement for the primary air  system is  to  get  the  primary air to  the
location where burning is  taking place in the  refuse bed.

     For  mass burn grate systems,  the  primary  air should be  introduced
through a  number of separately controllable  plenums underneath the grate.   In
current practice, between  four and six primary air control  regions  are
employed  along the grate  in order to have the capability of distributing  the
air  to the active burning  zones.  A high  pressure drop across the grate is
desirable  to ensure that the bed region supplied  by each plenum is completely
covered with air and there is minimal  bypassing around thicker regions of
refuse.

     Both  the  quantity and the  distribution of underfire air are important
parameters.  The quantity of underfire  air will directly impact the burning
rate of the MSW on the  system grate or hearth.   Therefore, the quantity of
undergrate air - along  with grate  speed  -  may be used as  a quick response
trim parameter for the steam production rate.   Over long averaging  times,  the
steaming rate is obviously controlled by  the MSW  feed  rate while  the quantity
and  distribution of undergrate air will  1) control the  location of  MSW

                                   8-2-3

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burnout and  2)  the mass averaged excess air of combustion products leaving
the lower furnace-  Generally, the distribution of  underfire air should  be
such that good carbon burnout  is achieved and the main flame is concentrated
on the burning  grates.   Air flows to the drying grates and finishing grates
are generally minimized,  consistent with moisture content  in the fuel and  low
organic content in the bottom ash.

     For  RDF fired  spreader-stoker units, primary air should be introduced
through the  traveling grate  to correspond to the distribution of RDF.   For
example,  some  RDF distributors attempt to spread the refuse uniformly over
the  grate   and  thus,   the  air must be  distributed  uniformly also.
Alternatively,  some distributors  throw the refuse toward  the end of  the
traveling grate  and the refuse is  carried to  the  other end by the grate
movement.   In this instance, separately-controlled underfire air plenums  are
necessary to  ensure proper primary air distribution.  Similar to mass-burn
systems,  a  high  pressure drop across the grate  may aid in the uniformity  of
air flow through the refuse bed.

     Finally, for  smaller multi-stage  starved-air  combustors, primary  air
control is  not  as critical as in larger units.   The  primary units are small
enough  that,  in  general, only the  amount  of primary air needs to  be
controlled.   Individually controlled underfire air plenums  are useful but  not
necessary.   However, uniformity of air flow across the bed  is still crucially
important and  can be accomplished  most effectively by  high grate pressure
drops.

8.4.3     Distribution of Overfire or Secondary Air

     The  most critical  requirement for complete combustion of the unburned
material  exiting  the primary  zone is mixing with air before the temperature
drops below  the  level required to  destroy the  organic  intermediates.   In
almost all current municipal waste combustion designs, secondary air is used
to aid the  lower  furnace mixing.   The correct  design  of the overfire  air
injection  scheme  is critical  to  the prevention of  unmixed zones  and
minimizing  the emission of trace  organics.   With proper overfire  air

                                  8-24-

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injection  schemes, particle carryover  can be reduced,  the  flame height can be
controlled  and  the furnace  gaseous  concentrations  can  be made to be more
uniform.

     For  any combustor  type, the overfire air must  be introduced in such a
way as to cover the entire furnace flow.  Only with complete coverage of the
flow can  the  adequate mixing  of the furnace gases be ensured.  Mixing is a
function  of  how well the injected  overfire air contacts the gases rising from
the burning refuse bed.   In  order for "good" mixing  to  occur, the overfire
air jets  must  penetrate into the gas flow at well  distributed locations and
they must entrain as much of  the  furnace gases as  possible within a short
distance  downstream from the  injection location.  For  mass burn systems, the
normal overfire air configuration involves rows of high velocity air jets on
the front and  rear walls.   Table  8-1  and Figure 8-8 provide a summary of
typical  overfire  air jet designs for large mass-burn  systems as provided by
manufacturers  for new systems.   The configurations  all  use front and rear
walls and multiple high velocity jets to completely  cover the furnace flow.
Refuse-derived-fuel-fired spreader  stoker designs also  employ overfire air
jets to  completely cover  the  furnace  flow.   For example,  Detroit Stoker
systems  have  OFA  jets  on  all  four walls  below the  height of the  fuel
distributor while Combustion  Engineering designs use tangentially-directed
high velocity  jets above  the  distributor level.  Again, the intent is to
achieve complete coverage of the flow  with penetrating  high velocity jets.

     Current designs  of small  multistage mass-burn systems and water cooled
rotary combustion units  also  use  overfire  air to  mix  the furnace gases.
Again,  the  design  and  operating  goal  is  to  achieve  flow coverage and
penetration.   However, different overfire air injection  schemes can be used
because  of  different furnace  configurations and smaller dimensions.  Many of
these  systems  employ duct injectors which  achieve the coverage and
penetration by  injecting  air  at  high  velocities into the flow or from the
center  line  through smaller multiple holes.
                                   8-25 ..

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TABLE 8-1.  OVERFIRE AIR JET CONFIGURATIONS FOR
            LARGE MASS BURN INCINERATORS
Manufacturers
Deutsche Babcock
(Browning Fern's)
Steinmuel 1 er
Von Roll
(Signal Resco)
Widmer & Ernst
(Blount)
Martin
(Ogden Martin)
Detroit Stoker
Riley Takuma
Combustion
Engineeri ng-db
Volund

OFA
% of Total Air
20 - 25
40
30
30
20 - 40
40
30 - 40
20
40 - 50

OFA
Configuration
Rows on Front and
Rear Walls
Side Wall Aspiration
Rows on Front and
Rear Walls at Nose
Rows on Front and
Rear Walls at Nose
Front and Rear Wall s
Interlaced
Offset Vortex
Front and Rear Wall s
Offset
2 Rows Each on Front
and Rear Walls
Rows on Front and Rear
Walls at 3 Levels
1 Row Front Wall
2 Rows Rear Wall
1 Row Side Walls
Perforated Ceramic
Plates-Side Walls
1 Row Back Wall
Controlled Air at
Feed and Exit of
Rotary Kiln
Jet Velocity
and Configuration
100 m/sec
70% penetration
80 m/sec
600 mm w.g.
80 mm dia.
50 m/sec
50 mm dia.
1 m separation
80 m/sec
70 mm dia.
50 - 100 mm dia.
450 mm w.g.
(Data Not
Available)
(Data Not
Available)
High Velocity
750 mm w.g.
Varies

                  8-26

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                                           II
Figure 8-8.  Typical designs of overfire air systems,
                         8-27

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     The need for  secondary air  to  completely cover and penetrate  the  flow
implies a  number of considerations for  the  design and  operation of the
injectors.  The injector parameters of importance include the following:

     0     Quantity of overfire air
     t     Number  and location of injectors
     0     Injector velocity
     0     Injector spacing
     0     Injection angle
     0     Injector shape
     0     Air supply pressure drop

These parameters are related to each other  and to the design of the furnace
configuration.  The optimum overfire jet configuration would involve  numerous
high-velocity  jets with  correspondingly  high pressure drops.  However, the
pumping energy  requirements (as indicated by the pressure drop and  quantity
of air)  may become excessive and the auxiliary power requirements will be too
high.   Thus,  good engineering practice  involves achieving penetration and
coverage  with  minimal pumping energy  (i.e. low pressure drop). Since the
design  of  the  overfire  air system  may not be  exact, it is appropriate to
allow for  a range of air flow, velocity and potentially even jet direction in
initial hardware design.  This would  permit  on-line optimization to  take
place.  The overfire air  design must  be tailored to the  overall  furnace
design  and operation.

Quantity of Secondary Air

     The amount  of  overfire or  secondary  air is vitally important.  For  a
given steam production rate and overall  excess air level, the quantity of air
available  as  overfire air  must be  balanced with the amount of air used as
underfire  air.   With too little overfire  air, the mixing cannot be  achieved
since insufficient momentum is available to  achieve cross stream mixing.  On
the other  hand, excessive overfire air can  result  in thermal quenching due to
the introduction of too  much  cold  air-  Current practice has established
20-40 percent  of the total  air as  overfire air as appropriate.  Extensive

                                    8-2&

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testing using  cold-flow modeling  in-furnace sampling,  and emissions testing
(especially  on mass-burn  systems)  have verified  the suitability of these
values.   Thus,  it appears  that  the design  of  overfire air supply  with
40 percent of  total air capacity and ensuring that the  system is operated
with  at let-st 20 percent overfire air  is appropriate to minimize emissions of
trace organic species.

Injector  Diameter and Velocity

     The  diameter of the overfire  air injectors and the initial jet velocity
can  be determined based on  the  required  quantity of overfire air and the
furnace  configuration.  Design procedures exist to determine  jet penetration
depths and trajectories.  The appropriate design goals would  include:

     •   The  trajectory of  the jet should cross  the center!ine of  the
         furnace duct before reaching two injector diameters  downstream.

     •   The depth of penetration  of  the jet should be at least 90 percent
         of  the furnace depth (in opposed wall or centerline injectors the
         penetration should reach  45  percent of the depth).

Demonstration that the appropriate jet  diameter and velocity has  been
selected can be based on either design equations or other  data such as cold-
flow modeling or furnace flow measurement data.

Number of Injectors and Location

     Once the  velocity and diameter of OFA  injectors have  been determined to
achieve  appropriate furnace flow penetration,  the number  and location of
nozzles  must be examined  to ensure adequate  flow coverage. The number of
nozzles  is of  course related to  the quantity of air available for use as
overfire  air.   There are a number of  appropriate configurations that would be
acceptable  for  achieving  the  coverage depending  upon the  furnace
configuration.  However,  a  cross-sectional view of the furnace showing the
                                   8-29

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location of  injectors is generally sufficient to illustrate the coverage of
the  furnace flow.

Air  Pressure

     The  pressure of overfire air  supply, AP, is related to  the jet velocity,
V by:
                                   + 460\
          V(ft/sec)  =  66.3\/ 	 MP (in w.g.)
                          V y    520    /
where T is  the  temperature of overfire  air.   No friction losses  are accounted
for  in  this  equation, thus actual  jet velocities will  be  7-10  percent lower
than predicted  by the above equation.

8.4.4     Verification of Appropriate  Air  Distribution

     The appropriate design  of an municipal waste combustor will  allow
control  of the various  combustion air  streams  and will be  sufficiently
flexible to  allow the air  to be distributed to the burning zones.   The
current report has attempted to underline  the fact that an MSW  or  RDF
combustor  is a combustion  system and that the  various  subsystems must be
carefully  integrated.   Recognizing  these  systems aspects,  an  important
question arises as to how  appropriate  adjustment of the subsystems can be
verified.   Current  practice  involves  use of a variety of indicators
including:

     •    Visual  inspection
     •    Exhaust concentration measurements  and
     •    In-furnace probing/mapping experiments.

The  use of visual  inspection is often extremely effective in  adjusting air
flow distribution  (at  least coarse distribution).  Such visual inspection can
be  used to guide adjustment of underfire air  distribution among  the  various
plenums beneath the  grate sections.  Exhaust concentrations of oxygen, and  CO

                                    8-30-

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coupled with  visual  observation of the flame can be used to  adjust overfire
air quantity and distribution.  Clearly, experimental verification  that there
are acceptably  low exhaust concentrations  of the spectrum of PCDDs/PCDFs is
proof  that the combustor  is  sufficiently  adjusted.  To  achieve  that
acceptable emission  level  (a level yet  to  be defined) almost  certainly
implies that an appropriate quantity of overfire has been injected and mixed
with the effluent  from the lower  furnace.   A number of combustion system
designers including Steinmueller/Dravo,  Martin Systems and Von Roll have used
in-furnace profiling of CO concentration to  develop  their  overfire  air
injection  scheme  and to establish  an  appropriate air flow  distribution in
particular furnaces.  The rationale  is  that  CO concentration is a direct
indication of  furnace mixing and  that peaks  in concentration of CO at the
fully mixed height are indicators  of  poor  air distribution.   Figure 8-4
illustrate a  relatively flat CO concentration profile at  full  load and a
highly  skewed in-furnace CO  profile at 60 percent load.

     In establishing a  new MSW  system a  combination  of  the above
verifications  is  required.   Visually  guided  adjustments  and exhaust gas
measurements  are  essential components.  In-furnace CO profiling may not be
necessary  for  every new unit  but is strongly suggested as a  diagnostic tool
but  it is strongly  suggested as  a  diagnostic tool  for  correcting  air
distribution  in facilities which  do  not  achieve sufficiently  low  trace
organic  emission.

     If in-furnace profiling  is to be  performed the CO measurements must be
taken over  the entire measurement plane with sufficient spatial  resolution to
cover the  entire  furnace flow.  Twenty-five  to fifty  sampling locations
should  be  adequate and a variation in CO concentration greater  than 50
percent  (e.g. from 1 percent to 1.5 percent when corrected to  the  same oxygen
concentration  level) may be  indicative  of  the need to adjust  air flows,
although it is essential  that  this criteria be  established more precisely by
extensive  field testing and  engineering analysis.  Despite the current lack
of precision  in this technique, CO concentration profiling can  provide an
indication  of combustion uniformity.
                                   8-31

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8-5       Combustion Monitoring Requirements

     In  the previous  sections, the  goals for design and  operation were
discussed which would  be  consistent with proper  combustion system operation.
In addition to these requirements, the system must be monitored continuously
and  controlled so that  the system functions in the proper operating mode.  In
other words, continuous monitoring and control  is required to help prevent a
failure condition.   This is particularly challenging when burning the ever-
changing  supply  of  refuse  which  has  a  relatively  low  fuel  value.
Fluctuations  in  the volatile and moisture content of  the  refuse  must be
accounted  for  properly  if the design  is to be implemented  correctly in
practice.   If the system is designed properly and tuned accordingly, then the
monitoring  and  control   system must  only ensure that this  system  remains
within  the  proper operating envelope.

     Three  continuous monitoring schemes  may  be  used to ensure that the
system  stays in compliance.  These include the following:

     •    Minimum flue gas carbon monoxide concentration  (corrected  for 63
         concentration)

     •    Minimum and maximum level of flue gas oxygen concentration

     •    Minimum furnace  temperature

These monitoring  variables, on their own, are  not  sufficient to ensure the
system is  not  emitting  PCDDs/PCDFs.   In particular, they  do not  provide
information  on the  degree of uniformity within the  furnace.  This is
crucially  important to ensuring destruction of  intermediates.  For example,
low  exhaust  mean  carbon monoxide  levels  are  indicative of the mean
conditions, not  the isolated pathways which could  lead to part per trillion
emissions of PCDD/PCDF species.  On the other hand,  once the system has  been
proven to  have  well-mixed conditions  by in-furnace profiling or by other
means, then flue gas CO  levels might be used to  indicate that no significant
                                   8-32-

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change has  taken place.   Field  measurements are required  to establish the
effectiveness  of  this type of monitoring.

     The  levels  of  these monitoring  parameters must be  specified along with
appropriate  averaging periods.   Discussions with manufacturers of new MSW
combustion equipment have provided  an  indication of the  currently achievable
level  of control  and  the  appropriate target.   Currently, many  of the
manufacturers already use  some  or all of these measurements  for monitoring
and control.   For carbon  monoxide,  it was  generally  agreed that 100 ppm
(corrected  to 12 percent C0£) was  achievable and appropriate.  Many systems
regularly  achieve continuous CO  levels in the range of  20-40  ppm.  However, s
even well-behaved systems can have momentary spikes in CO that  can exceed 100
ppm.   These momentary spikes due,, for  example, to a small  burst of volatiles
are not  necessarily of concern since  a well-designed system will dampen out
surges and  destroy intermediates.  Also, field and laboratory measurements
have suggested that CO is a conservative indicator of failure.  However, if
this CO excursion is too high, or  if it persists, then corrective action must
be taken.

     Recent data from mass burn  systems  have shown that well designed and
operated units can achieve  low  PCDD/PCDF levels from  the  combustion zone.
For example,  the new units  at Marion County,  Tulsa and Wurzberg and the
modified older  design, Von Roll  facility at  Quebec City,  were found to have
low PCDD/PCDF and other  trace organic emissions over a four hour average
 (sampling  time).  These  same units  were  found to have a rolling average
exhaust  CO  levels of less than 50 ppm over  the same averaging  period.  Thus,
this level  of CO is proven  to be acceptable  and achieveable with modern
design  and  operating practice.

     It has  been  pointed out that  the  total  combustion air flow should  not be
too  high or  too low if  the  emission of  trace organic species  is to be
minimized.   Current practice has recommended  that an exhaust 02 range of
between  6  and 12 percent  is  appropriate.  When the proper  excess air  level
has been established for  a  particular unit, then the control system  should
operate  within  a more restrictive range.   However, for  major corrective

                                   8-33

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action such  as  a system shutdown,  it is appropriate  to  establish  levels
outside the 6 to  12 percent  range.

     Gas  phase furnace  temperature monitoring is  required to ensure no  low-
temperature  excursions occur.  Measurements in the  furnace above the fully
mixed height  using suction pyrometry to shield radiation can indicate  that
the system is  being continuously operated in the same  thermal  environment.
In conjunction  with other design  and operating  procedures which  ensure
uniformity,  this measurement  prevents thermal   failure  modes.  The most
desirable  locations are low  in  the furnace but pyrometer  survivability  is  low
in this region.  For this reason an upper furnace  location  which is related
to the fully  mixed height  temperature of 1800°F is a reasonable compromise.
Simultaneous  measurement at both  locations during  tune-up would allow  the
upper appropriate furnace temperature requirements  to  be  defined.

8.6       System  Control
     The final element of a strategy to minimize emissions of trace  organics
is the  control  of the combustion  system relating to  the combustion  process.
A control  system is required  in  order to maintain  a desired steaming  rate
with changing refuse characteristics.  The control schemes  used by the
various manufacturers are  described in Section 5.   Many of the new systems
use multiple  loop automatic control  schemes which change refuse  charging
rates,  grate  feed rates,  underfire  and overfire air flows in response to
changes in  steam pressure  and  other  monitoring variables such as  flue gas
oxygen  or  furnace temperature.   The  key goal  of these  automatic control
systems is  to avoid failure modes  while maintaining  steaming rate.  For
example, one  notable failure  discussed in Section 8.1  is  the closure of
underfire  air flows during a  steam excursion which  drives the system to an
oxygen-starved  condition.  Such  control system flaws can be prevented by
monitoring  other  variables such  as  flue  gas oxygen.

     In addition to maintenance  of  steam pressure, the other aspect of
control is  the  system response  to  excursions in  monitored parameters.
Specifically,  one possible response to a CO excursion  or  for low temperatures

                                  8-34

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is to initiate auxiliary  fuel  combustion  via  an auxiliary burner.  These
auxiliary  burners should  burn  a clean  fossil  fuel  such as natural  gas or
light fuel oil with sufficient capacity  to dampen out rapidly the  results of
the excursion.  In this manner the inventory of refuse that remains  in the
furnace at the start of the  excursion  will  be burned  under  appropriate
conditions.   A  complete halt to refuse feeding  should not be necessary  unless
the excursion  persists.  However,  the feed  rate would have to be  reduced so
that the  steam  rate noes not exceed the desired level.

     The other  need for  auxiliary  fuel is  for startup.   As previously
mentioned, low temperature  has been specifically identified  as  a failure
mode.  To  avoid this failure mode on startup, auxiliary fuel should be used
to raise the furnace temperature  to the operating point  before waste is
added.  The  capacity of the auxiliary fuel  system should be consistent with
these needs; current practice suggests  that capacity equal  to 60  percent of
the heat  input  is appropriate.

     Finally,  low  load operation  is  of concern because  the combustion
conditions  may  deteriorate as the firing  rate is  reduced.  Lower  firing rates
may result  in lower temperatures and poorer mixing due to slower  overfire air
injection  velocities and lower combustion intensity-  A specific  load level
below which performance is unacceptable has not been established, and is most
likely system-specific.  In fact, special  design  and operating procedures can
be developed  to extend the range of operation for any particular  system.  For
example, steam jets might be used to supplement  overfire air jets  during low
load  operation.  Most manufacturers expressed  preference for operating above
80 percent but below 110 percent rated capacity.  If lower load  operation is
desired,  then additional testing such as  in-furnace CO profiling, temperature
profiling or  stack  emission  testing to  verify low exhaust organics
concentration at  the lowest firing rate may be  appropriate.

8.7      Minimization of Hydrocarbon Species and Other  Pollutants

     The  previous  sections have  described a strategy for  the  design and
operation  of municipal waste combustors to  minimize  the emission of trace

                                   8-35 -

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organics.   There is concern  that  its implementation  could result in higher
emissions  of  other pollutants.   In particular,  the impact on emission of
acids, particulate matter,  NOX and  metals must be evaluated.  It is expected
that there  will  be little  or  no  impact on emission of acids and particulate
matter but  ther3 may be adverse impacts on NOX and metals  emissions.

     The  design and operating  techniques for minimizing organic emissions is
not compatible with combustion-zone  NOX emission control.  High-temperature,
well-mixed  excess air conditions favor the formation of NOX from both thermal
fixation of molecular nitrogen  and the conversion  of  fuel nitrogen.  The
tradeoff  between NOX and  CO has  been observed  for some  time  for  large
furnaces where optimum conditions  for CO are not compatible with combustion
control  of NOX.   A similar tradeoff exists between NOX and hydrocarbons.
However,  NOX can  be controlled  through post-combustion  controls, as discussed
in Chapter  4.

     Combustion  modification  techniques  such  as air staging and flue gas
recircul ation which are designed  to moderate flame  temperatures have been
shown  to be effective for  NOX control.  For  example,  Volund Technology
employs  flue  gas recirculation in its refractory-lined combustors to control
NOX by lowering  the primary  zone  temperature.   However,  it is presently
unclear whether such control  strategies can be applied  universally or will be
compatible with  control strategies for hydrocarbons.   An alternative to
combustion  modification are  downstream processes which  would not interfere
with optimizing  the combustion zone for control of organic pollutants.  NH3
injection  above  the combustion  zone is currently being installed on new
systems  in  California (e.g.  City  of Commerce).  Other  NOX control strategies
that potentially could be  applied include reburning which may also aid in
hydrocarbon control and selective catalytic  reduction.  More research is
required  to  define  the  impact  of  combustion  control  techniques for
hydrocarbon control on NOX and  NOX control strategies.

     The  emission  of  heavy metals  may also be adversely impacted by design
and operating  practices for hydrocarbon control.  The  partitioning of metals
between  residuals, fly ash and  condensible fume depends strongly on the

                                   8-36

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temperature  and  stoichi ometry  that the metal  bearing  refuse experiences  in
the burning  refuse bed.  Data  was  presented earlier which indicates that
higher temperatures favor  vaporization of metals  and  that stoichi ometry
impacts the  condensible fraction.  Hence, changes  in  temperature to higher
values  and  modification  of the  air distribution might impact  metal
fume emissions  adversely-   The  magnitude of these  effects has not  been
established.  Post-combustion flue gas cleaning may  be  used to reduce levels
of metals  emissions, as discussed  in MWC study entitled  "Flue Gas Cleaning
Technology."

8.8       References

     Del linger,  B.  et  a!.,   "Laboratory Determination of High Temperature
     Decomposition  Behavior of Industrial Organic Materials".  Proceedings  of
     75th  APCA Annual Meeting, New Orleans, 1987.

     Haile, C. L.,  Blair and Stanley.   "Emissions  of PCDD and PCDF from a
     Resource Recovery Municipal  Incinerator."  USEPA/RTP,  1983.

     Haile, Blair,  Lucas and Walker.   "Assessment  of  Emissions of Specific
     Compounds  from a  Resource Recovery Municipal  Refuse  Incinerator."
     Washington, D.C.:  USEPA, Office of  Pesticides and  Toxic Substances, May
     1984.

     Hasselriis, F.   "Minimizing Trace  Organic Emissions  from Combustion  of
     MWS by Use  of Carbon Monoxide Monitors."  Proceedings of the Twelfth
     Biennial Conference, 1986  National Waste Processing  Conference.  ASME,
     New York, N.Y., p. 129,  1986.

     La Fond, R. K.,  J. C.  Kramlich,  W.  R.  Seeker,  and G.  S.  Samuelsen:
     Evaluation of  Continuous Performance Monitoring Techniques for Hazardous
     Waste  Incinerators, Jrnl. Air Pollut.  Control Assoc.  35,  658  (1985)
                                   8-37-

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9.0       HYDROCARBON CONTROL STRATEGY - SUMMARY

     The previously  presented information about  municipal waste combustion
points  to the  conclusion that good  combustion  engineering practices will
minimize  the emission of trace organic pollutants from the combustor/boiler
subsystem of  a  waste-to-energy  system.   Not only will  these practices
minimize emissions, they will  also  contribute to the operability of the  plant
because they  will  reduce corrosion  rates and  could  improve combustion
efficiency.   In  addition, air pollution control  devices may reduce  trace
organic emission levels even lower  (see the volume  entitled "Flue Gas
Cleaning Technology."

     This  report  has  synthesized what are  referred to as "good combustion
practices" from:   current  theories  on  the basic  mechanisms of PCDD/PCDF
formation and  destruction  in combustion systems, information provided by
manufacturers  on the design  of waste-to-energy  systems,  and operating/
emissions  data  from specific  plants.  The practices are designed:

     1.    To limit the formation of hydrocarbons

     2.    To maximize the destruction  of these  same compounds and their
          precursors prior to the exit of the combustor/boiler should they be
          formed

As such,  good  combustion practice  attempts  to  preclude conditions  which
promote formation of PCDDs/PCDFs and their precursors and  to ensure that the
environment experienced by  the gaseous  products  of waste combustion will
destroy both  PCDDs/PCDFs and their precursors if those compounds have been
formed.  These  conditions are:

     •    Mixing of fuel and  air to prevent the existence of long-lived  fuel -
          rich  pockets of combustion products

     •    Sufficiently high temperatures in the  presence of oxygen  for  the
          destruction of hydrocarbon species

                                   9-1 -

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     •     Prevention of quench zones  or  low temperature pathways  that would
          allow partially-reacted fuel  (solid or gaseous)  from  exiting the
          combustion chamber

     The  appropriate design of  a waste-to-energy system alone  is not
sufficient  to  ensure that  the  optimum combustion environment will exist in
practice.   The combustion  system must  also  be  operated continually in an
appropriate  manner and respond  properly to  changes in refuse combustion
characteristics in order  to  maintain proper conditions.  Only by examining
all aspects  of design and  operation of these systems can  the emissions of
trace organics  be minimized.

     This  final  section of the report will review these elements and indicate
the integration of the elements  into a combustion control strategy for trace
organics  such  as  PCDDs and PCDFs.   Finally, the remaining uncertainties and
issues will  be identified along  with recommendations on  research approaches
to address them.

9.1       Combustion Practices for Trace Hydrocarbon Emission Control of
          Municipal Waste Combustors

     Currently,  there are no definitive data on the mechanism of formation or
destruction  of PCDD/PCDF  in  municipal   waste  combustion  facilities.  The
available data indicates,  however, that trace quantities  of PCDD and PCDF
species can  be formed in the process of  burning heterogeneous fuels such as
MSW.  Several  theories propose  that PCDD/PCDF form more  easily from certain
similar  structure precursors such  as  chlorophenols and some species in  lignin
which are either  present  in  the feed or are  themselves formed during the
combustion  process.   However,  once formed,  both the  PCDD/PCDF and their
precursors  can be destroyed  if  exposed  to the appropriate  conditions.  The
intent of good  combustion design  practices for municipal  waste combustors is
not only  to minimize the formation of PCDD/PCDF or their precursors but also
to ensure that escape of  precursors from the combustion zone is minimized
because conditions might exist  downstream which will allow these precursors
to form PCDD/PCDF (e.g. by  fly  ash-catalyzed reactions).   The  appropriate

                                    9-2

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conditions  for the destruction  of  PCDD/PCDF and their precursors are  that the
species experience a given temperature after being mixed with  sufficient
oxygen.  The  challenge for the  designer is  that  no pocket of material can
escape  the  system which has not experienced these  conditions.

     The development of "good  combustion practices" to minimize emissions of
hydrocarbons  from  municipal waste  combustors  involves  three separate
elements:

     •    Design.  Design the  system to satisfy several  criteria which will
         ensure  that temperatures  and the degree of  mixing within the
         combustor are consistent with minimizing formation and  maximizing
         destruction of the trace  organics of concern.

     •    Operati on  Control .   Operate the  system in  a manner which  is
         consistent with the  design goal and provide facility controls  which
         prevent operation outside on established operating envelope.

     •    Verification.   Monitor to ensure  that the  system is continually
         operated in accordance with the design goals.

 If all   three  of these elements  are  satisfied, then the emission  of  trace
organics   from  the combustor  of  a municipal waste combustors  should be
minimized.

     This  project  has  identified the components  of each of these  elements
which  make  up  good  combustion  practices, and  that are  expected to be
important  in  the  control of PCDD/PCDF emissions from  municipal waste
combustors.   In addition, preliminary recommendations have been made on the
values  of  the  individual components.   Identification  of the elements was
based  upon  the  current design practices associated with combustion systems
that have  generally shown low PCDD/PCDF emissions.  The combustion control
elements and  preliminary component value recommendations are summarized in
Table  9-1,  9-2, and 9-3  for  mass  burn combustors, RDF-fired systems, and
starved-air  combustors, respectively.

                                   9-3 -

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      TABLE 9-1.   GOOD COMBUSTION PRACTICES FOR  MINIMIZING  TRACE ORGANIC
                    EMISSIONS  FROM MASS  BURN MUNICIPAL WASTE  COMBUSTORS
Element
 Design
 Operation/
  Control
 Verification
           Component
Temperature at fully mixed
height

Underfire air control
                Overfire air capacity  (not
                an  operating requirement)

                Overfire air injector
                design

                Auxiliary fuel  capacity
Excess air


Turndown restrictions


Start-up procedures


Use of auxiliary fuel


Oxygen in flue gas

CO in flue gas


Furnace temperature
                Adequate  air
                distribution
         Recommmendations
1800°F at fully mixed  height
At least 4 separately  adjustable
plenums.  One each under  the  drying
and burnout zones  and  at  least two
separately adjustable  plenums under
the burning zone

40% of total  air
That required for penetration  and
coverage of furnace cross-section

That required to meet start-up
temperature and 1800°F criteria  under
part-load operations

6-12? oxygen in flue gas  (dry  basis)
80-110% of design - lower limit  may
be extended with verification  tests

On auxiliary fuel to design
temperature

On prolonged high CO or low furnace
temperature

6-12% dry basis

50 ppm on 4 hour average - corrected
to 12% C02

Minimum of 1800°F (mean) at fully
mixed height across furnace

Verification Tests (see text Chapter
8 and 9)
                                        9-4

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      TABLE 9-2.   GOOD  COMBUSTION  PRACTICES  FOR MINIMIZING TRACE  ORGANIC
                   EMISSIONS FROM RDF COMBUSTORS
Element
 Design
 Operation/
  Control
 Verification
           Component
Temperature at fully mixed
height

Underfire air control
                Overfire air capacity
                (not necessary operation)

                Overfire air injector
                design

                Auxiliary fuel  capacity
Excess air


Turndown restrictions


Start-up procedures


Use of auxiliary fuel


Oxygen in flue gas

CO in flue gas


Furnace temperature
                Adequate  air
                distribution
                                                      Recommmendati ons
1800°F at fully mixed height
As required to provide  uniform  bed
burning stoichiometry (see  text)

40$ of total  air
That required for penetration  and
coverage of furnace cross-section

That required to meet start-up
temperature and 1800PF criteria  under
part-load operations

3-9% oxygen in flue gas (dry basis)
80-110? of design - lower limit  may
be extended with verification  tests

On auxiliary fuel to design
temperature

On prolonged high CO or low  furnace
temperature

3-9% dry basis

50 ppm on 4 hour average - corrected
to 12% C02

Minimum of 1800°F (mean) at  fully
mixed height

Verification Tests (see text Chapter
8 and 9)
                                       9-5.

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   TABLE 9-3.   GOOD COMBUSTION PRACTICES FOR MINIMIZING TRACE ORGANIC
                 EMISSIONS  FROM STARVED-AIR COMBUSTORS
     Element
Design
Operation/
 Control
Verification
         Component
Temperature at fully mixed
height

Overfire air capacity

Overfire air injector design


Auxiliary fuel capacity



Excess air


Turndown restrictions



Start-up procedures


Use of auxiliary fuel


Oxygen in flue gas

CO in flue gas


Furnace temperature


Adequate air distribution
     Recommmendations
1800°F at fully  mixed  height
80 percent of total  air

That required for  penetration and
coverage of furnace  cross-section

That required to meet  start-up
temperature and 1800°F criteria
under part-load conditions

6-12% oxygen in flue gas  (dry
basis)

80-110% of design  -  lower limit
may be extended with verification
tests

On auxiliary fuel  to design
temperature

On prolonged high  CO or  low
furnace temperature

6-12% dry basis

50 ppm on 4 hour average  -
corrected to 12% C02

Minimum of 1800°F  at fully mixed
plane (in secondary chamber)

Verification Tests (see  text  Chapter
8 and 9)
                                      9-6

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     It must be emphasized that the values  presented in these  tables are
preliminary  targets and require  verification to ensure their appropriateness.
As such,  they can be viewed as an  initial basis for  developing test plans for
field  evaluation  and pilot-scale  R&D studies.  Further  study may also
determine  that  some of  the elements  are more important  than  others in
ensuring  good combustion.

     As with any  general  principles,  the specific design  of individual
systems  must be  considered in  applying the recommendations in these  tables.
In particular,  several  combustion systems, such  as the mass burn  refractory
technologies of Volund and Enercon/Vicon and the  mass burn rotary  technology
of Westinghouse/O'Connor, incorporate differences from the typical  mass-burn
approach  that make  it  infeasible to  directly  apply  some  of  the
recommendations  in Table  9-1.   For such systems,  parameters such as  "fully
mixed  height"  will  have  to  be defined  based on  technology-specific
engineering analysis rather than  on the general  one meter rule suggested for
traditional  mass burn systems.

     Of course, the  final determinant of the performance of each system is
the measured level  of trace organics emitted from the system.  Whether these
levels indicate  acceptable performance  will  depend  on  emission  levels
established in  the facility's  permits, state standards or guidance,  and any
feder-al  guidance  or regulation  that  may  be established in the future.  If
emission measurements  indicate that the performance  of  a  system needs
improvement, in-furnace  CO profiling can be used to determine appropriate
adjustments to  the air  distribution.   Specifically, a flat in-furnace CO
concentration  indicates  sufficient  air adjustment and furnace  mixing.  A
precise  definition of "flat" considering spatial and temporal  variations is
not yet available and should be  developed as part of future test programs.

9.2       Research Recommendations

     The  "good combustion practices"  defined above were derived  from an
analysis  of the  available information which includes little direct evidence
relating  to  the appropriateness  of values recommended  in Tables 9-1, 9-2, and

                                   9-7

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9-3.  Further work is needed  to better  define and verify these  details for
control of  hydrocarbon  pollutants.  This work  is required  in  three major
areas:

     1.    Definition and verification of  target components and  levels

     2.    Mechanisms of PCDD/PCDF formation and destruction from MSW and RDF
          fired  systems

     3.    Tradeoffs with control of other pollutants

Figure  9-1  summarizes the  flow of information  required to  establish more
specific  design and operating recommendations for municipal  waste combustors
to minimize the emission of trace organic species.  This report  has used an
existing  data base to define "good combustion practices" for municipal waste
combustors.   In order to  define these  practices more specifically, better
(more comprehensive) field  data must be obtained, our understanding of the
mechanisms  of formation and destruction of the species of interest must be
broadened,  and  the tradeoffs  with other pollutants must be established.
Furthermore,  since  it is not possible to  test all municipal waste  combustors,
some generalization procedure must be available to extrapolate  the  data that
is collected on  operating combustors to the total population.

9.2.1      Combustion Control Guideline Definition and Verification

     There is little  information  relating  directly  the emissions of the
pollutants of concern with the design and  operational parameters of  municipal
waste combustors.   Thus, the definition  of "good combustion practices" must
be  considered  preliminary and further definition and  verification is
essential .   The approach  recommended is to examine directly the  impact of
design  and  operating conditions on PCDD/PCDF emissions while  monitoring key
performance  parameters  and  other pollutant  emissions from  representative
municipal  waste  combustors  operating under  a wide  range  of operating
conditions.   To establish  the required data  base these conditions should
include  testing  beyond the normal systems  operating envelope.

                                    9-8-

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      EXISTING
      DATA BASE
                 MECHANISTIC STUDIES
H                                DEFINITION OF
                                BEST COMBUSTION
                                  PRACTICE"
                                                    I
                                             IDENTIFICATION  OF
                                             RESEARCH NEEDS
                                                   i
                              FIELD EVALUATION
                                                    \
OTHER  POLLUTANTS
                                         GENERALIZATION PROCEDURE
                                               DESIGN AND
                                           OPERATING GUIDELINES
Figure 9-1.
Research program to establish design guidelines based upon  "Good
Combustion Practice".

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     The  tests on  operating combustors  should  be run on units which are
representative  of new combustion  systems i.e.  mass-burn  waterwall, RDF-
spreader stoker,  two-staged/starved-air, RDF fluidized bed.  The  selection
criteria  for the units to be tested  are as follows:

     •    Representative of the  units within a particular  combustion system
         class

     0    Capable of flexible operation in order to  generate the information
         on cause  and effect

     •    Availability of engineering design and operating data to help data
         interpretation

     •    Availability of in-furnace and stack sampling  access

     t    Willingness of owner/operator to participate  in  a  program of this
         type

The focus of these  field tests would be to determine the effect of design and
operation  on  both the emissions  of  PCDD/PCDF and other  pollutants.  In
addition,  it  will  be necessary to  establish the  in-furnace conditions  (e.g.
CO profile) which lead to high and low emissions of PCDD/PCDF.  Test series
should define  the baseline  conditions,  and the  impact  of furnace load,
combustion air  distribution,  firing  auxiliary fuel, excess air levels,
startup procedures and,  if  possible, fuel type.   Field tests are  expensive
and their  utility will be  expanded if procedures  are  available for on line
evaluation of  combustion  performance  factors.    Measurements will not be
possible over  the total  operating  range.  Spatial  and  temporal fluctuations
may  make the  data difficult  to  interpret.  Thus,  it  is  essential  that
engineering analysis tools  be available to interpret and rationalize the
data.
                                  9-10

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9-2.2     Mechanisms of PCDD/PCDF Formation and Destruction

     The combustion  control strategy  would be more valid  if  it were based
upon  a  better knowledge  of  the factors  controlling the formation  and
destruction  of PCDD/PCDF.   Several  chemical mechanisms have  been proposed
(see Chapter 4) but many  fundamental  physical/chemical issues remain that
should  be  addressed before a  controlling mechanism can be  accepted.  These
issues  include  the  following:

     t     Identify  conditions which favor  formation of PCDD/PCDF from refuse
          combustion

     •    Define the impact  of refuse  properties on PCDD/PCDF formation

     •    Define the temperature necessary to destroy PCDD/PCDF  species as  a
          function  of gas-phase environment

     •    Examine  the  role of  downstream  condensation and  formation
          mechanisms

Both laboratory scale  gas  phase kinetics  studies and  bench scale  refuse
burning studies would be  suitable for  addressing these issues.

9.2.3     Tradeoffs in Other  Pollutants

     As indicated  in Chapter  4, the combustion control strategy for PCDD/PCDF
may  have  detrimental impacts on other pollutants.  For example,  maximizing
temperature  and improving mixing may  promote NOX  formation.   Also,  improving
the  air distribution can  change conditions in the burning refuse bed which
can  impact the vaporization of heavy metals.   These changes could result  in
the  concentration metals  in the small  particulate fume that is  not  easily
controlled by air  pollution  control devices.

     For heavy  metals, the key issue is  the fate of metals in the refuse  as a
function  of  the combustion conditions.   The  research  needs  include  a  data

                                   9-T1

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base on  the  fate  of metals, control  of metals  by  particulate control  devices
the Teachability of metals from residuals,  and predictive methodologies  for
metal partitioning.   Carefully controlled bench-scale studies on  burning
refuse  could provide  a significant contribution  to  our understanding of  the
fate of  metals during combustion.   In  the area of NOX, development of  NOX
control  schemes  that  are  not detrimental  to PCDD/PCDF control are necessary
if more  stringent  NOX levels are enforced by local authorities.
                                     9-12

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