United States
Environmental
Protection Agency
Of flee of Solid Waste    Off Ice of Air      Off Ice of Research   EPA/530-Sw-87.02ici
and Emergency Response  and Radiation     and Development    June 1»7
Washington, DC 20460   Washington, DC 20460  Washington, DC 20460
&EPA
Municipal Waste
Combustion Study
                 Flue Gas Cleaning Technology
                          U.S. Environmental Protection Ag--:.'-•£

                          region 5, Library (j.L-16)
                          230 S. Dearborn 3U-r=.r., Room 167Q

                          Chicago, IL  6i'G")4

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                                                 EPA/530-SW-87-021d
                                                          June 1987
     MUNICIPAL WASTE COMBUSTION STUDY:
        FLUE GAS CLEANING TECHNOLOGY
                Prepared by:
             Charles B. Sedman
                    and
              Theodore G. Brna
   Air and Energy Engineering Laboratory
    U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
              Prepared for:
   U.S. Environmental Protection Agency
          Office of Solid Waste
           Washington. DC 20460

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                            EPA REVIEW NOTICE
This document has been approved for publication by the Office of Solid
Waste, U.S. Environmental Protection Agency.  Approval does not signify
that the contents necessarily reflect the view and policies of the Envir-
onmental Protection Agency, nor does the mention of trade names or com-
mercial products constitute endorsement or recommendation for use.

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                             TABLE OF CONTENTS
 EPA  Review Notice	      ii
 List of Figures	      iv
 List of Tables	       v
 Acknowl edgements	     vi i

 CHAPTER 1     INTRODUCTION	     1-1
 CHAPTER 2     PARTICIPATE MATTER CONTROL	     2-1
              2.1  Electrostatic Precipitators	     2-1
                  2.1.1  Principles of ESP Design	     2-1
                  2.1.2  Factors Affecting ESP Performance	     2-3
                  2.1.3  Design Considerations	     2-6
                  2.1.4  Performance of ESPs	     2-7
              2.2  Fabric Filters	     2-7
                  2.2.1  Theory and Principles of Filtration	     2-7
                  2.2.2  Gas Stream Factors that Affect Fabric Filter
                         Design and Operation	    2-12
                  2.2.3  Fabric Filter Systems	    2-14
              2.3  Wet Scrubbers	    2-19
 CHAPTER 3     GASEOUS EMISSION CONTROLS	     3-1
              3.1  Acid Gas Scrubbers (HC1 and HF only)	     3-1
              3.2  Post Combustion NOX Control	     3-1
                  3.2.1  Selective Catalytic Reduction (SCR)	     3-1
                  3.2.2  Wet NOX Removal  Processes	     3-6
CHAPTER 4    MULTIPOLLUTANT CONTROL SYSTEMS	     4-1
              4.1  Wet Scrubbing	     4-1
                  4.1.1  Wet Scrubbing for Acid Gases (HC1,  HF, and
                         S02)	     4-1
                  4.1.2  Wet Scrubbing for Acid Gases and NOX	     4-3
                  4.1.3  Performance of Wet Scrubbers	     4-4
             4.2  Dry and Semi-Dry Scrubbing	     4-5
                  4.2.1  Dry Injection Processes	     4-5
                  4.2.2  Semi-Dry Scrubbing	    4-13
             4.3  Combination Scrubbers	    4-16
                  4.3.1  Semi-Dry/Dry Scrubbing	    4-16
                  4.3.2  Semi-Dry/Wet Scrubbing	    4-20
CHAPTER 5    EFFECTIVENESS OF FLUE GAS CLEANING METHODS	     5-1
             5.1   Particulate Matter Control	     5-1
             5.2  Acid Gas Control	     5-1
             5.3  Post Combustion NOX Control	     5-3
             5.4  Post Combustion Organic Pollutant Control	     5-3
             5.5  Heavy Metals Control	     5-4
CHAPTER 6    OPERATION AND MAINTENANCE OF FLUE GAS CLEANING  SYSTEM	     6-1
             6.1   Electrostatic Precipitators	     6-1
             6.2  Fabric Filters	     6-5
             6.3  Scrubbers	     6-5
                  6.3.1  Lime/Limestone Wet Scrubbing	    6-10
                  6.3.2  Semi-Dry Scrubbing	    6-15
REFERENCES
                                      m

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LIST OF FIGURES
Figure
2-1
2-2
2-3
2-4

2-5
2-6
2-7
3-1
3-2
4-1
4-2
4-3
4-4
4-5
4-6
5-1

Principles of the electrostatic precipitation process 	
Diagram of a two-field, weighted-wire ESP 	
PM emissions vs. SCA for best-fit equations 	
Municipal waste incinerator equipped with dry flue gas

Small shaker-type fabric filter 	
Example of a large reverse-air fabric filter 	
Example of a small pulse-jet fabric filter 	

SCR options for municipal incinerators 	


Circulating fluid bed absorption (dry) process 	
Spray absorption (semi-dry) process 	
Semi -dry/dry scrubber 	

Saturation points of metal and metal compounds 	
Page
2-2
2-8
2-9

2-11
2-15
2-16
2-18
3-2
3-5
4-2
4-6
4-12
4-12
4-17
4-21
5-6
         1v

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                              LIST  OF  TABLES
Table                                                                 Page

 2-1      Partial  list of MWC/ESP applications	      2-3
 2-2      Plant test data	     2-10
 3-1      SCR plant designs  for  sludge Incinerator flue gas	      3-4
 4-1      Wet scrubber outlet  emissions	      4-4
 4-2      Inlet pollutant concentration and pollutant control with
          a  lime dry injection process	      4-7
 4-3      Summary  of key  operating  parameters	      4-8
 4-4      PCDD concentrations  (ng/Nm3  @ 82 02) in flue gas and
          efficiency of removal	      4-9
 4-5      PCDF concentrations  (ng/Nm3  @ 82 02) in flue gas and
          efficiency of removal	      4-9
 4-6      Percent  removal  of other  organics	     4-10
 4-7      Inlet/outlet metal concentrations (yg/Nm3 @ 82 02)	     4-10
 4-8      Hydrogen  chloride concentrations (@ 82 02) and removal
          efficiencies	     4-11
 4-9      Sulfur dioxide  concentrations (@ 82 02) and removal
          efficiencies	     4-11
 4-10      Outlet emission  guarantees for municipal waste
          combustors	     4-14
 4-11      Acid gas  emissions from municipal waste combustor	     4-15
 4-12      Trace  heavy  metals control by pilot plant FGC systems...     4-15
 4-13      Pollutant emissions  and control by semi-dry/dry
          scrubbing	     4-16
 4-14      Summary of emission  results  for tests of September -
          October,  1986 at the Marion  County, OR waste-to-energy
          facility	     4-18
 4-15      Permitted  emissions  from  the Marion County, OR waste-to-
          energy facility	     4-19

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                         LIST OF TABLES (cont'd)
Table
 4-16     Expected pollutant emissions from MWC equipped with
          semi -dry/wet scrubbi ng	       4-20
 5-1      Effectiveness of acid gas controls (2 Removal)	       5-2
 5-2      Spray dryer control of selected organic pollutants	       5-4
 6-1      Inspections for ESP	       6-2
 6-2      Typical  maintenance inspection schedule for a  fabric
              fi 1 ter system	       6-6
 6-3      Routine  inspection data for reverse air fabric filters..       6-8
 6-4      Routine  inspection data for pulse-jet fabric filters....       6-9
                                       vi

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                             ACKNOWLEDGEMENTS


     The authors express their appreciation to the numerous persons who
provided Information for this report.  Especially noteworthy were the con-
tributions of Claus Jorgensen (Acurex Corporation) regarding European
technology and test data; James D. Kilgroe and Julian W.  Jones (Air and
Energy Engineering Research Laboratory) for their constructive reviews of
the initial draft report; Abe Flnkelstein and Raymond Klicius (Environment
Canada) for NITEP program data and results; James Crowder and Peter Schindler
(EPA's Office of A1r Quality and Planning Standards)  regarding emissions
data and results; and Steve Green (EPA's Office of Solid  Waste) and
Glynda Wllkins (Radian Corporation) for their assistance  in revising and
finalizing the report.  Many useful comments were received from Industrial,
environmental, and other Interested organizations in  response to the
release of the draft report for public comment.  Most comments have been
addressed in the revised report as permitted by the data  and time available.
Although too numerous to mention the respondents here, their comments
are gratefully acknowledged and have, hopefully, resulted in a more com-
prehensive and soundly based report.
                                    vii

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                             1.0  INTRODUCTION


     This report presents the results of a recent study of flue gas cleaning
 technology applied to municipal waste combustors.  The Information presented
 here was developed during a comprehensive, Integrated study of municipal  waste
 combustion.  An overview of the findings of this study 1s Included In the
 Report  to Congress on Municipal Waste Combustion (EPA/530-SW-87-021a).   Other
 technical volumes Issued as part of the Municipal Waste Combustion Study
 Include:

     o  Emission Data Base for Municipal Waste Combustors
        (EPA/530-SW-87-021b)

     o  Combustion Control of Organic Emissions (EPA/530-SW-87-021c)

     o  Cost of Flue Gas Cleaning Devices (EPA/530-SW-87-021e)

     o  Sampling and Analysis of Municipal Waste Combustors
        (EPA/530-SW-87-021f)

     o  Assessment of Health Risks Associated with Exposure to Municipal
        Waste Combustion Emissions (EPA/530-SW-87-021g)

     o  Characterization of the Municipal Waste Combustion Industry
        (EPA/530-SW-87-021h)

     o  Recycling of Solid Waste (EPA/530-SW-87-0211)

     Until recently post combustion emission control technology was applied
 to waste incinerators strictly for particulate matter removal.  Early install-
 ations used either electrostatic precipitators (ESPs) or wet scrubbers  to
 meet local and state emission codes.  With the advent of 40CFR60 Subpart E,
 the New Source Performance Standards for municipal incinerators in the  early
 1970's, the ESP became the dominant choice.  This is reflected by recent
 studies which Indicate that 75 percent of conventional incinerators in  the
 United States use ESPs and 20 percent wet scrubbers.1

     Because several foreign governments are regulating pollutants other
 than particulate matter (notably Japan and some in Western Europe), many
advanced concepts for scrubbing acid gases—sulfur dioxide (503), hydro-
chloric acid (HC1), and hydrofluoric acid (HF)—have been commercially
applied.  State and local regulations 1n the U.S. are requiring the control
of these and other pollutants in new municipal waste combustion (MWC)
facilities and those now being planned.  These control systems Include  wet,
dry, and semi-dry scrubbers and fabric filters (FFs) or ESPs.  More recent
concern over trace metals emissions, organics such as dioxins and furans,
and nitrogen oxides have reinforced the selection of scrubbers as
well as post-combustion NOX control.
                                     1-1

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      The combustion  process  is  important in controlling organics, such as
 dioxins and furans.   Post  combustion emission control technology is not a
 substitute for good  combustion, although it may remove some pollutants
 resulting from poor  combustion.   Its use is aimed at controlling acid gases
 and particulate matter  to  permitted levels, but it may also effect some
 control of volatile  and semi-volatile organics and trace metals while
 achieving its  primary aims.   Post combustion emission control technology
 also offers an alternative for  controlling NOX downstream of the combustor
 or boiler if such  control  is  required.

      The major focus of this  report is on the use of scrubbers to control
 pollutants in  flue gas.  Scrubbers have been classified as c ther wet or
 dry depending  on whether the  cleaned flue gas leaving the scrubbing system
 is saturated or not.  With this classification, a wet scrubbing system
 emits a saturated  flue  gas (generally with water vapor), while a dry scrubber
 has a gaseous  effluent  which  is unsaturated.  In the U.S., S0£ scrubbers are
 called wet and dry when  the solids from the scrubbers are wet and dry,
 respectively.   Scrubber terminology in other countries often uses "semi-dry"
 or "wet/dry" and "dry"  scrubber.  The former usually applies to a spray
 dryer and fabric filter  or ESP  system in which the sorbent enters the spray
 dryer as  a slurry  or  solution, and the cleaned flue gas leaves the particu-
 late  (dust)  collector unsaturated.  A "dry" scrubber, as often used in the
 same  countries,  refers  to  a dry powdered sorbent being injected into-the
 flue  gas  duct  or a reactor upstream of the particulate collector (either a
 baghouse  or  ESP),  with  an  unsaturated, cleaned flue gas leaving this collec-
 tor.   Because  of references to scrubbing systems developed or being developed
 outside  the  U.S. in  this report,  semi-dry and wet/dry scrubbing in this
 report means that  a dirty, hot flue gas from a boiler (steam generator) or
 a  following  air  preheater  contacts a wet reagent in a spray dryer, but the
 gas stream leaves  the spray dryer unsaturated before entering the particulate
 collector  of the flue gas  cleaning system.   It Is also noted that in the U.S.
 a  scrubbing  system with either a  spray dryer or a dry-injected sorbent into
 the flue  gas upstream of the particulate collector is commonly called a "dry"
 scrubber.

      In-furnace  NOX control (thermal DeNOx and flue gas recirculation) is
 not discussed  in this report as it 1s more appropriate to a companion report
 In the Municipal Waste Combustion series.   However, post combustion NO
 control [selective catalytic reduction (SCR) and wet NOX scrubbing] is
 addressed  In Chapter 3 of  this report.

     The following chapters will describe briefly each generic control sys-
 tem, design and operating  considerations, and control effectiveness on selec-
 ted pollutants.  Control systems which reflect the most prevalent current
 U.S. practice—that of particulate matter control—are discussed initially,
 followed by gaseous controls, and finally,  the more advanced multipollutant
 control systems.  Chapter 5 summarizes the  effectiveness of the control
 systems discussed in the preceding three chapters for particulate matter,
 selected acid gas,  selected organic pollutants,  and selected trace heavy
metals.  Since  the  data from commercial  municipal  solid waste incinerators
are limited, pilot  plant data were also  considered in reporting the control
effectiveness of some pollutants.   The final  chapter, Chapter 6, addresses
 the operation and maintenance of flue  gas cleaning systems.
                                        1-2

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                       2.0  PARTICULATE MATTER CONTROL


 2.1         Electrostatic Precipitators3,4

 2.1.1       Principles of ESP Design

            Review of ESP Fundamentals.  The basic steps of the electro-
 static precipitation process are:(IT development of a current of negative
 molecular ions, from a high voltage corona discharge, that is used to charge
 dust particles in the gas stream; (2) development of an electric field in
 the gas space between the high voltage discharge electrode and the collection
 electrode that propels the negatively charged ions and particulate matter
 toward the collection electrode; and (3) removal of the collected particulate
 matter into hoppers by use of a rapping mechanism.  The basic principles of
 electrostatic precipitation are illustrated in Figure 2-1.

     The electrostatic precipitation process occurs within an enclosed chamber.
 A high voltage transformer and a rectifier converts the alternating current
 (a.c.) electrical power input into direct current (d.c.).  Suspended within
 the chamber are the grounded collection electrodes (metal plates) which are
 connected to the grounded steel framework of the supporting structure.
 Suspended between the collection plates are the high voltage discharge (wire)
 electrodes (corona electrodes) which are insulated from ground and negatively
 charged with voltages ranging from 20 kV to 100 kV.  The large difference in
 voltage between the discharge electrodes and collection electrodes and the
 interelectrode space charge of ions and charged particles create the electric
 field that drives the ions and particles toward the collection electrodes.
The particles may travel  some distance through the ESP before they are
 collected or they may be  collected, reentrained, and collected again.

          The last step of the process involves the removal of the dust from
 the collection electrodes.  In dry ESPs, this is accomplished by periodic
 striking of the collection and discharge electrode with a rapping device
 that is activated by a solenoid, air pressure, or gravity after release of
 a magnetic field, or through a series of rotating cams, hammers, or vibrators.
The dust is collected in  hoppers and then conveyed to storage or disposal.
 In wet ESPs, the collected dust is removed by an intermittent or continuous
 stream of water that flows down over the collection electrodes and into a
 receiving sump.

            History of MHC/ESP Applications.  Lurgi installed the first
electrostatic precipitator on a refuse incinerator in Zurich in 1927.  Three
more installations followed in Hamburg in 1930.  By 1968, this manufacturer
could claim 40 MWC/ESP installations in Europe and Japan, either in operation
or under construction. Measured collection efficiencies of seven precipita-
tors started up between 1961 and 1967 ranged from 98.9% to 99.92.

            A recent list of MWC/ESP installations tested between 1970 and
1985 is given in Table 2-1.5  The trend of lower emission levels in the
newer installations is evident in this table.  The Munich installation
                                     2-1

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    EIECTIOOE AT
            nuatirr
Wto
ocucro
rurittE
               OIJOU*GE
               »T ncaATin
                   7™\w ^ \fr^'^v  v ITi  w   \
                                                » •

                                                A\
            UNCHUPtD

            HXTIOIS
TO COLLCCTOt aiCTXOOE

MO rORHIKC OUST UTEJ
                                                              M104
Figure 2-1.   Principles  of the electrostatic precipitation process.
                                  2-2

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(Munich North, built in 1984) has only a two-field ESP  following  a  cyclone
and spray dryer (for acid gas control).  Nevertheless,  the  particulate
emissions are limited to 0.0104 gr/dscf by the 99.52 collection efficiency.
In addition, this installation achieves average removal  efficiencies  of 952
for HC1, 762 for S02, and 96.32 to 98.4% for heavy metals  in  particulate
form.


             TABLE 2-1.  PARTIAL LIST OF MWC/ESP APPLICATIONS5
Plant

S. W. Brooklyn, NY
South Shore, Brooklyn, NY
Dade, FL
Braintree, MA
Montreal, Canada
Chicago, IL
Washington, DC
Harrisburg, PA
Quebec City, Canada
Nashville, TN
Saugus, MA
Bamberg, West Germany
Munich, West Germany
Lausanne, Switzerland
Baltimore, MD
Westchester, NY
Avesta, Sweden
                        Sample/
                        Report
                         Date

                        1970-71
                        1970
                        1970
                        1970/78
                        1970/71
                        1971/75
                        1972
                        1974
                        1974
                        1978
                        1983
                        1983
                        1984
                        1984
                        1984
                        1985
                        1985
Pollution
 Control
Equipment
                                                            Particulate
                                                             Emissions
                                                              gr/dscf
ESP
ESP
ESP
ESP
ESP
ESP
ESP
ESP
ESP
ESP
ESP
Wet Scr/ESP
Dry Scr/ESP
Elec Scr/ESP
4 Field ESP
3 Field ESP
Cond Scr/ESP
0.114/0.146
0.056
0.027
0.108/0.083
0.013/0.08
0.025/0.03
0.0548
0.06
0.095
0.018
0.025
0.009
0.0104
0.014
0.01
0.016
0.0115
2.1.2
Factors Affecting ESP Performance
            Several important properties of the gas stream and particulate
matter determine how well an ESP will collect a given dust at a given dust
loading.  They include particle size distribution, gas flow rate, and partic-
ulate resistivity, which is influenced by the mineral composition and density
of the particulate matter and the process temperature.  These factors can
also affect the corrosiveness of the dust and gas and the ability to remove
the dust from the plates and wires.  Brief discussions of these properties
are given in the following paragraphs.
            Dust Particle Size
                     Distribution.
                                 'in
 ESP
 the
            	       performance is very sensi-
tive to the distribution of particle sizes in the combustion gas because
of variations in the effectiveness of particle charging by molecular ions.
Particle charging is least effective in the range from 0.1 to 1.0  um.   The
particulate penetration through an ESP (loss to the atmosphere) is typically a
                                       2-3

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maximum somewhere in this range.  On the other hand, the penetration of any
particles  larger than a few micrometers is due almost entirely to reentrain-
ment.  Thus, the variation in collection efficiency with particle size is
typically  much greater for an ESP than for a baghouse.

            Dust Electrical Resistivity.  Electrical current from the high
voltage corona discharge in an ESP passes through the dust layer on the
collection plate to reach electrical ground.  The product of the average
plate  current density (A/cm2) and the bulk electrical resistivity of the
collected  dust (ohm-cm) is a measure of the average electric field (V/cm)
within the collected dust layer.  When that average electric field becomes
too  large  (greater than about 5 to 10 kV/cm), electrical breakdown occurs
within voids in the dust layer.  That breakdown leads to sparking and/or
back corona that place the practical operating limit on the useful electri-
cal power  input to an ESP.  Thus, the value of dust resistivity bears upon
the useful electrification of an ESP.  The electrification is usually limited
below  desirable levels when the dust resistivity exceeds about 6 x lO^O ohm-cm.

     A dust resistivity that is too low is also undesirable.  The lower limit
is not well defined, but it is generally believed to lie in the range of 10?
to 10^ ohm-cm.  Low resistivity dust is easily precipitated, but it also dis-
charges quickly on the collecting plate.  The electrical force holding the
collected dust layer (against the scouring effect of the gas stream) dimin-
ishes, and reentrainment problems become severe.  In precipitators serving
municipal waste combustors, reentrainment problems may be associated with
carbonaceous particles resulting from incomplete combustion.

            Gas Flow Parameters.  Uniformity of gas velocity and gas tempera-
ture is essential to the optimum performance of a precipitator.  Large vari-
ations in gas temperature over the face of a precipitator can result in large
variations in dust resistivity.  The electrical operation of one entire pre-
cipitator field energized by a high voltage power supply will be limited by
that portion of the field where the collected dust has the highest resistivity.

     The average gas velocity through a precipitator determines the treatment
time for charging and collecting dust particles.  However, regions of low gas
velocity do not compensate for other regions of high gas velocity.  Optimum
ESP collection efficiency is achieved with uniform gas velocity.  The standard
deviation of a matrix of gas velocity measurements over the face of a precip-
itator divided by the average value usually does not exceed 0.15 in a modern
precipitator.  Too low a gas velocity leads to dust deposits in the duct
upstream of the ESP.  Too high a gas velocity leads to scouring of collected
ash from the plates of the ESP and high reentrainment.  The optimum average
gas velocity usually lies in the range of 3 to 6 ft/s. Considering that the
ash from municipal  waste combustion may have low electrical resistivity, with
the accompanying tendency to reentrain particles from the collecting plates,
the average gas velocity should lie at the lower end of this range.

     The total  volume gas flow (cfm) through an ESP is related to the designed
average gas velocity by the designed face area of the ESP.  Furthermore, the
total volume gas flow, compared with the size of the ESP, is mathematically
related to the collection efficiency of the ESP.  The specific collection


                                      2-4

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plate area (SCA, defined as the ratio of the total plate area to the volume
gas flow, ft^/lOOO cfm) appears in the exponent of the Deutsch equation which
gives the theoretical collection efficiency for dust particles of a specific
size.

            Combustion Parameters.  Stable and optimized ESP performance
depends on stable and complete fuel combustion.  However, control of the
combustion process 1s particularly difficult with municipal waste as the
fuel.  The fuel is usually wet.  Depending on the size and time of year,
moisture content can vary from 20 to 60 percent by weight.  Food wastes,
grass and tree clippings, and other wet wastes add to the moisture content
of  the material.  Large quantities of paper products and plastics help off-
set this problem somewhat.  The effect of a high moisture content is to
lower the heating value of the material charged.  At a fixed charging rate,
this in turn lowers the overall heat release rate and the combustion temper-
ature in the furnace.

     Another problem is the substantial quantity of noncombustible materials
that may be charged to the incinerator.  These materials include cans, other
metallic materials, rocks, dirt, etc.  Although paper products and plastics
tend to have low ash contents, the overall "ash" content of the charged
material is typically 20 to 30 percent by weight.  This high ash content also
contributes to a lower heating value.  Considering noncombustible and moisture
effects, heating values varying from 3000 to 6000 BTU/lb are not unusual.

     Another problem is the variation in size of fuel.  Good combustion
relies on even distribution of properly sized material on the grate.  The
underfire air will follow the path of least resistance.  If the distribution
of material on the grate is uneven, the gas will channel through the path of
least resistance and the remainder of the fuel bed will be "starved" for air.
In some instances the time/temperature relationship in the furnace may not
be sufficient to produce complete combustion.  Incomplete combustion can
result in ash that is high in carbon content and is easily reentrained from
the ESP collection plates.  Incomplete combustion can also result in con-
densed fumes of submicrometer particles that are more difficult to collect
and more likely to create undesirable visible emissions.

     Finally, the excess air in municipal waste combustion affects ESP per-
formance.  Increasing the amount of air in excess of stoichiometric require-
ments generally Improves combustion and raises combustion temperatures.  At
some point, however, no real increase in combustion efficiency (or conver-
sion of hydrocarbons to C02 and H20) results, and increasing the quantity
of combustion air actually decreases the flame temperature (because both
the combustion products and the excess air must be heated).  This can also
increase the flue gas volume leaving the incinerator.  Even after the gases
are cooled in an evaporative spray chamber or passed through a heat exchanger,
the result is a higher gas volume flow to the ESP, which reduces the SCA
and treatment time and increases the average gas velocity and the opportunity
for dust reentrainment.  Another report of this series^ provides a detailed
description of combustion processes.
                                      2-5

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 2.1.3       Design  Considerations

             The design  of  a  precipitator  to clean the flue gas from municipal
 waste combustion presents  no new problems  that  have not already been faced
 in applications to  coal  combustion,  cement kilns, and metallurgical refining.
 In fact,  a  great deal of ESP operating experience on hundreds of incinerator
 applications worldwide  has been acquired  by ESP manufacturers in Europe,
 Japan,  and  the  United States.  Several considerations of special emphasis
 in the  ESP  design process  are  reviewed briefly  in the following paragraphs,
 assuming  no acid gas controls  on the flue gas entering the ESP.

             Combustion  Efficiency.   The main source of difficulty in ESP appli-
 cations to  municipal waste combustors, especially mass burn units, is the var-
 iability  of waste material fired as  fuel.  Variations in the fuel and in the
 excess  air  supplied can  cause  large  variations  in the combustion efficiency
 and in  critical  parameters of  combustion  flue gas entering the ESP—gas
 volume  flow,  gas temperature,  and dust particle size distribution.  The
 precipitator design must take  into account worst case conditions.  This is
 a  familiar  problem  in electric generating plants firing a variety of coals.
 In municipal  incinerators, it  is essential to maintain and monitor good
 combustion.

             Acid Corrosion.  The acid dew point depends on the (variable)
 concentrations  of moisture and acid  vapors (S02, HC1) in the combustion
 flue  gas.   Corrosion problems  can be expected when the temperature in the
 ESP falls below  350*F without  acid gas control ahead of the ESP.  Heated
 purge air must  be supplied to  the high voltage bushings to keep them dry,
 and special  precautions  are  required during startup and shutdown.  Use of
 corrosion resistant materials, proper insulation, prevention of air inleakage,
 purging acid  gas  directly  after shutdown, and use of an indirect heating
 system  during shutdown can extend the life of the ESP.  Prevention of acid
 corrosion is  a  familiar  problem in electric generating plants firing high
 sulfur  coal  and  in  pulp  and  paper mills.

             Fine  Particle  Collection.  Particular attention must be paid to
 the collection  of submicrometer particles because these particles tend to
 be enriched  in  heavy metals.   Combustion variability complicates this design
 consideration because incomplete combustion tends to partition metals to
 the bottom ash.    As combustion improves, submicrometer fumes can be controlled
 and finer particles can  be generated.  Experience with modern utility fly
 ash precipitators has demonstrated that an ESP which is designed for high
 total mass collection efficiency (99.9%) will  also have satisfactory collec-
 tion of fine particles although lower than for large particles and the total
 particulate matter.

            Dust  Reentrainment.  Higher carbon content in the dust decreases
 the dust electrical  resistivity and  leads to increased dust reentrainment
 from the collecting  plates.  A uniform gas flow distribution, without large-
 scale turbulence, will  minimize reentrainment problems.  Proper design of the
aspect ratio (length or depth of the ESP to height of the plate)  and the gas
flow baffling in  the ESP are  essential  to uniform flow.  After the precipitator
 is brought on line,  the electrode rapping schedule and rapping intensity must


                                       2-6

-------
 be gradually  adjusted  (over  a  period of many weeks) to minimize emissions from
 rapping reentralnment.   Figures  2-2 depicts a typical ESP for existing municipal
 incinerators  in the  U.S.

 2.1.4      Performance  of ESPs6

            ESP control  data are available for total particulate matter, heavy
 metals, dioxins,  and acid gases  (see Reference 7).  The particulate matter data
 have been analyzed,  fitted to  a modified Deutsch-Anderson equation, and corre-
 lated with specific  collection area (SCA).  Figure 2-4 Illustrates this corre-
 lation for solid waste-fired industrial boilers.  The curves shown are based on
 emission data and may not reflect what vendors would guarantee for a given SCA.
 Table 2-2 shows the  data used  to develop the 1986 correlation, reflecting data
 for high SCA  ESP  applications  which were obtained after 1982.

      Very limited performance  data on control of heavy metals, dioxins, and
 add gases by ESPs alone are available.  Based on data from the Chicago North-
 west and Andover,  MA, tests, an ESP without an appreciable drop in flue gas
 temperature offers little or no control of volatile organics.  Data in
 Reference 6 show  that control  of arsenic, chromium, and lead emissions are
 less than the total  particulate control, suggesting that heavy metals are
 somewhat concentrated in the very fine particles.  The limited ESP data
 show no appreciable  reduction  in mercury emissions as mercury is mainly a
 vapor at normal operating temperatures.

 2.2         Fabric Filters

            Fabric filters have not generally been applied directly to flue
 gas  from municipal Incinerators, but rather as a sorbent collector and secondary
 reactor for dry and  semi-dry scrubbers as discussed later in Chapter 4.  The
 basic  theory  and  operation of  fabric filtration 1s presented here as background
 for  Chapter 4;  no  performance  data will be discussed in this section.

      Three  reasons that fabric filters have not been applied to Incinerator
 flue gas  are:   (1) attack by acid gases upon fabric, (2) fabric blinding by
 "sticky"  particles, and (3) baghouse fires caused by unstable combustion and
 carryover of  sparks  into the flue.  Electrostatic precipitators and wet scrub-
 bers  have  been  somewhat more forgiving to these phenomena and have generally
 been  applied.  However, with upstream scrubbing of acid gases and sorbent
 accumulation  on fabric materials, all  the above concerns are addressed and
 fabric  filters become a very attractive choice for particulate control as
 well as  control of other pollutants.

     Figure 2.3 shows a municipal solid waste incinerator facility with a
 flue gas  cleaning  system having a fabric filter although an ESP may be used
 In lieu of  the baghouse.

 2.2.1       Theory and Principles of Filtration8

            The five basic  mechanisms  for particulate collection by fibers
 In a fabric filter are:  (1)  Inertia! impaction,  (2) Brownlan diffusion, (3)
direct  interception,  (4)  electrostatic attraction,  and (5) gravitational
 settling.

                                      2-7

-------
Figure 2-2. Diagram of a two-field, weighted-wire ESP. 9
            (Courtesy of Western Precipitation.)
                              2-8

-------
    0.8-
    0.7-
    0.6-
s
®  C.5-
i
    0.4-
jo
    0.3-
     0.2-
     0.1-
              i     (
             SO   100
 I
200
                                                                             ,1962 correlation
                                                                             : 1986 correlation
      300

SCArn'rUDOOectm)
 I
400
 I
500
 I
600
           Figure  2-3.  PM emissions vs. SCA for best-fH  equations.
                                             2-9

-------
TABLE 2-2.  PLANT TEST DATA^
     (1986 Correlation)
PM Emissions, lb/106 Btu
0.266
0.167
0.167
0.092
0.082
0.083
0.071
0.099
0.040
0.059
0.050
0.053
0.036
SCA, ft2/ 1,000 cfm
138
145
134
240
246
260
297
288
343
347
630
616
474
            2-10

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IX)
I
                   A  TYPICAL  OGDEN  MARTIN  FACILITY
         1 Tipping Floor
         2 Re I use Holding Pit
         3 Feed Crane
         4 Feed Chute
         5 Martin Stoker Orate
         6 Combustion Air Fan
         7 Martin Residue Discharger and Handling System
         8 Combustion Chamber
         9 Radiant Zone (furnace)
         10 Convection Zone
         11 Superheater
12 Economizer
13. Dry Gas Scrubber
14. Baghouse or Electrostatic Preclpllalor
15 Fly Ash Handling System
16 Induced Draft Fan
17 Stack
16
              Figure 2.4.  Municipal waste  incinerator equipped with a dry flue gas cleaning system.
                          (Courtesy of Ogden Martin  Systems,  Inc.)

-------
 Inertial impaction is the dominant collection mechanism within  the  dust  cake.
 The forward motion of the particles results in impaction on  fibers  or  on
 already deposited particles.   Although impaction  increases with higher gas
 velocities, these high velocities reduce the effectiveness of Brownian
 diffusion.  Increasing the fabric and dust cake porosity by  use of  a less
 dense fabric or more frequent cleaning also reduces  diffusional  deposition.
 Except at low gas velocities, gravity settling of  particles  as  a method  of
 collection is usually assumed to be negligible.  Electrostatic  forces  may
 affect collection because of  the difference in electrical charge between
 the particles and the filter; however, the impact  on commercial-scale  equip-
 ment is not fully understood.  Sieving,  or particle  filtering,  occurs  when
 the particle is too large to  pass through  the fabric matrix.  It is not  a
 major mechanism for collecting particulate.   The combination of all these
 particle collection mechanisms results in  the high efficiency removal  of
 particulate matter.

             Dust Accumulation on Fabrics.   The fabric filtration process
 or the accumulation of particulate  on a  new fabric surface occurs in three
 phases:   (1)  early dust bridging of the  fabric substrate, (2) subsurface
 dust cake development,  and (3)  surface dust cake development.   The  fabric
 used in  a fabric filter is typically  a woven or felted  material, which forms
 the base on which particulate emissions  are  collected.   Woven fabrics  con-
 sist of  parallel  row   of yarns  in a square array.  The  open  spaces  between
 adjacent yarns  are occupied by  projecting  fibers called  fibrils.  Felted
 fabrics  are constructed of close, randomly intertwined  fabrics  that are
 compacted to  provide  fabric strength.

      In  the first phase,  particles  entering  a  new  fabric initially  contact
 the individual  fibers  and fibrils and  are  collected  by  the filtration mech-
 anisms.   These  deposited  particles, which  are  essentially lodged within  the
 fabric structure,  promote the  capture  of additional  particles.  As  these
 particles build  up during the  second  phase,  particle  aggregates form, bridg-
 ing of the  interweave  and interstitial spaces  occurs, and a more or less
 continuous  deposit is  formed.   In the  third  phase, particles continue  to
 collect  on  the  previous  deposit,  and  the surface dust cake is developed.

      The cleaning  cycle  (via  shaking,  reverse  air, or pulse jet) removes
 some  of  the surface cake.  After  a  few cleaning cycles,  theoretically a
 steady-state dust  cake  should be  formed, which  will  remain until the bag
 is  damaged, replaced,  or  washed.  Actually,  however,  the dust cake can vary
 significantly from cycle  to cycle,  particularly in applications involving
 utility  boilers or  metallurgical  processes.  This  remaining cake forms a base
 for the  collection  of particles when  the bag is put  back on line after cleaning.

2.2.2       Gas Stream Factors  that Affect Fabric  Filter Design and Operation

            A complete characterization of the effluent  gas stream is impor-
tant in  the design and operation of the fabric  filter system.   It should
include  the gas flow rate; minimum and maximum gas temperatures; acid dew
point; moisture content;  presence of large particulate matter;  presence of
sticky particulate matter; particulate mass  loading;  chemical,  adhesion,  and
abrasion properties of the particulate; and  presence  of  potentially explosive


                                       2-12

-------
gases or participate matter.  These data are used to design a collector with
the required degree of control or to optimize the operation of an existing
fabric filter, as illustrated by the following considerations:

       o  The size of a fabric filter system is determined by the gas
          volume to be filtered and the pressure drop at which the filter
          can be operated, given the fabric type, dust cake properties, and
          cleaning method.  The area of fabric surface (A) is determined by
          multiplying the total gas flow by the selected air-to-cloth
          ratio (A/C), which is based on the cleaning method or type of
          fabric filter.

       o  Penetration is related to the effective A/C ratio in the system,
          particularly if the A/C ratio is outside the optimum range for
          the specific application and type of fabric filter.  Therefore,
          the lowest possible face velocity (particle velocity at filter
          surface) consistent with economic constraints should be specified
          during the design phase.  This parameter should also be considered
          in the operation of an existing fabric filter, if process flow
          rates increase significantly or additional sources are added.

       o  Variations in gas stream temperature over time affect the opera-
          tion and design of a fabric filter.  The temperature of gases
          emitted from industrial processes may vary more than several
          hundred degrees within short periods of time.  It may fall
          below the gas moisture and acid dew points, or it may exceed
          the maximum temperature that the fabric will tolerate.  The tem-
          perature extremes must be determined before the filter fabric is
          selected and during evaluation of fabric filter performance.

       o  The particle size distribution of the dust must be considered
          in the design and operation of the collector.  Particle size
          distribution affects both the porosity of the dust cake and
          abrasion of the fabric.  The presence of fine particles in the
          gas stream can create a very compact dust cake and increase the
          static pressure drop through the cake.  These fine particles
          can also cause fabric bleeding if pulled through the fabric.
          The presence of large abrasive particles can reduce bag life
          and may necessitate the use of a precleaner or gas distribution
          devices in the collection system.

     Moisture content and acid dew point are important gas composition
factors.  Operating a fabric filter at close to the acid dew point introduces
substantial  risk of corrosion, especially in localized spots close to hatches,
in dead air pockets, in hoppers, or in areas adjacent to heat sinks, such as
external supports.  Allowing the operating temperature to drop below the
water and/or acid dew point, either during startup or at normal operation,
will usually cause blinding of the bags.  Acids or alkali materials can also
weaken the fabric and shorten its useful life.  Trace components, such as
fluorine, also can attack certain fabrics.
                                       2-13

-------
 2.2.3        Fabric Filter Systems

             Although  the basic participate  collection mechanisms are the
 same for all  fabric filters and  gas stream  factors affecting their perform-
 ance are relatively similar,  the equipment  itself and fabrics used in fabric
 filter  systems  may vary widely among vendors and applications.  Some of
 these variations are  necessary to meet various performance capability
 demands and  physical  characteristics; others are the products of individual
 contributions of the  numerous equipment and fabric vendors.

     Although fabric  filters  can be classified in a number of ways, the most
 common way is by their method of fabric cleaning:  shaker, reverse-air, and
 pulse-jet.

             Shaker-Type Fabric Filters.  A  conventional shaker-type fabric
 filter  is shown in Figure 2-5.   PTrffcu late-laden gas enters below the
 tube sheet and  passes from the inside bag surface to the outside surface.
 At  regular intervals  a portion of the dust  cake is removed by manual shaking
 (small systems) or mechanical shaking (large systems), the preferred method
 in  MWC applications.  Mechanical shaking of the filter fabric is normally
 accomplished by rapid horizontal motion induced by a mechanical shaker bar
 attached at  the top of the bag.  The shaking creates a standing wave in the
 bag and causes  flexing of the fabric.  The  flexing causes the dust cake to
 crack, and portions are released from the fabric surface.  The cleaning
 intensity is controlled by bag tension and  by the amplitude, frequency, and
 duration of  the shaking.  Woven fabrics are generally used in shaker-type
 collectors.  Because  of the low cleaning intensity, the gas flow is stopped
 before cleaning begins to eliminate particle reentrainment and to allow the
 release of the dust cake.  The cleaning may be done by bag, row, section,
 or  compartment.

     Gas flow through shaker-type fabric filters is usually limited to a low
 superficial  velocity  (numerically equals A/C ratio) of less than 3 ft/min.,
 typically ranging from 1 to 2 ft/min.  High A/C values can lead to excessive
 particle penetration  or blinding, which reduces fabric life and results in
 high pressure drop.  Typical A/C ratios are 2 to 6 cfm/ft2.

     Mechanical  shaker-type units differ with regard to the shaker assembly
 design,  bag length and arrangement, and type of fabric.  All sizes of con-
 trol systems can use  the shaker design.

            Reverse-Air fabric Filters.  A  large and typical reverse air
 filter is shown  in Figure 2-6.

     Regardless  of design differences,  the cleaning principle is the same.
Cleaning is accomplished by reversal of the gas flow through the filter
media.   The change in direction causes  the  surface contour of the filter
 surface  to change (relax) and promotes  dust-cake cracking.  The flow of gas
 through  the fabric assists in removal of the cake.   The reverse flow may be
 supplied by cleaned exhaust gases or by ambient air introduced by a secondary
fan.

                                      2-14

-------
                      OVERMOUNTED
                        EXHAUSTER
    DOOR
    DOOR
INLET
HANGER. \r"CHANNEL
                 BAG  NOZZLE/
               AND-RETAINER
       NLET CHAMBER
          AND
         HOPPER
         BAFFLE
           • -SUPPORT
                                 DISCHARGE  GATE
Figure 2-5.  Small  shaker-type fabric  filter.
                                             8
                         2-15

-------
Figure 2-6.  Example of  a large  reverse-air fabric filter.
                             2-16

-------
      In filters with inside bag dust collection,  cleaning  is  done with  com-
 partments isolated.  The filter bags may require  anticollapse rings  to  prevent
 closure of the bag and dust bridging.

      Reverse-air filters are usually limited to A/C  ratios  of less than 3
 cfm/ft2 and a range of about 1 to 2.5  cfm/ft2.  In general, the appropriate
 A/C ratio for a reverse-air unit should be  about  one-third  lower than for a
 similar shaker-type unit application.

             Pulse-Jet Fabric Filters.   In pulse-jet  fabric  filters,  filtering
 takes place on exterior bag surfaces.   A small pulse-jet fabric filter  is
 illustrated in Figure 2-7.   The bags supported by inner retainers (usually
 called cages) are suspended from a  tube sheet, an upper cell  plate.  Com-
 pressed air for cleaning is supplied through a manifold-solenoid assembly into
 blow pipes.   Venturis are sometimes mounted  in the bag entry  area to improve
 the pulse-jet effect and to protect the top  part  of  the bag.   The diffuser is
 placed at the gas inlet to  prevent  large particles from abrading lower  portions
 of the bag.

      During  cleaning,  a brief  (generally less than 0.2-second) pulse of com-
 pressed air  injected into the  top of the bag creates a traveling wave in the
 fabric,  which shatters the  cake and throws  it from the surface of the fabric.
 The  dominant  cleaning  mechanism in  a pulse-jet unit  is fabric  flexing.  Fel-
 ted  fabrics are  normally used,  and  the  cleaning intensity  (energy) is high.
 The  cleaning  usually proceeds  by  rows,  and all bags  in a row  are cleaned simul-
 taneously.  The  compressed-air  pulse, which  is delivered at 80 to 120 psi,
 results  in local  stoppage of the  gas flow.   The cleaning intensity is a func-
 tion  of  compressed-air pressure.  Pulse-jet  units can operate at substantially
 higher  A/C ratios  than the  previously discussed fabric filters because of their
 higher  cleaning  intensity.  Typical  ratios range from 5 to  10 cfm/ft2.

     The  plenum  pulse  cleaning  method is a variation of the pulse-jet clean-
 ing  mechanism; in  this  method,  an entire compartment of bags  is taken off-line
 and  pulsed with  compressed  air  from the  clean air plenum.

            Other  Designs and Modifications.  Fabric filters can be constructed
 either as a positive-pressure unit  with  the  fan upstream of the fabric filter
 or as a negative-pressure unit  with  the  fan  downstream of the unit.   Recent
 applications  have  employed  the  fan  downstream of the fabric filter.

     The use  of a  positive-pressure  fabric filter eliminates the need for
 ductwork and  a stack downstream of  the  unit, which reduces requirements for
 space and other materials but makes  sampling to determine particulate loading
more difficult.  Because positive-pressure units  are generally not exposed to
as high a static pressure as negative-pressure units, their housings  can
 sometimes be  constructed of a bolted light-gauge  material.   Any leaks from
 the fabric filter will enter the surrounding air  and increase  the fugitive
emissions from the unit.

     In negative-pressure units, the fan is  located  on  the  clean side of the
filter, where it is subject to less wear from dust abrasion.  The fabric
filter housing must be gas-tight, as any leaks will  draw  air in from  the

                                      2-17

-------
outside.  This outside air will normally cool the gas stream.   This  could
reduce the gas temperature below the dew point and cause condensation  on the
inside of the unit.  In some processes, introduction of outside air  increases
the risk of fire and/or explosion.  On the other hand,  leaks will  not  result
in fugitive emissions because ambient air is drawn into the unit.

2.3         Wet Scrubbers

            Wet scrubbers for particulate matter control in municipal  incin-
eration are not likely to be used in the future.  Although accounting  for
nearly one-fifth of all particle control systems on U.  S.  incinerators,1
wet  scrubbers have the following disadvantages:

        o   cannot meet current or future particulate matter emission
            requirements without very high pressure losses with accom-
            panying erosion and increased maintenance requirements,

        o   a liquid waste is generated, and

        o   water scrubbers will absorb acid gases to some extent  and,
            if not designed to handle acids, will have significant
            operating problems.

     In short, any wet scrubber applied to an incinerator  will  be  designed
for gas absorption/multipollutant control, and any particulate  matter  con-
trol  will  complement a fabric filter or electrostatic precipitator.  For
this  reason wet scrubber descriptions and functions are discussed  in
Chapters 3 and 4.
                                     2-19

-------
                                 oo
                                  o
                                  i:
                 \  \. •    _w_
 •                 A  i. - '  * * ^""^\ ^ ^"""
•f*^*"*^^"^M^*^^l*"^^^>^^^^^*^Tpa**^!^'?^*1l^^a**^tyfc^^TB W - j	 ^^ •*• »     ', ^rjf ^W
s
                                  ex
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                                  K
                                  I
                                  CM

-------
                        3.0  GASEOUS EMISSION CONTROLS

      Gaseous  emission  controls are designed for capture of a particular gas
 or gases  and  are  Intended for use in series with particulate control  devices.
 Although  several  wet scrubbers are available for specific hydrochloric acid
 mist control,  their use has been confined to special waste incinerator appli-
 cations.   Post-combustion NOX control systems have been applied in Japan to
 non-incinerator combustion scrubbers and more recently to municipal inciner-
 ators.  Consequently,  post-combustion NOX controls are also discussed below.

      3.1   Acid Gas Scrubbers (HC1 and HF only)10

           Add gas scrubbers are generally found in small waste incinerator
 applications  in convenient packaged form.  Using water or very dilute sodium
 solutions,  hydrochloric acid and hydrofluoric acids are easily absorbed with
 little  regard to  operation and maintenance.  However, in municipal incinerator
 applications  with particulate matter, sulfur oxides, and condensible  organics,
 exclusive  control of acid gases becomes more complex.

      Trace  alkali in scrubber water can react with flue gas components to form
 insoluble  salts;  therefore, the pH must be controlled to less than 4  to limit
 absorption  of  gases other than HC1 and HF and precipitation of insolubles.
 Calcium scrubbing is not likely for this reason, while clear liquor (sodium)
 scrubbing  is also potentially troublesome due to erosion and corrosion.

      Commercial practice in Europe has been to use water only scrubbing with
 minimal internals—spray towers or venturi scrubbers—at very low (0  to 1)
 pH  to minimize absorption of other gases.  Water is introduced at roughly
 one  liter per  20-25 mg HC1 to be absorbed.  Water injection rate is controlled
 by monitoring  solution conductivity.  Scrubber blowdown is neutralized with
 lime  before being discharged or further treated.  HC1 and HF removals over
 90 percent  are routinely achieved.

      Scrubbers are made of corrosion-resistant materials with mist eliminators
 and  rubber-lined stacks to minimize acid attack by the exhaust gases.  Figure
 3-1  illustrates a typical  system of this type.  Effluent regulations  in
 Europe are  already forcing Installations of this type to evaporate liquids
 and  send residue and solids to special  waste storage.  Newly enacted  S02
 removal requirements and restrictions on liquid wastes are rendering  single-
 stage acid  gas wet scrubbers obsolete.   Multistage scrubbers with effluent
 fed  to spray dryers upstream are currently considered state-of-the-art in
 Europe.  These are discussed in Chapter 4.

 3.2         Post Combustion NOX Control

 3.2.1       Selective Catalytic Reduction (SCR)H

            For post combustion NOX control,  selective catalytic reduction
 is the most advanced process,  with ammonia reducing NOX to nitrogen,  N2, and
water vapor in the presence of a catalyst.  This approach differs from the


                                     3-1

-------
LEGEND:
1  Gas-Gas  Heat Exchanger
2  YentuH  Scrubber
3  Stack
*  Lime  511o
5  Lime  Slater    ...
6  Neutralization Tank
±


3
                                                     Cleaned
                                                     Flue Gas
                        Waste
                        Liquor
                      Treatment
                         and
                       01 sposal
                      Figure 3-1.  Add gas scrubber.
                                                      10
                                      3-2

-------
 selective  non-catalytic reduction  (SNR) process tcommonly called Thermal
 De-N0x)  discussed  1n  the  combustion report of this series in several ways:

      o  Ammonia  is injected into the flue gas at lower temperature
         [150-400*C (302-752"F)] zones,
      o  A  catalyst is used to compensate for the lower driving force of
         reaction and  to better utilize ammonia,
      o  Higher NOX removal Is possible than for Thermal De-N0x,
      o  SCR can  be adversely affected by certain metals and acid gases,
         and
      o  Secondary  ammonia emissions are higher with Thermal De-N0x
         than with  SCR.

      Since virtually  all  of the NOX in combustion gases is in the form of
 nitrogen oxide (NO),  a small amount of oxygen -promotes the reaction as
 follows:

             4NO  +  4NH3 + 02	> 4N2 + 6^0                         (3-1)

      For typical applications on fossil-fuel-fired boilers, SCR removes
 60-85 percent  of the  NOX using 0.61-0.90 mole 1^3 per mole NOX, leaving a
 few ppm  unreacted  NH3  (although monitoring methods for NH3 are not avail-
 able  to  confirm  this  on a continuous basis).  The addition of larger
 amounts  of NH3 increases NOX removal but produces larger amounts of
 unreacted  N«3  (ammonia slip).  The optimum temperature for Reaction (3-1)
 is 300-400'C (572-752'F), so SCR is generally applied to flue gas at the
 economizer outlet.

      Earlier SCR problems such as catalyst poisoning by SOX, plugging,
 ammonium bisulfate deposition, production of $03, and erosion of cata-
 lyst  have  generally been overcome.   However, attack of conventional SCR
 catalysts  (which use base metal  with titanium dioxide) by hydrochloric
 acid  is  still a major problem.  This may be overcome by either (1) develop-
 ment  of  HCl-resistant catalyst or (2) location of SCR downstream of HC1
 controls.

      Two commercial applications of SCR on municipal incinerators in Japan
 began operation in late 1986.  Both use a special  low temperature,
 acid-resistant catalyst developed by Mitsubishi Heavy Industries (MHI).
 One,  a 150 ton/day plant in Tokyo,  is a retrofit application with very
 limited  space for  the catalyst.   Consequently, the design NOX removal  is
 only  30-40 percent from flue gas having 120 ppmv NOX and 500-800 ppmv  HC1.
The SCR unit Is located downstream of an ESP and upstream of a sodium-based
wet scrubber.

     The other SCR system, also  developed by MHI,  has been integrated
with  two new 65 ton/day Incinerators in Tokyo and follows the lime spray
dryer/baghouse systems.  This SCR system will treat a gas stream [^ 200*C
 ( ^392*F)]  containing about 50 ppmv HC1,  50 ppmv SOX, 30 mg/Nm3 dust,  and
150 ppmv NOX.  The system is designed to remove 73 percent NOX (40 ppmv at
outlet), but the  guarantee is only  33 percent NOX removal or an emission
NOX concentration of 100 ppmv.

                                      3-3

-------
     SCR 1s also being applied to sludge incinerators in Japan.   At least
15 SCR plants on municipal sludge incinerators have been constructed by
MHI.  Normally, impurities such as HC1 and trace metals degrade  SCR
catalysts, so the incinerator gas is typically subjected to sodium wet
scrubbing and, in some cases, a wet ESP to remove HC1 and trace  metals to
acceptable levels.  Then a special titanium-based honeycomb catalyst
developed by MHI is used to remove NOX by 80-90 percent.  Flue gas volumes
for  this application typically range from 2,500 to 108,000 Hm3/h.

     Design data for some sludge incinerator installations are presented
in Table 3-1.  As shown, uncontrolled NOX levels of 100-150 ppmv are
reduced by 80-90 percent with only 5 ppmv of ammonia slip.  Performance
data are not available at this time.


      TABLE 3-1.  SCR PLANT DESIGNS FOR SLUDGE INCINERATOR FLUE  GAS11
Gas Treated, Nm3/hr

S02, ppmv

NOX, ppmv

Temperature, *C

NH3/NOX molar ratio

NOX removal, %

NH3 slip, ppmv
108,000
10
100
350
1.0
90
5
40,000
50
150
400
1.0
80
5
24,000
20
130
400
1.0
90
5
2,500
20
130
400
1.0
90
5
     Three SCR options for incinerators are shown in Figure 3-2.12  option
I shows conventional SCR as practiced in coal-fired utility applications.
Option II uses SCR after the metals and acids have been removed,  but the
catalyst operates in a lower temperature range.  Such catalysts are in ad-
vanced development in both Europe and Japan, and two commercial applications
of low temperature catalysts in Japan were noted above.  Option III is
one of several possible schemes to utilize current commercially demonstrated
technology as discussed earlier.  Note that air preheat is replaced by
reheat of SCR inlet gas, incurring a substantial energy penalty.   Other
schemes could use auxiliary reheat, but in no case can bypass reheat be used,

     Noteworthy are the temperatures of operation.  Depending on  the
catalyst used, temperatures as low as 200*C (392*F) may be used,  and
reheat costs are reduced (see Figure 3-2, Option III).  Another benefit
is that noxious and difficult to scrub gases, such as mercaptans  and
sulfides, are decomposed by oxidation by about 80 percent.
                                     3-4

-------



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PM/HCI






f




                                                                                  CONVENTIONAL SCR (WITH
                                                                                  MCI RESISTANT CATALYST)
                                                      A/P
                    b.
to
I
en
        SCR WITH LOW  TEMPERATURE
        CATALYST
                                        A/P
                    c.
„     SCR WITH EXTENSIVE GAS REHEAT
                                                                                  I =     INCINERATOR
                                                                                  A/P=   AIRPREHEATER
                                                                                  PM/HCI = ESP/SCRUBBER, ETC.
                                                                                  GGH=   GAS GAS HEAT EXCHANGER
                                                                                  B =     REHEAT BURNER
                                    Figure 3-2.   SCR options for municipal  incinerators.
                                                                                         1 ?

-------
      Although this SCR process Is commercially  used,  Improvement efforts
 focus on two areas:  low temperature catalysts  (Option  II,  Figure 3-2) and
 high temperature catalysts resistant to  HC1  attack  and  metals poisoning.

      Operation and maintenance techniques  to ensure reliable operation
 include periodic inspection of the catalyst, with the partial replacement
 of the bed during annual  outages.   Routine monitoring of NOX emissions and
 ammonia slip by grab sampling is  necessary to determine the catalyst act-
 ivity and potential for buildup of ammonium  sulfate on  internal surfaces
 downstream.  The pressure drop across the  catalyst  bed  should be monitored
 to determine plugging and/or channeling, both of which  result in poor
 performance.

      The gradual loss in  catalyst activity is inevitable during SCR opera-
 tion.  If a minimum NOX removal is mandated, the NH3/NOX molar ratio can be
 gradually increased to compensate  for the  degradation of the catalyst.
 This approach entails increased ammonia  leakage and potential sulfate
 buildup downstream.  Alternatively,  the  catalyst may  be partially or totally
 replaced depending on the tradeoffs  of ammonia cost,  increased maintenance,
 and lower reliability versus catalyst cost.

 3.2.2  Wet NOX Removal  Processes11

        Wet processes  for  NOX removal  have several drawbacks:  (1) most
 nitrogen oxides are present as nitric oxide  (NO) which  is relatively un-
 reactive;  (2)  N02  is  relatively soluble  but  somewhat  less reactive than $03
 or  other acid  gases;  (3)  oxidation of NO to  N0£ is  difficult; and (4) nitrate
 and nitrite  byproducts  have  limited  use, requiring  expensive processing to
 produce a  salable  product,  such as fertilizer.

      The processes  which  have  been used  in Japan and  are planned in Germany
 for incinerators include  oxidation/reduction  and complex absorption.  In
 oxidation/reduction,  a  strong  oxidizing agent such  as sodium chlorite or
 ozone is added to  flue  gas  to  convert all NOX to N02  prior  to wet scrubbing
 (usually sodium-based).   The complex  absorption process uses ethylenediamine
 tetracetic acid (EDTA)  and  ferrous ions which promote NO absorption by
 forming  a complex  compound.

      Another wet NOX  process involves addition of ammonia upstream of
 a sodium hydroxide  spray  dryer in  Japan.  At  temperatures of 400 to 500*C
 (752  to  932T),  ammonia 1s  Injected at a molar ratio  of 0.35 NH3/NOX to get
 30  percent NOX  reduction  In  the spray dryer-ESP system.  Sodium sulfite is
 the only product waste; hence a reduction of  NOX to N£  is suspected, similar
 to  the SCR reactions.   Because the effect of HC1 on this process is not
 known,  the applicability of  this process to municipal  solid waste inciner-
ation 1s uncertain.

     Since the above wet NOX processes involve scrubbing where more reactive
species  (SOg, HC1) than NOX are present 1n the flue gas, all wet NOX schemes
are essentially multiple pollutant control  scrubbers.   They are discussed in
more detail 1n Chapter 4.
                                     3-6

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                     4.0  MULTIPOLLUTANT CONTROL SYSTEMS

     European and Japanese regulations generally require control  of gaseous
and particulate matter emissions; hence control systems which simultaneously
remove several pollutants are commonly used.  In addition, a growing number
of local and state permitting agencies in the United States recognize the
need for multipollutant control and, in essence, require the installation
of systems similar to those in Europe.  This section will  review  the
various types of multipollutant control systems commercially available
today and discuss their performance in controlling po"1  utants of  concern.

4.1         Wet Scrubbing

            The use of wet scrubbers for multipollutant control  has been
practiced since the early 1970s, notably in Europe and  Japan.  By addition
of alkali—sodium or calcium—a particulate scrubber also significantly
reduces the level of acid gases—S02, HC1 and HF—and converts them into
an aqueous salt solution or slurry requiring treatment and disposal.  More
recently wet scrubbers designed for acid gas removal have been augmented
by addition of chemicals to enhance NOX removal as well.  These augmenta-
tions involve expensive chemicals and increase the volume of scrubber waste.
4.1.1       Wet Scrubbing for Acid Gases (HC1, HF, and S02)
                                                           10
            Wet scrubbing for acid gases differs markedly from wet scrubb-
ing for particulate matter in several ways:  (1) emphasis is on gas/
liquid contact and not impingement of particles, (2) chemical additives
to the scrubber liquor require careful attention to the system chemistry
and accelerate the potential for corrosion and fouling of scrubber internals,
and (3) the volume of waste is substantially increased.  As a result,  sig-
nificantly more attention is paid to process conditions and effluent/exhaust
chemical composition with wet scrubbing for gases.

     Many types of wet scrubber are used for removing acid gases—spray
towers, centrifugal scrubbers, venturi scrubbers.  Scrubbers with internals,
such as packed-beds and trays, are less commonly used.  Figure 4-1 illus-
trates a typical wet scrubber installation for gas absorption.

     Gas enters the absorber where It is contacted with an alkaline solu-
tion.  For this discussion lime Is used as the reagent.  The lime solution
reacts with the add gases to form salts, which are generally insoluble
and may be removed by sequential clarifying, thickening, and vacuum
filtering.  The dewatered salts or sludges are then landfilled.

     In coal-fired utility boiler applications, the process features
potential closed-loop operation, wherein all water 1s recycled to the
process except for evaporative losses and residual moisture in the solid
waste.  Further, the suspended solids usually contain mostly calcium
sulfite which may be oxidized in an intermediate step by blowing air
through the liquor to form calcium sulfate or gypsum which is a poten-
tially salable by-product, as well as a more manageable, drier solid
waste.
                                     4-1

-------
                                                         Fresh
                                                         Mater
         Liquid
         Waste
                                                                          Flyash
                                                                            L1me
                                11
Lime from
 Vehicles
 LEGEND:

 1   Flue Gas  Inlet
 2   Absorber
 3   Hold Tank
 4   Hash Tray Tank
 5   Cl ar1 f1 er
 6   Thickener
 7   Vacuum Filter
 8   Pug Mill
 9   L1me Storage
10   Line Slaker
11   Dilution  Tank
12   Heat Exchanger
13   Stack
          Figure 4-1.  Line scrubbing for add gas and S02 removal
                                                                  13
                                      4-2

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      For  incinerator  lime scrubbing, the following chemical reactions occur:


             Ca(OH)2  + S02 —> CaS03 • 1/2H20 + 1/2H20              (4-1)

             CaS03  •  1/2H20 +  1/202 + 3/2H20 —> CaS04 • 2H20      (4-2)

             Ca(OH)2  + 2HC1 —> CaCl2 • 2H20                        (4-3)

             Ca(OH)2  + 2HF —> CaF2 • 2H20                          (4-4)


      However,  the presence of substantial HC1 in incinerator flue gas results
 in  a  significant portion of calcium chloride (CaCl2) in waste solids as well
 as  a  buildup of chlorides in the liquor such that waste handling and liquor
 recycle are more difficult.  Solids become more difficult to settle and
 dewater with CaCl2, and generation of a useful waste material such as gypsum
 is  impractical.  The  buildup of chlorides in liquor necessitates a purge
 stream to retain good scrubber performance and minimize plugging, scaling,
 and corrosion.  In  the case of incinerator flue gas, a single scrubber would
 operate in essentially an open loop, with large amounts of water consumption,
 treatment, and waste.

      A more practical scheme is to install a prescrubber just upstream of
 the absorber in Figure 4-1 which operates independently, with a separate
 liquor loop, primarily as a chloride/fluoride scrubber.  In this way the
 main absorber may act as an S02 scrubber (or S02/NOX scrubber in more
 advanced schemes) and generate more stable waste with liquor recycle.  This
 will be discussed in more detail in Section 4.3 when the hybrid scrubber
 systems are discussed.

 4.1.2        Wet Scrubbing for Acid Gases and NOX11

             In more recent advances, NOX control is possible by either
 adding a chemical which absorbs nitric oxide (NO), the primary constituent
 of  NOX, or oxidizes NO to N02 (nitrogen dioxide) which is more readily
 absorbed in scrubber liquor.

     In the former case,  a dissolved catalyst, ethylenediamine tetraacetic
acid (EDTA), is used in a sodium or ammonia solution with ferrous ion.  The
 sulfur oxides present are absorbed by either sodium sulfite or ammonium
 hydroxide, and these compounds enter into a complex series of reactions that
 take place after the NO is absorbed.  The overall reaction is as follows
 for ammonia scrubbing:

             2NO + 5S02 + 8NH3 + 8H20 	> 5(NH4)2S04                (4-5)

Since this process has not been operated on a municipal incinerator applica-
tion,  it is assumed that  chlorides and fluorides will be removed in a separate
loop to minimize adverse  effects.   This process is offered by one German
vendor,  Saarberg-Hoelter-Lurgi,  and is scheduled for installation on utility
boilers.   Little information  has been disclosed about process details, but

                                      4-3

-------
it is suspected that the EDTA process may well be one of the seven wet NOX
absorption processes recently announced for installation on incinerators  in
West Germany.14

     The other wet NOX absorption technique uses an oxidizing agent such
as ozone, chlorine dioxide, or sodium chlorite to oxidize NO to N02.   Then
the N0£ is scrubbed in a conventional sodium or magnesium scrubber along
with other acid gases.  The reactions are:


        NaC102 + 2NO —> NaCl + 2N02                                (4-6)

        ZNaOH + 2N02 + 1/202 —> 2NaN03 + H20                       (4-7)


     If a calcium scrubber is used, a catalyst must be added, and the result-
ing waste stream requires special treatment to avoid gaseous ammonia evolu-
tion from the waste liquor.

     These processes are discussed further in Section 4.3.

4.1.3       Performance of Wet Scrubbers10

            The performance of wet scrubbers is dependent on many factors,
and the data base is too limited on incineration applications for detailed
discussions of the effects.  Theoretically, the reaction of strong acid
gases (HC1, HF) proceeds rapidly with alkaline solutions and even mildly
acidic solutions.  Hence, HC1 and HF removal should be high (greater than
90 percent) in every case, assuming proper operation.  The reaction of S02
proceeds more slowly and over a limited pH range, the limiting factors
being the rate of S02 absorption and, for calcium systems, the dissolution
rate of solid caustic particles.  Thus, S02 removal may vary greatly over
a limited range of operation, depending on pH control, inlet S02 concentra-
tion, and many other factors discussed under operation and maintenance.
Since wet scrubbers operate at saturation [40-50*C (104-122*F) typically],
the quenching of flue gas by some 150 to 200'C (302-392*F) should reduce
considerably the volatile compounds, including organics and trace metals.

     Reported European outlet emissions for a wet scrubber designed for multi-
pollutant control, preceded by an electrostatic precipitator, are shown in
Table 4-1.

                 TABLE 4-1.  WET SCRUBBER OUTLET EMISSIONS10

                      HC1                  5-10 ppmv
                      HF                      1 ppmv
                      S02                    25 ppmv
                      Particulate Matter  <0.01 gr/dscf
                      Heavy Metals3      <0.001 gr/dscf
                      Hg                0.00002 gr/dscf
^Classes 1-3 of West German regulations.  Includes Cd, Tl, Hg, As, Co, Ni,
 Se, Te, Sb, Pb, kCr, Cu, Mn, V.

                                       4-4

-------
      It would be expected that volatile organic pollutants would also  be
removed by wet scrubbers because these pollutants would be condensed to form
particulate matter which would be collectible in the scrubber.   However, no
data are available to test this hypothesis.

4.2         Dry and Semi-Dry* Scrubbing

            Because of the difficulty in managing the buildup of chlorides
necessitating dual wet scrubbing and the burden of a liquid waste disposal,
the most popular flue gas scrubbers are the dry and semi-dry types.   Dry
scrubbing involves the injection of a solid powder such as lime or sodium
bicarbonate into the flue gas where acid gas removal occurs in the duct and
continues in the dust collector as sorbent and ash particles and condensed
volatile matter are captured.  In a semi-dry process, better known as spray
drying, the sorbent enters the flue gas as a liquid spray with sufficient
moisture to promote rapid absorption of acid gases but yet produces  only dry
solid particles entering the particle collector.

4.2.1       Dry Injection Processes10

            Dry injection of alkaline sorbent into circulating flue  gas
followed by a particle collector has been recently developed in Europe,
and more than 20 installations on incinerators are known in Europe and  Japan.
The best known and studied process of this type 1s the Malmo, Sweden-, install-
ation as shown in Figure 4-2.  A cyclone precollector removes a large amount  of
coarse fly ash prior to the waste heat boilers but does not play a role in
the process.  Dry calcium hydroxide (hydrated lime) is pneumatically injected
into a reactor where the gas rises and mixes with the sorbent.  The  reactor also
provides additional solids residence time to allow reactions to occur.   An
older electrostatic precipitator removes about 95 percent of the dust,  while  a
newer pulse-jet fabric filter removes the remainder.  In a new facility,  it  is
likely that a fabric filter would be used, although several units with  ESPs are
reported.

     One interesting fact about the Malmo installation is the recent attempt  to
increase acid gas removal.  A heat exchanger was Installed to lower  inlet  flue
gas from 250*C (482'F) to 160-180*C (320-356'F), and the ESP was downgraded  to
allow more sorbent to enter the fabric filter.  It is well known that fabric
filters may remove substantial amounts of acid gas in proportion to  the ratio
of sorbent on the fabric to the acid gas flow rate.  Also, a lower flue gas
temperature or higher relative humidity in the flue gas increases acid  gas
removal.
*A semi-dry scrubber 1s also known as wet/dry scrubbing and spray drying.   It
 consists of a spray dryer and a dry solids collection device (ESP or fabric
 filter).  In the U.S., dry scrubbing refers to a process 1n which solids  from
 the process are dry (I.e., the cleaned flue gas is unsaturated with water
 vapor).  Using the U.S. terminology, both a dry sorbent injection plus par-
 ticulate collection system and a spray dryer plus particulate collection
 system are dry scrubbers.


                                       4-5

-------
1. FURNACE AND BOILER
2. PRECOLLECTOR
3. WASTE HEAT BOILER NO. 1
4. REACTOR
5. ELECTROSTATIC PRECIPITATOR
6. FABRIC FILTER
7. WASTE HEAT BOILER NO. 2
8. LIME SILO
9.  LIME FEEDING
10. LIME RECIRCULATION
11. COARSE DUST CONVEYING
12. FINE DUST CONVEYING
13. DUST SILO
14. DUST HUMIDIFIER
15. DUSTBIN
 Figure 4-2.   Dry  absorption system, Malmo,  Sweden.
                                                     10
                            4-6

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     Performance of Dry Injection/Fabric Filter Systems

     The following normal control device inlet pollutant concentrations  and
removal data have been reported at Malmo:10


        TABLE 4-2.  INLET POLLUTANT CONCENTRATION AND POLLUTANT CONTROL
                    WITH A LIME DRY INJECTION PROCESS**

                              Inlet Concentration
       Pollutant              	(mg/Nm3)	            Removal  (%)

       Parti oil ate Matter             10                     Not reported
       HC1                           200                         80
       HF                              0.2                       98
       S02                           150                         50
       Cd                              0.002                     99+
       Pb                              0.04                      99+
       Zn                              0.17                      99+
       Hg                              0.04                      90


     It should also be noted that dioxin emissions were reported below the
detectable limit (0.1 ng/Nm3), but this is likely due to very efficient
combustion efficiency in the incinerators (reported as 99.8 percent).
Recent data from Canada on a slipstream pilot plant indicate very high
control of pollutants in municipal solid waste flue gas with dry scrubbing
(lime injection followed by fabric filtration) for temperatures from
110-140'C (230-284*F) when humidification precedes the scrubber.  Effective
pollutant control was also obtained with a wet/dry or semi-dry (spray
dryer plus fabric filter) system at 140'C (fabric filter inlet temperature
with and without recycle from this flue gas cleaning system).  Only the
mercury and S0£ removals appear to vary with flue gas temperature.
Mercury capture was 90 percent or better at 110-140*C but essentially  nil
with flue gas at about 200*C (392*F) into the fabric filter.  S0£ capture
ranged from 96 percent at 110'C to 29 percent for the 200*C fabric filter
inlet temperature.  Removal efficiencies for other trace organics (chloro-
benzenes, polychlorinated biphenyls, and chlorophenols) were generally 95
percent or higher over the temperature range of 110-140*C.   Polycyclic
aromatic hydrocarbons removal was about 85 percent over the same tempera-
ture range but rose to 98 percent at 200*C, while the removal of the
other trace organics fell to the 55-60 percent neighborhood at 200'C.
Tables 4-3 through 4-9 summarize the test conditions and results reported
by Environment Canada for the Quebec City test program.15

     A second type of dry injection process is a circulating fluid bed-absorp-
tion process shown in Figure 4-3.  The process is offered semi-dry for
utility applications and totally dry for incinerator applications.   A  few
                                   4-7

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              TABLE 4-3.  SUMMARY OF KEY OPERATING  PARAMETERS^
Dry System4

Incinerator
Steam flow, kg/h x TO3
Gas temperature 9
boiler outlet, "C
Fabric Filter
Pressure drop,
cm water gauge
Flow Rate
Lime, kg/h
Dry Flue Gas
Inlet, Nm3/hb
Midpoint, Nm3/h&
Outlet, Nm3/hb
Temperature
Inlet to pilot plant, *C
Inlet to fabric
filter/C
Flue Gas Composition at
Outlet to Pilot Plant
C02, %
02. *
CO, ppm
THC, ppm
Particulate Loading
Inlet to pilot plant,
mg/Nm3 @ 81 02
no'c
32.8
303
15.7

3.7
3600
4400
4230

267
113
7.1
12.7
140
5
7710
125*C
33.4
287
14.4

3.6
3730
4430
4350

258
125
7.4
12.4
180
4
7260
140'C
33.2
291
14.9

3.7
3520
4120
4170

261
142
7.5
12.5
220
5
6250
>200'C
33.0
287
14.7

3.6
3010
3650
3600

253
209
7.3
12.9
160
4
5740
Wet/Dry System3
140*C
32.1
278
15.2

3.5
3650
4180
4220

254
140
8.3
11.8
130
4
5560
140"C
"Recycle"
31.3
293
15.9

3.5
3560
4110
4090

263
141
7.5
12.5
170
4
7190
aTemperatures shown are nominal  values  at the  midpoint (fabric  filter  inlet) of
 the pilot plant flue gas cleaning system.
^Reference conditions are 25*C and 101.325  kPa.
                                         4-8

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     TABLE 4-
4-4.  PCDD CONCENTRATIONS (nq/NmJ  @  82  0?)  IN
      EFFICIENCY OF REMOVAL^
FLUE GAS AND

Inlet, ng/Nm3
Midpoint, ng/Nm3
Outlet, ng/Nm3
Efficiency
Inlet/midpoint, X
Overall , X

110'C
580
310
0.2

47
>99.9
Dry
125*C
1400
570
NDC

60
>99.9
System3
140*Cb
1300
540
NDC

57
>99.9

>200'C
1030
1140
6.1

(11)
99.4
Wet/ Dry
140'C
1100
840
NDC

24
>99.9
System4
140'C
"Recycle"
1300
1270
0.4

2
>99.9
aTemperatures shown are nominal values at the midpoint (fabric  filter  inlet)  of
 the pilot plant flue gas cleaning system.
^Based on one test.
CND = Not detected.
TABLE 4.5  PCDF CONCENTRATIONS (ng/Nm3 Q 8X 02)  IN FLUE GAS AND EFFICIENCY
           OF REMOVAL**

Inlet, ng/Nm3
Midpoint, ng/Nm3
Outlet, ng/Nm3
Efficiency
Inlet/midpoint, X
Overall, X

no'c
300
270
2.3

11
99.3
Dry
125'C
940
440
NDC

54
>99.9
System3
140'Cb
1000
630
1.0

37
99.9

>200'C
560
490
1.2

13
99.8
Wet/Dry
140'C
660
690
NDC

-4
>99.9
System3
140'C
"Recycle"
850
1030
0.9

-21
99.9
3Temperatures shown are nominal  values at the midpoint (fabric filter inlet)  of
 the pilot plant flue gas cleaning system.
^Based on one test.
CND = Not detected.

                                        4-9

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                TABLE 4-6.  PERCENT REMOVAL OF OTHER ORGANICSlS
                                     Dry Systerna
                      	    Wet/Dry  System3
                                           140'C
110'C    125*C    140'cb  >200*C   140'C   "Recycle1
 Chlorobenzenes

 Polychlorinated
    blphenyls

 Polycyclic aromatic
    hydrocarbons

 Chlorophenols
 95

 72


 84


 97
 98

>99


 82


 99
 98

>99


 84


 99
62

54



98



56
>99

>99



>99



 99
 99

>99



 79



 96
 a Temperatures shown  are  nominal  values at  the midpoint  (fabric filter inlet)
  of the pilot plant  flue gas  cleaning system.
 bBased on one test.
        TABLE  4-7.   INLET/OUTLET METAL CONCENTRATIONS  (Ug/Nm3 @ 82 02)15
Dry System3
Metal
Zinc
(Zn)
Cadmium
(Cd)
Lead
(Pb)
Chromium
(Cr)
Nickel
(N1)
Arsenic
(As)
Antimony
(Sb)
Mercury
(Hg)
Location
Inlet
Outlet
Inlet
Outlet
Inlet
Outlet
Inlet
Outlet
Inlet
Outlet
Inlet
Outlet
Inlet
Outlet
Inlet
Outlet
110'Cb
99000
7
1300
0.4
41000
4
3100
0.4
1000
1.3
150
0.02
2000
0.2
440
40
125'Cb
108000
5
1300
0.4
44000
3
1900
0.4
1800
0.4
100
0.04
800
0.4
480
13
140*C
93000
6
1500
NDd
34000
5
2000
1
1300
0.7
130
0.04
1000
0.6
320
20
>200*C
91000
10
1000
0.6
35000
6
1900
0.5
800
2
80
0.07
1500
n.5
450
610
Wet/Dry
140*C "
77000
5
1200
ND<1
36000
1
1400
0.2
700
1.3
110
0.04
1000
0.3
190
10
System3
140"C
Recycle"
88000
6
1100
NDd
34000
6
1700
0.7
2500
2
130
0.03
2200
0.6
360
19
aTemperatures shown are nominal values at the midpoint (fabric filter inlet of
 the pilot plant flue gas cleaning system.
bBased on one test, except for mercury which is based on two tests.
     concentrations are rounded off for simplicity.
    = Not detected.
                                        4-10

-------
      TABLE 4-8.  HYDROGEN CHLORIDE CONCENTRATIONS (9 82 02)  AND  REMOVAL
                  EFFICIENCIES!*
Dry System3

Stoi chi ome trie Ratio
Inlet, ppm
Midpoint, ppm
Outlet, ppm
Eff. to midpoint, %
Eff. Overall, X
no*c
1.16
423b
15b
7b
96
98
125°C
1.03
464C
69b
9b
85
98
140'C
1.04
475b.e
129b,e
2gb,e
73
94
>200*C
1.49
392C,e
196b,e
9ic,e
50
77
Wet/Dry System3
140*C
1.19
366d
I49b
29b
59
92
140'C
"Recycle"
1.10
470b
152b
42b
68
91
3Temperatures shown are nominal values at the midpoint (fabric filter  inlet)  of
 the pilot plant flue gas cleaning system.
bBased on continuous gas monitors.
cBased on manual sampling (15-30 minute tests).
dBased on a combination of continuous gas monitors and manual  sampling (15-30
 minute tests).
     test only.
       TABLE 4-9.  SULFUR DIOXIDE CONCENTRATIONS (0 8% 02)  AND REMOVAL
                   EFFICIENCIES15
Dry System3

Stoichlometric Ratio
Inlet, ppm
Midpoint, ppm
Outlet, ppm
Eff. to midpoint, X
Eff. Overall, X
no*c
1.16
119
24
4
80
96
125*C
1.03
118
65
10
45
92
140*Cb
1.04
99
64
41
35
58
>200*C
1.49
117
103
83
11
29
Wet/Dry System3
140*C
1.19
106
67
35
37
67
140"C
"Recycle"
1.10
106
70
43
35
60
3Temperatures shown are nominal values at the midpoint (fabric filter inlet)
 of the pilot plant flue gas cleaning system.
bBased on one test.
                                      4-11

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   LIME
FLUE GAS
1. LIME SILO
2. REACTOR
3. CYCLONE
4. OUST COLLECTOR
5. STACK
6. WASTE SILO
                                              DRY WASTE
     Figure 4.3  Circulating fluid bed absorption  (dry)  process.
                                                                10
                                      1.  LIME FEEDER
                                      2.  LIMESLAKER
                                      3.  FEEDTANK
                                      4.  HEAD TANK
                                      5.  SPRAY ABSORBER
                                      6.  DUST COLLECTOR
                                      7.  STACK
                                                  DRY WASTE
          Figure 4-4.   Spray  absorption  (semi-dry)  process.
                                                           10
                                    4-12

-------
 installations  on  incinerators  are  currently  in operation in Western
 Europe,  but the process  has  some significant advantages for an ESP retro-
 fit application discussed  in the following paragraphs.

      Flue  gas  enters  the vertical  reactor equipped with a venturi-type
 gas disperser, where  gas velocity  is  reduced and dry lime introduced.
 The reactor forms  a fluidized  layer of fly ash and sorbent reacting with
 acid gases.  Eventually all  dry solids are entrained and enter the dust
 separator  just upstream of the electrostatic precipitator.  A substantial
 portion  of this collected product  is  reintroduced into the fluid bed.

      Important features of this process are  high gas/solid mass transfer
 rates and  recycle  which improves the  normally low sorbent utilization of
 dry injection  processes.  Also, reactors and precollectors are small
 volume vessels compared to wet and semi-dry  process vessels.  The important
 control  parameter  is  the pressure drop across the entrained bed, which is
 maintained at  around  20 mbar by controlling  the recycle solids.  Recycle
 product  is mechanically conveyed to the venturi section and fed by gravity
 to  the system.  The fresh reagent rate is controlled by stack gas monitors.

      It  should be  mentioned  that the  circulating fluid bed has found only
 limited  use  for incinerator  applications, primarily because emissions of
 heavy metals cannot be adequately controlled to meet many European require-
 ments.   Addition of moisture to the flue gas, which could condense heavy
 metals,  enlarges the  fluid bed considerably, making spray drying more
 attractive  for new sources.  It is also possible that high CaCl2 concen-
 trations may complicate the  bed dynamics considerably.

     As  a  retrofit process for ESPs,  this system has several desirable
 features including:

         o  Substantial acid gas removal upstream of the ESP,
         o  Simple operation of small vessels,
         o  No wet solids handling, and
         o  Better utilization of sorbent than simple dry injection.

 Typical  outlet emissions data^ for the variables studied are as follows:

         HC1            75 mg/m3 (90S removal)
         HF             0.06-0.13 mg/m3 (98-99% removal)
        Cd             0.3 mg/m3

4.2.2     Semi-Dry Scrubbing 10

            Semi-dry scrubbing is also called spray dryer scrubbing, dry
scrubbing  (a misleading title), or wet/dry scrubbing.   In this process,the
sorbent is injected as a liquid or liquid slurry and the product collected
as a dry  solid.  For the purposes of this discussion,  it will be referred
to as semi-dry  scrubbing to differentiate among true dry scrubbing (dry
sorbent injection  plus dust collection) and  true wet scrubbing.


                                  4-13

-------
      Semi-dry  scrubbing  has many  variations:  sorbent may be injected
 through liquid nozzles or  rotary  atomizers, the sorbent may be screw-fed
 or pneumatically  blown in  dry and rewetted by water-only nozzles, or it
 may be injected wet  or dry into a fluidized bed with overhead water
 sprays.  The ensuing discussion will focus upon one of these systems—the
 spray dryer absorbei—because it  is the most common and successful.

      Figure 4-4 illustrates a typical spray drying process for incinerator
 applications.   Lime  1s slaked, mixed with water, and then pumped as a slurry
 to a head tank, an option  for spray drying.  Depending on the inlet concen-
 tration of pollutants, slurry 1s  metered into the spray absorber (shown
 with a rotary  atomizer in  Figure  4-4).  Flue gas heat is sufficient to
 dry the slurry into  a solid powder within the reactor vessel, and part of
 the solids are collected 1n the bottom of the absorber vessel while the
 remainder are  collected  In the particle collector.  Recycle of solids
 back to the feed  tank may  be selected as an option if sorbent utilization
 is very low or higher removals of gaseous pollutants are desired.

      The control  of  this process  is relatively simple for incinerator
 applications.   The spray dryer outlet flue gas is controlled at temperatures
 well above its saturation  value.  This precludes any sorbent from contact-
 ing downstream surfaces as a wet  powder leading to solids buildup.  It also
 assures operation well above the  dew points of any acid gases.  Because of
 the presence of calcium chloride  (a very hygroscopic, difficult-to-dry
 solid),  temperatures  are typically controlled at 110*-160'C (230-320'F)
 by  limiting the amount of  water injected.  In view of the Environment Canada
 data15,  it appears important that fine atomization of the feed slurry and
 good gas/slurry droplet mixing be attained to achieve reliable spray dryer
 operation  at a  temperature below  140*C (284"F).

      A  second  control loop is usually provided based on the pollutant
 emission  levels in stack gases to regulate the addition of reagent to the
 system.  Therefore with liquid and solids regulated by separate control
 loops,  the solids composition of  the slurry may vary somewhat.  It is typ-
 ically  low (~  10 percent)  for incinerator applications and easily managec
 In  many  cases,  the sorbent rate is fixed at a conservatively high rate to
 ensure  low stack emissions, but waste disposal plus sorbent operating
 costs are  increased.

     Many additional  details of operation of spray drying are given in
 References 10,  16, and 17.  Compared with other systems, considerable
 emissions  testing has been performed on both pilot- and full-scale spray
 dryer Installations.  Outlet emission guarantees for spray dryer plus
 fabric  filters  and spray dryer plus ESP are as follows:10

 TABLE 4-10.   OUTLET EMISSION GUARANTEES FOR MUNICIPAL WASTE COMBUSTORS10

FGC System          Spray Dryer + Fabric Filter  Spray Dryer + ESP
Flue Gas Component  Concentration  Removal,?     Concentration Removal,%

HC1                 20-60 ppmv      88-99         20-60 ppmv    90-98
HF                  2-5 ppmv        50-80         2-5 ppmv      50-80
S02                 20-70 ppmv      67-90        50-175 ppmv    15-50
Particulate         0.013-0.02 gr/scf            0.015-0.04 gr/scf

                                      4-14

-------
     From these guarantees, performance data seem similar for either  appli-
cation except for S02 and participate.  Results from these installations  at
130-140'C (266-284*F) flue gas outlet temperatures show these similarities.
However, when the heavy metal and dioxin/furan removals were examined at
the pilot plant level, dramatic differences were noted.

     First, actual removal of acid gases and particles has substantially
exceeded guaranteed levels as shown in a typical case using a fabric  filter
at 140*C (284*F):

     TABLE 4-11.  ACID GAS EMISSIONS FROM MUNICIPAL WASTE COMBUSTORlO

       Acid Gas     Guarantee, mg/Nm3     Actual Emissions, mg/Nm3

         HC1               55                    12-35
         HF                 5                      0.2
         S02               70                    20-70

     On a pilot plant, the following heavy metal removals were observed
for ESP and fabric filter applications at the same flue gas conditions:

     TABLE 4-12.  TRACE HEAVY METALS CONTROL BY PILOT PLANT FGC SYSTEMS^

                   Spray Dryer + ESP, %  Spray Dryer + Fabric Filter. %

     Hg*                   35-40                   75-85
     Pb                    65-75                   95-98
     Cd                    95-97                   95-97
     As                    93-98                   95-98
     Particulate       99.2-99.4               99.8-99.9
aVapor only.

     Further, when dioxins and furans were measured in the pilot plant,  from
48-89 percent of the dioxins and 64-85 percent of the furans were controlled
by the spray dryer plus ESP.  However, a spray dryer plus fabric filter  im-
proved removal in every case, usually by a substantial margin.   In fact  at
110'C (230*F), over 99 percent control of both dioxins and furans was observed
out of the fabric filter.

     Recent Canadian data*5 (see Tables 4-3 through 4-9) on a slipstream
pilot plant show high pollutant capture for a lime spray dryer/fabric filter.
The removal of dioxins/furans approached 100 percent for a fabric filter
inlet gas temperature of 140"C (284"F).  S02 and HC1 removals were 60 and
over 90 percent, respectively.  Metals recovery was over 99 percent except
for mercury which was about 95 percent at 140*C.  The overall removal
efficiencies for other trace organics in the MWC flue gas (chlorobenzenes,
chlorophenols, polychlorinated biphenyls, and polycyclic aromatic hydro-
carbons) were 99 percent or more at 140"C without recycle.  With product
recycle, the removal of polycyclic hydrocarbons and chlorophenols fell to
79 and 96 percent, respectively.  The control efficiencies need to be
verified on full-scale operating facilities.


                                        4-15

-------
      Tests  at the Marion County, OR, MWC facility in 1986 were made to
 determine compliance with  permit conditions.  Results for units (boilers)
 1  and 2  are shown in Table 4-13, and the discharge permit emission limits
 are  shown in Table 4-14 so that a comparison of test results with the
 discharge permit emissions may be made.18  Each unit has a rated capacity
 of 275 tons/day  (tpd) and  is equipped with a semi-dry/dry scrubber/fabric
 filter system (see Figure  4-5).  Lime slurry is fed to the quench reactor
 (spray dryer)  and a cement kiln dust is supplied to the dry reactor
 (venturi) just upstream of of the fabric filter.  Annual compliance tests
 for  the  Oregon Department  of Environmental Quality are also slated at
 this  facility  in June 1987.

      Comparison  of values  in the preceding tables show that NOX emissions
 from  the Marion County facility were about 15Z higher than that specified in
 its discharge  permit.  All of the other test results showed values lower
 than  the permit  limits.

 4.3          Combination Scrubbers

             Several innovative processes have been recently developed spe-
 cifically for  incinerator  applications which utilize combinations of existing
 scrubber technology.  These Include a dry/semi-dry process and wet/dry
 processes.

 4.3.1        Semi-Dry/Dry Scrubbing19

             Figure 4-5 depicts the seml-dry/dry scrubbing process which
 has been commercially operated In Japan and 1s currently installed on three
 U. S.  facilities.  The process is relatively simple, c.nsisting of a quench
 reactor,  dry venturi  reactor, and baghouse.  The quench reactor is essen-
 tially an upflow spray dryer with multiple sprays of lime slurry used to
ensure reliability.   An upflow system is claimed to ensure against large
 droplets (due  to sprayer malfunction) Impinging on downstream surfaces.

     The quenched gas then enters a venturi reactor where a dry powder of
 calcium  silicate/lime composition 1s Introduced.  This addition reportedly
 Increases the particle size of fly ash and sorbent such that the baghouse
 pressure loss  (and hence need for bag cleaning) is minimized.   This also
 results  In the Hrne  and calcium silicate sorbents being retained on the
filters  for very long times,  reportedly up to eight hours.

     With the combination  of  a more reactive sorbent (calcium silicate)
and very  long retention time  1n the gas stream, high removals of acid
gases would be expected.   Reported data19 for a commercial system are
summarized below:

  TABLE 4.13.  POLLUTANT EMISSIONS AND CONTROL BY SEMI-DRY/DRY
               SCRUBBING19

     Pollutant    Emission Concentration                Removal,  %

     Particulate  0.0004-0.039 gr/dscf or 1-9 mg/Nm3    98.7  - 99.1
     HC1           1-4  ppmv                              95 (average)
     S0£           0-5  ppmv
     Hg            85  g/Nm3

                                    4-16

-------
FLUE GAS
              SOLIDS   L*-
                         LIME
                                   CALCIUM SILICATE
AIR
                               1. QUENCH REACTOR (SPRAY DRYER)
                               2. DRY VENTURI
                               3. BAGHOUSE
                               4. STACK
               Figure 4-5.  Semi-dry/dry  scrubber.
                                                   19

-------
                              • ..   *»vfliiiniM
                                                          UI- i j i \j<\  1 1. j i o ur
                                                                                I t-MDtK  "  ULIUtJLK
                                    AT THE MARION COUNTY, OR, WASTE-TO-ENERGY FACILITY^
                  Flue  Gas  from Boiler 1   Flue Gas from Boiler 2
Flue Gas In Main Stack
oo
Pollutant
NOX
S02
CO
TSP
Pb normal
Pb bypassb
Be
TCDD
VOC
Fluorides
Hg
HCl
Corrected TSP
gr/dscf @
12% C02
Avg Ib/hr
60.1
10.3
1.9
3.7
0.003
—
2.4x10-7
1.9x10-8
0.2
0.046
0.026
0.61
0.016
Other Requirement


Max Ib/hr
69.0
12.8
2.0
4.7
—
0.02
2.7x10-7
2.3x10-8
0.2
0.071
	
0.73
0.021
Visible Em1
Avg Ib/hr
48.4
10.6
2.2
0.8
—
—
<2. 0x10-7
	
0.1
	
0.034
2.7
0.004
sslons
Max Ib/hr
53.2
17.9
2.6
1.2
—
—
—
—
0.2
	
	
2.9
0.006
0%
Avg Ib/hr
108.5
20.9
4.1
4.5
0.006
	
<4.4xlO"7 <
3.8x10-8
0.3
0.092
0.060
3.3


091. 5%d tpy
435
83.8
16
18
0.02
	
cl. 8x10-6
1.5x10-7
1.2
0.37
0.24
13


Max Ib/hr
122.2
30.7
4.6
5.9
	
0.04
5.4x10-7
4.5x10-8
0.4
0.14
0.068
3.6


eiooz* tpy
535
134
20
25.8
	
0.2
2.4x10-6
2.0x10-7
1.8
0.61
0.30
16


     aFac111ty availability.
     &Data based on one  test  of  5  minutes duration during which the baghouse was  bypassed.

-------
TABLE 4-15.   PERMITTED  EMISSIONS  FROM THE MARION COUNTY, OR, WASTE-TO-ENERGY
              FACILITY18
           Flue Gas  from
           Boiler  1
Flue Gas from
Boiler 2
Pollutant
NOX
S02
CO
TSP
Pb
Be
TCDD
VOC
Fluorides
Hg
HCl
(Ib/hr)
47.0
36.5
27.5
10.0
00.26
1.45x10-6
8.5x10-7
1.65
0.8
0.085
< 11.5
(Ib/hr)
47.0
36.5
27.5
10.0
00.26
1.45x10-6
8.5x10-7
1.65
0.8
0.085
< 11.5
Other Requirement
Emission Limits Main Stack
for Flue Gas from Both Boilers
         (gr/dscf)
(Ib/hr)  at 12? CO?)  (tpy)
                                                 94.0

                                                 73.0

                                                 55.0

                                                 20.0

                                                00.52

                                               2.9xlO-6

                                               1.7x10-6

                                                 3.1

                                                 1.6

                                                 0.17

                                              < 23
                                0.030
                       290

                       220

                       170

                        61

                       1.6

                     8.8xl016

                     5.1x1016

                       9.6

                       4.8

                       0.51

                    <  69
Opacity   Visible Emissions
     10Z
                                    4-19

-------
     Design conditions for this system are typically 90 percent HC1  removal,
 80 percent S02 removal, and 0.008 gr/dscf particulate ratter emission.

 4.3.2       Semi-Dry/Wet Scrubbing10

            A flow sheet of this process is shown in Figure 4-6.  As dis-
 cussed earlier, a wet scrubber system designed for multipollutant control
 will likely require separate treatment for HC1 and S02 if high removals of
 both are desired and liquid wastes are to be minimized.  The primary feature
 of this process is separate scrubbing for HC1 and S02, and the significant
 innovation is location of an upstream spray dryer which disposes of  liquid
 effluent from both wet scrubbers.

     Flue gas enters a spray chamber in which spent scrubber liquor  is
 introduced through a series of nozzles, or a rotary atomizer.  The reported
 retention time for gases is 4-5 seconds.  Gas then enters a particulate
 collector (usually an ESP, but it is not clear that a fabric filter  would
 not work as well) and then to the two-stage wet scrubbing system. The  first
 stage is essentially an HC1 scrubber (described in Section 3.1) operated
 with water injection in a venturi column at a pH of between 0 and 1.
 Water injection is controlled by monitoring the solution conductivity,  and
 the blowdown stream is neutralized with lime externally.

     A second option is to raise the pH to 3-4 and inject a sodium chlorite
 (NaC102) solution, which results .n oxidation of nitric oxide (NO) to nitro-
 gen dioxide (N02), described in Section 3.2.2.

     The second stage scrubber, also a venturi, is operated at a higher
 pH (5-6) with sodium hydroxide for S02 removal (and NOX removal if required).
 Both stages are designed to intentionally minimize internal surfaces to
 avoid scaling and erosion.  Blowdown from the second stage is sent to a
 sludge holding tank prior to being sent back to the spray dryer.

     No plant is currently known to be operating with this system.  However,
 several plants were in the design or construction stage at the end of 1986
 and incorporate all features,  including NOX removal in the wet scrubbers.
 Since no data for this system are available, the performance is assumed to
 be at least equivalent to that described for two-stage wet scrubbing:10

TABLE 4-16.   £XP£CT£D POLLUTANT EMISSIONS FROM MWC EQUIPPED WITH SEMI-DRY/
             WET SCWJBBINGiO

        Pollutant                       Emissions Concentration
        HC1                             10 mg/Nm3 (8 ppmv)
        SO?                             50 mg/Nm3 (25 ppmv)
        HP                             0.5 mg/Nm3 (0.78 ppmv)
        Particulate                     10 mg/Nm3 (0.0057 gr/dscf)
        Heavy Metals (group 1-3)        1 mg/Nm3 (0.00057 gr/dscf)
        Hg (vapor)                      0.5 mg/Nm3 (0,000028 gr/dscf)
                                 4-20

-------
1.  FLUE GAS
2.  EXHAUST GAS
3.  SPRAY DRYER
4.  ELECTROSTATIC PRECIPlTATOR OR FABRIC FILTER
5.  GAS GAS HEAT EXCHANGER
6.  VENTURI SCRUBBER
7.  NEUTRALIZATION TANK
8.  SLUDGE TANK
9.  LIME SILO
10. LIME SLAKER
11. SODIUM HYDROXIDE STORAGE

12. SODIUM AIR TANK
13. DRY WASTE
              Figure  4-6.   Sem-dry/wet  scrubber.
                                                 10

-------
     As discussed in Section 3.2.2, NOX removal  by oxidation/absorption
has been practiced in Japan with 30-90 percent NOX removal  when sodium or
magnesium is the second stage scrubber reagent.   These systems use chlorine
dioxide, C102, rather than NaC102, however.

     The mixing of streams containing chlorides, fluorides, sulfites,  sulfates
and perhaps nitrates appears to be a complicating factor.   Problems relating
to corrosion are dealt with by using Has telloy steel  and rubber linings in
critical scrubber components.  However, when  the mixed streams are handled,
co-precipitation (scale) due to saturation must be addressed.    Also,  the
amount of water to be spray dried is limited  by  the evaporative capacity
of the flue gas and the minimum approach to saturation desired.  Thus, waste
liquor must necessarily be very concentrated  with respect to dissolved
and suspended solids.
                                       4-22

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5.0         EFFECTIVENESS OF FLUE GAS CLEANING METHODS

5.1         Participate Matter Control

            Participate matter control for Incinerator flue gas  applica-
tions 1s a simple matter of choice of control  device and proper  operation
of that device.  Wet scrubbers are relatively  Ineffective for particle
control, removing 80 to 95 percent at normal operating ranges.  Very  high
pressure losses are required to remove fine particles, and the erosion
and corrosion potential 1n acidic gas streams  makes this a poor  choice
from economic and reliability standpoints.

     Electrostatic precipitators are the most widely used and are the
most versatile control systems.  Very low emission levels are achievable
(<0.02 gr/dscf) at high ratios of collector plate surface area to gas flow
volume in the range of 500 min./ft or greater.  Fabric filters are seldom
used without upstream sorbent injection due to a potential for fires  and/or
blinding by sticky particles.  However, fabric filters are also  capable
of control to less than 0.02 gr/dscf and are less sensitive to operational
upsets that disrupt ESP performance.

5.2         Acid Gas Control

            Control of acid gases (HC1, HF, and $03) requires scrubbing
or devices for gas/liquid or gas/solid contact.  Water alone is  a reasonably
effective sorbent for very reactive acid gases such as HC1 and HF, but an
alkali sorbent (or control of liquid pH to the 5 or greater range) is
necessary for substantial S02 control.  Totally dry sorbents require  sub-
stantial residence time in the gas for effective acid gas control.  Injec-
tion of sorbent into a duct must be complemented by either a fluid bed
reactor, humidiflcation, a fabric filter dust collector, or combinations
of these to be effective.

     Spray drying or semi-dry Injection of sorbent is more effective  than
dry injection, with Increasing acid gas control as the approach  to satura-
tion temperature is decreased, either by waste heat recovery or  water
Injection/humidificatlon.  The most effective  control of acid gases is  by
alkali scrubbers operating at saturation, or wet scrubbing, but  this  has
to be weighed against the amounts of waste water generated.

     Pilot plant Canadian data show dry lime injection to be effective
for removing over 90S of the inlet HC1 for a fabric filter inlet tempera-
ture of 140*C (284*F) or less.  In the same plant, the S02 removal was
over 90% for 125'C (257'F), but only 58% at the 140*C filter inlet temp-
erature.  The stoichiometrie ratio was about 1.1 (based on both  HC1 and
S02)  in these tests.

     Combination dry, semi-dry scrubbers control add gases perhaps more
effectively than once-through spray drying and are probably similar in  effec-
tiveness to spray drying with recycle, depending on approach to  saturation
                                   5-1

-------
          TABLE 5-1.  EFFECTIVENESS OF ACID GAS CONTROLS (% REMOVAL)
                                                      Pollutant
 Control System                              HC1	HF	SQ2
 Dry  Injection + Fabric Filter3               80         98          50
 Dry  Injection + Fluid Bed Reactor/ESP^       90         99          60
 Spray Dryer-ESP                              95+        99         50-70
   (Recycle)c                               (95+)      (99)       (70-90)
 Spray Dryer-Fabric Filter                    95+        99         70-90
   (Recycle)C                               (95+)      (99)       (80-95)
Dry/Spray Dryerd                             95+        99          90+
Wet Scrubber6                                95+        99          90+
Wet/Dry Scrubber6                            95+        99          90+
a  T = 160-180'C (320-356'F)
b  T = 230*C (446'F)
C  T = 140-160'C (284-320*F)
d  T = 200'C (392*F)
e  T = 40-50*C (104-122'F)
T is the temperature at the exit of the control device.
                                       5-2

-------
 temperature.  Combination wet-dry systems are the potentially most effective
 system  for add gas control but are Increasingly complex as the number of
 pollutants targeted Increase.  Table 5-1 summarizes the above discussion.
 The reader 1s cautioned that the reagent requirements and solid/liquid
 wastes  are not factored 1n, and this table only reflects systems as oper-
 ated.   Any of these techniques may be enhanced by more reactive sorbents
 or  operation at more favorable temperatures.

      In summary, effective acid gas control is possible with dry, semi-dry,
 and wet scrubbers.  HC1 and HF are relatively easy to control, while S02
 control  1s more difficult and 1s favored by wet or semi-dry systems with
 lower flue gas temperatures.  Although not discussed due to lack of data,
 very effective sulfur trioxide control was reported on a spray dryer pilot
 plant.   Should $03 control also become a concern, systems which contact the
 gas with wet or dry sorbent prior to a particulate control device should
 be  encouraged, since after scrubbing, $03 apparently becomes an aerosol and
 1s  amenable to capture.  Control systems with particle collectors upstream of
 the scrubber have historically reported poor $03 control effectiveness.

 5.3        Post Combustion NOX Control

            Probably the most difficult and expensive pollutant to control
 1s  NOX,  primarily due to the unreactivity of NO which is 95 percent or more of
 the total uncontrolled NOX.  The most effective control is selective, catalytic
 reduction (SCR) which currently must be preceded by acid gas and heavy metals
 control  to be effective.  (However, a low temperature, acid-resistant catalyst
 has recently been applied to three small municipal incinerators 1n Japan, but
 performance data are not yet available.) If the thermal penalties are accept-
 able, then SCR can remove 80-90 percent of NOX with a NH3/NO molar ratio of
 1.0 and about 5 ppmv NH3 slip.  Use of special lower temperature, HCl-resistant
 catalysts in the future can make SCR more attractive.  Potentially less effec-
 tive and more complicated NOX control  may be .chieved by an oxidation step
 integrated into sodium- or magnesium-based wet scrubbing.  Due to the liquid
 waste potential, this may be best applied to the combination wet-dry scrubber
 system described in Section 4.3.2.  Using SCR, a NOX control of 30 to 50
 percent would be expec*ed.

 5.4         Post Combustion Organic Pollutant Control

            Control  of dioxins and furans,  as well as other trace organic
 compounds, Is not well  understood because the mechanism of capture is not
 known.  Likely,  condensation and capture as a particle is significant, and
attack and capture by caustic reagents is also probable.  These capture phe-
nomena are best addressed by lowering flue gas temperatures, subjecting the
VOCs 1n the flue gas to caustic sorbent, and collecting the product on a highly
efficient particle collector.  Limited data (mostly for pilot plants)  show
 that spray drying followed by fabric filtration is very effective for YOC
control  and superior to spray dryer/ESP control.  Also lower flue gas temper-
atures favor Increased  YOC control.  Reference 13 is a good discussion of
                                      5-3

-------
 these  observations.  The results are summarized In Table 5-2, where COD
 refers to chlorinated d1benzo-paradioxlns
 and CDF to chlorinated dibenzofurans.


     TABLE 5-2.  SPRAY DRYER CONTROL OF SELECTED ORGANIC POLLUTANTS17

                             Control System (% Removal)
Compound

 D1oxlns:

tetra CDD

penta CDD
hexa CDD
hepta CDD
octa CDD

 Furans;

tetra CDF
penta CDF
hexa CDF
hepta CDF
octa CDF
    SD + ESP    SD + FF @ High Temp.
       48

       51
       73
       83
       89
       65
       64
       82
       83
       85
<52

 75
 93
 82
 NA
 98
 88
 86
 92
 NA
                     SD + FF 0 Low  Temp.
>97

>99.6
>99.5
>99.6
>99.8
>99.4
>99.6
>99.7
>99.8
>99.8
     Reference 7 notes that only limited data have been collected on control
device efficiencies for dioxins and furans, with only outlet concentrations
being reported for most tests.  Unfortunately, test data and methodologies
are lacking to compare the effectiveness of various control systems on organic
pollutants.  However, the superiority of a sorbent on a fabric filter for
control is evident from Table 5-2.  The data shown were based on tests in a
single pilot plant, and thus should be used with caution.

     The results of Canadian tests, based on an Incinerator flue gas slip-
stream and the use of dry lime Injection (110-140*C) and lime spray drying
(140"C), each with a downstream fabric filter for dust collection, show high
overall removal efficiencies (99+Z) for dioxins and furans for the fabric
filter Inlet temperatures noted.  With dry lime injection, the removal of
other organlcs (chlorobenzenes, chlorophenols, and polychlorinated biphenyls)
fell markedly at 200'C compared with inlet fabric filter temperatures of
110-140'C (230-284'F).
5.5
Heavy Metals Control
            The control of heavy metals 1s similar to organic pollutant control
in that the effective control of particles and low flue gas temperatures are
major factors.  Sorbents, however, are not suspected to play a major role.
                                      5-4

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 Toxic  metals enter  the collectors as solids, liquids, and vapors, and as the
 flue gas  cools,  the  vapor portion converts to collectible solids and liquids.
 Figure 5-1  illustrates various  heavy metals as they appear in flue gas and
 their  relative  theoretical concentrations  (vapor pressures) as a function of
 flue gas  temperature.

     From Figure 5-1, it can be deduced that reduction of flue gas tempera-
 tures  below 200°C (392°F) and high efficiency particulate collection should
 result in a very large reduction of metals, except for mercury (Hg), arsenates
 (As2d3)2, and selenium (Se02 and See).  Corresponding reductions of these
 compounds proceed dramatically as temperatures are lowered.  With the metals
 at  there  saturation  temperatures, each is expected to be reduced by 90 percent
 for each  additional  temperature drop of 11 to 17*C (20 to 30*F).  If this
 temperature effect is true, then wet scrubbing or wet/dry scrubbing which
 operates  at saturation [ <\, 40"C (104'F)] will be most effective for total
 heavy  metals control, while most dry and semi-dry systems will be effective
 for practically all  metals except mercury, arsenic, and selenium.

     Reported metals control  data generany show 95-98 percent control  or
 greater for most heavy metals except mercury.  Vapor phase mercury control
 has been  reported as follows:  75 to 85 percent control with spray dryer plus
 baghouse; 35 to 45 percent control with spray dryer plus ESP.1^ This is im-
 portant in that vapor control is possible with fabric filters and ESPs,
 although  limited data show the former to be clearly superior.  Wet scrubbers
would appear to be ideal  for  mercury control, but the collection of mercury
 vapors  via condensation and capture is not well  documented.  Therefore, the
choice  of the most effective  mercury control  is  still the subject of contro-
versy  (see Reference 10).
                                    5-5

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            ttOOOO
             10000
Concentration
   mg/rn^
                              Measured He, Concentrations hi raw fas
          Figure 5-1.   Saturation points of metal  and metal compounds.
                                                                         16
                                              5-6

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     6.0      OPERATION AND MAINTENANCE OF FLUE GAS CLEANING SYSTEM

              This section is intended to summarize good operating and main-
tenance (04M) practices for flue gas controls typically applied to municipal
waste incinerators.  Advanced controls, such as selective catalytic reduction
and wet oxidation-absorption NOX control, are not addressed here,  as exper-
ience with these systems is too limited to warrant a discussion.

6.1           Electrostatic Precipitators9

              Poor performance of an electrostatic precipitator can be divided
into fundamental problems, mechanical problems, and operational problems.
Fundamental problems include design inadequacies such as poor gas  flow distri-
bution, inadequate collector area, or unstable energization equipment.  These
are best addressed by replacement or redesign of problem areas and are inde-
pendent of the 0AM program.  Mechanical problems include electrode misalignment,
wire breakage, cracked collector surfaces, air inleakage, cracked  insulators,
plugged hoppers, and dust deposits.  Defective components should be replaced
once the cause of the problem has been identified.  Operational problems,
which are those last addressed by O&M programs, include process upsets,
inadequate power input, electrical problems, rapper failures, and  dust removal
valve failures.

     A good performance monitoring program for ESPs is recommended, which
includes measurement of key operating parameters, performance tests, and
monitoring and recordkeeping of key operating parameters.  These parameters
include gas volume flow, velocity, and temperature; chemical composition
of gases and particles; particle concentrations, size distribution, and resis-
tivity; and power input to the ESP.  Use of on-line instrumentation (such
as voltage and current meters, spark meters, rapper monitors, transmissometers,
and hopper level indicators) are essential for proper ESP operation.  Periodic
performance testing for particulate concentrations, such as Reference Methods
5 or 17 and Method 9, are useful tools in evaluating long-term or  gradual
effects not easily monitored.

     O&M practices include development of procedures for startup,  shutdown,
and routine operation.  Inspections on a daily, weekly, and annual basis
are recommended as shown in Table 6-1.

     Inspection procedures and detailed 0AM guidance for ESPs may  be found
in Reference 9.
                                     6-1

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                     TABLE 6-1.  INSPECTIONS FOR ESP9
 Dail;
  o Corona power levels (i.e., primary current, primary voltage, secondary
    current, secondary voltage) by field and chamber,  (twice a shift)

  o Process operating conditions [i.e., firing rates, steam flow or load
    (Ib/h), flue gas temperature, flue gas oxygen, etc.].  The normal
    operator's log may serve this purpose,  (hourly)

  o Rapper conditions (i.e., rappers out, rapper sequence, rapper intensity,
    rapping frequency by field and chamber).

  o Dust discharge system (conveyors, air locks, valves for proper
    operation, hopper levels, wet-bottom liquor levels).

  o Opacity (i.e., absolute value of current 6-minute average and range
    of magnitude of rapper spiking) for each chamber duct if feasible.
    (2-hour intervals)

  o Abnormal operating conditions (i.e., bus duct arcing, T-R set control
    problems, T-R set trips excessive sparking),  (twice  a shift)

  o Audible air inleakage (i.e., location and severity).


Weekly;

  o Trends analysis (plot gas load V-I curves for each field and chamber
    and other key parameters to check for changes in values as compared
    with baseline).

  o Check and clean or replace T-R set cabinet air filters and insulator
    purge air and heating system filters.

  o Audible air inleakage (i.e., location and severity).

  o Abnormal  conditions  (i.e.,  bus duct arcing, penthouse and shell  heat
    systems,  insulator heaters, T-R set oil  levels,  and temperature).

  o Flue gas  conditions  exiting the ESP (i.e.,  temperature and oxygen
    content).

  o More extensive  rapper checks (also optimize rapper operation if  needed).
                                     6-2

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                            TABLE  6-1  (Cont'd)

Annually:

  o Transformer Enclosure

      HV line,  insulators,  bushings, and  terminals
      Electrical  connections
      Broken surge arresters

  o High-Voltage  Bus  Duct

      Corrosion of duct
      Wall  and  post insulators
      Electrical  connections

  o Penthouse,  Rappers,  Vibrators

      Upper rapper rod  alignment
      Rapper rod  insulators
      Ash  accumulation
      Insulator clamps
      Lower rapper rod  alignment
      Support insulator heaters
      Dust in penthouse area
      Corrosion in penthouse area
      Water inleakage
      HV connections
      HV support  insulators
      Rapper rod  insulator  alignment

  o Collecting  Surface  Anvil Beam

      Hanger rods
      Ash  buildup
      Weld  between anvil beam and  lower rapper rod

  o Upper Discharge Electrode Frame Assembly

      Welds between hanger  pipe and hanger frame
      Discharge frame support bolts
      Support beam welds
      Upper frame levelness and alignment to gas stream

  o Lower Discharge Electrode Frame Assembly

      Weight guide rings
      Levelness of frame
      Distortion  of the  frame
                                  6-3

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                            TABLE 6-1 (Cont'd)

  o Stabilization Insulators

      Dust buildup and electrical tracking
      Broken Insulators

  o Collecting Electrodes

      Dust deposits; location and amount
      Plate alignment
      Plate plumbness
      Plate warpage

  o Discharge Electrode Assembly

      Location of dust buildup and amount
      Broken wires
      Wire alignment
      Weight alignment and movement

  o Hoppers

      Dust buildup
      Level  detectors
      Heaters
      Vibrators
      Chain wear, tightness,  and alignment
      Dust buildup in corners and walls

o  Dust Discharge System

      Condition of valves,  air locks,  conveyors

o  General

      Corrosion
      Interlocks
      Ground system
      Turning vanes,  distribution plates,  and  ductwork
                                  6-4

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 6.2           Fabric Filters8

              Poor performance of fabric filters can be categorized by
 (1) problems that affect all fabric filters, regardless of type, and
 (2) problems that are characteristic of a particular cleaning system design.
 The first category includes fabric failure, dust discharge problems, corrosion,
 and improper maintenance considerations.

     Fabric failures that occur Immediately after the baghouse goes on line
 are generally caused by improper Installation or manufacturing defects.   With
 proper design and operation, these failures are usually isolated in early
 stages of operation.  Generally reverse-air and shaker-type fabric filters
 are more prone to initial failures than pulse-jet systems.  Other fabric
 failures may be caused by high temperatures, condensation, chemical degrada-
 tion, too high air-to-cloth ratio, high pressure drops, and bag abrasion.
 Dust discharge failures generally result from cool  spots 1n the dust hopper,
 air leakage, or failure (either mechanical  or human error) to keep the dust
 level in the hopper manageable.  Also bag cleaning  systems have failures
 generic to the system type.  Failure to clean bags  properly results in
 increased pressure drop, bag failure, and reduced bag life.

     An effective monitoring program that Includes  performance evaluation
and data collection is a necessity.  The most important parameters to record
and evaluate are the opacity of exhaust gases and pressure drop across an
individual compartment or the entire baghouse.  As  a general rule, these are
checked daily to determine if operation is  within the normal range for that
system.

     04M procedures include established startup, operating, and shutdown
procedures which emphasize avoiding dew point conditions through cold gas
bypass, auxiliary heat, and system purges.   Preventive maintenance practices
include periodic inspections of components  as shown in Tables 6-2 through
6-4.

     Inspection procedures and detailed 04M guidance for fabric filters
may be found in Reference 8.

6.3           Scrubbers

              Although wet scrubbers with liquid wastes are not expected to
be Installed in quantity, operation and maintenance procedures are presented
briefly here.   The more likely to be used dry scrubber systems are discussed
separately from wet scrubbers.  Considerations are  based on calcium-based
scrubbing for both systems, sodium-based systems being unlikely to be used
because of soluble waste disposal restrictions and  cost considerations.^
                                    6-5

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                TABLE 6-2.  TYPICAL MAINTENANCE INSPECTION
                   SCHEDULE FOR A FABRIC FILTER SYSTEMS
Inspection
frequency
       Component
             Procedure
Dally
Stack and opacity monitor

Manometer



Compressed air system


Collector
Weekly
              Damper valves
              Rotating equipment and
              drives

              Dust removal system
Filter bags
              Cleaning system
              Hoppers
Check exhaust for visible dust.

Check and record fabric pressure
loss and fan static pressure.
Watch for trends.

Check for air leakage (low
pressure).  Check valves.

Observe all indicators on
control panel and listen to
system for properly operating
subsystems.

Check all isolation, bypass, and
cleaning damper valves for
synchronization and proper operation.

Check for signs of jamming, leakage,
broken parts, wear, etc.

Check to ensure that dust is being
removed from the system.

Check for tears, holes, abrasion,
proper fastening, bag tension, dust
accumulation on surface or in
creases and folds.

Check cleaning sequence and cycle
times for proper valve and timer
operation.  Check compressed air
lines including oilers and filters.
Inspect shaker mechanisms for
proper operation.

Check for bridging or plugging.
Inspect screw conveyor for proper
operation and lubrication.
                                   6-6

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                            TABLE 6-2 (Cont'd)
 Inspection
 frequency
       Component
             Procedure
Monthly
 "uarterly
Shaker mechanism

Fan(s)




Monitor(s)


Inlet plenum



Access doors

Shaker mechanism
Semi-
annually
Annually
Motors, fans, etc,
Collector
Inspect for loose bolts.

Check for corrosion and material
buildup and check Y-belt drives
and chains for proper tension and
wear.

Check accuracy of all indicating
equipment.

Check baffle plate for wear; if
appreciable wear is evident,
replace.  Check for dust deposits.

Check all gaskets.

Tube type (tube hooks suspended
from a tubular assembly):  Inspect
nylon bushings in shaker bars and
clevis (hanger) assembly for wear.

Channel shakers (tube hooks
suspended from a channel bar
assembly):  Inspect drill bushings
in tie bars, shaker bars, and
connecting rods for wear.

Lubricate all  electric motors,
speed reducers, exhaust and
reverse-air fans, and similar
equipment.

Check all bolts and welds.
Inspect entire collector thoroughly,
clean, and touch up paint where
necessary.
                                   6-7

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    TABLE 6-3.  ROUTINE INSPECTION DATA FOR  REVERSE AIR FABRIC FILTERS8
o Routine Inspection Data
  Stack
  Fabric Filter
 Average  Opacity
 Opacity  During the Cleaning Cycles (for each compartment)

 Inlet and Outlet Static Pressures
 Inlet Gas Temperature
 Rate of  Dust Discharge (Qualitative Evaluation)
 Presence or Absence of Audible Air Infiltration
 Presence or Absence of Clean Side Deposits
 Ripping  Strength of Discarded Bags
o Baseline and Diagnostic  Inspection Data
  Stack
  Fabric  Filter
  Stack Test
 Fan
Average Opacity
Opacity During the Cleaning Cycles (for each compartment)

Date of Compartment Rebagging
Inlet Static Pressure (Average)
Outlet Static Pressure (Average)
Minimum, Average, and Maximum Gas Inlet Temperatures
Average 02 and C02 Concentrations (Combustion Sources Only)
Time to Complete a Cleaning Cycle of all Compartments
Length of Shake Period
Length of Null Period
Bag Tension (Qualitative Evaluation)
Rate of Dust Discharge (Qualitative Evaluation)
Presence or Absence of Audible Air Infiltration
Presence or Absence of Clean Side Deposits

Emission Rate
Gas Flow Rate
Stack Temperature
02 and C02 Content
Moisture Content

Fan Speed
Fan Motor Current
Gas Inlet and Outlet Temperatures
Damper Position
                                  6-8

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    TABLE 6-4.  ROUTINE  INSPECTION  DATA FOR PULSE-OET FABRIC FILTERS^
o Routine Inspection  Data

  Stack
  Fan

  Fabric Filter
Average Opacity
Duration and Timing of Puffs

None

Inlet and Outlet Gas Temperatures
Inlet and Outlet Static Pressures
Presence or Absence of Clean Side Deposits
Air Reservoir Pressure
Audible Checks for Air Inleakage
Qualitative Solids Discharge Rate
o Diagnostic Inspection  Data

  Stack
  Fan
  Fabric  Filter
Average Opacity
Peak Opacity During Puffs
Duration and Timing of Puffs

Inlet Gas Temperature
Speed
Damper Position
Motor Current

Inlet Gas Temperature
Outlet Gas Temperature
Inlet Static Pressure
Outlet Static Pressure
Inlet 02 and C0£ Content (Combustion Sources)
Outlet 02 and C02 Content (Combustion Sources)
Qualitative Solids Discharge Rate
Air Reservoir Pressure
Frequency of Cleaning
Presence or Absence of Clean Side Deposits
Audible Air Infiltration
                                  6-9

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6.3.1         Lime/Limestone Wet Scrubbing*3

              Extensive surveys of lime/limestone wet scrubbing technology
have shown the eight major areas of failure listed below in order of decreasing
likelihood of failure:

              o mist eliminators
              o ductwork
              o absorber
              o stack
              o fans
              o pipes and valves
              o thickener
              o dampers

     Design considerations allow ease in addressing these problems but
are too detailed to be presented here.  They are discussed extensively
in Reference 13.  Inspection procedures for each major unit operation
are also detailed in this reference.

     04M practices, preventive maintenance procedures, and unscheduled
maintenance procedures are presented  in detail  in the following discussion.

     Standard 04M procedures for lime/limestone scrubbers include:

     o Variable Load Operation.  With each change in load, the operator
       must check the system to verify that all in-service modules are
       operating in a balanced condition.   As the acid gas concentrations
       in the inlet flue gas change,  the FGD system should be able to
       accommodate and compensate for such change.  Operator surveillance
       of system performance is needed, however, to verify proper system
       response (e.g., slurry recirculation  umps can be added and removed
       from service as the acid gas concentration increases or decreases).

     o Verification of Flow Rates.  The easiest method of verifying liquid
       flow rates is for an operator  to determine the discharge pressure
       in the slurry recirculation  spray header with a hand-held pressure
       gauge  (permanently mounted pressure gauges frequently plug in
       slurry service).   Flow 1n slurry piping  can be checked by touching
       the pipe.   If the piping 1s  cold to the  touch at the normal operating
       temperature of 52-54*C (125-130*F), the  line may be plugged.

     o  Routine  Surveillance of Operation.   Visual inspection of the
       absorbers  and reaction tanks can identify scaling, corrosion, or
       erosion  before they seriously  impact the operation of the system.
       Visual  observation  can identify leaks, accumulation of liquid or
       scale  around process piping, or discoloration on the ductwork
       surface  resulting  from inadequate or deteriorated lining material.
                                     6-10

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 o M1st Eliminators.  Many techniques have been employed to Improve
  mist collection and minimize operational problems.  The mist
  eliminator can be washed with a spray of process makeup water or a
  mixture of makeup water and thickener overflow water.  Successful
  long-term operation without mist eliminator plugging generally
  requires continuous operator surveillance, both to check the
  differential pressure across the mist eliminator section and to
  visually inspect the appearance of the blade surface during shut-
  down periods.

 o Reheaters.  Inline reheaters are frequently subject to corrosion
  by chlorides and sulfates.  Plugging and deposition can also occur
  but are more rare.  Usually, proper use of soot blowers prevents
  these problems.

 o Reagent Preparation.  Operational procedures associated with
  handling and storage of solid reagent are generic to solids
  handling.  Operation of pumps, valves, and piping in the slurry
  preparation equipment 1s similar to that in other slurry service.

 o Pumps, Pipes, and Valves.  Operating experience has shown that
  pumps, pipes, and valves can be significant sources of trouble in
  the abrasive and corrosive environments of lime/limestone FGD systems.
  The flow streams of greatest concern are the reagent feed slurry,
  the slurry recirculation loop, and the slurry bleed streams.  When
  equipment is temporarily removed from slurry service, it must be
  thoroughly flushed.

 o Thickeners.  Considerable operator surveillance is required to
  minimize the suspended solids in the thickener overflow so that
  this liquid can be recycled to the system as supplementary pump
  seal water, mist eliminator wash water, or slurry preparation
  water.  For optimum performance, the operator must maintain
  surveillance of such parameters as underflow slurry density,
  flocculant feed rate,  inlet slurry characteristics, and turbidity
  of the overflow.

o Waste Disposal.  For untreated waste slurry disposal, operation of
  both the discharge to  the pond and the return water equipment
  requires attention of  the operating staff.  In addition to normal
  operations, the pond site must be monitored periodically for proper
  water level,  embankment damage, and security for protection of the
  public.   Landfill  disposal  Involves the operation of secondary
  dewatering equipment.   Again,  when any of the process equipment is
  temporarily removed from service,  it must be flushed and cleaned
  to prevent deposition  of waste solids.  For waste treatment
  (stabilization  or  fixation),  personnel are required to operate the
  equipment and to maintain proper process chemistry.
                              6-11

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     o Process Instrumentation and Controls.  Operation of the FGD system
       requires more of the operating staff than surveillance of automated
       control loops and attention to indicator readouts on a control  panel.
       Manual control and operator response to manual  data indication  are
       often more reliable than automatic control  systems and are often
       needed to prevent failure of the control system.  Many problems
       can be prevented when an operator can effectively Integrate manual
       with automated control techniques.

     Preventive Maintenance Programs.  Preventive  maintenance is the
practice of maintaining system components in such  a  way as to prevent
malfunctions during periods of operation and to extend the life of the
equipment.  The goal of preventive maintenance is  to increase availability
of the FGD system by eliminating the need for emergency repair ("reactive
maintenance").

     The term preventive maintenance is synonymous with periodic maintenance.
Such procedures may be as simple as lubrication of a pump or as complex
as complete disassembly for inspection and overhaul.  Some of the more
important preventive maintenance procedures by subsystems are summarized
in the following sections:

     o Absorbers.   Of primary concern in the absorber  module is the
       integrity of the structural  materials.   Maintenance personnel
       should enter and inspect the absorber module  at least semi-annually.

     o Mist Eliminators.  Scale deposits typically are the chief maintenance
       factor with mist eliminators.   The mist eliminator may be subject
       to nonuniform flow or a faulty wash system.   Wash spray pressure
       should be monitored.   Mist eliminators  should be inspected during
       forced or scheduled  outages.

     o Reheaters.   Both inline and indirect reheaters  are subject to
       scaling and corrosion.   In addition to  visual inspection,  pressure
       testing and measurement of heat transfer efficiency are useful  in
       quantifying the  magnitude of a reheater problem.   In a direct
       reheat system,  the mixing chamber and the air heating equipment
       must be checked  routinely.

     o Dampers.  Fans. Ductwork,  and  Chimneys.   All points in the system
       must be checked  for  integrity  of lining materials and for damage
       resulting from collection of  condensation products in stagnant  air
       spaces (e.g.,  duct elbows and  corners).   Components located in  the
       wet portion of  the system are  subject to scaling and corrosion.
       Upstream  fans  and ductwork  may be subjected to  erosion.
                                    6-12

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 o Reagent Preparation.  Reagent preparation subjects the ball  mill
  or slaker to abrasive wear.  Because the equipment sees intermittent
  service, it should be inspected visually each time it is placed in
  service.  Annual disassembly is also needed to check for excessive
  wear.

 o Reagent Feed.  Maintenance of the reagent slurry feed system is
  critical because failure of this equipment strongly impacts  the
  FGD system operation.  The slurry storage tank should be checked
  daily for leakage and associated equipment inspected for proper
  operation.

 o Pumps, Pipes, and Valves.  Slurry pumps are normally disassembled
  at least annually.The purpose of the inspection is to verify
  lining integrity and to detect wear and corrosion or other signs
  of potential failure.  Bearings and seals are checked but not
  necessarily replaced.  Pipelines also must be periodically dis-
  assembled or tested in other ways (e.g., hand-held nuclear and
  ultrasonic devices) both for solids deposition and for wear.
  Valves must be serviced routinely, especially control valves.

 o Thickeners.   Thickener coatings should be inspected periodically
  to prevent corrosion.  Drag rakes, torque arms, and support  cables
  must also be inspected for wear.

o Waste Disposal  Equipment.  Secondary dewatering devices, mixing
  components,  and transport equipment must also have periodic  main-
  tenance to check for abrasive wear and solids deposition.  Vacuum
  filters, both drum and belt type,  require periodic replacement of
  the filter media.  In a centrifuge, both the scroll coating  and
  the bowl surfaces are subject to wear.

o Process Instruments and Controls.   All  electronic equipment  (pH,
  flow,  pressure, temperature,  level, vibration, noise, and continuous
  monitors)  must be calibrated  periodically.   Numerous installation
  and maintenance techniques have proved beneficial in ensuring the
  reliability  of  sensors.   Ease of access to  the sensors is very
  important.   The sensors should be  cleaned and calibrated routinely.
  Experience with process  instrumentation and controls in FGD  systems
  has shown  that a good preventive maintenance program begins  with
  daily  operating procedures.  Proper use of  instruments will  include
  daily  flushing  of most instrument  lines in  slurry service just
  before monitoring of process  variables.  Routine comparison  of the
  instruments  in  a process stream with similar instruments in  parallel
  streams  can  point out incipient failures.  Operating data, especially
  from  the startup test program,  can also indicate potential problem
  areas.
                                   6-13

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     Unscheduled Maintenance.  Even the most rigorous preventive  maintenance
program will not prevent random failures to which the maintenance staff
must respond.  Most malfunctions are correctable by unscheduled (reactive)
maintenance.  In some situations, usually during initial  system startup,
design modifications may be required to bring the system  into compliance
with operating standards.  Each subsystem of the FGD system is subject to
malfunctions from a variety of causes.   The discussion that follows
Introduces these problems and the probable responses.

     o Absorbers.  Structural failure of absorber internals and recycle
       pump suction screens have occurred as a  result of  excessive vibra-
       tion, uncorrected corrosion damage, or high pressure differentials.
       These malfunctions must be repaired Immediately before operation
       is resumed.

     o Mist Eliminators.  Failure of the mist eliminator  is typically due
       to scaling and plugging.   The scale may  be removed either  by
       thorough washing or by mechanical  methods, in which maintenance
       personnel  enter the absorber and manually chip away the scale
       deposits.

     o Reheaters.  Reheater malfunctions  include tube failures in inline
       reheaters, damper problems in bypass  reheat,  or nonuniform flows
       in indirect  reheaters.   Correction of these problems •  11  probably
       necessitate  changes in equipment design.

     o Fans.   Fans  can develop vibrations resulting  from  deposition of
       scale  in wet service or from erosion  of  blades in  dry  service.
       The cause  of the vibration must  be eliminated and  the  fan  repaired
       and rebalanced.

     o Ductwork.  Most problems  associated with  ducts develop  over a long
       period.  Sudden  or  gross  failures,  such  as a  major  leak, call for
       immediate  repair.   Temporary  repair or patching  may  suffice until
       the next scheduled  outage.   Acid condensation  in a  chimney  can
       cause  lining  deterioration  and subsequent damage to  the base metal.
       These  problems  are  usually  Identified during  preventive maintenance
       inspections  and  require long-term  solutions.

    o  Reagent Feed.  Malfunctioning components  such  as slakers must be
       repaired in  accordance  with  the manufacturer's  instructions.  Some
       facilities have  experienced  trouble with  plugging of the lime/lime-
       stone  feeder  due  to  intrusion of moisture.  Correction  of  these
       problems will probably  necessitate  changes  in  equipment design.

    o  Pumps. Pipes, and Valves.  Excessive wear  of  the impeller  or
       separation of the lining from the  pump casing  is a common  problem.
      Operation of a slurry pipeline with Insufficient flow velocity can
      cause  clogging.  High flow velocity or extended service  can cause
      erosion.  Malfunction and binding of a valve actuator are  typically
      caused by wear-induced misalignment.
                                        6-14

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      o Thickeners.  The thickener underflow can become plugged because of
       excessive solids in the slurry or failure of the underflow pump.
       A plugged underflow or rapidly settling waste solids will produce
       a heavy  "blanket" in the bottom of the thickener.  The rake must
       then be  raised so that the torque remains within acceptable limits.
       If the torque cannot be kept within limits, the thickener must be
       drained  and the sludge blanket removed manually.

 6.3.2         Semi-dry Scrubbing20

              Although little information is published on OiM of dry or
 semi-dry scrubbing systems, some vendor-specific information has been
 compiled and condensed into a general guide for the spray dryer/absorber
 unit  operation.  Keep in mind that the particle collector is an integral
 part  of the overall system and information in 6.1 or 6.2 should be
 added to the guidelines below in developing an integrated OiM program.

     The following operations are key OiM considerations in spray dryer
 operation:

     Temperature Measurements - A key operating parameter is the approach
 to adiabatic saturation temperature.  The "approach" is the difference between
 the dry and wet bulb temperatures at the exit of the spray dryer (or fabric
 filter as appropriate).  While instrumentation is available to monitor
 the wet bulb temperature, the reliability of such instrumentation is not
 as high as desired, and frequent manual wet bulb measurements are recommended
 for systems operated close to the adiabatic saturation temperature.
 Also, the spray dryer outlet thermocouples should be regularly checked for
 proper calibration.

     Slurry Preparation.   S02 removal performance depends greatly on lime
 reactivity.While most lime vendors produce a consistently high quality
 product, the quicklime reactivity should be checked frequently using the
 ASTM C110-10, 3-minute temperature rise test.  The slaker outlet tempera-
 ture should also be checked routinely to verify exit temperatures in the
 desired range of 77-89*C (170*F-190'F).  This is a good indicator of
 proper slaker operation,  suitable quicklime reactivity, and proper slaker
 exit solids content in the product slurry.

     The lime and atomizer feed preparation systems require handling of
 slurries with high solids concentrations.  Settling of solids can lead to
buildup and pluggage of the handling equipment.  Routine checks should be
made for proper mixing of solids in the slurry and for buildup of solids
1n piping,  on tank walls,  etc.  In particular, screens or strainers need
to be checked frequently  for pluggage.
                                   6-15

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      The  atomizer  feed  slurry solids content can be an important parameter
 for  assuring  proper  solids drying and to assure that maximum recycle
 ratios  are  being employed.  The solids contents are generally continuously
 measured  by and controlled to some type of density meter reading.  These
 meters  are  notorious for calibration drift and should be checked for cali-
 bration regularly.   Some systems installed on utility boilers check density
 meter calibration  as often as once per day.

      Atomizers.  If  rotary atomizers are used in the spray system, the
 atomizer  drive system requires continuous monitoring.  The atomizer drive
 lubrication and cooling system should be continuously monitored and
 maintained according to the manufacturer's instructions.  The vibrations
 produced  by the drive system should also be monitored either continuously
 or at regular intervals.  Improper operation of the atomizer drive system
 at normal operating  speed can result in immediate and usually catastrophic
 failure.  The atomizer should always be dynamically balanced prior to use.
 The  atomizer  wheel or disk should also be periodically inspected for
 nozzle blockage, solids buildup, and excessive nozzle surface wear.

     Two-fluid nozzle or pressure atomization is mechanically a much
 simpler process than rotary atomization and requires less rigorous
monitoring.   Nozzles should be periodically inspected for blockage, wear,
and  corrosion.  Nozzles should be properly positioned inside the reactor
vessel,  and all  fittings should be free of leaks.   Liquid and air pressure
should be monitored and kept at the appropriate levels to assure proper
atomization.  The spray dryer vessel  should be periodically inspected to
ensure that a nozzle or several  nozzles are not partially plugged and
producing a coarse spray which has caused a buildup of wet solids on
vessel walls.
                                     6-16

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 REFERENCES

  1.  Municipal Waste Combustion Study - Data Gathering Phase.  Preliminary
     Draft.  Radian Corporation, October 16. 1985, Part I, Chapter 4.0.

  2.  Municipal Haste Combustion Study;  Combustion Control of Organic
     Emissions!EPA/530-SW-87-Q21C, June 1987.

  3.  DuBard, J. L.. Southern Research Institute.  Private Communication  to
     Dale L. Harmon, EPA/AEERL, October 8, 1986.

  4.  DuBard, J. L., Southern Research Institute.  Private Communication  to
     Dale L. Harmon, EPA/AIERI, October 28, 1986.

  5.  Clarke, M. J., "Emission Control Technologies for Resource Recovery."
     Presented at the Symposium on Environmental Pollution in the Urban
     Area, Booklyn, NY, Subsection of the American Chemical Society,
     March 15, 1986.

  6.  Review and Update of PM Emissions Database for Solid
     Waste-Fired Industrial Boilers.  Memorandum.  Radian Corporation,
     August 29, 1986.

  7.  Municipal Waste Combustion Study;  Emission Data Base for Municipal
     Waste Combustors. EPA/530-SW-87-021b. June 1987.

  8.  Operation and Maintenance Manual for Fabric Filters.  EPA-625/1-86-020,
     June 1986, pp. 2-1 through 2-17.
 9.  Operation and Maintenance Manual for Electrostatic Preci pita tors.
         -625/1-85-017, September 1985, pp, 5-1 through 6-72.
Ope re
EPA-(
10.  Assessment of Flue Gas Cleaning Technology for Municipal  Waste Combustion.
     Draft Report.  Acurex Corporation, September 1986.

11.  Ando, J. Recent Developments 1n S02 and NOX Abatement Technology for
     Stationary Sources in Japan,  JJraft Report,  (to be published as an
     EPA report in 1987).

12.  Ellison, William H.  "Status of German FGD and DeNOx."  Presented at the
     Third Annual Pittsburgh Coal Conference, Pittsburgh, PA,  September 9,  1986.

13.  Flue Gas Desulfurizatipn Inspection and Performance Evaluation.
     EPA-625/1-85-019, October 1985, pp. 99-222.

14.  Scrubber-Adsorber Newsletter.  Mcllvalne Co., Northbrook, IL, July 30,
     1986, No. 145, pp 3-6.

15.  The National Incinerator Testing and Evaluation Program:  Air Pollution
     Control  Techno!oqylEnvironment Canada, Ottawa, Ontario.Report
     EPS 3/UP/2, September 1986.
                                      R-l

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16.  Moeller, J. T., Jorgensen, C., and Fallenkamp,  F.,  "Dry  Scrubbing  of
     Toxic Incinerator Flue Gas by Spray Absorption."  Presented  at ENVITEC
     83, Dusseldorf, W. Germany, February 21-24,  1983.

17.  Neilsen, K. K., Moeller, J. T., and Rasmussen,  S.,  "Reduction of
     Dioxins and Furans by Spray Dryer Absorption from Incinerator Flue
     Gas."  Presented at Dioxin 85, Bayreuth,  W.  Germany,  September 16-19,
     1985.

18.  Environmental Test Report:  Marlon County Solid Waste-to-Energy Facility.
     Ogden Projects, Inc., Emeryville, CA.   Report No.  107, November 21, 1986.

19.  Teller,  A. J., "The Landmark Framingham,  Massachusetts,  Incinerator."
     Presented at the Hazardous Materials Management Conference,  Philadelphia,
     PA, June 5-7, 1984
20.  Design and Selection  Considerations  for  Spray  Dryer  Based Flue Gas
     Desulfurization Systems.EPA  Contract 68-02-2994, WA 1/081. Radian
   ^
   Jm	
Corporation, September 30,  1986,  pp.  5-1  to  5-3.
                                    R-2

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