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
-------
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
-------
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
§
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
-------
1
\/
^{ *tpn \ ^.
^- r""™"\
^A )
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
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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
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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
-------
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
-------
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
-------
ttOOOO
10000
Concentration
mg/rn^
Measured He, Concentrations hi raw fas
Figure 5-1. Saturation points of metal and metal compounds.
16
5-6
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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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