UFC 3-430-03
15 May 2003
UFC 3-430-03
15 May 2003
Any copyrighted material included in this UFC is identified at its point of use.
Use of the copyrighted material apart from this UFC must have the permission of the
copyright holder.
U.S. ARMY CORPS OF ENGINEERS (Preparing Activity)
Record of Changes (changes are indicated by \1\ ... /1/)
Change No.
This UFC supersedes TM 5-815-1, dated 9 May 1988. The format of this UFC does not conform to
UFC 1-300-01; however, the format will be adjusted to conform at the next revision. The body of
this UFC is a document of a different number.
UFC 3-430-03
15 May 2003
The Unified Facilities Criteria (UFC) system is prescribed by MIL-STD 3007 and provides
planning, design, construction, sustainment, restoration, and modernization criteria, and applies
to the Military Departments, the Defense Agencies, and the DoD Field Activities in accordance
with USD(AT&L) Memorandum dated 29 May 2002. UFC will be used for all DoD projects and
work for other customers where appropriate. All construction outside of the United States is
also governed by Status of forces Agreements (SOFA), Host Nation Funded Construction
Agreements (HNFA), and in some instances, Bilateral Infrastructure Agreements (BIA.)
Therefore, the acquisition team must ensure compliance with the more stringent of the UFC, the
SOFA, the HNFA, and the BIA, as applicable.
UFC are living documents and will be periodically reviewed, updated, and made available to
users as part of the Services’ responsibility for providing technical criteria for military
construction. Headquarters, U.S. Army Corps of Engineers (HQUSACE), Naval Facilities
Engineering Command (NAVFAC), and Air Force Civil Engineer Support Agency (AFCESA) are
responsible for administration of the UFC system. Defense agencies should contact the
preparing service for document interpretation and improvements. Technical content of UFC is
the responsibility of the cognizant DoD working group. Recommended changes with supporting
rationale should be sent to the respective service proponent office by the following electronic
form: Criteria Change Request (CCR). The form is also accessible from the Internet sites listed
UFC are effective upon issuance and are distributed only in electronic media from the following
Whole Building Design Guide web site
Hard copies of UFC printed from electronic media should be checked against the current
electronic version prior to use to ensure that they are current.
Chief Engineer
Naval Facilities Engineering Command
The Deputy Civil Engineer
DCS/Installations & Logistics
Department of the Air Force
Dr. GET W. MOY, P.E.
Director, Installations Requirements and
Office of the Deputy Under Secretary of Defense
(Installations and Environment)
Chief, Engineering and Construction
U.S. Army Corps of Engineers
TM 5-815-1
MAY 1988
This manual has been prepared by or for the Government and, except to the extent
indicated below, is public property and not subject to copyright.
Copyright material included in this manual has been used with the knowledge and
permission of the proprietors and is acknowledged as such at point of use. Anyone
wishing to make further use of any copyrighted materials, by itself and apart from
this text, should seek necessary permission directly from the proprietors.
Reprints or republications of this manual should include a credit substantially as
follows: :Joint Departments of the Army and Air Force, U.S., Technical Manual
If the reprint or republication includes a copyrighted material, the credit
should also state: "Anyone wishing to make further use of copyrighted
materials, by itself and apart from this text, should seek necessary
permission directly from the proprietors."
TM 5-815-1/AFR 19-6
ject material relating to the topic of this manual can be
found at the end of this manual. Also included is a
glossary listing abbreviations and a brief definition of
terminology used in the text.
1-3. Unique control problems
Military facilities have air pollution control problems
which are unique to their mission. Among the
problems are those associated with classified waste
disposal, ammunition, plant wastes, chemical warfare
wastes, hazardous toxic waste, and radioactive wastes.
Each will require a consultant or a specialist to help
solve the unique problem. Therefore, each unique
problem will require special handling on a case-to-case
basis. The manual does not include any information on
treatment of emissions, or the incineration of these
unique materials.
1-1. Purpose
a. This manual is designed to facilitate the identification of air pollutant emission rates, and the selection of
control equipment required to meet local, state, and
federal compliance levels. Presented herein are fuel
classifications, burning equipment types, emission rate
factors, emission measuring techniques, control equipment types, and control methods. Also included are
discussions of stack dispersion techniques, and control
equipment selection.
b. Each control equipment chapter provides performance data and equipment limitations which aid in
the comparative selection of control equipment types.
Each chapter includes a discussion of the basic control
theory, various equipment types, collection efficiency,
pressure drop, operating requirements and limitations,
application, materials of construction, and advantages
and disadvantages in relation to other type control
1-2. Scope
a. This manual has been limited to the application of
control equipment to fuel burning boilers and incinerators for the purpose of reducing point-source emission rates. A procedural schematic for its use is
illustrated in figure 1 - 1. Although the selection of a
site, a fuel, and burning equipment are outside the
scope of this manual, there are alternatives available to
the engineer in arriving at the least-cost solution to air
pollutant problems. Once these factors have been
decided, boiler or incineration emission rates and
reduction requirements can be estimated using chapters 2 and 3.
b. If emission rates are in compliance with local,
state, and federal regulations for point-sources, their
effect on local air quality must yet be ascertained. Such
factors as stack height and prevailing meteorological
conditions, while affecting ambient pollution levels, do
not have an effect on point-source emission rates. They
are considered in this manual only to make the reader
aware of their importance. These factors are unique for
each particular site, and usually warrant expert consultation. If emission rates for a boiler or incinerator
are above local, state or federal requirements, or if airquality regulations might be violated, selection of a
pollution control device will be required. The technical
and cost selection of control equipment are embodied
in this manual.
c. Appendix A contains a list of references used in
this manual. A bibliography listing publications of sub-
1-4. Economic considerations
The selection of one particular type of design for a
mechanical system for a given application when two or
more types of design are known to be feasible must be
based on the results of a life cycle cost analyses, prepared in accordance with the requirements of the
Department of Defense Construction Criteria Manual
(DOD 4270. 1-M). Standards for the conduct of all
economic studies by and for the Department of the
Army and the Department of the Air Force are
contained in AR 11-28 and AFR 178-1, respectively.
Subject to guidance resulting from implementation of
Executive Order 12003 and related guidance from
DOD, the cited economic analysis techniques are to
remain valid. The basic underlying principles and the
most commonly used techniques of economic analysis
are described in some detail in a variety of publications
and standard textbooks on engineering economy such
as Principles of Engineering Economy by Grant,
Arisen, and Leavenworth; guides published by
professional organizations such as the American
Institute of Architects’ Life Cycle Cost Analysis-a
Guide for Architects; and handbooks prepared by
government agencies such as the Naval Facilities
Engineering Command's "Economic Analysis
Handbook”, NAVFAC P-442. Clarification of the basic
standards and guidelines for a particular application
and/or supplementary standards for guidelines which
may be required for special cases may be obtained by
request through normal channels to Headquarters of
the particular service branch involved.
TM 5-815-1/AFR 19-6
TM 5-815-1/AFR 19-6
solid, semi-solid, liquid, or gaseous waste at specified
rates, so that the residues contain little or no combustible material. In order for an incinerator to meet these
specifications, the following principles of solid fuel
combustion generally apply:
— Air and fuel must be in the proper proportion,
— Air and fuel, especially combustible gases, must
be properly mixed,
— Temperatures must be high enough to ignite
both the solid fuel and the gaseous components,
— Furnace volumes must permit proper retention
time needed for complete combustion,
— Furnace configurations must maintain ignition
temperatures and minimize fly-ash entrainment.
2-1. Incineration
This chapter describes and quantifies whenever possible the air pollution particulate emissions which are the
direct result of the incineration process.
a. Incineration process. The incineration process
consists of burning solid, semisolid, liquid, or gaseous
waste to produce carbon dioxide, water, and ash. It is
an efficient means of reducing waste volume. The
solid, incombustible residue of incineration is inert,
sanitary, and sensibly odorless.
b. Emissions. Incineration contributes to air pollution. The polluting emissions are ash, hydrocarbons,
sulfur oxides (SOX), nitrous oxides (NOX), chlorides,
and carbon monoxide. Estimating absolute quantities
of these pollutants is not an exact science, hut historical
testing data from typical incinerators allow estimates of
emissions to be made. Also, measurement methods for
incinerator emissions are sufficiently advanced to permit actual data to be obtained for any existing incinerator. These measurements are preferred in all cases
over analytical estimates.
c. Pollution codes. Air pollution particulate emissions must be considered in regard to federal, state and
local pollution codes. In general, incinerators cannot
meet current pollution code requirements without particulate control devices.
2-2. Types of incinerator waste materials
Waste materials are classified as shown in table 2-1.
An ultimate analysis of a typical general solid waste is
shown in table 2-2. Because of the wide variation in
composition of waste materials, an analysis of the
actual material to be incinerated should be made before
sizing incineration equipment.
2-3. Function of incinerators
Incinerators are engineered apparatus capable of withstanding heat and are designed to effectively reduce
2-4. Effect of waste properties
The variability of chemical and physical properties of
waste materials, such as ash content, moisture content,
volatility, burning rate, density, and heating value,
makes control of incineration difficult. All of these factors affect to some degree the operating variables of
flame-propagation rate, flame travel, combustion temperature, combustion air requirements, and the need
for auxiliary heat. Maximum combustion efficiency is
maintained primarily through optimum incinerator
2-5. Types of incinerators
a. Municipal incinerators. Incinerators are classified
either as large or small units, with the dividing point at
a processing rate of 50 tons of waste per day. The trend
is toward the use of the smaller units because of their
lower cost, their simplicity, and lower air emission
control requirements. There are three major types of
municipal incinerators.
(1) Rectangular incinerators. The most common
municipal incinerator is the rectangular type.
The multiple chamber units are either refractory lined or water cooled and consist of a
combustion chamber followed by a mixing
chamber. The multicell units consist of two
or more side-by-side furnace cells connected
to a common mixing chamber. Primary air is
fed under the grate. Secondary air is added in
the mixing chamber to complete combustion.
A settling chamber often follows the mixing
chamber. Ash is removed from pits in the
bottom of all of the chambers.
TM 5-815-1/AFR 19-6
(2) Vertical circular incinerators. Waste is usually fed into the top of the refractory lined
chamber. The grate consists of a rotating
cone in the center surrounded by a stationary
section with a dumping section around it.
Arms attached to the rotating cone agitate the
waste and move the ash to the outside.
Primary air is fed underneath the grate.
Overfire air is fed into the upper section of
the chamber.
(3) Rotary kiln incinerators. Rotary kiln incinerators are used to further the combustion of
waste that has been dried and partially
burned in a rectangular chamber. The waste
is mixed with combustion air by the tumbling
action of the kiln. Combustion is completed
in the mixing chamber following the kiln
where secondary air is added. The ash is
discharged at the end of the kiln.
b. Industrial and commercial incinerators. Industrial and commercial incinerators generally fall into six
categories. The capacities of these incinerators generally range from a half to less than 50 tons per day. They
are usually operated intermittently.
(1) Single chamber incinerators. Single chamber
incinerators consist of a refractory lined combustion chamber and an ash pit separated by
a grate. There is no separate mixing
TM 5-815-1/AFR 19-6
fluidize. Waste is fed above the bed and then
mixes with the media where it burns.
Fluidized bed incinerators are normally self
sustaining and require an auxiliary fuel
burner only for startup. Fluidizing air is
supplied by a centrifugal blower. Ash leaves
the fluidized bed incinerator when it becomes
fine enough to be carried out by the flue gas.
Fluidized bed incinerators are capable of
burning most types of liquid or solid waste.
c. Sludge incinerators. Sludge incinerators handle
materials high in water content and low in heat content.
Two types of incinerators are normally used for sludge
(1) Multiple hearth incinerators. Multiple hearth
incinerators consist of vertically stacked
grates. The sludge enters the top where the
exiting flue gas is used to drive off the
moisture. The burning sludge moves through
the furnace to the lower hearths. Ash is
removed from under the last hearth.
(2) Fluidized bed incinerator. Fluidized bed
incinerators are particularly well suited for
sludge disposal because of the high heat
content of the bed media. Heat from the
combustion of the sludge is transferred to the
bed media. This heat is then transferred back
to the incoming sludge, driving off the
chamber. An auxiliary fuel burner is
normally provided underneath the grate. The
units are normally natural draft (no fans).
Emissions from single chamber units are high
because of incomplete combustion.
Multiple chamber incinerators. Multiple
chamber refractory lined incinerators normally consist of a primary chamber, a mixing
chamber and a secondary combustion chamber. The primary chamber is similar to a
single chamber unit. Air is fed under the
grate and through overfire air ports.
Secondary air is added in the mixing
chamber. Combustion is completed in the
secondary combustion chamber where some
settling occurs. These units are also normally
natural draft.
Conical incinerators. Conical incinerators
known commonly as "tee-pee" burners have
been used primarily in the wood products
industry to dispose of wood waste. Since
they cannot meet most local particulate
emission requirements, and since wood
waste is becoming more valuable as a fuel,
conical incinerators are being phased out.
Trench incinerators. Trench incinerators are
used for disposal of waste with a high heat
content and a low ash content. The
incinerator consists of a U-shaped chamber
with air nozzles along the rim. The nozzles
are directed to provide a curtain of air over
the pit and to provide air in the pit.
Controlled-air incinerators. Controlled-air
incinerators consist of a refractory lined primary chamber where a reducing atmosphere
is maintained and a refractory lined
secondary chamber where an oxidizing
atmosphere is maintained. The carbon in the
waste burns and supplies the heat to release
the volatiles in the waste in the form of a
dense combustible smoke. Overfire air is
added between chambers. The smoke is
ignited in the secondary chamber with the
addition of air. Auxiliary fuel burners are
sometimes provided in the secondary
chamber if the mixture does not support
combustion. Air for this type of incinerator is
provided by a forced draft fan and is
controlled by dampers in order to provide the
incinerators are efficient units with low
particulate emission rates.
Fluidized bed incinerators. Fluidized bed
incinerators consist of a refractory lined vertical cylinder with a grid in the lower part
that supports a bed of granular material, such
as sand or fine gravel. Air is blown into the
chamber below the grid causing the bed to
2-6. Particulate emission standards
The Clean Air Act requires all states to issue regulations regarding the amount of particulate emission
from incinerators. Each state must meet or exceed the
primary standards set forth by the federal act, limiting
particulate emissions for incinerators with a charging
rate of more than 50 tons per day of solid to .08 grains
per standard cubic foot (gr/std ft3) of dry gas at 12
percent carbon dioxide (CO2). Federal guidelines for
sewage sludge incinerators limit emissions to 1.3
pounds (lbs) per ton of dry sludge input and opacity to
20 percent maximum. No federal guidelines currently
exist for gaseous emissions. State and local regulations
may meet or exceed the federal guidelines. These regulations are subject to change and must be reviewed
prior to selecting any air pollution control device.
2-7. Particulate emission estimating
In order to select a proper pollution control device, the
quantities of particulate emissions from an incinerator
must be measured or estimated. Measurement is the
preferred method. For new incinerator installations
where particulate emissions must be estimated, tables
2-3 and 2-4 should be used unless concurrent data
guaranteed by a qualified Vendor is provided.
a. Factors affecting emission variability. The quantity and size of particulate emissions leaving the furnace of an incinerator vary widely, depending upon
TM 5-815-1/AFR 19-6
TM 5-815-1/AFR 19-6
such factors as incinerator design, refuse type, incinerator capacity, method of feeding, and method of
operation. Improved incinerator performance reduces
both dust loading and mean particle size.
(1) Incinerator capacity. Large incinerators burn
refuse at higher rates creating more turbulent
gas flow conditions at the grate surface.
Rapid, turbulent, combustion aided by the
use of more underfire air causes particle
suspension and carry over from the
incinerator grate surface resulting in higher
emission rates for large incinerators.
(2) Underfire air flow. The effect of increasing
underfire grate air flow is to increase particulate emission rate.
(3) Excess air Excess air is used to control combustion efficiency and furnace temperatures.
Incinerators are operated at levels of excess
air from 50 percent to 400 percent. However,
particulate emission levels increase with the
amount of excess air employed. Increases in
excess air create high combustion gas
velocities and particle carry over. Excess air
is important as a furnace temperature control
because incomplete combustion will occur at
furnace temperatures below 1400 degrees
Fahrenheit, and ash slagging at the grate surface and increased NOX emissions will occur
above furnace temperatures of 1900 degrees
(4) Opacity. For information on the use of
visible opacity measurement as an aid to
achieving efficient combustion, see
paragraph 3-8.
b. Data reduction. The state regulations for particulate emissions are expressed in a variety of units. The
following techniques permit the user to reduce particulate test data to grains per dry standard cubic foot at 12
percent CO2, as well as to convert other particulate
concentration units, as used by some states, to this
(1) Test data conversion to grains per dry standard cubic foot at 12 percent CO2. Equation
2-1 applies.
Cs at 12 percent CO2 '
(eq. 2-1)
(tm % 460)
× C
where: Cs at 12 percent CO2 particulate
concentration in grains per dry standard
cubic foot at gas conditions corrected to 12
percent CO2 and standard temperature of 68
degrees Fahrenheit.
= particulate concentration
at test conditions in grains
per dry cubic foot of gas
tm = gas temperature at the test
equipment conditions
CO2 = percent by volume of the
CO2 in the dry gas
TM 5-815-1/AFR 19-6
= barometric pressure in
inches of mercury at the
test equipment conditions.
(2) To convert particulate loadings given as
pounds per 1000 pounds of dry gas at 50
percent excess air, equation 2-2 applies.
where: C at 50 percent EA = pounds of
particulate per 100 pounds of gas at 50
percent excess air
Percent carbon is by weight from the ultimate analysis of the refuse. The GCV and tons of refuse must be
consistent with the ultimate analysis. If the ultimate
analysis is on a dry basis, the GCV and tons of refuse
must be on a dry basis.
(5) To convert grains per dry standard cubic foot
at 7 percent O2 to grains per dry standard
cubic foot at 12 percent CO2, equation 2-8
= Molecular weight of the
gas sample
M = .16 CO2 + .04 O2 + 28
(eq. 2-4)
where: N2 = percent N2 from Orsat
O2 = percent O2 from Orsat
CO = percent CO from Orsat
CO2 = percent CO2 from Orsat
(3) To convert grains per dry standard cubic foot
at 50 percent excess air to grains per dry
standard cubic foot at 12 percent CO2, equation 2-5 applies.
(4) To convert pounds of particulate per ton of
refuse charged to grains per dry standard
cubic foot at 12 percent CO2, equation 2-6
where: GCV = gross calorific value of
waste, British thermal
units (Btu)/lb
= carbon F factor, std
ft3/million (MM) Btu
(6) To convert pounds of particulate per million
British thermal units fired to grains per dry
standard cubic foot at 12 percent CO2, equation 2-9 applies.
2-8 Sample calculations
a. An industrial multichamber incinerator burns a
type I waste at 10 percent moisture of the analysis
shown below. What is the estimated particulate emission rate in grains per dry standard cubic foot at 12
percent CO2?
Waste Analysis (Percent by Weight on Wet Basis)
50 percent
Heating value
8500 Btu/lb
(1) Table 2-3 lists industrial multichamber incinerators as having a particulate emission
factor of 7 lb/ton of refuse.
(2) Using equation 2-7,
(3) Using equation 2-6,
b. Test data from an incinerator indicates a particulate concentration of 0.5 gr/ft3 at 9 percent CO2. Correct the particulate concentration to grains per dry
standard cubic foot at 12 percent CO2. Test conditions
were at 72 degrees Fahrenheit and a barometric pressure of 24 inches of mercury.
TM 5-815-1/AFR 19-6
(1) Using equation 2-1,
c. The emission rate of an incinerator is 10 lb/1000
lb of dry flue gas at 50 percent excess air. The Orsat
analysis is 8.0 percent O2, 82.5 percent N2, 9.5 percent
CO2 and 0 percent CO. Convert the emission rate to
grains per dry standard cubic foot at 12 percent CO2.
Waste Analysis
35 percent by weight on dry basis
Heating Value 6500 Btu/pound as fired
21 percent
(1) In order to use equation 2-7, the percent carbon and the heating value must be on the
same basis.
(1) Using equation 2-3,
d. An incinerator burning waste of the analysis
shown below has a measured emission rate of 5
pounds/ MMBtu. What is the expected particulate
emission rate in grains per dry standard cubic foot at
12 percent CO2?
(2) Using equation 2-4,
M=.16(9.5) + .04(8.0) + 28 = 29.84
(2) Using equation 2-7,
(3) Using equation 2-2,
(3) Using equation 2-9.
= 6.46 gr/std ft3
TM 5-815-1/AFR 19-6
Generation processes
The combustion of a fuel for the generation of steam or
hot water results in the emission of various gases and
particulate matter. The respective amounts and chemical composition of these emissions formed are dependent upon variables occurring within the combustion
process. The interrelationships of these variables do not
permit direct interpretation by current analytical
methods. Therefore, most emission estimates are based
upon factors compiled through extensive field testing
and are related to the fuel type, the boiler type and size,
and the method of firing. Although the use of emission
factors based on the above parameters can yield an
accurate first approximation of on-site boiler
emissions, these factors do not reflect individual boiler
operating practices or equipment conditions, both of
which have a major influence on emission rates. A
properly operated and maintained boiler requires less
fuel to generate steam efficiently thereby reducing the
amount of ash, nitrogen and sulfur entering the boiler
and the amount of ash, hydrocarbons, nitrogen oxides
(NOx ) and sulfur oxides (SOx) exiting in the flue gas
stream. Emissions from conventional boilers are discussed in this chapter. Chapter 13 deals with emissions
from fluidized bed boilers.
(2) Residuals. Residual fuel oils (No.4, No.5,
No.6) contain a greater amount of ash, sediment, sulfur, and nitrogen than is contained in
distillates. They are not as clean burning as
the distillate grades.
c. Gaseous fuel. Natural gas, and to a limited extent
liquid petroleum (butane and propane) are ideally
suited for steam generation because they lend themselves to easy load control and require low amounts of
excess air for complete combustion. (Excess air is
defined as that quantity of air present in a combustion
chamber in excess of the air required for stoichiometric
combustion). Emission levels for gas firing are low
because gas contains little or no solid residues,
noncombustibles, and sulfur. Analyses of gaseous fuels
may be found in "Perry's Chemical Engineering
d. Bark and wood waste. Wood bark and wood
waste, such as sawdust, chips and shavings, have long
been used as a boiler fuel in the pulp and paper and
wood products industries. Because of the fuel's relatively low cost and low sulfur content, their use outside
these industries is becoming commonplace. Analyses
of bark and wood waste may be found in
Techniques for Particulate Emissions from Stationary
Sources”. The fuel's low heating value, 4000-4500
British thermal units per pound (Btu/lb), results from
its high moisture content (50-55 percent).
e. Municipal solid waste (MSW) and refuse derived
fuel (RDF). Municipal solid waste has historically been
incinerated. Only recently has it been used as a boiler
fuel to recover its heat content. Refuse derived fuel is
basically municipal solid waste that has been prepared
to burn more effectively in a boiler. Cans and other
noncombustibles are removed and the waste is reduced
to a more uniform size. Environmental Protection
Agency, "Control Techniques for Particulate Emissions
from Stationary Sources" gives characteristics of refuse
derived fuels.
Types of fuels
a. Coal. Coal is potentially a high emission producing fuel because it is a solid and can contain large
percentages of sulfur, nitrogen, and noncombustibles.
Coal is generally classified, or “ranked”, according to
heating value, carbon content, and volatile matter. Coal
ranking is important to the boiler operator because it
describes the burning characteristics of a particular
coal type and its equipment requirements. The main
coal fuel types are bituminous, subbituminous,
anthracite, and lignite. Bituminous is most common.
Classifications and analyses of coal may be found in
"Perry's Chemical Engineering Handbook".
b. Fuel oil. Analyses of fuel oil may be found in
"Perry's Chemical Engineering Handbook".
(1) Distillates. The lighter grades of fuel oil
(No.1, No.2) are called distillates. Distillates
are clean burning relative to the heavier
grades because they contain smaller amounts
of sediment, sulfur, ash, and nitrogen and can
be fired in a variety of burner types without a
need for preheating.
Fuel burning systems
a. Primary function. A fuel burning system provides
controlled and efficient combustion with a minimum
emission of air pollutants. In order to achieve this goal,
a fuel burning system must prepare, distribute, and mix
the air and fuel reactants at the optimum concentration
and temperature.
TM 5-815-1/AFR 19-6
A fuel oil heated above the proper viscosity
may ignite too rapidly forming pulsations and
zones of incomplete combustion at the burner
tip. Most burners require an atomizing viscosity
between 100 and 200 Saybolt Universal
Seconds (SUS); 150 SUS is generally specified.
(5) Municipal solid waste and refuse derived fuel
burning equipment. Large quantities of MSW
are fired in water tube boilers with overfeed
stokers on traveling or vibrating grates. Smaller
quantities are fired in shop assembled hopper or
ram fed boilers. These units consist of primary
and secondary combustion chambers followed
by a waste heat boiler. The combustion system
is essentially the same as the "controlled-air"
incinerator described in paragraph 2-5(b)(5).
The type of boiler used for RDF depends on the
characteristics of the fuel. Fine RDF is fired in
suspension. Pelletized or shredded RDF is fired
on a spreader stoker. RDF is commonly fired in
combination with coal, with RDF constituting
10 to 50 percent of the heat input.
b. Types of equipment.
(1) Traveling grate stokers. Traveling grate stokers
are used to burn all solid fuels except heavily
caking coal types. Ash carryout from the
furnace is held to a minimum through use of
overfire air or use of the rear arch furnace
design. At high firing rates, however; as much
as 30 percent of the fuel ash content may be
entrained in the exhaust gases from grate type
stokers. Even with efficient operation of a grate
stoker, 10 to 30 percent of the particulate
emission weight generally consists of unburned
(2) Spreader stokers. Spreader stokers operate on
the combined principles of suspension burning
and nonagitated type of grate burning. Particulate emissions from spreader stoker fired
boilers are much higher than those from fuel
bed burning stokers such as the traveling grate
design, because much of the burning is done in
suspension. The fly ash emission measured at
the furnace outlet will depend upon the firing
rate, fuel sizing, percent of ash contained in the
fuel, and whether or not a fly ash reinjection
system is employed.
(3) Pulverized coal burners. A pulverized coal
fired installation represents one of the most
modern and efficient methods for burning most
coal types. Combustion is more complete
because the fuel is pulverized into smaller particles which require less time to burn and the
fuel is burned in suspension where a better
mixing of the fuel and air can be obtained.
Consequently, a very small percentage of
unburned carbon remains in the boiler fly ash.
Although combustion efficiency is high, suspension burning increases ash carry over from
the furnace in the stack gases, creating high
particulate emissions. Fly ash carry over can be
minimized by the use of tangentially fired
furnaces and furnaces designed to operate at
temperatures high enough to melt and fuse the
ash into slag which is drained from the furnace
bottom. Tangentially fired furnaces and slag-tap
furnaces decrease the amount of fuel ash
emitted as particulates with an increase in NOx
(4) Fuel oil burners. Fuel oil may be prepared for
combustion by use of mechanical atomizing
burners or twin oil burners. In order for fuel oil
to be properly atomized for combustion, it must
meet the burner manufacturer's requirements
for viscosity. A fuel oil not heated to the proper
viscosity cannot be finely atomized and will not
burn completely. Therefore, unburned carbon
or oil droplets will exit in the furnace flue gases.
3-4. Emission standards
The Clean Air Act requires all states to issue regulations regarding the limits of particulate, SOx and NOx
emissions from fuel burning sources. State and local
regulations are subject to change and must be reviewed
prior to selecting any air pollution control device.
Table 31 shows current applicable Federal Regulations
for coal, fuel oil, and natural gas. The above allowable
emission rates shown are for boilers with a heat input
of 250 million British thermal units (MMBtu) and
3-5. Formation of emissions
a. Combustion parameters. In all fossil fuel burning
boilers, it is desirable to achieve a high degree of combustion efficiency, thereby reducing fuel consumption
and the formation of air pollutants. For each particular
type fuel there must be sufficient time, proper temperature, and adequate fuel/air mixing to insure complete combustion of the fuel. A deficiency in any of
these three requirements will lead to incomplete
combustion and higher levels of particulate emission in
the form of unburned hydrocarbon. An excess in time,
temperature, and fuel/air mixing will increase the boiler
formation of gaseous emissions (NOx). Therefore,
TM 5-815-1/AFR 19-6
service regarding fuel selection, such as AR 420-49 for
the Army's use.
3-7. Emission factors
Emission factors for particulates, SOx and NOx, are
presented in the following paragraphs. Emission factors
were selected as the most representative values from a
large sampling of boiler emission data and have been
related to boiler unit size and type, method of firing
and fuel type. The accuracy of these emission factors
will depend primarily on boiler equipment age,
condition, and operation. New units operating at lower
levels of excess air will have lower emissions than estimated. Older units may have appreciably more. Therefore, good judgement should accompany the use of
these factors. These factors are from, Environmental
Protection Agency, "Compilation of Air Pollutant
Emission Factors". It should be noted that currently
MSW and RDF emission factors have not been established.
a. Particulate emissions. The particulate loadings in
stack gases depend primarily on combustion efficiency
and on the amount of ash contained in the fuel which
is not normally collected or deposited within the boiler.
A boiler firing coal with a high percentage of ash will
have particulate emissions dependent more on the fuel
ash content and the furnace ash collection or retention
time than on combustion efficiency. In contrast, a
boiler burning a low ash content fuel will have particulate emissions dependent more on the combustion efficiency the unit can maintain. Therefore, particulate
emission estimates for boilers burning low ash content
fuels will depend more on unit condition and operation.
Boiler operating conditions which affect particulate
emissions are shown in table 3-2. Particulate emission
factors are presented in tables 3-3, 3-4, 3-5 and 3-6.
b. Gaseous emissions.
(1) Sulfur oxide emissions. During combustion,
sulfur is oxidized in much the same way carbon
is oxidized to carbon dioxide (CO2). Therefore,
almost all of the sulfur contained in the fuel will
be oxidized to sulfur dioxide (SO2) or sulfur
trioxide (SO3) in efficiently operated boilers.
Field test data show that in efficiently operated
boilers, approximately 98 percent of the fuelbound sulfur will be oxidized to SO2, one percent to SO3, and the remaining one percent
sulfur will be contained in the fuel ash. Boilers
with low flue gas stack temperatures may produce lower levels of SO2 emissions due to the
formation of sulfuric acid. Emission factors for
SOx are contained in tables 3-3, 3-4, 3-5, and
(2) Nitrogen oxide emissions. The level of nitrogen
oxides (NOx) present in stack gases depends
upon many variables. Furnace heat release rate,
temperature, and excess air are major variables
there is some optimum value for these three
requirements within the boiler's operating range which
must be met and maintained in order to minimize
emission rates. The optimum values for time,
temperature, and fuel-air mixing are dependent upon
the nature of the fuel (gaseous, liquid or solid) and the
design of the fuel burning equipment and boiler.
b. Fuel type.
(1) Gaseous fuels. Gaseous fuels burn more readily
and completely than other fuels. Because they
are in molecular form, they are easily mixed
with the air required for combustion, and are
oxidized in less time than is required to burn
other fuel types. Consequently, the amount of
fuel/air mixing and the level of excess air
needed to burn other fuels are minimized in gas
combustion, resulting in reduced levels of
(2) Solid and liquid fuels. Solid and liquid fuels
require more time for complete burning
because they are fired in droplet or particle
form. The solid particles or fuel droplets must
be burned off in stages while constantly being
mixed or swept by the combustion air. The size
of the droplet or fired particle determines how
much time is required for complete combustion, and whether the fuel must be burned on a
grate or can be burned in suspension. Systems
designed to fire solid or liquid fuels employ a
high degree of turbulence (mixing of fuel and
air) to complete combustion in ‘the required
time, without a need for high levels of excess
air or extremely long combustion gas paths. As
a result of the limits imposed by practical boiler
design and necessity of high temperature and
turbulence to complete particle burnout, solid
and liquid fuels develop higher emission levels
than those produced in gas firing.
3-6. Fuel selection
Several factors must be considered when selecting a
fuel to be used in a boiler facility. All fuels are not
available in some areas. The cost of the fuel must be
factored into any economic study. Since fuel costs vary
geographically, actual delivered costs for the particular
area should be used. The capital and operating costs of
boiler and emission control equipment vary greatly
depending on the type of fuel to be used. The method
and cost of ash disposal depend upon the fuel and the
site to be used. Federal, state and local regulations may
also have a bearing on fuel selection. The Power Plant
and Fuel Use Act of 1978 requires that a new boiler
installation with heat input greater than 100 MMBtu
have the capability to use a fuel other than oil or
natural gas. The Act also limits the amount of oil and
natural gas firing in existing facilities. There are also
regulations within various branches of the military
TM 5-815-1/AFR 19-6
affecting NOx emission levels, but they are not
the only ones. Therefore, while the emission
factors presented in tables 3-3, 3-4, 3-5, and 36 may not totally reflect on site conditions, they
are useful in determing if a NOx emission
problem may be present. Factors which
influence NOx formation are shown in table 3-7.
3-8. Opacity
Visual measurements of plume opacity (para 5-3j) can
aid in the optimization of combustion conditions. Particulate matter (smoke), the primary cause of plume
opacity, is dependent on composition of fuel and efficiency of the combustion process. Smoke varies in
color but is generally observed as gray, black, white,
brown, blue, and sometimes yellow, depending on the
conditions under which certain types of fuels or
materials are burned. The color and density of smoke
is often an indication of the type or combustion
problems which exist in a process.
a. Gray or black smoke is often due to the presence
of unburned combustibles. It can be an indicator that
fuel is being burned without sufficient air or that there
is inadequate mixing of fuel and air.
b. White smoke may appear when a furnace is operating under conditions of too much excess air. It may
also be generated when the fuel being burned contains
TM 5-815-1/AFR 19-6
TM 5-815-1/AFR 19-6
excessive amounts of moisture or when steam atomization or a water quenching system is employed.
c. A blue or light blue plume may be produced by
the burning of high sulfur fuels. However; the color is
only observed when little or no other visible emission
is present. A blue plume may also be associated with
the burning of domestic trash consisting of mostly
paper or wood products.
d. Brown to yellow smoke may be produced by processes generating excessive amounts of nitrogen dioxide. It may also result from the burning of semi-solid
tarry substances such as asphalt or tar paper encountered in the incineration of building material waste.
3-9. Sample problems of emission estimating
a. Data Conversion. Pounds per million Btu (lb/
MMBtu) to grains per standard cubic foot (gr/std ft3)
dry basis is accomplished by equation 3-1.
TM 5-815-1/AFR 19-6
b. Sample Problem Number 1. An underfed stoker
fired boiler burns bituminous coal of the analysis
shown below. If this unit is rated at 10 MM Btu per
hour (hr) of fuel input, what are the estimated emission
(b) 65 pounds/ton x ton/2000 pounds = .0325
pound of particulate/pound of coal
(1) Using table 3-3 (footnote e), particulate emissions are given as 5A pound/ton of coal
where A is the percent ash in the coal.
(a) 5x13% ash = 65 pounds of particulate/ton
of coal.
Using table 3-3, SO2 emissions are given as
38S pound/ton of coal, where S is the
percent sulfur in the coal.
(a) 38 x .7% sulfur = 26.6 pounds of SO2/ton
of coal
(b) 26.6 pounds/ton = ton/2000 pounds =
.0133 pound of SO2/pound of coal
TM 5-815-1/AFR 19-6
TM 5-815-1/AFR 19-6
(3) Using table 3-3, NOx emissions are given as
15 pounds/ton of coal.
(a) 15 pounds/ton x ton/2000 pounds = .0075
pound of NOx/pound of coal
pounds/MMBtu, the required removal efficiency is determined as,
(5) If the oxygen in the flue gas is estimated at 5
percent by volume, what is the dust concentration leaving the boiler in grains/standard cubic foot (dry)?
Using equation 3-1
(4) If particulate emission must be reduced to .2
TM 5-815-1/AFR 19-6
c. Sample Problem Number 2. A boiler rated at 50
MMBtu/hr burns fuel oil of the analysis shown below.
What are the estimated emission rates?
(2) Using table 3-5 (footnote d), NOx emissions
are given as 120 pound/MCF of natural gas.
(1) Using table 3-4, particulate emissions are
given as [10(S) + 3] pound/I 000 gal, where
S is the percent sulfur in the fuel oil.
(2) Using table 3-4, SO2 emissions are given as
157S pound/1000 gal, where S is the percent
sulfur in the fuel oil.
(3) Using table 3-4, NOx emissions are given as
[22 + 400 (N)2] pound/1000 gal, where N is
the percent nitrogen in the fuel oil.
d. Sample Problem Number 3. A commercial boiler
rated at 10 MMBtu/hr fires natural gas with a heating
value of 1000 Btu/ft3. What are the estimated particulate and NOx emission rates?
(1) Using table 3-5, particulate emissions are
given as a maximum of 15 pound per million
cubic feet (MC F) of natural gas.
e. Sample Problem Number 4. A spreader stoker
fired boiler without reinjection burns bark and coal in
combination. The bark firing rate is 2000 pound/hr.
The coal firing rate is 1000 pound/hr of bituminous
coal with an ash content of 10 percent and a heating
value of 12,500 Btu/pound. What is the estimated
particulate emission rate from this boiler?
(1) Using table 3-6, the bark firing particulate
emission rate is given as 50 pounds/ton of
50 pounds/ton x ton/2000 pounds x 2000
pound/hr = 50 pounds/hr of particulate from
(2) Using table 3-3, the coal firing particulate
emission rate for a heat input of 12.5
MMBtu/hr is 13A pounds/ton of fuel.
(13 x 10) pound/ton x 1000 pound/hr x
ton/2000 pound = 65 pounds/hr of
particulate from coal.
(3) The total particulate emission rate from the
boiler is,
50 pounds/hr from bark + 65 pounds/hr
from coal = 115 pounds/hr
TM 5-815-1/AFR 19-6
Stack emissions
The discharge of pollutants from the smokestacks of
stationary boilers and incinerators is regulated by both
Federal and State Agencies. A permit to construct or
modify an emission source Will almost certainly be
a. The emissions must comply with point source regulations, dependent upon characteristics of the point
source, and also with ambient air quality limitations
which are affected by physical characteristics of the
location and the meteorology of the area of the new
b. The permitting procedure requires that estimates
be made of the effect of the stack emissions on the
ambient air quality. Predictive mathematical models
are used for arriving at these estimates.
c. Due to the time requirements and the complexity
of the process and the highly specialized nature of
many of the tasks involved, it is advisable to engage
consultants who are practiced in the permitting
procedures and requirements. This should be done at
a very early stage of planning for the project.
Air quality standards
a. Federal Standards — Environmental Protection
Agency Regulations on National Primary and Secondary Ambient Air Quality Standards (40 CER 50).
b. State standards. Federal installations are also
subject to State standards.
c. Emission levels. One must file for a New Source
Review application if, after use of air pollution control
equipment, the new boiler or incinerator will result in
increased emissions of any pollutant greater than a
specified limit. Proposed modifications of existing
boilers and incinerators that will cause increases in
pollutant emissions greater than certain threshold levels
("de minimis" emission rate) require New Source
d. General determinants for steps required for permitting. Steps required for a New Source Review
depend upon the location of the new source, characteristics of the other sources in the area, and on discussions with the State Air Pollution Control Agencies,
possibly the EPA, and how well one is current with the
changes in regulations and administrative practices.
Because of the constantly changing picture, it is usually
very beneficial to engage an air quality consultant to
aid in planning permitting activities.
e. Technical tasks. The principal technical tasks that
are required for the permitting effort in most cases may
be summarized as follows:
(1) Engineering studies of expected emission
rates and the control technology that must
be used.
(2) Mathematical modeling to determine the
expected impact of the changed emission
(3) Collection of air quality monitoring data
required to establish actual air quality concentrations and to aid in analysis of air
quality related values. All technical tasks
are open to public questioning and critique
before the permitting process is completed.
f. New Source Review program steps. The steps
required in a New Source Review vary. However, it is
always required that a separate analysis be conducted
for each pollutant regulated under the Act. Different
pollutants could involve different paths for obtaining a
permit, and may even involve different State and Federal Agencies.
(1) Attainment or nonattainment areas. A concern which must be addressed at the
beginning of a New Source Review is
whether the location is in a "nonattainment"
or “attainment” area. An area where the
National Ambient Air Quality Standards
(NAAQS) are not met is a "nonattainment"
area for any particular pollutant exceeding
the standards. Areas where the National
Ambient Air Quality Standards (NAAQS)
Permit acquisition process
a. New Source Review. The state agency with jurisdiction over pollution source construction permits
should be contacted at the very beginning of the project
planning process because a New Source Review (NSR)
application will probably have to be filed in addition to
any other State requirements. A New Source Review
is the process of evaluating an application for a "Permit
to Construct” from the Air Quality Regulatory Agency
having jurisdiction.
b. Planning. Consideration of air quality issues very
early in the planning process is important because engineering, siting, and financial decisions will be affected
by New Source Review. Engineering and construction
schedules should include the New Source Review process which can take from 6 to 42 months to complete
and which may require the equivalent of one year of
monitoring ambient air quality before the review process can proceed.
TM 5-815-1/AFR 19-6
Consider the questions related to prevention of significant deterioration and
nonattainment. If it is found the facility
will be a major source, determine for
which areas and pollutants you will have
to follow PSD rules. Determine possible
"off-sets" if any will be required.
(g) List the tasks and steps required for a permit and estimate the costs and time increments involved in the review process.
Coordinate the New Source Review
schedule with the facility planning
schedule and determine how the New
Source Review will affect construction
plans, siting, budgetary impact, schedules
and the engineering for controls
that are being met are designated as an
"attainment" area. Designation of the area
as "attaining", or "nonattaining", for each
pollutant encountered determines which of
the two routes is followed to procure a
permit. Note that the area can be attaining
for one pollutant and nonattaining for
another pollutant. If this occurs one must
use different routes for each of the
pollutants and would have to undertake
deterioration" (PSD) and "nonattainment"
(NA) analyses simultaneously.
(2) Attainment area. If the proposed source is
in an "attainment" area, there is a specified
allowed maximum increase, or "increment",
of higher air pollutant concentrations. The
upper limit of this increment may be well
below the prevailing National Ambient Air
increment" concept is intended to "prevent
significant deterioration" of ambient air
quality. The new source might be allowed
to consume some part of the increment’‘ as
(3) Nonattainment area. If the proposed new
source is in a "nonattainment" area, it may
have to be more than off-set by decreases
of emissions from existing sources,
resulting in air cleaner after addition of the
new source than before it was added. In the
absence of pollutant reductions at an
existing source which is within
administrative control, it may be necessary
to negotiate for, and probably pay for,
emission reductions at other sources.
(4) Summary of permitting path. The steps
listed below present a summary of the
permitting steps:
(a) Formulate a plan for obtaining a construction permit. It is usually advisable to
engage a consultant familiar with the permitting procedures to aid in obtaining the
(b) Contact state regulatory agencies.
(c) Determine if the modification could
qualify for exemption from the New
Source Review process.
(d) Determine if the proposed facility will be
considered a "major source" or "major
modification" as defined by the
(e) Determine if, and how, with appropriate
controls, emissions can be held to less
than "de minimis" emission rates for the
pollutant so New Source Review
procedures might be avoided.
Mathematical modeling
a. Modeling requirement. Air quality modeling is
necessary to comply with rules for proposed sources in
both attaining and nonattaining areas. Modeling is a
mathematical technique for predicting pollutant concentrations in ambient air at ground level for the specific site under varying conditions.
b. Modeling in attainment areas. Modeling is used,
under PSD rules, to show that emissions from the
source will not cause ambient concentrations to exceed
either the allowable increments or the NAAQS for the
pollutant under study. It may be necessary to model the
proposed new source along with others nearby to demonstrate compliance for the one being considered.
c. Modeling in nonattainment areas. Modeling is
used to determine the changes in ambient air concentrations due to the proposed new source emissions
and any off-setting decreases which can be arranged
through emissions reduction of existing sources. The
modeling then verifies the net improvement in air
quality which results from subtracting the proposed
off-sets from the new source emissions.
d. Monitoring. Modeling is also used to determine
the need for monitoring and, when necessary, to select
monitoring sites.
e. Guideline models. EPA's guideline on air quality
recommends several standard models for use in regulatory applications. Selection requires evaluation of
the physical characteristics of the source and surrounding area and choice of a model that will best simulate
these characteristics mathematically. Selection of the
proper model is essential because one that greatly overpredicts may lead to unnecessary control measures.
Conversely, one that under-predicts ambient pollution
concentration requires expensive retrofit control measures. Because of the subtleties involved, it is usually
advisable to consult an expert to help select and apply
the model.
TM 5-815-1/AFR 19-6
For a New Source Review, monitoring may be
required to obtain data which shows actual baseline air
quality concentrations. If monitoring is required,
prepare a monitoring plan that includes monitor siting,
measurement system specifications, and quality
assurance program design. Once the plan is ready, it
should be reviewed with the relevant agencies.
Presentation and hearings
Factors affecting stack design
a. Design of the stack has a significant effect on the
resulting pollutant concentrations in nearby ambient
air. Stack emission dispersion analysis is used to determine increases in local air pollution concentrations for
specific emission sources. Factors which bear upon the
design of stacks include the following:
— Existing ambient pollutant concentrations in
the area where the stack will be located
— Meteorological characteristics for the area
— Topography of the surrounding area
b. Specific regulations having to do with stack
design have been promulgated by the EPA to assure
that the control of air pollutant shall not be impacted by
stack height that exceeds "good engineering practice”
or by any other dispersion technique. These regulations
have a direct bearing on the specific location and
height of a stack designed for a new pollution source.
After a New Source Review application is prepared, it
must be reviewed with the appropriate agency. Often
a public hearing will be necessary and the application
will have to be supported with testimony. At the
hearing, all phases of work will be subject to public
scrutiny and critique.
TM 5-815-1/AFR 19-6
In order to evaluate the nature and magnitude of air
pollution, establish remedial measures, and determine
control programs, it is necessary to test for the existence of pollutants. In the upgrading of existing installations, compliance is determined through "point source
emission rate tests." Revisions to the regulations
regarding air pollution test requirements for federal
installations appear in the Federal Register.
standards. For the determination of possible violations
of air quality, the continuous monitoring of pollutant
concentrations is normally required for a one-year
period. Air quality measurements are a function of the
sampling site, the local meteorology, the methods used,
and the existing pollutant concentration in the
atmosphere. Personnel knowledgeable and experienced
in meteorology and air quality testing are needed to
conduct and evaluate air-quality measurements.
b. Sampling technique. The criteria for instrumentation, calibration, and use of EPA-approved sampling
techniques are covered under 40 CFR 53
Environmental Protection Agency Regulations on
Ambient Air Monitoring Reference and Equivalent
Methods. See table 5-2.
Stack and source measurement techniques
The point source emission rate test methods and
requirements are covered under Environmental Protection Agency Regulations on Standards of Performance for New Stationary Sources, 40 CFR 60 and
subsequent revisions. The techniques are listed in table
5.3 Meteorological and ambient air measurement
a. Measurements. Air quality measurements are
used to trace emission sources and determine if these
sources comply with federal, state, and local air quality
Continuous sampling is the recommended
technique for obtaining the most reliable
information concerning the variation of
pollutant concentration in the real
atmosphere. Discrete sampling can be used
for plume tracking and random checking.
Discrete sampling should be used with
caution, however, when measuring any of
several pollutants that have daily variations.
(For example, ozone has very low concentrations at night.) In addition, use of
discrete sampling methods will often result
TM 5-815-1/AFR 19-6
In order to evaluate the nature and magnitude of air
pollution, establish remedial measures, and determine
control programs, it is necessary to test for the existence of pollutants. In the upgrading of existing installations, compliance is determined through "point source
emission rate tests." Revisions to the regulations
regarding air pollution test requirements for federal
installations appear in the Federal Register.
Stack and source measurement techniques
The point source emission rate test methods and
requirements are covered under Environmental Protection Agency Regulations on Standards of Performance for New Stationary Sources, 40 CFR 60 and
subsequent revisions. The techniques are listed in table
standards. For the determination of possible violations
of air quality, the continuous monitoring of pollutant
concentrations is normally required for a one-year
period. Air quality measurements are a function of the
sampling site, the local meteorology, the methods used,
and the existing pollutant concentration in the
atmosphere. Personnel knowledgeable and experienced
in meteorology and air quality testing are needed to
conduct and evaluate air-quality measurements.
b. Sampling technique. The criteria for instrumentation, calibration, and use of EPA-approved sampling
techniques are covered under 40 CFR 53
Environmental Protection Agency Regulations on
Ambient Air Monitoring Reference and Equivalent
Methods. See table 5-2.
Meteorological and ambient air measurement
a. Measurements. Air quality measurements are
used to trace emission sources and determine if these
sources comply with federal, state, and local air quality
Continuous sampling is the recommended
technique for obtaining the most reliable
information concerning the variation of
pollutant concentration in the real
atmosphere. Discrete sampling can be used
for plume tracking and random checking.
Discrete sampling should be used with
caution, however, when measuring any of
several pollutants that have daily variations.
(For example, ozone has very low concentrations at night.) In addition, use of
discrete sampling methods will often result
TM 5-815-1/AFR 19-6
(1) Total suspended particulates. The high
volume air sample is the federal reference
method for measuring total suspended
particulates. Air is drawn (at 40 to 60
ft3/min) through a glass fiber filter by means
of a blower, and suspended particles having
an aerodynamic diameter between 100 and
1.0 micron are collected. The suspended
particulate is calculated by dividing the net
weight of the particulate by the total air
volume samples and is reported in ug/m3.
(2) Coefficient of haze (C OH). A few states
have standards for a particulate measurement
called the coefficient of haze. This measurement is reported in units of COH/1000 linear
feet of sampled air. In this method, air is
drawn through a small spot on a circle of
filter paper until the equivalent of a 1000 feet
long column of air of the diameter of the spot
has passed through the filter paper.
Transmittance through this spot then serves
as a measurement of particulate material
collected on the filter. There are considerable
doubts as to the usefulness and true meaning
of COH data, since the transmittance
recorded is a function of the nature of the
particulate as well as the total weight
(3) Dustfall (settleable particulates). Several
states have standards for the amount of particulate that settles out of the air over a given
length of time (one common unit is grams/
square meter/30 days). The method of
collection is generally the dust bucket. A dust
bucket is a 15-inch deep metal or plate container with a 6-inch opening that is exposed
to the air generally for a period of one month.
Dust buckets should be partially filled with
distilled water (or antifreeze) which prevents
the transporting of dust out of the buckets by
strong winds. This water also acts as a wash
at analysis time. After evaporating the water,
the remaining material is weighed and the
residues are converted to the required units.
i. Traceable compounds. Test methods for compounds other than those for which standards exist are
often useful in evaluating stack dispersion. If unusual
fuel additives are used, or if incinerators are used to
dispose of specialized materials, laboratory chemists
can often devise sampling methods to measure these
compounds in the atmosphere.
j. Ringelmann standards. Particulate matter such as
soot, fly ash, and droplets of unburned combustibles
present in exhaust gases tend to impart blackness or
opacity to a plume. It is assumed that the darker the
shade of gray or black, the greater the concentration of
particulate matter present in a plume. The Ringelmann
in economically unacceptable manpower
requirements. In these cases, sampling with
continuous instruments and recording on
data charts provides a lower cost solution.
(2) Air quality regulations require the measurement of extremely small pollutant concentrations (1/100 of a part per million by
volume). Sensitive instruments capable of
detecting small concentrations are needed.
c. Sampling method for carbon monoxide. The federal reference method for measuring carbon monoxide
is the instrumental nondispersive infrared technique. A
typical instrument consists of a reference cell filled
with CO free air, and a sample, or detector, cell. The
difference in transmittance of infrared radiation passing
through the sample cell and the reference cell is sensed
by a photon detector. The difference is a measure of
the optical absorption of the CO in the sample cell and
is proportional to the CO concentration in the sample.
The signal from the detector is amplified and used to
drive an output meter as a direct measure of CO
concentration. This method is precise and accurate.
d. Sampling method for sulfur dioxide. The WestGaeke sulfuric acid method is the Federal reference
method for measuring sulfur oxides. The West-Gaeke
method is a discrete bubbler technique which involves
bubbling ambient air through an impinger for 24 hours.
Sulfuric acid is added to the absorber to eliminate
interferences from oxides of nitrogen. SO2 is collected
in a tetrachloromercurate solution. When acid bleach
pararosaniline is added to the collected SO2 together
with formaldehyde, a red-violet compound is formed
which is then measured spectrophotometrically. This
method is a discrete instrumental sampling method, but
may be modified for continuous use.
e. Sampling method for oxidants and ozone. The
instrumental-chemiluminescence method is the federal
reference method for measuring ozone. Upon mixing
ambient air and ethylene in the testing instrument,
ozone reacts with the ethylene to emit light. This light
is measured by a photomutiplier. If the air and ethylene
flow rates are constant, and the proportion of air and
ethylene therefore known, the resulting signal can be
related to ozone concentration. Analyzers are calibrated with a known ozone concentration.
f. Sampling method for nitrogen dioxide. The federal reference method for NO2 is the indirect measurement of the concentration of nitrogen dioxide by
photometrically measuring the light intensity of wavelengths greater than 600 nanometers resulting from the
gas phase chemiluminescent reaction of nitric oxide
(NO) with ozone (O3).
g. Sampling method for total hydrocarbons. Gas
chromatography flame ionization is the federal reference method of measuring total hydrocarbons.
h. Sampling method for particulates.
TM 5-815-1/AFR 19-6
Chart offers a set of standards with which to measure
the opacity of an effluent plume. By the comparison of
the blackness of a plume to the blackness of a series of
graduated light diffusers, a Ringelmann number corresponding to a percent opacity can be assigned to the
plume (see table 5-3). It should be noted that while
Ringelmann numbers give a relative indication of
plume opacity, they bear no direct relationship to
plume particulate loading. They should supplement but
not replace point-source emission tests.
have a removable cover. On double wall stacks
sampling ports may consist of a 4-inch diameter pipe
extending from 4 inches outside the stack to the inner
edge of the inner stack wall. Accessible sampling ports
shall be provided and located so that the cross sectional
area of the stack or flue can be traversed to sample the
flue gas in accordance with the applicable current
federal or state regulations for fuel burning equipment.
Air pollution project contacts
U.S. Army Environmental Hygiene Agency (AEHA),
Aberdeen Proving Grounds, MD, may be contacted for
the respective service air pollution projects on the following:
a. Source testing to characterize pollutants for
design controls.
b. Consultation on test performance standards and
witnessing tests.
Flue gas sampling ports
c. Testing of installed air pollution abatement equipment for compliance with regulatory standards.
Sampling ports are approximately 4 inches in diameter,
extend out approximately 4 inches from the stack, and
TM 5-815-1/AFR 19-6
The cyclone is a widely used type of particulate collection device in which dust-laden gas enters tangentially
into a cylindrical or conical chamber and leaves
through a central opening. The resulting vortex motion
or spiraling gas flow pattern creates a strong
centrifugal force field in which dust particles, by virtue
of their inertia, separate from the carrier gas stream.
They then migrate along the cyclone walls by gas flow
and gravity and fall into a storage receiver. In a boiler
or incinerator installation this particulate is composed
of fly-ash and unburned combustibles such as wood
char. Two widely used cyclones are illustrated in figure
be handled and high collection efficiencies are needed
a multiple of small diameter cyclones are usually
nested together to form a multicyclone. A unit of this
type consists of a large number of elements joined
together with a common inlet plenum, a common
outlet plenum, and a common dust hopper. The
multicyclone elements are usually characterized by
having a small diameter and having axial type inlet
vanes. Their performance may be hampered by poor
gas distribution to each element, fouling of the small
diameter dust outlet, and air leakage or back flow from
the dust bin into the cyclones. These problems are
offset by the advantage of the multicyclone’s increased
collection efficiency over the single high efficiency
cyclone unit. Problems can be reduced with proper
plenum and dust discharge design. A typical fractional
efficiency curve for multi-cyclones is illustrated in
figure 6-6.
e. Wet or irrigated cyclone. Cyclones may be operated wet in order to improve efficiency and prevent
wall buildup or fouling (See fig. 6-7). Efficiency is
higher for this type of operation because dust particles,
once separated, are trapped in a liquid film on the
cyclone walls and are not easily re-entrained. Water is
usually sprayed at the rate of 5 to 15 gallons per 1,000
cubic feet (ft3) of gas. Wet operation has the additional
advantages of reducing cyclone erosion and allowing
the hopper to be placed remote from the cyclones. If
acids or corrosive gases are handled, wet operation
may result in increased corrosion. In this case, a
corrosion resistant lining may be needed. Reentrainment caused by high values of tangential wall
velocity or accumulation of liquid at the dust outlet can
occur in wet operation. However, this problem can be
eliminated by proper cyclone operation. Wet operation
is not currently a common procedure for boilers and
Cyclone types
a. Cyclones are generally classified according to
their gas inlet design and dust discharge design, their
gas handling capacity and collection efficiency, and
their arrangement. Figure 6-2 illustrates the various
types of gas flow and dust discharge configurations
employed in cyclone units. Cyclone classification is
illustrated in table 6-1.
b. Conventional cyclone. The most commonly used
cyclone is the medium efficiency, high gas throughput
(conventional) cyclone. Typical dimensions are illustrated in figure 6-3. Cyclones of this type are used
primarily to collect coarse particles when collection
efficiency and space requirements are not a major consideration. Collection efficiency for conventional
cyclones on 10 micron particles is generally 50 to 80
c. High efficiency cyclone. When high collection
efficiency (80-95 percent) is a primary consideration in
cyclone selection, the high efficiency single cyclone is
commonly used (See figure 6-4). A unit of this type is
usually smaller in diameter than the conventional
cyclone, providing a greater separating force for the
same inlet velocity and a shorter distance for the particle to migrate before reaching the cyclone walls. These
units may be used singly or arranged in parallel or
series as shown in figure 6-5. When arranged in parallel they have the advantage of handling larger gas volumes at increased efficiency for the same power consumption of a conventional unit. In parallel they also
have the ability to reduce headroom space requirements below that of a single cyclone handling the same
gas volumes by varying the number of units in operation.
d. Multicyclones. When very large gas volumes must
Cyclone collection efficiency
a. Separation ability. The ability of a cyclone to
separate and collect particles is dependent upon the
particular cyclone design, the properties of the gas and
the dust particles, the amount of dust contained in the
gas, and the size distribution of the particles. Most
efficiency determinations are made in tests on a geometrically similar prototype of a specific cyclone
design in which all of the above variables are
accurately known. When a particular design is chosen
it is usually accurate to estimate cyclone collection
efficiency based upon the cyclone manufacturer’s
TM 5-815-1/AFR 19-6
efficiency curves for handling a similar dust and gas.
All other methods of determining cyclone efficiency
are estimates and should be treated as such.
b. Predicting cyclone collection efficiency. A particle size distribution curve for the gas entering a cyclone
is used in conjunction with a cyclone fractional
efficiency curve in order to determine overall cyclone
collection efficiency.
(1) A particle size distribution curve shows the
weight of the particles for a given size range
in a dust sample as a percent of the total
weight of the sample. Particle size
TM 5-815-1/AFR 19-6
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TM 5-815-1/AFR 19-6
distributions are determined by gas sampling
and generally conform to statistical
distributions. See figure 6-8.
(2) A fractional cyclone efficiency curve is used
to estimate what weight percentage of the
particles in a certain size range will be
collected at a specific inlet gas flow rate and
cyclone pressure drop. A fractional efficiency
curve is best determined by actual cyclone
testing and may be obtained from the cyclone
manufacturer. A typical manufacturer’s fraction efficiency curve is shown on figure 6-9.
(3) Cyclone collection efficiency is determined by
multiplying the percentage weight of particles
in each size range (size distribution curve) by
the collection efficiency corresponding to that
size range (fractional efficiency curve), and
adding all weight collected as a percentage of
the total weight of dust entering the cyclone.
Cyclone pressure drop and energy
a. Pressure drop. Through any given cyclone there
will be a loss in static pressure of the gas between the
inlet ductwork and the outlet ductwork. This pressure
drop is a result of entrance and exit losses, frictional
losses and loss of rotational kinetic energy in the
exiting gas stream. Cyclone pressure drop will increase
as the square of the inlet velocity.
b. Cyclone energy requirements. Energy requirements in the form of fan horsepower are directly proportional to the volume of gas handled and the cyclone
resistance to gas flow. Fan energy requirements are
estimated at one quarter horsepower per 1000 cubic
feet per minute (cfm) of actual gas volume per one
inch, water gauge, pressure drop. Since cyclone
pressure drop is a function of gas inlet and outlet areas,
cyclone energy requirements (for the same gas volume
and design collection efficiency) can be minimized by
reducing the size of the cyclone while maintaining the
same dimension ratios. This means adding more units
in parallel to handle the required gas volume. The
effect on theoretical cyclone efficiency of using more
units in parallel for a given gas volume and system
pressure drop is shown in figure 6-10. The increased
collection efficiency gained by compounding cyclones
in parallel can be lost if gas recirculation among
individual units is allowed to occur.
TM 5-815-1/AFR 19-6
a. Particulate collection. Cyclones are used as particulate collection devices when the particulate dust is
coarse, when dust concentrations are greater than 3
grains per cubic foot (gr/ft3), and when collection efficiency is not a critical requirement. Because collection
efficiencies are low compared to other collection
equipment, cyclones are often used as pre-cleaners for
other equipment or as a final cleaner to improve
overall efficiency.
b. Pre-cleaner. Cyclones are primarily used as precleaners in solid fuel combustion systems such as
stoker fired coal burning boilers where large coarse
particles may be generated. The most common application is to install a cyclone ahead of an electrostatic
precipitator. An installation of this type is particularly
TM 5-815-1/AFR 19-6
TM 5-815-1/AFR 19-6
TM 5-815-1/AFR 19-6
TM 5-815-1/AFR 19-6
efficient because the cyclone exhibits an increased collection efficiency during high gas flow and dust loading
conditions, while the precipitator shows and increase in
collection efficiency during decreased gas flow and
dust loading. The characteristics of each type of
equipment compensate for the other, maintaining good
efficiency over a wide range of operating flows and
dust loads. Cyclones are also used as pre-cleaners
when large dust loads and coarse abrasive particles
may affect the performance of a secondary collector.
They can also be used for collection of unburned
particulate for re-injection into the furnace.
c. Fine particles. Where particularly fine sticky dust
must be collected, cyclones more than 4 to 5 feet in
diameter do not perform well. The use of small diameter multicyclones produces better results but may be
subject to fouling. In this type of application, it is
usually better to employ two large diameter cyclones in
d. Coarse particles. when cyclones handle coarse
TM 5-815-1/AFR 19-6
TM 5-815-1/AFR 19-6
particles, they are usually designed for low inlet
velocities 5-10 feet per second (ft/sec). This is done to
minimize erosion on the cyclone walls and to minimize
breakdown of coarser particles that would normally be
separated, into particles too fine for collection.
e. Limited space. In cases where cyclones must be
erected in limited space, smaller diameter multicyclones have an obvious space advantage over larger
diameter units. Small cyclones also have the advantage
of increased efficiency over a single unit handling the
same gas capacity, although this advantage is sometimes lost by uneven gas distribution to each unit with
resultant fouling of some elements.
Cyclone performance
a. Collection efficiency and pressure drop. For any
given cyclone it is desirable to have as high a collection
efficiency and as low a pressure drop as possible.
Unfortunately, changes in design or operating variables
which tend to increase collection efficiency also tend to
increase pressure drop at a greater rate than the collection efficiency. Efficiency will increase with an increase
in particle size, particle density, gas inlet velocity,
cyclone body or cone length, and the ratio of body
diameter to gas outlet diameter. Decreased efficiency
is caused by an increase in gas viscosity, gas density,
cyclone diameter; gas outlet diameter; and inlet widths
or area. The effect on theoretical collection efficiency
of changing the dimensions of an 8 inch diameter
cyclones is shown in figure 6-11. The effects of
changing gas inlet velocity, grain loading, particle
specific gravity, gas viscosity, and particle size
distribution on a 50 inch diameter cyclone are shown
in figures 6-12 and 6-13. These figures illustrate the
dependence of cyclone collection efficiency on those
variables and the importance of maintaining proper gas
inlet conditions.
b. Field performance. The actual in-field performance of cyclone units will vary because of changes in
operating conditions such as dust load and gas flow.
Table 6-2 illustrates the optimum expected performance of cyclone units for particulate removal
application in combustion processes.
Cyclone operation
TM 5-815-1/AFR 19-6
a. Erosion. Erosion in cyclones is caused by
impingement and rubbing of dust on the cyclone walls.
Erosion becomes increasingly worse with high dust
loading, high inlet velocities, larger particle size, and
more abrasive dust particles. Any defect in cyclone
design or operation which tends to concentrate dust
moving at high velocity will accelerate erosion. The
areas most subject to erosive wear are opposite the
inlet, along lateral or longitudinal weld seams on the
cyclone walls, near the cone bottom where gases
reverse their axial flow, and at mis-matched flange
seams on the inlet or dust outlet ducting. Surface irregularities at welded joints and the annealed softening of
the adjacent metal at the weld will induce rapid wear.
The use of welded seams should be kept to a minimum
and heat treated to maintain metal hardness. Continuous and effective removal of dust in the dust outlet
region must be maintained in order to eliminate a high
circulating dust load and resultant erosion. The cyclone
area most subject to erosion is opposite the gas inlet
where large incoming dust particles are thrown against
the wall, and in the lower areas of the cone. Erosion in
this area may be minimized by use of abrasion resistant
metal. Often provisions are made from removable linings which are mounted flush with the inside surface of
the shell. Erosion resistant linings of troweled or cast
refractory are also used. Dust particles below the 5 to
10 micron range do not cause appreciable erosion
because they possess little mass and momentum. Erosion is accelerated at inlet velocities above approximately 75 ft/sec.
b. Fouling. Decreased collection efficiency,
increased erosion, and increased pressure drop result
from fouling in cyclones. Fouling generally occurs
either by plugging of the dust outlet or by buildup of
TM 5-815-1/AFR 19-6
materials on the cyclone wall. Dust outlets become
plugged by large pieces of extraneous material in the
system, by overfilling of the dust bin, or by the breakoff of materials caked on the cyclone walls. The
buildup of sticky materials on the cyclone walls is
primarily a function of the dust properties. The finer or
softer the dust, the greater is the tendency to cake on
the walls. Condensation of moisture on the walls will
contribute to dust accumulations. The collector should
therefore be insulated to keep the surface temperature
above the flue gas dew point. Wall buildup can
generally be minimized by keeping the gas inlet
velocity above 50 ft/sec.
c. Corrosion. Cyclones handling gases containing
sulfur oxides or hydrogen chloride are subject to acid
corrosion. Acids will form when operating at low gas
temperatures, or when the dust hopper may be cool
enough to allow condensation of moisture. Corrosion
is usually first observed in the hopper or between
bolted sections of the cyclone inlet or outlet plenum
spaces where gasketing material is used and cool
ambient air can infiltrate. Corrosion at joints can be
minimized by using welded sections instead of bolted
sections. Ductwork and hoppers should be insulated
and in cold climates the hoppers should be in a weather
protected enclosure. Heat tracing of the hoppers may
be necessary.
TM 5-815-1/AFR 19-6
d. Dust hopper design. A properly designed dust
hopper should be air tight and large enough to prevent
the dust level from reaching the cyclone dust outlets.
Dust hoppers are usually conical or pyramidal in shape
and are designed to prevent dust buildup against the
walls. All designs should include a means of
continuous removal of dust from the hopper to a
storage bin, with an adequate alarm system to indicate
a malfunction. Bin level alarms are frequently used for
this purpose. On negative pressure systems, hoppers
and removal system must be air tight. If hot unburned
combustibles or char are present in the collected
particulate, introduction of fresh air can cause a hopper
fire. Pneumatic ash transport systems are not
recommended for ash containing unburned
combustibles or char for the same reason.
Selection of materials
a. Conditions. Cyclones can be constructed of a
variety of types of metals. The type of materials
specified is dependent upon the erosion characteristics
of the dust, the corrosion characteristics of the gases,
and the operating temperature of the cyclone.
Generally, cyclones are constructed of mild steel or
cast iron. (See para 7-5 for additional information on
materials selection for pollution control systems).
b. Erosion. Erosion is the single most important
criterion in specifying the materials for cyclone con-
TM 5-815-1/AFR 19-6
Advantages and disadvantages
a. Advantages. The advantages of selecting cyclones
over other particulate collection devices are:
— No moving parts,
— Easy to install and replace defective parts,
— Constructed of a wide variety of materials,
— Minimum space requirements,
— Designed to handle severe service conditions
of temperature, pressure, dust loading,
erosion, corrosion, and plugging,
— Can be designed to remove liquids from gas,
— Low capital costs,
— Low maintenance costs.
b. Disadvantages. The disadvantages of selecting
cyclones over other particulate collection devices are:
— Lower collection efficiency,
— Higher collection efficiencies (90-95 percent)
only at high pressure drops (6 inches, water
— Collection efficiency sensitive to changes in
gas flow, dust load, and particle size
— Medium to high operating costs.
struction. Erosion life of a cyclone may be extended by
using harder and thicker grades of steel. A stainless
steel of 400 Brinell rating or better is normally chosen
for cyclones subject to erosive conditions. When erosion is extreme, it is necessary to provide for replaceable liners in cyclone construction. Liners are made of
hard stainless steels or erosion resistant refractory. In
low temperature fly ash applications, cyclones of mild
steel or iron can be used because dust loadings are
generally too small to cause appreciable erosion. Cast
iron is most often used in multicyclones in boiler service.
c. Temperature. Cyclones operated above 800
degrees Fahrenheit cannot be constructed of mild
steels because the metal will creep and form ridges or
buckled sections. Above 800 degrees Fahrenheit,
nickel-copper bearing steel such as Monel is used to
provide added strength. when temperatures are in
excess of 1000 degrees Fahrenheit, nickel-chromium
steel of the 400 series is used in conjunction with
refractory linings. Silica carbide refractories provide
excellent protection against erosion and high
temperature deformation of the cyclone metal parts.
TM 5-815-1/AFR 19-6
A scrubber utilizes a liquid to separate particulate or
gaseous contaminants from gas. Separation is achieved
through mass contact of the liquid and gas. Boiler
emissions to be controlled include fly ash and sulfur
oxides. Incinerator emissions to be controlled include
fly ash, sulfur oxides and hydrogen chloride.
Types of scrubbers
(2) Preformed spray scrubbers. A preformed
spray scrubber (spray tower) is a device
which collects particles or gases on liquid
droplets and utilizes spray nozzles for liquid
droplet atomization (figure 7-2). The sprays
are directed into a chamber suitably shaped
to conduct the gas through the atomized
liquid droplets. Spray towers are designed
for low pressure drop and high liquid
consumption. They are the least expensive
method for achieving gas absorption because
of their simplicity of construction with few
internals. The operating power cost is low
because of the low gas pressure drop. Spray
towers are most applicable to the removal of
gases which have high liquid solubilities.
Particle collection efficiency is good for
particles larger than several microns in
diameter. Pressure drops range from 1 to 6
inches, water gauge.
(3) Centrifugal scrubbers. Centrifugal scrubbers
are cylindrical in shape, and impart a
spinning motion to the gas passing through
them. The spin may come from introducing
gases to the scrubber tangentially or by
directing the gas stream against stationary
swirl vanes (figure 7-2). More often, sprays
are directed through the rotating gas stream
to catch particles by impaction upon the
spray drops. Sprays can be directed outward
from a central spray manifold or inward
from the collector walls. Spray nozzles
mounted on the wall are more easily
serviced when made accessible from the outside of the scrubber. Centrifugal scrubbers
are used for both gas absorption and particle
collection and operate with a pressure drop
ranging from 3 to 8 inches, water gauge.
They are inefficient for the collection of
particles less than one or two microns in
(4) Impingement and entrainment scrubbers.
Impingement and entrainment scrubbers
employ a shell which holds liquid (figure 73). Gas introduced into a scrubber is
directed over the surface of the liquid and
atomizes some of the liquid into droplets.
These droplets act as the particle collection
and gas absorption surfaces. Impingement
and entrainment scrubbers are most
a. Low energy scrubbers. Low energy scrubbers are
more efficient at gaseous removal than at particulate
removal. A low energy scrubber utilizes a long liquidgas contact time to promote mass transfer of gas. Low
energy scrubbers depend on extended contact surface
or interface between the gas and liquid streams to
allow collection of particulate or gaseous emissions.
(1) Plate-type scrubbers. A plate-type scrubber
consists of a hollow vertical tower with one
or more plates (trays) mounted transversely
in the tower (figure 7-1). Gas comes in at the
bottom of the tower; and must pass through
perforations, valves, slots, or other openings
in each plate before exiting from the top.
Liquid is usually introduced at the top plate,
and flows successively across each plate as
it moves downward to the liquid exit at the
bottom. Gas passing through the openings in
each plate mixes with the liquid flowing over
the plate. The gas and liquid contact allows
the mass transfer or particle removal for
which the plate scrubber was designed.
Plate-type scrubbers have the ability to
remove gaseous pollutants to any desired
concentration provided a sufficient number
of plates are used. They can also be used for
particle collection with several sieve
(perforated) plates combining to form a
sieve-plate tower. In some designs, impingement baffles are placed a short distance
above each perforation on a sieve plate,
forming an impingement plate upon which
particles are collected. The impingement
baffles are below the level of liquid on the
perforated plates and for this reason are
continuously washed clean of collected
particles. Particle collection efficiency is
good for particles larger than one mircron in
diameter. Design pressure drop is about 1.5
inches of water for each plate.
TM 5-815-1/AFR 19-6
frequently used for particle collection of
particles larger than several microns in
diameter. Pressure drops range from 4 to 20
inches, water gauge.
(5) Moving bed scrubbers. Moving bed
scrubbers provide a zone of mobile packing
consisting of plastic, glass, or marble spheres
where gas and liquid can mix intimately
(figure 7-3). A cylindrical shell holds a
perforated plate on which the movable
packing is placed. Gas passes upward
through the perforated plate and/or down
over the top of the moving bed. Gas
velocities are sufficient to move the packing
material when the scrubber is operating
which aids in making the bed turbulent and
keeps the packing elements clean. Moving
bed scrubbers are used for particle collection
and gas absorption when both processes
must be carried out simultaneously. Particle
collection efficiency can be good down to
particle sizes of one micron. Gas absorption
and particulate collection are both enhanced
when several moving bed stages are used in
series. Pressure drops range from 2.5 to 6
inches water gauge per stage.
b. High energy scrubbers. High energy scrubbers
utilize high gas velocities to promote removal of particles down to sub-micron size. Gas absorption efficiencies are not very good because of the co-current
movements of gas and liquid and resulting limited gas/
liquid contact time.
TM 5-815-1/AFR 19-6
(1) Venturi scrubbers. The venturi scrubber utilizes a moving gas stream to atomize and
accelerate the liquid droplets (figure 7-4). A
convergent-divergent nozzle is used to
achieve a gas velocity of 200 to 600 feet per
second (ft/ sec) which enhances liquid
atomization and particulate capture.
Collection efficiency in a gas atomized
venturi scrubber increases with pressure
drop. Pressure drops of 25 inches water
gauge or higher are utilized to collect submicron particles. Scrubbers of the gas
atomized type have the advantage of adjustment of pressure drop and collection
efficiency by varying gas velocity. The gas
velocity is controlled by adjusting the area of
the venturi throat. Several possible methods
for doing this are illustrated in figure 7-5.
This can be used to control performance
under varying gas flow rates by maintaining
a constant pressure drop across the venturi
throat. Due to the absence of moving parts,
scrubbers of this type may be especially
suitable for the collection of sticky particles.
Disadvantages include high pressure drop
for the collection of sub-micron particles and
limited applicability for gas absorption.
(2) Ejector venturi. The ejector venturi scrubber
utilizes a high pressure spray to collect particles and move the gas. High relative velocity
between drops and gas aids in particle
collection. Particle collection efficiency is
good for particles larger than a micron in
diameter. Gas absorption efficiency is low
because of the co-current nature of the gas
and liquid flow. Liquid pumping power
requirements are high and capacity is low
making this type impractical for boiler or
incinerator emissions control.
(3) Dynamic (wetted fan) scrubber This
scrubber combines a preformed spray,
packed bed or centrifugal scrubber with an
integral fan to move the gas stream through
the scrubber. Liquid is also sprayed into the
fan inlet where the rotor shears the liquid
into dispersed droplets. The turbulence in
the fan increases liquid/ gas contact. This
type of scrubber is effective in collection of
fine particulate. Construction of this
scrubber is more complex due to the necessity of the fan operating in a wet and possibly
corrosive gas stream. The design must
prevent build-up of particulates on the fan
c. Dry scrubbers. Dry scrubbers are so named
because the collected gas contaminants are in a dry
(1) Spray dryer. The spray dryer is used to
remove gaseous contaminants, particularly
sulfur oxides from the gas stream. An
alkaline reagent slurry is mechanically
atomized in the gas stream. The sulfur
oxides react with the slurry droplets and are
absorbed into the droplets. At the same time,
TM 5-815-1/AFR 19-6
the heat in the gas stream evaporates the
water from the droplets leaving a dry
powder. The gas stream is then passed
through a fabric filter or electrostatic
precipitator where the dry product and any
fly ash particulate is removed. The scrubber
chamber is an open vessel with no internals
other than the mechanical slurry atomizer
nozzles. The vessel is large enough to allow
complete drying of the spray before
impinging on the walls and to allow enough
residence time for the chemical reaction to
go to completion. A schematic of the system
is shown in figure 7-6. Refer to chapters 8
and 9 for discussion of the fabric filter or
electrostatic precipitator.
(2) Gravel bed. The gravel bed, while referred
to as a dry scrubber, is more a filter using
sized gravel as the filter media. A bed of
gravel is contained in a vertical cylinder
between two slotted screens. As the gas
passes through the interstices of the gravel,
particulates impact on, and are collected on
the gravel surface. Sub-micron size particles
are also collected on the surface because of
their Brownian movement. Dust-laden
gravel is drawn off the bottom and the dust
is separated from the gravel by a mechanical
vibrator or pneumatic separator. The cleaned
gravel is then conveyed up and dumped on
top of the gravel bed. The cylindrical bed
slowly moves down and is constantly
TM 5-815-1/AFR 19-6
TM 5-815-1/AFR 19-6
a. Particulate removal. Scrubbers may be used as
control devices on incinerators and boilers for fly ash
collection. The plate, spray, venturi, and moving bed
types have been successfully applied; however, their
application has been limited because they require:
— more energy than dry particulate collection
devices of the same collection efficiency,
— water supply and recovery system,
— more extensive solid waste disposal system,
— system to control the scrubbing process in
response to gas flow rate changes.
b. In making decisions on applicability to a particular
process, figure 7-7 is useful in determining all
components which must be taken into consideration.
c. Gaseous removal. Scrubbers have been used primarily for the removal of sulfur oxides in stack gases.
(See chapter 10 for a more detailed description of
sulfur oxides (SOx) control techniques.) However, as
new control systems are devised, simultaneous removal
of gases and particulate material will become the
accepted procedure for designing scrubbers for
combustion processes.
Treatment and disposal of waste
Wet scrubber systems are designed to process exhaust
streams by transfer of pollutants to some liquid
medium, usually water seeded with the appropriate
reactants. Liquid effluent treatment and disposal are
therefore an essential part of every wet scrubber system. Installation and maintenance of the associated
components can add appreciably to the system capital
and operating costs. The degree of treatment required
will depend upon the methods of disposal or recycle
and on existing regulations. Required effluent quality,
environmental constraints, and availability of disposal
sites must be established before design of a treatment
facility or the determination of a disposal technique can
proceed. In many industrial applications the scrubber
liquid wastes are combined with other plant wastes for
treatment in a central facility. Design of this waste
treatment should be by an engineer experienced in
industrial waste treatment and disposal.
Selection of materials
a. General conditions. When choosing construction
materials for scrubber systems, certain pertinent operating parameters should be considered. The metal surface of an exhaust gas or pollution control system will
behave very differently in the same acid mist environment, depending on conditions of carrier gas velocity,
temperature, whether the conditions are reducing or
oxidizing, and upon the presence of impurities. For
example, the presence of ferric or cupric iron traces in
acids can dramatically reduce corrosion rates of
stainless steels and titanium alloys. On the other hand,
traces of chloride or fluoride in sulfuric acid can cause
TM 5-815-1/AFR 19-6
severe pitting in stainless steels. This condition is frequently encountered in an incinerator which burns
large quantities of disposable polyvinyl chloride (PVC)
b. Temperature. Corrosion rates generally increase
with increases in exhaust temperatures. This is due to
the increased mobility of ions and increased reaction
rates. However, in cases where the corrosion process
is accelerated by the presence of oxygen, increasing the
acid temperature eventually boils out dissolved oxygen,
rapidly diminishing corrosion rate. This is the case with
Monel, a nickel-copper alloy.
c. Velocity. Often the corrosion resistance of an alloy
depends on the existence of an adhering oxide layer on
its surface. A high exhaust gas velocity can remove or
erode the surface layer. Once removed, this layer cannot be renewed because the oxide film is washed away
as it forms.
d. State of oxidation. Under reducing condition,
Monel is very resistant to moderate sulfuric-acid concentrations. Under oxidizing conditions, or in the presence of oxidizing ions, however, very rapid corrosion
occurs. The reverse is true of stainless steels which are
resistant to oxidizing acid environments, but are
attached by acids under reducing conditions. The
equipment designer should select materials based on
individual case conditions including temperature, abrasion, pH, etc.
Auxiliary equipment
a. Gas transport.
(1) Ducts and stacks. Large boiler plant stacks
have a wind shield of reinforced concrete or
of steel, with a separate inner flue or
numerous flues of steel, acid-resistant brick,
and occasionally, stainless steel. The space
between the inner flue and the outer wind
shield may be insulated with a mineral wool
wrapping. This is to prevent the condensation of acid dew on the inside of the metal
chimney, which occurs below dew point
temperature, and also to prevent acid “smut”
from being blown out of the chimney. Acid
smut is a term for ash particles contaminated
with acid. It is heavy and tends to fall out of
the gas plume soon after exiting from the
TM 5-815-1/AFR 19-6
pressure piping. Considerations must also be
made for weatherproofing against freezing
(2) Pumps. Centrifugal pumps are used to
supply the scrubbing liquid or recycled slurry
to the scrubber nozzles at the required
volume flow rate and pressure. Where no
solids are present in the liquid, bare metal
pumps, either iron or stainless steel
construction, are used. In recycle systems
with solids in the liquid, special rubber-lined
or hard-iron alloy pumps are used to control
erosion of the pump internals. These are
generally belt driven to allow selection of the
proper speed necessary for the design
capacity and head. Solids content must still
be controlled to limit the maximum slurry
consistency to meet the scrubber and pump
c. Entrainment separation. After the wetted gas
stream leaves the scrubbing section, entrained liquid
droplets must be removed. Otherwise they would rain
out of the stack and fall on the surrounding area.
Removal can be by gravity separation in an expanded
vessel with lowered velocity or a cyclonic separator
can swirl out the droplets against the vessel wall.
Knitted wire or plastic mesh demisters or chevron or
“zig-zag” vanes can be located at the scrubber outlet to
catch any droplets.
d. Process measurement and control. The scrubber
control system should be designed to follow variations
in the boiler or incinerator gas flow and contaminant
load to maintain outlet emissions in compliance with
selected criteria.
(1) Measurements. Measurement of data from
the process to provide proper control should
include inlet gas flow rate, temperature and
pressure, scrubber gas pressure drop, liquid
pressure, flow rate, solids consistency, pH,
and outlet gas temperature. Selection of
instrumentation hardware should be on an
individual application basis.
(2) Control. Pressure drop across a scrubber can
be referenced as an indication of
performance following initial or periodic,
outlet gas testing. In a variable throat
venturi, for instance, this pressure drop can
be used to control the throat opening,
maintaining constant performance under
varying gas volume flow rates. Measurement
of scrubber slurry solids consistency can be
used to control bleed-off of high solids slurry
and make-up with fresh water. If sulfur
dioxide (SO2) is being controlled then
measurement of scrubber liquid pH can
control make-up of caustic to maintain
efficiency of SO2 removal. Complete
specification or design of a control system
must be on a case-by-case basis.
stack. In smaller plants, stacks may be a
single wall steel construction with insulation
and lagging on the outer surface. For wet
scrubbing practice, chimneys for vapor-saturated gases containing corrosive substances
may be made of rubber-lined steel,
fiberglass-reinforced resin or other
corrosion-resistant material. With materials
that have a limited maximum temperature,
provisions must be made to protect the stack
from high temperatures because of loss of
scrubbing liquid. Chimney or stack velocities
are generally 30 ft/ sec to prevent reentrainment of moisture from the stack wall
which would rain down around the plant.
Sometimes cones are fitted at the top to give
exit velocities as high as 75 ft/sec. The chief
reason for high velocities is to eject the gases
well away from the top of the stack to
increase the effective height and to avoid
downwash. Downwash can damage the
metal structure supporting the stack, the
stack itself, or the outside steel of a lined
metal stack. (For a more detailed analysis of
the meteorological considerations involved
in stack design, see chapter 4.)
(2) Fans. In a wet scrubber system the preferred
location for the boiler or incinerator
induced-draft fan is upstream of the
scrubber. This eliminates the need for
special corrosion-resistant construction
required to handle the wet downstream gas.
The fan should be selected to resist build-up
of dry ash or erosion of the rotor surfaces.
For high dust load applications a radial blade
or radial tip blade fan is more durable. In a
dry scrubber application the fan should be
downstream of the scrubber in the clean gas
stream. Here a more efficient air-foil or
squirrel-cage rotor can be used.
b. Liquid transport.
(1) Pipework. For most scrubbing duties, the
liquid to be conveyed will be corrosive.
There exists a wide variety of acid resistant
pipework to choose from, but generally
speaking, rubber-lined steel pipe has high
versatility. It is easy to support, has the
strength of steel, will withstand increases in
temperature for a short time and will not
disintegrate from vibration or liquid
hammer. Fiberglass filament wound plastic
pipe is also suitable for a very wide range of
conditions of temperature, pressure, and
chemicals. The chief disadvantage of rubberlined pipe is that it cannot be cut to size and
has to be precisely manufactured with
correct lengths and flange drilling. Site
fabrication is not possible. Most piping is
manufactured to ANSI specifications for
TM 5-815-1/AFR 19-6
Advantages and disadvantages
a. Advantages. The advantages of selecting scrubbers over other collection devices are:
Capability of gas absorption for removal of
harmful and dangerous gases,
High efficiency of particulate removal,
Capability of quenching high temperature
exhaust gases,
Capability of controlling heavy particulate
b. Disadvantages. The disadvantages of selecting
scrubbers over other collection devices are:
— Large energy usage for high collection efficiency,
— High maintenance costs,
— Continuous expenses for chemicals to
remove gaseous materials,
— Water supply and disposal requirements,
— Exhaust gas reheat may be necessary to
maintain plume dispersion,
— Weather proofing is necessary to prevent
freezeup of equipment.
TM 5-815-1/AFR 19-6
Electrostatic precipitator (ESP)
Types of electrostatic precipitators
a. Two stage ESPs. Two stage ESPs are designed so
that the charging field and the collecting field are independent of each other. The charging electrode is
located upstream of the collecting plates. Two stage
ESPs are used in the collection of fine mists.
b. Single stage ESPs. Single stage ESPs are designed
so that the same electric field is used for charging and
collecting particulate s Single stage ESPs are the most
common type used for the control of particulate
emissions and are either of tube or parallel plate type
construction. A schematic view of the tube and parallel
plate arrangement is given in figure 8-1.
(1) The tube type precipitator is a pipe with a
discharge wire running axially through it. Gas
flows up through the pipe and collected particulate is discharged from the bottom. This
type of precipitator is mainly used to handle
small gas volumes. It possesses a collection
efficiency comparable to the parallel plate
types, usually greater than 90 percent. Water
washing is frequently used instead of rapping
to clean the collecting surface.
(2) Parallel plate precipitators are the most commonly used precipitator type. The plates are
usually less than twelve inches apart with the
charging electrode suspended vertically
between each plate. Gas flow is horizontal
through the plates.
b. Cold precipitation. Cold precipitators are
designed to operate at temperatures around 300
degrees Fahrenheit. The term “cold” is applied to any
device on the low temperature side of the exhaust gas
heat exchanger. Cold ESPs are also generally of the
single stage, parallel plate design. They are smaller in
construction than hot precipitator types because they
handle smaller gas volumes due to the reduced temperature. Cold precipitators are most effective at collecting particles of low resistivity since particle
resistance to collection is greater at lower temperatures. These precipitators are subject to corrosion
due to the condensation of acid mist at the lower temperatures.
An electrostatic precipitator is a device which removes
particles from a gas stream. It accomplishes particle
separation by the use of an electric field which:
— imparts a positive or negative charge to the
— attracts the particle to an oppositely charged
plate or tube,
— removes the particle from the collection
surface to a hopper by vibrating or rapping
the collection surface.
plate design. It has the advantage of collecting more
particulate from the hot gas stream because particle
resistance to collection decreases at higher
temperatures. The ability to remove particles from the
collection plates and hoppers is also increased at these
temperatures. However, hot precipitators must be large
in construction in order to accommodate the higher
specific volume of the gas stream.
Modes of operation.
All types of ESPs can be operated at high or low temperatures, with or without water washing (table 8-1).
a. Hot precipitation. A hot precipitator is designed
to operate at gas temperatures above 600 degrees
Fahrenheit and is usually of the single stage, parallel
c. Wet precipitation. A wet precipitator uses water
to aid in cleaning the particulate collection plates. It
may employ water spray nozzles directed at the collection plates, or inject a fine water mist into the gas
stream entering the precipitator. Wet precipitators
enhance the collection efficiency of particulates by
reducing reentrainment from the collection plates. Care
should be taken so that water addition does not lower
gas temperature below the dewpoint temperature, thus
allowing the formation of acids. A wet precipitator can
be of either plate or tube type construction.
Electrostatic precipitators are among the most widely
used particulate control devices. They are used to control particulate emissions from the electric utility
industry, industrial boiler plants, municipal incinerators, the non-ferrous, iron and steel, chemical,
cement, and paper industries. It is outside the scope of
this manual to include all of these application areas.
Only applications to boilers and incinerators will be
a. Boiler application. Parallel plate electrostatic
precipitators are commonly employed in the utility
industry to control emissions from coal-fired boilers.
Cold type precipitators are the prevalent type because
TM 5-815-1/AFR 19-6
they are most easily retrofitted. In the design of new
installations, the use of hot precipitators has become
more common, because of the greater use of lower
sulfur fuels. Low sulfur fuels have higher particle
resistivity and therefore particulate emissions are more
difficult to control with cold precipitation. Figure 8-2
may be used for estimating whether hot precipitators or
cold precipitators should be selected for a particular
sulfur content of coal.
b. Wood refuse boiler applications. An ESP can be
used for particulate collection on a wood fired boiler
installation if precautions are taken for fire prevention.
The ESP should be preceded by some type of
mechanical collection device to prevent hot glowing
char from entering the precipitator and possibly starting
a fire.
c. Incinerator application. Until relatively recently,
ESPs were used for pollution control on incineration
units only in Europe. In the United States, however, the
ESP is now being viewed as one of the more effective
methods for the control of emissions from incinerators.
The major problem associated with the use of
precipitators on incinerators is high gas temperatures.
Temperatures up to 1800 degrees Fahrenheit can be
encountered at the incinerator outlet. These temperatures must be reduced before entering a precipitator. Several methods can be used to accomplish
this temperature reduction:
— mixing of the gas with cooler air;
— indirect cooling such as waste heat boilers,
— evaporative cooling in which droplets of
water are sprayed into the gas.
TM 5-815-1/AFR 19-6
The performance of an electrostatic precipitator is predominantly affected by particle resistivity, particle size,
gas velocity, flow turbulence, and the number of
energized bus sections (electrically independent sections) in operation.
a. Particle resistivity. Particle resistivity is an electrical property of a particle and is a measure of its
resistance of being collected. Particle resistivity is
affected by gas temperature, humidity, sodium content,
and sulfur trioxide (SO3) content. See figure 8-3.
b. Collection plate area. Collection plate area, and
gas volume, affect electrostatic precipitator performance. The basic function relating these factors is shown
in equation 8-1.
c. Bus sections. The number of energized bus sections in a precipitator has an effect upon collection
efficiency. A power loss in one energized bus section
will reduce the effectiveness of the precipitator. See
figure 8-4.
d. Turbulence. Turbulence in the gas flow through
an electrostatic precipitator will decrease its collection
efficiency. For proper operation all segments of the
flow should be within 25 percent of the mean flow
Description of components
a. Shell. The shell of an ESP has three main functions: structural support, gas flow containment, and
insulation. Shell material is most commonly steel; if
necessary, insulation can be applied to the exterior to
prevent heat loss. Brick or concrete linings can be
installed on shell interiors if gas stream corrosion of the
metal may occur. Corrosion resistant steel can also be
used as a lining, but the cost may be uneconomical and
at times prohibitive. Since the shell is also used for
structural support, normal civil engineering precautions
should be taken in the design.
b. Weighted wire discharge electrodes. Wires vary
in type, size, and style. Provision is made to keep the
TM 5-815-1/AFR 19-6
discharge wire from displacement by attachment to a
suspended weight. The wires can be made stiff consisting of a formed sheet, or they can be simple variations
of the normal straight round wire such as being barbed
or pronged. Steel alloys are commonly used for wire
construction, but actually any conducting material with
a proper configuration and sufficient tensile strength
can be used.
(1) Rigid frame discharge electrodes. Rigid
frame designs incorporate a framework which
supports the discharge electrodes. By using
the rigid frame design the need for wire
weights is eliminated since the frame keeps
the wires properly supported and aligned.
(2) Rigid electrodes. The rigid electrode design
uses electrodes that have sufficient strength to
stay in alignment their entire length. The elec-
trodes are supported from the top and kept in
alignment by guides at the bottom. Rigid electrodes are the least susceptible to breakage.
c. Collection electrodes. There are numerous types
of collection electrodes designed to minimize
reentrainment and prevent sparking. The material used
in construction, however, must be strong enough to
withstand frequent rapping. In order to insure correct
electrode application, it is wise to see if the electrode
chosen has exhibited good performance at similar
d. Hoppers. A hopper is used to collect ash as it falls
from the precipitator. The hopper should be designed
using precautions against corrosion in the precipitator
as any leakage due to corrosion will enhance entrainment. If the precipitator is dry, a hopper angle should
be chosen that will prevent bridging of collected dust.
TM 5-815-1/AFR 19-6
safe rodding out the hoppers should they become
e. Rappers. Rappers are used to remove dust from
the discharge and collection electrodes. Rappers are
usually one of two types, impulse or vibrator. The
vibrator type removes dust from the discharge electrode by imparting to it a continuous vibration energy.
They are used to remove dust from the collection electrodes. Impulse rappers consist of electromagnetic
solenoids, motor driven cams, and motor driven hammers. Important features to note in choosing rappers
are long service life without excessive wear and
flexible enough operation to allow for changing
precipitator operating conditions. Low intensity
rapping of plates (on the order of one impact per
minute) should be used whenever possible to avoid
damage to the plates. visual inspection of the effect of
rapping on reentrainment is usually sufficient to
determine a good rapping cycle.
f. High tension insulators. High tension insulators
serve both to support the discharge electrode frame
and also to provide high voltage insulation. The materials used are ceramic, porcelain, fused silica and
alumina. Alumina is the most common. The insulators
must be kept clean to prevent high voltage shorting and
resultant equipment damage. Compressed air or steam
can be used for this purpose.
g. Four point suspension. Rigid electrode and rigid
frame units may utilize a four point suspension system
to support the discharge electrode framework in each
chamber. This type of suspension system assures a
better alignment of the discharge and collection electrodes. This in turn provides a more consistent operation.
h. Distribution devices. Perforated plates, baffles or
turning vanes are usually employed on the inlet and
outlet of an ESP to improve gas distribution. Improper
distribution can cause both performance and corrosion
problems. These distribution devices may require rappers for cleaning.
i. Model testing. Gas flow models are used to determine the location and type of distribution devices. The
models may include both the inlet and outlet ductwork
in order to correctly model the gas flow characteristics.
Gas flow studies may not be required if a proven precipitator design is installed with a proven ductwork
Hoppers must be sized so that the amount of dust
collected over a period of time is not great enough to
overflow and be reentrained. Seals also must be provided around the outlet to prevent any air leakage. If
the precipitator is wet, the hopper should allow
removal of sludge in a manner compatible with the
overall removal system. In general the collected dust in
the hoppers is more free flowing when kept hot. The
hop-pers should be insulated and should have heaters
to maintain the desired temperatures. Hoppers heaters
will also prevent the formation of acids that may occur
at low temperatures. Provisions should be made for
8-7 Control systems
The electric power control system is the most important component system of any E SP. The basic components of this system are: step-up transformer; high
voltage rectifier; voltage and amperage controls; and
a. Automatic power control. By utilizing a signal
from a stack transmissionmeter the power level in the
precipitator can be varied to obtain the desired performance over a wide range of operating conditions.
TM 5-815-1/AFR 19-6
b. High voltage transformer. The standard iron core
transformer is the only instrument generally used to
step-up the input voltage. The only care that need be
taken is that the transformer is of superior quality and
able to put out the quantity of voltage required by the
precipitator. Transformers are designed to withstand
high ambient temperatures and electrical variations
induced by sparking. For high temperature operation,
the most common transformer cooling method is liquid
c. High voltage rectifier. Silicon rectifiers are the
latest advance in rectifying circuitry. They are solid
state devices which have a few of the disadvantages of
the other types of rectifiers. An assembly of silicon
rectifiers is used for lower rated current sets, typically
500 miliamperes (mA).
d. Voltage and amperage controls. Controls are
needed to insure that the precipitator is supplied with
the maximum amount of voltage or power input, and
to control the effects of sparking. The most modern
method of accomplishing these aims is through the use
of silicon controlled rectifiers (SCR). Other modern
control devices are saturable reactors and thyristors
(four element, solid state devices). Voltage control can
also be accomplished by tapped series dropping
resistors, series rheostats, tapped transformer primaries, and variable inductances.
e. Auxiliary control equipment. As with any control
TM 5-815-1/AFR 19-6
device, gas flow should be monitored either by readout of amperage from the fans or by measuring static
pressure. It is also useful to have sensors which
measure the sulfur dioxide (SO2) concentration and
temperature of the inlet gas stream in order to
determine the dew-point temperature.
Advantages and disadvantages
a. Advantages.
(1) The pressure drop through a precipitator is a
function of inlet and outlet design and precipitator length. Pressure drop rarely exceeds
0.5 inches, water gauge.
(2) The ESP can be designed to have 99.9 + percent collection efficiency.
(3) Silicon control rectifiers and other modern
control devices allow an electrostatic
precipitator to operate automatically.
(4) Low maintenance costs.
b. Disadvantages.
(1) Due to the size of a typical ESP and the
erratic nature of most processes (especially if
frequent start-up and shutdowns occur) the
temperature in different parts of the structure
could at times drop below the acid dew point.
Corrosion can cause structural damage and
allow air leakage.
(2) An ESP is sensitive to its design parameters.
A change in the type of coal used, for
(3) High capital costs.
(4) If particulate emission concentrations are
high, a mechanical precleaner may be necessary.
(5) High voltages are required.
(6) No SO2 control is possible with an ESP.
TM 5-815-1/AFR 19-6
Fabric filtration
Fabric filters are used to remove particles from a gas
stream. Fabric filters are made of a woven or felted
material in the shape of a cylindrical bag or a flat
supported envelope. These elements are contained in
a housing which has gas inlet and outlet connections, a
dust collection hopper; and a cleaning mechanism for
periodic removal of the collected dust from the fabric.
In operation, dust laden gas flows through the filters,
which remove the particles from the gas stream. A
typical fabric filter system (baghouse) is illustrated in
figure 9-1.
more difficult. A closed suction system is
illustrated in figure 9-2.
b. Filter shape and arrangements.
(1) The cylindrical filter is the most common
filter shape used in fabric filtration. The
principal advantage of a cylindrical filter is
that it can be made very long. This maximizes
total cloth area per square foot of floor space.
Cylindrical filters are arranged to
accommodate each of the basic flow
configurations shown in figure 9-3.
(2) A panel type filter consists of flat areas of
cloth stretched over an adjustable frame. (See
figure 9-3.) Flow directions are usually
horizontal. Panel filters allow 20 to 40
percent more cloth per cubic foot of collector
volume and panels may be brushed down if
dust build-up occurs. However, panel-type
filters are not widely used in boiler and
incinerator applications.
Types of filtering systems
The mechanisms of fabric filtration are identical
regardless of variations in equipment structure and
design. In all cases, particulates are filtered from the
gas stream as the gas passes through a deposited dust
matrix, supported on a fabric media. The dust is
removed from the fabric periodically by one of the
available cleaning methods. This basic process may be
carried out by many different types of fabric filters with
a variety of equipment designs. Filtering systems are
differentiated by housing design, filter arrangement,
and filter cleaning method.
a. Housing design. There are two basic housing configurations which apply to boiler and incinerator flue
gas cleaning. These are closed pressure, and closed
(1) The closed pressure baghouse is a completely
closed unit having the fan located on the dirty
side of the system. Toxic gases and gases with
high dew points are handled in this type of
baghouse. Fan maintenance problems arise
due to the fact that the fan is in the dirty gas
stream before the baghouse. The floor of the
unit is closed and the hoppers are insulated. A
closed pressure baghouse is illustrated in
figure 9-2.
(2) The closed suction is the most expensive type
of baghouse, with the fan being located on the
clean gas side. The closed suction baghouse
is an all-welded, air-tight structure. The floor
is closed, and the walls and hopper are
insulated. Fan maintenance is less than with
the pressure type, but inspection of bags is
c. Cleaning methods. A fabric cleaning mechanism
must impart enough energy to the cloth to overcome
particle adhering forces without damaging the cloth,
disturbing particle deposits in the hopper; or removing
too much of the residual dust deposit on the filter. The
cleaning period should be much shorter than the filtering period. The correct choice of cleaning method for
a particular application will greatly enhance the performance of the fabric filter system. An incorrectly
matched cleaning method can result in high pressure
drops, low collection efficiency, or decreased bag life.
A performance comparison of the various cleaning
methods is given in table 9-1.
(1) Mechanical shake. Some baghouses employ
a type of mechanical shaking mechanism for
cleaning. Bags are usually shaken from the
upper fastenings, producing vertical, horizontal, or a combination of motions, on the bag.
All bags in a compartment may be fastened to
a common framework, or rows of bags are
attached to a common rocking shaft. After the
bags have been shaken, loosened dust is
allowed to settle before filtration is resumed.
The entire cleaning cycle may take from 30
seconds to a few minutes. Some designs
incorporate a slight reversal of gas flow to aid
in dust cake removal and settling, as any
TM 5-815-1/AFR 19-6
slight flow in the direction of normal filtration
will greatly reduce the effectiveness of
cleaning. For this reason a positive sealing
type valve is recommended for baghouse inlet
and outlet. Shaker baghouses are normally
used in small capacity systems or systems
with a large number of filtering
(2) Reverse flow without bag collapse. This
cleaning method is used with a dust that
releases fairly easily from the fabric. (See
figure 9-4). A low pressure reversal of flow is
all that is necessary to remove deposited dust
from fabric. To minimize flexure and wear;
the fabric is supported by a metal grid, mesh,
or rings, sewn into the bag. Any flow that is
reversed through the filter must refiltered.
This results in increased total flow, requiring
a greater cloth area, and producing a higher
filtering velocity. This net increase in flow is
normally less than 10 percent. Reverse
pressures range from 125 pounds/square inch
(lb/in2) down to a few inches, water gauge.
The gentle cleaning action of reverse flow
allows the use of glass fabric bags in high-
temperature applications.
(3) Reverse flow with bag collapse. Even though
flexure can be detrimental to the bag, it is
frequently utilized in order to increase the
effectiveness of cleaning in a reverse
baghouse. Filter bags collecting dust on the
inside of the fabric are collapsed by a burst of
reverse air which snaps the dust cake from
the cloth surface. The bags do not collapse
completely but form a cloverleaf type pattern.
Collapse cleaning uses the same equipment
arrangement as reverse flow without bag
collapse. One design sends a short pulse of air
down the inside of the bag, along with the
reverse flow, to produce increased flexure
and cleaning as is illustrated in figure 9-5.
The principal disadvantage of flexural
cleaning is the increased fabric wear. If the
dust cake fails to be removed completely, the
bag will stiffen in that area and cause wear in
adjacent areas during cleaning.
(4) Reverse-flow heating. With a reverse flow
cleaning system it may be necessary to have a
reverse flow heating system. This system
TM 5-815-1/AFR 19-6
would be used to keep the gas temperatures
in the baghouse above the acid dew point
during the cleaning cycle.
(5) Pulse-jet. A pulse jet system is illustrated in
figure 9-6. A short blast of air at 29 to 100 lb/
in2 is directed into the top of the filter. This
blast is usually sent through a venturi which
increases the shock effect. As the pulse starts
down the filter tube, more air is drawn in
through the top. This combination causes the
flow within the bag to temporarily reverse,
bulges the fabric, and releases the dust cake
from the outside of the filter tube. The whole
process occurs in a fraction of a second which
enables a virtually continuous filtering flow.
Filter elements can be pulsed individually, or
in rows. With a multicompartment baghouse,
a whole section may be pulsed at one time
through a single venturi. The pulse produces
less fabric motion than in shaking and also
allows tighter bag spacing. A pulse-jet cleaning system requires no moving parts for
cleaning and is designed to handle high gas
flows per square foot of cloth area (air to
cloth ratio). However; this system requires a
compressed air system with a timer
mechanism and control air solenoid valve for
automatic cyclic cleaning. Pulse-jet
TM 5-815-1/AFR 19-6
a. Fabric type. The two basic types of fabric used in
filtration are woven and felted. The woven fabric acts
as a support on which a layer of dust is collected which
forms a microporous layer and removes particles from
the gas stream efficiently. A felted material consists of
a matrix of closely spaced fibers which collect particles
within its structure, and also utilizes the filter cake for
further sieving. Filtering velocities for woven fabrics
are generally lower than felts because of the necessity
of rebuilding the cake media after each cleaning cycle.
It is necessary that woven fabrics not be overcleaned,
as this will eliminate the residual dust accumulation
that insures rapid formation of the filter cake and high
collection efficiencies. Felts operate with less filter
cake. This necessitates more frequent cleaning with a
higher cleaning energy applied. Woven products, usually more flexible than felts, may be shaken or flexed
for cleaning. Felts are usually back-washed with higher
pressure differential air and are mainly used in pulsejet baghouses. However, felted bags do not function
well in the collection of fines because the very fine
particles become embedded in the felt and are difficult
to remove in the cleaning cycle.
b. Fiber. The basic structural unit of cloth is the
single fiber. Fiber must be selected to operate satisfactorily in the temperature and chemical environment of
the gas being cleaned. Fiber strength and abrasion
resistance are also necessary for extended filter life.
The first materials used in fabric collectors were natural fibers such as cotton and wool. Those fibers have
limited maximum operating temperatures (approximately 200 degrees Fahrenheit) and are susceptible to
degradation from abrasion and acid condensation.
Although natural fibers are still used for many applications, synthetic fibers such as acrylics, nylons, and
Teflon have been increasingly applied because of their
superior resistance to high temperatures and chemical
attack (table 9-2).
(1) Acrylics offer a good combination of abrasion
resistance and resistance to heat degradation
under both wet and dry conditions. An outstanding characteristic of acrylics is the ability
to withstand a hot acid environment, making
them a good choice in the filtration of high
sulfur-content exhaust gases.
(2) An outstanding nylon fiber available for
fabric filters is Nomen, a proprietary fiber
developed by Dupont for applications
requiring good dimensional stability and heat
resistance. Nomen nylon does not melt, but
degrades rapidly in temperatures above 700
degrees Fahrenheit. Its effective operating
limit is 450 degrees Fahrenheit. When in
contact with steam or with small amounts of
water vapor at elevated temperatures, Nomen
exhibits a progressive loss of strength.
However, it withstands these conditions better
baghouses are used when dust concentrations
are high and continuous filtering is needed.
Fabric characteristics and selection
Fabric filter performance depends greatly upon the
correct selection of a fabric. A fabric must be able to
efficiently collect a specific dust, be compatible with
the gas medium flowing through it, and be able to
release the dust easily when cleaned. Fiber, yarn
structure, and other fabric parameters will affect fabric
performance. At the present time, the prediction of
fabric pressure drop, collection efficiency, and fabric
life is determined from past performance. It is
generally accepted practice to rely on the experience of
the manufacturer in selecting a fabric for a specific
condition. However, the important fabric parameters
are defined below to aid the user in understanding the
significance of the fabric media in filtration.
TM 5-815-1/AFR 19-6
TM 5-815-1/AFR 19-6
than other nylons and many other fibers.
Because of Nomen's high abrasion resistance,
it is used in filtration of abrasive dusts or wet
abrasive solids and its good elasticity makes
it ideal for applications where continuous
flexing takes place. All nylon fabrics provide
good cake discharge for work with sticky
(3) Teflon is the most chemically resistant fiber
produced. The only substances known to
react with this fiber are molten alkali metals,
fluorine gas at high temperature and pressure,
and carbon trifluoride. Teflon fibers have a
very low coefficient of friction which
produces excellent cake discharge properties.
This fact, coupled with its chemical inertness
and resistance to dry and moist heat
degradation, make Teflon suitable for
filtration and dust collection under severe
conditions. Its major disadvantages are its
poor abrasion resistance and high price. For
these reasons, Teflon would be an economical
choice only in an application where extreme
conditions will shorten the service life of
other filter fibers. It should be noted that the
toxic gases produced by the decomposition of
Teflon at high temperatures can pose a health
hazard to personnel and they must be
removed from the work area through
c. Yarn type. Performance characteristics of filter
cloth depend not only on fiber material, but also on the
way the fibers are put together in forming the yarn.
Yarns are generally classified as staple (spun) or filament.
(1) Filament yarns show better release characteristics for certain dusts and fumes,
especially with less vigorous cleaning
(2) Staple yarn generally produces a fabric of
greater thickness and weight with high permeability to air flow. Certain fumes or dusts
TM 5-815-1/AFR 19-6
undergoing a change of state may condense
on fiber ends and become harder to remove
from the fabric.
d. Weave. The weave of a fabric is an important
characteristic which affects filtration performance. The
three basic weaves are plain, twill, and satin.
(1) Plain weave is the simplest and least
expensive method of fabric construction. It
has a high thread count, is firm, and wears
(2) Twill weave gives the fabric greater porosity,
greater pliability, and resilience. For this reason, twill weaves are commonly used where
strong construction is essential.
(3) Satin fabrics drape very well because the
fabric weight is heavier than in other weaves.
The yarns are compacted which produces
fabric body and lower porosity, and they are
often used in baghouses operating at ambient
e. Finish. Finishes are often applied to fabrics to
lengthen fabric life. Cotton and wool can be treated to
provide waterproofing, mothproofing, mildewproofing,
and fireproofing. Synthetic fabrics can be heat-set to
minimize internal stresses and enhance dimensional
stability. Water repellents and antistatic agents may
also be applied. Glass fabrics are lubricated with
silicon or graphite to reduce the internal abrasion from
TM 5-815-1/AFR 19-6
brittle yarns. This has been found to greatly increase
bag life in high temperature operations.
f. Weight. Fabric weight is dependent upon the density of construction, and fiber or yarn weight. Heavier
fabric construction yields lower permeability and
increased strength.
Materials and construction
a. Collector housing. Small unit collectors can be
assembled at the factory or on location. Multicompartment assemblies can be shipped by compartment or
module (group of compartments), and assembled onsite. Field assembly is disadvantageous because of the
need for insuring a good seal between panels, modules
and flanges. Baghouse collector wall and ceiling panels
are constructed of aluminum, corrugated steel, or con-
crete, the limitations being pressure, temperature, and
corrosiveness of the effluent. The metal thickness must
be adequate to withstand the pressure or vacuum
within the baghouse and sufficient bracing should be
provided. If insulation is needed, it can be placed
between wall panels of adjacent compartments and
applied to the outside of the structure. Pressure-relieving doors or panels should be included in the housing
or ductwork to protect equipment if any explosive dust
is being handled. An easy access to the baghouse
interior must be provided for maintenance.
Compartmented units have the advantage of being able
to remain on-line while one section is out for
maintenance. Walkways should be provided for access
to all portions of the cleaning mechanism. Units with
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TM 5-815-1/AFR 19-6
Auxiliary equipment and control
a. Instrumentation. Optimum performance of a fabric filter system depends upon continuous control of
gas temperature, system pressure drop, fabric pressure,
gas volume, humidity, condensation, and dust levels in
hoppers. Continuous measurements of fabric pressure
drop, regardless of the collector size, should be provided. Pressure gages are usually provided by the filter
manufacturer. With high and with variable dust loadings, correct fabric pressure drop is critical for proper
operation and maintenance. Simple draft gages may be
used for measuring fabric pressure drop, and they will
also give the static pressures at various points within
the system. Observation of key pressures within small
systems, permits manual adjustment of gas flows and
actuation of the cleaning mechanisms.
b. The number and degree of sophistication of pressure-sensing devices is relative to the size and cost of
the fabric filter system. High temperature filtration will
require that the gas temperature not exceed the
tolerance limits of the fabric and temperature displays
are required to indicate whether necessary dilution airdampers or pre-cooling sprays are operating correctly.
A well-instrumented fabric filter system protects the
investment and decreases chances of malfunctions. It
also enables the operating user to diagnose and correct
minor problems without outside aid.
c. Gas preconditioning. Cooling the inlet gas to a
fabric filter reduce the gas volume which then reduces
required cloth area; extends fabric life by lowering the
filtering temperature; and permits less expensive and
durable materials to be used. Gas cooling is mandatory
when the effluent temperature is greater than the maximum operating temperature of available fabrics. Three
practical methods of gas cooling are radiation convection cooling, evaporation, and dilution.
(1) Radiation convection cooling enables fluctuations in temperature, pressure, or flow to be
dampened. Cooling is achieved by passing the
gas through a duct or heat-transfer device and
there is no increase in gas filtering volume.
However, ducting costs, space requirements,
and dust sedimentation are problems with this
(2) Evaporative cooling is achieved by injecting
water into the gas stream ahead of the
filtering system. This effectively reduces gas
temperatures and allows close control of
filtering temperatures. However, evaporation
may account for partial dust removal and
incomplete evaporation may cause wetting
and chemical attack of the filter media. A
visible stack plume may occur if gas
temperatures are reduced near to or below the
dew point.
(3) Dilution cooling is achieved by mixing the gas
steam with outside air. This method is
inexpensive but increases filtered gas volume
requiring an increase in baghouse size. It is
possible the outside air which is added may
also require conditioning to control dust and
moisture content from ambient conditions.
bags longer than 10 to 12 feet should be provided with
walkways at the upper and lower bag attachment
b. Hopper and disposal equipment. The dust-collection hopper of a baghouse can be constructed of the
same material as the external housing. In small light
duty, hoppers 16 gage metal is typical. However, metal
wall thicknesses should be increased for larger
baghouses and hopper dust weight. The walls of the
hopper must be insulated and should have heaters if
condensation might occur. The hopper sides should be
sloped a minimum of 57 degrees to allow dust to flow
freely. To prevent bridging of certain dusts, a greater
hopper angle is needed, but continuous removal of the
dust will also alleviate bridging. If dust bridging is a
significant problem, vibrators or rappers may be
installed on the outside of the hopper. The rapping
mechanism can be electrically or pneumatically operated and the size of the hopper must be sufficient to
hold the collected dust until it is removed. Overfilled
hoppers may cause an increased dust load on the filter
cloths and result in increased pressure drop across the
collector assembly. Storage hoppers in baghouses
which are under positive or negative pressure warrant
the use of an air-lock valve for discharging dust. Since
this will prevent re-entrainment of dust or dust blowout. A rotary air valve is best suited for this purpose.
c. For low solids flow, a manual device such as a
slide gate, trip gate, or trickle valve may be used,
however, sliding gates can only be operated when the
compartment is shut down. For multicompartmented
units, screw conveyors, air slides, belt conveyors or
bucket conveying systems are practical. When a screw
conveyor or rotary valve is used, a rapper can be
operated by a cam from the same motor.
Energy requirements.
The primary energy requirement of baghouses is the
power necessary to move gas through the filter. Resistance to gas flow arises from the pressure drop across
the filter media and flow losses resulting from friction
and turbulent effects. In small or moderately sized
baghouses, energy required to drive the cleaning mechanism and dust disposal equipment is small, and may
be considered negligible when compared with primary
fan energy. If heating of reverse air is needed this will
require additional energy.
a. Incinerators. Baghouses have not been widely
used with incinerators for the following reasons:
(1) Maximum operating temperatures for fabric
filters have typically been in the range of 450
TM 5-815-1/AFR 19-6
d. Wood refuse boiler applications. It is not recommended that a baghouse be installed as a particulate
collection device after a wood fired boiler. The possibility of a fire caused by the carry over of hot glowing
particles is to great.
Significant testing has shown that emissions from a
fabric filter consist of particles less than 1 micron in
diameter. Overall fabric filter collection efficiency is 99
percent or greater (on a weight basis). The optimum
operating characteristics attainable with proper design
of fabric filter systems are shown in table 9-3.
Advantages and disadvantages
a. Advantages.
(1) Very high collection efficiencies possible
(99.9 + percent) with a wide range of inlet
grain loadings and particle size variations.
Within certain limits fabric collectors have a
constancy of static pressure and efficiency,
for a wider range of particle sizes and concentrations than any other type of single dust
(2) Collection efficiency not affected by sulfur
content of the combustion fuel as in ESPs.
(3) Reduced sensitivity to particle size distribution.
(4) No high voltage requirements.
(5) Flammable dust may be collected.
(6) Use of special fibers or filter aids enables submicron removal of smoke and fumes.
(7) Collectors available in a wide range of configurations, sizes, and inlet and outlet locations.
b. Disadvantages.
(1) Fabric life may be substantially shortened in
the presence of high acid or alkaline
atmospheres, especially at elevated temperatures.
(2) Maximum operating temperature is limited to
550 degrees Fahrenheit, unless special fabrics
are used.
(3) Collection of hygroscopic materials or condensation of moisture can lead to fabric plugging, loss of cleaning efficiency, large
pressure losses.
(4) Certain dusts may require special fabric treatments to aid in reducing leakage or to assist in
cake removal.
(5) High concentrations of dust present an explosion hazard.
(6) Fabric bags tend to burn or melt readily at
temperature extremes.
to 550 degrees Fahrenheit, which is below the
flue gas temperature of most incinerator
(2) Collection of condensed tar materials
(typically emitted from incinerators) could
lead to fabric plugging, high pressure drops,
and loss of cleaning efficiency
(3) Presence of chlorine and moisture in solid
waste leads to the formation of hydrochloric
acid in exhaust gases, which attacks fiberglass
and most other filter media
(4) Metal supporting frames show distortion
above 500 degrees Fahrenheit and chemical
attack of the bags by iron and sulphur at temperatures greater than 400 degrees Fahrenheit
contribute to early bag failure. Any fabric
filtering systems designed for particulate control of incinerators should include:
— fiberglass bags with silica, graphite, or teflon
lubrication; or nylon and, teflon fabric bags
for high temperature operation, or stainless
steel fabric bags,
— carefully controlled gas cooling to reduce
high temperature fluctuations and keep the
temperature above the acid dew point,
— proper baghouse insulation and positive sealing against outside air infiltration. Reverse air
should be heated to prevent condensation.
b. Boilers. Electric utilities and industrial boilers
primarily use electrostatic precipitators for air pollution
control, but some installations have been shown to be
successful with reverse air and pulse-jet baghouses.
The primary problem encountered with baghouse
applications is the presence of sulphur in the fuel which
leads to the formation of acids from sulphur dioxide
(SO2) and sulphur trioxide (SO3) in the exhaust gases.
Injection of alkaline additives (such as dolomite and
limestone) upstream of baghouse inlets can reduce SO2
present in the exhaust. Fabric filtering systems
designed for particulate collection from boilers should:
— operate at temperatures above the acid dew
— employ a heated reverse air cleaning method,
— be constructed of corrosion resistant material,
— be insulated and employ internal heaters to
prevent acid condensation when the
installation is off-line.
c. SO2 removal. The baghouse makes a good control
device downstream of a spray dryer used for SO2
removal and can remove additional SO2 due to the passage of the flue-gas through unreacted lime collected
on the bags.
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Formation of sulfur oxides (SOx)
a. Definition of sulfur oxide. All fossil fuels contain
sulfur compounds, usually less than 8 percent of the
fuel content by weight. During combustion, fuel-bound
sulfur is converted to sulfur oxides in much the same
way as carbon is oxidized to CO2. Sulfur dioxide (SO2)
and sulfur trioxide (SO3) are the predominant sulfur
oxides formed. See equations 10-1 and 10-2.
b. Stack-gas concentrations. In efficient fuel combustion processes, approximately 95 percent of the
fuel-bound sulfur is oxidized to sulfur dioxide with 1
to 2% being coverted to sulfur trioxide.
c. Factors affecting the formation of SOx.
(1) 503 formation increases as flame temperature
increases. Above 3,150 degrees Fahrenheit,
503 formation no longer increases.
(2) SO3 formation increases as the excess air rate
is increased.
(3) SO3 formation decreases with coarser
(3) When choosing a higher quality fuel, as in
changing from residual to distillate fuel oil,
modest modifications, such as changing
burner tips, and oil feed pumps, are required.
c. Changes in fuel properties. Consideration of possible differences in fuel properties is important. Some
examples are:
(1) Higher ash content increases particulate emissions.
(2) Lower coal sulfur content decreases ash
fusion temperature and enhances boiler tube
(3) Lower coal sulfur content increases fly-ash
resistivity and adversely affects electrostatic
precipitator performance.
(4) Low sulfur coal types may have higher
sodium content which enhances fouling of
boiler convection tube surfaces.
(5) The combination of physical coal cleaning
and partial flue gas desulfurization enables
many generating stations to meet SO2
standards at less expense than using flue gas
desulfurization alone.
d. Modification of fuel. Some possibilities are:
(1) Fuels of varying sulfur content may be mixed
to adjust the level of sulfur in the fuel to a low
enough level to reduce SO2 emissions to an
acceptable level.
(2) Fuels resulting from these processes will
become available in the not too distant future.
Gasification of coal removes essentially all of
the sulfur and liquification of coal results in a
reduction of more than 85% of the sulfur.
e. Applicability of boiler conversion from one fuel
type to another. Table 10-1 indicates that most boilers
can be converted to other type of firing but that policies
of the agencies must also be a consideration.
Available methods for reducing SOX
a. Fuel substitution. Burning low sulfur fuel is the
most direct means of preventing a SOx emissions problem. However, low sulfur fuel reserves are decreasing
and are not available in many areas. Because of this,
fuel cleaning technology has receive much attention.
There are presently more than 500 coal cleaning plants
in this country. At present, more than 20% of the coal
consumed yearly by the utility industry is cleaned.
Forty to ninety percent of the sulfur in coal can be
removed by physical cleaning, depending upon the type
of sulfur deposits in the coal. As fuel cleaning technology progresses and the costs of cleaning decrease,
fuel cleaning will become a long term solution
available for reducing sulfur oxide emissions.
b. Considerations of fuel substitution. Fuel substitution may involve choosing a higher quality fuel
grade; or it may mean changing to an alternate fuel
type. Fuel substitution may require any of the following
(1) Alternations in fuel storage, handling, preparation, and combustion equipment.
(2) When changing fuel type, such as oil to coal,
a new system must be installed.
TM 5-815-1/AFR 19-6
— adjusting turbine control valves to insure
proper lift
— adjusting preheater seals and feedwater heaters
— insuring cleanliness of heat transfer surfaces,
such as condensers, superheaters, reheaters,
and air heaters.
h. Limestone injection. One of the earliest techniques used to reduce sulfur oxide emission was the
use of limestone as a fuel additive. This technique
involves limestone injection into the boiler with the
coal or into the high temperature zone of the furnace.
The limestone is calcined by the heat and reacts with
the SO2 in the boiler to form calcium sulfate. The
unreacted limestone, and the fly ash are then collected
in an electrostatic precipitator, fabric bag filter, or
other particulate control device. There are a number of
problems associated with this approach:
(1) The sulfur oxide removal efficiency of the
additive approach is in the range of 50 to
70% in field applications. However, it is
considered feasible that when combined with
coal cleaning, it is possible to achieve an
overall SO2 reduction of 80 percent.
(2) The limestone used in the process cannot be
(3) The addition of limestone increases
particulate loadings. In the precipitator this
adversely affects collection efficiency.
(4) The effects of an increased ash load on
slagging and fouling as well as on particulate
collection equipment present a group of
problems which must be carefully considered.
(5) The high particulate loadings and potential
boiler tube fouling in high heat release boilers
tend to cause additional expense and technical
problems associated with handling large particulate loadings in the collection equipment.
(6) There have been many claims over the years
regarding the applicability of fuel additives to
the reduction of sulfur oxide emissions. The
United States Environmental Protection
Agency has tested the effect of additives on
residual and distillate oil-fired furnaces. They
conclude that the additives have little or no
i. Flue gas desulfurization (FGD). There are a
variety of processes which have demonstrated the
ability to remove sulfur oxides from exhaust gases.
Although this technology has been demonstrated for
some time, its reduction to sound engineering practice
and widespread acceptance has been slow. This is
particularly true from the standpoint of high system
reliability. The most promising systems and their
performance characteristics are shown in table 10-2.
j. Boiler injection of limestone with wet scrubber. In
this system limestone is injected into the boiler and is
f. Approach to fuel substitution. An approach to fuel
substitution should proceed in the following manner:
(1) Determine the availability of low sulfur fuels.
(2) For each, determine which would have sulfur
emissions allowable under appropriate
(3) Determine the effect of each on particulate
emissions, boiler capacity and gas temperatures, boiler fouling and slagging, and
existing particulate control devices.
(4) Identify the required equipment modifications, including transport, storage, handling,
preparation, combustion, and control equipment.
(5) For the required heat output calculate the
appropriate fuel feed rate.
(6) Determine fuel costs.
(7) Determine the cost of boiler and equipment
modification in terms of capital investment
and operation.
(8) Annualize fuel costs, capital charges, and
operating and maintenance costs.
(9) With the original fuel as a baseline, compare
emissions and costs for alternate fuels.
(g. Modification to boiler operations and maintenance.
(1) A method of reducing sulfur oxides emissions
is to improve the boiler use of the available
heat. If the useful energy release from the
boiler per unit of energy input to the boiler
can be increased, the total fuel consumption
and emissions will also be reduced.
(2) An improvement in the boiler release of
useful energy per unit of energy input can be
achieved by increasing boiler steam pressure
and temperature. Doubling the steam drum
pressure can increase the useful heat release
per unit of energy input by seven percent.
Increasing the steam temperature from 900 to
1000 degrees Fahrenheit can result in an
improvement in the heat release per unit of
energy input of about 3.5 percent.
(3) Another way to maximize the boiler's output
per unit of energy input is to increase the
attention given to maintenance of the correct
fuel to air ratio. Proper automatic controls
can perform this function with a high degree
of accuracy.
(4) If additional emphasis can be put on maintenance tasks which directly effect the boilers
ability to release more energy per unit of
energy input they should be considered a
modification of boiler operations. Items
which fall into this category are:
— Washing turbine blades
— adjusting for maximum throttle pressure
TM 5-815-1/AFR 19-6
TM 5-815-1/AFR 19-6
o. Dry furnace injection of limestone. In this system,
dry ground limestone is injected into the boiler where
it is calcined and reacts with the 502 formed during
combustion of the fuel. The flue gases containing the
sodium sulfate, sodium sulfite, unreacted limestone,
and fly ash all exit the boiler together and are captured
on a particulate collector. The cleaned flue gases pass
through the filter medium and out through the stack
(fig 10-1a).
p. Magnesium oxide (MgO) scrubber This is a
regenerative system with recovery of the reactant and
sulfuric acid. As can be seen in figure 10-2 the flue gas
must be precleaned of particulate before it is sent to the
scrubber. An ESP or venturi scrubber can be used to
remove the particulate. The flue gas then goes to the
MgO scrubber where the principal reaction takes place
between the MgO and SO2 to form hydrated magnesium sulfite. Unreacted slurry is recirculated to the
scrubber where it combines with makeup MgO and
water and liquor from the slurry dewatering system.
The reacted slurry is sent through the dewatering system where it is dried and then passed through a recovery process, the main step of which is calcination. High
reliability of this system has not yet been obtained. SO2
removal efficiencies can be high, but scaling and corrosion are major problems.
q. Wellman Lord process. Sodium sulfite is the
scrubbing solution. It captures the SO2 to produce
sodium bisulfite, which is later heated to evolve SO2
and regenerate the sulfite scrubbing material. The SO2
rich product stream can be compressed or liquified and
oxidized to sulfuric acid, or reduced to sulfur. Scaling
and plugging are minimal problems because the
sodium compounds are highly soluble in water. A
Wellman-Lord unit has demonstrated an SO2 removal
efficiency of greater than 90 percent and an availability
of over 85 percent. The harsh acid environment of the
system has caused some mechanical problems (See
figure 10-3).
r. Catalytic oxidation. The catalytic oxidation process uses a high temperature (850 degrees Fahrenheit)
and a catalyst (vanadium pentoxide) to convert SO2 to
SO3. The heated flue gas then passes through a gas heat
exchanger for heat recovery and vapor condensation.
Water vapor condenses in the heat exchanger and combines with SO3 to form sulfuric acid. The acid mist is
then separated from the gas in an absorbing tower. The
flue gas must be precleaned by a highly efficient particulate removal device such as an electrostatic precipitator preceding the cat-ox system to avoid
poisoning the catalyst. The major drawback of this
system is that it cannot be economically retro-fitted to
existing installations (fig 10-4).
s. Single alkali sodium carbonate scrubbing. In
order to eliminate the plugging and scaling problems
associated with direct calcium scrubbing, this FGD
system was developed. As shown in figure 10-5, the
process is a once through process involving scrubbing
calcined to lime. The lime reacts with the SO2 present
in the combustion gases to form calcium sulfate and
calcium sulfite. As the gas passes through a wet scrubber, the limestone, lime, and reacted lime are washed
with water to form sulfite. As the gas passes through a
wet scrubber, the limestone, lime, and reacted lime are
washed with water to form a slurry. The resulting
effluent is sent to a settling pond and the sediment is
disposed by landfilling. Removal efficiencies are below
50% but can be reliably maintained. Scaling of boiler
tube surfaces is a major problem.
k. Scrubber injection of limestone. In this FGD system limestone is injected into a scrubber with water to
form a slurry (5 to 15% solids by weight). The
limestone is ground into fines so that 85% passes
through a 200-mesh screen. CaCO3 absorbs SO2 in the
scrubber and in a reaction tank where additional time
is allowed to complete the reaction. Makeup is added
to the reusable slurry as necessary and the mixture is
recirculated to the scrubber. The dischargable slurry is
taken to a thickener where the solids are precipitated
and the water is recirculated to the scrubber.
Limestone scrubbing is a throwaway process and
sludge disposal may be a problem. At SO2 removal
efficiencies of about 30%, performance data indicate
that limestone scrubbers have a 90% operational
reliability. Plugging of the demister, and corrosion and
erosion of stack gas reheat tubes have been major
problems in limestone scrub-hers. Figure 10-1 shows
a simplified process flow-sheet for a typical limestone
scrubbing installation.
l. Scrubber injection of lime. This FGD process is
similar to the limestone scrubber process, except that
lime (Ca(OH)2) is used as the absorbent. Lime is a
more effective reactant than limestone so that less of it
is required for the same SO2 removal efficiency. The
decision to use one system over the other is not clearcut and usually is decided by availability.
m. Post furnace limestone injection with spray drying. In this system, a limestone slurry is injected into a
spray dryer which receives flue gas directly from the
boiler. The limestone in the slurry reacts with the SO2
present in the combustion gases to form calcium
sulfate and calcium sulfite. The heat content of the
combustion gases drives off the moisture resulting in
dry particulates exiting the spray dryer and their
subsequent capture in a particulate collector following
the spray dryer. The particulates captured in the
collector are discharged as a dry material and the
cleaned flue gases pass through the filter to the stack
(fig 10-la).
n. Dry, post furnace limestone injection. Ground dry
limestone is injected directly into the flue gas duct prior
to a fabric filter. The limestone reacts in the hot
medium with the SO2 contained in the combustion
gases and is deposited on the filter bags as sodium sulfate and sodium sulfite. The dry particulate matter is
then discharged to disposal and the cleaned flue gases
pass through the filter medium to the stack (fig 10-lb).
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TM 5-815-1/AFR 19-6
with a solution of sodium carbonate or sodium hydroxide to produce a solution of dissolved sodium sulfur
salts. The solution is then oxidized to produce a neutral
solution of sodium sulfate. Because it is a throwaway
process, the cost of chemicals make it an unattractive
SOx removal process when burning high sulfur fuels
(greater than 1 percent).
t. Dual alkali sodium scrubbing.
(1) The dual alkali SOX removal system is a
regenerative process designed for disposal of
wastes in a solid/slurry form. As shown in
figure 10-6, the process consists of three
basic steps; gas scrubbing, a reactor system,
and solids dewatering. The scrubbing system
utilizes a sodium hydroxide and sodium
sulfite solution. Upon absorption of SO2 in
the scrubber, a solution of sodium bisulfite
and sodium sulfite is produced. The scrubber
effluent containing the dissolved sodium salts
is reacted outside the scrubber with lime or
limestone to produce a precipitate of calcium
salts containing calcium sulfate. The
precipitate slurry from the reactor system is
dewatered and the solids are deposed of in a
landfill. The liquid fraction containing
soluable salts is recirculated to the absorber.
Double alkali systems can achieve efficiencies
of 90 - 95% and close to 100% reagent
(2) This system is designed to overcome the
inherent difficulties of direct calcium slurry
scrubbing. All precipitation occurs outside the
scrubber under controlled reactor conditions.
The principal advantages of the dual alkali
system are:
(a) Scaling problems associated with direct
calcium-based scrubbing processes are
significantly reduced.
(b) A less expensive calcium base can be
(c) Due to high solubility and concentration
of active chemicals, lower liquid volumes
can be used thereby lowering equipment
(d) Slurries are eliminated from the
absorption loop, thereby reducing
plugging and erosion problems.
(e) A sludge waste, rather than a liquid waste,
is produced for disposal.
(f) High SO2 removal efficiency (90% or
u. Absorption of SO2.
(1) Activated carbon has been used as an absorbent for flue-gas desulfurization. Activated
carbon affects a catalytic oxidation of 502 to
SO3, the latter having a critical temperature of
425 degrees Fahrenheit. This allows absorption to take place at operating temperatures.
The carbon is subsequently regenerated in a
separate reactor to yield a waste which is used
in the production of high grade sulfuric acid,
and the regenerated absorbent. There are
serious problems involved in the regeneration
of the absorbent, including carbon losses due
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to attrition, chemical decomposition, serious
corrosion problems, and danger of
combustion of the reactivated carbon.
(2) Zeolites are a class of highly structured aluminum silicate compounds. Because of the regular pore size of zeolites, molecules of less
than a certain critical size may be
incorporated into the structure, while those
greater are excluded. It is often possible to
specify a certain zeolite for the separation of
a particular material. Zeolites possesses
properties of attrition resistance, temperature
stability, inertness to regeneration techniques,
and uniform pore size which make them ideal
absorbents. However, they lack the ability to
catalyze the oxidation of SO2 to SO3 and thus
cannot desulfurize flue-gases at normal
operating temperatures. Promising research is
under way on the development of a zeolite
material that will absorb SO2 at flue-gas
temperatures by oxidation of SO3 and
subsequently store it as a sulphate in the pores
of the zeolite.
v. Cost of flue-gas desulfurization. The actual
capital and operating costs for any specific installation
are a function of a number of factors quite specific to
the plant and include:
— Plant size, age, configuration, and locations,
— Sulfur content of the fuel and emission
control requirements,
— Local construction costs, plant labor costs,
and cost for chemicals, water, waste disposal,
— Type of FGD system and required equipment,
— Whether simultaneous particulate emission
reduction is required.
Procedure to minimize SOX emission
a. Efficiency requirement. The SOx removal efficiency necessary for any given installation is dependent
upon the strictest regulation governing that installation.
Given a certain required efficiency, a choice can be
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made among the different reduction techniques. This
section shows how a rational basis can be utilized to
determine the best method.
b. Boiler modification. This technique is useful in
reducing SOx emissions by 0 to 6% depending upon
the boiler. For industrial boilers operating above 20%
excess-air the use of proper control equipment or low
excess-air combustion will usually reduce emissions by
4 to 5%. If the operating engineer is not familiar with
boiler optimization methods, consultants should be utilized.
c. Fuel substitution. This method can be used for
almost any percent reduction necessary. Availability
and cost of the fuel are the major factors to be considered. Fuels can be blended to produce the desired sulfur input. Care must be taken, however, so that the ash
produced by the blending does not adversely affect the
boiler by lowering the ash fusion temperature or causing increased fouling in the convection banks.
d. Flue-gas desulfurization. Various systems are
available for flue-gas desulfurization. Some of these
systems have demonstrated long term reliability of
operation with high SOx removal efficiency. Lime/limestone injection and scrubbing systems have been most
frequently used. It must be recognized that each boiler
control situation must be accommodated in the overall
system design if the most appropriate system is to be
installed. The selection and design of such a control
system should include the following considerations:
(1) Local SO2 and particulate emission requirements, both present and future,
(2) Local liquid and solid waste disposal regulations,
(3) Local market demand for recovered sulfur,
(4) Plant design limitations and site characteristics,
(5) Local cost and availability of chemicals, utilities, fuels, etc.,
(6) Added energy costs due to process pumps,
reheaters, booster fans, etc.
Sample problems.
The following problems have been provided to
illustrate how to determine the maximum fuel sulfur
content allowable to limit SO emission to any
particular level.
a. Approximately 90 to 97 percent of fuel sulfur is
oxidized to sulfur dioxide (SO2) during combustion.
This means that for every lb of sulfur in the fuel,
approximately 2 lbs of sulfur oxides will appear in the
stack gases. (The atomic weight of oxygen is ½ that of
sulfur.) Since most of the sulfur oxides are in the form
of SO2, emissions regulations are defined in these units.
To estimate maximum probable SO2 emissions, the following equation applies:
b. Assume a fuel-oil burning boiler must limit emissions to .35 lbs/MMBtu. What is the maximum allowable sulfur content if No.6 Residual fuel-oil is to be
(1) From table 10-3, Typical Analysis of Fuel-Oil
Types, an average heating value of 18,300
TM 5-815-1/AFR 19-6
Btu/lb for No.6 residual fuel has been
assumed. Maximum allowable sulfur content
is determined as:
d. Assume a boiler installation burns No.4 fuel-oil
with a heating value of 19,000 Btu/lb. What is the
maximum fuel sulfur content allowable to limit SOx
emissions to .8 lbs/MMBtu?
(2) Table 10-3 shows that No.5 and No.6 fuel
oils have fuel sulfur contents in excess of
.32%. If No.4 fuel oil is chosen, a fuel with
less than .32% sulfur may be available.
c. Assume a fuel-oil burning boiler must limit SOx
emission to .65 lbs/MMBtu. If No.6 residual fuel oil is
to be used, can SOx emission limits be met?
(1) From table 10-3, the minimum sulfur content
in No.6 fuel oil is .7%. If .7% sulfur fuel can
be purchased, the heating value of the fuel
must be:
(2) Since the heating value of No. 6 fuel oil is
generally between 17,410 and 18,990 Btu/lb,
SOx emission limits cannot be met using this
fuel. If we assume a No.6 fuel-oil with one
percent sulfur and a heating value of 18,600
Btu/lb is used the percent SOx removal efficiency that will be required is determined as:
e. Assume a coal burning boiler must limit SOx
emissions to 1 lb/MMBtu. If sub-bituminous coal with
a heating value of 12,000 to 12,500 Btu/lb (see table
10-4) is to be used what is the maximum allowable
fuel sulfur content?
f. Since coal of this low sulfur content is not available, what SOx removal efficiency would be required
burning 1% sulfur coal?
TM 5-815-1/AFR 19-6
TM 5-815-1/AFR 19-6
TM 5-815-1/AFR 19-6
Formation of nitrogen oxides.
a. Nitrogen oxides (NOx). All fossil fuel burning
processes produce NOx. The principle oxides formed
are nitric oxide (NO) which represents 90-95 percent
(%) of the NOx formed and nitrogen dioxide (NO2)
which represents most of the remaining nitrogen
b. NOx formation. Nitrogen oxides are formed primarily in the high temperature zone of a furnace where
sufficient concentrations of nitrogen and oxygen are
present. Fuel nitrogen and nitrogen contained in the
combustion air both play a role in the formation of
NOx. The largest percentage of NOx formed is a result
of the high temperature fixation reaction of
atmospheric nitrogen and oxygen in the primary
combustion zone.
c. NOx concentration. The concentration of NOx
found in stack gas is dependent upon the time, temperature, and concentration history of the combustion
gas as it moves through the furnace. NOx concentration
will increase with temperature, the availability of oxygen, and the time the oxygen and nitrogen simultaneously are exposed to peak flame temperatures.
tions produce more NOx. The more bulk mixing of fuel
and air in the primary combustion zone, the more turbulence is created. Flame color is an index of flame
turbulence. Yellow hazy flames have low turbulence,
whereas, blue flames with good definition are considered highly turbulent.
c. Burner number. The number of burners and their
spacing are important in NOx emission. Interaction
between closely spaced burners, especially in the center
of a multiple burner installation, increases flame
temperature at these locations. The tighter spacing
lowers the ability to radiate to cooling surfaces, and
greater is the tendency toward increased NOx emissions.
d. Excess air. A level of excess air greatly exceeding
the theoretical excess air requirement is the major
cause of high NOx emissions in conventional boilers.
Negotiable quantities of thermally formed NOx are
generated in fluidized bed boilers.
e. Combustion temperature. NOx formation is
dependent upon peak combustion temperature, with
higher temperatures producing higher NOx emissions.
f. Firing and quenching rates. A high heat release
rate (firing rate) is associated with higher peak temperatures and increased NOx emissions. A high rate of
thermal quenching, (the efficient removal of the heat
released in combustion) tends to lower peak temperatures and contribute to reduced NOx emissions.
g. Mass transportation and mixing. The concentration of nitrogen and oxygen in the combustion
zone affects NOx formation. Any means of decreasing
the concentration such as dilution by exhaust gases,
slow diffusion of fuel and air; or alternate fuelrich/fuel- lean burner operation will reduce NOx
formation. These methods are also effective in
reducing peak flame temperatures.
h. Fuel type. Fuel type affects NOx formation both
through the theoretical flame temperature reached, and
through the rate of radiative heat transfer. For most
combustion installations, coal-fired furnaces have the
highest level of NOx emissions and gas-fired
installations have the lowest levels of NOx emissions.
i. Fuel nitrogen. The importance of chemically
bound fuel nitrogen in NOx formation varies with the
temperature level of the combustion processes. Fuel
nitrogen is important at low temperature combustion,
but its contribution is nearly negligible as higher flame
temperatures are reached, because atmospheric nitro-
Factors affecting NOx emissions
a. Furnace design and firing type. The size and
design of boiler furnaces have a major effect on NOx
emissions. As furnace size and heat release rates
increase, NOx emissions increase. This results from a
lower furnace surface-to-volume ratio which leads to
a higher furnace temperature and less rapid terminal
quenching of the combustion process. Boilers generate
different amounts of NOx according to the type of
firming. Units employing less rapid and intense burning
from incomplete mixing of fuel and combustion gases
generate lower levels of NOx emissions. Tangentially
fired units generate the least NOx because they operate
on low levels of excess air, and because bulk misting
and burning of the fuel takes place in a large portion of
the furnace. Since the entire furnace acts as a burner;
precise proportioning of fuel/air at each of the individual fuel admission points is not required. A large
amount of internal recirculation of bulk gas, coupled
with slower mixing of fuel and air, provides a combustion system which is inherently low in NOx production
for all fuel types.
b. Burner design and configuration. Burners operating under highly turbulent and intense flame condi-
TM 5-815-1/AFR 19-6
gen contributes more to NOx formation at higher temperatures.
NOx reduction techniques
(3) Changing from a higher to a lower NOx
producing fuel is not usually an economical
method of reducing NOx emissions because
additional fuel costs and equipment capital
costs will result. For additional information
on fuel substitution, see paragraph 10-3. In
doing so, it should be noted that changing
from coal to oil or gas firing is not in
accordance with present AR 420-49.
b. Load reduction. Load reduction is an effective
technique for reducing NOx emissions. Load reduction
has the effect of decreasing the heat release rate and
reducing furnace temperature. A lowering of furnace
temperature decreases the rate of NOx formation.
(1) NOx reduction by load reduction is illustrated
in figure 11-1. As shown, a greater reduction
a. Fuel selection. Reduction of NOx emissions may
be accomplished by changing to a fuel which decreases
the combustion excess air requirements, peak flame
temperatures, and nitrogen content of the fuel. These
changes decrease the concentration of oxygen and
nitrogen in the flame envelope and the rate of the NOx
formation reaction.
(1) The specific boiler manufacturer should be
consulted to determine if a fuel conversion
can be performed without adverse effects.
The general NOx reduction capability of
initiating a change in fuel can be seen
comparatively in table 11-1.
(2) A consideration when comtemplating a
change in fuel type is that NOx emission
regulations are usually based on fuel type.
Switching to a cleaner fuel may result in the
necessity of conforming to a more strict
emission standard.
TM 5-815-1/AFR 19-6
(2) The successful application of LEA firing to
any unit requires a combustion control system
to regulate and monitor the exact
proportioning of fuel and air. For pulverized
coal fired boilers, this may mean the
additional expense of installing uniform
distribution systems for the coal and air
(3) Low excess air firing is a desirable method of
reducing NOx emission because it can also
improve boiler efficiency by reducing the
amount of heat lost up the stack. Consequently, a reduction in fuel combustion will
sometimes accompany LEA firing.
d. Low excess air firing with load reduction. NOx
emissions may be reduced by implementing a load
reduction while operating under low excess air conditions (table 11-2). This combined technique may be
desirable in an installation where NOx emissions are
extremely high because of poor air distribution and the
resultant inefficient operation of combustible equipment. A load reduction may permit more accurate control of the combustion equipment and allow reduction
of excess air requirements to a minimum value. NOx
reduction achieved by simultaneous implementation of
load reduction and LEA firing is slightly less than the
combined estimated NOx reduction achieved by separate implementation.
e. Two-stage combustion. The application of delayed
fuel and air mixing in combustion boilers is referred to
as two stage combustion. Two-stage combustion can
be of two forms. Normally it entails operating burners
fuel-rich (supplying only 90 to 95 percent of
stoichiometric combustion air) at the burner throat, and
admitting the additional air needed to complete
combustion through ports (referred to as NO ports)
located above and below the burner. There are no ports
to direct streams of combustion air into the burner
flame further out from the burner wall thus allowing a
gradual burning of all fuel. Another form of two-stage
combustion is off-stoichiometric firing. This technique
involves firing some burners fuel-rich and others airrich (high percentage of excess air), or air only, and is
usually applied to boilers having three or more burner
levels. Off-stoichiometric firing is accomplished by
staggering the air-rich and fuel-rich burners in each of
the burner levels. Various burner configuration tests
have shown that it is generally more effective to
operate most of the elevated burners air-rich or air
only. Off-stoichiometric firing in pulverized coal fired
boilers usually consists of using the upper burners on
air only while operating the lower levels of burners
fuel-rich. This technique is called overfire air
(1) Two-stage combustion is effective in
reducing NOx emissions because: it lowers
the concentration of oxygen and nitrogen in
the primary combustion zone by fuel-rich
firing; it lowers the attainable peak flame
temperature by allowing for gradual
in NO2 is attainable burning gas fuels because
they contain only a small amount of fuelbound nitrogen. Fuel-bound nitrogen
conversion does not appear to be affected by
furnace temperatures, which accounts for the
lower NOx reductions obtained with coal and
oil firing. Some units such as tangentially
fired boilers show as much as 25 percent
decrease in NOx emissions with a 25 percent
load reduction while burning pulverized coal.
(2) Although no capital costs are involved in load
reduction, it is sometimes undesirable to
reduce load because it may reduce steam
cycle efficiency.
c. Low excess air firing (LEA). In order to complete
the combustion of a fuel, a certain amount of excess air
is necessary beyond the stoichiometric requirements.
The more efficient the burners are in misting, the
smaller will be the excess air requirement. A minimum
amount of excess air is needed in any system to limit
the production of smoke or unburned combustibles;
but larger amounts may be needed to maintain steam
temperature to prevent refractory damage; to complete
combustion when air supply between burners is unbalanced; and to compensate for instrument lag between
operational changes. Practical minimums of excess air
are 7 percent for natural gas, 3 to 15 percent for oil
firing, and 18 to 25 percent for coal firing.
(1) Since an increase in the amount of oxygen
and nitrogen in a combustion process will
increase the formation and concentration of
NOx, low excess air operation is the first and
most important technique that should be
utilized to reduce NOx emissions. A 50
percent reduction in excess air can usualy
reduce NOx emissions from 15 to 40 percent,
depending upon the level of excess air
normally applied. Average NOx reductions
corresponding to a 50 percent reduction in
excess air for each of the three fuels in
different boiler types are shown in table 11-2.
Reductions in NOx emission sup to 62 percent
have been reported on a pulverized coal fired
boiler when excess air is decreased from a
level of 22 percent to a level of 5 percent.
TM 5-815-1/AFR 19-6
mixing accompanying the increased
combustion air/ gas volume. Gas recirculation
does not significantly reduce plant thermal
efficiency but it can influence boiler
operation. Radiation heat transfer is reduced
in the furnace because of lower gas
temperatures, and convective beat transfer is
increased because of greater gas flow.
(2) The extent of the applicability of this
modification remains to be investigated. The
quantity of gas necessary to achieve the
desired effect in different installations is
important and can influence the feasibility of
the application. Implementing flue-gas
recirculation means providing duct work and
recycle fans for diverting a portion of the
exhaust flue-gas back to the combustion air
windbox. It also requires enlarging the
windbox and adding control dampers and
instrumentation to automatically vary flue-gas
recirculation as required for operating
conditions and loads.
h. Steam or water injection. Steam and water injection has been used to decrease flame temperatures and
reduce NOx emissions. Water injection is preferred
over steam because of its greater ability to reduce temperature. In gas and coal fired units equipped with
standby oil firing with steam atomization, the atomizer
offers a simple means for injection. Other installations
require special equipment and a study to determine the
proper point and degree of atomization. The use of
water or steam injection may entail some undesirable
operating conditions, such as decreased efficiency and
increased corrosion. A NOx reduction rate of up to 10
percent is possible before boiler efficiency is reduced
to uneconomic levels. If the use of water injection
requires installation of an injection pump and attendant
piping, it is usually not a cost-effective means of
reducing NOx emissions.
combustion of all the fuel; and it reduces the
amount of time the fuel and air mixture is
exposed to higher temperatures.
(2) The application of some form of two stage
combustion implemented with overall low
excess air operation is presently the most
effective method of reducing NOx emissions
in utility boilers. Average NOx reductions for
this combustion modification technique in
utility boilers are listed in table 11-3.
However, it should be noted that this
technique is not usually adaptable to small
industrial boilers where only one level of
burners is provided.
f. Reduced preheat temperature. NOx emissions are
influenced by the effective peak temperature of the
combustion process. Any modifications that lower
peak temperature will lower NOx emissions. Lower air
preheat temperature has been demonstrated to be a
factor in controlling NOx emissions. However, reduced
preheat temperature is not a practical approach to NOx
reduction because air preheat can only be varied in a
narrow range without upsetting the thermal balance of
the boiler. Elimination of air preheat might be expected
to increase particulate emissions when burning coal or
oil. Preheated air is also a necessary part of the coal
pulverizer operation on coal fired units. Jn view of he
penalties of reduced boiler efficiency and other disadvantages, reduced preheat is not a preferred means of
lowering NOx emissions.
g. Flue-gas recirculation. This technique is used to
lower primary combustion temperature by recirculating
part of the exhaust gases back into the boiler combustion air manifold. This dilution not only decreases
peak combustion flame temperatures but also
decreases the concentration of oxygen available for
NOx formation. NOx reductions of 20 to 50 percent
have been obtained on oil-fired utility boilers but as yet
have not been demonstrated on coal-fired units. It is
estimated that flue gas recirculation has a potential of
decreasing NOx emissions by 40 percent in coal-fired
(1) Flue gas recirculation has also produced a
reduction on CO concentrations from normal
operation because of increased fuel-air
Post combustion Systems for NOx
a. Selective catalytic reduction (SCR) of NOx is
based on the preference of ammonia to react with NO,
rather than with other flue-gas constitutents. Ammonia
is injected so that it will mix with flue-gas between the
economizer and the air heater. Reaction then occurs as
this mix passes through a catalyst bed. Problems
requiring resolution include impact of ammonia on
downstream equipment, catalyst life, flue-gas
monitoring, ammonia availability, and spent-catalyst
b. Selective noncatalytic reduction (SNR) Ammonia
is injected into the flue-gas duct where the temperature
favors the reaction of ammonia with NOx in the fluegas. The narrow temperature band which favors the
reaction and the difficulty of controlling the temperature are the main drawbacks of this method.
TM 5-815-1/AFR 19-6
c. Copper oxide is used as the acceptor for SO2
removal, forming copper sulfate. Subsequently both
the copper sulfate which was formed and the copper
oxide catalyze the reduction of NO to nitrogen and
water by reaction with ammonia. A regeneration step
produces an SO2 rich steam which can be used to manufacture by-products such as sulfuric acid.
Step-by-step NOx reduction method
a. Applicability. The application of NOx reduction
techniques in stationary combustion boilers is not
extensive. (However, NOx reduction techniques have
been extensively applied on automobiles.) These techniques have been confined to large industrial and utility
boilers where they can be more easily implemented
where NOx emissions standards apply, and where
equipment modifications are more economically justified. However some form of NOx control is available
for all fuel-burning boilers without sacrificing unit
output or operating efficiency. Such controls may
become more widespread as emission regulations are
broadened to include all fuel-burning boilers.
b. Implementation. The ability to implement a particular combustion modification technique is dependent
upon furnace design, size, and the degree of equipment
operational control. In many cases, the cost of conversion to implement a modification such as flue-gas
recirculation may not be economically justified. Therefore, the practical and economic aspects of boiler
design and operational modifications must be
ascertained before implementing a specific reduction
(1) Temperature reduction through the use of
two stage combustion and flue-gas
recirculation is most applicable to high heat
release boilers with a multiplicity of burners
such as utility and large industrial boilers.
(2) Low excess air operation (LEA) coupled with
flue-gas recirculation offers the most viable
solution in smaller industrial and commercial
size boilers. These units are normally
designed for lower heat rates (furnace
temperature) and generally operate on high
levels of excess air (30 to 60%).
c. Compliance. When it has been ascertained that
NOx emissions must be reduced in order to comply
with state and federal codes, a specific program should
be designed to achieve the results desired. The
program direction should include:
— an estimate of the NOx reduction desired,
— selection of the technique or combination
thereof, which will achieve this reduction;
— an economic evaluation of implementing each
technique, including equipment costs, and
changes in operational costs;
— required design changes to equipment
— the effects of each technique upon boiler
performance and operational safety.
d. Procedure. A technical program for implementing
a NOx reduction program should proceed with the aid
of equipment manufacturers and personnel who have
had experience in implementing each of the NOx
reduction techniques that may be required in the
following manner:
(1) NOx emission test. A NOx emission test
should be performed during normal boiler
load times to ascertain actual on-site NOx
generation. This test should include recording
of normal boiler parameters such as: flame
temperature; excess air; boiler loads; flue-gas
temperatures; and firing rate. These
parameters can be referred to as normal
operating parameters during subsequent
changes in operation.
(2) Reduction capabilities. The desired reduction
in NOx emissions, in order to comply with
standards, should be estimated based on measured NOx emission data. Specific NOx reduction techniques can then be selected based
on desired reductions and reduction capabilities outlined in preceding paragraph 11-3.
(3) Equipment optimization. Any realistic program for NOx reduction should begin with an
evaluation and overhaul of all combustion
related equipment. A general improvement of
boiler thermal efficiency and combustion efficiency will reduce the normal level of NOx
emissions. Of major importance are:
(a) the cleanliness of all heat transfer surfaces
(especially those exposed to radiative heat
(b) maintaining proper fuel preparation (sizing, temperature, viscosity),
(c) insuring control and proper operation of
combustion equipment (burners nozzles,
air registers, fans, preheaters, etc.),
(d) maintaining equal distribution of fuel and
air to all burners.
(4) Low excess air operation. Low excess air
operation is the most recommended modification for reducing NOx emission. Possible
reductions are given in preceding table 11-2.
How-ever, a control system is needed to
accurately monitor and correct air and fuel
flow in response to steam demands. Of the
control systems available, a system incorporating fuel and air metering with stack gas O2
correction will provide the most accurate
control. A system of this nature will generally
pay for itself in fuel savings over a 2 to 3-year
period, and is economically justified on
industrial boilers rated as low as 40,000 lb of
(5) Flue-gas recirculation. Flue-gas recirculation
is the second most effective NOx reduction
technique for boilers where two stage
combustion cannot be applied. Low excess
TM 5-815-1/AFR 19-6
design must accompany any application of
flue-gas recirculation which effectively lowers
furnace temperature and thus, radiative heat
transfer. Convective heat transfer is also
increased by increased gas flow due to the
dilution of combustion air. It is advisable to
consult boiler manufacturers as to the
applicability of flue-gas recirculation to their
e. Summary. The potential and applicability of each
NOx reduction technique is summarized in table 11-4.
air operation and flue-gas recirculation must
be implemented simultaneously from a design
point of view. LEA operation may require
installation or retrofitting of air registers to
maintain proper combustion air speed and
mixing at reduced levels or air flow. Flue gas
recirculation will require larger air registers to
accommodate the increased volume of flow.
Therefore, simultaneous application of LEA
operation and flue-gas recirculation may
minimize the need for redesign of burner air
registers. Knowledge of furnace thermal
TM 5-815-1/AFR 19-6
TM 5-815-1/AFR 19-6
Principles of selection
a. Selection of emission control equipment is made
in three basic steps.
(1) Performance. The control equipment must be
capable of continuously controlling the emission of the pollutant below the permitted
quantities. The equipment type and design
should have a proven record of meeting the
required removal or collection efficiency and
the manufacturer should guarantee the
equipment for continuous performance.
(2) Construction. The materials of construction
should be compatible with the characteristics
and constituents in the flue gases. Materials
should be resistant to erosion and corrosion
and should be suitable for the operating temperatures. The unit should have adequate
access manholes and service platforms and
stairs to inspect and maintain the equipment.
Units should be adequately insulated and
weather protected.
(3) Operation. Where more than one design or
type of device can provide the necessary
pollution control it then becomes necessary to
evaluate the various designs based on a lifecycle cost-analysis, and the ease of operation.
b. Preliminary information which is needed to properly select pollution control equipment are as follows:
(1) Site-specific emissions limitations for the
stack serving the particular boiler or
incinerator must be determined for the
applicable source and ambient condition. This
information is to be derived from existing
federal, state and local regulations.
(2) Obtain detailed descriptions of the boiler or
incinerator including the combustion control
system(s) and all support auxiliaries including
outline drawings available from the manufacturers; and the predicted uncontrolled, gaseous emissions established for the units.
(3) For the particular fuel to be burned,
determine the method of firing and maximum
continuous rated heat input per British
Thermal Units per hour (BTU’s/Hr) along
with applicable combustion calculations for
normal and upset operating conditions. This
may require a fuel analysis. In the case of coal
firing the analysis should include ultimate and
proximate properties and an analysis of the
residual ash.
(4) Obtain required construction and operations
permit forms from applicable regulatory
agencies, complete, and submit where
(5) Obtain the requirements and restraints for disposing of the collected pollutant. Under some
circumstances such as preliminary studies it
becomes necessary to calculate the process
data and then use empirical data to estimate
the emission quantities.
c. The U.S. Environmental Protection Agency (EPA)
has published a Technical Manual 'AP-42" and
excerpts from the EPA publication have been
reproduced and included in Chapters 2 and 3 of this
manual to be used as a guide for predicting the emissions that will be generated by various fuels and combustions apparatus.
d. Present emissions control requirements and laws
are complicated and stringent, and emission control
equipment represents a significant portion of the combustion equipment costs. Inadequately specified or
applied control devices could be a very costly error. It
is advisable wherever possible to utilize qualified engineers experienced in boiler or incinerator plant designs
and operation of such tasks. It is beneficial for the
engineer to also have experience in securing necessary
Flue gas properties
a. Gas properties influence the design and performance of the pollution control equipment. When working with a particular emission standard or code the gas
properties must be converted to the units used in the
codes, such as lbs per million BTU; gr/ACFM;
DSCFM at 32; DSCFM at 68; DSCFM corrected to 8
percent 02.
b. Flow rate. The flow-rate of exhaust gases generated in the combustion process must be measured or
calculated to determine the required volumetric size of
the collection equipment. Flow-rate variations result in
velocity changes and thus influence collector efficiency
and pressure drop. It is necessary therefore to obtain
maximum, average, and minimum values for a cyclical
or modulating operation.
c. Temperature. Gas temperature affects gas volume
(and simultaneously collector volume) and materials of
TM 5-815-1/AFR 19-6
Particulate properties
a. Particulate properties that must be determined for
control equipment selection and design are described
below. Appropriate test methods are listed in table 5-1.
b. Concentration (loading). Particulate loading is a
measurement of particulate concentration in flue gases
(see this manual, chapters 2 and 3) expressed in grains
per cubic foot. Particulate loading is used as a criteria
to design and select applicable collection equipment.
Fluctuations in loading (for example: soot blowing in
boilers) must be noted and maximum, minimum, and
average values should be recorded. High grain loadings
may require a series system of control devices to meet
particulate emissions and air quality standards. For
instance, a cyclone followed by an electrostatic precipitator or baghouse.
c. Particle size. The particle size analysis affects the
collection efficiency for each control device. Fine particulate collection requires high-efficiency equipment
such as venturi scrubbers, electrostatic precipitators, or
fabric filters.
d. Resistivity. Particulate resistivity is a limiting
factor in the design of electrostatic precipitators.
Resistivity must be determined if an electrostatic precipitator is to be selected to control particulate emissions. As a general guideline, resistivity above 1010
ohm-cm normally rules out the use of electrostatic precipitation unless provisions are made for particulate
electrical conditioning.
e. Handling characteristics. Particle-handling
characteristics influence dust-handling systems (ductwork, collector structure, hoppers, conveyors) and
materials of construction. Dust-handling characteristics
include flow properties, abrasiveness, hygroscopicity,
moisture content, agglomerating tendencies. These
properties, including specific gravity and bulk density
should be evaluated in the design of a dust-collecting
f. Chemical composition. Chemical composition of
particulate affects materials of construction and design
of the collector and ash disposal equipment as does
carrier gas composition.
construction for the collector. Temperature may also
limit use of certain collectors. For instance, temperatures above 550 degree Fahrenheit rule out the use
of fabric filters.
d. Pressure. Carrier gas pressure must be known or
calculated to determine the structural requirements for
the collector under operating and upset conditions.
e. Viscosity. Gas viscosity is a measure of molecular
activity in a gas stream. The greater the gas viscosity,
the greater the resistance to particle migration across
the stream normal to gas flow. Since gas viscosity
increases with gas temperature, it is an important factor
in the performance of dry particulate collection
devices. viscosity effects can be minimized if equipment is properly specified.
f. Moisture content. Moisture content affects the
performance of collection equipment and the choice of
construction materials. It is important to know the dew
point of the exhaust gas, as temperatures below dew
point allows acid vapors to condense and attack structural surfaces. This is a particular concern with boiler
flue-gas which often contains a significant amount of
sulfuric acid vapor.
g. Chemical composition. Chemical composition
primarily affects the choice of construction materials
for a collector. Collectors must be suitably protected to
handle corrosive gases.
h. Toxicity. Handling of toxic gases requires special
treatment and equipment and must be reviewed on an
individual basis. This manual does not address incineration of toxic or hazardous wastes.
Application of emission control systems for boilers.
As a result of current, stringent, stack emission regulations, applications of certain conventional emissions
control systems have evolved that provide satisfactory
performance when properly sized and specified. Referenced are CFR40 part 60 for new source performance
standards (NSPS) only, as ambient regulations have
wide variation from site-to-site requiring investigation
for each location. Following is a brief description of the
most common combustion sources, fuels, and control
devices employed:
a. Natural gas fired power boiler. NSPS cover particulates; sulfur dioxide SO2; nitrogen dioxide NOx;
and opacity.
(1) External devices are not usually required.
Properly adjusted combustion controls,
burner(s), furnace designs, and gas monitoring are sufficient to meet the performance
(2) Even though natural gas is a relatively clean
fuel, some emissions can occur from the
combustion reaction. For example, improper
operating conditions, including items such as
poor mixing and insufficient air, may cause
large amounts of smoke, carbon monoxide,
and hydrocarbons to be produced. Moreover,
because a sulfur-containing mercaptan is
added to natural gas for detection proposes,
small amounts of sulfur oxides will also be
produced in the combustion process.
(3) Nitrogen oxides are the major pollutants of
concern when burning natural gas. Nitrogen
dioxide emissions are a function of the temperature in the combustion chamber and the
rate of cooling of the combustion products.
TM 5-815-1/AFR 19-6
(2) SO2. Use wet scrubbing system with a low
pressure drop.
(3) NOx . May be controlled by utilizing limited
excess-air firing; flue gas recirculation; staged
combustion; or combinations of these.
(4) Opacity. May be controlled by limiting or collecting the particulates and by properly
adjusted and designed combustion controls
with good burner and furnace designs.
d. Pulverized coal-fired power boiler. NSPS cover
limitations for particulates; SO2; NOx; and opacity.
Methods of modifying or controlling emissions are discussed in the following.
(1) Particulates.
(a) Control by use of electrostatic
(b) Control by use of fabric filters
(c) Control by use of venturi scrubber
(d) Control by combination of a mechanical
collector followed by either (a), (b), or
(c), above
(2) SO2..
(a) Use suitable wet scrubber (can double for
both SO2 and particulates)
(b) Use suitable dry scrubber followed by
fabric filters or electrostatic precipitator
(c) Selection of a wet or dry scrubbing
system is determined by evaluating the
economics (installation and operating
costs) and the disposal of the collected
(3) NOx. Ensure that the burner and furnace are
designed for limited excess-air firing and
staged combustion. In some cases it may be
necessary to have a second stage air fan
designated as an NOx control fan in order to
gain compliance.
(4) Opacity. This may be controlled by
particulate removal and properly adjusted
combustion controls. In some cases this could
be the more stringent requirement for
particulate removal.
e. Spreader and mass feed stoker coal fired boilers
with a traveling grate. NSPS cover limitations for particulates; SO2; NOx; and opacity. Methods of modifying
or controlling emissions are discussed in the following.
(1) Particulates.
(a) Control by use of electrostatic precipitator
(b) Control by use of suitable fabric filter
(c) Control by use of suitable wet scrubber
(d) Control by a combination of a mechanical
collector followed by either (a), (b), or
(c) above
(2) SO2.
(a) Use suitable wet scrubber (can double for
both SO2 and particulate).
(b) Use suitable dry scrubber followed by
either a fabric filter or an electrostatic
Emission levels generally vary considerably
with the type and size of unit and are also a
function of loading.
(4) In some large boilers, several operating modifications have been employed for NOx
control. In staged combustion, for example,
including off-stoichiometric firing, also called
"biased firming," some burners are operated
fuel-rich, some fuel-lean, while others may
supply air only. In two-staged combustion,
the burners are operated fuel-rich (by
introducing only 80 to 95 percent
stoichiometric air) with combustion being
completed by air injected above the flame
zone through second-stage “NOx -ports”. In
staged combustion, NOx emissions are
reduced because the bulk of combustion
occurs under fuel-rich, reducing conditions.
b. Distillate oil fired power boilers. NSPS cover particulates; SO2; NOx; and opacity. Methods of modifying
or controlling emissions are discussed in the following.
(1) Particulate. The user should note that in most
cases external pollution control devices are
not required for boilers firing No.1 or No.2
fuel oils.
(2) SOx. Most distillates will contain sulfur quantities low enough so that no treatment will be
necessary. However, a fuel analysis must be
reviewed as some distillates can have as much
as one percent sulfur. When the sulfur content
produces SO2 emissions in excess of the
allowable a wet scrubbing system will be
(3) NOx. Control requires the proper combustion
controls, and burners and furnaces designed
to limit NOx generation from high combustion
temperatures. Usually NOx reductions are
accomplished by limiting excess air firing and
staged combustion. Large utility system units
sometimes also employ flue-gas recirculation
in addition to the other methods.
(4) Opacity. This may be controlled by proper
air-fuel ratios; good combustion controls;
limiting particulate emissions; and proper
engineering design of the burners and furnace
c. Residual oil fired power boilers. NSPS cover particulates; SO2; NOx; and opacity. Methods of modifying
or controlling emissions are discussed in the following.
(1) Particulate control.
(a) When using low-sulfur oils, cyclonic
mechanical collectors are usually
adequate. On larger utility size units
electrostatic precipitators are employed to
limit particulate emissions.
(b) For emissions from combustion of highsulfur oils a wet scrubbing system can be
used for both SO2
removal and
particulate control.
TM 5-815-1/AFR 19-6
when ponding is not viable. The dry ash
should be cooled and conditioned with
water before being transported for land
fill disposal.
g. Coal fired fluidized bed boilers. NSPS cover limitation for particulates; SO2; NOx; and opacity. Methods of modifying or controlling emissions are discussed
in the following.
(1) Particulates. Control by use of fabric filter or
an electrostatic precipitator. Most units will
not require a mechanical collector in series
with the baghouse or electrostatic
precipitator. However, if high dust loadings
are anticipated an in-line mechanical collector
in series with the baghouse or electrostatic
precipitator may be justified.
(2) SO2. Controlled by the metering (feeding) of
lime stone into the fluidized fuel bed.
(3) NOx. The comparatively low furnace temperatures experienced in fluidized bed boilers
limits the heat generated NOx formation. No
special devices or controls are required for
NOx control on fluidized bed units.
(4) Opacity. Controlled by particulate removal
and properly adjusted and designed
combustion controls.
(5) Ash handling and removal systems. Can be
dry or wet and may be automated cycles or
continuous ash removal utilizing equipment
and methods previously discussed.
(3) NOx. Control by specifying furnace and combustion air controls designed to maintain limited flame temperatures under operating conditions.
(4) Opacity. Control by particulate removal and
properly adjusted combustion controls. This
can be the more stringent requirement for
particulate removal.
f. Wood waste and bark fired boilers. NSPS cover
limitation for particulates and opacity. Methods of
modifying or controlling emissions are discussed in the
(1) Particulates.
(a) Control by use of a mechanical collector
followed by either a scrubber or an electrostatic precipitator.
(b) Control by use of wet scrubber.
(c) Control by use of electrostatic
(d) Control by use of gravel bed filter.
(2) Opacity. Opacity is controlled by particulate
collection and properly adjusted combustion
controls. The "as-fired" condition of wood
waste fuel will impact the choice of
particulate control equipment.
(a) Hogged bark and wood chips with 45%
to 55% moisture usually require a
mechanical collector followed by a
scrubber or an E SP. Material collected in
the mechanical collector is a combination
of char, ash, and sand. The material is
classified to separate the char from the
ash/sand mixture so the char can be
reinjected into the furnace combustion
zone. The ash/sand mixture is discharged
by gravity or conveyor to a holding tank
which can be either wet or dry. All ashhopper discharge openings must be protected from air infiltration by rotary-seal
discharge valves or an air-lock damper
arrangement, to prevent ignition of hot
(b) Dry wood wastes that are chipped to less
than 1" x ½” size may not require the
mechanical collector and reinjection
equipment. Gas clean-up equipment of
choice may then be either the scrubber or
electrostatic precipitator. Ash discharge
hoppers need to be protected by seal
valves or air locks in all cases.
(c) Fabric filters are avoided because of the
potential for burning the fabric with hot
char carry over.
(d) Ash handling is usually accomplished
using a hydraulic conveying system
discharging to an ash settling pond.
(e) Screw conveyors or drag-chain conveyors
are acceptable alternatives for dry
handling of ash from wood-fired boilers
Municipal solid waste-fired boilers
(MSW) and boilers using refuse
derived fuels(RDF)
a. Municipal solid waste fired boilers fall in the same
emission regulation category as an incinerator. Compliance is only required for particulate emission regulations.
b. Boilers using refuse derived fuels must meet the
incinerator regulations and are also required to meet
emission standards for any other fuels fired in the
boiler. In most states the allowable emissions are
calculated on the ratio of fuels fired and which cover
control of particulate, SO2, NOx, and opacity.
(1) Particulats Use mechanical collectors as a
primary device followed by either a fabric
filter or an electrostatic precipitator. The ESP
is favored when there is co-firing with coal in
the MSW boiler. Without coal co-firing,
resistivity of the particulate can be extremely
high. Wet scrubbers should be avoided
because of possible odor pick up.
(2) SO2. SO2 formation is a function of the sulfur
content in the refuse and fuel. In most cases
no SO2 removal devices are necessary.
However, when required a dry scrubber
system followed by either a baghouse or an
electrostatic precipitator is preferred.
TM 5-815-1/AFR 19-6
(3) NOx. Furnace design and firing methods are
used to limit NOx. Two-step combustion is
employed. The primary zone is fired with limited air to maintain a reducing atmosphere
and the secondary zone uses an oxidizing
atmosphere to provide a controlled low-temperature flame with minimum excess air.
(4) Opacity. Opacity is controlled by limiting particulate emissions and by properly designed
combustion controls.
12-6. Applications of emission control
systems for incinerators
Refuse incinerators are type categorized as: municipal;
industrial; commercial; and sludge. NSPS cover particulate emissions only. However, incineration of many
solid, liquid, and gaseous wastes will produce noxious
gases that require special treatment.
a. Municipal incinerators. Optimum control of
incinerator particulate emissions begins with proper
furnace design and careful operation. A proper design
includes: a furnace/grate system appropriate to the
waste; an adequate combustion gas retention time and
velocity in the secondary combustion chamber; a suitable underfire and overfire air system; and establishing
the optimum underfire/overfire air ratios.
(1) for compliance with NSPS it is necessary to
utilize gas cleaning equipment and to
optimize operating conditions for the furnace.
(2) Particulates. May be controlled with mechanical collectors; settling chambers; after
burners; and low efficiency scrubbers used as
precleaners. These must be followed by an
electrostatic precipitator or a high efficiency
venturi/orifice scrubber for final cleaning.
Fabric filters may be used if emissions gas
temperature is maintained below the
maximum temperature rating of fabric media
being used. This will usually require water
spray injection for evaporative cooling of the
gas stream.
(3) Odor control is frequently required and can
be accomplished with after-burners
strategically located in the furnace to oxidize
the odorous gases.
b. !Industrial and commercial incinerators. Design
of the incinerators and emissions control requirements
are greatly influenced by the composition of the solid
waste that is incinerated.
(1) Single chamber and conical (Teepee) type
incinerators will not meet current NSPS emission requirements.
(2) Multiple
chamber incinerators with
fluidized-bed incinerators including sludge
incinerators may be equipped with one or
more of the previously discussed or following
gas-cleaning systems to meet NSPS.
(3) When particulates are the controlled
pollutant, primary collection devices
commonly used are:
mechanical collectors; wetted baffles; and
spray chambers.
(4) The final collection fo small particulate material is usually accomplished with one of the
following devices:
— venturi or orifice-type scrubber -electrostatic
— fabric filter.
c. Incinerator vapor and odor control. Objectionable vapors and odors in incinerator exhaust streams
sometimes necessitate specialized control systems.
Odorous components present downstream of conventional cleaning systems are usually organic in gaseous or fine particulate form. Several methods
available for their control are discussed below.
(1) Afterburners. Direct thermal incineration can
be utilized to oxidize odorous fumes. A fume
incineration system, or afterburner, basically
consists of a gas or oil-fired burner mounted
to a refractor-lined steel shell. Odorous
vapors and particulate matter are exposed to
a high temperature flame (1200 to 1400
degrees Fahrenheit) and are oxidized into
water vapor and carbon dioxide. The
principal advantages of direct thermal
incineration of odorous pollutants are
simplicity, consistent performance, easy
modification to accommodate changes in
standards, and ease of retrofit. The major disadvantage is the uncertainty and expense of
fuel supply usually natural gas.
(2) Vapor condenser. Vapor condensers are utilized to control obnoxious odors, particularly
m processes where the exhaust gases contain
large quantities of moisture. Condensers can
be either the direct contact type, or shell and
tube surface condensers. The resulting condensate is rich in odorous material and can be
sewered of treated and disposed of by other
conventional methods. (See paragraph 7-4 for
further information on treatment and disposal
of waste materials.) Condensers are often
used in conjunction with an afterburner. In
such a system, exhaust gases are condensed
to ambient temperature before incineration,
reducing gas stream volume by as much as 95
percent and reducing moisture content.
Lowering gas volume and moisture content
can substantially reduce the cost and fuel
requirements of the afterburner assembly.
(3) Catalytic oxidation. Incineration of odorous
pollutants in the presence of a suitable
catalyst can lower the temperature required
for complete combustion and reduce the
overall reaction time. Advantages of catalytic
oxidation are:
TM 5-815-1/AFR 19-6
Technical evaluation of control
a. Given the site-specific ambient air quality
requirements, and the NSPS emissions limitations, and
then comparing them with the uncontrolled emissions
data for the combustor, it becomes possible to make a
selection of various emissions controls systems to meet
the emission restraints. Required is a knowledge of the
various emissions control devices and their application
to specific problems including their sizing and
b. Other factors which must be evaluated in selecting
control equipment include: site compatibility; disposition of the collected pollutant; installation and
operation costs; maintainability; and the ability to
provide continuous protection during operation of the
combustion units. Tables 12-1 and 12-2 offer a comparison of these characteristics to serve as an aid in the
final determination of the best control system for a
particular application.
c. Specific operating characteristics that should be
compared in evaluating suitable collection equipment
are listed below. Each control device section of this
manual should be consulted for specific descriptions of
various control equipment.
(1) Temperature and nature of gas and particles.
Collection equipment must be compatible
with operating temperatures and chemical
composition of gas and particles.
(2) Collector pressure loss. The power requirement for gas-moving fans can be a major cost
in air pollution control.
(3) Power requirement. Electrostatic precipitators, scrubbers, and fabric filters have
additional electrical requirements beside fan
(4) Space requirement. Some control equipment
requires more space than others. This factor
may, in certain cases, preclude the use of
otherwise satisfactory equipment.
(5) Refuse disposal needs. Methods of removal
and disposal of collected materials will vary
with the material, plant process, quantity
involved, and collector design (chap 6, 7, and
9). Collectors can be unloaded continuously,
or in batches. Wet collectors can require
additional water treatment equipment and if
the pollutation control device uses water
directly or indirectly, the supply and disposal
of used water must be provided for.
12-8. Tradeoffs and special considerations
a. Design considerations. In order to design equipment to meet air pollution control requirements, the
top output or maximum ratings should be used in the
selection of control equipment. The additional cost for
extra capacity is negligible on the first cost basis, but a
later date addition could cost a substantial sum. It
should also be noted whether the dust-generating process is continuous or cyclic, since an average dust concentration design may not satisfy high emissions at
start-up or shut-down. Cyclic operation could also lead
to problems in terms of equipment performance relative to high or low temperatures and volumes. Ductwork providing good gas distribution arrangements for
a specific volume could cause significant problems if
the gas volume were to increase or decrease.
b. Reliability of equipment. Since particulate control
equipment is relatively expensive, and due to the fact
that it is usually an integral part of the power
generation process, it is of utmost importance that the
equipment provide reliable service. Wrong choices of
fabric for fabric filters; wrong materials of construction
for wet scrubbers; the wrong choice of a multicyclone
to achieve high efficiency on fine particles; can all lead
to collector outages, or complete failure. Collector
failures may be accompanied by a loss of production or
by expensive replacement with new devices. Evaluation trade-offs should be made between costs for an
auxiliary control unit and the cost of shutting down the
entire process due to collector failure.
c. Space allowance. Special consideration by the
design engineer must be given to provide space in the
planned plant layout for adding more pollution control
equipment in the future. Future plant modifications will
in most cases have to meet more stringent standards
than the existing NSPS.
d. Gas cooling. When high temperature (greater than
450 degrees Fahrenheit) exhaust gasses are being
handled, a study should be made on the cost of installing equipment to operate at the elevated temperature
versus the cost and effects of gas cooling.
e. Series operation of collectors. Dust collectors
may be used in series operation for the following
(1) A primary dust collector acts as a precleaner
— Smaller units required because lower gas
temperatures reduce gas volume,
— Less oxygen required in the effluent stream
since catalyst promotes efficient use of oxygen,
— Lower NOx emissions due to lower flame
temperatures and reduced oxygen loads.
(4) The principle disadvantages are:
— High initial capital equipment costs
— Periodic replacement of expensive catalysts
(5) Absorbers. Absorption systems for odor control involve the use of selected liquid absorbents to remove odorous molecules from
effluent gases. The gas to be absorbed should
have a high solubility in the chosen absorbent
or should react with the absorbing liquid.
Various methods are used to affect intimate
contact of liquid absorbent and gaseous
TM 5-815-1/AFR 19-6
TM 5-815-1/AFR 19-6
TM 5-815-1/AFR 19-6
(3) Utilizing a primary and secondary collector in
series provides some flexibility to the system
in the event there is a failure of one of the
f. Wet vs. dry collection. Factors to be taken into
consideration in a comparison of wet and dry collection
— Solubility of aerosol
— Ultimate pH of scrubbing liquor
— Liquor corrosion and erosion potential
— Special metals or protective coatings
— Availability of make-up water
— Disposal and treatment of waste water
— Space required for liquid-handling equipment
— -Vapor plume visibility
— Operating and installed costs
— Maintenance and operation
g. Summary. A summary of the general guidelines in
the selection of emission control equipment for boiler
flue gases is provided in table 12-3.
to prevent plugging, reduce abrasions, or
reduce the dust loading to the secondary
collector. The addition of a precleaner adds
pressure drop and costs, and should only be
applied where the performance of the
secondary is inadequate without a primary
collector of the type proposed.
(2) Mechanical collectors of the multicyclone
type are usually the first choice for primary
collector service. They are low cost; provide
reliable collection of large diameter
suspended solids in the 85 percent collection
efficiency range; and can be specified in a
wide variety of wear resistant metals. There
are very few NSPS applications where the
single or (in series) double mechanical
collector can meet the particulate emission
standards. Consequently, a final cleaning
device of high efficiency on small size
particulate should follow the mechanical
TM 5-815-1/AFR 19-6
TM 5-815-1/AFR 19-6
13-2. Types of fluidized bed boilers.
a. Fluidized bed combustion has now progressed
through the first and into the second and third generation of development. Fluidized bed technology is not
new but has been revived in this country because of
fuel costs and the availability of poor quality fuels.
Commercial and industrial power plants now have a
third type of solid fuel boiler to consider for steam
requirements. Economics, fuel pricing, availability of
low grade fuels and environmental considerations have
made the fluidized bed boiler a viable option to
evaluate along with the stoker or pulverized coal fired
units. The units can with care be designed to burn a
number of fuels including low grade coals, lignite, coal
mine wastes (culm), refinery gas, woodwastes, waste
solvents, sludge, etc.
b. Fluidized bed combustion offers the ability to
burn high sulfur coal and meet environmental requirements without the use of scrubbers. The mixture of
fuel and limestone is injected in such a way that the
fuel and limestone are distributed across the bed. The
fuel and limestone are kept in turbulent motion by
upward air flow from the bottom of the furnace. The
furnace combustion takes place at about 1550 degrees
Fahrenheit to 1750 degrees Fahrenheit. Control of
sulfur dioxide and nitrogen oxide emissions in the
combustion chamber without the need for additional
control equipment is one of the major advantages over
conventional boilers.
a. Fluidized bed boilers cover a variety of systems.
There is no unique design. An industrial fluidized bed
boiler could assume several possible configurations
depending on such factors as bed pressure, the choice
between natural or assisted circulation, the gas velocity
in the bed, fuel and air distribution systems, bed design
and method of achieving high carbon utilization and
control of sulfur dioxide.
b. There are four types which will be given consideration for control of sulfur dioxide and nitrogen oxide
emissions. These are shown in figure 13-1 and size is
also compared for a 50 million Btu/hour heat imput
c. The types can further be demonstrated by comparing them as stationary fluid bed (bubbling bed) or
circulating bed designs. To determine this type, the
relationship between the gas velocity and the differential pressure in the fluidized bed must be established.
Figure 13-2 shows this relationship for various bed
d. The fluidized bed is a system in which the air
distributed by a grid or distribution plate, is blown
through the bed solids developing a "fluidized condition." Fluidization depends largely on the particle size
and the air velocity. At low air velocities, a dense
defined bed surface forms and is usually called a bubbling fluidized bed. With higher air velocities, the bed
13-1. Fluidized bed boilers.
TM 5-815-1/AFR 19-6
particles leave the combustion chamber with the flue
gases so that solids recirculation is necessary to maintain the bed solids. This type of fluidization is called
circulating fluidized bed.
e. The mean solids velocity increases at a slower rate
than does the gas velocity, as illustrated in figure 13-3.
Therefore, a maximum slip velocity between the solids
and the gas can be achieved resulting in good heat
transfer and contact time with the limestone, for sulfur
dioxide removal. When gas velocity is further
increased, the mean slip velocity decreases again.
These are the operating conditions for transport reactor
or pulverized coal boiler. The design of the fluidized
bed falls between the stoker fired boiler and the pulverized coal boiler using the bed expansion.
f. The shallow fluidized bed boiler operates with a
single bed at a low gas velocity. A shallow bed minimizes fan horsepower and limits the free-board space.
The bed depth is usually about 6 inches to 9 inches and
the free-board heights are only four to five feet.
Desulfurization efficiency of a shallow bed is poor,
with only about 60 to 80 percent removal, because SO2
does not have adequate time to react with the limestone
before moving out of the shallow bed. The shallow bed
fluidized boiler is of the bubbling bed design. The shallow bed will be of very limited use because of its poor
sulfur dioxide removal.
g. A deep fluidized bed boiler is a bubbling bed
(1) The bed depth is usually 3 feet to 5 feet deep
and the pressure drop averages about one
inch of water per inch of bed depth. The bulk
of the bed consists of limestone, sand, ash, or
other material and a small amount of fuel.
The rate at which air is blown through the bed
determines the amount of fuel that can be
reacted. There are limits to the amount of air
that can be blown through before the bed
material and fuel are entrained and blown out
TM 5-815-1/AFR 19-6
of the furnace. Conversely, when air flow is
reduced below the minimum fluidizing
velocity, the bed slumps and fluidization
(2) The fuel feed systems available are either
under-bed feed system or over-the-bed feed
system. The under-bed feed system is quite
complex. It requires coal at less than 8
percent surface moisture and crushed to
about 6 MM top size to minimize plugging
the coal pipes. Operating and maintenance
costs are usually high for the under-bed feed
system. The major advantage of the underbed feed system is that with use of recycle
combustion efficiency approaches 99 percent.
The over-bed feed system is an adaptation of
the spreader stoker system for conventional
boilers. This system has a potential problem
of effective carbon utilization. Carbon
elutriation can be as high as 10 percent.
(3) Some bubbling bed units have sectionalized
or modular design for turndown or load
response. This allows a section to be cut in or
out as required. Some are actually divided
with water cooled or refractory walls. This
type unit should be matched to the facility
demand pro-file to avoid continual bed
slumping and operator attention. When
continuous stopping of sections is required to
control load for extended periods, the
fluidized bed boiler may become a big user of
auxiliary fuel to maintain bed temperature.
(4) Major limitations of the bubbling bed design
are high calcium/sulfur ratios, low
combustion efficiency, limited turndown
without sectionalization of the furnace bottom
and complexity of the under bed feed system
required to minimize elutriation of unburned
fines. Typical fluidized bed combustors of
this type are shown in figures 13-4 and 13-5.
h. In the circulating fluidized bed boiler, the fuel is
fed into the lower combustion chamber and primary air
is introduced under the bed.
(1) Because of the turbulence and velocity in the
circulating bed, the fuel mixes with the bed
material quickly and uniformly. Since there is
not a definite bed depth when operating, the
density of the bed varies throughout the system, with the highest density at the level
where the fuel is introduced. Secondary air is
introduced at various levels to ensure solids
circulation, provide stage combustion for NOx
reduction, and supply air for continuous fines
combustion in the upper part of the combustion chamber.
(2) Combustion takes place at about 1600
TM 5-815-1/AFR 19-6
degrees Fahrenheit for maximum sulfur
retention. The hot gases are separated from
the dust particles in a cyclone collector. The
materials collected are returned to the
nonmechanical seal, and ashes are removed at
the bottom. The hot gases from the cyclone
are discharged into the convection section of
a boiler where most of the heat is absorbed to
generate steam. Typical fluidized bed boilers
of this type are as shown in figure 13-6.
(3) Major performance features of the circulating
bed system are as follows:
(a) It has a high processing capacity because
of the high gas velocity through the
(b) The temperature of about 1600 degrees
Fahrenheit is reasonably constant
throughout the process because of the
high turbulence and circulation of solids.
The low combustion temperature also
results in minimal NOx formation.
Sulfur present in the fuel is retained in the
circulating solids in the form of calcium
sulphate soit is removed in solid form.
The use of limestone or dolomite
sorbents allows a higher sulfur retention
rate, and limestone requirements have
been demonstrated to be substantially less
than with bubbling bed combustor.
The combustion air is supplied at 1.5 to 2
psig rather than 3-5 psig as required by
bubbling bed combustors.
It has a high combustion efficiency.
It has a better turndown ratio than bubbling bed systems.
TM 5-815-1/AFR 19-6
(g) Erosion of the heat transfer surface in the
combustion chamber is reduced, since the
surface is parallel to the flow. In a
bubbling bed system, the surface
generally is perpendicular to the flow.
i. In the dual bed fluidized combustor, combustion
and desulfurization take place in two separate beds,
allowing each different reaction to occur under optimal
(1) The lower bed burns coal in a bed of sand,
fluidized from below by the combustion air
and gases, and maintained at a steady
equilibrium temperature by the extraction of
energy through in-bed steam generator tubes.
The bed depth is more shallow than the conventional bubbling bed design.
(2) The flue gas then travels through an upper
bed of finely ground limestone where
desulfurization takes place. The dual bed
design allows coals to be burned at about
desulfurization takes place at about 1550
degrees Fahrenheit. The upper bed also
serves to catch unburned coal particles that
may have escaped to complete combustion of
any unburned carbon.
(3) A dual bed can be utilized on capacities up to
200,000 pounds per hour of steam. The
major advantages are: shop fabrication; can
be retrofitted to some existing oil and gas
fired boilers; enhanced combustion efficiency
by allowing the lower bed to operate at 1750
degrees Fahrenheit; lower free-board heights
required; and better load following. A typical
dual bed fluidized combustor is shown in
figure 13-7.
TM 5-815-1/AFR 19-6
a. Fuel Application.
(1) A wide range of high grade and low grade
fuels of solid, liquid or gaseous type can be fired. The
primary applications are fuels with low heating value,
high sulfur, waste materials, usually the least
expensive. Fuel can be lignite, coal washing waste
(culm), high sulfur coal, delayed petroleum coke, or
waste material that would not burn satisfactorily in a
conventional boiler. The fluidized bed boiler has the
ability to burn most any residual fuel and reduce
emissions by removal of sulfur compounds in the
limestone bed.
(2) A complete evaluation of fuels to be burned
should be given consideration in selection of
the equipment. Many factors including
heating value, moisture, ash fusion
temperature, sulfur content, and ash content
will affect the system configuration.
(3) Fuel sizing is important. For coal it is recommended that it not be run-of-mine. It should
be crushed to avoid large rocks and pieces of
coal causing problems in the bed. Coal sizing
is important and will vary with each fluidized
bed manufacturer. Typically, sizing will vary
from 0 — ¼ inch x 0 for overfeed systems to
¼ inch x 0 for underfeed systems.
TM 5-815-1/AFR 19-6
b. Process application.
(1) The fluidized bed can be utilized to control
SO2 emissions when high sulfur fuels are
used. Also reduction of SO2 emissions can be
achieved when nonattainment areas are looking for additional steam for process. The
capability of fluidized bed combustion to
control emissions makes this technology
particularly suited for applications where
stringent emissions control regulations are in
(2) Steam generation in a fluidized bed boiler
versus a conventional boiler will not be
economical when using compliance coal for
control of sulfur dioxide emissions. However,
several studies indicate that fluidized bed
boilers are competitive with conventional coal
fired boilers that include flue-gas
desulfurization systems. Facility location may
dictate Best Available Control Technology
(BACT) be used to control SO2 and NO2
(3) Nitrogen oxide emissions can be controlled
with a fluidized bed boiler. The fluidized bed
boiler generates very little thermal nitrogen
oxide because of the low temperature of
(4) Pressurized fluidized bed boilers continue in
research and development. Higher efficiency
designs for utility applications involve considerably higher initial costs and design
complexity. Also, a cost effective way to
clean up the hot flue gases before they reach
the turbine has not been found.
(5) The fluidized bed boiler can be used to
incinerate low grade fuels that would be
normally considered waste residues.
TM 5-815-1/AFR 19-6
Fluidized bed performance
a. With the exception of a baghouse or precipitator,
which is required for particulate removal, additional
gas cleaning devices are not required for environmental
control with fluidized bed systems.
b. Fluidized bed boilers are able to remove sulfur
dioxide directly in the combustor. This is accomplished
by using limestone in the fluid bed. The limestone
calcines to form calcium oxide (CaO) and then reacts
with SO2 to form calcium sulfate as follows:
Materials and construction
The materials used for construction of fluidized bed
units are similar to those used in conventional boilers
depending on the design pressure and temperature of
the system.
a. In-bed tubes. The fluidized bed boilers that have
in-bed tubes have experienced high erosion rates in
some cases. Vertically oriented tubes are less prone to
erosion than the horizontal ones. Where in-bed tubes
are used, consideration should be given to use of
thicker walls on the tubes and their metallurgy. Wear
fins can be installed to reduce erosion. Also, some
corrosion may be experienced due to the reducing
atmosphere in the lower regions.
b. Fluidized bed. The fluidized bed or bottom of the
combustor section varies considerably with each type
of design. The method used for air distribution is
important in maintaining uniform fluidization across
the bed. Some units have had problems with plugging
of the air openings. The bottom is castable refractorylined on some units. Others have heat transfer tubes
The ideal temperature range for desulfurization in a
fluidized bed is about 1600 degrees Fahrenheit.
c. A bubbling fluidized bed boiler will require a
higher calcium to sulfur ratio for control of SO2, while
the circulating fluidized bed boiler can achieve similar
SO2 removal with the Ca/S ratio of 1.5 to 2. See figure
d. Nitrogen oxide is controlled by distribution of
primary air under the bed and secondary air part way
up the combustor. The staging of combustion limits the
nitrogen oxide to that which is formed only by fuelbound nitrogen. Thermally formed nitrogen oxide is
negligible in the fluidized bed. See figure 13-9 for
predicted nitrogen oxide emissions.
e. Several fluidized bed boiler manufacturers are
now offering performance guarantees based upon
experience in the bubbling, circulating, and dual bed
TM 5-815-1/AFR 19-6
protected with abrasion resistant refractory in regions
where the gas flow changes directions.
c. Cyclone. In the circulating fluidized bed unit, the
cyclone separator is lined with refractory to minimize
abrasion and prevent heat losses.
d. Ash cooler. The ash cooler is also refractory lined
to increase life of the unit due to abrasion of the solids
being handled.
Auxiliary equipment
a. The following briefly describes the major components of auxiliary equipment for the fluidized bed
(1) Materials handling for fuel and limestone.
The handling of fuel and limestone will vary
depending on the source of supply and the
type of delivery. Delivery is usually by truck
or rail car.
(2) The conveying systems for the fuel and limestone can be either a pneumatic or a mechanical system. The mechanical system may be
belt, chain, bucket, or screw conveyor, or a
combination of these.
(3) Coal can be stored in open piles or storage
silos. From storage, coal is fed to a crusher or
dryer as required for efficient burning. Crushing of the coal is required when it is run-ofmine, for efficient burning, elimination of
rocks in the bed, high moisture content, high
ash content and when pneumatic conveying is
(4) Drying of the coal is recommended when the
fuel moisture content exceeds fifteen percent
for all fluidized bed boilers except the
circulating fluidized bed boiler. The flue gas
from the fluidized bed can be used for drying
the fuel.
b. Coal feed stream slitter. The dual bed unit has a
proprietary stream slitter which permits accurate feed
of coal to multipoints under the bed for maximum
combustion efficiency.
TM 5-815-1/AFR 19-6
Advantages and disadvantages
a. Advantage:
(1) Low SO2 emissions
(2) Low NOx emission due to staged combustion
and changing of the primary to secondary air
(3) Only fuel bound nitrogen converted to NOx
(thermally formed NOx is negligible)
(4) High combustion efficiency, (as high as 99
plus percent)
(5) High turn-down and load following ability
(6) Uses a variety of fuels including:
— high sulfur
— low BTU
— high ash
— low cost
— waste materials
(7) High boiler efficiency (85 to 90 plus percent)
(8) Load changes greater than 5% per minute
(9) No
sootblower may be used
(10) No slagging of coal ash
(11) Low maintenance
(12) Dry ash
(13) Broad tolerance to changes in coal quality
(14) Sulfur removal w/o need for scrubbers
b. Disadvantages:
(1) Bed turn-down capability not clear
` (2) Startup procedures more complex
(3) Control response almost instantaneous
(4) Use of partial bed slumping as load control
mechanism for bubbling bed
(5) Requirement of a free-board for combustion
efficiency for bubbling bed
(6) Corrosion susceptibility in bubbling bed
(7) Calcium-to-sulfur ratio greater than 2.5
causes degradation of boiler efficiency
(8) Fluidized bed is a newer technology than conventional boilers
(9) Complex under-bed fuel-feed system required
for some bubbling beds
c. Startup burners. Startup burners are supplied in
the bed or air ducts to heat the bed up to coal ignition
temperature. The startup burner can be used for low
loads. Usually it is capable of carrying about 20 percent
or more of boiler capacity.
d. Fluidized bed heat exchanger. The fluid bed heat
exchanger is used to cool the ash to about 750 degrees
Fahrenheit. The coolant can be feedwater or any process fluid which requires heating. The metallurgy of the
heat exchanger must be compatible with the fluids it is
e. Flue gas clean-up for particulate. Either an electrostatic precipitator or a baghouse may be used for
particulate control. Basic guidelines established for
determining which type unit to use on a conventional
coal fired unit may be used to select the particulate
control device for a fluidized bed boiler. Electrostatic
precipitators can encounter resistivity problems
because of the low sulfur content in the particulate to
be collected.
f. Ash-handling systems.
(1) The ash-handling systems are similar to ashhandling systems for conventional boilers.
The bottom ash does have to be cooled prior
to disposal. Most of the ash-handling systems
are dry, and the ash can be sold for use in
other products.
(2) Some potential uses of the ash are: aggregate
in concrete; road base ingredients; stabilization of soil embankments; pozzolan in
masonry units and mortar; agriculture and
livestock feeds extender; and neutralization of
spent acid wastes.
TM 5-815-1/AFR 19-6
Government Publications.
Department of Defense (DOD)
DOD 4270. 1-M
Departments of Army, Air Force, and Navy
AR 11-28
Economic Analysis and Program Evaluations for Resource
Facilities Engineering - Heating, Energy Selection and Fuel
Storage, Distribution and Dispensing Systems
Economic Analysis Handbook
AR 420-49/AFR 178-1
Construction Criteria Manual
Executive Department, The White House, Washington, D.C. Excecutive Order No.12003
(July 1977)
Relating to Energy Policy and Conservation.
Envrionmental Protection Agency (EPA), 401 M Street SW, Washington D.C. 20005
AP 42 (May 1983)
Compilation of Air Pollutant Emission Factors
EPA45O/3-81-005 (Sept.1982)
Control Techniques for Particulate Emissions from
Stationary Sources
Government Printing Office, N. Capital Street, NW, Washington, D.C. 20001
Part 50, Title 40, Code of Federal Regulations
Environmental Protection Agency Regulations on National
Primary and Secondary Ambient Air Quality Standards
Part 60, Title 40, Code of Federal Regulations
Environmental Protection Agency Regulations on Standards
of Performance for New Stationary Sources
Part 53, Title 40, Code of Federal Regulations
Environmental Protection Agency Regulations on Ambient
Air Monitoring Reference and Equivalent Methods
Nongovernment Publications
TAPPI Journal, Technical Association of the Pulp and Paper Industry, P.O. Box 105113, Atlanta, GA. 30348
Dec.1982, (pp.53-56).
Fluidized Bed Steam Generation - An Update, by I.G. Lutes.
McGraw Hill Publishing Company, 1221 Avenue of the Americas, New York, New York 10001
Fifth Edition (1973)
Perry’s Chemical Engineering Handbook by R.H. Perry.
Addison-Wesley Publishing Company, Inc., Jacob Way, Redding, Massachusetts 01867
Industrial Electrostatic Precipitators by Harry J. White
Air Pollution Control Association, P.O. Box 2861, Pittsburgh, Pennsylvania 15236
APCA #69-162 (1969)
The Effect of Common Variables on Cyclone Performance
by J.W. Schindeler.
John Wiley & Sons 605 Third Avenue, New York, New York 10158
Fourth Edition (1964)
Principles of Engineering Economy by E.L. Grant, W.G.
Foster Wheeler Energy Corporation, 110 South Orange Avenue, Livingston, New Jersey 07039
The Technology and Economics of Fluidized Bed
Combustion Engineering, Inc., 1000 Prospect Hill Road, Windsor; Connecticut 06095
TIS-7537 (1984)
Circulating Fluid Bed Steam Generation by L. Capuano, K.
Ataby, S.A. Fox
TM 5-815-1/AFR 19-6
Acid Dew Point
Actual Combustion Air
Air Register
Air-to-Cloth Ratio
Atmospheric Stability
Blinding (blinded)
Boiler (thermal) Efficiency
Cloth Area
Cloth Weight
Combustion Air Windbox
Combustion Efficiency
Continuous Automatic Filtering
Critical Temperature
Dilution Air
Entrainment Spray
Excess Air
Felted Fabrics
Flue Gas
Temperature at which acid vapor condenses to form acid droplets.
The total amount of air supplied for complete combustion and equal to the
theoretical plus the excess air.
A type of burner mounting which admits secondary air to the combustion
The rate of volumetric capacity of a fabric filter (volume of air or gas in
ft3 /min per ft2 of filter fabric) commonly expressed as ft/min. Also called
filtering velocity, superficial face velocity, and filtration rate.
Scale adopted by the American Petroleum Institute to indicate the specific
gravity of a fluid. (API gravity for a liquid rises as its temperature rises.)
Non-combustible mineral matter which remains after a fuel is burned.
Degree of non-turbulence in the lower atmosphere.
The breaking of a liquid into a multitude of tiny droplets.
Loading or accumulation of filter cake to the point where the capacity rate
is diminished. Also termed "plugging" (Plugged).
Ratio of useful heat in delivered steam to the theoretical gross heat in the
fuel supplied.
A device which positions a flame in a desired location by delivering fuel
(and sometimes air) to that location. Some burners may also atomize the
fuel, and some mix the fuel and air.
To render a substance friable by the expulsion of its volatile content through
The total amount of cloth area in the form of bags or envelopes in a fabric
filter system.
A measure of filter fabric density. It is usually expressed in ounces per
square yard or ounces per square foot.
Scrubbing spray liquid and exhaust gas flowing in the same direction.
Inlet plenum for preheated combustion air.
The actual combustion heat released divided by the maximum possible heat
that can be released by combustion of a fuel.
A fabric filter unit that operates continuously, without interruption for
cleaning. The flow pattern through the system is relatively constant.
Temperature above which the substance has no liquid-vapor transition.
The air added downstream of the combustion chambers in order to lower
the exhaust gas temperature (In incinerators).
Atomized liquid downstream of scrubber spray nozzles.
Electrostatic precipitator.
The air remaining after a fuel has been completely burned (also, that air
which is supplied in addition to the theoretical quantity required).
Structures built up by the interlocking action of the fibers themselves,
without spinning, weaving, or knitting.
A continuous fiber element.
All gases which leave the furnace by way of the flue, including gaseous
products of combustion and water vapor; excess oxygen and nitrogen.
TM 5-815-1/AFR 19-6
Fly Ash
Gas Absorption
Heat Content
Heat Release Rate (firing rate)
High Temperature Fixation
Horizontal Front Wall Firing
Horizontal Opposed Firing
Intermittent Filtering System
In. Water
Mass Transport
Multiple Chamber In-Line
Multiple Chamber Retort
Mulicompartment Baghouse
Overfire Air (Secondary Air)
Orsat Analysis
Permeability (of fabric)
Peak Flame Temperature
Plant Thermal Efficiency
Plenum (or Plenum Chamber)
Porosity (Fabric)
Preheated Air
Primary Combustion Temperature
Suspended particles, charred paper, dust, soot, or other partially burned
matter; carried in the gaseous by-products of combustion. (Sometimes
referred to as particulate matter, or pollutants).
A process for removing a gas constituent from an exhaust gas stream by
chemical reaction between the constituent to be removed and a scrubbing
Unit of weight, equal to 1 lb.
The sum total of latent and sensible heat stored in a substance minus that
contained at an arbitrary set of conditions chosen as the base or zero point.
Usually expressed as Btu/lb, Btu/gal, Btu/ft3 for solid, liquid and gaseous
fuels, respectively.
The amount of heat liberated during the process of complete combustion
and expressed in Btu/hr/ft3 of internal furnace volume.
Reaction between nitrogen and oxygen at a high temperature in air
forming nitrogen oxides.
Horizontal furnace firing with all burners located in the front wall.
Horizontal furnace firing with burners located on directly opposing walls.
Particle to liquid adherence from collision.
A flow pattern in a fabric filter system which is saw-tooth-like. The flow
continually decreases until it is stopped. Then cleaning takes place and flow
is then again resumed at an increased value which again decreases, etc.
Inches of water column used in measuring pressure. One inch of water
column equals a pressure of .036 lb/in2.
Any process or force that causes a mass to flow through an open system.
Unit of length, equal to 1 millionth of a meter.
An incinerator design that allows combustion gases to flow straight through
the incinerator with 90-degree turns in only the vertical direction.
An incinerator design that causes combustion gases to flow through 90degree turns in both horizontal and vertical directions.
A compartmented filter baghouse that permits a uniform gas flow pattern
as compartments are taken offline for cleaning.
Nitrogen oxides.
Any air controlled with respect to quantity and direction, which is sup-plied
beyond the fuel bed (as through ports in the walls of the primary combustion
chamber) for the purpose of completing combustion of materials in gases
from the fuel bed. (Also used to reduce operating temperatures within the
furnace and referred to as secondary air).
An apparatus used to determine the percentages (by volume) of CO2, O2,
and CO in flue gases.
The ability of air (gas) to pass through filter fabric, expressed in ft3 of air per
mm. per ft2 of fabric with .5" H2O pressure differential.
The highest temperature achieved in the primary combustion zone.
The actual power output of a plant divided by the theoretical heat input rate.
Part of a piping or duct flow system having a cross-sectional area considerably larger than that of any connecting ducts pipes or openings.
A term often used interchangeably with permeability. (Actually a percentage
of voids per unit volume).
Air heated prior to its use for combustion, frequently by hot flue gases.
Temperature measured at the flame.
TM 5-815-1/AFR 19-6
Reentrainment (re-entrainment)
Residual Dust Accumulation
Tangential Firing
Theoretical Air
Theoretical Flame Temperature
Thread Count
Underfire Air
Woven Fabric
Volume Flow Rate
Particles reentering the gas stream after having been captured in a
particulate collection device.
The fairly stable matrix of dust that remains in a woven fabric after it is
cleaned. It accounts for the relatively high collection efficiency of a woven
fabric immediately after cleaning.
Four-cornered fuel firing to create a swirling flame pattern in a furnace.
The exact amount of air required to supply oxygen for complete combustion
of a given quantity of a specific fuel.
The maximum possible flame temperature from burning a fuel.
The number of warp and filling yarns per inch in woven cloth.
Any air controlled with respect to quantity and direction, forced or induced
and supplied beneath the grate, that passes through the fuel bed.
The quantity (measured in units of volume) of a fluid flowing per unit of
time, as ft3/min or gal/hr.
Fabric produced by interlacing strands at approximate right angles.
Lengthwise strands are called warp yarns and cross-wise strands are called
filling yarns.
Actual Cubic Feet per Minute
Best Available Control Technology
British Thermal Units per Hour
Carbon Dioxide
Carbon Monoxide
Cubic Feet per Minute
Degrees Fahrenheit
Department of Defense
Dry Standard Cubic Feet per Minute
Electrostatic Precipitators
Feet per Minute
Feet per Second
Flue Gas Desulfurization
Grains per Cubic Foot
Grains per Square Meter
Grains per Standard Cubic Foot
Micro-grams per Cubic Meter
Million British Thermal Units
Municipal Solid Waste
National Ambient Air Quality Standards
Navy Energy and Environmental Support Activity
New Source Review
New Source Performance Standards
Nitrogen Dioxide
Nitric Oxide
Nitrogen Oxides
(Deg F)
(E SP's)
Std. Ft3)
(Mu Btu)
(MM Btu)
TM 5-815-1/AFR 19-6
Non-attainment (in conjunction w/PSD)
Pounds per Square Inch
Prevention of Significant Deterioration
Refuse Derived Fuels
Square Feet
Square Meter
Sulfur Dioxide
Sulfur Oxides
Sulfur Trioxide
U.S. Environmental Protection Agency
U.S. Army Environmental Hygiene Agency
TM 5-815-1/AFR 19-6
"Coal Cleaning to Improve Boiler Performance and Reduce SO2 Emissions," Power; Sept.1981.
"Combustion-Fossil Power Systems", Combustion Engineering Co., Inc., 1981.
"Fuel Flexibility High on List of Design Objectives for New Station," Power; May 1982.
Kelly, A.J., and Oakes, E., Pyropower Corporation, "Circulating Fluidized Bed Combustion Systems and Commercial Operating Experience," 1982 TAPPI Engineering Conference, San Francisco, CA.
Kelly, W.R., J.M. Rourke, and Mullin, D.E., "Industrial Application of Fluidized-Bed Cogeneration System."
Chemical Engineering Progress, January 1984.
Lund, T., Lurgi Corporation, "Lurgi Circulating Bed Boiler: Its Design and Operation," Seventh International
Conference on Fluidized Bed Combustion, Philadelphia, PA. 1982.
"Pollution Control," Power; April 1982.
Ramsdell, Roger G., "Practical Design Parameters for Hot and Cold Precipitators," Paper presented at the
American Power Conference, Chicago, Illinois, May 1973.
Schwieger; R. and J., "Special Report - Fluidized Bed Boilers", Power, August 1982, S1-S16.
Smith, J.W., Babcock & Wilcox, 'AFBC Boiler Development Accomplishments and Requirements. " 1982 TAPPI
Engineering Conference, San Francisco, CA.
Steam-Its Generation and Use, 38th edition, Babcock and Wilcox, 1972.
Yip, H. H. and Engstrom, F., Pyropower Corporation and Ahlstrom Company, "Operating Experience of Commercial Scale Pyroflow Circulating Fluidized Bed Combustion Boilers." Seventh International Conference On
Fluidized Bed Combustion, Philadelphia, PA. 1983.
The proponent agency of this publication is the Office of the Chief of Engineers, United States Army.
Users are invited to send comments and suggested improvements on DA Form 2028 (Recommended
Changes to Publications and Blank Forms) direct to HQ USACE (CEEC-EE), WASH., DC 20314-1000.
By Order of the Secretaries of the Army and the Air Force:
Brigadier General, United States Army
The Adjutant General
Larry D. Welch, General, USAF
Chief of Staff
William 0. NATIONS, Colonel, USAF
Director of Information Management
and Administration
General, United States Army
Chief of Staff
Army: To be distributed in accordance with DA Form 12-34B, requirements for Energy Monitoring and
Control Systems (EMCS).
Air Force: F
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