United States
Environmental Protection
Agency
Air Pollution Training institute
MO 20
Environmental Researcn Center
Research Triangle Park. NC 27711
-150 I--2-X
June. '982
&EPA
APTI
Course SI:431
Air Pollution Control Systems
for Selected Industries
Self-instructional
Guidebook
sis
-------
v^w ^tates
fcnvrronmental Protection
Agency
Air Pollution Training Institute
MD20
Environmental Research Center
Research Triangle Park, NC 27711
EPA 450/2-82-006
June, 1983
Air
APTI
Course Sj:431
Air Pollution Control Systems
for Selected Industries
Self-instructional
Guidebook
Developed by:
David S. Beachler
James A. Jahnke, Ph.D.
Gerald T. Joseph, P.E.
Marilyn M. Peterson
Northrop Services, Inc.
P.O. Box 12313
Research Triangle Park, NC 27709
Under Contract No.
68-02-3573
EPA Project Officer
R. E. Townsend
United States Environmental Protection Agency
Office of Air, Noise, and Radiation
Office of Air Quality Planning and Standards
Research Triangle Park, NC 27711
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Notice
This is not an official policy and standards document. The opinions and selections are those of the
authors and not necessarily those of the Environmental Protection Agency. Every attempt has been
made to represent the present state of the art as well as subject areas still under evaluation. Any
mention of products or organizations does not constitute endorsement by the United States
Environmental Protection Agency.
The authors request that any material abstracted from this manual be appropriately referenced as
a matter of professional courtesy in the following manner:
Beachler, D. S.; Jahnke, J. A.; Joseph, G. T.; and Peterson. M. M. 1983. Air Pollution
Control Systems for Selected Industries—Self-instructional Guidebook. APTI Course SI:431,.'
EPA 450/2-82-006.
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Table of Contents
Page
Course Introduction vii
Description vii
Goals and Objectives vii
Requirements for Successful Completion vii
Materials viii
Using the Guidebook viii
Instructions for Completing the Final Examination viii
Lesson 1. Air Pollution Control 1-1
Lesson Goal and Objectives 1-1
Overview 1-1
Paniculate Emission Control 1-2
Gaseous Emission Control 1-5
General Concerns 1-6
Lesson 2. Cyclones 2-1
Lesson Goal and Objectives 2-1
Introduction 2-1
Particle Collection Mechanisms 2-2
Cyclone Construction 2-4
Cyclone Operating Parameters 2-8
Lesson 3. Fabric Filtration 3-1
Lesson Goal and Objectives 3-1
Introduction 3-1
Bag Designs 3-2
Baghouses 3-3
Filtration Designs 3-5
Types of Bag Cleaning 3-6
Baghouse Components 3-11
Bag Failure 3-14
Air-to-Cloth Ratios 3-14
Industrial Applications of Baghouses 3-15
Lesson 4. Electrostatic Precipitators 4-1
Lesson Goal and Objectives 4-1
Introduction 4-1
ESP Description 4-2
Particle Collection 4-4
Precipitator Components 4-6
ESP Operation 4-13
111
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Page
Lesson 5. Wet Collectors • '• • 5"!
Lesson Goal and Objectives ''.--... 5-1
T J • 5-1
Introduction a A
E a
Principles of Operation
Wet Collector Devices 5"5
Advantages and Disadvantages of Wet Collection Control Systems 5-12
Lesson 6. Adsorbers °-l
Lesson Goal and Objectives 6-1
Introduction 6-1
Theory of Adsorption 6-2
Adsorbent Materials 6-4
Adsorption Process 6-5
Factors Affecting Adsorption 6-6
Adsorption Control Systems 6-8
Lesson 7. Combustion Equipment 7-1
Lesson Goal and Objectives 7-1
Introduction 7-1
Combustion Process 7-2
Combustion Equipment Used to Control Gaseous Emissions 7-6
Applications 7-12
Lesson 8. Condensation 8-1
Lesson Goals and Objectives 8-1
Introduction 8-1
Condensation Principles 8-2
Condensers 8-2
Lesson 9. Fossil Fuel-Fired Steam Generators 9-1
Lesson Goal and Objectives 9-1
Introduction 9-1
The Boiler 9-1
Air Pollution Emissions 9-10
Air Pollution Control Equipment 9-11
New Source Performance Standards 9-13
Other Potential Emission Points 9-14
IV
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Page
Lesson 10. Steel Milk 10-1
Lesson Goal and Objectives 10-1
Introduction 10-1
Coke Oven Batteries 10-4
Blast Furnaces 10-5
Steel Furnaces 10-8
Sinter Plants 10-12
Steel Processing 10-12
Air Pollution Emissions 10-18
New Source Performance Standards 10-26
Summary 10-27
Lesson 11. Petroleum Refineries 11-1
Lesson Goal and Objectives 11-1
Introduction 11-1
Refining Process 11-2
Air Pollution Emissions 11-9
Air Pollution Control Methods 11-13
Air Pollution Regulations 11-15
Lesson 12. Portland Cement Plants 12-1
Lesson Goal and Objectives 12-1
Introduction. 12-1
Producing Cement 12-2
Air Pollution Emissions 12-7
Air Pollution Control Equipment 12-10
New Source Performance Standards 12-11
Lesson 13. Acid Plants 13-1
Lesson Goal and Objectives 13-1
Introduction 13-1
Sulfuric Acid Manufacture 13-1
Nitric Acid Manufacture 13-4
Air Pollution Emissions 13-7
Air Pollution Control Methods 13-8
New Source Performance Standards 13-11
Lesson 14. Municipal Incinerators 14-1
Lesson Goal and Objectives 14-1
Introduction 14-1
Incinerator Operation 14-1
Air Pollution Emissions 14-12
Air Pollution Control Equipment 14-13
New Source Performance Standards 14-14
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Page
Lesson 15. Kraft Pulp Mills 15'1
Lesson Goal and Objectives 15-1
Introduction 15-1
Kraft Method 15-2
Air Pollution Emissions 15-13
Air Pollution Control Methods 15-14
New Source Performance Standards 15-17
Lesson 16. Nonferrous Smelters 16-1
Lesson Goal and Objectives 16-1
Introduction 16-1
Basic Operations of Nonferrous Metal Production 16-2
Extracting Metals from Sulfide Ore 16-9
Air Pollution Emissions from Extraction Operations 16-19
Air Pollution Control Methods 16-20
New Source Performance Standards 16-21
Lesson 17. Asphalt Concrete Plants 17-1
Lesson Goal and Objectives 17-1
Introduction 17-1
Producing Asphalt 17-2
Air Pollution Emissions 17-8
Air Pollution Control Methods 17-10
New Source Performance Standards 17-12
VI
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Course Introduction
Description
This course is an introduction to the fundamental operating characteristics of paniculate and gaseous
pollutant emission control systems. It reviews physical, chemical, and engineering principles of con-
trol devices and the application of control systems to several industrial processes. Major topics
include:
• Principles of gaseous emission control equipment, including absorbers, combustors, condensers,
and adsorbers
• Principles of paniculate emission control equipment, including cyclones, fabric filters, elec-
trostatic precipitators, and scrubbers
• Application of control equipment to selected industries—power plants, municipal incinerators,
asphalt batch plants, cement plants, acid plants, steel mills, petroleum refineries, kraft pulp
mills, and smelters
Goal and Objectives
Goal
To familiarize you with the operation of air pollution control equipment and with nine industrial
processes, their air pollution emission points, and equipment used to reduce their emissions.
Objectives
Upon completing this course, you should be able to:
1. describe the collection mechanisms used to capture panicles and gases.
2. identify and describe the operation of four types of air pollution control devices used to reduce
paniculate emissions from industrial sources.
3. identify and describe the operation of four types of air pollution control devices used to reduce
gaseous emissions from industrial sources.
4. name one air pollution control device used to collect both panicles and gases emitted from
industrial sources.
5. briefly describe the operation of industrial processes discussed in this course, such as acid plants,
steel mills, and cement plants.
6. recognize the major air pollution emission points of these nine industrial processes.
7. describe air pollution control techniques for industrial sources presented in this course.
Requirements for Successful Completion
In order to receive 5.0 Continuing Education Units (CEUs) and a certificate of course completion,
you must:
1. take a mail-in final examination.
2. achieve a final examination grade of at least 70%.
vu
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Materials
Reading
This guidebook—supplementary reading materials are not required.
Using the Guidebook
This book directs your progress through the course. The first eight lessons describe paniculate and
gaseous emission control equipment. The next nine lessons describe nine large industrial sources and
the control equipment used to reduce emissions for each.
To complete a review exercise, place a piece of paper across the page covering the questions below
the one you are answering. After answering the question, slide the paper down to uncover the next
question. The answer for the first question will be given on the right side of the page separated by a
line from the second question, as shown here. All answers to review questions will appear below and
to the right of their respective questions. The answer will be numbered to match the question. Please
do not write in this book. Complete each review exercise in the lessons. If you are unsure about a
question or answer, review the material preceding the question. Then proceed to the next section.
Review Exercise
i.
2.
3.
*-*.,
Question lonlo
nlli cllu ylloiiuli<
Question oli oul
It iiluoiiytc <>
Question > w lot
jjonll i clUi yllon
1. Answer
nlio
2. Answer
Hi "nil1 _
Instructions for Completing the Final Examination
Contact the Air Pollution Training Institute if you have any questions about the course or when you
are ready to receive a copy of the final examination.
After completing the final exam, return it and the answer sheet to the Air Pollution Training
Institute. The final exam grade and course grade will be mailed to you.
Air Pollution Training Institute
Environmental Research Center
MD 20
Research Triangle Park, NC 27711
Vlll
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Lesson 1
Air Pollution Control
Lesson Goal and Objectives
Goal
To provide you with a brief history of air pollution control
regulations and introduce some fundamental concepts under-
lying the use of control devices on industrial sources for both
paniculate matter and pollutant gases.
Objectives
At the end of this lesson, you should be able to:
1. recognize categories of Federal standards that specify air
pollution emission limits for industrial sources.
2. recognize collection mechanisms in control devices for
paniculate matter.
3. recognize collection mechanisms in control devices for
pollutant gases.
4. identify the three main parameters used for either
judging the performance of or designing air pollution
control equipment.
Overview
Industrial sources can emit a significant amount of paniculate
matter and pollutant gases into the atmosphere. In order to
improve and protect the quality of the air, these emissions can
be reduced by using different types of control devices.
Air cleaning devices have reduced paniculate and gaseous
pollutants from various industrial sources for many years.
Originally, air pollution control equipment was used to control
pollutants only if they were a nuisance, were highly toxic, or if
they had some recovery value. Now, because of legislation such
as the 1970 Clean Air Act and the 1977 Clean Air Act Amend-
ments, more stringent control is required for many major
industries.
The Federal government has set standards for pollutant
levels in the ambient air. These standards, known as the
National Ambient Air Quality Standards (NAAQS), are
specified for pollutants such as sulfur dioxide (SO2), carbon
1-1
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monoxide (CO), ozone (O3), nitrogen dioxide (NO2), par-
ticulate matter, and lead (Pb). In order that these ambient stan-
dards can be attained, industrial source emission standards
have also been set. These source standards limit the pollutant
concentration that can be emitted, and in some cases, specify
the efficiency of the control equipment that must be installed
on the source. Many new industrial sources are subject to
regulations called New Source Performance Standards (NSPS).
A new source is one that is contracted and installed at a facil-
ity after the date emission standards are proposed for that
industry. New Source Performance Standards (NSPS) that have
been promulgated are first published in the Federal Register
and then in the U.S. Code of Federal Regulations. NSPS are
set for a number of industrial source categories such as acid
plants, cement plants, and fossil-fuel-fired steam generators.
NSPS regulations specify emission limits and occasionally the
type of control equipment that must be installed on various
industrial sources.
In most cases, air pollution control equipment is installed on
industrial sources to reduce emissions in order to meet regula-
tions. However, it is possible to reduce emissions by other
methods. Changing fuel sources, modifying or changing raw
materials, or using alternative production procedures also can
reduce emissions without adding on control equipment. These
methods are usually considered before installing expensive con-
trol equipment.
Particulate Emission Control
Cyclones, baghouses, electrostatic precipitators and wet scrub-
bers are used to reduce paniculate emissions from industrial
sources. All of these devices collect paniculate matter (par-
ticles) by mechanisms involving an applied force. The simplest
collection force is gravity. Large particles moving slow enough
in a gas stream can be overcome by gravity and be collected.
Gravity is responsible for collecting particles in a simple device
such as a settling chamber.
The settling chamber was one of the first devices used to
control paniculate emissions; however, it is very rarely used
today. Because its effectiveness in collecting particles is very
low, it cannot be used to meet most air pollution regulations.
However, the settling chamber can be used as a precleaner for
other paniculate emission control devices — to remove very large
particles. A typical settling chamber is presented in Figure 1-1.
The unit is constructed as a long horizontal box with an inlet,
chamber, outlet, and dust collection hoppers. The velocity of
the particle-laden gas stream is reduced in the chamber. All
1-2
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particles in the gas stream are subject to the force of gravity.
At the reduced gas velocity in the chamber, the large particles
(greater than 40 /un in diameter) are overcome and fall into
the hoppers.
Figure 1-1. Gravity settling chamber.
Another_collection force used for particle capture is
centrifugal force. The shape or curvature of the collector
causes the gas stream to rotate in a spiral motion. Larger par-
ticles move toward the outside of the wall by virtue of their
momentum (Figure 1-2). The particles lose kinetic energy there
and are separated from the gas stream. Particles are then over-
come by gravitational force and collected. Centrifugal and
gravitational forces are both responsible for particle collection
in a cyclone.
In both fabric filters and wet collectors, three separate forces
are responsible for particle collection: impaction, direct
interception, and diffusion. In a fabric filter, the target object
for panicle capture is a stationary filter bed supported by the
fabric. In a wet collector, the target object is a water droplet.
Figure 1-2. Centrifugal force.
1-3
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Consider the case of an individual fiber in a fabric filter.
Impaction occurs when the particle is so large that it cannot
follow the gas streamlines around the stationary impaction
target. It hits or impacts on the fiber (Figure 1-3). Direct
interception is a special case of the impaction mechanism. The
center of a particle may follow the streamlines formed around
the fibers. A collision will occur if the distance between the
particle center and the collection surface is less than the par-
ticle radius (Figure 1-4). Particles below 0.1 /nn in
aerodynamic diameter undergo Brownian motion, randomly
moving or diffusing throughout the gas volume. The
mechanism of diffusion is responsible for the collection of par-
ticles which are so small that they become affected by collisions
of molecules in the gas stream. The randomly moving particles
then move or diffuse through the gas to impact on the fiber
and are collected (Figure 1-5).
The other primary particle collection mechanism involves
electrostatic forces. The particles can be naturally charged, or,
as in most cases involving electrostatic attraction, be charged
by subjecting the particle to a strong electric field. The
charged particles migrate to an oppositely charged collection
surface (Figure 1-6). This is the collection mechanism responsi-
ble for particle capture in both electrostatic precipitators and
charged droplet scrubbers. In an electrostatic precipitator, par-
ticle collection occurs because of electrostatic forces only. In a
charged droplet scrubber, particle removal occurs by the com-
bined effects of impaction, direct interception, diffusion, and
electrostatic attraction. Particles are charged in these scrubbers
to enhance both diffusion and direct interception.
Particle collection can occur from the combined effect of the
mechanisms discussed. In addition, particles can agglomerate
or grow in size through cooling or increasing humidity or from
electrostatic effects. Agglomerated particles thus have a larger
Partii
Figure 1-3. Impaction.
Particle
Figure 1-4. Direct interception.
Figure 1-5. Diffusion.
Particle -l^\
0 »••*• •H^£f + I
Figure 1-6. Electrostatic attraction.
1-4
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aerodynamic diameter and can be collected by impaction,
interception, or gravitational forces. Many factors influence the
choice of a control device used to reduce industrial paniculate
emissions. If emitted material can be used or reused in the
process, dry collection should be used. If the pollutant has
little economic value, collection should be accomplished and
the pollutant disposed of safely and economically. The
industrial process and potential control devices must both be
carefully reviewed. The conversion of an air pollution problem
into a water pollution problem can create a more difficult
disposal problem.
Gaseous Emission Control
Gaseous pollutants are emitted from a variety of industrial
processes. Those frequently controlled include sulfur dioxide
(SO»), nitrogen oxides (NO,), nonmethane organics (NMO)
and carbon monoxide (CO).
Absorbers, adsorbers, combustors (incinerators), and con-
densers are used to control gaseous emissions. The use of a par-
ticular device depends on the physical and chemical properties
of both the pollutant and the exhaust stream. More than one
device may^be able to control emissions from a given source.
For example, vapors (gaseous emissions) generated from
loading gasoline into tank trucks may be controlled by any of
these devices. On the other hand, absorbers are most com-
monly used for reducing SO, emissions generated as a result of
burning fossil fuels.
As with particle collection, gases are collected by various
mechanisms. Gases can be removed by an operation called
absorption—gases are dissolved in a liquid. A gaseous pollutant
exhaust stream contacts and is dissolved by the liquid (Figure
1-7). The liquid used most often is water since it is inexpensive,
readily available, and can dissolve a number of pollutants.
Gases can also be removed by an operation called
adsorption—gaseous pollutants adhere to a solid surface
(Figure 1-8). Activated carbon is the solid most often used
since many hydrocarbon vapors and odorous organic com-
pounds from industrial exhaust streams adhere to the carbon.
Gaseous pollutants can also be controlled by burning a
gaseous pollutant (organic) in a chamber to form harmless
products—carbon dioxide and water. Auxiliary fuels are
burned in the chamber in order to raise the pollutant's
temperature to the point where it will readily oxidize. Natural
gas and oil are commonly used as auxiliary fuels to incinerate
gaseous pollutants.
X Water
'" droplets
Figure 1-7. Absorption.
Carbon
gas
Figure 1-8. Adsorption.
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Gaseous vapor can be condensed into liquid droplets. This is
usually accomplished by reducing the temperature of the
pollutant-laden gas stream until liquid droplets form. Con-
densers are used to remove water vapor and also condensible
organic compounds from industrial exhaust streams.
General Concerns
Collection Efficiency
The performance of air pollution control equipment is often
judged in terms of its collection efficiency. Collection efficiency
is defined as the percentage reduction in pollutant concentra-
tion between the inlet and outlet of the control device; or
pollutant concentration _ pollutant concentration
Collection at the inlet at the outlet ,.„_
efficiency = : X 100%
(by weight) pollutant concentration
at the inlet
A high value for efficiency indicates that a greater degree of
control is achieved on the source. A low value indicates that
lesser control occurs with more pollutants emitted into the
atmosphere.
Emission.limits are usually set by existing air pollution
regulations. The control to be achieved depends on the allowed
outlet concentration and the quantity of emissions generated
from the process. For example, assume that a source emits
1800 mg/m3 paniculate matter from its stack (uncontrolled).
If the regulation states that the maximum allowable emission
rate cannot exceed 90 mg/m3, then the collection efficiency ,
must be:
Collection = 1800-90
efficiency 1800
= 0.95 or 95%
In order to meet the regulations in this case, a control device
having 95% collection efficiency must be installed on this source.
Air pollution control equipment is often designed specifically
for the industrial source on which it is installed. Several design
factors should be considered. One is the concentration or grain
loading of paniculate pollutants in the process exhaust stream.
Pollutant concentration is typically expressed in terms of
pounds per cubic foot (lb/ft3), grains per cubic foot (gr/ft3),
and grams per cubic meter (g/m3). For gaseous pollutants,
concentration is expressed in terms of parts per million (ppm)
by volume, e.g., mVlO'm3, ft3/106ft3. Both the level and fluc-
tuation of grain loading are very important. Some control
devices, such as fabric filters, are relatively unaffected by high
levels or great fluctuations in particle concentration. Others,
1-6
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such as electrostatic precipitators, generally do not function
effectively with large fluctuating concentration levels. Another
related problem can occur when the exhaust gas velocity
changes rapidly. Some control devices are designated to operate
at specific exhaust gas velocities. Large changes in gas velocities
can drastically affect the unit's collection efficiency.
Particle Characterization
Particle characteristics such as size, shape, and density must be
considered when designing control systems. Paniculate matter
is the finely divided solid or liquid material that exists as par-
ticles in the stack gas. Particle size is usually expressed in terms
of the aerodynamic diameter. The aerodynamic diameter
describes how the particle moves in a gas stream. Particle
diameters are measured in units of micrometers (/tm). Particles
with large diameters (> 10 pm) can be collected in cyclones.
Particles having small diameters (< 5 /tin) must be collected in
more sophisticated devices such as scrubbers, baghouses, or
electrostatic precipitators. Thus, the collection efficiency of a
specific control device depends on the size of the particles in
the exhaust stream. Devices called impactors are commonly
used to determine the particle size distribution of exhaust
streams from industrial sources. The impactor is inserted into
the stack aad a sample stream is pulled into the impactor. Par-
ticles impact on collection or impaction plates according to
their aerodynamic size. Additional information on particle size
devices can be obtained from APTI Course 413 Control of Par-
ticulate Emissions—Student Manual EPA 450/2-80-066.
Pressure Drop
Another important characteristic of control devices is the effect
they have on the flow of exhaust gas in an industrial process. A
resistance to the flow of gas can build up, especially if the gas
must be forced through small constrictions or openings.
Pressure drop is a measure of the air resistance across a system.
Pressure drop, also called gas pressure drop, describes the
pressure loss between the inlet and outlet sections of the control
device. Collectors with large pressure drops would require
larger fans (and greater power requirements) to either push or
pull the exhaust gas through the system. An increase in
pressure drop means that there is a larger pressure loss in the
system. Some control devices such as venturi scrubbers are
designed to operate at high pressure drops [as great as 254 cm
H2O (100 in. H2O)]. On the other hand, electrostatic
precipitators are designed to operate at much lower pressure
drops [usually less than 2.54 cm H2O (1.0 in. HZO)].
1-7
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In order to reduce pollutant emissions from industrial proc-
esses, the control system should be designed to meet emission
limitations at minimum cost with maximum reliability. The
basic trade-offs involve decisions between collection efficiency,
installation costs, and operating costs. This course will review
the many control techniques which have been used to meet the
requirements of air pollution regulations for various industrial
sources.
Review Exercise
1. Maximum pollutant levels set by the Federal government for
specific air pollutants in the ambient air are referred to as
a. New Source Performance Standards.
b. Special Source Emission Standards.
c. National Clean Air Act Regulations.
d. National Ambient Air Quality Standards.
2. The NSPS regulations are designed to ensure that all new
plants have
a. the same minimum emission requirements.
b. identical control devices.
c. none of the above
1. d. National Ambient
Air Quality Standards.
3. Impaction, direct interception, and diffusion can all be
responsible for particle collection in
a. fabric filters.
b. adsorbers.
c. condensers.
d. incinerators.
2. a. the same minimum
emission requirements.
4. All control devices used to collect paniculate matter
operate using
a. electrostatic attraction.
b. water.
c. an applied force.
3. a. fabric filters.
In a simple control device such as the settling chamber,
large particles moving slow enough in a gas stream can be
overcome by
a. centripetal force.
b. gravity.
c. centrifugal force.
d. impaction.
4. c. an applied force.
5. b. gravity.
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6. How does centrifugal force cause particles to separate from
the gas stream? What shape encourages this separation?
If a panicle is so large that it cannot follow the gas stream-
lines around a fiber or droplet, it will the object.
a. bypass
b. diffuse through
c. impact on
d. electrically charge
6. The gas stream rotates
in a spiral fashion. The
particles' momentum
causes them to break
from the gas stream
and hit the walls of
the device. The device
is usually in a curved
or circular shape.
8. A high value of collection efficiency means that
a. the source does not need a control device.
b. a high percentage of the pollutant is collected.
c. a high percentage of the pollutant is emitted.
7. c. impact on
9. If.
is large or constantly fluctuating, some control
devices may not function efficiently.
a. particle concentration
b. grain loading
c. gas velocity
d. all the above
8. b. a high percentage of
the pollutant is
collected.
10. Aerodynamic diameter describes how a particle moves in
a gas stream. It has a direct bearing on
a. the collection efficiency of a specific control device.
b. the ability of the panicle to be collected.
c. both a and b
9. d. all the above
11.
is a measure of the resistance to gas movement
through a control device.
a. Collection efficiency
b. Particle-to-panicle density
c. Pressure drop
d. none of the above
10. c. both a and b
12. An increase in pressure drop means that the pressure loss
in the system is larger/smaller.
13. True or False? All control devices must operate at high
pressure drops to be efficient.
11. c. Pressure drop
12. larger
13. False
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Lesson 2
Cyclones
Lesson Goal and Objectives
Goal
To familiarize you with the paniculate-removal device used by
many industries—the cyclone—its collection mechanisms, dif-
ferent designs in use, and operating principles.
Objectives
At the end of this lesson, you should be able to:
1. explain how particles are collected in cyclones.
2. list the four major cyclone design features.
3. recall why a variety of inlet designs have been developed.
4. identify different uses for different cyclone designs.
5. list three advantages of using cyclones to collect
paniculatejnatter.
6. list three possible problems associated with cyclone use.
7. relate efficiency and pressure drop to cyclone operation.
Introduction
The cyclone is a simple mechanical device commonly used to
remove relatively large particles from gas streams. Cyclones
have a distinctive and easily recognized form (Figure 2-1) and
can be found in almost any industrial area of a town or
city—at lumber companies, feed mills, cement plants,
smelters—and at many other industrial sites. They can be
located on the roof of a plant or beside a building. They range
in size from a few centimeters in diameter when used in
analytical equipment to several meters in diameter when used
for air pollution control.
In industrial applications, cyclones are often used as
precleaners for the more sophisticated air pollution control
equipment such as electrostatic precipitators or baghouses.
Cyclones are more efficient for removing paniculate matter
than are settling chambers, but are less efficient than either
wet scrubbers, baghouses, or electrostatic precipitators.
Cyclones used as precleaners are often designed to remove
more than 80% of the particles that are greater than 20 /un in
diameter. Smaller particles that escape the cyclone can then be
collected by more efficient control equipment.
Figure 2-1. Common cyclone.
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Cyclones are relatively inexpensive to construct since they
have no moving parts. Fans move the gas through the system
but are an auxiliary device. They are usually inexpensive to
operate.
The first cyclone design was patented in 1886. This is the
basic design shown in Figure 2-1. Since 1886, experience has
led to both improved design and to unique modifications.
Cyclones in series, or banks of small cyclones in parallel
(multiclones), can be used to effectively remove particles
having diameters of approximately 5 to 10 /un. Changing
details in cyclone design and in relative dimensions has led to
improved efficiencies without always increasing costs of
operation.
Particle Collection Mechanisms
Cyclones force the incoming gas stream to twist and turn in a
spiral fashion. Large particles entering with the gas stream
cannot turn with the gas because of their momentum or iner-
tia. As a result, they break out of the gas stream and hit the
wall of the cyclone. The particles then fall down the wall and
are collected in a hopper. Figure 2-2 shows a top view of this
action.
The spiral pattern of gas flow (Figure 2-3) is developed by
the manner in which the gas is introduced. It enters along the
side of the cyclone body wall and turns a number of times to
spiral down to the bottom, much like the funnel of a tornado.
When the gas reaches the bottom of the cyclone, it reverses
direction and flows up the center of the tube, also in a spiral
fashion. This spiral or vortex pattern turns in the same direc-
tion when it goes up as when it was descending.
In the cylindrical section of the cyclone, particulate matter is
forced to the wall. Or, in other words, the particles move
towards the wall by the action of an "apparent" centrifugal
force. In the cone section, the body is tapered to give the gas
enough rotational velocity to keep the particles against the
wall. This helps prevent reentrainment — the return of collected
particles back into the gas stream. As the particulate matter
falls to the bottom, it is collected in the hopper and is either
continuously or periodically removed.
Figure 2-2. Cyclone particle collection.
Figure 2-3. Gas flow in a cyclone.
2-2
-------
Review Exercise
1. The removal efficiency of cyclones is generally greater than
that of
a. electrostatic precipitators.
b. baghouses.
c. gravity settling chambers.
d. all the above
2. Cyclones used as precleaners are normally designed to
remove particles that are larger than /un in
diameter at better than efficiency.
a. 0.1 urn, 80%
b. 20 iaa, 80%
c. 1 iaa, 90%
d. 5 iaa, 100%
1. c. gravity settling
chambers.
3. In the figure below, draw and label the direction of gas
flow and indicate where the particles are collected.
2. b. 20 iaa, 80%
2-3
-------
4.
5.
6.
In cyclones, panicles are collected
a. because their inertia causes them to break through gas
streamlines.
b. because they are electrostatically attracted to the walls.
c. because the gravitational forces overcome the centripetal
forces.
d. by sieving action.
The cyclone is given its name because of the characteristic
spiral motion of the gas in the device. How many direc-
tions are present in a cyclone?
a. one descending direction, only
b. two directions: one descending and one ascending
c. three directions: one descending, one in the hopper,
and one ascending
True or False? The upward gas spiral rotates in the same
direction as does the downward gas spiral.
4. a. because their
causes them to
through gas
streamlines.
5. b. two directions
descending and
ascending
inertia
break
: one
one
6. True
Cyclone Construction
Cyclones can be designed in a number of different ways. The
most common design is the tangential entry cyclone already
shown in Figure 2-3. This type of cyclone has four major
design features: inlet, cyclone body, dust discharge system, and
outlet (Figure 2-4).
Inlet
Let us first consider different inlet designs. Gas coming into
the cyclone must be transformed from straight flow into a cir-
cular pattern to form the vortex. Problems can arise in the
inlet if turbulence develops and inhibits vortex formation. For
this reason, modifications of the basic tangential entry have
been used (Figure 2-5).
Inlet deflector vanes added to a tangential entry can narrow
and force the gas stream to move against the wall. Helical and
involute entries can help provide a smoother transition of the
gas into a vortex pattern. The success of these modifications is
sometimes marginal, although increased efficiencies have been
reported.
2-4
-------
Outlet
Inlet
Body
Tangential entry
Tangential entry
with deflector vanes
Dust discharge
system
Figure 2-4. Cyclone features.
Helical entry
Involute entry
Figure 2-5. Cyclone entries.
0.5 De
Body
The particle removal efficiency of a cyclone depends to a great
extent upon the cyclone's dimensions. A designer of cyclones
must first know both the anticipated volumetric gas flow rate
through the system and what is expected in terms of removal
efficiency. The most important dimension is the diameter of
the body. A longer cyclone in relation to its diameter will pro-
vide for more vortex revolutions and thus more chances for
particle collection. Also, small-diameter cyclones collect small
particles more efficiently than do large-diameter cyclones.
Smaller body diameters create larger separation forces.
Cyclones less than 0.25 m (10 in.) in diameter are generally
considered to have high efficiency, although efficiencies can
vary depending on the relative cyclone dimensions. Figure 2-6
shows an example of relative dimensions in a high-efficiency
tangential entry cyclone.
Figure 2-6. High-efficiency cyclone
dimensions.
2-5
-------
Dust Discharge System
Collected paniculate matter should be removed from the
cyclone or else it will be reentrained and sucked back up in the
inner vortex. A number of methods can be used for either
periodic or continual removal of the collected material. A
manual slide gate (Figure 2-7) at the bottom of the cone is one
of the simplest constructions allowing periodic removal (Figure
2-7). A rotary valve (Figure 2-8) can provide continuous
removal.
Outlet
Modifications to gas outlets have been developed in attempts to
improve the operating characteristics of the cyclone. The gas
leaving a cyclone will normally continue moving in a circular
pattern. If this flow could be straightened without producing
turbulence, the amount of energy needed to move the gas
through the system could be reduced. This has been accom-
plished to some extent by outlet devices similar to those shown
in Figure 2-9, the involute scroll outlet, and Figure 2-10, the
outlet drum.
Figure 2-7. Slide gate.
::;'£yit^r'V.
Figure 2-8. Rotary valve.
Figure 2-9. Involute scroll outlet.
Figure 2-10. Outlet drum.
2-6
-------
Other Configurations
We have just discussed a number of design features of the com-
mon tangential entry cyclone. Other mechanical configura-
tions, that can still take advantage of centrifugal action to
remove particles from a gas stream, are possible.
The large cyclonic separator shown in Figure 2-11 is a simple
design that also uses the principle of centrifugal action. The
cyclonic separator is often used after wet collectors—control
devices that use water sprays to trap particles in large droplets.
These large water droplets contain entrained paniculate mat-
ter, and can be removed quite effectively by this simple
cyclone. Here the gas and droplets enter tangentially at the
bottom of the drum, forming a vortex. The large droplets are
forced against the walls and drained off at the bottom.
The axial inlet cyclone shown in Figure 2-12 is used in the
multicyclone arrangement. Here the gas inlet is parallel to the
axis of the cyclone body. The gas enters from the top and is
directed into a vortex pattern by the vanes attached to the cen-
tral tube. This helps prevent the turbulence around the
entrance that can be a problem in tangential entry cyclones.
In the multicyclone (Figure 2-13), axial cyclones are
arranged in parallel. The dirty gas enters uniformly through all
of the individual cyclones. The large number of inlets enables
small high-~efficiehcy cyclones to be used without greatly
impeding the process gas flow. The inlet vanes are, however,
prone to plugging.
Figure 2-11. Cyclonic separator.
Figure 2-12. Axial inlet cyclone.
Figure 2-13. Multicyclone.
2-7
-------
Cyclone Operating Parameters
In cyclones, collection efficiency generally increases as the
pressure drop increases. The small openings that create high
inlet pressure also produce a high gas velocity through the
cyclone. This higher velocity results in greater centrifugal force
on the particles, and thus greater collection efficiency. The
pressure drop in conventional tangential cyclones can range
from 5 to 40 cm (2 to 16 in.) of water.
The pressure drop and efficiency depend on the relative
dimensions of the cyclone. The effect of changes in these
dimensions and changes in gas stream parameters is sum-
marized in Table 2-1.
Table 2-1. Changes in performance characteristic.
Cyclone and process design changes
Increase cyclone size (Dc)
Lengthen cylinder (Lc)
Lengthen cone (Zc)
Increase exit tube diameter (Dc)
Increase inlet area — maintaining velocity
Increase dust concentration
Increase particle size and/ or density
Pressure drop
Decreases
Decreases slightly
Decreases slightly
Decreases
Decrease!
Decrease for
large increases
No change
Efficiency
Decreases
Decreases*
Increases
Decreases
Decreases
Increases
Increases
Source: Bhatia and Cheremisinoff, 1977.
*Some studies indicate efficiency does not always decrease when the cylinder is lengthened.
Cyclones provide one of the least expensive methods of
removing relatively large panicles from gas streams. They can
be made of almost any type of material—a distinct advantage
if condensed acids are present. They have been operated at
temperatures higher than 1000°C (1832°F) using refractory
linings, and have also been constructed to operate at high
pressures. Capital costs are low, and operating costs are also
low compared to those associated with more complicated
systems.
Problems do arise, however. Sticky or agglomerating
materials can plug the smaller-diameter high-efficiency
cyclones. Varying gas flow rates will affect cyclone performance
since particle collection efficiency depends upon the incoming
gas velocity. Also, hard, sharp-edged particles can rapidly wear
the internal surfaces of the cyclone.
Overall, cyclones are a common type of control device used
to prevent paniculate matter from entering the atmosphere.
They are widely used in industry to control emissions from
dryers, crushers, incinerators, and kilns; and as precleaners for
the control equipment that we will be discussing in subsequent
lessons.
2-8
-------
Review Exercise
1. List the four major design features of the cyclone.
2. Different types of inlets have been developed to avoid
problems of
1. • inlet
• body
• dust discharge system
• outlet
3. Cyclones with smaller inlet and exit areas, or smaller body
diameters, are more/less efficient than are cyclones with
larger diameters.
2. turbulence
4. What will eventually happen if paniculate matter is not
removed from the cyclone collection hopper?
a. The gas will make more turns in the cyclone body.
b. Cyclone efficiency will increase.
c. Paniculate matter will be reentrained into the inner
vortex.
3. more
5. Axial cyclones are commonly used in
a. conjunction with wet scrubbers.
b. baghouses.
c. multicyclones.
4. c. Paniculate matter
will be reentrained into
the inner vortex.
6. Areas to consider when designing a cyclone for a specific
application, are
a. size and relative dimensions.
b. gas temperature.
c. incoming gas volumetric flow rate.
d. dust concentration.
e. all the above
5. c. multicyclones.
7. Pressure drop is related to
a. barometric pressure.
b. the amount of energy needed to move gas through a con-
trol device.
c. collection efficiency in a cyclone.
d. b and c only
e. a and b only
6. e. all the above
7. d. b and c only
2-9
-------
8. List at least three advantages of using a cyclone to collect
paniculate matter.
9. List at least three possible problems associated with the use of
cyclones for collecting particulate matter.
8. • low capital costs
• low operating costs
• construction with
variety of materials
• no moving parts
• possible plugging
problems
• limited efficiency in
collection of small
panicles
• efficiency sensitive to
varying inlet flow
rates
• wear problems
References
Bethea, R. M. 1978. Air Pollution Control Technology. New York: Van Nostrand Reinhold,
pp. 117^44. "
Bhatia, M. V. and Cheremisinoff, P. N. 1977. Cyclones. Air Pollution Control and Design Hand-
book. P. N. Cheremisinoff and R. A. Young, eds. pp. 281-316, New York: Marcel Dekker, Inc.
Caplan, K. 1964. All About Cyclone Collectors. Air Eng. Sept.: 28-38.
Caplan, K. 1977. Source Control by Centrifugal Force and Gravity. Air Pollution Vol. IV
Engineering Control of Air Pollution, A. C. Stern, ed. pp. 97-148, New York: Academic Press.
Danielson, J. A., ed., 1973. Air Pollution Engineering Manual, Research Triangle Park, NC: U.S.
Environmental Protection Agency, pp. 91-99.
Doerschlag, C. and Miczek, G. 1977. How to Choose a Cyclone Dust Collector. Chem. Eng.
Feb.: 64-72.
Hesketh, H. E. 1979. Air Pollution Control. Ann Arbor: Ann Arbor Science Publishers,
pp. 184-193.
Koch, W. H. and Licht, W. 1977. New Design Approach Boosts Cyclone Efficiency, Chem. Eng.
Nov.: 79-88.
Lapple, C. E. 1950. Gravity and Centrifugal Separation, Industrial Hygiene Quarterly 11: 40-48.
Leith, D. and Mehta, D. 1973. Cyclone Performance and Design. Atmos. Environ. 7: 527-549.
Schneider, A. G. 1975. Mechanical Collectors. Handbook for the Operation and Maintenance of
Air Pollution Control Equipment, F. L. Cross, Jr. and H. E. Hesketh, eds. pp. 41-68, Westport:
Technomic Publishing.
2-10
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Lesson 3
Fabric Filtration
Lesson Goal and Objectives
Goal
To familiarize you with the paniculate emission removal device
used by many industries—the baghouse—its operating principles
and different designs in use.
Objectives
At the end of this lesson, you should be able to:
1. explain how particles are collected in baghouses.
2. list five major baghouse components.
3. list three bag-cleaning mechanisms and recall how each
works.
4. recall two ways fabric filters are constructed.
5. recalt-the reason why different fibers are used to make
bag fabric.
6. list three conditions that can shorten the operating life of
a bag.
7. define air-to-cloth ratio.
8. list three industries that use baghouses to control par-
ticulate emissions.
Introduction
Fabric filtration is one of the most common techniques used to
collect paniculate matter. Fabric filtration systems use a filter
material such as nylon or wool to remove panicles from
industrial exhaust gas. The panicles are retained on the fabric
material, while the cleaned gas passes through the material.
The collected panicles are then removed from the fabric
filter by a cleaning mechanism: by mechanical shaking or using
blasts of air. The removed particles are stored in a collection
hopper until they are disposed of or are reused in the process.
3-1
-------
Design
Fabric filters used in industry are usually called baghouses. A
baghouse consists of the following components:
• filter medium and support
• filter cleaning device
• collection hopper
• shell
The particle collection surface is composed of the filtering
material and a support structure. Most U.S. baghouse designs
use long cylindrical tubes that contain felted fabric or woven
cloth as the filtering medium. The cloth can be supported at
the top and bottom of the bag by metal rings or clasps, or by
an internal cage that supports the entire bag (Figure 3-1). Dust
is collected on either the inside or the outside of the fabric
material depending on the baghouse design.
Some European baghouse designs use an envelope filter
arrangement as shown in Figure 3-2. The envelope filter con-
sists of felted or woven fabric supported by a metal retaining
cage. The metal cage keeps the fabric taut as the dust filters
through and collects on the outside of the material. Clean air
passes out the open end of the envelope.
Recently, cartridge filters have been used for filtering par-
ticulate matter from small industrial processes. The cartridge
filters are similar to truck air cleaner filters and are approx-
imately 0.61 m (1.86 ft) long (Figure 3-3). Dust is collected on
the outside of the cartridge while clean air flows out through
the center.
I Meta
Metal cap
Anti-collapse
ring
Clasp-
Internal
- support
cage
*-End cap
Figure 3-1. Bags and support.
Support cage
Figure 3-2. Envelope filter.
Figure 3-3. Cartridge filter.
3-2
-------
Baghouses
Baghouses are usually constructed using many cylindrical bags
that hang vertically in the baghouse (Figure 3-4). The number
of bags can vary from a few hundred to a thousand or more
depending on the size of the baghouse. When dust layers have
built up to a sufficient thickness, the bag is cleaned, causing
the dust particles to fall into a collection hopper. Bags can be
cleaned by a number of methods. Particles are stored in the
hopper and are usually removed by a pneumatic or screw con-
veyer. The baghouse is enclosed by sheet metal to contain the
collected dust and to protect the bags from atmospheric
environmental conditions.
Hopper
Figure 3-4. Typical baghouse.
3-3
-------
The envelope baghouse consists of compartments that con-
tain envelopes of fabric that are mounted on frames and
attached to the walls of the collector (Figure 3-5).
Figure 3-5. Envelope baghouse.
Cartridge systems (Figure 3-6) operate similarly to baghouses
that use bag tubes. Cartridge baghouses are usually used on
smaller industrial processes—those handling exhaust flow rates
of less than 1416 mVmin (50,000 cfm).
Figure 3-6. Cartridge baghouse.
3-4
-------
Review Exercise
1. The four major components of a baghouse are:
, , and
2. Baghouses use either _
as the filtering media.
., or.
1. filter medium and
support,
filter cleaning device,
collection hopper,
shell
3. Most U.S.
4. Dust clean
baghouse designs use many cylindrical bags that
in the baghouse.
fA from th* hags is rnlWtcH in a ,.,,,,
2. bags, envelopes,
cartridges
3. hang vertically
4. hopper
Filtration Designs
There are -two filtration designs used in baghouses: interior
filtration and exterior filtration. In baghouses using interior
filtration, particles are collected on the inside of the bag. The
dust-laden gas enters through the bottom of the collector and
is directed inside the bag by diffuser vanes or baffles and also a
cell plate. The cell plate is a thin metal sheet surrounding the
bag openings. The cell plate separates the clean gas section
from the baghouse inlet. The particles are filtered by the bag
and clean air exits through the outside of the bag (Figure 3-7).
In interior filtration systems, bags are held at the top by dif-
ferent types of attachments. For reverse air cleaning baghouses,
this attachment is a spring and metal cap (Figure 3-8). For
shaker cleaning baghouses, bags are attached at the top by a
hook (Figure 3-9).
Hook
Figure 3-9. Shaker bags.
Figure 3-7. Interior filtration.
Figure 3-8. Reverse air bags.
3-5
-------
In exterior filtration systems, dust is collected on the outside
of the bags. The filtering process goes from the outside of the
bag to the inside with clean gas exiting through the inside of
the bag (Figure 3-10). Consequently, some type of bag support
is necessary, usually an internal bag cage or rings sewn into the
bag fabric. Bags are attached at the top to a tube sheet and
are closed at the bottom by an end cap.
Types of Bag Cleaning
A number of cleaning mechanisms are used to remove caked
particles from bags. The three most common are shaking,
reverse air, and pulse jet.
Shaking
Shaking can be done manually, but is usually performed
mechanically in industrial-scale baghouses. Small baghouses
handling exhaust streams of less than 14.2 mVmin (500 cfm)
are frequently cleaned by hand levers. However, thorough
cleaning is rarely achieved since a great amount of effort must
be used for several minutes to remove dust cakes from the
bags. In addition, these small units do not usually have a
manometer-installed on them to give pressure drop readings
across the baghouse. These readings are used to determine
when bag cleaning is necessary. Therefore, manual shaker
baghouses are rarely used for controlling paniculate emissions.
Mechanical shaking is accomplished by using a motor that
drives a shaft to move a rod connected to the bags. It is a low
energy process that gently shakes the bags to remove deposited
particles. The shaking motion and speed depend on the ven-
dors' designs and the composition of dust deposited on the bag.
The shaking motion can be in either a horizontal or vertical
direction, with the horizontal being the most often used. The
tops of the bags in shaker baghouses are sealed or closed and
are supported by a hook or clasp. Bags are usually shaken at
the top by moving the frame where the bags are attached. This
causes the bags to ripple and release the dust (Figure 3-11).
The flow of dirty gas is stopped during the cleaning process.
Therefore, the baghouse must be compartmentalized to be
useable on a continuous basis. Shaker baghouses usually use
interior filtration (dust collected on the inside of the bags).
Shaker bags are usually 15.2 to 45.7 cm (6 to 18 in.) in
diameter and up to 12.2 m (40 ft) in length.
A typical shaker baghouse is shown in Figure 3-12. The bags
are attached to a shaft that is driven by an externally mounted
motor. The bags are shaken, and the dust falls into a hopper
Figure 3-10. Exterior filtration.
Horizontal
Vertical
Figure 3-11. Shaking.
3-6
-------
located below the bags. The duration of the cleaning cycle is
usually from 30 seconds to a few minutes.
The frequency of the cleaning depends on the type of dust,
the concentration of dust, and the pressure drop across the
baghouse. The baghouse usually has two or more compart-
ments to allow one compartment to be shut down for cleaning.
Figure 3-13 is a detailed view of the shaking mechanism.
The bags are attached in sets of two rows to mounting frames
across the width of the baghouse. A motor drives the shaking
lever, which in turn causes the frame to move and the bags to
shake.
Shaking should not be used when collecting sticky dusts. The
shaking force needed to remove sticky dust could tear or rip
the bag. Bag wear on the whole can be a problem at the bot-
tom of the bag where it is attached to the cell plate, but the
greatest wear is usually at the top of the bag where the support
loop attaches to the bag. Proper shaking frequency is also
important to prevent premature bag failure.
Reverse Air
Reverse air, the simplest cleaning mechanism, is accomplished
by stopping the flow of dirty gas into the compartment and
backwashing thejrompartment with a low pressure flow of air.
Dust is removed by merely allowing the bags to collapse, thus
causing the dust cake to break and fall into the hopper (Figure
3-14). The cleaning action is very gentle, allowing the use of
less abrasion-resistant fabrics such as fiberglass. Reverse air
cleaning is generally used with woven fabrics. Cleaning fre-
quency varies from 30 minutes to several hours, depending on
the inlet dust concentration. The cleaning duration is approx-
imately 10 to 30 seconds; the total time is 1 to 2 minutes for
the valve to open and close, and for the dust to settle.
Reverse air baghouses are usually compartmentalized to per-
mit a section to be off-line for cleaning. Although dust can be
collected on either the inside or the outside of the bag, it
usually is collected on the inside. The bag is open at the bot-
tom and sealed by a metal cap at the top. The bag contains
rings to keep it from completely collapsing during the cleaning
cycle. Complete collapse of the bag would prevent the dust
from falling into the hopper. Bags are supported by small steel
rings sewn to the inside of the bag. The rings are placed every
10 to 46 cm (4 to 18 in.) throughout the bag length,
depending on the length and diameter of the bag. Reverse air
baghouses use very large bags (as compared to shaker or pulse
jet baghouses) ranging from 20 to 46 cm (8 to 18 in.) in
diameter and from 6.1 to 12.2 cm (20 to 40 ft) in length.
Occasionally bags in shaker baghouses are as large as those in
reverse air baghouses.
Shaking mechanism
Figure 3-12. Typical shaker baghouse.
Shaking lever
Figure 3-13. Detail of a shaking
mechanism.
Backwash
Figure 3-14. Reverse air cleaning.
3-7
-------
Cleaning air is supplied by a separate fan —usually much
smaller than the main system fan since only one compartment
is cleaned at a time. A typical reverse air cleaning baghouse is
shown in Figure 3-15.
Compartments filtering
Compartment being
cleaned
Reverse air fan
Main fan
Figure 3-15. Typical reverse air baghouse.
Pulse Jet
A third commonly used bag-cleaning mechanism is pulse jet or
pressure jet cleaning. Approximately 40 to 50 percent of the
new baghouse installations in the U.S. use pulse jet cleaning.
The pulse jet cleaning mechanism uses a high pressure jet of
air to remove the dust from the bag. Bags in the baghouse
compartment are supported internally by rings or cages. Bags
are held firmly in place at the top by clasps and have an
enclosed bottom (usually a metal cap). Dust-laden gas is
filtered through the bag, depositing dust on the outside surface
of the bag. Pulse jets are used for cleaning in an exterior filtra-
tion system.
The dust cake is removed from the bag by a blast of com-
pressed air injected into the top of the bag tube. The blast of
high pressure air stops the normal flow of air through the
filter. The air blast develops into a standing or shock wave that
causes the bag to flex or expand as the shock wave travels
down the bag tube. As the bag flexes, the cake fractures and
deposited particles fall from the bag (Figure 3-16). The shock
wave travels down and back up the tube in approximately 0.5
seconds.
)\ ^ Cleaning air
Figure 3-16. Pulse jet cleaning.
3-8
-------
The blast of compressed air must be strong enough for the
shock wave to travel the length of the bag and shatter or crack
the dust cake. Pulse jet units use air supplied from a common
header that feeds into a nozzle located above each bag (Figure
3-17).
Compressed air supply
Blow pipe
Figure 3-17. Typical pulse jet baghouse with air supply.
Most pulse jet baghouses have bag tubes that are 10.2 to
16.2 cm (4 to 6 in.) in diameter. The length of the bag is
usually around 2.4 to 3.7 m (8 to 12 ft), but can be as long as
7.6 m (25 ft). Pulse jet baghouses use smaller bags than shaker
and reverse air baghouses.
Review Exercise
1. Mechanical shaking is accomplished by using a _
that drives a shaft to shake the dust-laden bags.
2. True or False? In a shaker baghouse, the flow of dirty air
into a compartment is shut down during bag cleaning.
1. motor
2. True
3-9
-------
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
The shaking motion causes the dust cake to break and fall
into the .,
Bag cleaning frequency depends on the dust type, the dust
ron'-pntration, anH fhp , across the baghouse.
Reverse air cleaning is very gentle, allowing the use of less
abrasion-resistant fabrics such as woven
( v
Cleaning air in a reverse air baghouse is usually supplied by
a „ , . ,,-
Reverse air baghouses use large bags with lengths that
range from
a. 3 to 5 feet.
b. 20 to 40 feet.
c. 5 to 10 feet.
d. 75 to 100 feet.
True or False? During reverse air cleaning, the flow of dirty
air into the compartment is stopped.
Pulse jet cleaning is accomplished by
a. shaking eSch bag in the compartment while the damper is
closed.
b. a blast of compressed air into each bag.
c. reversing the flow of air into the baghouse compartment
and gently shaking the bags.
In a pulse jet baghouse, dust is removed from the
of the bag when the bag is cleaned.
In a pulse jet baghouse, the dust collects on the outside of
the bag. Therefore, the bag must be supported, usually by
True or False? Pulse jet air is supplied from a common
header that feeds into a nozzle located above each bag.
Pulse jet baghouses use bags that are usually
a. 12 to 16 inches in diameter and 20 to 40 feet long.
b. 4 to 6 inches in diameter and 8 to 12 feet long.
c. 16 to 24 inches in diameter and 15 to 25 feet long.
True or False? In pulse jet cleaning, the flow of dirty air
into the compartment must be stopped before cleaning is
initiated.
3. hopper
4. pressure drop
5. glass (fiberglass)
6. separate fan
7. b. 20 to 40 feet.
8. True
9. b. a blast of compressed
air into each bag.
10. outside
11. a metal cage (or rings)
12. True
13. b. 4 to 6 inches in
diameter and 8 to 12
feet long.
14. False
3-10
-------
Baghouse Components
Bags
Tubular bags vary in length and diameter depending on
baghouse design and manufacturer. The length varies from 3
to 12 m (10 to 40 ft) and the diameter is usually between 10
and 46 cm (4 and 18 in.)- Bags are hung vertically in the
baghouse (Figure 3-18). Reverse air baghouses use long bags, 6
to 12 m (20 to 40 ft), with large diameters, 30 to 46 cm (12 to
18 in). Pulse jet baghouses use smaller bags, 2.4 to 3.7 m (8 to
12 ft) with diameters of 10 to 16 cm (4 to 6 in.).
Housing
Baghouses are constructed as single or compartmental units.
The single unit is generally used on small processes that are not
in continuous operation such as grinding and sanding proc-
esses. Compartmental units consist of more than one baghouse
compartment and are used in continuous operating processes
with large exhaust volumes such as electric melt steel furnaces
and industrial boilers. In both cases, the bags are housed in a
shell made of a rigid metal material. Occasionally, it is
necessary to include insulation with the shell when treating
high temperature flue gas. This is done to prevent moisture or
acid mist from condensing in the unit since acid or water could
cause corrosion and rapid deterioration of the baghouse.
Metal cap
Ann-collapse
ring
Clamp
Thimble
Figure 3-18. Typical bag for reverse
air baghouse.
Hoppers
Hoppers are used to store the collected dust temporarily before
it is disposed of in a landfill or reused in the process. Dust
should be removed as soon as possible to avoid packing that
would make removal very difficult. Hoppers are usually
designed with a 60° slope to allow dust to flow freely from the
top of the hopper to the bottom discharge opening. Some
manufacturers add devices to the hopper to promote easy and
quick discharge. These devices include strike plates, poke
holes, vibrators, and rappers. Strike plates are simply pieces of
flat steel that are bolted or welded to the center of the hopper
wall. If dust becomes stuck in the hopper, rapping the strike
plate several times with a mallet will free this material. Hopper
designs also usually include access doors or ports. Access ports
provide for easier cleaning, inspection, and maintenance of the
hopper (Figure 3-19).
Strike
Figure 3-19. Hopper.
3-11
-------
Discharge Devices
A discharge device is necessary for emptying the hopper and to
close off the unit to the atmosphere. Discharge devices can be
manual or automatic. Some discharge devices used on bag-
houses include manual slide gates, rotary airlock devices, screw
conveyors, and pneumatic conveyors. Pneumatic conveyors use
compressed air or a blower to convey dust from the hopper.
Filter Construction
Woven, felted, and membrane materials are used to make bag
filters. Woven filters are made of yarn with a definite repeated
pattern. They are used with low-energy cleaning methods such
as shaking and reverse air. Felted filters are composed of ran-
domly placed fibers compressed into a mat and attached to
some loosely woven backing material. They are usually used
with higher-energy cleaning systems such as pulse jet cleaning.
Membrane filters are made by laminating various types of
fibers together to form a membrane.
Fibers
The fibers used for fabric filters vary depending on the
industrial application to be controlled. Some fibers are made
from natural fibers such as cotton or wool. These fibers have
temperature limitations (<212°F or 100°C) and only average
abrasion resistance. Cotton is readily available — making it very
popular for low-temperature, simple applications. Wool with-
stands moisture very well and can be made into thick felts easily.
Synthetic fibers such as nylon, Orion®, and polyester have
slightly higher temperature limitations and chemical
resistances. Synthetic fibers are more expensive than natural
fibers. Polypropylene is the most inexpensive synthetic fiber
and is used in many industrial applications, such as foundries,
coal crushers, and food industries. Nylon is the most abrasive-
resistant synthetic fiber—making it useful in applications that
filter abrasive dusts. Polyester or Dacron® has good overall
qualities to resist acids, alkalines, and abrasion and is relatively
inexpensive —making it useful in many industrial processes such
as metal smelting or iron casting.
Nomex® is a registered trademark of fibers made by
DuPont. DuPont makes the fibers, but not the fabrics or bags.
Nomex® is widely used because of its relatively high
temperature resistance and its resistance to abrasion. It is used
for filtering dusts from cement coolers, asphalt batch plants,
ferroalloy furnaces, and coal dryers.
Other fibers such as Teflon® and Fiberglas® (or glass) can
be used in relatively high temperature situations. Teflon® has
very good resistance to acid attack (except fluorine) and can
withstand continuous temperatures of up to 230 °C (445 °F).
3-12
-------
Fiberglas® or glass is often used in baghouses that filter very
high temperature (up to 260 °C or 500 °F) flue gas. Glass fibers
are usually lubricated in some fashion so that they will slide
over one another without breaking or cutting during the clean-
ing cycle. Silicon graphite and Teflon are commonly used as
lubricants and will help retain the upper service-temperature
limits. Glass fibers are susceptible to breakage and require a
very gentle cleaning cycle. Both Teflon and glass have been
used to remove paniculate emissions generated from industrial
and utility coal-fired boilers.
Another material used to make bags is Gore-Tex® mem-
brane. Gore-Tex® membrane is laminated with a variety of
fibers such as Fiberglas®, polyester, and Nomex® to produce
felt fibers. Some reports have indicated very good emission
reduction, relatively low pressure drops, increased bag life, and
higher air-to-cloth ratios using this material in metal industries,
chemical industries, food industries, and coal-fired boilers.
Table 3-1 lists a number of typical fibers used for fabric
filters. The properties of the listed fibers include temperature
limits, acid and alkali resistance, and abrasion resistance.
Table 3-1. Typical fabrics used for bags.
Generic
name
Natural
fiber
cellulose
Polydefin
Natural
Fiber
protein
Polyamide
Acrylic
Polyester
Aromatic
polyamide
Fluoro-
carbon
Clau
Fiber
Cotton
Polypro-
pylene
Wool
Nylon*
Orion*
Dacron*
Nomex*
Teflon*
Fiberglas*
or glass
Maximum temperature
Continuous
°F
180
190
200-216
200-225
240
275
400
400-
450
500
•c
82
88
93-102
93-107
116
135
204
204-
232
260
Surges
•F
225
200
250
250
260
325
425
500
550
•c
107
93
121
121
127
163
218
260
288
Acid
resistance
poor
good to
excellent
very good
poor to
fair
good to
excellent
good
poor to
good
excellent
except
poor to
fluorine
fair to
good
Alkali
resistance
very good
very good
poor
good to
excellent
fair co
good
good
good to
excellent
excellent
except
poor to
trifluoride.
chlorine
and
molten
alkaline
metals
fair to
good
Flex
abrasion
resistance
very good
excellent
fair to good
excellent
good
very good
excellent
fair
fair
Sources: Bethea. 1978: EPA. 1979: Theodore and Buonicore, 1976.
3-13
-------
Bag Failure
Three conditions can shorten the operating life of a bag. They
are abrasion, high temperature, and chemical attack. The
chief design variable is the upper temperature limit of the
fabric. The process exhaust temperature will determine which
fabric material should be used for dust collection. Exhaust-gas
cooling may be feasible, but one must be careful to keep the
exhaust gas hot enough to prevent moisture or acid from con-
densing on the bags.
Another problem frequently encountered in baghouse opera-
tion is that of abrasion. Bag abrasion can result from bags rub-
bing against each other, from the type of bag cleaning used in
the baghouse, or where dust enters the bag and contacts the
fabric material. For instance, in a shaker baghouse, vigorous
shaking may cause premature bag deterioration, particularly at
the points where the bags are attached. In pulse jet units, the
continual, slight motion of the bags against the supporting
cages can also seriously affect bag life. This is the single biggest
maintenance problem associated with baghouses.
Bag failure can also occur by chemical attack to the fabric.
Changes in dust composition and exhaust gas temperatures
from industrial processes can greatly affect the bag material. If
the exhaust gas stream is lowered to its dew point or if a new
chemical species is created, the design of the baghouse (fabric
choice) may be completely inadequate. Proper fabric selection
and good process operating practices can help eliminate bag
deterioration caused by chemical attack.
Air-to-Cloth Ratios
One important baghouse design variable is air-to-cloth ratio.
Air-to-cloth (A/C) ratios describe how much dirty gas passes
through a given surface area of filter in a given time. A high
air-to-cloth ratio means a large volume of air passes through
the fabric. A low air-to-cloth ratio means a small volume of air
passes through the fabric. Air-to-cloth ratios are usually
expressed in units of (cmVs)/cm2 of cloth [(ft3/min)/ft2]. Air-
to-cloth ratios vary depending on a number of factors such as
the bag cleaning mechanism, the filter material, and the
filtered dust particles. For instance, reverse air baghouses used
for filtering flyash from boilers use glass or Fiberglas® bags.
Fiberglas® bags cannot withstand high filtering stress (high air-
to-cloth ratios). Too high an air-to-cloth ratio results in
excessive pressure drops, reduced collection efficiency, bags
becoming caked solidly with dust, and rapid bag deterioration.
Therefore, the air-to-cloth ratio must be kept low, usually less
than 1 to 3 (cmVs)/cm2
3-14
-------
On the other hand, a pulse jet baghouse used for filtering
asphalt-batch-plant dust usually uses thick felted
Nomex® bags. Since these bags are strong, they can withstand
high air-to-cloth ratios—usually between 2.5 and 7.5
(cmVs)/cm*. Typical air-to-cloth ratios for reverse air, shaker,
and pulse jet baghouses are given in Table 3-2.
Industrial Applications of Baghouses
Fabric filters have been used for paniculate emission reduction
for many industrial applications. Baghouses have been
designed to collect particles in the submicron range with
99.9*% control efficiency. They have occasionally been used to
remove particles and then recirculate the clean air back into
the plant to help supplement heating needs. Baghouses have
been used in the chemical, steel, cement, food, phar-
maceutical, metal-working, aggregate, and carbon-black
industries.
One relatively new baghouse application is filtering flyash in
both industrial and utility boilers. Here, baghouses are
becoming as popular as electrostatic precipitators for removing
99.9*% of the paniculate matter from the flue gas.
Table 3-2. Typical air-to-cloth ratios.
Bighouse
cleaning
method
Shaking
Revere* air
Pulse jet
Air-uxloth ratios
1-3 (cmVs)/cm«
0.5-2.0 (cm»/s)/cm«
2.5-7.5 (cm'/s)/cm«
2-6 (ftVmin)/ft'
1-4 (ftVmin)/ft'
5-15(ftVmin)/ft«
Note: Air-io-cloih ratios are occaiionally given as 2.0:1
instead of 2.0 (cm'/s)/cm'.
Review Exercise
1. Bag filters (bags) are made from
materials.
2- filters are made from y
repeated pattern.
.V filters are composed of
that are compressed into a mat and
woven backing material.
4. The bags in a baghouse are housed
is usually made of metal (steel).
5. Sometimes it is necessary to use
or
am and have a definite
randomly placed fibers
attached to a loosely
in a that
with the shell
to prevent moisture or acid from condensing in the baghouse.
6. The dust is temporarily stored in a
1. woven, felted
2. Woven
3. Felted
4. shell
5. insulation
6. hopper
3-15
-------
7.
8.
9.
10.
Two natural fibers used for fahrir filters are , and
Wool and cotton are inexpensive but are susceptible to
failure at
Two fabrics that are good for use in high -temperature
(>200°C) industrial processes are
a. Teflon® and Fiberglas®.
b. nylon and wool.
c. cotton and Orion®.
d. polypropylene and Dacron®.
Three conditions that shorten bag operating life are
i , and
7. wool, cotton
8. high temperatures
9. a. Teflon® and
Fiberglas®.
10. abrasion,
high temperature,
chemical attack
References
Bethea, R. M. 1978. Air Pollution Control Technology: An Engineering Analysis Point of View.
New York: Van Nostrand Reinhold Company.
Cheremisinoff, P. N. and Young, R. A., eds. 1977. Air Pollution Control and Design Hand-
book, Part 1. New York: Marcel Dekker, Inc.
Cross, F. L. and Hesketh, H. E., eds. 1975. Handbook for the Operation and Maintenance of Air
Pollution Control Equipment. Westport, CN: Technomic Publishing Co., Inc.
Environmental Protection Agency (EPA). October, 1981. APTI Course 413, Control of Particu-
late Emissions, Student Manual. EPA 450/2-80-066.
Environmental Protection Agency (EPA). February, 1980. Survey of Dry SO2 Control Systems
EPA 600/7-80-030. y
Environmental Protection Agency (EPA). 1979. Paniculate Control by Fabric Filtration on Coal-Fired
Industrial Boilers. EPA 625/2-79-021.
Environmental Protection Agency (EPA). 1976. Capital and Operating Costs of Selected Air Pol-
lution Control Systems. EPA 450/3-76-014.
Environmental Protection Agency (EPA). 1973. Air Pollution Engineering Manual. 2nd ed. AP-40.
Frederick, E. R. 1974. Some Effects of Electrostatic Charges in Fabric Filtration./. Air Poll
Control Assoc. 24:1164-1168.
Hesketh, H. E. 1979. Air Pollution Control. Ann Arbor: Ann Arbor Science Publishers Inc.
3-16
-------
Kaplan, S. M. and Felsvang, K. 1979. Spray Dryer Absorption of SO, from Industrial Boiler
Flue Gas. 86th National AICHE Meeting Paper, Houston, Texas, April, 1979.
Kraus, M. N. 1979. Baghouses: separating and collecting industrial dusts. Chem. Eng. 86:94-106.
McKenna, J. D. and Greiner, G. P. 1981. "Baghouses." Air Pollution Control Equip-
ment—Selection, Design, Operation and Maintenance, ed. by Theodore, L. and Buonicore, A. J.
Englewood Cliffs. NJ: Prentice Hall Inc.
Neveril, R. B., Price, J. U. and Engdahl, K. L. 1978. Capital and Operating Costs of Selected
Air Pollution Control Systems—I./. Air Poll. Control Assoc. 28:829-836.
Proceedings: Symposium on the use of fabric filters for the control of sub-micron particulates,
April 8-10, 1974, Boston, MA./. Air Poll. Control Assoc. 24:1139-1197, 1974.
Proceedings: The User and Fabric Filtration Equipment III, October 1-3, 1978. Niagara Falls, NY.
Air Pollution Control Association Specialty Conference.
Reigel, S. A. and Applewhite, G. D. 1980. "Operation and Maintenance of Fabric Filter Systems."
Operation and Maintenance for Air Paniculate Control Equipment, ed. by Young, R. A. and
Cross, F. L. Ann Arbor, MI: Ann Arbor Science.
Stern, A. C. ed. 1977. Air Pollution. Third Edition. Volume IV. New York: Academic Press.
Sittig, M. 1977. Particulates and Fine Dust Removal Processes and Equipment. New Jersey: Noyes
Data Corporation.
Theodore, L. and Buonicore, A. J. 1976. Industrial Air Pollution Control Equipment for Par-
ticulates. Cleveland: CRC Press.
3-17
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Lesson 4
Electrostatic Precipitators
Lesson Goal and Objectives
Goal
To familiarize you with the paniculate emission removal device
used by many industries—the electrostatic precipitator—its
operating principles, and the different designs in use.
Objectives
At the end of this lesson, you should be able to:
1. recall how particles are collected in an electrostatic
precipitator (ESP).
2. list four major components of an ESP.
3. recall the location of a hot-side precipitator.
4. recognize three types of rappers and recall their operation
principles. -
5. define resistivity and recall how it can be altered to
improve ESP performance.
6. define aspect ratio and specific collection area.
7. list three industrial applications for ESPs.
Introduction
Electrostatic precipitators (ESPs) have been used to reduce par-
ticulate emissions in many industrial applications for over fifty
years. ESPs have been designed to collect particles with
diameters of from 0.1 /on to 10.0 /un; collection efficiency is
considered high, sometimes exceeding 99%. The ability of
ESPs to handle large exhaust gas volumes at temperatures
between 175 and 700°C (350 to 1300°F) makes them very
attractive to many industries. This ability is particularly
desirable for cement kiln emission reduction and for control of
emissions from basic oxygen steel furnaces in the steel industry
where flue gas enters the precipitators at temperatures greater
than 350°C (660°F). ESPs are commonly used for paniculate
emission reduction for black liquor operations in the pulp and
paper industry, for blast furnaces and sintering operations in
the steel industry, and for fly ash control from industrial and
utility boilers.
4-1
-------
ESP Description
An electrostatic precipitator, depicted in Figure 4-1, contains six
essential components. Each of these components will be discussed
in detail in this lesson:
• discharge electrode • rapper
• collection electrode • hopper
• electrical system • shell
The discharge electrode is usually a small-diameter metal wire.
This electrode is used to ionize the gas (that charges the particles)
and to create a strong electric field. The collection electrode is
either a tube or a flat plate with an opposite charge relative to
that of the discharge electrode. The collection electrode collects
charged particles. The electrical system consists of high voltage
components used to control the strength of the electric field
between the discharge and collection electrodes.
The rapper imparts a vibration or shock to the electrodes,
removing the collected dust. Rappers remove dust that has
accumulated on both collection electrodes and discharge elec-
trodes. Occasionally, water sprays are used to remove dust from
collection electrodes. These precipitators are called water-walled
ESPs. Hoppers are located at the bottom of the precipitator.
Hoppers are used to collect and temporarily store the dust
removed during the rapping process. The shell structure encloses
the electro_des and supports the entire ESP.
ESPs that use plates as collection electrodes are called plate
precipitators. ESPs that use tubes for collection electrodes are
called tubular precipitators.
Rapper
. Electrical
system
Shell
Discharge
electrode
Collection
electrode
Hopper
Figure 4-1. Typical electrostatic precipitator.
4-2
-------
Plate ESPs
Plate electrostatic precipitators are used more often than
tubular precipitators. A high voltage creates an intense electric
field which charges the particles as the flue gas passes through
the precipitator. Dirty gas flows into a chamber consisting of a
series of discharge electrodes—wires equally spaced along the
center line of adjacent plates (Figure 4-2). Discharge electrodes
are approximately 0.13 to 0.38 cm (0.05 to 0.15 in.) in
diameter. Collection plates are usually between 6 and 12 m (20
and 40 ft) high. The plates are usually spaced from 20 to 30
cm (8 to 12 in.) apart.
Tubular ESPs
Tubular precipitators consist of cylindrical collection electrodes
(tubes) with discharge electrodes (wires) located in the center of
the cylinder (Figure 4-3). Dirty gas flows into the tubes where
particles are charged. Charged particles are collected on the
inside walls of the tubes. Collected dust or liquid is removed by
washing the tubes with water sprays located directly above the
tubes. Tubular precipitators are generally used for collecting
mists or fogs. Tube diameters typically vary from 0.15 to 0.31
m (0.5 to 1 ft), with length usually varying from 1.85 to 4.0 m
(6 to 15 ft)L
Hot-side ESPs
Hot-side ESPs are electrostatic precipitators placed in locations
where the flue gas temperature is relatively high. They can be
either tubular or plate. Hot-side precipitators are used in high
temperature applications such as in the collection of utility and
industrial boiler fly ash. A hot-side precipitator is located
before the combustion air preheater in a boiler, whereas a
cold-side precipitator is located after the air preheater. The
flue gas temperature here for hot-side precipitators is in the
range of 320 to 420 °C (608 to 790 °F). The use of hot-side pre-
cipitators also helps reduce corrosion and hopper plugging.
However, hot-side precipitators have some disadvantages. Since
the temperature of the flue gas is higher, the volume of gas to
be treated in the ESP is larger. Consequently, the overall size
of the precipitator will be larger. Another major disadvantage
includes structural and mechanical problems that occur in the
precipitator shell and support structure. Structural distortions
stem mainly from differences in thermal expansion between the
shell and the support structure.
—' electrode
Figure 4-2. Gas flow through a
plate precipitator.
Collection
electrodes
Figure 4-3. Gas flow through a
tubular precipitator.
4-3
-------
Particle Collection
Charging the Particles in the Precipitator
Since the majority of precipitators have plates as collection
electrodes, plate ESPs will be used for this discussion. Particles
suspended in flue gas are charged as they pass through elec-
trostatic precipitators. A high-voltage, pulsating, direct current
is applied to an electrode system consisting of a small diameter
discharge electrode and a collection electrode. The discharge
electrode is usually negatively charged. The collecting plate is
usually grounded. The applied voltage is increased until it pro-
duces a corona discharge which can be seen as a luminous blue
glow around the discharge electrode. The corona causes gas
molecules to ionize. The negative gas ions that are produced
migrate toward the grounded collection electrode. The negative
gas ions bombard the particles suspended in the flue gas
stream, imparting a negative charge to them. Negatively
charged particles then migrate to the collection electrode and
are collected (Figure 4-4).
\ ' =
\ •• *
. •--•%-& i
Gas
ions /"\
-0 !
^•".-0 5
Charged
particles
Figure 4-4. Particle charging.
Discharging the Particles at the Collection Electrode
When a charged particle reaches the grounded collection elec-
trode, the charge on the particle is only partially discharged.
The charge is slowly leaked to the grounded collection plate. A
portion of the charge is retained and contributes to the inter-
molecular cohesive and adhesive forces that hold the particles
4-4
-------
onto the plates. Particles are held to the plates by adhesive
forces. Newly arrived particles are held to the collected par-
ticles by cohesive forces. The dust layer is allowed to build up
on the plate to a thickness of 0.08 to 1.27 cm (O.OS to 0.5 in.),
and then the rapping cycle is initiated. Rapping cycles are
initiated on a set-timed cycle.
Rapping the Particles into the Hopper
Periodically rapping the precipitator plates is necessary to
maintain the continuous flue gas cleaning process. The plates
are rapped while the ESP is on-line; the gas flow continues
through the precipitator and the applied voltage remains con-
stant. In wet-walled precipitators, tubes are cleaned by water
sprays. In most other precipitators, deposited dry particles are
dislodged by sending mechanical impulses or vibrations to the
plates. Plates are rapped when the accumulated dust layer is
relatively thick (0.08 to 1.27 cm). This allows the dust layer to
fall off the plates as large aggregate sheets and helps eliminate
dust reentrainment. Most precipitators have adjustable rappers
so that rapper intensity and frequency can be changed
according to dust concentration in the flue gas.
Dislodged dust falls from the plates into the hopper. The
hopper is a single collection bin with sides sloping approx-
imately 60° to allow dust to flow freely from the top of the
hopper to the discharge opening. Dust should be removed as
soon as possible to avoid (dust) packing. Packed dust is very
difficult to remove. Most hoppers are emptied by some type of
rotary discharge device, screw conveyor, or pneumatic con-
veyor. A typical hopper with a screw conveyor is shown in
Figure 4-5.
Figure 4-5. Hopper and screw
conveyor.
Review Exercise
1.
2.
3.
Tn an HeofXWW'O preripitatnr, the ... electrode is
normally a small-diameter metal wire.
The charged particles migrate to and are collected on
the
are used to remove dust from both the collection
electrodes and the discharge electrodes.
1. discharge
2. collection electrode
3. Rappers
4-5
-------
In a single stage, high voltage ESP, the applied voltage is
increased until it produces a(an)
a. extremely high alternating current for particle charging.
b. corona discharge which can be seen as a blue glow around
the discharge electrode.
c. corona spark that occurs at the collection electrode.
5. True or False? Particles are usually charged by negative gas
ions that are migrating toward the collection electrode.
4. b. corona discharge
which can be seen as a
blue glow around the
discharge electrode.
6. As dust particles reach the grounded collection electrode,
their charge is
a. immediately transferred to the collection plate.
b. slowly leaked to the grounded collection electrode.
c. cancelled out by the strong electric field.
5. True
Particles are held onto the collection plates by
a. a strong electric field force.
b. a high pulsating direct current.
c. intermolecular cohesive and adhesive forces.
d. electric sponsors.
6. b. slowly leaked to the
grounded collection
electrode.
8. True or False? During the rapping process, the voltage is
turned down to about 50% of the normal operating voltage
to allow the rapped particles to fall freely into the hopper.
7. c. intermolecular
cohesive and adhesive
forces.
8. False
Precipitator Components
Discharge Electrodes
The discharge electrodes in many U.S. precipitator designs are
thin round wires varying from 0.13 to 0.38 cm (0.05 to 0.15
in.) in diameter. Most common designs use wires approximately
0.25 cm (0.1 in.) in diameter. The discharge electrodes consist
of vertically hung wires supported at the top and held taut and
plumb by a weight at the bottom. The wires are usually made
from high carbon steel, but have also been constructed of
stainless steel, copper, titanium alloy, Inconel®, and alumi-
num. The weights are made of cast iron and are generally 11.4
kg (25 Ibs) or more.
Discharge wires are usually supported to help eliminate
breakage from mechanical fatigue. The wires move under the
4-6
-------
influence of aerodynamic and electrical forces and are subject
to mechanical stress. The weights at the bottom of the wire are
attached to guide frames to help maintain wire alignment.
Attaching the weights will prevent them from falling into the
hopper in the event that the wire breaks (Figure 4-6). The bot-
tom and top of each wire are usually covered with a shroud of
steel tubing. The shrouds help minimize sparking and conse-
quent metal errosion by sparks at these points on the wire.
The size and shape of the electrodes are governed by the
mechanical requirements of the system. Most U.S. designs have
traditionally used thin, round wires for corona generation.
Some designers have also used twisted wire, square wire,
barbed wire, or other configurations. Some of these are
illustrated in Figure 4-7.
Upper guide frame
Figure 4-7. Typical discharge
electrodes.
European precipitator manufacturers favor the use of rigid
support frames for discharge electrodes. The frames may con-
sist of coiled spring wires, serrated strips, or needle points
mounted on a supporting strip. An example is shown in Figure
4-8. The purpose of the rigid frame is to eliminate the possible
swinging of the discharge wires. These designs have been used
as successfully as the U.S. wire designs. Many U.S. vendors are
now using rigid frame discharge electrodes.
Weight
Figure 4-6. Guide frames and
jhroudi for discharge
Figure 4-8. Rigid frame discharge
electrode design.
4-7
-------
Collection Electrodes
Most U.S. precipitators use plate collection electrodes because
they treat large gas volumes and usually have high collection
efficiency. The plates are generally made of carbon steel.
However, plates are occasionally made of stainless steel or an
alloy steel for special flue gas stream conditions. The plates
range from 0.05 to 0.2 cm (0.02 to 0.08 in.) in thickness.
Plates are spaced from 20 to 30 cm apart (8 to 12 in.). Normal
spacing for high efficiency ESPs is 20 to 23 cm (8 to 9 in.).
Plates are usually between 6 and 12 m (20 to 40 ft) high.
Collection plates are constructed in a number of shapes as
shown in Figure 4-9. These plates are solid-sheets that are
sometimes reinforced with structural stiffeners to increase plate
strength. In some cases, the stiffeners act as baffles to help
reduce particle reentrainment losses.
VVVTL
' K, ' : ' 5-'
\I
iii
Figure 4-9. Typical collection
plates.
Shell
The shell structure encloses the electrodes and supports the
precipitator components in a rigid frame to maintain proper
electrode alignment and configuration (Figure 4-10). The sup-
port structure is especially critical for hot-side precipitators
because precipitator components can expand and contract
when the temperature differences between the ESP (400 °C)
and the ambient atmosphere (20 °C) are large. Excessive
temperature stresses can literally tear the shell and hopper
joints and welds apart.
Collecting plates and discharge electrodes are normally sup-
ported from the top so that the elements hang vertically
because of the force of gravity. This allows the elements to
expand or contract with temperature changes without binding
or distorting.
Shells, hoppers, and connecting flues should be covered with
insulation to conserve heat, and to prevent corrosion due to
water vapor and acid condensation on internal precipitator
components. Insulation will also help minimize temperature
differential stresses, especially on hot-side precipitators. Ash
hoppers should be insulated and heated. Cold fly ash has a
tendency to cake; therefore, it is extremely difficult to remove.
The precipitator should also be designed to provide easy
access to strategic points of the collector for internal inspection
of electrode alignment, for maintenance, and for cleaning elec-
trodes, hoppers, and connecting flues during outages.
4-8
-------
Insulation
Rappers
Dust that has accumulated on collection and discharge elec-
trodes is removed by rapping. Dust deposits are generally
dislodged by mechanical impulses or vibrations imparted to the
electrodes. A rapping system is designed so that rapping inten-
sity and frequency can be adjusted for varying operational con-
ditions. Once the operating conditions are set, the system -must
be capable of maintaining uniform rapping for a long time.
4-9
-------
Hammer
Collection plates are rapped by a number of methods. One
rapper system uses hammers mounted on a rotating shaft as
shown in Figure 4-11. As the shaft rotates, the hammers drop
(by gravity) and strike anvils that are attached to the collecting
plates. Rappers can be mounted on the top or on the side of
collection plates.
Rapping intensity is controlled by the weight of the hammers
and the length of the hammer mounting arm. The frequency
of rapping can be changed by adjusting the speed of the
rotating shafts. Thus, rapping intensity and frequency can be
adjusted for the varying dust concentration of the flue gas.
Figure 4-11. Typical hammer/anvil rappers for
collection plates.
Magnetic Impulse
Another rapping system used for many U.S. designs consists of
magnetic impulse rappers to remove accumulated dust layers
from collection plates. A magnetic impulse rapper has a steel
plunger that is raised by a current pulse in a coil. The raised
plunger then drops back, due to gravity, striking a rod con-
nected to a number of plates within the precipitator as shown
in Figure 4-12. Rapper frequency and intensity are easily
regulated by an electrical control system. The frequency may
be one rap every few minutes to one rap an hour with an
intensity of 10 to 24 g's (Katz, 1979). Magnetic impulse rappers
usually operate more frequently but with less intensity than do
rotating hammer/anvil rappers.
Figure 4-12. Typical magnetic
impulse rappers for
collection plates.
4-10
-------
Electric Vibrator
The discharge or corona electrodes must also be rapped to pre-
vent excessive dust deposit buildup that will interfere with
corona generation. This is usually accomplished by the use of
air or electric vibrators that gently vibrate the discharge wires.
Vibrators are usually mounted externally on precipitator roofs
and are connected by rods to the high tension frames that sup-
port the corona electrodes (Figure 4-13). An insulator, located
above the rod, electrically insulates the rapper while
mechanically transmitting the rapping force.
Tumbling Hammers for Rigid Frame Discharge Electrodes
Rigid frame discharge electrodes are rapped by tumbling ham-
mers. The tumbling hammers operate similarly to the hammers
used to remove dust from collection electrodes. The hammers
are arranged on a horizontal shaft. As the shaft rotates, the
hammers hit an impact beam which transfers the shock, or
vibration, to the center tubes on the discharge system, causing
dust to be removed (Figure 4-14).
Transformer-Rectifier Sets
High voltage equipment controls the strength of the electric
field generated between the discharge and collection electrodes.
This is accomplished by using transformer-rectifier (T-R) sets.
The transformer steps up the voltage from 400 volts to approx-
imately 20,000 to 70,000 volts. This high voltage helps to
increase particle movement to the collection plates. The rec-
tifier converts alternating current to direct current. Direct (or
undirectional current) is required for electrical precipitation.
Most modern precipitators use solid-state silicon rectifiers and
oil or askerel-filled high voltage transformers.
High voltage
frame
Impact
beam
Tumbling
hammers
Rapper
Rapper
insulator
Wire
support
channel
Figure 4-13. Typical vibrator
rappers uied for
discharge electrodes.
_ Discharge
wire
Center
tube
Figure 4-14. Tumbling hammers for
rigid frame discharge
electrodes.
4-11
-------
Review Exercise
1.
2.
3.
4.
5.
6.
The discharge electrodes in most U.S. precipitator designs are
a. thin plates usually between 20 and 50 feet high.
b. thin round wires varying from 0.05 to 0.15 inch in
diameter.
c. rigid frames.
Collection plates are usually spaced
a. more than 15 inches apart.
b. from 8 to 12 inches apart.
c. less than 4 inches apart.
Two rappers used for removing accumulated dust from
collection electrodes are
a. electric vibrator rappers.
b. pneumatic rappers.
c. magnetic impulse rappers.
d. hammer and anvil rappers.
True or False? Occasionally, discharge electrodes must be
rapped to prevent excessive dust deposits on them that inter-
fere with corona generation.
The transformer-rectifier sets are used
a. to step down the voltage and convert alternating current to
direct current.
b. to step up the voltage to approximately 20,000 to 70,000
volts and to convert alternating current to direct current.
c. to step up the voltage to approximately 70,000 volts and to
convert direct current to alternating current.
True or False? Electrostatic precipitators cannot be used for
collecting dust from cement kilns or basic oxygen steel fur-
naces because the flue gas temperatures are too high.
1. b. thin round wires
varying from 0.05 to
0.15 inch in diameter.
2. b. from 8 to 12 inches
apart.
3. c. magnetic impulse
rappers.
d. hammer and anvil
rappers.
4. True
5. b. to step up the voltage
to approximately 20,000
to 70,000 volts and to
convert alternating cur-
rent to direct current.
6. False
4-12
-------
ESP Operation
ESPs have been used in many industrial applications. The
design of the ESP depends on various process variables such as
flue gas temperature and flow rate, dust concentration, and
the physical and chemical properties of the dust.
Resistivity
Particle resistivity is a condition of the particle in the flue gas
that can drastically affect ESP collection efficiency. Resistivity
describes the resistance of the collected dust layer (on the
plates) to the flow of electric current. Particles that have high
resistivity are more difficult to collect than those having normal
resistivity. This is because the collected dust layer tends to
break down the flow of electric current from the discharge
electrode to the collection electrode. Particles that have high
resistivity do not leak their charge to ground upon arrival at
the collection plate. Consequently, ESP performance is
reduced. High resistivity problems occur most frequently when
low sulfur coal is burned in boilers. The collection efficiency of
some ESPs has been reduced as much as 50% due to resistivity
problems (White, 1974).
High resistivity, can be reduced by adjusting the temperature
and moisture content of the flue gas flowing into the ESP. Par-
ticle resistivity can be decreased by increasing the gas
temperature above 260°C (500°F) or by reducing it below
150°C (300 °F). Hot-side precipitators have frequently been used
to combat resistivity problems, where the flue gas temperature
into the ESP is greater than 260 °G. However, it has been
reported that the efficiency of hot-side ESPs is quite sensitive to
the composition of fly ash, and since the composition of fly ash
is highly variable, reliable operation can be difficult.
Increasing the moisture content of the flue gas also lowers
resistivity. This can be accomplished by spraying water or inject-
ing steam into the duct work preceding the ESP. In both
temperature adjustment and moisture conditioning, the flue gas
must be above the dew point to prevent corrosion problems to
the precipitator.
Other conditioning agents such as sulfuric acid, sulfur triox-
ide, ammonia, sodium chloride, and soda ash have also been
used to reduce panicle resistivity (White, 1974). For coal fly ash,
the resistivity can be lowered by injecting approximately 10 to 30
ppm sulfur trioxide into the flue gas ducts preceding the ESP.
Specific Collection Area
The specific collection area (SCA) is defined as the ratio of the
collection surface area to the gas flow rate into the ESP.
Increasing the surface area for a given flue gas flow rate will
4-13
-------
generally increase the collection efficiency of the precipitator.
Typical designs use an SCA of 20 to 25 m* of collecting surface
for every 1000 mVhr gas flow rate (350 to 400 ft* per 1000
acfm). The general range of SCA is between 11 and 45 m*
plate surface area per 1000 mVhr gas flow rate (200 to 800 ft*
per 1000 acfm).
Aspect Ratio
Large ESPs typically use plates that are 9.2 m (30 ft) high. The
length of the plates varies, but generally, the longer the plates
are, the better the collection efficiency will be. The aspect
ratio is defined as the ratio of the total length to height of col-
lector surface. The aspect ratio for ESPs can range from 0.5 to
2.0. For 99.5* percent collection efficiency, the precipitator
design should have an aspect ratio of greater than 1.0.
Therefore, if the plate is 9.2 m high, the length should be at
least 9.2 m.
ESPs have been used in many industries to reduce par-
ticulate emissions. ESPs have been installed on over a thousand
power plants throughout the U.S. to control fly ash emissions.
These units have been designed in some cases to collect
micron-sized particles and have collection efficiencies exceeding
99%. ESPs are generally more efficient in collecting very small
particles than cyclones and scrubbers. ESPs are relatively inex-
pensive to operate compared to baghouses and scrubbers since
the pressure drops across ESPs are low. ESPs also are very
useful for filtering high-temperature flue gas, such as in a
power plant or cement kiln.
Review Exercise
1. When the collected dust layer resists the flow of electric
current, the condition is called
a. normal resistivity.
b. high resistivity.
2. True or False? A change in particle resistivity can affect
ESP performance.
1. b. high resistivity.
3. High resistivity can be reduced by
a. burning low sulfur coal.
b. increasing the temperature above 260°C.
c. spraying water or injecting steam into the duct work that
precedes the ESP.
d. all the above
e. b and c only
2. True
3. e. b and c only
4-14
-------
4. Increasing the collection surface area for a given flue gas
flow rate will generally increase/decrease the collection effi-
ciency of the precipitator.
5. The ratio of the total length of a collection surface to its
height is called the
a. specific collection area.
b. corona discharge ratio.
c. acfm.
d. aspect ratio.
e. none of the above
4. increase
6. The
for ESPs can range from 0.5 to 2.0.
Collection efficiency is generally better when this is greater
than
5. d. aspect ratio.
6. aspect ratio, 1.0
References
Anderson,-E. 1924. Report, Western Precipitator Co., Los Angeles, CA, 1919. Trans. Amer. Inst.
Chem. Eng. 16:69.
Bethea, R. M. 1978. Air Pollution Control Technology—an Engineering Analysis Point of View.
New York: Van Nostrand Reinhold Company.
Cheremisinoff, P. N. and Young, R. A., eds. 1977. Air Pollution Control and Design Handbook
Part 1. New York: Marcel Dekker, Inc.
Cross, F. L. and Hesketh, H. E., eds. 1975. Handbook for the Operation and Maintenance of Air
Pollution Control Equipment. Westport, Conn.: Technomic Publishing Co., Inc.
Deutsch, W. 1922. Ann. Phys. (Leipzig) 68:335.
Environmental Protection Agency (EPA). 1981. APTI Course 413 Control of Paniculate Emissions-
Student Manual. EPA 450/2-80-066.
Environmental Protection Agency (EPA). 1979. Paniculate Control by Fabric Filtration on Coal
Fired Industrial Boilers. EPA 625/2-79-021.
Environmental Protection Agency (EPA). 1976. Capital and Operating Costs of Selected Air
Pollution Control Systems. EPA 450/3-76-014.
Environmental Protection Agency (EPA). 1973. Air Pollution Engineering Manual. 2nd ed. AP-40.
Hesketh, H. E. 1979. Air Pollution Control. Ann Arbor: Ann Arbor Science Publishers, Inc.
Katz, J. 1979. The An of Electrostatic Precipitators. Munhall, Pennsylvania: Precipitator
Technology, Inc.
Rose, H. E. and Wood, A. J. 1956. An Introduction to Electrostatic Precipitation in Theory and
Practice. London: Constable and Company.
4-15
-------
Schmidt, W. A. 1949. Ind. and Eng. Chem. 41:2428.
Sittig, M. 1977. Particulates and Fine Dust Removal Processes and Equipment. New Jersey: Noyes
Data Corporation.
Stern, A. C. ed. 1977. Air Pollution. Third Edition. Volume IV. New York: Academic Press.
Theodore, L. and Buonicore, A. J. 1976. Industrial Air Pollution Control Equipment for Particu-
lates. Cleveland: CRC Press.
White, H. J. 1977. Electrostatic Precipitation of Fly Ash. APCA Reprint Series./, of Air Poll.
Control Assoc.
White, H. J. 1974. Resistivity Problems in Electrostatic Precipitation,/, of Air Poll. Control
Assoc. 24:315-338.
White, H. J. 1963. Industrial Electrostatic Precipitation. Reading, Massachusetts: Addison-Wesley.
4-16
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Lesson 5
Wet Collectors
Lesson Goal and Objectives
Goal
The purpose of this lesson is to familiarize you with the con-
struction and operation of wet collectors—control devices used
to remove paniculate matter and pollutant gases from exhaust
gas streams.
Objectives
At the end of this lesson, you should be able to:
1. describe the principles involved in the collection of par-
ticles by using water droplets.
2. list at least three factors that affect the absorption of
gases in liquids.
3. list at_leastjtwo methods in which power can be applied
to collect paniculate matter in a wet scrubber.
4. describe the construction of a spray tower and its use in
collecting paniculate matter.
5. explain what happens in a venturi throat.
6. describe the design of a packed tower and its use in col-
lecting gaseous pollutants.
7. list three advantages of using wet collectors for pollu-
tion control.
8. list three disadvantages of using wet collectors for
pollution control.
Introduction
Wet collectors provide a versatile means of removing both par-
ticles and pollutant gases from the exhaust stream of many
industrial processes. These devices use water to make small,
hard-to-collect panicles easier to collect by incorporating them
in larger water droplets. Gases can be absorbed by virtue of
their solubility in water or by adding chemicals to the water.
Wet collectors can be constructed in all sizes. They can be
small enough to accommodate low volumetric gas flow rates of
small chemical processes, or they can be designed large enough
to remove gases such as sulfur dioxide from large coal fired
power boilers.
5-1
-------
Wet collectors can achieve a wide range of efficiencies for
the removal of either paniculate matter or gases. A multitude
of different types of scrubbers are commercially available. For
removing particles, a very important design parameter is the
power input into the scrubber, or pressure drop across the
scrubber. Generally, the more power you use to run a wet col-
lector, the more particles you can remove. On the other hand,
wet scrubbers constructed to remove pollutant gases such as
SO2, NO, or HC1 depend more heavily on mechanical and
chemical engineering design, and not so much on the pressure
drop across the scrubber.
The versatility of scrubbers does not come without problems.
Requirements for higher efficiencies for either the removal of
gases or particles lead to higher operating costs. By-products
are hard to recover and an air pollution problem can quickly
be transformed into a water pollution problem.
For paniculate matter removal, wet collectors are more
effective than settling chambers or cyclones, but generally less
efficient than baghouses or electrostatic precipitators unless
operated at high pressure drops.
Wet scrubbers provide an alternative to incineration, adsorp-
tion or condensation devices, if the pollutant gases are soluble
or can be made to react with chemicals in the scrubbing liquid.
Principles of Operation
Collection of Particles
There are two principle wet scrubber collection mechanisms.
The first is inertial impaction. When droplets are injected
into a gas stream, moving panicles cannot always avoid hitting
them. Because of their inertia, the particles can't follow the
streamlines around the droplets (Figure 5-1) and as a result,
impact into the droplet. The particles become trapped in the
larger drops which allows them to be more easily collected.
Very small particles can also be collected by the diffusion
mechanism. Panicles are continually bombarded by gas
molecules as they move in a gas stream. This bombardment
can cause them to first move one way and then another in a
random manner (diffuse through the gas). This random motion
can eventually cause the particles to bump into a water droplet
and be collected (Figure 5-2).
Wet collector systems must provide for two operations for
pollutant removal. One is the process of bringing the pollutant
(either particles or pollutant gases) into contact with the liquid.
The other operation is to remove the liquid from the gas
stream (Figure 5-3). The removal of the liquid may at first
appear relatively simple, but small water droplets can be dif-
ficult to separate from the gas.
Figure 5-1. Inertial impaction.
Figure 5-2. Diffusion.
Figure 5-3. Contact and separation.
5-2
-------
Many mathematical expressions have been developed to help
design and describe the performance of scrubbing systems. For
particle collection, one basic concept has been widely used.
This is the concept of contact power, and states:
"When compared at the same power consumption,
all scrubbers give substantially the same degree of
collection of a given dispersed dust regardless of the
mechanism involved and regardless of whether the
pressure drop is obtained by high gas flow rates or
high water flow rates."*
In other words, you pay for what you get. As more power is
applied, more particles are removed, assuming that the power
is applied effectively. There are, however, several exceptions to
this generalization.
Collection of Gases
Pollutant gases such as SO, or HC1 move about because of
their thermal activity and because they are bombarded by
other molecules. When a molecule such as HC1 diffuses over to
a water droplet or to a liquid surface, it can become absorbed
or dissolve in the liquid. A system designed to remove gases by
the absorption process must provide an absorbent (the liquid)
which will readily incorporate the absorbate (the gas, HC1).
In absorption-systems, the gases and liquid are contacted by
slow, turbulent mixing. This is important for several reasons.
First, in most absorption systems, the conditions are selected
so that the process is gas phase controlled. This means that the
rate of removal depends upon how fast it takes the pollutant
gas molecule to reach the absorbing liquid—not on how long it
takes for it to be absorbed (since this is chosen to be rapid).
The turbulent mixing helps contact the gas molecules with the
liquid.
Secondly, gases are absorbed better in clean liquid. If,
through turbulent mixing, fresh liquid contacts pollutant gases,
absorption is improved.
And thirdly, slow mixing allows greater time for diffusion to
occur—for the gases to migrate to liquid droplets or to liquid
films.
A number of process variables are also important for gas
absorption. For example, as shown in Figure 5-4, the gas
solubility will increase if the pressure is increased. However, the
gas solubility decreases as temperature is increased. Chemicals
can also be added to the liquid to react with the gases and
remove them. For example, many of the flue gas desulfuriza-
tion (FGD) systems use lime or limestone to react with SOj to
form sulfates which are then removed.
Pressure—
Temperature—
Figure 5-4. Process variables.
'Reference: Lapple, C. E. and Kamack, H. J. 1955. Perfor-
mance of Wet Dust Scrubbers. Chem. Eng. Prog. 51:110-121.
5-3
-------
The mathematical expressions developed to describe the
removal of pollutant gases by liquid scrubbing are basically the
chemical engineering expression of gas-liquid equilibrium.
Temperatures, pressures, solubility, and types of reactants (if
any) must be specified in order to calculate removal effec-
tiveness using these expressions.
Review Exercise
1. Wet collectors are pollution control devices that use a liquid
to remove or from an exhaust gas
stream.
2. True or False? Wet scrubbers can be designed for small
locations.
1. particles
pollutant gases
3. Wet scrubbers designed to remove pollutant gases depend
mostly on
a. mechanical and chemical engineering design.
b. hopper design.
c. power input.
d. body-diameter.
2. True
4. List at least three problems associated with wet scrubbers.
3. a. mechanical and
chemical engineering
design.
5. Wet collectors use large/small water droplets to capture
large/small dust particles.
4. • by-products hard to
recover
• water pollution
problems
• high operating costs
for high efficiencies
6. Inertial impaction is the
a. primary mechanism for collecting gases in wet scrubbers.
b. process of random motion of molecules in a gas.
c. primary mechanism for collecting particles in wet
scrubbers.
5. large, small
6. c. primary mechanism
for collecting particles
in wet scrubbers.
5-4
-------
7.
8.
9.
10.
Diffusion is a physical process
a. in which small particles move randomly to bump into a
target.
b. in which gas molecules will move to a liquid surface.
c. resulting from the collision caused by the thermal motion
of molecules.
d. all the above
The concept of contact power states that
a. as more hardware is added to a scrubber, the efficiency
will increase.
b. the greater the solubility of a pollutant gas, the greater
the removal efficiency.
c. as more power is applied to a scrubber, more SOX will be
removed.
d. as more power is applied to a wet collector, more particles
will be removed.
Scrubbers designed to remove gases generally use liquids in
which the gases are
a. insoluble.
b. very soluble.
As the temperature of a liquid increases, gas solubility
increases/decreases?
7. all the above
8. d. as more power is
applied to a wet col-
lector, more particles
will be removed.
9. b. very soluble.
10. decreases
Wet Collector Devices
Wet collectors are constructed in all shapes and sizes. They can
vary from a simple chamber fitted with spray nozzles to com-
plicated systems using baffles, motors, sprays, and other hard-
ware. One classification system categorizes wet collectors by the
way in which contact is made between the gas stream and the
water. For example, spray scrubbers send water at high
pressure through nozzles to generate the droplets that bombard
incoming dust particles. Here, the energy is applied to force
water through the nozzles.
Four categories can be used to classify wet collectors in this
manner (Figure 5-5). In removing pollutants, wet collectors can
use energy from the liquid stream, the gas stream, a
mechanically driven rotor, or a combination of these methods.
Liquid
stream
Combination
Figure 5-5. Energy sources.
5-5
-------
In this section, examples of scrubbers in each of these
categories will be discussed (Table 5-1).
Table 5-1. Scrubber categories.
Energy source
Device
Spray scrubbers
Venturi scrubben
Orifice scrubbers
Vertical spray rotor
Moving bed scrubbers
Packed towers
The components used to construct wet scrubbers are limited
in number. The designer of scrubbing systems may utilize only
one or several of the design features listed below:
Spray nozzles
Venturi throats
Impingement surfaces
• plates
• baffles
• packing
Spray inducing orifices
Cyclonic openings
Mechanically driven rotors
The combinations possible among these items have led to
many commercially available scrubbers.
Spray Towers
Spray towers are simple in design and construction. They
generally consist of a cylindrical or rectangular chamber with
one or more levels of spray nozzles as shown in Figure 5-6. The
nozzles produce droplets that fan out into the chamber to
impact the paniculate matter or to absorb gases contained in
the polluted gas stream. The flow is generally countercurrent,
as shown in this figure. That is, the direction of gas flow is
opposite to the direction of liquid flow.
Figure 5-6. Spray tower.
5-6
-------
Several different types of spray nozzles can be used (Figure
5-7). In the impingement nozzle, high pressure liquid strikes a
plate or pin to give a spray of uniformly-sized droplets. The
helically shaped solid cone nozzle can provide a wide spray and
be less subject to plugging than the impingement nozzle. The
principal energy requirements of spray towers come from the
need of forcing liquid through the nozzles at high pressure so
that the fine droplets will be produced.
Spray towers can be used effectively for gas absorption if the
contaminant gas is highly soluble. For example, spray towers
are used to remove HC1 gas from the tail gas exhaust from the
manufacture of hydrochloric acid. Spray towers are adequate
for the collection of coarse particles larger than 10 to 25 /xm in
diameter. Smaller size particles can also be collected if the
liquid inlet nozzle pressure is increased.
Venturi Scrubbers
Venturi scrubbers provide a means of using the energy of a
moving gas stream to atomize liquid into droplets. In the ven-
turi, gas is forced through a constriction which narrows the gas
flow (Figure 5-8). In order to get through this narrow part of
the device, the gas must move at a relatively higher velocity. If
water is introduced into this narrow throat, the high velocity
gas will shear it into droplets. These droplets then serve as
targets for particle collection.
Impingement
Helical
Figure 5-7. Spray nozzles.
Figure 5-8. Venturi scrubber.
5-7
-------
Venturi scrubbers can have the highest collection efficiencies
for particles of any wet scrubbing system. The gas pressure
drop across the device can range from 2 to 40 cm of water (5
to 100 in. of water) in commercial systems, although pressure
drops of 8 to 24 cm of water (20 to 60 in. of water) are more
common. The velocity of the gas in the throat can range from
30 to 250 m/s (100 to 800 ft/sec) as a result of the pressure
difference across the throat. Operating at a high pressure drop,
a venturi can remove particles less than 0.5 fan in diameter.
The venturi is merely a system designed to use the energy in
the gas stream to atomize liquid. The water droplets incor-
porating paniculate matter must be collected after leaving the
venturi. Large bottom inlet cyclonic separators are often used
for this purpose (Figure 5-9).
Mist eliminators are often included in wet scrubbing systems
to remove liquid droplets before the gas enters the atmosphere.
Figure 5-10 shows examples of wire mesh and chevron mist
eliminators. Droplets impinge upon their surfaces and drain off
(due to gravity) for collection.
Figure 5-9. Venturi scrubber with
bottom inlet cyclonic
separator.
Wire mesh
Figure 5-10. Mist eliminators.
Venturi scrubbers can also be used for the absorption of
pollutant gases. The short time that the gas is in contact with
the liquid, however, limits the collection efficiency of the
device for gases. For the collection of paniculate matter,
increased energy input increases collection efficiency in the
venturi. For gas absorption, almost the direct opposite holds
true. An increase in pressure drop decreases residence time,
usually allowing less time for absorption. Venturi scrubbers
have been used successfully in power plants to remove both fly
ash and SO2. There are many variations to venturi design.
Systems have been constructed where water is sprayed in at the
throat (Figure 5-11), where the throat size can be adjusted to
accommodate varying gas flows (Figure 5-12), or where a series
of rods placed close together give a series of small venturi
throats (Figure 5-13).
Water sprays
Figure 5-11. Spray venturi.
5-8
-------
Orifice Scrubbers
Another method of utilizing gas stream energy is to design a
system in which the process gas is forced through a pool of
liquid. The gas moves through a restricted passage or orifice to
atomize the water. The larger particles in the incoming gas
stream impinge upon the surface of the pool and are collected.
Smaller particles impact upon the droplets produced by the
high velocity gas skimming over the surface of the water.
In the orifice scrubber shown in Figure 5-14, baffles serve as
impingement surfaces for droplet collection. The drops fall to
the bottom of the scrubber and the paniculate matter settles
out to form a sludge that must be periodically removed.
Orifice scrubbers are used principally for the collection of
paniculate matter, especially sticky or agglomerating materials.
As a medium pressure drop device, they have a moderate col-
lection efficiency for panicles around 1 fan in diameter (better
than 50% efficiency).
Figure 5-12. Adjtutable throat venturi.
Figure 5-13. Venturi rods.
Figure 5-14. Orifice scrubber.
5-9
-------
Review Exercise
1. Wet collectors can remove particulate matter from flue gases
by the application of energy. List at least three ways in which
this energy can be applied.
2. Which one of the following could not be used in a wet
collector?
spray nozzles
baffles
cyclonic openings
filter bags
1. • from the liquid stream
• from the gas stream
• mechanically driven
rotors
• combination of
methods
3.
4.
True or False? A spray tower can collect only particulate
matter.
True or False? Large fans are needed to provide high
pressure drops for particle collection in spray towers.
2. filter bags
3. False
5. Which one of the following would be characteristic of a
venturi scrubber?
4. False
•f
6. The primary energy contribution for particle collection in a
venturi scrubber comes from
a. the gas stream.
b. the liquid stream.
c. spray rotors.
5. b.
7. The common range of pressure drops in a venturi scrubber are
a. from 0.4 to 0.8 cm H2O (1 to 2 in. H2O).
b. from 8 to 24 cm H2O (20 to 60 in. H2O).
c. from 40 to 80 cm H2O (100 to 200 in. HfO).
6. a. the gas stream.
8. Gas absorption occurs more readily in wet scrubbers if the
gas/liquid contact or residence time is long/short.
7. b. from 8 to 24 cm H2O
(20 to 60 in. H2O)
9. An important feature of an orifice scrubber is
a. the liquid spray nozzle.
b. the pool of liquid.
c. the venturi throat.
d. the swimming pool.
8. long
9. the pool of liquid.
5-10
-------
Moving Bed Scrubbers
A device that uses energy from both the liquid stream and the
gas stream is the moving bed scrubber. As shown in Figure
5-15, process gas is injected at the bottom of the device, keep-
ing a bed of plastic spheres in constant motion. Water is
sprayed from nozzles over these moving spheres. Panicles in the
gas stream can impact both on the liquid covering the spheres,
and the droplets directed down from the spray nozzle. The gas
will also atomize liquid circulating within the moving bed to
provide further opportunity for particle collection. The con-
tinuous movement of the bed, combined with the washing
effect of the water sprays, minimizes plugging by collected par-
ticulate matter.
These systems can also be used for gas absorption and have
been used to remove both particulate matter and SOS from
power plant exhausts.
Vertical Spray Rotor
An example of a wet collector operated by mechanical means is
the vertical (spray) rotor (Figure 5-16). Here, a whirling rotor
submerged in a pool of liquid produces a fine droplet spray.
Process gas passes through the spray and particles are subse-
quently collected.
Packed Towers
The packed tower or packed column is most often used for gas
absorption. In these systems, specially designed packing
materials are placed in the scrubber. Liquid is sprayed on the
packing and allowed to flow through the system. As a result, a
liquid film covers the material, providing a large surface area
for gas to come into contact with the liquid. A large'liquid sur-
face area enhances the chances of pollutant gases being
absorbed. Particles (> 3 /im in diameter) in the gas stream can
also be effectively collected, but they can quickly plug the
small passages between the packing and make the whole system
inoperative. Several common types of packing material are
shown in Figure 5-17. They can be made of plastic, metal, or
ceramic.
£2° ° °"o"o*»o2 °'
L0«A"Pa?»o«
Mist
eliminator
Mobile
packing
Rotor
Figure 5-16. Vertical spray rotor.
Figure 5-17. Packing materials.
5-11
-------
Figure 5-18 shows a typical packed tower. This flow arrange-
ment is commonly referred to as counter current flow. Here
liquid flows down through the columns and the gas stream
being treated flows upwards. The countercurrent flow device is
quite effective for removing pollutant gases since the cleanest
gas contacts the purest absorbing liquor throughout the tower
(this maximizes pollutant solubility). Other factors that are
important in removing gases in these systems are tower
diameter, packing height, gas and liquid flow rates, and com-
position of the scrubbing liquor.
Advantages and Disadvantages of Wet
Collection Control Systems
Wet collectors offer many design opportunities for air pollution
control systems. In addition to serving as the primary means of
control, they can also be used as pre- or post-collectors or as
gas-conditioning systems that can be used in conjunction with
other collection equipment. Some advantages of using a wet
collector system follow. The wet collector system
• collects gases and particulate matter. This can be an
advantage for small industrial process industries unable to
afford separate control systems.
• has relatively small space requirements. Systems
can be designed for small locations, roof mounting,
etc.
• handles high temperature, high humidity gas streams.
Temperature limits and condensation problems in
baghouses and ESPs can be avoided since wet scrubbers
cool incoming gases and wash away accumulated par-
ticulate matter.
• has the ability to humidify a gas stream. The scrubber
can reduce the temperature and volume of an unsaturated
gas stream at high temperature by the process of evapora-
tion. Smaller ducting and fan sizes can then be used
downstream of the collector.
• keeps fire and explosion hazards at a minimum. Fire
and explosion hazards connected with various dry dusts
can be eliminated by using water as the control medium.
• has no secondary dust sources. Once the particulate mat-
ter is collected, it cannot escape from hoppers or in
transport. The collected slurries resulting from wet scrub-
bers can possibly be more easily handled than dry dust.
Figure 5-18. Packed tower.
5-12
-------
Although wet collectors have many advantages, they may not
be suitable for all applications. Some relative disadvantages
follow. The scrubber system can cause
• corrosion problems. Water and absorbed gaseous pollu-
tants can form highly corrosive acid solutions; therefore,
choosing proper construction materials for the control
system is important.
• meteorological problems. Highly humidified exhaust gases
can produce a wet, visible steam plume, especially during
cold weather; fog and precipitation from the plume may
cause local meteorological problems.
• water pollution. Adequate precautions must be taken
before scrubber waste liquid is disposed; settling ponds
and sludge clarifiers are often included in the design of
wet collector systems to meet waste water regulations.
• by-product recovery difficulty. Recovery of dust for
reuse is difficult when wet collectors are used; costs
associated with dewatering and drying the scrubber sludge
may make other control methods more practical.
In addition, costs may run high because of the high pressure
drop and power requirements. High collection efficiencies for
paniculate matter are attainable only at high pressure drops;
the increased fan power required to move the exhaust gases
through the scrubber may result in significant operating costs.
Review Exercise
1. A moving bed scrubber uses energy from
a. the gas stream.
b. the liquid stream.
c. a mechanical rotor.
d. both the gas stream and a mechanical rotor.
e. both the gas stream and the liquid stream.
Packing materials are used in wet collectors to
a. provide a weight to the system.
b. provide a large liquid film surface area.
c. provide high liquid flow rates.
d. provide high gas flow rates.
1. e. both the gas stream
and the liquid stream.
2. b. provide a large liquid
film surface area.
5-13
-------
3. The packed tower shown below is an example of.
flow.
4. True or False? In countercurrent flow, the cleanest gas
comes into contact with the dirtiest liquid.
3. countercurrent
5. List three advantages of using wet scrubbers as air pollution
control systems.
4. False
6. List three disadvantages of using wet scrubbers as air pollution
control systems.
5. • Relatively small
space requirement
• Collects both gases
and liquids
• Handles high
temperature gases
• Can humidify a gas
stream
6. • Corrosion problems
• Meteorological
problems
• Water pollution
• Difficulty of
recovering by-products
References
American Petroleum Institute. 1974. Manual on Disposal of Refinery Wastes—Volume on Atmos-
pheric Emissions. API Publication 931, Chapter 13-Filters and Wet Collectors for the Removal
of Paniculate Matter.
Bethea, R. M. 1978. Air Pollution Control Technology—An Engineering Analysis Point of View.
New York: Van Nostrand Reinhold Co., pp. 253-328.
5-14
-------
Bhatia, M. V. 1977. Packed Tower and Absorption Design. In Air Pollution Control and Design
Handbook. P. N. Cheremisinoff and R. A. Young, eds. New York: Marcel Dekker, Inc.
Busch, J. S., MacMath, W. E. and Lin, M. S. 1973. Part I —The Basic Scrubber. Pollut. Eng.
Jan. 1973: 28-32.
Calvert, S. 1977. How to Choose a Paniculate Scrubber. Chem. Eng. Aug. 1977: 54-68.
Calvert, S. 1977. Get Better Performance from Particulate Scrubbers. Chem. Eng. Oct. 1977:
133-140.
Cheremisinoff, P. N. and Young, R. A. 1974. Wet Scrubbers—A Special Report. Pollut. Eng.
May 1974: 33-43.
Cheremisinoff, P. N. and Young, R. A. 1977. Air Pollution Control and Design Handbook. New
York: Marcel Dekker, Inc.
Dickie, L. 1967. All About Wet Collectors-Part I. Air Eng. Jan. 1967: 14-19.
Dickie, L. 1967. All About Wet Collectors - Part II. Air Eng. Feb. 1967: 24-27.
Environmental Protection Agency (EPA). 1969. Control Techniques for Particulate Air Pollutants.
AP-51, pp. 50-81.
Environmental Protection Agency (EPA). 1972. Wet Scrubber System Study. EPA-R2-72-118a.
NTIS Report PB-213016. Research Triangle Park, NC.
Hesketh, H. E. 1979. Air Pollution Control. Ann Arbor: Ann Arbor Science, pp. 217-261.
Lapple, C. E. and Kamack, H. J. 1955. Performance of Wet Dust Scrubbers. Chem. Eng. Prog.
51:110-1?!.
Marchello, J. M. 1976. Control of Air Pollution Sources. New York: Marcel Dekker, Inc.,
pp. 187-207.
McCarthy, J. E. 1980. Scrubber Types and Selection Criteria. Chem. Eng. Prog. May 1980:
58-62.
Perry, J. H. and Chilton, C. H. (eds). 1973. Chemical Engineers Handbook 5th ed. New York,
McGraw-Hill Book Co., pp. 20-94 to 20-104.
Semrau, K. T. 1977. Practical Process Design of Particulate Scrubbers. Chem. Eng. Sept. 1972:
87-91.
Sherwood, T. K. and Pigford, R. L. 1952. Absorption and Extraction. New York: McGraw Hill
Book Co.
Strauss, W. 1975. Industrial Gas Cleaning. Oxford: Pergamon Press, pp. 367-408.
Theodore, K. and Buonicore, A. J. 1975. Industrial Control Equipment for Gaseous Pollutants.
Vol. I. Cleveland: CRC Press.
Theodore, L. and Buonicore, A. J. 1976. Industrial Air Pollution Control Equipment for
Particulates. Cleveland: CRC Press, pp. 191-250.
5-15
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Lesson 6
Adsorbers
Lesson Goal and Objectives
Goal
To familiarize you with the principles of adsorption and the
use of adsorbers in industry to reduce gaseous pollutant
emissions.
Objectives
At the end of this lesson, you should be able to:
1. describe the basic principles of adsorption.
2. list three factors influencing the efficiency of adsorption
equipment.
3. list four industries that use adsorption equipment to
control gaseous emissions.
4. descibe the basic operation of adsorption equipment.
5. recognize the most commonly used adsorbent material.
Introduction
Adsorption is a mass transfer process in which gas molecules
are removed from an air stream because they adhere to the
surface of a solid. In an adsorption system, the contaminated
air stream is passed through a layer of solid particles referred
to as the adsorbent bed. As the contaminated air stream passes
through the adsorbent bed, the pollutant molecules adsorb or
"stick" to the surface of the solid adsorbent particles. Even-
tually the adsorbent bed becomes "filled" or saturated with the
pollutant. The adsorbent bed must then be disposed of and
replaced, or the pollutant vapors must be desorbed before the
adsorbent bed can be reused.
The attractive forces that hold gas molecules to the surface
of a solid are present whenever most gases and solids come in
contact. However, certain solids exhibit a strong attraction for
specific types of gases. In addition, solids used as adsorbents
have very large surface areas, enabling them to hold large
volumes of pollutant vapor. Designers utilize these factors in
selecting the adsorbent for the type of pollutant to be removed.
For example, in air pollution control, one of the most widely
6-1
-------
used adsorbents is activated carbon, since it has a strong
attraction and large capacity for adsorbing hydrocarbon vapors
and odorous or toxic organic compounds.
Adsorbed gases are not destroyed, but merely "stored" on the
surface of the carbon until they can be removed by desorption.
Therefore, most adsorption systems consist of two or more car-
bon adsorber beds operating on a timed adsorbing and desorb-
ing cycle. Depending on the nature of the contaminant gases
being adsorbed, the desorbed gases can either be sent to a
recovery system or some other final disposal process.
Theory of Adsorption
The process of adsorption is analogous to using a sponge to
mop up water. Just as a sponge soaks up water, a porous solid
(the adsorbent) is capable of capturing gaseous pollutant
molecules. The air stream carrying the pollutants must first be
brought into contact with the adsorbent. The pollutant
molecules then diffuse into the pores of the adsorbent (internal
surface) where they are adsorbed (Figure 6-1). The majority of
the gas molecules are adsorbed on the internal pore surfaces.
Not all of the pollutant molecules that come in contact with
the adsorbent will be immediately adsorbed. The attractive
forces between the surface of the adsorbent (solid) and the gas
molecules must be greater than the forces that tend to keep the
molecules in motion in the air stream. The strength of the
attractive forces depends on the chemical structure of both the
gas molecule and the solid. When molecules are adsorbed (stop
moving) they lose their kinetic energy of motion in the form of
heat. Therefore all adsorption processes are exothermic—
meaning they give off heat.
Adsorption processes can be classified as either physical or
chemical. The basic difference between physical and chemical
adsorption is the type of bond that is formed when the gas
molecule is adsorbed.
Migrating
gas molecules
.t. . . ". T
Figure 6-1. Mechanism of adsorption.
Physical Adsorption
An imbalance of forces exists on the surface of any solid. These
forces are the same as those that hold the molecules together
throughout the entire body of the solid—except that the forces
at the surface are the "leftover ones." To satisfy the imbalance
of forces, gas molecules are attracted and physically held to the
walls of the pores in the solid.
In physical adsorption, the strongest bonds are formed
between a gas molecule and the surface of an adsorbent that
exhibit the same polarity. Substances are catagorized as polar
6-2
-------
or nonpolar depending on their electron distribution. Polar
substances are those that exhibit a separation of positive and
negative charges within the surface atoms of the substance.
Water is a prime example of a polar substance. Nonpolar
substances have no distinct positive or negative charge. Most
organic vapors are considered nonpolar because of their sym-
metry. Therefore, organic vapors are strongly attracted to non-
polar adsorbent materials, while water vapor is strongly
attracted to other polar adsorbents.
An important characteristic of physical adsorption is that the
chemical nature of the pollutant molecule remains unchanged.
Since the pollutant molecule is only being held to the adsor-
bent by weak forces of cohesion, the adsorption process is
readily reversible. Therefore, the pollutant molecules can be
readily removed (desorbed) from the adsorbent and recovered
for future use.
Chemical Adsorption
Chemical adsorption or chemisorption results from a chemical
interaction between the pollutant molecule and the adsorbent.
The pollutant molecule is held to the surface of the adsorbent
by forming a chemical bond. In these instances, a sharing or
exchange of electrons takes place and new compounds or
substances ^an be formed. Therefore, chemisorption b not
easily reversible. In some instances, the pollutant molecules
may be impossible to remove from the adsorbent. Few
industrial adsorption systems use chemical adsorption, because
it would be extremely costly to regenerate or replace the absor-
bent bed.
Review Exercise
1. Most pollutant molecules are adsorbed on the internal surface
within each adsorbent particle referred to as the
2. Adsorption can be classified as a
or
1. pore
process by the way bonds are formed. The
is most commonly used.
process
All adsorption processes are
a. endothermic.
b. exothermic.
c. isothermal.
d. polar.
2. physical, chemical,
physical
3. b. exothermic
6-3
-------
4. True or False? Polar adsorbents have a much higher attraction
for nonpolar gases than they do for polar gases.
5. Physical adsorption is a
pollutant adsorbed can be readily recovered.
a. irreversible
b. chemical bonding
c. reversible
d. endothermic
. process meaning that the
4. False
5. c. reversible
Adsorbent Materials
Several materials are used commercially as adsorbing agents.
The most common adsorbents used industrially are activated
carbon, silica gel, activated alumina (alumina oxide), and
zeolites or molecular sieves. The important characteristics that
determine the effectiveness of an adsorbent are its chemical
structure and nature, total surface area, pore size distribution
(diameters ^jf its pore), and particle size.
As previously discussed, in physical adsorption, polar gas
molecules prefer polar adsorbents, while nonpolar adsorbents
are best for removing nonpolar gases. Since water vapor (polar)
is present in most pollutant exhaust streams, polar adsorbents
are not very effective in air pollution control systems. Polar
adsorbents quickly fill with water and have no room to adsorb
the pollutant gases. Of the nonpolar adsorbents, activated car-
bon is the one primarily in use. Because of its nonpolar sur-
face, activated carbon is used to control organic pollutants
such as solvents, odors, toxic gases, and gasoline vapors.
Activated carbon can be manufactured from a variety of
organic feedstocks such as wood, coal, coconut or other nut-
shells, and petroleum byproducts. The carbon particles are
"activated" by heating the particles in the absence of air. This
heating or activation process produces a highly porous carbon
particle with the extremely high internal surface (pore) area
necessary for good adsorption. Manufacturers of activated car-
bons can vary adsorptive properties by adjusting the
temperature, oxygen amount, or feedstock used during activa-
tion. The total surface area of activated carbon can range from
600 to 1600 square meters per gram of carbon (2.9 X 10" to
7.8x 106 ftVlb). This is equivalent to having the surface area
of 2 to 5 football fields in one gram of carbon.
6-4
-------
Adsorption Process
A variety of configurations are used to bring a contaminated
air stream into contact with an adsorbent. The most common
configuration is to pass the air stream down through a fixed
volume or bed of adsorbent material. As the contaminant-
laden air stream passes through the bed of adsorbent, contami-
nant gases are being adsorbed; therefore, portions of the adsor-
bent bed become saturated or "filled" with these gases. When
the adsorbent becomes saturated, it cannot continue to adsorb
contaminant gases. Figure 6-2 illustrates the adsorption process
occurring as a gas stream passes down through an adsorber
bed.
Saturated area of bed
TWY
Active adsorbing area of bed
11
I *
§1
12
II
2 o
I'S
•+ Breakpoint
Volume of contaminated gases
Figure 6-2. Breakthrough curve.
The exhaust stream contains the pollutant at an initial con-
centration of c,. The exhaust stream is passed down through the
bed of carbon granules. Most of the pollutant is adsorbed. The
gas stream leaves the adsorber relatively pollutant free (denoted
at Ci). This process continues as the carbon granules are filled
with the pollutant. Eventually, most of the carbon in the
adsorber becomes saturated with the pollutant. At this time,
the carbon bed can no longer adsorb all the pollutant gases
6-5
-------
and some pass through the system and are exhausted into the
atmosphere. This is referred to as the breakthrough point
(breakpoint) of the bed, c3 on the graph.
In air pollution control, even trace amounts of pollutants
exhausted to the atmosphere are undesirable. To achieve con-
tinuous system operation, the carbon must be periodically
replaced or the pollutant must be desorbed from the carbon
bed before breakthrough occurs. In desorption or regeneration,
the pollutant vapors are removed from the used bed in
preparation for the next cycle. Most commercial adsorption
systems are the regenerable type.
Factors Affecting Adsorption
A number of factors influence the performance of an adsorp-
tion system. The following variables determine the efficiency of
a physical adsorption system. The carbon capacity is the
amount of gaseous pollutants that a given volume of carbon
can adsorb at a given temperature and pressure.
Temperature
As the temperature increases, the amount of vapors that can
be adsorbed^ decreases. Figure 6-3 illustrates this concept.
Increasing the operating temperature is the technique most
often used for desorbing the carbon bed. As a general rule,
adsorber operating temperatures are kept below 55°C (130°F)
to ensure proper operation. However, this depends on the
adsorbant material and pollutant to be adsorbed.
Pressure
Adsorption capacity increases with an increase in the pressure
of the system (Figure 6-4). Pressure changes do not affect
adsorption capacity as much as temperature changes. However,
some systems do use a decrease in pressure to desorb vapors
from the adsorber.
Gas Velocity
The contact or residence time between the contaminant stream
and adsorbent is determined by the gas velocity through the
adsorber. The slower the contaminant stream flows through
the adsorbent bed, the greater the probability of a contami-
nant molecule hitting an available site and being adsorbed.
Particulate Matter Content
If present in the gas stream, paniculate matter can reduce
adsorber efficiency, increase the pressure drop, and eventually
plug the bed. Any micron-sized particle of dust or lint that is
Temperature—
Figure 6-3. The relationship of carbon
capacity to temperature.
Pressure—
Figure 6-4. The relationship of carbon
capacity to pressure.
6-6
-------
not filtered can cover the surface of the adsorbent. This greatly
reduces the surface area available to the gas molecules for
adsorption. Covering active adsorption sites with dust particles
is referred to as blinding or deactivation. To avoid this situa-
tion, almost all industrial adsorption systems are equipped with
some type of paniculate matter removal device that pretreats
the gas stream, usually a filter.
Review Exercise
A solid adsorbent material is
a. activated carbon.
b. molecular sieves.
c. silica gel.
d. all the above
2.
adsorbents will prefer to adsorb any water vapor
that may be present in a gas stream.
1. d. all the above
3. Because of its nonpolar surface,
is used to
control emissions of organic solvents, odors, gasoline vapors,
and other organic vapors.
2. Polar
4. True oF False? Surface areas contained in one gram of carbon
can often be equivalent to the surface area of two to five
football fields.
3. activated carbon
To achieve continuous operation of an adsorption system,
the activated carbon must be periodically
a. replaced.
b. desorbed.
c. regenerated.
d. any of the above
4. True
6. True or False? When a carbon bed becomes saturated, it can
no longer remove all the pollutant vapors.
5. d. any of the above
7. The
occurs when pollutant vapors pass through
the adsorber bed and are then exhausted into the atmosphere
without being adsorbed.
6. True
8. In physical adsorption, the capacity of the adsorbent (i.e.,
activated carbon) as the temperature of the
system increases.
a. increases
b. decreases
c. remains the same
7. breakthrough point
(breakpoint)
8. b. decreases
6-7
-------
9. Adsorbent capacity _
increases.
a. increases
b. decreases
c. remains the same
, as the pressure of the system
10. Reduction in adsorber efficiency, due to covering of the
active adsorption sites by particulate matter, is referred to as
9. a. increases
10. blinding or
deactivation
Adsorption Control Systems
Adsorber systems have a number of different shapes and gas
flow patterns. The most common industrial adsorption system
is referred to as the fixed bed (Figure 6-5). The fixed bed
system consists of a vessel that contains a carbon bed. The
depth of the carbon bed is usually between 0.3 and 1.2 m (1
and 4 ft) thick, depending on the concentration of contami-
nant in the gas stream. The contaminated gas stream is first
passed through a filter to remove any dust that could blind the
bed and decrease efficiency. The contaminated gas stream then
passes down through the fixed bed of carbon. The cleaned
gases are exhausted out a stack. Upward air flow through the
carbon bed is usually avoided to eliminate the risk of
entraining carbon particles in the exhaust stream.
As previously discussed, a fixed amount of carbon can only
adsorb a certain volume of contaminant before the carbon
becomes saturated. To continue to be effective, the carbon in
the system must either be replaced or the contaminant vapors
must be desorbed. Replacement of the carbon is only feasible
in small, low-volume air filtering systems. Adsorbers used for
air pollution control are designed to allow for an inplace
desorption or regeneration cycle. From the example of the
sponge, regeneration is analogous to squeezing the sponge dry
so that it may be used again.
Fixed bed adsorption systems use multiple beds. One or
more of the beds treats the process exhaust while the others are
regenerated or lay idle. Regeneration is accomplished by rever-
sing the conditions that promote adsorption. The most com-
mon way to desorb the vapors is by increasing the temperature
or decreasing the pressure of the system.
Solvent
recovery
system
Steam inlet
for regeneration
Figure 6-5. Fixed bed adsorber.
6-8
-------
Thermal Swing—Steam Stripping
Because injecting steam into the adsorber is simple and
relatively inexpensive, it is the most common desorption tech-
nique. This is referred to as thermal swing or steam stripping.
Heating the bed breaks the weak bond between the contami-
nant molecules and the carbon, allowing the contaminant to
be swept away in the desorbing gas stream. The advantages to
using steam for desorption follow.
• At high pressure, the steam's temperature (100°C) is high
enough to desorb most solvents of interest without damag-
ing the carbon or the desorbed vapors. If the temperatures
were too high, desorbed vapors could be polymerized or
cracked, sometimes forming undesirable compounds.
• Steam readily condenses in the adsorber bed releasing its
(the steam) heat of condensation, aiding in desorption.
• Many organic compounds can be easily separated and
recovered from the effluent stream by condensation,
distillation, or in some cases, decanting.
• Steam is a more concentrated source of heat than is hot
air, so it very quickly and effectively raises the
temperature of the adsorber bed.
A typical two bed adsorption system is shown in Figure 6-6.
The adsorption-desorption cycle starts by sending the contami-
nant air stream to bed "B" for treatment. After a predeter-
mined length of time (before breakthrough occurs), adsorber
"B" is taken off line by switching the contaminant air stream
to bed "A." Steam is then injected into bed "B" to desorb the
contaminant. The steam passes up through the bed counter-
current to the flow of the contaminant air stream. The steam
and the desorbed vapor stream can then be sent to a recovery
system where the contaminant vapors (if valuable) can be
recovered for reuse. Before bed "B" is returned to service, it is
allowed to cool and dry.
Some disadvantages associated with steam regeneration
are:
• The water from the condenser/recovery system could
pose a water pollution problem unless it is sent to a waste-
water treatment facility.
• Some organic compounds may react with water to produce
corrosive substances. Corrosive substances can greatly
reduce the life of the adsorption equipment unless expen-
sive corrosive resistant materials are used.
• A hot, wet carbon bed will not effectively remove organic
vapors. The bed may need to be cooled and dried, to
ensure adequate removal at the beginning of a new cycle.
Vapor inlet
"A"
.Condenser
-Vapor inlet
Condenser
Steam source
Figure 6-6. Two bed adsorber system.
6-9
-------
Applications
Adsorber systems are used mainly for the control of organic sol-
vent emissions from air streams that are relatively free of par-
ticulate matter. Examples of solvent-using operations are
drycleaning, degreasing, surface coating, rubber processing,
and flexographic and gravure printing. Adsorption systems
have also been used to control toxic or odorous vapors
discharged from food processing plants, rendering plants,
sewage treatment plants, and many chemical manufacturing
processes (such as producing fuels, cements, fertilizers, and
pharmaceutical products).
Review Exercise
1. Regeneration of an adsorber can be accomplished by
increasing/decreasing the temperature.
2. Using steam to desorb vapors from an adsorber bed is an
example of desorption.
1. increasing
3. True or False? Steam is used to desorb vapors because it is
a more Concentrated source of heat than is hot air; and
because steam condenses in the adsorber bed, it releases addi-
tional heat to aid in desorption.
2. thermal swing or steam
stripping
4. Steam used for regeneration usually flows
flow of contaminant vapors.
a. cocurrent
b. countercurrent
c. crosscurrent
d. parallel
to the
3. True
5. A problem that may arise from using steam to desorb
organic vapors is that
a. the effluent from the condenser could pose water pollution
problems.
b. the organics can react with the water and form corrosive
liquids.
c. a hot, wet carbon bed will not effectively remove organic
vapors.
d. all the above
4. b. countercurrent
6. In a fixed bed adsorption system, the contaminant air stream
usually passes up/down through the fixed bed.
5. d. all the above
6. down
6-10
-------
7. The desorbed vapors and the steam used to clean the
saturated bed are usually
a. recycled to the plants' water system.
b. recycled to the other adsorber on line.
c. sent to a recovery system.
d. all the above
8. Fixed bed carbon adsorption systems have been used to
control
a. organic emissions.
b. solvent degreasing emissions.
c. dry cleaning emissions.
d. all the above
7. c. sent to a recovery
system.
8. d. all the above
References
Bethea, R. M. 1978. Air Pollution Control Technology. New York: Van Nostrand Reinhold.
Calgon Corporation. Air Purification with Activated Carbon, Technical Bulletin. Pittsburgh.
Cannon, T. E. 1977. Carbon Adsorption Applications In Air Pollution Control and Design
Handbook, PrN. Cheremisinoff and R. A. Young, eds. New York: Marcel Dekker, Inc.
Cerny, S. and Smesek, M. 1970. Active Carbon. New York: Elsever Publishing Company.
Chemical Engineering, 1977. Beaded Carbon Ups Solvent Recovery. August 29.
Dubinin, M. M. and Saverina, E. 1936. The Porosity and Sorptive Properties of Active Carbon.
Aota Physicochem. USSR, 4. 647.
Environmental Protection Agency (EPA). 1973. Package Sorption Device System Study.
EPA-R2-73-202. Research Triangle Park, NC.
Hellman, T. M. 1977. Odor Control By Adsorption. Air Pollution Control and Design
Handbook, P. N. Cheremisinoff and R. A. Young, eds. New York: Marcel Dekker, Inc.
Kovach, L. J. 1978. Gas-Phase Adsorption and Air Purification. Carbon Adsorption Handbook,
P. N. Cheremisinoff and F. Ellerbush, eds. Ann Arbor: Ann Arbor Science Publishers, Inc.
Perry, J. H. ed. 1973. Chemical Engineers Handbook. 5th ed. New York: McGraw Hill Book Co.
Parmele, C. S., O'Connell, W. L., and Basdekis, H. S. 1979. Vapor-Phase Adsorption Cuts
Pollution, Recovers Solvent. Chem. Engr. 86:58-70 (December 31, 1979).
Stem, A. C. ed. 1977. Air Pollution. Third Edition. Volume IV. New York: Academic Press.
Sutcliffe Speakman Co. 1963. Solvent Recovery with Active Carbon. Technical Bulletin, Bronxville,
New York.
Union Carbide. Purasiv HR for Hydrocarbon Recovery. Technical Bulletin. New York.
Vic Manufacturing. Carbon Adsorption/Emission Control Technical Bulletin. Minneapolis.
Woods, F. J. and Johnson, J. E. 1964. NRL Report 6090.
6-11
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Lesson 7
Combustion Equipment
Lesson Goal and Objectives
Goal
To familiarize you with the principles of combustion and the
use of combustion systems (incinerators and flares) in industry
to reduce pollutant gaseous emissions.
Objectives
At the end of this lesson you should be able to:
1. recognize the relationship between time, temperature,
and turbulence in the combustion process.
2. describe the operation of direct flame incinerators,
catalytic incinerators, and flares in the control of com-
bustible pollutants.
Introduction
Combustion is a process that commonly has many
uses—heating, cooking, and producing electricity. In air pollu-
tion control, combustion is used to control hydrocarbon and
other organic vapors, gaseous pollutants including S, Cl, F,
and combustible paniculate matter emitted from various indus-
tries. If a process emits a pollutant exhaust stream containing
organic vapors, the exhaust stream can be sent to an incin-
erator where the vapors are burned to form carbon dioxide and
water.
Combustion systems are relatively simple devices capable of
achieving high gaseous removal efficiencies. Combustion
systems generally consist of a set of burners and a refractory-
lined (heat resistant) chamber. The burners supply the air and
fuel to provide heat. The heat produced ignites the organic
vapors while the chamber acts as a "holding" space that allows
time for all of the vapors to completely oxidize. Because of the
high cost of fuels, combustion systems often include a device or
system to recover heat from the hot exhaust gases.
7-1
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Some problems may occur when combustion is used for con-
trolling gaseous pollutants. With many organic compounds,
incomplete combustion (oxidation) can result in the formation
of aldehydes or organic acids. If formed, these compounds
could create additional pollution problems. Also, oxidizing
organic vapors that contain sulfur or halogens will produce
unwanted pollutants, such as sulfur dioxide, hydrochloric acid,
or hydrofluoric acid. When these are formed, an additional
control device may be required before the gas stream can be
exhausted into the atmosphere.
Combustion Process
Oxidation is a process in which oxygen chemically combines
with various elements. Oxidation is exothermic, meaning that
as it occurs, heat is generated. Combustion or thermal
incineration is simply an accelerated oxidation process. The
combustible material is heated (raised to its ignition
temperature), which causes oxygen to rapidly combine with the
material and, in turn, give off additional heat. For example,
when organic vapors are raised to their ignition temperature,
the hydrogen and carbon molecules will rapidly combine
(react) with any oxygen present to form carbon dioxide and
water and produce more heat to aid the oxidation process.
Factors Affecting Combustion
In air pollution control systems, the ultimate goal of an
incinerator is to achieve complete combustion, or oxidation, of
the pollutant gases and combustible particulate matter, if
present. Incomplete combustion or partial oxidation of the
pollutant gases can result in the formation of new pollutants
that may be more toxic or corrosive than were the original
ones. To achieve complete combustion of any combustible mix-
ture, the following conditions must be provided: a temperature
high enough to ignite or promote rapid oxidation of the com-
bustible mixture; turbulent mixing of all the combustibles with
air (oxygen); and a residence time sufficient for complete oxi-
dation. These conditions are referred to as the three T's of
combustion. In addition to these three factors, the amount of
air (oxygen) present is also critical to the completeness of the
combustion process. These factors will be discussed in more
detail in the following sections.
7-2
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Temperature
The rate at which combustible materials are oxidized is greatly
affected by the operating temperature of the incinerator. In
general, the higher the operating temperature, the faster the
oxidation reaction will proceed (Figure 7-1). To be efficiently
incinerated, waste gases must be heated to their ignition tem-
perature. If temperatures fall below the ignition temperatures,
not all of the waste gases will be oxidized; i.e., incomplete
combustion will occur.
In an incinerator, heat is added to the waste gas stream by
burning auxiliary fuel. The more fuel bumed, the higher the
cost of operating the incinerator. The amount of auxiliary fuel
that must be burned depends on the amount of heat that must
be added to reach the operating temperature minus any heat
produced from burning the hydrocarbons in the waste gas
stream. If the waste gas stream has a high heat content, then
little auxiliary fuel is needed; therefore, operating costs can be
minimized. However, regardless of the heat source, the main
concern is to ensure that the entire waste gas stream is heated
to the required operating temperature.
To ensure complete combustion, most incinerators operate at
temperatures higher than the waste gas ignition temperature.
Thermal destruction of most compounds occurs between 590
and 650°G-(1110 and 1200°F). Incinerators generally operate
at 700 to 820 °C (1300 to 1500°F) with residence times of
approximately 0.1 to 0.6 sec.
Residence Time
To ensure complete combustion of a mixture, both the
operating temperature and the residence time must be
specified. Residence time is measured from the time the waste
gas stream reaches the system's operating temperature until the
time the waste gas leaves the combustion chamber. The
residence time is, therefore, determined by the size of the com-
bustion chamber and the flow rate of the gas through this
chamber.
100
80
60
40
20
0.01 second
residence
time
600 1000 1400 1800
Temperature, °F
Source: EPA, 1972
Figure 7-1. Effect of temperature
on the rate of pollutant
destruction.
7-3
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Changing the operating temperature affects the required
residence time, and vice versa. As the operating temperature is
increased, the oxidation reaction proceeds faster, and complete
combustion occurs in a shorter time (Figure 7-2). The reverse is
also true—as the operating temperature is decreased, the
residence time must be increased to assure adequate
combustion.
Turbulence
Turbulence refers to the mixing processes involved in incinera-
tion. Proper mixing is important in the combustion process for
two reasons. First, for complete combustion to occur, every
combustible molecule must come in contact with oxygen
molecules. If not, unreacted combustible molecules will be
exhausted from the incinerator. Second, to reach the operating
temperature, the entire waste gas stream must be mixed with
the heat source. The waste gas stream can be heated in two
ways:
1. by direct contact with the auxiliary fuel flame, and/or
2. by mixing the waste gas with the hot combustion products
downstream of the flame.
Although direct contact of the waste gases with the flame is the
quickest way to reach operating temperature, it is not always
feasible. Banging too large a volume of waste gases in contact
with the flame can cause flame cooling—a reduced tempera-
ture in the area immediately surrounding the flame.
Oxygen Requirements
Oxygen must be present for combustion to occur. To achieve
complete combustion of a compound, sufficient oxygen must
be available to convert all the carbon present to COZ and the
hydrogen to HZO. This quantity of oxygen is referred to as the
stoichiometric, or theoretical, amount and is the minimum
amount of oxygen required. To ensure complete combustion,
almost all industrial combustion systems use more than the
stoichiometric amount of air (oxygen). This extra volume of air
is referred to as excess air. The amount of excess air is usually
kept low since heating the excess air to the operating
temperature of the incinerator requires additional fuel, and
thus reduces efficiency. Too much excess air can cause the
reaction to be quenched and lead to incomplete combustion in
the chamber.
600 1000 1400 1800
Temperature, °F
Source: EPA, 1972
Figure 7-2. Effects of temperature and
residence time on rate of
pollutant destruction.
7-4
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Review Exercise
1. The process of.
oxidation process.
is simply an accelerated
2. When complete combustion of a gas containing only
hydrogen and carbon occurs, are the products
formed.
1. combustion
3. Which of the following is not one of the three T's of
combustion?
a. time
b. thermal barrier
c. temperature
d. turbulence
2. COz and water
4. True or False? In general, the higher the operating
temperature, the faster the oxidation reaction (or combus-
tion) will occur.
3. b. thermal barrier
To ensure efficient combustion, the waste gas must be heated
to at least its
a. ignition temperature.
b. boiling point.
c. vapor pressure.
d. critical temperature.
4. True
6. In most incinerators, the operating temperature is main-
tained by
5. a. ignition temperature.
To ensure complete combustion, most incinerators
operate at
a. 220 to 300 °C (430 to 570°F)
b. 500 to 610°C (930 to 1130°F)
c. 700 to 820°C (1300 to 1500°F)
d. 1300 to 1500°C (2370 to 2700 °F)
6. burning an auxiliary
fuel.
8. As the operating temperature of the incinerator is
increased, the combustion reaction proceeds slower/faster.
and therefore requires a shorter/longer residence time for
complete combustion.
7. c. 700 to 820°C
(1300 to 1500°F)
9. True or False? The waste gases to be incinerated can be
heated only by mixing them at the flame.
8. faster, shorter
9. False
7-5
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10. The minimum, or theoretical, amount of oxygen required
to achieve complete combustion of a compound is referred to
as the amount.
11. If more than the theoretical, or minimum, amount of air is
used, this quantity of air is referred to as
10. stoichiometric
11. excess air
Combustion Equipment Used
to Control Gaseous Emissions
Three kinds of combustion equipment are used to control
gaseous pollutants. In a flare, all the combustible material is
oxidized in the flame. In a thermal oxidizer, the combustible
gases first pass over or around the burner flame and then into
a chamber where the gas flow is slowed, allowing time for com-
plete oxidation. A catalytic oxidizer is similar to a thermal
oxidizer. The main difference is that after passing through the
flame area, the gas then passes through a catalyst bed that
promotes oxidation at a lower temperature than is necessary in
a thermal oxidizer.
Flares
Flares are used to dispose of large volumes of intermittent or
emergency releases of combustible gases from industrial
sources. Flares have been used mainly at oil refineries and
chemical plants that handle large volumes of combustible gas.
Flares are simply burners that have been designed to oxidize
varying rates of fuel —and to do so smokelessly. In general,
flares can be classified as either elevated or ground-level. Flares
are elevated to eliminate any potential fire hazard at ground
level. Ground-level flares must be completely enclosed to con-
ceal the flame. In order to handle all plant emergencies, either
type of flare must be capable of operating over a wide range of
waste gas flow rates.
An elevated flare is essentially a hollow pipe with a set of
burners at the top. The burners at the top are referred to as
the flare tip. Flare tips are designed to provide good mixing of
air and waste gas, so that the waste gas will burn without
smoking. Steam jets can be used to mix the air and waste gas.
Figure 7-3 is an example of a flare tip with steam injection
points.
Waste gas Steam injection
burner , point
Pilot light
Figure 7-3. Flare tip.
7-6
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Most ground flares consist of multiple burners enclosed
within a refractor (heat resistant) shell. The shell encloses the
flame to eliminate noise, luminescence, and safety hazards.
The waste gas is introduced through a jet or venturi to provide
turbulent mixing. The flare system requires a stack for proper
release of the effluent gases. Figure 7-4 shows a ground flare
composed of two chambers designed to incinerate different
gases and a liquid-waste stream.
Burners
Liquid waste
injectors
Figure 7-4. Ground flare.
Although the flare is designed to eliminate waste gas disposal
problems, it can present safety and operational problems of its
own. Several problems are associated with the operation of a
flare system:
• Thermal radiation. Heat given off to the surrounding area
may be unacceptable.
• Light. Luminescence from the flame may be a nuisance if
the plant is located in an urban area.
• Noise. Mixing at the flare tip is done by jet Venturis,
which can cause excess noise levels in nearby
neighborhoods.
• Smoke. Incomplete combustion can result in toxic or
obnoxious emissions.
• Energy consumption. Flare systems waste energy in two
ways: first, by keeping the pilot flame constantly lit and
second, by not using the potential recovery value of the
waste gas being flared.
7-7
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Thermal Oxidizers
Thermal oxidizers use a flame in a chamber to convert com-
bustible material to carbon dioxide and water. Thermal oxi-
dizers are commonly referred to as organic vapor incinerators
or afterburners. These devices have a chamber lined with a
refractory material, and have one or more sets of burners. A
typical single flame incinerator is depicted in Figure 7-5. The
waste gas stream is passed through the burner area where it is
heated to above its ignition temperature. The hot gas then
passes through one or more chambers where the flow of gas is
slowed to allow complete combustion. Additional fuel and/or
excess air can be added, if necessary, through the burners.
— —Figure 7-5. Single flame incinerator.
To achieve adequate mixing and heating of the waste gas
stream, a variety of burner arrangements can be used in ther-
mal oxidizers. One burner design is the multijet, illustrated in
Figure 7-6. In this system, many small flames are created
instead of just one large flame. Only a portion of the waste gas
directly contacts these flames while the remainder is mixed
downstream (in front of the mixing plate) with the hot gases of
combustion.
Adjustable mixing plate
Gap
Multijet
burners
Stationary baffle
Auxiliary
fuel
Figure 7-6. Multijet burner.
7-8
-------
Thermal oxidizers typically operate at temperatures of
between 700 and 820 °C (1300 and 1500°F) with a residence
time of 0.1 to 0.6 seconds. Typical operating temperatures of
thermal oxidizers used in various industries can be found in the
Afterburner System Study, EPA-R2-062.
Because of the rising cost and limited availability of fuels,
heat recovery is an integral part of many incinerator systems.
Since the exhausted gases from the combustion chamber are at
high temperature, a system to recover the heat is often quite
feasible and cost effective. Figure 7-7 shows an example of a
typical heat recovery system.
Heat recovery system
Figure 7-7. Heat recovery system.
The waste gas from the process enters the heat exchanger
and passes through the inside of numerous tubes. The hot
exhaust gas from the combustion chamber passes over the out-
side of the tubes, heating the tubes and the waste gas. A heat
recovery system can reduce the amount of fuel needed to heat
the inlet waste gas to the operating temperature of the
incinerator, thereby reducing operating costs.
Catalytic Oxidizers
A catalyst is a substance that causes or speeds a chemical reac-
tion without undergoing a change itself. In catalytic incinera-
tion, a waste gas is passed through a layer of catalyst—the
catalyst bed. The catalyst causes the oxidation reaction to pro-
ceed at a faster rate and at lower temperatures than in thermal
oxidation. A catalytic oxidizer (catalytic incinerator) operating
in a 370 to 480 °C (700 to 900 °F) range can achieve the same
efficiency as a thermal incinerator operating at between 700
and 820 °C (1300 and 1500°F). This can result in a 40 to 60%
fuel savings, which substantially reduces operating costs. It
7-9
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should be noted that catalytic oxidation produces the same end
products (CO2 and H2O) and gives off the same heat of com-
bustion as does thermal incineration.
The most effective and commonly used catalysts for oxida-
tion reactions come from the noble metals group. Platinum,
either alone or in combination with other noble metals, is by
far the most commonly used. Platinum is desirable because it
gives a high oxidation activity level at low temperatures, is
stable at high temperatures, and is chemically inert. Palladium
is another noble metal that exhibits these properties and is
sometimes used in catalytic incinerators.
Since catalytic oxidation is a surface reaction, a cheap sup-
port material is coated with the noble metal. The support
material can be made of ceramic, such as alumina, silica-
alumina, or it can be made of a metal such as nickel-
chromium. The support material is arranged in a matrix shape
to provide high surface area, low pressure drop, uniform flow
of the waste gas through the catalyst bed, and a structurally
stable surface. Structures that provide these characteristics are
pellets, a honeycomb matrix, or a mesh matrix. Figure 7-8
shows a typical honeycomb catalyst module, which is the most
common. The support material for this is usually ceramic, but
can be metal.
Figure 7-8. Typical catalyst shapes.
7-10
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A catalytic incinerator is shown in Figure 7-9. A catalytic
incinerator contains a preheat section (burner area), where
part of the waste gas is raised to operating temperature. The
burners are the same as those used for thermal incineration.
The remaining portion of the waste gas is mixed with the hot
products of combustion before it passes over the catalyst bed.
This ensures a uniform waste gas temperature as the waste gas
passes over the bed. After the hot flue gases pass over the bed,
they may be sent to a heat recovery system.
Heating chamber
Catalyst
Figure 7-9. Catalytic incinerator.
Operating Limitations of Catalytic Oxidizers
The main problem in catalytic incineration is deactivation—the
reduction in the effectiveness of the catalyst. Certain con-
taminants, if present in the waste gas stream, cause a loss of
catalyst activity.
Paniculate matter in the waste gas stream will coat the sur-
face of the catalyst, reducing its effectiveness. Certain metals
(such as phosphorous, bismuth, arsenic, antimony, mercury,
zinc, lead, and tin) can chemically combine with the catalyst,
thereby deactivating the catalyst. Deactivation of the catalyst in
this manner is referred to as catalyst poisoning. Sulfur and
halogen compounds can also cause a reduction in catalyst
effectiveness. However, catalysts absorbing these compounds
can be regenerated. Finally, all catalysts deteriorate with nor-
mal use. A catalyst bed normally lasts from three to five years
before it must be replaced.
7-11
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Applications
Both thermal and catalytic incinerators are used to control
gaseous and paniculate emissions that are combustible. These
systems have been used on ovens, driers, coating lines, smoke-
houses and cookers. Table 7-1 lists some of the processes that
use incinerators. The main difference between thermal and
catalytic incinerators is that catalytic incinerators operate at
lower temperatures. However, the effectiveness of a catalytic
incinerator is reduced if certain contaminants are present.
Table 7-1. Applications of afterburners.
Adhesive cape curing
Asphalt blowing
Brake lining ovens
Cat cracker regenerator off gas
Charcoal broilers
Coil and scrip coating lines
Core ovens
Cupola furnace stacks
Deep fat frying
Fat rendering
Fiber glass curing
Herbicide and insecticide
manufacturing off gas
Lithographing ovens
Meac smokehouses
Metal-coaling ovens
Metal reclaiming
Pulp and paper
Packing house effluents
Paint baking ovens
Paine removal facilities
Phchalic anhydride manufaccuring off gas
Plastic curing ovens
Printing presses
Quench bach oil fumes
Resin and paint cooking
Roofing paper machine hoods
Rubber curing
Solvent degreasing
Solvent manufacturing off gas
Sulfur plant tail gas
Textile dryers
Varnish bum-off
Varnish kettles
Vinyl sponge curing
Wire enameling
Source: EPA, 1972.
Review Exercise
1.
2.
In general, flares can be classified as either
-------
4. A
., or vapor incinerator, is a device that uses a
flame and a chamber to convert combustible material to
carbon dioxide and water.
a. flare
b. thermal oxidizer
c. catalytic oxidizer
d. direct combustor
5. Thermal incinerators generally operate between
and with a residence time of 0.1 to 0.5 seconds.
a. 150 and 260°C (300 and 500 °F)
b. 370 and 480°C (700 and 900 °F)
c. 700 and 820 °C (1300 and 1500°F)
d. 1150 and 1370°C (2100 and 2500°F)
4. b. thermal oxidizer
6. True or False? In any incinerator, all of the waste gas must
come in direct contact with the flame.
5. c. 700 and 820 °C
(1300 and 1500°F)
7. In this illustration, name the system indicated by the
question mark.
6. False
8. In a catalytic incinerator, the oxidation occurs at a
temperature than in a thermal incinerator.
7. heat recovery device,
or heat exchanger
Catalytic incinerators generally operate in the temperature
range between
a. 150 and 260°C (300 and 500°F).
b. 370 and 480°C (700 and 900°F).
c. 700 and 820°C (1300 and 1500°F).
d. 1150 and 1370°C (2100 and 2500°F).
8. lower
10. True or False? Catalytic incinerators are generally more
expensive to operate because they use more fuel.
9. b. 370 and 480°C
(700 and 900 °F).
10. False
7-13
-------
11.
12.
13.
In catalytic incinerators, the most effective and commonly
used catalysts are noble metals .such 3-s .. and
True or False? Generally, catalytic incinerators use catalytic
structures made entirely of the noble metals.
True or False? All catalysts deteriorate or lose activity with
normal use.
11. platinum,
palladium
12. False. The noble
is merely coated
cheaper support
substance.
metal
on a
13. True
References
Bethea, R. M. 1978. AIT Pollution Control Technology. New York: Van Nostrand Reinhold.
Environmental Protection Agency (EPA). 1972. Afterburner Systems Study. EPA-R2-062. Research
Triangle Park, NC.
Environmental Protection Agency (EPA). 1973. Air Pollution Engineering Manual, AP-40. Research
TriangleJ>ark,_ NC.
Environmental Protection Agency (EPA). 1977. Controlling Pollution from the Manufacturing and
Coating of Metal Products. Research Triangle Park, NC.
Environmental Protection Agency (EPA). 1978. Study of Systems for Heat Recovery from After-
burners. Industrial Gas Cleaning Institute, Contract No. 68-02-1473 (TASK 23). Research Triangle
Park, NC.
Gottschlich, C. F. 1977. Combustion. Air Pollution Vol. IV Engineering Control of Air Pollution,
A. C. Stern, ed. New York: Academic Press.
Mueller, J. H. 1977. Heat Recovery. Air Pollution Control and Design Handbook, Part 1.
P. N. Cheremisinoff and R. A. Young, eds. New York: Marcel Dekker, Inc.
North American Combustion Handbook. 1965. North American Manufacturing Co. Cleveland,
Ohio.
Ross, R. D. 1977. Thermal Incineration. Air Pollution Control and Design Handbook, Part 1.
P. N. Cheremisinoff and R. A. Young, eds. New York: Marcel Dekker, Inc.
Snape, T. H. 1977. Catalytic Incineration. Air Pollution Control and Design Handbook, Part 1.
P. N. Cheremisinoff and R. A. Young, eds. New York: Marcel Dekker, Inc.
Straitz, J. F. 1980. Flaring with Maximum Energy Conservation. Pollution Engineering. 12:47-49.
Waid, D. E. 1974. How to Design Air Pollution Equipment that Will Not Become Obsolete.
Combustion. 45:4-12.
7-14
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Lesson 8
Condensation
Lesson Goal and Objectives
Goal
To familiarize you with the principles of condensation and with
the use of condensers in industry to reduce gaseous pollutant
emissions.
Objectives
At the end of this lesson, you should be able to:
1. describe the basic principles of condensation.
2. recognize the operation principles of direct contact and
surface condensers.
3. recognize operation differences between contact and sur-
face condensers.
4. list three industries that use condensers to reduce gaseous
emissions.
5. recognize the limitations of condensers as primary devices
for the control of gaseous pollutants.
Introduction
Condensation is the process of reducing a gas or vapor to a
liquid. Any gas can be reduced to a liquid by lowering its
temperature and/or increasing its pressure. The most common
approach is to reduce the temperature of the gas stream, since
increasing the pressure of a gas is very costly.
Condensers are simple, relatively inexpensive devices that
usually use water to cool and condense a vapor stream. Since
these devices are usually not capable of reaching low tem-
peratures (below 21 °C or 70 °F), high removal efficiencies are
not obtained unless the vapors will condense at high
temperatures (usually above 38 °C or 100°F). Condensers are
typically used as pretreatment devices. They are used ahead of
incinerators, absorbers, or adsorbers to reduce the total gas
volume to be treated by these more expensive pieces of equip-
ment. Used in this manner, they help reduce the overall cost of
the control system.
8-1
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Condensation Principles
When a hot vapor stream contacts a cooler surface, heat is
transferred from the hot gases to the cooler surface. As the
temperature of the vapor stream is lowered, the average kinetic
energy of the gas molecules is reduced. Also, the volume that
these vapors occupy is reduced. Ultimately the gas molecules
are slowed down and crowded together so closely that the
attractive forces between the molecules cause them to condense
to a liquid.
Two conditions aid condensation: low temperature (so that
the kinetic energy of the gas molecules is low) and high
pressure (so that the molecules are brought close together).
The actual conditions at which a particular gas molecule will
condense depend on its physical and chemical properties.
Condensers
Condensers fall into two basic categories: contact condensers
and surface condensers. In a contact condenser, the coolant
and vapor stream are physically mixed. They leave the con-
denser as a single exhaust stream. In a surface condenser, the
coolant is separated from the vapors by tubular heat-transfer
surfaces. The coolant and condensed vapors leave the device by
separate exits. Surface condensers are commonly called shell-
and-tube heat exchangers. The temperature of the coolant is
increased, so these devices also act as heaters.
Direct Contact Condensers
Contact condensers are simple devices such as spray towers,
steam or water jet ejectors, and barometric condensers. These
devices bring the coolant, usually water, into direct contact
with the vapors as illustrated in Figures 8-1, 8-2, and 8-3. The
liquid stream leaving the condenser contains the coolant plus
the condensed vapors. If the vapor is soluble in the coolant
then absorption also occurs. Absorption increases the amount
of contaminant that can be removed under the given
conditions.
Spray tower condensers (Figure 8-1) are the same as spray
absorbers. The vapors enter the bottom of the tower, and
coolant is sprayed down over them. Baffles are usually added
to ensure adequate contact between coolant and vapors. Ejec-
tors (Figure 8-2) and barometric condensers (Figure 8-3)
operate in a similar manner as spray tower condensers; the only
difference is that they use liquid sprays to move the vapor
8-2
-------
stream. In both of these devices, the coolant is sprayed into a
venturi throat. This spraying action creates a vacuum that
moves the vapor stream through the condenser. Therefore, a
fan is not required with these systems.
Entrainment separator
Spray nozzles
Figure 8-1. Spray condenser (direct contact).
Discharge
Figure 8-2. Jet ejector condenser
(direct contact).
Water
inlet
Figure 8-3. Barometric condenser
(direct contact).
8-3
-------
Surface Condensers
A surface condenser is usually in the form of a shell-and-tube
heat exchanger (Figure 8-4). The device consists of a circular
or oval cylindrical shell into which the vapor stream flows.
Inside the shell are numerous small tubes through which the
coolant flows. Vapors contact the cool surface of the tubes,
condense, and are collected—while noncondensed vapors are
sent for further treatment.
Vapor Liquid
outlet inlet
Liquid
reversing
channel
Condensate
outlet Liquid
outlet
Figure 8-4. Shell-and-tube heat exchanger.
The entire stream of coolant flows through parallel tubes.
The cooling liquid goes in one end of the tubes, reverses direc-
tion at the reversing channel, passes back through another set
of tubes, and then exits. The uncondensed vapor stream passes
over and around the tube bundle. Some vapor condenses on
the tubes, and is collected in the bottom portion of the shell.
Some vapor does not condense and leaves through the vapor
outlet. The shell-and-tube heat exchanger requires a large
number of tubes and a low gas velocity through the exchanger
to provide adequate heat transfer.
Comparison of Contact and Surface Condensers
Since, in contact condensers, coolant is merely sprayed on the
vapors, these systems are simpler in design, less expensive, and
more flexible in application than are surface condensers.
However, contact condensers require more coolant, and,
because of direct mixing, produce 10 to 20 times the amount
of wastewater (condensate) than do surface condensers. Since
the wastewater from a contact condenser is contaminated with
vapors, it cannot be reused and can pose a water disposal
8-4
-------
problem. If the condensed vapors have a recovery value, sur-
face condensers are usually used, since their condensate can be
recovered directly.
Applications
Condensers are usually used in combination with other air
pollution control devices. Condensers are used ahead of
incinerators, absorbers, or adsorbers to reduce the total gas
volume that must be treated by these more expensive pieces of
control equipment. Condensers are used to control organic
emissions from petroleum refining, petrochemical manufactur-
ing, drycleaning, degreasing, rendering, and surface coating
industries. In addition, condensation processes using a
refrigerant as the coolant are being used to recover gasoline
vapors at bulk gasoline terminals. These systems are more com-
plicated in design and more expensive to install and operate
than are simple water-cooled condensers.
Review Exercise
1. As thetemperature of a gas stream is cooled, the average
kinetic energy of the gas molecules is
a. increased.
b. reduced.
c. remains the same.
2. True or False? Condensation can occur by decreasing the
pressure of a gas stream (at a constant temperature).
1. b. reduced.
3. Condensers are usually pretreatment devices used to reduce
the total that would otherwise be treated by
more expensive control equipment.
2. False
4. What are the two conditions that promote condensation?
3. gas volume
5. Generally, the removal efficiencies of condensers are
because of the temperature range at which they
usually operate.
a. low
b. high
c. 100%
d. 10%
4. low temperature,
high pressure
5. a. low
8-5
-------
6.
7.
8.
9.
10.
11.
12.
Condensers ran be classified as ,_ or
A shell-and-tube heat exchanger is an example of a
condenser.
In an ejector or a barometric condenser, „,, rather
than fans, are used to move the waste gas stream through the
condenser.
A simple spray tower is an example of a _ , , „
condenser.
Shell-and-tube condensers separate the vapor stream from
the , minimizing the amount of condensate.
In a shell-and-tube condenser, the cooling liquid usually
passes through the . side of the exchanger, while
the vapor stream passes through sides.
If the rnndensed vapors have a rerovery valuer _ _
condensers are usually used, since their condensate can be
recovered directly.
—
6. surface,
contact
7. surface
8. water sprays
9. contact
10. coolant
11. tube,
shell
12. surface
References
Bell, K. J. and Ghaly, M. A. 1972. An approximate generalized design method for multicomponent/
partial condensers. Am. Inst. Chem. Engrs. Symp. Series. Vol. 69. No. 131, pp. 72-79.
Kern, D. Q,. 1950. Process Heat Transfer. New York: McGraw Hill Book Co.
Lord, R. C., Minton, P. E., Silusser, R. P. 1970. Design of Heat Exchangers. Chem. Engr
77:96-118 (Jan. 26).
Marchello, J. M. 1976. Control of Air Pollution Sources. New York: Marcel Dekker, Inc.
McCabe, W. L. and Smith, C. J. 1967. Unit Operations of Chemical Engineering. New York:
McGraw Hill Book Co.
Perry, J. H. ed. 1973. Chemical Engineers Handbook 5th ed. New York: McGraw Hill Book Co.
Peters, M. S., and Timmerhaus, K. D. 1968. Plant Design and Economics for Chemical Engineers
New York: McGraw Hill Book Co.
Standiford, F. C. 1979. Effect of Non-Condensables on Condenser Design and Heat Transfer.
Chemical Engineering Progress, pp. 59-62.
8-6
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Lesson 9
Fossil Fuel-Fired Steam Generators
Lesson Goal and Objectives
Goal
To familiarize you with steam boilers—their basic components, how they generate electricity, the
pollutant emissions produced, and the methods used to reduce pollutant emissions.
Objectives
At the end of this lesson, you should be able to:
1. recognize the purpose of a boiler.
2. identify the five basic components of a boiler.
3. name three air pollution emissions produced when fossil fuel is burned.
4. recognize control devices used to reduce paniculate emissions from boilers.
5. recognize control devices used to reduce gaseous emissions from boilers.
Introduction
Steam and electric power are produced by burning fuel in a boiler. Fuels most commonly used are
natural gas, oil, and coal—referred to as fossil fuels. The chemical energy contained in the fuel is
convened to thermal energy in the boiler. The thermal energy heats water contained in tl^e boiler
tubes to make steam. Steam can be used for processes such as generating electricity, space heating,
brewing beer, or producing chemicals.
The Boiler
A simple representation of a boiler, turbine, and generator is shown in Figure 9-1. Water is boiled to
produce steam. Steam drives a turbine which in turn rotates a magnet that is inside a large coil of
wire, which is the generator. Electric current is produced by moving the magnet in a coil of wires.
Boilers used to produce steam to generate electricity are similar to this simple representation.
9-1
-------
Generator
Boiler
Figure 9-1. Simplified schematic of steam generation.
A boiler consists of a burner (or burners) or a fuel spreader and grate to combust fuel, a firebox
where combustion occurs, heat exchange equipment such as boiler tubes, and fans to provide combustion
air and also to remove combustion flue gas products. Boilers or heaters are grouped as fire tube or water
tube boilers.
9-2
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Fire tube boilers are generally small and medium-sized industrial boilers. These units are usually
sold and packaged with burners, blowers, or other equipment all mounted in the same framework. In
fire tube boilers, combustion products pass through the inside of heat exchanger tubes while water
and, eventually, steam are contained outside the tubes by an outer shell (Figure 9-2). These units are
generally used to produce low pressure steam and heat for industrial applications.
Heat exchanger tubes
Water
Outer shell
Fuel
9-2a. Single-pan fire tube boiler.
Water
Heat
exchanger
tubes
Combustion
chamber
Water
9-2b. Four-pass fire tube boiler.
Figure 9-2. Typical fire tube boilers.
9-3
-------
Water tube boilers are constructed in a wide range of sizes. All large steam generators are water
tube boilers. In water tube boilers, hot combustion products pass over tube sections that contain
water. Water is boiled to make steam that is collected in steam drums in the furnace. Water tube
boilers are used when large amounts of high pressure steam are needed.
A typical water tube boiler is shown in Figure 9-3. The five main sections are the fire wall (or
water tubes), superheater, convection tubes, economizer, and air preheater. Each tube section acts as
a heat absorbing surface that extracts the heat from the flue gas as it passes through the boiler.
Firebox
' Superheater
• Convection tubes
Economizer
I
Fuel and Combustion
combustion air *" to firebox
Figure 9-3. Typical water tube boiler.
9-4
-------
The firebox, or furnace, (Figure 9-4) is virtually surrounded by water tubes. Hot combustion
products (flue gas) at temperatures approximately 1400 °C (2550°F) radiate heat to the water tubes.
Water in the tubes turns into steam, the steam circulates, and it is collected in a steam drum or
drums located in the convection tube section (Figure 9-5). Hot flue gas is pulled through the boiler
by an induced draft fan. Hot gas passes over the convection tube section (or sections) which are
located in the upper portion of the boiler. Steam and water that may have condensed when reaching
the steam drum are heated as the hot gas moves over and around the convection tubes.
Flue gas
~1400°C
Figure 9-1. Firebox with water tubes.
- 1300°C
~600°C
Figure 9-5. Convection section and superheater.
9-5
-------
The superheater section is another bank of boiler tubes where steam from the boiler drum is
heated to very high temperatures (Figure 9-5). A modern boiler will produce steam at a pressure of
approximately 13,790 kPa (2000 psi). Steam in a boiler drum is called saturated steam. The
temperature of saturated steam at 13,790 kPa (2000 psi) is 336°C (637°F). Steam at atmospheric
pressure, 101 kPa, is 100°C (212°F). When steam at 13,790 kPa (2000 psi) from the boiler drum
passes through a superheater, its temperature may be raised by 167°C (300 °F) or more to approx-
imately 538°C (1000°F). This superheated steam has several advantages over ordinary steam. It is
hotter; therefore, boiler efficiency is increased. Also, since superheated steam is "drier," it does not
easily condense into water droplets that can corrode and erode turbine parts. The flue gas passes over
the convection section and superheater at a temperature of approximately 1300°C (2370 °F) and
leaves at approximately 600°C (1110°F).
Another section of tubes is the economizer shown in Figure 9-6. The economizer heats the feed-
water before it is delivered to the steam drum. Steam is drawn from the steam drum to the turbine
as the demand for electricity increases. An equivalent amount of water, called make-up water, is
pumped through the economizer tubes to heat the water before it enters the steam drum. A bank of
tubes is placed in the boiler where the flue gas passes over the tubes at approximately 316°C (600°F).
Cold feedwater inside the tubes is heated to a temperature approaching the temperature of water in
the boiler [100°C (212°F)].
Steam drum
Economizer
~250°C
Water in
(~ 20°C)
Figure 9-6. Economizer.
9-6
-------
The air preheater is a tube section that preheats the air used for burning the fuel in the furnace
(Figure 9-7). This section takes out the last little bit of usable heat from the flue gas. Cool air is
forced through a small bank of tubes by a small forced draft fan. The hot flue gas passes the tubes,
transferring heat to the cool combustion air. The "warmed" combustion air is diverted to the burners
where it is used to burn the fuel. The flue gas leaving the air preheater to the stack is now at a
temperature of approximately 148 to 204 °C (300 to 400 °F).
Figure 9-7. Air preheater
Many modern large water tube boilers use regenerative air preheaters. Regenerative air preheaters
are large heat exchanger wheels that contain heat absorbing materials (Figure 9-8). In these devices,
hot flue gas flows through one portion of the wheel while cool, clean combustion air passes through
the remaining portion. Heat is stored in the absorbing material through which the hot flue gas flows.
As the wheel revolves, the cold combustion air passes through these hot surfaces and becomes heated.
This preheated air is sent to the burners and is burned with fuel in the firebox. The absorbing
material of the wheel is constructed of corrugated sheet metal plates. The plates, arranged in a
9-7
-------
honeycomb matrix, provide both maximum heat transfer and air flow between the plates. These
devices are more efficient than shell-and-tube heat exchangers and can usually operate at pressure
drops less than 10 cm H2O (4 in. H,O).
Hot flue gas
Preheated
combustion
To stack
exhaust
Cold air
Figure 9-8. Heat wheel air preheater.
Other equipment is included in the boiler design. Pumps are used to move steam and water
throughout the system. Low pressure pumps transfer water from rivers or lakes through treatment
processes into the boiler. High pressure pumps circulate steam through the convection section,
superheater, and turbine. Condensers produce a vacuum at the exhaust end of the turbine to
increase the turbine efficiency. Condensers also recover condensed steam that is reused in the boiler.
Fuel feeding equipment such as coal pulverizers or oil atomizers are used to provide fuel to the
burners. If coal is burned, bottom ashes must be removed by conveyors and hoppers.
In modern boilers used for producing steam and generating electricity, careful attention is given to
the plant operation. Water must be treated to prevent scaling problems in boiler tubes. Cooling
water must also be treated before it is returned to lakes or rivers since chemicals are usually added to
cooling water to prevent algae buildup in the cooling tower. Ashes and other pollutants in the flue
gas must be removed before the flue gas enters the atmosphere, further complicating the design of
the plant.
9-8
-------
Review Exercise
1. A boiler consists of a burner (or burners) or a fuel spreader
and grate to combust fuel, a firebox, heat exchange equip-
ment such as , and to provide combus-
tion air and to remove flue gas products.
2. In
, boilers, combustion products pass through the
inside of heat exchanger tubes while water and steam are
contained outside the tubes by an outer shell.
1. boiler tubes,
fans
3. In boilers, hot combustion products pass over
tube sections that contain water. Water is boiled to make
steam that is collected in steam drums.
2. fire tube
4. The
is a section of tubes that heats the feed-
water before it is delivered to the boiler.
a. convection
b. economizer
c. superheater
d. air preheater
3. water tube
5. True or False? The temperature of steam in the superheater
can be as high as 538 °C (1000°F).
4. b. economizer
6. True or False? The air preheater tube section is located in
the firebox.
5. True
7. Steam circulates through boiler tubes and is collected in steam
drums located in the tube section.
6. False
8. The purpose of a boiler is to convert energy in
fossil fuels to energy, which in turn heats
to make
7. convection
8. chemical,
thermal,
water,
steam,
9-9
-------
Air Pollution Emissions
Air pollution emissions generated from burning fossil fuels in a boiler are paniculate matter, sulfur
dioxide (SO2), nitrogen oxides (NOX), and carbon monoxide (CO). These are emitted in varying
amounts depending on the fuel burned and the boiler's operating conditions. A complete listing of
emission factors for particulates, SO2, and NO, emitted from boilers is given in the EPA publication
AP-42.
Particulate Matter
Paniculate matter is emitted from a boiler stack when fossil fuel is burned in the furnace. Since coal
usually contains a higher content of ash than fuel oil or gas, the paniculate emissions from coal-fired
boilers are usually greater than those from oil- or gas-fired boilers. In fact, when natural gas is
burned in a boiler, paniculate emissions are almost nil. Coal-fired boilers produce different amounts
of paniculate emissions depending on ash content of the coal and the way the fuel is burned in the
furnace. For instance, when coal is pulverized and burned in a pulverized coal-fired boiler, the par-
ticulate emissions are higher than those from a cyclone-fired or spreader stoker-fired boiler. Cyclone
boilers bum a coarser coal than pulverized coal-fired boilers, resulting in coarser fly ash in the
exhaust gas. The furnace is a cylinder where the combustion air enters the boiler tangentially. In a
spreader stoker-fired boiler or a mass-fed stoker-fired boiler, coal is fed onto a grate where it is
burned. Ashes are removed by a ram, vibrating grate, or traveling chain grate. Many large industrial
boilers are spreader stoker-fired units. Small boilers are mass-fed stoker-fired units.
For fuel-oil combustion, paniculate emissions vary depending on the grade and composition of the
fuel burned, the type and size of the boiler, and the firing and loading practices used. Loading prac-
tice refers to the percent capacity—such as 50%, 75%, or 100% of the boiler's rated capacity—at
which the boiler is operated. The amount of particulate emissions resulting from burning fuel oil
depends mostly on the grade of the fuel burned. Fly ash emissions are greater when burning heavy
residual oils (#6 and #5 grades) than when burning lighter distillate oils (#2 grade). Paniculate emis-
sions are also a function of the sulfur content when burning residual oil. The higher the sulfur con-
tent for residual oil, the higher the paniculate emissions generated. This is because high sulfur
residual oil contains more ash, sulfur, and heavy organic compounds that are difficult to burn cleanly.
Sulfur Dioxide
Sulfur dioxide emissions occur when the sulfur contained in the fuel is oxidized to SO2. Therefore,
the lower the amount of sulfur contained in the fuel, the lower the resulting emissions will be when
the fuel is burned. Natural gas contains very little sulfur and, consequently, a very small amount of
SO2 is emitted from a gas-fired boiler. Fuel oils contain varying amounts of sulfur in the oil. The
heavier oils usually contain more sulfur than the lighter oils. SO2 emissions resulting from burning
coal will depend on the amount of sulfur contained in the coal. Low sulfur western coal usually has a
sulfur content of less than 1% whereas some high sulfur eastern coals contain between 3 and 6%
sulfur. The higher the sulfur content in the coal, the larger the amount of SO2 emitted.
Nitrogen Oxide
When fossil fuels are burned in a furnace, nitrogen oxides (NOX) are formed by two processes. In the
first, the nitrogen and oxygen contained in the combustion air react at the high temperatures in the
furnace to form nitrogen oxide (NO). In the second, the nitrogen compounds contained in the fuel
are oxidized to form NO. The important factors that affect the formation of nitrogen oxides
are: flame and furnace temperature, residence time that the combustion products are at the flame
9-10
-------
temperature, the nitrogen and oxygen content of the combustion air, and the nitrogen content of the
fuel that is burned. In large boilers, approximately 95% of the NO, is in the form of NO, the
remainder is nitrogen dioxide (NOZ).
In boilers, nitrogen oxide emissions will vary depending on which fuel is burned. If coal is burned,
NO, emissions will be high because coal has a high percentage of nitrogen and the temperature of
the flame is high. For fuel oil, the NO. emissions will vary depending on how much nitrogen is con-
tained in the fuel oil and also on the conditions in the furnace. NO, emissions from gas-fired boilers
occur mainly because of the high temperatures in the furnace since natural gas contains very little
nitrogen.
Carbon Monoxide
Carbon monoxide (CO) is formed as a result of the incomplete combustion of the fuel (see Lesson 7).
If the boiler is operated properly, the CO emissions will be relatively low regardless of the fuel that is
burned.
Table 9-1 summarizes emissions at fossil fuel-fired steam generators and lists the major emission
points for each emission.
Table 9-1. Potential source) of atmospheric emissions
within foMil fuel-fired steam generators.
Emission
Particulate matter
Sulfur dioxide
Nitrogen oxide
Souice
Oil-fired boUen
Coal-fired boilers
Coal-handling operations*
Ash disposal operations*
Oil-fired boilers
Coal-fired boilers
Gas-fired boilers
Oil-fired boilers
Coal-fired boilers
•Fugitive emissions.
Air Pollution Control Equipment
Particulate Emission Control
Paniculate emissions are controlled by using multicyclones, scrubbers, electrostatic precipitators
(ESPs), and baghouses. Multicyclones are used to remove large particles and therefore can be used
before scrubbers, ESPs, or baghouses. ESPs have been designed to remove particles with a 99.94-
percent efficiency. One problem with using ESPs occurs when low sulfur coal is burned in the boilers
(see Lesson 4). Since this dust is difficult to collect because of resistivity, it is usually treated by
adjusting the flue gas temperature, adding moisture to the gas stream, or using an additive such as SOS.
When baghouses are used to reduce dust from coal-fired boilers, the temperature of the incoming
flue gas must be carefully controlled (see Lesson 3). The flue gas temperature must be high enough
to prevent water or acid from condensing in the baghouse. However, the temperature must be low
enough to keep the fabric material in the baghouse from deteriorating. Materials such as Teflon® or
Fiberglas® are frequently used for bags. Baghouses and ESPs have been used to control paniculate
emissions from coal-fired boilers. ESPs have been used to control particulate emissions from oil-fired
boilers. No particulate emission control equipment is necessary on gas-fired boilers since natural gas
is a relatively clean fuel.
9-11
-------
Sulfur Dioxide Control
Sulfur dioxide (SOZ) is emitted from coal-fired and oil-fired boilers burning fuels that contain sulfur.
SOZ emissions have been controlled by both wet and dry scrubbing. These are usually called flue gas
desulfurization (FGD) processes. The two most popular wet scrubbing methods are lime and
limestone scrubbing. Approximately 75% of all installed FGD systems use a lime or limestone slurry
as the scrubbing liquor. Here SO* reacts with the lime or limestone slurry to form calcium sulfite and
calcium sulfate sludge. The sludge must be disposed of in a pond or landfill. Other wet scrubbing
systems include the Dual Alkali, Wellman-Lord, and Magnesium Oxide processes. Additional infor-
mation on these FGD processes can be found in EPA 450/2-81-005 Control of Gaseous
Emissions—Student Manual. Most wet scrubbing FGD systems are capable of reducing SOZ emissions
by 90%.
In dry FGD scrubbing, an alkaline slurry is injected in a spray dryer and the resulting dry particles
are collected in a baghouse or electrostatic precipitator (Figure 9-9). Spray dryers are vessels where
hot flue gases are contacted with a fine, wet, alkaline spray. The high temperature of the flue gas,
121 to 204°C (250 to 400°F), evaporates the moisture from the alkaline spray, leaving a dry product.
The dry product is collected in a baghouse or ESP. Dry scrubbing FGD systems can remove approx-
imately 75 to 90% of the SO2 emissions.
Baghouse
Figure 9-9. Spray dryer with baghouse.
Nitrogen Oxide Control
Nitrogen oxides are emitted from gas-, oil- and coal-fired boilers. These emissions can be reduced by
two control methods: combustion modifications and flue gas treatment. Combustion modifications
are changes made in the operation and design of the furnace. Some of the more widely used combus-
tion modification techniques include the use of low excess air, staged combustion, flue gas recircula-
tion, and low-NOz burners. These combustion modifications alter the combustion conditions in the
furnace. This can be accomplished by:
• reducing the peak flame temperature,
• reducing the residence time the combustion products remain in the chamber, or
• changing the mixing rate of the fuel and air.
NOZ emissions can be reduced from 10 to 40% depending on the fuel burned and the combustion
conditions in the furnace. Advanced combustion modification processes may be able to further
reduce NO* emissions in the future.
9-12
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Nitrogen oxide emissions can also be reduced by treating the flue gas after it leaves the combustion
zone. Flue gas treatment methods include the Exxon Thermal DE-NO,, Selective Catalytic Reduction
(SCR), and the Shell UOP processes. These processes have been used in Japan to reduce NO, emis-
sions from utility boilers. Pilot projects are currently being tested in the United States at a number of
utilities. Full scale units are expected to be installed in the next few years. The Exxon process has
reduced NO* emissions by 60%, and the SCR and Shell UOP processes have reduced NO, emissions
by 90%. Flue gas treatment methods are much more costly than combustion modifications. Addi-
tional information on flue gas treatment and combustion modification techniques can be obtained
from EPA 450/2-81-005, Control of Gaseous Emissions—Student Manual.
New Source Performance Standards
EPA has promulgated New Source Performance Standards (NSPS) for fossil fuel-fired steam
generators with heat input greater than 73 megawatts (250 X 10* Btu/hr) heat input. These standards
establish emission limits for utility boilers. There are two standards: one for steam generators
installed after August 17, 1971, and one for electric utility steam generators installed after September
18, 1978. The NSPS for utility boilers is given in Table 9-2. Standards for industrial boilers are forth-
coming and will be published in the Federal Register.
9-13
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Table 9-2. New Source Performance Standards for emissions from fouil fuel-fired steam
generators rated at greater than 73 MW (thermal) or 250 x 10' Btu/hr heat input.
Emissions
Paniculate*
SO,
NO,
NSPS
Subpart D; new
sources after
Augun 17. 1981
Subpart Da; new sources
(electric utility boilers)
built after September 18,
1978
Subpan D; new
sources after
August 17, 1981
Subpan Da; new sources
(electric utility boilers)
built after September 18,
1978
Subpan D; new
sources after
August 17, 1971
Subpan Da; new sources
(electric utility boilers)
built after September 18,
1978
Metric units
(ng/J)
43
13
340
520
340
and 90% SO,
reduction is
required unless
SO, emissions
are less than
86
520
and 90% SO,
reduction
required unless
SO, emissions
are less than
260
then 70% SO,
reduction
is required
86
130
300
86
130
210
260
340
English units
(lb/10- Btu)
0.1
0.03
0.8
1.2
0.8
and 90% SO,
reduction is
required unless
SO, emissions
are less than
0.2
1.2
and 90% SO,
reduction
required unless
SO, emissions
are less than
0.6
then 70% SO,
reduction
is required
0.2
0.3
0.7
0.2
0.3
0.5
0.6
0.8
Fuel
Solid, liquid or gaseous
Solid, liquid or gaseous
Liquid
Solid (coal)
Liquid or gaseous
Solid (coal)
Gaseous
Liquid
Solid (except lignite)
Caseous (except coal derived)
Liquid (except coal derived)
Subbituminous coal
Bituminous/ anthracite coal
and lignite
Lignite mined in NC, SD,
and Montana burned in
a slag-top furnace
"Opacity is not to exceed 20% for periods over six minutes in one hour, and opacity is never to exceed 27%
for all fuels.
Other Potential Emission Points
Boiler stacks are not the only sources of potential air pollution at power plants and industrial boiler
facilities. Fugitive paniculate emissions can occur at coal-fired plants from coal-handling and ash-
handling operations and from wind blowing over coal storage piles. These emissions can be controlled
by using covers and water sprays when possible. Fugitive particulate emissions can also occur from
ash handling, storage and disposal operations.
9-14
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Fossil fuel-fired steam generators are potentially one of the largest manmade air pollution sources.
However, by using air pollution control devices and by proper operation of the boilers and associated
equipment, these plants can operate with a minimum amount of air pollution emitted into the
atmosphere.
Review Exercise
1. True or False? The amount of paniculate emissions from
coal-fired boilers depends on the way the coal is burned in
the furnace.
2. For fuel-oil combustion, the amount of particulate matter
emitted depends on the and of the
fuel burned, the type and size of the boiler, and the firing
and loading practices used.
1. True
3. In a boiler, SO* emissions occur when the
contained in the fuel is oxidized.
2. grade,
composition
4. The higher/lower the sulfur content in coal, the higher the
SO* emitted when the coal is burned.
3. sulfur
5. True or False? In a boiler, nitrogen oxides are formed
because the nitrogen content in the fuel is oxidized forming
NO, and the nitrogen and oxygen contained in the combus-
tion air react at high temperatures to form NO.
4. higher
6. In large boilers, the majority of NO, emitted (approximately
95%) is in what form?
a. N4O
b. NOt
c. NO
d. SO,
5. True
7. In a boiler burning coal and using high flame temperatures,
NO, emissions will usually be high/low because coal usually
contains a large/small amount of nitrogen.
6. c. NO
8. True or False? In a boiler, the amount of carbon monoxide
emitted depends solely on the amount of carbon contained
in the fuel.
7. high
large
8. False
9-15
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9. Paniculate matter emitted from a boiler is usually reduced
by using a
a. baghouse.
b. scrubber.
c. electrostatic precipitator.
d. any of the above
10. True or False? Sulfur dioxide (SOZ) emissions from a boiler are
usually reduced by using a cyclone.
9. d. any of the above
11. In dry FGD scrubbing, an alkaline spray is injected into a
with particle collection in a
baghouse or electrostatic precipitator.
10. False
12. True or False? In a boiler, NO, emissions can be reduced by
using either combustion modification techniques or flue gas
treatment.
11. spray dryer,
dry
12. True
References
Environmental Protection Agency (EPA). 1982. APTI Course SI:412 Baghouse Plan Review—Student
Guidebook. 450/2-82-005.
Environmental Protection Agency (EPA). 1981. APTI Course 415 Control of Gaseous Emissions-
Student Manual. EPA 450/2-81-005.
Environmental Protection Agency (EPA). 1981. APTI Course 413 Control of Paniculate Emissions-
Student Manual. EPA 450/2-80-066.
Environmental Protection Agency (EPA). 1981. Compilation of Air Pollutant Emission Factors
AP-42.
Environmental Protection Agency (EPA). 1973. Second edition. Air Pollution Engineering Manual
AP-40.
TPC Training Systems. 1974. How Power Plants Work. Barrington, Illinois: Technical Publishing
Company.
9-16
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Lesson 10
Steel Mills
Lesson Goal and Objectives
Goal
To familiarize you with the general operation of a steel mill, potential air pollution emission points,
and the control equipment used to reduce these emissions.
Objectives
At the end of the lesson, you should be able to:
1. describe each of the basic processes of furnaces used in refining iron ore into steel listed below:
• coke ovens,
• blast furnaces,
• basic oxygen furnaces,
• electric furnaces, and
• sinter plant.
2. list four sources of air pollution and emissions from each.
3. describe the control equipment or operating procedures used to reduce emissions from air pollu-
tion sources in a steel mill.
Introduction
Iron and steel are refined metals used for making many different products. Iron is an element mined
as iron ore in many States, but is found in great abundance in Minnesota and Michigan. Iron is very
rarely mined as a pure metal. The ore consists of iron oxides and contains impurities such as carbon,
silicon, phosphorus, and sulfur. Therefore, iron and steel metals are produced by refining the ore.
Steel is produced by refining iron metal. The steelmaking process is analogous to making jelly from
fruit. To make jelly, the fruit is cooked, the juices are strained, and ingredients are added to give the
jelly its desired flavor. To make steel, iron ore is melted in a number of furnaces, impurities are
removed in the form of slag, and other materials are added to give the steel its desired properties
such as hardness, strength, or elasticity.
Many processes are used to make the final steel product. The processes used depend on the initial
raw materials and the desired final products. A typical integrated steel mill will consist of many refin-
ing, shaping, and forming processes as shown in Figure 10-1. These include producing coke in coke
ovens, melting pig iron in blast furnaces, refining iron into steel in basic oxygen or electric furnaces,
reheating ingots in soaking pits, reclaiming usable materials in sinter plants, rolling steel products in
blooming, slabbing, and billet mills, and forming and coating the steel to give the finished product.
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Limestone and
iron ore mining
Figure 10-1. Steel process—making steel.
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Soaking pit
Teeming and
stripping ingou
Figure 10-1 (continued). Steel process—shaping and forming.
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Coke Oven Batteries
Coke, the chief fuel used in blast furnaces, is the residue remaining from coking certain grades of
bituminous coal. Coke is produced in a coke oven battery by driving off the volatile compounds in
coal, leaving a strong residue that contains a high percentage of carbon and relatively few impurities.
Coke must be strong enough to support the weight of limestone and iron ore in the blast furnace. A
typical coke oven battery is shown in Figure 10-2. Coke oven batteries are frequently referred to as
coke batteries.
Volatile materials
by-products plant
Regenerative
chamber
Coke quench car
Coke guide
sleeve
Figure 10-2. Typical coke oven battery.
In a coke oven, coal is heated in the absence of air in specially designed refractory chambers.
Volatile material is driven off (but not burned) and then piped to distillation and separation columns
where valuable by-products such as phenols, napthalene, benzene, toluene, and ammonium sulfate
are extracted. After these chemicals are removed, some of the gas (approximately 40%) is returned
to the coke oven to supply heat.
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Coke batteries consist of three main parts —coking chambers (or ovens), heating chambers (or
flues), and regenerative chambers—all constructed with refractory brick. A coke battery usually has
40 to 100 coking chambers (or ovens) and each chamber is approximately 4 to 7 m (12 to 20 ft) high,
9 to 15 m (30 to 50 ft) long, but only 0.3 to 0.6 m (1 to 2 ft) wide. Doors are at both ends of the
coking chamber. The coking chambers are separated by heating chambers, so that a heating
chamber is on each side of a coking chamber. Regenerative chambers are located underneath to sup-
port the battery and to control the flow of hot air and coke oven gas used in the heating chambers.
Coal is charged into the coking chamber through ports on the top of the oven—a process called
the charging cycle. A measured amount of coal (16 to 20 tons) for an oven charge is drawn from a
storage bin and moved to the oven that is to be charged. The coal is hauled by a rail (larry) car that
travels on top of the battery. After the oven has been charged, the ports are sealed and the coal
begins to fuse. The top of the coal charge is leveled by a long bar that is inserted through the
pushing side of the battery. The leveling bar goes through a chuck door located on the oven door.
The coking cycle lasts from 15 to 20 hours until all the volatile material in the coal is removed, at
which time the coke is ready to be pushed. The doors at both ends of the coking chamber are then
removed, and a long arm, or pushing ram, pushes the coke into a coke quench car (open rail car).
The car then moves to a quench station where the red hot coke is quenched with water sprays to stop
it from burning. It is then stored until needed in the blast furnace. After the pushing cycle is com-
pleted, the doors are cleaned and reset back into place, and that oven is ready to be charged with
new coal.
Blast Furnaces
Iron oxides are reduced to iron, commonly called pig iron, in blast furnaces. The furnaces are large,
steel shells lined with fire brick. Blast furnaces have hearth diameters ranging from 4.5 to 13.5 m (15
to 45 ft) with overall heights up to 60 m (200 ft). The blast furnace also has three or four stoves that
are fire-brick lined steel shells. The stoves are used to heat the air used in the furnace (Figure 10-3).
Blasts of air are injected through nozzles, called tuyeres, located near the bottom of the furnace.
Figure 10-4 shows the flow of raw materials through a blast furnace. Iron ore, coke, and limestone
are the materials charged into blast furnaces. Measured proportions of each are hoisted to the top of
the furnace in skip cars. These materials are charged into the furnace through a bell-shaped hopper.
Furnaces are rated according to the amount of pig iron produced in one day. Production ratings
range from 907 metric tons (1000 tons) per day for small furnaces up to 9070 metric tons (10,000
tons) per day for very large ones. As the coke burns, carbon monoxide forms and reduces the iron
oxide to iron. Limestone removes the impurities in the iron ore by reacting with the impurities to
form slag. Iron, as a white-hot liquid, flows to the bottom of the furnace as it is reduced. The slag
also flows to the bottom of the furnace. Since slag is lighter than the molten iron, it floats on top of
the iron and helps prevent the iron from being reoxidized. The furnace is tapped every four to five
hours. The pig iron flows into refractory-lined torpedo-shaped rail cars and the slag into refractory-
lined slag cars. The pig iron is poured into molds or transferred to steel furnaces to be refined into
steel. The slag is cooled and then usually processed into gravel, Portland cement, or some other
building material.
Iron ore is also occasionally agglomerated into pellets which are used in the blast furnace. Pellets
are formed by rolling a mixture of fine iron ore with a clay binder. Most pellet plants are located at
or near the iron ore mining sites.
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Blast furnace stove
. - Air
Figure 10-3. Blast furnace.
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Skip car
Heanh diameter
Blast furnace gai
Figure 10-4. Flow of materials in a blast furnace.
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Steel Furnaces
Pig iron, as it comes out of the blast furnace, contains about 5% carbon and other impurities such as
silicon, phosphorus, manganese, and sulfur. Pig iron is refined into steel by oxidizing the impurities
in a basic oxygen furnace (EOF), an electric furnace, or an open hearth furnace. Most steel is pro-
duced in electric or basic oxygen furnaces. In the U.S. in 1981, basic oxygen furnaces produced
about 61% of all steel, electric arc furnaces about 28%, and open hearth furnaces 11%. The use of
open hearth furnaces has declined during the last 15 years because basic oxygen furnaces and electric
furnaces are more efficient. The melting time of an open hearth furnace is usually four times that of
a basic oxygen furnace and twice that of an electric furnace.
Basic Oxygen Furnaces
The basic oxygen process was a great technological breakthrough that helped increase U.S. steel pro-
ductivity in the early 1960's. Developed in Austria in 1953, this process reduced the melting time
required from about eight or nine hours (in an open hearth furnace) to about 45 minutes. This new
method reduced the necessary investment capital and appreciably reduced the operating costs.
The EOF vessel is a pear-shaped steel shell lined with special refractory brick. The vessel, closed at
the bottom and open at the top, sits on a rocking mechanism so it can be moved to different posi-
tions for furnace charging, oxygen blow, and steel tapping, as shown in Figure 10-5. Charging lasts
for about five minutes and consists of approximately 70% molten pig iron and 30% cold scrap steel.
After charging is completed, the furnace is returned to the upright position for the oxygen blow. A
water-cooled oxygen lance is then lowered into the vessel. Pure oxygen is blown into the bath of
molten pigjron to agitate the molten metal, add heat to the process, and oxidize any of the
impurities contained in the pig iron. This raises the temperature of the molten bath to about 1725°C
(3000°F) and lasts for about 25 minutes. Unwanted impurities are oxidized, and lime and other
fluxes are added to the molten metal to combine with these impurities to form slag.
After the oxygen blow is completed, the steel is first tested to check for the desired temperature
and chemical components. The furnace is then tilted to pour the molten steel out of a taphole that is
near the top of the vessel. The steel flows into a ladle where any substances such as chromium,
manganese, or nickel are added to bring the steel to the batch specifications. Slag is poured into slag
pots and sent for further processing or for disposal. Since the basic oxygen process requires pig iron,
it is used in conjunction with blast furnaces and coke ovens.
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Water-cooled
oxygen lance
Steel tapping
Figure 10-5. Basic oxygen furnace and positions.
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Electric Furnaces
Electric furnaces have been used for making steel since the early 1900's. Scrap metal is used almost
exclusively as the raw material to produce many different varieties of alloy, specialty, and carbon
steel. Hot pig iron is very rarely used in the charge, thereby eliminating the need for blast furnaces
and coke ovens at the plant.
Large graphite or carbon electrodes are lowered from holes in the roof of electric furnaces. Elec-
trodes may range from 25 to 61 cm (10 to 24 in.) in diameter, depending on the size of the furnace.
The body of the furnace consists of a steel shell lined with refractory brick. The removable top is also
lined with refractory brick. A modern 200-ton furnace has a shell 7.3 m (24 ft) in diameter. The roof
swings aside for charging. After the furnace has been charged with scrap, the roof is replaced and
the electrodes are lowered until they are sitting a few inches above the scrap, as shown in Figure
10-6. When the current is turned on, the scrap begins to melt. The current arcs from one electrode
to the scrap, passes through the scrap, and arcs to another electrode. Intense heat (3000°C) is
generated by the arcs and the electrical resistance of the charge (scrap), quickly melting the charge.
A full charge is melted in approximately one to three hours. The power consumed is approximately
500 kWh per ton of steel produced.
During the melt, lime is occasionally added to combine with impurities to form slags. Most fur-
naces also use water-cooled oxygen lances that are inserted into the side wall of the furnace
chambers. The lance is used to help oxidize the impurities in the charge and to quickly form fluid
slags. Slags are removed periodically by tilting the furnace to allow them to flow into slag pots. A
small container of molten metal is periodically taken from the furnace and tested for chemical com-
position. Once the desired constituents of the steel have met the batch specification, the furnace is
tapped (see Figure 10-6) and the steel is poured into ingots or sent to a continuous casting process.
Electric steelmaking offers some advantages over the basic oxygen process—the temperature of the
furnace can be closely regulated, very high temperatures are obtainable, and using scrap material for
the furnace charge eliminates the need for a blast furnace and coke ovens. The electric process does
take longer, sometimes as long as four hours for a melt, as compared to 45 minutes for the basic
oxygen process.
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Electrodes
Melting period
Steel tapping
Figure 10-6. Electric arc furnace.
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Sinter Plants
Sintering reclaims some materials such as flue dusts, iron ore fines, coke breeze, mill scale, and
turnings—converting them into a high quality blast furnace feed. Sinter plants are usually found only
at very large integrated steel plants.
In the sinter plant, iron-bearing materials are mixed with fuel (usually coke breeze) and flux
(usually limestone) and placed on a traveling grate (Figure 10-7). This bed of materials is ignited by
burning coke oven gas or natural gas in burners located at the inlet of the traveling grate. As the bed
moves along the traveling grate, air is pulled down through the bed to burn it, forming a fused,
porous red-hot sinter. At the discharge, the bed of fused sinter is broken into smaller chunks by tooth
and comb breakers and pug mills. The hot sinter is then sent to an air cooler that blows air through
the sinter to cool it. The cooled sinter is then crushed and screened to the appropriate size. The
sinter is now ready to be used as blast furnace feed to supplement the iron ore.
To storage
Windbox
exhaust
Cooler
Figure 10-7. Sintering process.
Steel Processing
After the molten steel is tapped from the basic oxygen furnace, the electric furnace, or an open
hearth furnace, it is poured into ingot molds or into a continuous caster.
Ingot Molds
In 1981, approximately 79% of the steel produced in the U.S. was cast into ingot molds. The molds
are removed from the solid ingots when they are cooled (Figure 10-8). The ingots range in size from
less than one ton to 100 tons or more depending on the processing operation. After the steel
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solidifies, the molds are stripped off the ingots and the ingots are placed into a soaking pit to reheat
the steel to an even temperature. Soaking pits, called reheat furnaces, are large furnaces lined with
refractory brick and are generally fired with natural gas or fuel oil (Figure 10-9). After the ingots are
heated for approximately two hours (until they are red hot), they are then sent to a primary rolling
mill.
The ingots pass through massive rollers that reduce them into blooms, slabs, or bars. These are
sent to additional rolling mills and other locations where the steel is made into the final products.
Figure 10-8. Ingot mold*.
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Figure 10-9. Reheating ingots in soaking pits.
Continuous Casting
A continuous caster produces a semifinished form without the ingot-pouring and soaking-pit opera-
tions. Hot steel from the EOF shop or electric furnace is poured directly from the ladle into the
caster. The metal flows from a shallow refractory-lined holding basin into a water-cooled mold
(Figure 10-10). The cooling causes the liquid steel to form a semisolid in the shape of the mold in the
caster. The continuous bar, bloom, or slab is withdrawn at a steady rate and then cut with a saw or
oxygen torch. The finished pieces are cooled further with water sprays. The bars, blooms, or slabs
are now ready for further processing in rolling and final-shaping mills.
From this point the steel can be sent to strip mills, rail mills, wire and rod mills, scarfing mills,
pickling tanks, seamless pipe mills, and final-coating operations. Since these processes do not
generate as much paniculate matter or sulfur dioxide emissions as the coke ovens, blast furnaces, or
steel furnaces, they will not be covered in this course.
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o
I
I—I
01
• Steel from BOF or electric furnace
Figure 10-10. Continuoui calling operation.
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Review Exercise
Three materials used in a blast furnace are
a. coke, iron ore, limestone.
b. coke, coal, sulfur oxides.
c. carbon monoxide, coal, limestone.
d. limestone, iron ore, coal.
Coke is produced in a coke-oven battery by driving off the
compounds in the coal.
a. sulfur
b. carbon
c. volatile
d. docile
1. a. coke, iron ore,
limestone.
Two chambers in the coke-oven battery sit side-by-side with
another chamber underneath. The two chambers that sit
side-by-side are and The one that
sits underneath is the
chamber.
a. coking and regenerative; heating
b. coking and heating; regenerative
c. coking and charging; oxidative
d. degenerative and heating; coking
2. c. volatile
4. Coal is_usually charged into a coke battery from the
while coke is removed from the
a. side, top
b. left side, bottom
top, side
c.
d. top, bottom
3. b. coking and
heating; regenerative
What happens to the hot coke after it is pushed from the
coke battery?
a. It is sent as is to the blast furnace.
b. It is heated with hot liquid.
c. It is quenched with water sprays.
d. none of the above
4. c. top, side
In a blast furnace, iron oxides are refined into
a. steel.
b. pig iron.
c. coke.
d. limestone.
5. c. It is quenched with
water sprays.
In a blast furnace, blasts of air are injected through nozzles
located in the of the furnace.
a. top
b. bell-shaped hopper
c. traveling grates
d. bottom
6. b. pig iron.
7. d. bottom
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8. Impurities are removed from iron ore by using
form slag.
a. carbon monoxide
b. limestone
c. pig iron
d. sulfates
to
9. Which is lighter: slag or molten pig iron?
8. b. limestone
10. Pig iron is usually
a. made into Portland cement.
b. used for building materials.
c. refined into steel.
9. slag
11. True or False? The BOF process of refining steel takes
approximately eight or nine hours as compared to the open
hearth process which takes approximately 45 minutes.
10. c. refined into steel.
12. The BOF is
a. rigid
b. closed
c. tilted differently
d. moved by a crane
for charging and tapping.
11. False. The reverse is
true.
13. BOF and electric furnaces are lined with
conserve heat and protect the steel shell.
a. limestone-
b. refractory brick
c. cast iron
d. graphite
to
12. c. tilted differently
14. The purpose of the sinter plant is to
a. improve the quality of coal.
b. refine the impurities in the iron ore.
c. convert waste materials into high quality blast furnace
feed.
d. both a and b
13. b. refractory brick
14. c. convert waste
materials into high
quality blast furnace
feed.
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Air Pollution Emissions
Air pollution emissions are generated at many processes in a large integrated steel plant. Particulate
matter is generated mainly from blast furnaces, coke ovens, basic oxygen furnaces, electric furnaces,
and sinter plants. Sulfur dioxide emissions can be generated by burning coke oven gas that has not
been desulfurized. Hydrogen sulfide and paniculate emissions are generated when slag is processed.
If the steel mill has an in-plant boiler, paniculate matter and SOj emissions can be generated from
burning coal or high-sulfur fuel oil. These emissions can be controlled by techniques discussed in
Lesson 9, Fossil Fuel-Fired Steam Generators.
Controlling Blast Furnace Emissions
The blast furnace has the potential of emitting paniculate matter and SOj. The gas generated within
the furnace during the refining process contains a high percentage of carbon monoxide (approx-
imately 30%), water vapor, nitrogen, carbon dioxide, hydrogen, and paniculate matter. Once the
paniculate matter is removed, the gas can be used as fuel in the blast furnace and in other processes
throughout the plant. Blast furnace gas has a low heating value of approximately 2.79X 10s to
3.73 x 106 J/m3 (75 to 100 Btu/ft3). The paniculate matter is removed from the blast furnace gas
before it is burned in other parts of the plant. Electrostatic precipitators and high-energy wet scrub-
bers are preceded by a cyclone which removes large dust particles. The collected paniculate matter
can be recycled in the sinter plant.
Particulate matter and some SOZ emissions are generated when iron is cast. Emissions occur when
the furnace is tapped, and molten iron and slag flow in runners into pig iron and slag cars. These
emissions are called cast house emissions, because they are emitted from troughs and runners that are
located over the casting area of the blast furnace. There may be from two to six runners per cast
house, and their length may vary from 8 to 25 m (26 to 80 ft).
Blast furnace cast house emissions can be controlled by a number of techniques. Most control
systems are designed to have a hood or a number of hoods over the taphole and runners. The hoods
connect to duct work and a fan, sending the exhaust to a baghouse to remove paniculate matter
(Figure 10-11). Some systems use covered runners in addition to the hoods and baghouse arrange-
ment to collect the emissions. One other system uses one large hood at the top of the cast house that
evacuates the total cast house emissions into a baghouse. A review of these control techniques is given
in the Proceedings—AIT Pollution Control in the Iron and Steel Industry Specialty Conference,
APCA, April 1981.
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Figure 10-11. Blast furnace cast houie emission control system.
Controlling Coke Oven Battery Emissions
In a coke battery, emissions occur from a number of locations or operations (Figure 10-12). These
include the following:
• charging operations,
• topside leaks,
• pushing operations,
• quenching operations,
• door leaks, and
• coke battery combustion stacks.
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Gas collector main
Heating flue
Charging
port lid
Dropsleeve
Figure 10-12. Coke oven battery charging operations.
Charging Emissions
Each coke oven has three to five charging ports on top of the oven. During charging, the port lids
are removed and coal is evenly charged into each port, usually one at a time. Coal falls through a
dropsleeve on the larry car hopper that is tightly sealed against the charging port. Surface moisture
and some light hydrocarbons contained in the coal are vaporized as the fine coal comes into contact
with the heated refractory walls of the oven. This can be seen as a cloud of steam, brown smoke, and
dust.
To control charging emissions, many steel plants use a method called staged charging in which
alternate ports are charged at different times. The larry car loaded with coal travels to the oven to be
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charged. The port lids are removed and dropsleeves on the larry car hoppers are lowered to the
charging ports to make a tight seal. Steam aspirating nozzles located in the top of the stand pipe are
turned on to help suck coke oven gas into the gas collector main. Coal is charged sequentially, one or
two hoppers at a time. As coal falls into the oven it forces air in the oven to escape. If the system
functions correctly, the displaced air leaves through standpipes to the gas collector main as shown in
Figure 10-12. As soon as a port (or ports) has been charged, the dropsleeves are raised, the lids are
replaced, and another port is charged. After all the ports have been charged, the coal in the oven is
leveled with a leveling bar (or rod).
Another method of coal charging is called pipeline charging—where coal is charged directly into
the top side of the oven by a pipe. The coal is blown through the pipe that is connected above the
door and leveling rod. Coal is usually preheated in a coal preheating facility to a temperature of
approximately 150°C (300 °F). Each oven is charged with preheated coal at the proper charging
sequence. Although this method has the potential to reduce charging emissions by eliminating the
larry car, in practice, the emission rate has sometimes been as high as that of the traditional larry car
charging method.
Topside Emissions
Topside emissions can occur around the charging port lids and stand pipe lids while the coking cycle
is in progress. Frequently, lids are sealed with a clay mixture to reduce emissions. They should be
inspected regularly and replaced if found to be cracked or warped.
Pushing Emissions
The majority of paniculate emissions from a coke oven occur when it is pushed, especially if the coke
battery is poorly maintained. The emissions can be very heavy if the coke is green. Green coke con-
tains a high amount of unvolatilized hydrocarbons and dust particles. It can occur if coke is removed
from the oven too early or if the oven is not properly heated. The coke battery operator should make
sure the coke has been in an oven for the proper coking time before pushing, and not to overfill an
oven when charging it. This will help reduce the number of green pushes that will occur. Good
operating and maintenance practices are very important.
A number of methods have been used to control emissions from coke pushing. Sheds that com-
pletely enclose the coke side of the coke battery (where the coke falls into the railcar after it has been
pushed) have been used successfully. Figure 10-13 illustrates a shed design. The emissions are trapped
in a shed, sucked into ducts by a large fan, and removed by a scrubber, a wet electrostatic
precipitator, or a baghouse.
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To scrubber,
ESP, or baghouse
Figure 10-13. Shed design for coke pushing emissions.
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Another coke side-emission control system uses a movable hood attached to the coke quench car.
The hood usually covers the whole length of the coke quench car and is connected to a fixed duct by
a movable seal (Figure 10-14). The ductwork runs along the length of the coke battery and is con-
nected to a scrubber, electrostatic precipitator, or a baghouse.
To scrubber,
ESP, or baghouse
Figure 10-14. Coke quench car connected to a fixed duct.
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A third design uses an enclosed coke quench car with a mobile gas-cleaning device (scrubber). The
quench car is enclosed with a hood on all sides except where the hot coke falls into the car. The
quench car traps the emissions during the push and they are scrubbed (Figure 10-15). A good review
of all three of these pushing control systems is given in the Proceedings of Control of Air Emissions
from Coke Plants, APCA, April 1979.
Figure 10-15. Coke quench car with mobile scrubber.
Quenching Emissions
Quenching the hot coke with water sprays also produces emissions. These occur at the quenching
tower and can be seen as a billowy gray-colored steam plume. Paniculate matter can be reduced
somewhat by using baffles located in the top of the quench tower. Occasionally, coke plants will
quench coke with dirty water. This practice can cause emissions of ammonia, phenol, and other air
pollutants and should be avoided if possible. One practice that is used in the Soviet Union and in a
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number of European countries to reduce quenching emissions is called dry quenching. Dry quenching
uses inert gases as a medium to collect heat from the hot coke and transport that heat to waste-heat
boilers. This practice helps eliminate quenching emissions and water cleanup problems (Linsky,
1975). However, this practice is not used in the U.S. at the present time.
Door Leaks
Emissions from door leaks occur frequently at poorly maintained coke batteries. Leakage will occur if
doors located on either side of the battery are not properly sealed. The two types of doors commonly
used are the knife-edged, or self-sealing, door and the luted door. The knife-edged door has
adjustable metal strips on the door which contacts the flat door jamb to form a good seal as long as
they are clean and not warped. Some older coke batteries use luted doors that require a clay mixture
(or luting) that is applied to each door after it has been replaced on the oven. Careful maintenance
of both types of doors should include cleaning and inspecting the doors and jambs between each
coking cycle and prompt attention to sealing any visible leaks during each cycle.
Combustion-Stack Emissions
Paniculate and SO* emissions can be emitted from coke battery combustion stacks. Coke oven gas,
cleaned in the by-products plant, is burned in the battery to supply heat. If the coke battery is old,
or if the refractory in the flues is cracked, paniculate matter can be emitted from the combustion
stack. Good maintenance and operating practices can help reduce this problem.
Sulfur dioxide emissions are emitted from the coke battery combustion stack if the coke oven gas
has not been desulfurized. Coke oven gas, after it has gone through the by-products plant, still con-
tains a high concentration of hydrogen sulfide gas (H2S). When this gas is burned in the coke bat-
tery, SOj is emitted. These emissions can be controlled by liquid absorption or wet oxidative proc-
esses. Current coke oven gas desulfurization technology includes using the Vacuum Carbonate and
Sulfiban liquid absorption processes and the Stretford wet oxidative process (Massey and Dunlap, 1975).
Summary
Coke ovens can be one of the largest sources of emissions in a steel mill. Control equipment and
techniques for reducing coke oven emissions are constantly being changed and improved. In addition,
coke oven operators and maintenance persons must be trained in order to keep process and control
equipment operating properly.
Controlling Basic Oxygen Furnace Emissions
Paniculate and carbon monoxide (CO) emissions occur during the oxygen blow. The furnace can use
two different hood systems: an open hood, which has a 0.5 to 0.6 m (1.5 to 2.0 ft) clearance over the
furnace opening, and a movable closed hood that fits tightly around the furnace cop. The second
design minimizes the air drawn into the exhaust fan and control device. With an open hood, carbon
monoxide emissions are usually reduced by burning them in the hood. In a closed-hood system, a gas
is produced that is high in CO content. This gas is either flared at the stack or used as fuel in
another location in the mill. The hot gases generated by the oxygen blow are approximately 1090 to
1650°C (2000 to 3000°F). These are usually cooled by water sprays located above the hood. The
cooled paniculate-laden gases are sent to an electrostatic precipitator or scrubber (scrubbers only, for
closed hoods) to remove the particulate matter.
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Paniculate emissions also occur during charging and tapping operations. The furnace is tilted to
different positions for these operations. If the furnace uses an open hood, the hood can be adjusted
somewhat to help suck in the particulate emissions. If the furnace uses a closed hood, a separate set
of hoods ducted to a baghouse is used to capture the charging and tapping emissions.
Particulate emissions also occur at the reladling station, where the hot metal is received from the
blast furnace and transferred to the EOF shop ladles. These emissions are usually small, submicron
iron oxide panicles, and kish. Kish is graphite that separates from the pig iron as it slowly cools.
These emissions are usually sucked into a hood, then ducted to, and removed by, a baghouse.
Controlling Electric Furnace Emissions
During the scrap melting cycle, particulate matter and carbon monoxide emissions are generated.
Their emission rate depends on the type and amount of scrap charged into the furnace and whether
an oxygen lance is used in the furnace. The majority of the carbon monoxide is oxidized into carbon
dioxide while it is in the furnace.
Paniculate matter can be collected by a number of systems. Most electric steel furnaces have some
type of evacuation duct connected directly to the roof of the electric furnace. The duct is usually
connected to a baghouse, ESP, or scrubber to remove paniculate emissions generated during the melt
cycle. Charging and tapping emissions can be collected by one large hood located in the roof of the
building, a smaller canopy hood located above the furnace, or by two small hoods located above steel
tapping and slag ladles. The trapped emissions are usually ducted to, and collected in, a baghouse.
Some collection systems have been designed to totally enclose the furnace shop and evacuate the
emissions through the building roof to a baghouse. Large air volumes must be sucked into the hood
to evacuate" the building. This system can be used to collect charging, tapping, and melt cycle emis-
sions without additional hooding.
Baghouses are the most popular device used to reduce electric furnace emissions. The gas stream
into the baghouse can be very hot and is usually cooled by air dilution, radiation cooler tubes, or
water sprays. Many baghouses use Dacron® bags. Electrostatic precipitators are not frequently used
because iron oxide dust has high dust resistivity and is difficult to collect.
Controlling Sinter Plant Emissions
Particulate emissions are generated at two points in the sinter plant —at the windbox and at the
coolers (see Figure 10-7). Occasionally SO2 emissions can be emitted from the windbox if the coke
used as fuel in the sinter contains a high content of sulfur or is coke oven gas (that has not been
desulfurized) is used to ignite the bed of materials. However, this is usually not that much of a prob-
lem. Hydrocarbon emissions can occur from the windbox if the mill scale and turnings are extremely
oily.
Electrostatic precipitators, scrubbers, and cyclones have been used to control windbox emissions.
However, ESPs are usually used due to their high collection efficiency. If oily scrap is used as feed
material, care must be taken to prevent ESP collection plates from being coated with tarry par-
ticulate matter. Controlling the amount of oily mill scale processed in the sinter plant can help
alleviate this problem. In addition, hydrocarbon emissions can also be reduced by limiting the
amount of oily feed material going into the sinter plant.
Baghouses have also been used (but not often) to control windbox emissions at some steel plants.
Baghouses and scrubbers have been used to control emissions from the cooler. The temperature of
the exhaust gases from the cooler are approximately 150 to 200°C (300 to 400°F). Therefore, glass
bags are used to filter gases in this temperature range.
10-26
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New Source Performance Standards
There are two New Source Performance Standards (NSPS) for iron and steel plants—Subpart N,
which affects basic oxygen furnaces and Subpart AA, which affects electric arc furnaces. NSPS for
coke ovens are forthcoming in the near future. The NSPS for basic oxygen furnaces states that the
paniculate emissions emitted from the control device installed on the furnace must be less than
50 mg/dscm (0.022 gr/dscf). Also, the opacity must be less than 10%, except that the opacity can be
between 10 and 20% once during a production melt cycle.
The NSPS for electric arc furnaces states that the paniculate emissions must be less than
12 mg/dscm (0.0052 gr/dscf). The opacity of a stack on the control device must be less than 3%.
The opacity from the shop (building) must be 0%, except: (1) shop opacity can be between 0 and
20% during charging periods, and (2) shop opacity can be between 0 and 40% during tapping
periods. A continuous opacity monitor is required to be installed and operated on the stack of the air
pollution control device.
Summary
Table 10-1 lists the major air pollution sources at a steel mill and the control techniques used to
reduce the emissions. The steel processes covered in this lesson are not the only potential sources of
air pollution. Others included are pickling tanks and the processes of rolling, scarfing, shaping, and
coating. These emissions must also be controlled, some with careful process design and others with
small air pollution control devices. In addition, fugitive paniculate emissions from roads all over the
plant can be a major source of paniculate matter at a steel plant. These emissions can be controlled
by paving, water sprays, and careful traffic routing. One accepted bubble plan has been granted by
the EPA to Armco Steel in Middletown, Ohio. Armco is controlling fugitive paniculate emissions
from roads and storage piles, instead of putting control equipment on some steel processes.
Iron and steel plants can be a major source of air pollution. However, by installing air pollution
control devices and by using good operation and maintenance practices on the control devices, these
plants can operate cleanly and efficiently.
10-27
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Table 10-1. Process emissions and control techniques.
Process
Emissions
Control device or technique
Blast furnace
Blast furnace gas
Cast house emissions
Paniculate
Paniculate (fugitive)
Cyclone followed by wet scrubber
Cyclone followed by ESP
Total building evacuation with exhaust to baghouse
Hoods over tap hole, runners and troughs ducting exhaust to baghouse
Covered runners and troughs, and fume suppression
Coke oven
Coke oven gas
Combustion stack
Charging
Pushing
H,S. SO2
Paniculate
Paniculate
Paniculate
Stretford process
Sulfiban process
Vacuum Carbonate process
Electrostatic precipitator or baghouse
Staged charging, operation and maintenance
Pipeline charging I not emission control techniques.
Hot larry car charging J but charging methods
Shed with exhaust to scrubber, baghouse, or ESP
Hooded coke quench car attached to fixed duct with exhaust to
scrubber, baghouse, or ESP
Enclosed coke quench car with mobile scrubber
Quenching
Topside
Door leaks
Paniculate,
ammonia, phenol
Paniculate (and
occasionally partially-
burned organics)
Paniculate (and
partially-burned
organics)
Use clean quench water and baffles in quench towers
Dry quench towers (not used in U.S.)
Replace lids immediately after charging cycle is completed
Clay mixture around lids to form tight seal
Knife edge doors making sure doors are thoroughly cleaned
before being replaced (after pushing cycle is completed)
Luting doors with clay mixture to obtain tight seal
Basic oxygen furnace (BOF)
Oxygen blow period
Tapping or charging
Reladling stations
Paniculate
Paniculate
Paniculate
Closed or open hood with gas cooling followed by scrubber or ESP
Total building evacuation to baghouse
Local hoods with gases ducted to ESP or scrubber
Furnace enclosure with gases ducted to scrubber
Local hoods above ladle with gases ducted to baghouse
Electric furnace
Melt period
Charging and capping
Paniculate
Paniculate
Evacuation duct on furnace cop with gases ducted to baghouse,
ESP, or scrubber
Large hood located in the roof of building with gases ducted
co baghouse
Hood located above furnace with gas ducted to baghouse
Hoods over furnace, capping ladle and slag ladle wich gases
ducted to baghouse
Sinter plant
Windbox
Coolers
Paniculate
(sometimes SO2 and
organics)
Paniculate
Baghouse
Electrostatic precipitator
Scrubber
Gravel-bed filter
Scrubber
Baghouse
10-28
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Review Exercise
1. Potential pollutant emissions from
a. paniculate matter.
b. SO,.
c. CO.
d. all the above
a blast furnace are
2. Paniculate matter is usually removed before the blast furnace
gas is burned, primarily by a ,.,., ._ or
3. Cast house emissions occur from
located all over the casting area.
a. blooms and billets
b. ingots and molds
c. runners and troughs
d. runners and disks
4. Most control systems use
and runners.
a. hoods
b. water sprays
c. bellows
d. chemicals
5. List at least four locations and/ or
oven emissions occur.
6. Coke oven charging emissions can
a. staged charging.
b. sheds.
c. water sprays.
and
over the taphole
operations where coke
be controlled by
7. Green coke contains
a. unvolatilized hydrocarbons.
b. no paniculate matter.
c. dust panicles.
d. sulfur dioxides.
e. both a and c
8. Why does green coke occur?
a. The coke is removed too early.
b. The coke is left in oven too long.
c. The oven is in need of repair.
d. both a and c
1. d. all the above
2. electrostatic
precipitator or
wet scrubber
3. c. runners and troughs
4. a. hoods
5. • charging operations
• topside leaks
• pushing operations
• quenching operations
• door leaks
• coke battery
combustion stacks
6. a. staged charging.
7. e. both a and c
8. d. both a and c
10-29
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9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
Coke oven pushing emissions are controlled by
a. sheds enclosing coke pushing area.
b. movable hood attached to coke car.
c. enclosed quench car with scrubber.
d. all the above
Particulate emissions from quenching towers are reduced by
a. tents.
b. flares.
c. baffles.
d. ammonia sprays.
llsina ,.. should be avoided in quenching
operations.
a. clean water
b. baffles
c. dirty water
Knife-edged doors
a. have tapered edges that are self-sealing.
b. have flat edges that are self-sealing.
c. have tapered edges that must be luted with clay.
Coke oven gas, even after going through the by-products
plant, still contains a high concentration of
a. paniculate matter.
b. dust~particles.
c. ammonia.
d. hydrogen sulfide.
e. both a and b
Particulate emissions are sometimes controlled during the
oxygen blow in a BOF by a(an)
a. closed hood, tightly fitting around furnace top.
b. closed hood with clearance above furnace.
c. large open hood located at top of BOF shop.
d. both a and c
True or False? Hot gases generated during the oxygen blow
are cooled by water sprays located above the hood.
The scrap melting cycle in the electric furnace produces
and ... emissions.
Since is oxidized in the fiirnare, i( does not
need to be controlled.
,.,_ ... are the most popular rnnrrnl Hevires used to
reduce electric furnace emissions.
a. Electrostatic precipitators
b. Gravity settling chambers
c. Baghouses
d. Wet scrubbers
8. d. both a and c
9. d. all the above
10. c. baffles.
11. c. dirty water
12. a. have tapered edges
that are self-sealing.
13. d. hydrogen sulfide.
14. a. closed hood, tightly
fitting around furnace
top.
15. True
16. paniculate matter and
carbon monoxide
17. carbon monoxide
18. c. Baghouses
10-30
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19. Electric furnace charging and tapping emissions can be
collected by
a. one large hood located in the building's roof.
b. a small canopy hood located above the furnace.
c. quenching sprays located above the furnace.
d. both a and b
20. True or False? Paniculate emissions generated at the windbox
of the sinter plant are usually collected by wet scrubbers or
electrostatic precipitators.
19. d. both a and b
21. NSPS have been promulgated to control emissions from
a. basic oxygen furnaces.
b. electric furnaces.
c. blast furnaces.
d. open hearth furnaces.
e. both a and b
20. True
21. e. both a and b
References
Barnes, T. M., et. al. 1975. Control of Coke Oven Emissions. Iron and Steelmaking (Quarterly)
No. 3. -
Environmental Protection Agency (EPA). March, 1978. Operation and Maintenance of Paniculate
Control Devices on Selected Steel and Ferroalloy Processes. EPA 600/2-78-037.
Environmental Protection Agency (EPA). 1973. Field Surveillance and Enforcement Guide for
Primary Metallurgical Industries. EPA 450/3-73-002.
Frederic, E. R. ed. 1981. Proceedings: Air Pollution Control In the Iron and Steel Industry—
Specialty Conference. APCA. April 21-23. Chicago, IL.
Frederick, E. R. ed. 1979. Proceedings: Control of Air Emissions From Coke Plants. APCA. April
17-19, 1979. Pittsburgh, PA.
Huntz, O. and Blacksown, R. W. 1978. Recent Developments in Machine Scarfing of Continuous
Cast and Rolled Steel. Iron and Steel Engineer. January 1978.
Linsky, B. et al. 1975. Dry Coke Quenching, Air Pollution and Energy; A Status Report. JAPCA.
September 1975, Volume 25, No. 9.
Massey, M. J. and Dunlap, R. W. Economics and Alternatives for Sulfur Removal from Coke
Oven Gas. JAPCA. October 1975, Volume 25, No. 10.
Sharp, J. D. 1967. Electric Steelmaking. Cleveland: CRC Press.
U.S. Code of Federal Regulations. 1981. 40 Protection of Environment—Parts 53 to 80.
Washington: U. S. Government Printing Office.
10-31
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Lesson 11
Petroleum Refineries
Lesson Goal and Objectives
Goal
To familiarize you with the general operation of a refinery, potential air pollution emission points,
and the control equipment or procedures used to reduce these emissions.
Objectives
At the end of this lesson, you should be able to:
1. describe the four basic procedures involved in the refining of crude oil:
• separating,
• converting,
• treating, and
• blending.
2. list eight ah- pollution sources and the types of pollutants they emit.
3. describe the control equipment or procedures used to reduce emissions from sources of air
pollution in a petroleum refinery.
Introduction
Crude oil is a mixture of hydrocarbon compounds and small amounts of sulfur, nitrogen, oxygen,
and various metals. The chemical and physical characteristics of crude oils differ significantly; some
crude oils are clear, volatile liquids, while others are heavy, viscous sludges. The exact chemical (pro-
portions of the various organic compounds) and physical characteristics of a crude oil depend on its
original geographic location. A petroleum refinery takes one or more crude oils and converts, or
refines, these oils into useful products such as gasoline, heating oils, lubricating oils, and a wide
variety of chemicals and solvents. The refining process involves first separating the crude oil into its
various components. The less desirable or usable components are then sent to treating processes to
remove impurities such as sulfur and metals and/or to conversion processes to increase the yield
(amount produced) of the more desirable products (i.e., gasoline).
In discussing the emission sources at a refinery, an understanding of the entire refining process is
important. A refinery is comprised of many individual process units, each designed and operated to
process specific crude oils and produce specific products. The number and complexity of the process
units at each refinery varies significantly. Therefore, it is impossible to label one flow diagram as
"typical" of all refineries. Although each refinery is unique, the process units at any refinery can be
categorized into four basic procedures:
• separating,
• converting,
• treating, and
• blending.
11-1
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The following discusses the important process units, not only in terms of production but also
atmospheric emissions, for each of these refining procedures.
Refining Process
A refinery is made up of many interconnected process units. As we said, no single flow diagram can
be used to show the relationship of the process units at all refineries. Figure 11-1 presents a simplified
schematic of the layout of the basic refining operation. Most refineries are designed to maximize the
production of gasoline (light distillates). Some small refineries may only perform crude separation
and some limited treating. Large refineries, however, have converting and additional treating
processes. Although each process unit in a refinery performs a different function (separating, con-
verting, etc.), the equipment used in any process unit is very similar. Process units generally consist of
reaction vessels or towers, piping, pumps, compressors, valves, and heat exchangers. Therefore, the
physical appearances of the different process units are similar, making any refinery look like intricate
masses of pipes and towers (see Figure 11-2).
Key to major
processes
Separating
O
Treating
A
Convening
O
Blending
O
To hydrotreater
and hydrocracker
• Hydrogen •»
Gas
Specific
steps with key
to major process
Gas
Figure 11-1. Typical processing steps in a petroleum refinery.
11-2
-------
Figure 11-2. Typical petroleum refinery.
Separating
The first step in the refining process is to separate the crude oil into its various components.
Separating the components is accomplished by heating the crude oil and then removing the various
components according to their boiling ranges. This separation process is referred to as distillation, or
jractionation. Figure 11-3 illustrates a two-stage separation operation. This two-stage separation con-
sists of a crude atmospheric distillation tower and a vacuum distillation tower.
Before entering the crude distillation tower, the crude oil is heated to approximately 400 °C
(750°F) in a furnace. The heated crude oil enters the distillation tower and the vapor (lighter
hydrocarbons) begins to rise up through the tower. The tower consists of many trays spaced an equal
distance apart. Components having a low molecular weight pass up through these trays, while the
heavier components begin to condense on the lower trays. At various points in the tower, liquid is
drawn off the trays. Components with the lowest boiling point (also low molecular weight) are drawn
off the top of the tower while the liquid with the higher boiling point leaves the bottom of the tower.
The liquid leaving the bottom of the tower is sent to the vacuum distillation tower for further separa-
tion. The sidestreams can be sent to storage to await additional processing or blending as final
product.
The operation of the vacuum distillation tower is similar to the crude distillation tower, except that
the pressure in the tower is lower (vacuum). The vacuum tower is used to separate the heavier por-
tion of the crude (from the bottom of the atmospheric distillation tower) into fractions. Operating
under a vacuum enables the heavier compounds to vaporize at lower temperatures. If heavy crude is
distilled at high temperatures it tends to decompose or polymerize, possibly plugging operating equip-
ment and reducing the percent yield of product produced. The vacuum is usually created by using a
steam ejector.
11-3
-------
Light gases
Naptha
Vacuum
producing
system
HeavV Gas oils
gas oils
Light
gas oils
Heater
Distillation columns
Heavy
residuals
Figure 11-3. Separating: two-stage distillation.
Converting
Conversion processes are used to increase the quantity and the quality of the end products at a
refinery. Unlike the separation processes which merely separate the crude oil according to molecular
weight, conversion processes change the chemical structure of the components. The less desirable
components are converted to more salable products. For example, most crude oils contain only a
small percentage of gasoline. To meet the high demand for gasoline, the heavy gas oils are converted
to gasoline, increasing the average yield to over 40% gasoline from a barrel of crude.
11-4
-------
Various conversion processes are used in a refinery. Some of the more common ones are listed in
Table 11-1. The most common process is to "crack" the high molecular-weight, high boiling-point
compounds (heavy fuel oils) into smaller, low molecular-weight, low boiling-point compounds
(gasoline). Several cracking processes have been used, including thermal cracking, catalytic cracking,
and hydrocracking. Catalytic cracking is the most important (both from an environmental and end-
product viewpoint). Of the several different catalytic cracking processes, fluid catalytic cracking
(FCC) is by far the most widely used.
Table 11-1. Conversion processes.
1. Cracking
A. Thermal cracking
1. Visbreaking or viscosity breaking—Used to reduce the viscosity of some residual fractions so
that they may be blended into fuel oils. Visbreaking is a mild form of cracking that is carried
out in a heating furnace and distillation tower.
2. Coking—Severe form of thermal cracking. Converts low-value residual fuel oil to higher-value
gas oil plus petroleum coke. The coking process is carried out at high temperatures and low
pressures.
B. Catalytic cracking— Breaks down the heavier distillate fractions into the lighter, more valuable
gasoline. The catalytic cracking process can be catagorized as a fluidized bed. moving bed, or
fixed bed (once-through). By far, the most predominant is the fluidized bed design.
C. Hydrocracking—Used to upgrade a wide variety of feedstocks into more valuable products. In
hydrocracking, the feedstocks are cracked in the presence of hydrogen, causing the cracked
products to become saturated with hydrogen. Because hydrogen a used, hydrocracking also
improves the quality of the products by desulfurizing and denitrifying the feedstocks. A wide
variety of catalysts and system designs are used in hydrocracking.
2. Catalytic reforming—Used to rearrange the structure of low-octane compounds to yield high-octane
compounds. The process converts straight-chained naptha to ringed- or branched-structured gasoline
components. The reforming process also produces hydrogen which is used in other refinery processes.
3. Alkyiation—Joins together two different structured compounds, and some synthetic chemicals. One
of the compounds made is a branched or ringed molecule, while the other is a double-bonded
molecule. Sulfuric or hydrofluoric acid is used to catalyze the reaction.
4. Isomerization—la similar to catalytic reforming in that it is used to rearrange the structure of a com-
pound. The isomerization process is usually applied to butane or mixtures of pentane and hexane to
increase their octane rating.
11-5
-------
Figure 11-4 illustrates one design for a fluid catalytic cracking unit (FCCU). In this design, the
feed stream (heavy gas oils) is heated and then mixed with the hot, regenerated powder-like catalyst.
The liquid gas oils begin to vaporize, crack into smaller compounds, and rise to the reactor carrying
the catalyst with them. Steam is added to the reactor to fluidize the catalyst and to strip the
hydrocarbon products from the catalyst. The gaseous products rise through the reactor vessel, pass
through cyclones to remove entrained catalyst particles, and are removed from the top of the reactor.
Gas to fractionator
Steam
atalyst
Regenerator <
Liquid gas oil/catalyst
Liquid (heavy gas oils)
Figure 11-4. Simplified fluid catalytic cracking unit.
During the reaction, coke deposits form on the finely-powdered catalyst particles. These coke
deposits must be continually removed to maintain catalyst activity. The spent catalyst is withdrawn
from the reactor and sent back to the regenerator. In the regenerator, a controlled amount of air is
added to burn off the coke without harming the catalyst. The gases in the regenerator pass through
cyclones that separate the regenerated catalyst dust from the combustion gases. This "cyclone" may
be a single cyclone, dual cyclones, or multiple large diameter cyclones. (Chapter 2 covers operating
principles of cyclones.) The exhaust gases from the cyclones contain paniculate matter, carbon
monoxide, sulfur oxides, nitrogen oxides, and hydrocarbon vapors.
11-6
-------
Treating
Treating, or "purification" processes are used to remove sulfur, nitrogen, and metal compounds from
the intermediate feedstocks. Removing these compounds improves the quality of the products by
eliminating odors, discoloration, and in some instances, producing a more stable finished compound.
These compounds must also be removed to avoid damaging the catalysts used in the conversion proc-
esses such as catalytic cracking and catalytic reforming.
Numerous treating processes can be used, depending on the contaminant that must be removed.
Presently, the most common treatment process for all types of petroleum products is hydrotreating.
In hydrotreating, the petroleum products are passed over a catalyst and hydrogen gas is injected into
the reactor. The hydrogen reacts with the sulfur in the oil and converts the sulfur into hydrogen
sulfide. The hydrogen sulfide is then removed and may be recovered as elemental sulfur by further
processing. In addition to removing sulfur, hydrotreating can also be used to saturate (add hydrogen
to) many unstable double-bonded compounds; thereby reducing their tendency to polymerize (form
gummy substances). When hydrotreating is used to remove sulfur, it is referred to as
hydrodesulfurization.
Many other treatment processes are available to remove undesirable contaminants. In general, the
petroleum products can be washed with acids (such as HSSO4) or caustic bases (such as NaOH) to
remove sulfur compounds (especially mercaptans). Removing these compounds improves the quality
and/or performance of the petroleum products. Filtration, absorption, and air blowing are other
processes used to purify the petroleum products.
Blending
Blending involves 'mixing different fractions of the "refined" petroleum products to produce over
2000 finished products. Products are blended to meet specifications as to the vapor pressure, vis-
cosity, sulfur content, boiling point, or octane number desired. Blending is a closed process that
occurs in pipelines or tanks that use pumps and valves to mix the various petroleum factions.
Review Exercise
1. True or False? The chemical composition of crude oil is exactly
the same for all geographic locations.
2. Four basic procedures involved in the refining of crude
oil are:
1. False
3. Separating the crude oil into its fractions is accomplished by
heating the crude and removing its components according to
their
a. crystal size.
b. boiling point.
c. octane number.
2. • separating
• treating
• converting
• blending
3. b. boiling point
11-7
-------
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
Two stage distillation usually occurs in and
distillation towers.
Tn a distillation tower, the components with the
boiling point ( ... molecular weight) are drawn off
at the top of the tower.
a. lowest, lowest
b. highest, lowest
c. lowest, highest
d. highest, highest
The feed stream into the vacuum distillation tower comes
from the top/bottom of the atmospheric tower.
Operating a distillation tower under a vacuum causes the
components to vaporize at . , temperatures.
a. higher
b. lower
c. indeterminable
True or False? Conversion processes change the chemical
structure of the less desirable components to make them more
salable.
The conversion process used to break high molecular- weight,
high boiling-point compounds into smaller, low molecular-
weight, low boiling-point compounds is
a. thermal cracking.
b. catalytic cracking.
c. hydrocracking.
d. any of the above
By far, the most widely used of the catalytic cracking
processes is . . ._ _
In the fluid catalytic cracking process, the catalyst is
continually regenerated by
a. adding steam.
b. burning off the coke which deposits on the catalyst.
c. cooling the catalyst.
d. washing the catalyst with a caustic solution of NaOH.
The exhaust gases from the catalyst regenerator may contain
a. paniculate matter.
b. CO.
c. hydrocarbons.
d. all the above
processes are used tr> remove sulfur, nitroaen, or
metal compounds from the intermediate petroleum products.
4. atmospheric and
vacuum
5. a. lowest, lowest
6. bottom
7. b. lower
8. True
9. d. any of the above
10. fluid catalytic cracking
11. b . burning off the coke
which deposits on the
catalyst.
12. d. all the above
13. Treating
11-8
-------
14. True or False? In addition to removing sulfur, hydrotreating
can also be used to saturate many unstable double-bonded
compounds, reducing their tendency to polymerize.
15. Washing petroleum products with acids or bases removes
Ifi, _ ., .._ _ involves mixing the different petroleum fractions
to produce a finished product.
14. True
15. sulfur compounds
16. Blending
Air Pollution Emissions
Refineries can emit paniculate matter, hydrocarbons (volatile organic compounds), sulfur oxides,
nitrogen oxides, and carbon monoxide. The type and quantity of pollutants emitted from a refinery
depends on many factors such as type of crude, capacity, complexity of the processing, air pollution
control equipment, and the degree of maintenance and housekeeping practices used in the plant.
Only a few processes emit pollutants directly into the atmosphere through exhaust stacks. Unfor-
tunately, there are a large number of potential fugitive emission sources in the refinery. Table 11-2
lists, by type of emission, the potential emission sources in a typical refinery.
Table 11-2. Potential sources of atmoipheric emissions within refineries.
Type of emission
Paniculate matter
Sulfur oxides
Nitrogen oxides
Hydrocarbons
Carbon monoxide
Odors
Source
Catalytic cracker, fluid coking, catalyst regeneration,
process heaters, boilers, decoking operations, incinerators
Sulfur recovery unit, catalytic cracking, process heaters,
boilers, decoking operations, unit regenerations, treating
units, flares
Process heaters, boilers, catalyst regeneration, flares,
catalytic cracking
Storage tanks, loading operations, waste water treating,
catalyst regeneration, barometric condensers, process
heaters, boilers, cooling towers, vacuum jets, equipment
leaks (pumps, valves, blind changing)
Catalyst regeneration, decoking, compressor engines,
incinerators, catalytic cracking
Treating units, drains, tank vents, barometric condensers,
sumps, oil-water separators
Source: EPA, March 1979.
11-9
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Particulate Matter
The potential sources of paniculate matter at a refinery are catalyst regenerators, boilers, process
heaters, fluid cokers, decoking operations and incinerators. The major potential source of paniculate
matter is from catalyst regenerators used in the catalytic cracking units. Coke, which forms on the
surface of the catalyst during the cracking process, must be burned off in regenerating units. Since
flue gas from the regenerator contains heavy concentrations of catalyst dust, it must be sent to a con-
trol device.
Carbon Monoxide
Of the sources listed in Table 11-2 the major emitter of CO is the regenerator for the catalytic crack-
ing unit. In the regenerator, a controlled amount of air is used to burn off the coke from the
catalyst, resulting in CO formation. Other sources of CO emissions include any combustion process
that is operated improperly.
Sulfur Oxides
Crude oils from different parts of the world vary significantly in their sulfur content. Sulfur exists in
crude oils in many forms, such as HjS or attached to organic compounds (mercaptans). The major
sulfur compound generated by refining is hydrogen sulfide. Nearly all refinery processes generate
some gases which contain hydrogen sulfide or other low molecular weight sulfur compounds. The
largest potential sources of these gases are from hydrotreating, cracking, and reforming processes.
The exhaust gases from these processes are used as fuel in process heaters and boilers, and would
result in sulfur oxtde emissions if the sulfur compounds are not removed.
Another potential source of sulfur emissions is from the sour water strippers. Aqueous solutions
from many refinery processes are contaminated with hydrogen sulfide. (In a refinery, streams
containing HjS are referred to as sour.) The hydrogen sulfide is removed from the water by steam
stripping in sour water strippers. The stripped hydrogen sulfide can then be sent to a sulfur recovery
unit, which reduces the potential SOj emissions resulting from the combustion of HZS.
In addition, refineries also emit SOf emissions from the burning of residual fuel oil in boilers and
process heaters. The quantity of SO2 emissions depends on the percent of sulfur contained in the
fuel.
Nitrogen Oxide
Nitrogen oxide emissions result from the combustion of fuel in process heaters and boilers, internal
combustion engines used to drive compressors, and catalyst regenerators. For nitrogen oxide emis-
sions, the amount emitted depends more on the design of the combustion unit than on the type of
fuel.
Hydrocarbons or Volatile Organic Compounds
As can be seen from Table 11-2, most of the equipment in a refinery has the potential to emit
hydrocarbon vapors. Hydrocarbon vapors are released into the atmosphere in two ways. The vapors
can be entrained in the exhaust gas and released through a stack, or they can be emitted from equip-
ment whenever the oils being processed are exposed to the atmosphere — either intentionally or by
leaks. The amount of emissions depends on the volatility (vapor pressure) of the oils being processed.
11-10
-------
Potentially, the largest source of hydrocarbon emissions is from the storage of liquid in large tanks.
In storage tanks, the space over the top of the liquid can become saturated with vapors of the stored
liquid. These saturated vapors can be forced into the atmosphere through a number of different
mechanisms such as:
• expansion and contraction of the vapors due to heating during the day and cooling at night,
• displacement of the vapors during filling, and
• direct evaporation.
Wastewater separators are another large source of hydrocarbon emissions. Contaminated
wastewater comes from many sources such as equipment cleaning, leaks, spills, process wastewater,
and rain run-off. Wastewater is collected in drain systems and sent to gravity separators where the oil
is skimmed from the top of the water. Hydrocarbons will be emitted whenever wastewater is exposed
to the atmosphere. The emissions points could be open drains and drainage ditches, manholes, and
the surface of the separator or treatment pond.
In addition, the following are also potential sources of hydrocarbon emissions in a refinery:
• vacuum producing systems—these systems are primarily used hi vacuum distillation. A vacuum
is created by reducing the pressure (removing the gases) in the vessel. Hydrocarbon emissions
result when the gases evacuated from the vessel are released to the atmosphere.
• process turnaround—turnaround refers to shutting down a unit for repair or inspection and
then starting it up again. During turnaround, the vessel must be depressurized, (referred to as
blowdown) flushed out, and then ventilated to provide a safe working atmosphere. Hydrocarbon
emissions occur during depressurization if the gases in the vessel are vented to the atmosphere.
• equipment leaks—hydrocarbon emissions occur when the volatile material flowing through
refinery components leaks. Many types of equipment in a refinery can leak; the most important
are valves, pumps, compressors, pressure relief devices, flanges, and other connectors.
• loading facilities—generally, most petroleum products leave the refinery through a pipeline.
However, loading gasoline and other light oils into tank trucks for delivery can result in
hydrocarbon emissions by vapor displacement or evaporation.
• steam ejectors and barometric condensers on vacuum distillation units.
• cooling towers—cooling towers are used to remove excess process heat occurring during the
refining process. Hydrocarbon emissions from cooling towers will occur if organic products leak
into the cooling water (in the heat exchangers), if contaminated process water is used in the
cooling tower, or if treated waste water is used as make-up water for the cooling tower.
11-11
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Review Exercise
1. True or False? The type and quantity of pollutants emitted
from a refinery are the same for every refinery in the U.S.
2. The major potential source of particulate emissions at a
refinery is from
1. False
3. The major potential source of carbon monoxide at a refinery
is from
a. the catalyst regenerator on the FCC unit.
b. storage tanks.
c. vacuum systems.
d. oil-water separators.
2. the catalyst regenerator
on the FCC unit
4. What is the average percent of sulfur in crude oil?
a. 0.1%
b. 10%
c. 2.0%
d. none of the above
3. a. the catalyst
regenerator on the
FCC unit.
5. The largest potential source of H2S in a refinery is from
a. hydrotreating.
b. reforming.
c. cracking. ~
d. sour water stripping.
e. any of the above
4. d. none of the above;
it varies for different
crudes.
6. Nitrogen oxide emissions result from
a. equipment leaks.
b. combustion of fuel in heaters and boilers, and catalytic
crackers.
c. storage tanks.
d. all the above
5. e. any of the above
7. True or False? Potentially, the largest source of hydrocarbon
emissions from a refinery is from storage tanks.
6. b. combustion of fuel
in heaters and boilers,
and catalytic crackers.
8. vapors are emitted from wastewater systems
whenever the wastewater is exposed to the atmosphere.
a. Particulate
b. Hydrocarbon
c. Carbon dioxide
d. Ozone
7. True
9. List three sources of fugitive hydrocarbon emissions.
8. b. Hydrocarbon
10.
refers to shutting down a unit for repairs or
inspection and then starting it up again.
9. valves, pumps,
compressors, flanges,
and pressure relief
devices
10. Process turnaround
11-12
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Air Pollution Control Methods
The following discussion on air pollution control devices is categorized by the type of equipment or
process operation used in the refinery.
Catalyst Regeneration Unit
Flue gas from the regenerators of catalytic cracking units may contain paniculate matter (catalyst
fines), carbon monoxide, hydrocarbons, nitrogen oxides, and sulfur oxides. The flue gas is usually
first sent to a multicyclone. The multicyclone removes the large particles and recycles the recovered
catalyst. From the multicyclone, the flue gas from a conventional fluid catalytic cracking unit
(FCCU) is generally sent to a waste heat boiler, also called a CO boiler. The boiler reduces emissions
of carbon monoxide and hydrocarbons by burning them, which generates usable heat. The exhaust
from the waste heat boiler, which still contains a high concentration of the fine catalyst particles, is
sent to a high collection efficiency control device. The most common control device used is the elec-
trostatic precipitator. In some cases, scrubbers are used to control both SO2 and paniculate emis-
sions. Occasionally, some of the light ends (that are high in sulfur content) from the catalytic crack-
ing units are sent to the sulfur recovery plant.
Since the early 1970s, high temperature catalyst regeneration techniques have been developed. New
catalysts and regenerator designs have reduced the amount of CO and eliminated the need for a
waste heat (CO) boiler. However, a high-efficiency paniculate control device is still required to col-
lect the very fine catalyst panicles.
Storage Tanks -
To prevent evaporative hydrocarbon losses from storage tanks, the tanks can be fitted with special
roofs. Tanks storing volatile liquids (i.e., gasoline) are equipped -with floating roofs. Floating roof
tanks reduce evaporative losses by minimizing vapor spaces (the space between the liquid in the tank
and the top of the tank). A floating roof is constructed so that it floats on the surface of the stored
liquid. The roof rises and falls as the liquid is filled or emptied from the tank. The roof, or deck, is
usually about half a foot smaller in diameter than the tank wall. In order that the entire surface of
liquid is covered, the roof is equipped with a sliding seal that attaches to the roof and fits snuggly
against the tank wall (see Figure 11-5).
Internal
External
Figure 11-5. Floating roof tanks.
11-13
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Another method used to prevent evaporative hydrocarbon losses from storage tanks is to install a
vapor recovery system. Figure 11-6 is a flow diagram of a simplified vapor recovery system. Generally
several storage tanks are connected to a single vapor recovery unit. Vapors generated in the storage
tanks are piped to an absorption tower. Chilled gasoline is sprayed over these vapors. Liquid
recovered in the tower is sent back to storage.
Liquid
gasoline
Absorption
column
Figure 11-6. Vapor recovery unit (using absorption system).
Wastewater Separators
Depending on the volatility and quantity of waste oil in the water, the hydrocarbon emissions from
the wastewater separators may need to be controlled. One control method is to cover the separator,
minimizing the amount of oily wastewater exposed to the atmosphere. Two general types of covers
are used: (1) a solid cover with all openings sealed or (2) a floating pontoon cover with seals to
enclose the space between the cover's edge and the wall of the separator.
Vacuum Producing Systems
All vacuum producing systems discharge a stream of gaseous hydrocarbons while generating a
vacuum. These hydrocarbon emissions can be prevented by piping the vapors to an incinerator or by
using them as fuel in a process heater or boiler in the refinery.
11-14
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Process Unit Turnarounds
Atmospheric emissions resulting from shutdown of a process unit can be greatly reduced if the vapors
are combusted as fuel gas or sent to a flare.
Equipment Leaks
The major sources of fugitive hydrocarbon leaks in a refinery are: pumps, compressors, valves,
flanges, pressure relief devices, and process drains (EPA, 1978). The major solution to control these
emissions is locating the sources that are leaking. While this can be a momumental task (for example,
a large refinery may easily have over 10,000 valves), the savings in product may pay for the leak
detection repair program. For a refinery to effectively maintain equipment that may leak, it must
have an ongoing inspection and maintenance program. In an inspection and maintenance program,
potential leaking equipment is periodically checked and, if found to be leaking, is repaired. A log
form showing when the equipment was checked, if it leaked, and what repairs were made is an essen-
tial part of the program. Additional information on controlling leaking equipment can be found in
APTI Course 417 Controlling VOC Emissions from Leaking Process Equipment (EPA, August 1982).
Sulfur Recovery Plants
Sulfur compounds removed from the various petroleum fractions are converted to H2S, collected, and
sent to a sulfur recovery plant. The most common sulfur recovery plant is referred to as a Claus
plant. The Claus process uses catalytic reactors to convert the HtS to elemental sulfur. The exhaust
gas (referred to as tail gas) from the Claus unit contains HSS and SO2.
A number of commercial systems are used to reduce the sulfur emissions from the tail gas of the
Claus plant. Presently, four processes have demonstrated their ability to meet new SO2 compliance
levels (EPA, 1980). The four processes are the Beavon, Shell Claus Offgas Treating (SCOT),
Wellman-Lord, and the Institut Francais du Petrole (IFF).
Process Heaters and Boilers
Emissions of paniculate matter, nitrogen oxides, and hydrocarbons from process heaters and boilers
are controlled by proper operation of the unit to ensure complete combustion. Emissions of sulfur
dioxide are normally kept to a minimum by burning low-sulfur distillate oils or gas. Removal of
sulfur from these fuels prior to burning helps reduce SOZ emissions. If the refinery burns high sulfur
fuels, flue gas treatment (such as that used on power plants) may be required (see Lesson 9).
Air Pollution Regulations
Presently, the New Source Performance Standards (NSPS) for petroleum refineries specifically
regulate:
• paniculate matter emissions, carbon monoxide emissions, and opacity from the catalyst
regenerator of a fluid catalytic cracker.
• sulfur dioxide emissions from fuel gas combustion devices.
• sulfur dioxide emissions from Claus sulfur recovery plants.
11-15
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The emission limit for these pollutants is listed in Table 11-3. Note that the NSPS defines a fuel gas
combustion device as any equipment used to burn fuel gas, such as process heaters, boilers and
flares. Exempted from the SOt standard are fluid coking units, fluid catalytic cracking waste heat
boilers, facilities in which gases are burned to produce sulfur or sulfuric acid, and gases sent to a
flare due to a process-upset or emergency.
NSPS also exist for any facility that stores petroleum liquids in tanks with a capacity larger than
151,416 liters (40,000 gallons). The NSPS for storage tanks is divided into two parts: one for tanks
constructed after March 8, 1974* but prior to May 19, 1978 and those constructed after May 18,
1978. The NSPS require some type of floating roof or vapor recovery system on tanks storing
petroleum liquids with a vapor pressure greater than 10.3 kPa (1.5 psia) and a vapor recovery system
on tanks storing liquids with a vapor pressure greater than 76.6 kPa (11.1 psia). Tanks constructed
after May 18, 1978 are also required to have certain types of sealing devices between the floating roof
and the tank wall and owners are required to periodically inspect these seals to ensure that they meet
specific minimum "gap" requirements (the gap is the distance between the seal and tank wall). Note,
there are two types of floating roofs (internal and external), each having a separate secondary seal
requirement and allowable gap measurement.
Table 11-3. New Source Performance Standards for Petroleum Refineries.
Pollutant
Paniculate matter
_ Carbon monoxide
Sulfur dioxide
Hydrocarbon
Process unit affected
Catalyst regenerator built or
modified after June 11, 1973
Catalyst regenerator built or
modified after June 11, 1973
Any fuel gas combustion device built
or modified after June 11, 1973
Claus sulfur recovery units built
after October 4, 1976
Claus plants with a capacity of 30
long tons per day or less at small
refineries are exempted
Storage tank
Limit
S 1.0 kg/1000 kg of coke bum-off
(l.Olb/lOOOlb)
£ 30% opacity except for one six-minute
average period in an hour
< 0.050% by volume in exhaust
gases (5000 ppm)
Prohibits burning of fuel containing
more than 230 mg H,S/dscm (0.10 gr
H,S/dscf).
or
install flue gas treatment to limit the
release of SOt to the atmosphere
If emissions are controlled by an
oxidation or reduction system followed
by incineration, the unit may not emit
more than 0.025% by volume of SO,
corrected to zero percent oxygen
If emissions are controlled by a reduction
system not followed by incineration, the
unit may not emit more than 0.030% by
volume of reduced sulfur compounds and
0.0010% by volume of H,S calculated as
SO, corrected to zero percent oxygen
See 40 CFR, Part 60, subparts K and K.
In order to reduce hydrocarbon emissions, the EPA is currently working on NSPS for fugitive
hydrocarbon emissions from refineries. In general, these standards would not require the installation
of control equipment, but instead would require the refinery to institute an inspection and
maintenance program to locate and repair leaking process equipment. Other regulations that are
presently being developed by EPA are NESHAPs for benzene and NSPS for catalytic crackers.
*If the tank capacity is greater than 245,000 liters (65,000 gallons) the standard applies to tanks
installed after June 11, 1973 but prior to May 19, 1978.
11-16
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In addition to these New Source Performance Standards, EPA has published a series of Control
Technique Guidelines. These guidelines list reasonably available control technology (RACT) for
reducing volatile organic (hydrocarbon) emissions from certain processes. The emission sources
covered by these control technique documents are:
• vacuum producing systems,
• wastewater separators,
• process turnarounds,
• storage tanks, and
• fugitive emissions.
These RACT guidelines are generally applicable to existing refineries located in areas that are
nonattainment for ozone.
For more information concerning the RACT guidelines, see APTI Course 482 Sources and Control
of Volatile Organic Air Pollutants (EPA, March 1981).
Review Exercise
1. Control equipment installed on the regenerator of a
catalytic cracking unit generally consists of a
a. multiclone.
b. waste heat boiler.
c. electrostatic precipitator.
d. all the above
2. To prevent evaporative hydrocarbon losses from storage
tanks, the tanks can be fitted with
a. a floating roof.
b. a vapor recovery system.
c. an overfill weir.
d. either a or b
1. d. all the above
3. Floating roof tanks reduce hydrocarbon losses by
a. minimizing the vapor space.
b. maximizing the vapor space.
c. causing vapor lock.
d. exhausting the vapors to a waste heat boiler.
2. d. either a or b
4. From the following illustration, what is point A?
3. a. minimizing the
vapor space.
4. a roof seal
11-17
-------
5. One control option for reducing hydrocarbon emissions from
wastewater separators is to
4. a roof seal
6. Piping the vapors produced at a process to an incinerator
or burning them as fuel can be control methods for vapors
generated from
a. vacuum producing systems.
b. process unit turnarounds.
c. Glaus plants.
d. all the above
e. a and b only
5. cover the separator
Reducing equipment leaks (fugitive hydrocarbon emissions)
usually involves using what control option?
a. installing an incinerator
b. instituting an inspection and maintenance program
c. installing a precipitator
d. They cannot be controlled by any processes.
6. e. a and b only
8. True or False? To reduce sulfur emissions, sulfur compounds
are removed from the various fractions and sent to a
Glaus plant to eventually be recovered as elemental sulfur.
7. b. instituting an
inspection and
maintenance program
Tail gas from a Glaus plant
a. is sent to gasoline blending.
b. contains HZS and SOj which may have to be controlled.
c. is the largest source of hydrocarbon emissions.
d. all the above
8. True
10. The NSPS for catalyst regenerators of fluid catalytic
crackers regulates emissions of
a. paniculate matter.
b. opacity.
c. carbon monoxide.
d. sulfur dioxide.
e. all the above
f. a, b and c only
g. a, b and d only
h. a and d only
i. c and d only
j. only the lonely.
9. b. contains H2S and
SOj which may have
to be controlled.
11. True or False? The storage of petroleum liquids in tanks
larger than 40,000 gallons is prohibited by the NSPS.
10. f. a, b and c only
11. False. The NSPS
requires some type of
floating roof or vapor
recovery system.
11-18
-------
References
Environmental Protection Agency (EPA). June 1978. Control of Volatile Organic Compound Leaks
from Petroleum Refinery Equipment. EPA 450/2-78-036.
Environmental Protection Agency (EPA). August 1982. Controlling VOC Emissions from
Leaking Process Equipment. EPA 450/2-82-015.
Environmental Protection Agency (EPA). October 1977. Control of Refinery Vacuum Producing
Systems, Waste-water Separators, and Process Unit Turnarounds. EPA 450/2-77-025.
Environmental Protection Agency (EPA). July 1974. Field Surveillance and Enforcement Guide for
Petroleum Refineries. EPA 450/3-74-042.
Environmental Protection Agency (EPA). March 1980. Petroleum Refinery Enforcement Manual.
EPA 340/1-80-0008.
Environmental Protection Agency (EPA). May 1981. Sources and Control of Volatile Organic Air
Pollutants. EPA 450/2-81-011.
Environmental Protection Agency (EPA). March 1979. A Review of Standards of Performance for
New Stationary Sources—Petroleum Refineries. EPA 450/3-79-008.
Environmental Protection Agency (EPA). APTI Course 482 Sources and Control of Volatile Organic
Air Pollutants—Instructor's Guide. EPA 450/2-81-010.
Environmental Protection Agency (EPA). 1980. Assessment of Atmospheric Emissions from Petroleum
Refining (5 volumes). EPA 600/2-80-075.
11-19
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Lesson 12
Portland Cement Plants
Lesson Goal and Objectives
Goal
To familiarize you with processes at cement plants, the pollutant emissions produced by them, and
the devices used to reduce these emissions.
Objectives
At the end of this lesson, you should be able to:
1. recognize the process steps necessary to produce cement.
2. recall the pollutant emissions resulting from the production of cement.
3. list the major pollutant emission points (if any) in each process step involved in cement
production.
4. recall the control device used to reduce pollutant emissions at a given emission point.
Introduction
Portland cement, commonly called "cement," is a fine, powdery material. When mixed with water,
cement forms a paste that hardens to form a rocklike mass. When rock, gravel, or sand are mixed
with the cement, the resultant mixture is termed concrete. When made into concrete, Portland
cement resembles the building stone that is quarried on the isle of Portland, England; hence, its
name.
Cement is manufactured using a combination of various raw materials such as limestone, cement
rock, sand, iron ore, clay, and shale. Most of these raw materials (stones) are mined at or near the
cement plant. These raw materials are then crushed to the proper size and blended to give the
proper proportions of chemical compounds, mainly silica, alumina, and calcium oxide. The blended
mixture is then sent to a kiln where it is heated to a very high temperature. Heating chemically
changes the raw mix to clinker—a gravel-like material. Gypsum is added to the clinker, and this mix-
ture is ground into Portland cement.
Cement plants are referred to as either wet or dry. The processes are very similar, except that in
the wet process, water is added to the raw materials before or during grinding. The addition of water
forms a slurry, which helps to ensure a more uniform blend of materials. However, almost all new or
planned cement plants will use the dry process, since it can be twice as energy efficient as can the
wet process (EPA, 1979).
12-1
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Producing Cement
The manufacturing of Portland cement can be divided into four steps:
1. quarrying and crushing of raw materials,
2. grinding and blending of raw materials,
3. cement clinker production, and
4. finish grinding and packaging.
Cement rock, limestone, clay, shale, and other stones are usually mined from open-pit quarries at
or near the cement plant. These raw materials are then transported to the primary crusher by dump
trucks, rail cars, or conveyors (Figure 12-1). The primary crusher reduces the rocks to diameters
ranging from 15 to 25 cm (6 to 10 in.). After the rock is broken by the primary crusher, it is con-
veyed to a secondary crusher where the rocks are crushed to diameters from 2 to 2.5 cm (% to 1 in.).
Vibrating
screen
crusher
Primary crusher—
jaw type
Figure 12-1. Quarrying and crushing operations.
Rocks from the secondary crusher are conveyed to raw material storage bins. Each type of raw
material (rocks) is stored in a separate storage silo. From storage they will be sent to grinding and
blending operations.
12-2
-------
The second step is the preparation (grinding and blending) of the raw materials for feeding into
the kiln (Figure 12-2). The raw materials are ground to approximately 0.01 cm (0.029 in.) either
before, during, or after blending. The preparation process depends on whether the cement is pro-
duced by the wet or the dry process. In the dry process, as shown in Figure 12-2a, the raw materials
are proportioned onto a conveyor and put through air separators (cyclones) where they are dried by
heated air. The air separators allow the proper-sized material to be pumped to the dry mixing and
blending silos. Any oversized material is sent to the grinding mill and then recycled.
Dust recycle/collection
system
Mixing, blending, and
storage silos
Pneumatic pump loads
storage bins
Figure 12-2a. Grinding and blending operations—dry process.
12-3
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In the wet process, Figure 12-2b, water is combined with the raw materials in the grinding mill,
forming a slurry. After grinding, the slurry is then screened (oversized material is sent back to the
grinding mill) and stored in huge, open tanks. The advantage of the wet process is that it is easier to
produce a more uniform (homogeneous) mixture of the raw materials. The big disadvantage, though,
is that much more heat must be used in the kiln to evaporate the added water.
Mixing and
blending tanks/
Storage tank
for feed to kiln
Water in
From raw
material storage
Figure 12-2b. Grinding and blending operations—wet process.
12-4
-------
In the third step, Figure 12-3, the blended feed material (either wet or dry) is fed into the kiln and
heated. The kiln is fired with coal, oil, or gas at the opposite end from where the feed is introduced.
The hot combustion gas flows countercurrent to the feed material, exposing the feed to higher and
higher temperatures. Heating this material to above 1595°C (2900 °F) causes a chemical reaction
(called fusion) to occur, and a new mineralogical compound, cement clinker, is produced. Cement
clinker is a round, marble sized, glass-hard material that forms at approximately 1595°C. At the flame
end of the kiln, the clinker drops from the kiln into a cooler, where its temperature is quickly
reduced. From the cooler, the clinker is stored in tanks for later use or sent directly to the finishing
mills.
Flue gas
Feed from
mixing and
blending
Exhaust
Exhaust to treatment
Clinker
cooler
X
To storage silos
Figure 12-3. Cement clinker production.
12-5
-------
The final step of cement production is the finish grinding and packing (Figure 12-4). The clinker
is finely ground (to approximately 0.0043 cm [0.0017 in.J in diameter) along with a small amount of
gypsum. The gypsum controls the setting time of the cement. The final product (Portland cement) is
either bagged or stored for bulk shipping.
Air
separator
Finished
product
storage
Finishing grinding
mill
. Figure 12-4. Finish grinding and packaging.
12-6
-------
Review Exercise
1. True or False? Cement and concrete refer to the same
mineralogical substance.
2. Name the four steps involved in the production of Portland
cement.
1. False
3. Portland cement can be produced by a
process.
, or
2. • quarrying and
crushing
• grinding and blending
• clinker production
• finish grinding and
packaging
4. Clinker is produced in the
a. ball mill
b. kiln
c. grinding mill
d. cooler
3. wet (or) dry
4. b. kiln
Air Pollution Emissions
Paniculate matter is the primary pollutant emitted from the manufacture of Portland cement.
Because of the raw materials used, almost every operation in the cement plant has the potential to
produce dust. Figure 12-5, a flow diagram of a typical cement plant, indicates the potential sources
of dust emissions. Exhaust from the kiln is the largest source of paniculate emissions in the cement
plant. The second and third largest sources are the clinker cooler and the dry milling system, respec-
tively. Many other potential fugitive emission points, called transfer points, are at the end of all
material-conveying devices.
Combustion gases from the fuel used to heat the kiln are also present. These include both SO2 and
NO,. However, it has been reported that very little of the SO2 that is generated is emitted into the
atmosphere (EPA, 1979). As SOt is generated, it reacts with both calcium and other alkaline oxide
panicles to form sulfates and is eventually incorporated into the clinker (EPA, 1979). NOX emissions
are much greater than SO» emissions. Cement kilns can be a potentially large source of NOX emis-
sions. Four primary factors contribute to the formation of NO,:
1. the flame and kiln temperature,
2. the residence time of the combustion gases at this temperature,
3. the rate of cooling of the gases, and
4. the quantity of excess air present.
Control of these factors may result in a sharp reduction of the NO* emissions. However, at present,
NOX control equipment in cement plants is not used because of the absence of regulations and
because of the high technology costs.
12-7
-------
Key:
Emission points
Product flow
Flue gas
Figure 12-5. Potential emission points at cement plants
12-8
-------
The change from the wet to the dry production process has both a positive and a negative effect
on the amount of paniculate emissions. With a dry process, fugitive particulate emissions are
increased (over wet process emissions) from grinding, mixing, blending, storing, and feeding raw
materials into the kiln. However, the dry processes generally use a suspension preheater for heating
the feed going into the kiln. A suspension preheater is similar in operation to a cyclone. The exhaust
containing the fines from the suspension preheater is sent to a collection system, therefore reducing
the uncontrolled particulate emissions from the kiln. It also ensures better contact of the kiln exhaust
gases with the feed material, which may increase sorption of SOS from the kiln exhaust gases.
Review Exercise
1 . What is the primary pollutant produced from the manu-
facture of cement?
a. ozone
b. NO,
c. SO,
d. paniculate matter
2. List, in order of highest to lowest emissions, the three
largest sources of dust in the cement plant.
3. Which pollutant generated by cement production is usually
incorporated into the process?
a. ozone
b. NO,
c. SO,
d. particulate matter
4. True or False? NOZ emissions are not emitted from cement
plants because plants have effective NO, control devices.
1. d. paniculate
matter
2. exhaust gases from the
kiln, from the clinker
cooler, from the dry
milling system
3. c. S02
4. False
12-9
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Air Pollution Control Equipment
Cyclones, gravel bed filters, electrostatic precipitators, and baghouses are all used to control par-
ticulate emissions from various points in a cement plant. Although high-energy wet scrubbers
(Venturis) have been installed at a few cement plants, they are not generally used (EPA, 1975). Table
12-1 summarizes the advantages and disadvantages of the control equipment used throughout a
Portland cement plant.
Table 12-1. Advantages and disadvantages of control devices
for cement operations. ,
Operation
Quarrying
Crushing and
grinding
Raw material
storage
Integral
preheater
and kiln
Kiln
Clinker cooler
Finish
grinding
Finished
material
storage
Packaging and
shipping
Mechanical collectors
Not applicable
Not applicable
Not applicable
Integral pan of pre-
heater countercurrent gas
and material flow; high-
energy controls neces-
sary to meet opacity
requirements
Used as precleaners for
high-energy devices
Used as precleaners for
high-energy devices
Cannot meet opacity
requirements
Cannot meet opacity
requirements
Cannot meet opacity
requirements
Baghouses
Not applicable
Very good
Very good
Very good; must con-
tend with temperature
reduction, dewpoint
Very good; must con-
tend with temperature
reduction, dewpoint
Very good, must con-
tend with abrasive
panicles; gravel-bed
filters have been
recently introduced
Very good
Very good
Very good
Electrostatic precipitators
Not applicable
Not economically feasible;
low flow volumes
Not economically feasible
Very good, if gas stream is
properly conditioned
Very good, must contend with
dewpoint panicle resistivity
and potential explosion
problems
Must contend with combina-
tion of clinker dust and
moisture, possibly coating
ESP interior with cement
Very good on large mills
Not economically feasible;
low air flow volumes
Not economically feasible;
low air flow volumes
Source: EPA, September 1975.
Fugitive Emissions
Small baghouse systems are used at the numerous transfer points in a cement plant to recover and
recycle the dust. Small baghouses are also used to control dust generated from the grinding and the
packaging operations. Emissions generated at transfer points are not exhausted through a stack; i.e.,
they are fugitive emissions. Therefore, adequate control requires the installation of a hood capable of
capturing the emissions and a fan that can move the dust to the baghouse.
Kiln Emissions
A baghouse or electrostatic precipitator is required to meet current regulations to control dust from a
kiln. Most control systems also include cyclones to remove the larger-sized particles before they enter
12-10
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the baghouse or electrostatic precipitator. Generally, the dust collected in the cyclone can be recycled
back into the kiln. Presently, neither the baghouse nor the electrostatic precipitator has proven to be
more effective than the other for controlling emissions from the kiln.
When baghouses are used to control kiln emissions, the gas temperature is of primary importance,
especially from dry-process kilns. Kiln exhaust gases should be cooled to below 260 °C (500 °F) before
being sent to the baghouse. Higher temperatures will accelerate the deterioration of the bag fabrics.
However, when baghouses are used on wet-process kilns, the baghouse may need insulating to prevent
water vapor from condensing on the bags.
Effective operation of an electrostatic precipitator depends on the resistivity of the dust being col-
lected (see Lesson 4). Wet-process kiln gases generally exhibit the proper moisture and temperature
characteristics to reduce panicle resistivity and allow effective electrostatic precipitation. When
precipitators are used on dry-process kilns, water cooling and conditioning are occasionally used to
overcome the problems of high resistivity and high temperature.
A special problem arises when electrostatic precipitators are used to control kiln emissions. During
kiln startup, the temperature of the kiln is raised slowly to reduce the loss of the heat resistant
(refractory) lining. While the kiln is warming up, combustible gases will be present in the exhaust
stream (especially with coal-fired kilns). Electrostatic precipitators cannot be activated in the presence
of combustibles. The internal arcing of the precipitator could cause a fire or explosion. Use of a
cyclone preceding the precipitator helps to minimize the excessive emissions during startup. Periods
of excessive emissions during startup, malfunction, or shutdown are specifically exempted from the
Federal New Source Performance Standards for cement kilns.
Clinker Cooler Emissions
The relatively small particle size of clinker cooler dust requires a high efficiency control device to
meet present regulations. Baghouses are the primary control device used on clinker cooler exhaust
gases. One gravel bed filter system was installed on a clinker cooler (EPA, 1975). A gravel bed filter
system generally consists of a cyclone and a silica gravel bed filter. The cyclone removes the large
particles, and the remaining dust is removed in the gravel bed—which is insensitive to operating
temperatures. These collectors handle operating temperatures of up to 540 °C (1000°F) with no gas
cooling or conditioning required.
New Source Performance Standards
The Federal government has promulgated New Source Performance Standards (NSPS) for Portland
cement plants. The NSPS apply to plants under construction on or after August 17, 1971, and
impose limits for paniculate matter emissions and opacity. Table 12-2 summarizes the NSPS for
Portland cement plants.
Table 12-2. New Source Performance Standards
for Portland cement plants.
Process
Kiln
Clinker cooler
Other facilities
Paniculate matter emission limit
Ib/ton of feed to kiln
0.30
0.10*
kg/Mg of feed to kiln
0.15
0.05*
Opacity
<20%
<10%
<10%
•These limits are based on kg/Mg (Ib/t) of dry feed to the kiln.
12-11
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Review Exercise
Which of the following are generally not used by the
cement industry to reduce pollutant emissions?
a. baghouses
b. mechanical collectors
c. wet collectors
d. electrostatic precipitators
2. Which control device is generally used at the transfer points
in cement plants?
a. baghouse
b. mechanical collector
c. wet scrubber
d. electrostatic precipitator
1. c. wet collectors
3. Which control device(s) is(are) usually the primary dust con-
troller at the kiln?
a. baghouse
b. mechanical collector
c. electrostatic precipitator
d. all the above
e. both a and c, above
2. a. baghouse
A predefining device often used with kilns is the
a. baghouse.
b. electrostatic precipitator.
c. cyclone.
d. absorber.
3. e. both a and c, above
5. With control devices commonly used on wet-process kilns,
extensive thermal insulation is used to prevent
4. c. cyclone.
6. Since clinker cooler dust is relatively large/small, a control
device should be of high/low efficiency.
5. condensation of water
vapor within the
control device
Which of the following control devices should not be
used during kiln startup?
a. baghouses
b. venturi scrubbers
c. electrostatic precipitators
d. cyclones
6. small, high
7. c. electrostatic
precipitators
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8. A(n)
cannot be activated in the presence of
combustible materials.
a. baghouse
b. venturi scrubber
c. electrostatic precipitator
d. cyclone
The NSP.S opacity limit for emissions from kilns in cement
plants is
a. 10%.
b. 20%.
c. 30%.
d. 40%.
8. c. electrostatic
precipitator
10. The NSPS opacity limit for clinker cooler operations in
cement plants is
a. 10%.
b. 20%.
c. 30%.
d. 40%.
9. b. 20%.
11. Compliance with emission regulations and opacity regulations
is usually waived during
a. startup. -
b. shutdown.
c. malfunction.
d. all the above
e. none of the above
10. a. 10%.
11. d. all the above
References
Environmental Protection Agency (EPA). April, 1977. Compilation of Air Pollutant Emission
Factors. AP-42.
Environmental Protection Agency (EPA). September, 1975. Inspection Manual for Enforcement
of New Source Performance Standards: Portland Cement Plants. EPA 340/1-75-001.
Environmental Protection Agency (EPA). March, 1979. A Review of Standards of Perfor-
mance for New Stationary Sources—Portland Cement Industry. EPA 450/3-79-012.
12-13
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Lesson 13
Acid Plants
Lesson Goal and Objectives
Goal
The purpose of this lesson is to familiarize you with the manufacturing methods, emissions generated,
and control techniques used at sulfuric and nitric acid plants.
Objectives
At the end of this lesson, you should be able to:
1. describe three steps involved in both the sulfuric and nitric acid manufacturing processes.
2. list the air pollutants emitted from sulfuric and nitric acid plants and identify the primary air
pollution emission points in the process.
3. describe at least two techniques used to control air pollutant emissions from sulfuric and nitric
acid plants.
4. describe the format of the emission standards used for acid plants.
Introduction
Sulfuric and nitric acids are basic chemicals used in a number of industries. These acids are impor-
tant in the production of organic and inorganic chemicals, explosives, and plastics. They are also
important in the manufacture of chemical fertilizers. Over 50% of the sulfuric acid produced is used
in the manufacture of phosphate fertilizers from phosphate rock. Approximately 70% of the nitric
acid produced is consumed in making liquid and solid ammonium nitrate fertilizers.
The production methods for making sulfuric acid and nitric acid are similar. Each starts with a
basic feedstock material such as sulfur or ammonia. The feedstock is oxidized with the aid of a
catalyst, and the oxidized product is combined with water in an absorption device to form the acid.
Atmospheric emissions occur primarily after the absorption step in both the sulfuric and nitric acid
manufacturing processes.
Control methods may include changes in plant design or the use of add-on control equipment.
Commonly, extra absorbing systems are used to both collect the product and reduce pollutant
emissions.
Sulfuric Acid Manufacture
Sulfuric acid (HtSO4) is produced today in the United States by the contact process. In this method,
sulfur dioxide and air come in contact with a catalyst, forming sulfur trioxide. The sulfur trioxide is
dissolved in a solution of 98% sulfuric acid and 2% water. The sulfur trioxide combines with the
water in the absorbing solution to form more sulfuric acid.
13-1
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Sulfur dioxide used in the contact process can be produced in a number of ways. The most com-
mon method is to burn elemental sulfur in the presence of air. Sulfur dioxide emitted from the
smelting of copper, zinc, and lead ores is commonly converted to sulfuric acid using the contact
process. Feedstock materials can also be obtained by burning iron pyrites (iron-sulfur compounds),
hydrogen sulfide, and acid wastes generated by petroleum refineries.
About 68% of the sulfuric acid produced in the United States is made by burning elemental
sulfur. The contact process is relatively uncomplicated when the sulfur dioxide is obtained in this way
(Figure 13-1). Here sulfur is introduced into the sulfur burner in molten form. The combustion air
used for burning is taken from the atmosphere and dried with concentrated sulfuric acid, which picks
up water quite readily. The dry air is preheated before it enters the combustion chamber. This
method of combustion will produce an exhaust stream containing 7 to 14% sulfur dioxide by volume.
The hot gas from the burner is then sent to a waste heat boiler where it is cooled. The cooled gas
is filtered to remove dust and is then sent to the converter, where the sulfur dioxide is oxidized to
sulfur trioxide using a catalyst. The catalyst is generally vanadium pent oxide. It is placed in a
number of beds through which the gas must pass. The temperature is maintained near 427 °C
(800°F) during this process in order to achieve maximum conversion efficiency.
The gas exiting the converter is cooled to about 230 to 260 °C (450 to 500 °F) in an economizer and
then enters the absorption tower. Since sulfur trioxide is not readily soluble in water, concentrated
(98%) sulfuric acid is used to help absorb the gas. Sprays of the concentrated acid come in contact
with the sulfur trioxide in an absorption tower. The sulfur trioxide combines with 2% water con-
tained in the acid to form more sulfuric acid.
Fuming sulfuric acid (also called oleum — an acid made up of sulfuric acid and sulfur trioxide in a
1:1 ratio) can also be produced. The product from the absorption tower can be sent to the oleum
tower where the sulfur trioxide combines with the 100% sulfuric acid. The oleum is drawn off from
the tower and, with the addition of water, sulfuric acid of varying concentrations can be produced.
Sulfuric acid can also be produced from smelter gases. These gases must first be cleaned to remove
impurities such as paniculate matter and water vapor. Since dust can fill the voids in the catalyst
material and reduce its activity, cyclones, scrubbers, or electrostatic precipitators are used to remove
the material before it enters the drying tower.
13-2
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Sulfur
in
Sulfur burner
SO, to SO, converter
Economizer
98% H,SO. out
Figure 13-1. Sulfuric acid preparation.
13-3
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Nitric Acid Manufacture
Nitric acid (HNOS) is made commercially by processes involving the oxidation of ammonia.
Ammonia is oxidized with air as the gases are passed over a platinum catalyst to form nitric oxide.
The nitric oxide is, in turn, oxidized to nitrogen dioxide and then absorbed in water to form nitric
acid. A nitric acid plant can be described by separating it into three basic steps (Figure 13-2):
1. Converter or oxidation section—oxidation process — ammonia oxidized to nitric oxide (NO).
2. Cooler-condenser section—oxidation process—nitric oxide oxidized to nitrogen dioxide (NOZ).
3. Absorber section—nitrogen dioxide absorbed in water to form nitric acid (HNOS).
In the first step, fine wire mesh made of platinum and rhodium helps to convert the ammonia to
nitric oxide at a temperature of 800°C (1470°F). Preheated air is used in this oxidation process.
In the second step—converting nitric oxide to nitrogen dioxide—the rate of reaction is much
slower. However, the rate can be increased if lower temperatures and higher pressures are used. The
nitric oxide is, therefore, cooled before this process, and high gas pressure (up to 500 kPa [73 psig])
may be used to speed the reaction.
In the third step, 50 to 60% nitric acid is produced by absorbing the nitrogen dioxide in water.
This reaction is slow, but the absorption rate can again be increased by lowering the temperature
and increasing the pressure (see Lesson 5). The formation of nitric acid in the absorption step
releases NO as a by-product in the reaction. This can be reoxidized to NO2, but this secondary reac-
tion is not always complete. As a result, both nitric oxide and nitrogen dioxide can escape in the
exhaust from the absorber. As with sulfuric acid plants, the absorber is the primary point of pollu-
tant emissions in a nitric acid plant.
Nitric acid plants differ in how the various steps of the process are operated. The oxidation and
absorption steps can be run either at the same or at different pressures. In this sense, nitric acid
plants are categorized as having either single-pressure or dual-pressure processes. In the single-
pressure process, low or medium pressures of 400 to 800 kPa (60 to 120 psig) are used for both steps.
In the dual-pressure or combination-pressure process, the ammonia is oxidized at low pressure^ on
the order of 300 to 500 kPa (45 to 75 psig). The NOt is absorbed at pressures of from 800 to 1400
kPa (120 to 210 psig).
13-4
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Flue gas
Ammonia
oxidizer
Water
Ammonia (NH,)
NO
NO,
Condeniate acid \
Ammonia (NH§) in
Figure 13-2. Nitric acid preparation.
13-5
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Review Exercise
Sulfuric and nitric acid are important chemicals in the
manufacturing of
a. kraft pulp.
b. fertilizer.
c. cement.
d. asphalt concrete.
2. Sulfuric acid is produced from
a. elemental sulfur.
b. smelter exhaust gases.
c. iron pyrites.
d. acid wastes.
e. all the above
1. b. fertilizer.
3. The vanadium pentoxide catalyst used in the converter of a
sulfuric acid plant oxidizes
a. sulfur to sulfur dioxide.
b. sulfuric acid to fuming sulfuric acid.
c. sulfur dioxide to sulfur trioxide.
2. e. all the above
4. The liquid used in the absorption tower of a sulfuric acid
plant is
a. water,
b. 2% sulfuric acid.
c. 98% sulfuric acid.
d. oleum.
3. c. sulfur dioxide to
sulfur trioxide.
5. Nitric acid is made from
a. ammonia.
b. ammonium nitrate.
c. exhaust gases produced by fossil fuel combustion.
d. petroleum.
4. c. 98% sulfuric acid.
6. Fill in the blanks. The three principle steps involved in
producing nitric acid are:
a. conversion of to
b. oxidation of NO to
c. absorption of
in H2O to form
5. a. ammonia.
7. The second step of the nitric acid manufacturing process
is favored at
a. high temperatures and low pressures.
b. low temperatures and high pressures.
8. Name the two categories of nitric acid processes.
6. a. ammonia (NH3) to
nitric oxide (NO)
b. nitrogen dioxide
(NO,)
c. nitrogen dioxide
(NOt) in H2O to form
nitric acid (HNO3).
7. b. low temperatures and
high pressures.
8. • single pressure
• dual pressure
13-6
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Air Pollution Emissions
Sulfuric Acid Plants
Sulfur dioxide, sulfur trioxide, and sulfuric acid mist are the principal air pollutants occurring from
contact sulfuric acid plants. The emission of these pollutants depends on plant operation. In many
systems, emission standards can be met by changing operating conditions or by slightly modifying the
production process.
Sulfur dioxide is primarily emitted from the absorption towers. The conversion of sulfur dioxide to
sulfur trioxide is an equilibrium process that depends on temperature and the individual gas concen-
trations. Under some conditions, significant amounts of sulfur dioxide may remain unconverted. This
sulfur dioxide will not be absorbed as readily as the sulfur trioxide in the absorption tower and will
escape into the atmosphere.
Sulfur trioxide can be emitted directly from the absorber in cases where the tower is not operating
properly. The emissions will result in the formation of acid mist in the absorber plume.
Acid mist is formed by sulfur trioxide coming into contact with water vapor. Cooling sulfuric acid
below the dew point causes sulfuric acid mist particles, ranging from 0.3 to 5.0 fim in diameter, to
form. Emitted into the atmosphere, these particles will scatter light to produce a dense, white plume.
Acid mist is difficult to remove in the absorber, and once formed in the process, will pass through
the absorber and be emitted. Extraneous water vapor in the process gas stream is the primary cause
of mist formation. The water vapor can appear if the sulfur burned contains a high organic content,
if the process air is inadequately dried, or if the strength of the acid in the absorbing tower is too
low. Also, if nitrogen oxides are produced during the sulfur burning process, they can be carried
through the absorber to oxidize the sulfur dioxide emitted from the absorber. The sulfur trioxide
produced will then convert to acid mist in the atmospheric plume. Table 3-1 summarizes sulfuric
acid plant emissions.
Table 3-1. Potential sources of atmospheric emissions
within fulfuric acid plants.
Emisrion
Sulfur dioxide
Sulfur trioxide
Sulfuric acid
Source
Absorption towers
13-7
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Nitric Acid Plants
The absorption unit of a nitric acid plant is the primary emission point for this industry. Emissions of
nitric oxide and nitrogen dioxide with trace amounts of nitric acid mist can occur. Nitrogen dioxide
will give a reddish brown color to the plume from the absorber. The opacity of the plume will
depend on the amount of nitrogen dioxide and acid mist exiting the absorber.
Emissions from nitric acid plants, as with those from sulfuric acid plants, primarily depend on how
the plant is operated. Operating variables affecting nitrogen oxide emissions include:
• the amount of air supplied to the oxidizer and absorber,
• system pressures and temperatures, and
• gas stream throughput rates.
Table 3-2 summarizes nitric acid plant emissions.
Table 3-2. Potential sources of atmospheric emissions
within nitric acid plants.
Emission
Nitric oxide
Nitrogen dioxide
Nitric acid
Source
Absorption cowers
Air Pollution Control Methods
Sulfuric Acid Plants
Sulfur dioxide and sulfuric acid mist are the principal pollutants controlled in sulfuric acid plants.
Sulfur dioxide emissions are minimized by producing acid using the dual-absorption process (also
called the double-contact process) instead of using the single-absorption process discussed previously.
New and modified sulfuric acid plants built in the United States today use this manufacturing
method rather than adding alkali scrubbers or other control devices to remove sulfur dioxide. Acid
mist is commonly controlled by adding filter-type mist eliminators after the absorption tower.
Figure 13-3 illustrates a dual-absorption system. Note the addition of another absorption tower. In
this manufacturing method, the exhaust gases from the first absorber are sent back to the converter.
Most of the sulfur trioxide has been removed in the absorber, leaving an exhaust gas richer in sulfur
dioxide. This gas is sent to the bottom stage of the converter and reacted to form sulfur trioxide.
The resulting gas stream is then sent to the second absorber to produce more sulfuric acid.
A number of benefits are derived from this technique. SOt emissions from a single-absorption
system are generally above EPA standards, but the standards can easily be met using a dual-
absorption system. The dual-absorption system recovers sulfur dioxide which otherwise would be lost
to the atmosphere. Although the initial cost of these systems is higher than for the single-absorption
units, the types of equipment and operating methods are all similar. Overall, the increased efficiency
in sulfuric acid production and the capability of meeting emissions standards have encouraged the
use of the dual-absorption process.
13-8
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SO, fr
sulfur burner
Gas filter
SO, to SO,
convener
Absorbing,
tower
Economizer
Dilution
tank
H,S04
Figure 13-3. Sulfuric acid dual-absorption (double-contact) system.
13-9
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Acid mist emitted from the absorption towers can be controlled using fiber mist eliminators. Glass
or fluorocarbon fibers are commonly used in a number of configurations. Tubular mist eliminators,
operating in a manner similar to the exterior filtration method in baghouse systems, represent one
such configuration (Figure 13-4). The collected sulfuric acid drains to the bottom of the tank and is
returned to the process. A collection efficiency of virtually 100% can be achieved for mist particles
greater than 3.0 fan in diameter with such a system.
Fiber tubes
Figure 13-4. Vertical-tube mist eliminator installation.
Fibers packed between stainless steel mesh frames to form panels are also commonly used to collect
the acid mist (see Lesson 5). These panels can be arranged vertically or horizontally to remove mist
effectively.
Electrostatic precipitators can also be used to remove acid mist, but filter pads are generally used
in practice. Precipitators are, however, frequently used to clean the feed gas introduced to the con-
tactor. This is generally the case in acid plants where sulfur dioxide is produced from the combustion
of sulfide ores or acid sludges.
13-10
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Nitric Acid Plants
A number of methods are used to control the nitrogen oxide emissions from nitric acid plants. Two
methods that have received the widest application are extended absorption and catalytic reduction.
Extended absorption is similar to the dual-absorption process used in sulfuric acid plants. In the
nitric acid plant, a second absorption tower is added in series to the existing absorber. Water is used
to absorb the nitrogen oxides emitted from the first absorber. A weak acid is formed which is then
recycled to the first absorber to increase the overall yield from the plant.
Catalytic reduction requires the addition of a catalytic unit which will convert the nitrogen oxides
to nitrogen. To do this, methane is mixed with the exhaust gas and the mixture is passed over a
platinum or palladium catalyst. The reactions that occur produce heat and harmless products such as
carbon dioxide, water vapor, and nitrogen. Another catalytic method uses ammonia to reduce
nitrogen dioxide to nitrogen by passing the gas mixture over a platinum catalyst.
Other control methods have been used to reduce emissions from nitric acid plants. Among these
have been wet scrubbing, molecular sieve adsorption, and chilled absorption. These methods gen-
erally have higher energy requirements or higher operating costs than extended absorption or
catalytic reduction. In fact, because of the fuel requirements for catalytic reduction, the extended
absorption method is most commonly applied to new nitric acid production units constructed today.
New Source Performance Standards
Acid plants are regulated by the States and by the Federal EPA under the New Source Performance
Standards. The Federal standards apply to new or modified production units constructed after
August 17, 1971. These standards are summarized in Table 13-3.
Table 13-3. New Source Performance Standards for acid plants.
Type of acid
plant
Sulfuric acid*
Nitric acid*
Process unit
Any process equipment
Any process equipment
Pollutant
SO,
Acid mist
NO.
Emission level
2.0 kg/metric ton of acid produced
(4.0 Ib/ton of acid produced)
0.075 kg/metric ton of acid produced
(0.15 Ib/ton of acid produced)
1.5 kg/metric ton of acid produced
(3.0 Ib/ton of acid produced)
•Opacity is not to exceed 10%.
13-11
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Review Exercise
What are the principal air pollutants emitted from sulfuric
acid plants?
a. dust panicles
b. sulfur dioxide (SO2)
c. sulfur trioxide (SO3)
d. sulfuric acid mist
2. What are the principal air pollutants emitted from nitric acid
plants?
a. nitric oxide (NO)
b. nitrogen dioxide (NOZ)
c. nitrogen (Nz)
d. ammonia (NH5)
1. b. sulfur dioxide (SO2)
d. sulfuric acid mist
Sulfuric acid plants and nitric acid plants are similar in their
operation. The principal pollutant emissions occur from
similar components in each type of plant. What is the main
source of acid plant emissions?
a. catalytic oxidation system
b. heat recovery system
c. absorption tower
2. a. nitric oxide (NO)
b. nitrogen dioxide
(NO,)
4. What is .the primary method used to meet SO2 emissions
standards in newly constructed sulfuric acid plants?
a. application of flue gas desulfurization systems
b. use of dual-absorption systems
c. application of mist eliminators
d. use of single-absorption systems
3. c. absorption tower
5. What is the most common method used to control sulfuric
acid mist emissions from sulfuric acid plants?
a. application of flue gas desulfurization systems
b. use of dual-absorption systems
c. application of mist eliminators
d. application of electrostatic precipitators
4. b. use of dual-
absorption systems
What are the two most common methods used to control
NO, emissions from nitric acid plants?
a. extended absorption
b. application of molecular sieves
c. catalytic reduction
d. aklaline scrubbing
5. c. application of
mist eliminators
6. a. extended absorption
c. catalytic reduction
13-12
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7. What is the NSPS SO» emission standard for sulfuric acid
plants?
a. 1.5 kg SOx/metric ton of acid produced
b. 2.0 kg SOx/metric ton of acid produced
c. 1.5 ng SOt/Joule heat input
d. 2.0 ppm SO,
8. What is the NSPS NO, emission standard for nitric acid
plants?
a. 2.0 kg NOj/Joule heat input
b. 3.0 ng NO,/metric ton of acid produced
c. 1.5 kg NOi/metric ton of acid produced
d. 10%
7. b. 2.0 kg SOi/metric
ton of acid produced
8. c. 1.5 kg NO,/metric
ton of acid produced
References
Environmental Protection Agency (EPA). 1966. Atmospheric Emissions from Nitric Acid Manufactur-
ing Processes. EPA/AP-27.
Environmental Protection Agency (EPA). 1965. Atmospheric Emissions from Sulfuric Acid
Manufacturing Processes. EPA/AP-13.
Environmental Protection Agency (EPA). 1979. A Review of Standards for New Stationary
Sources—Nitric Acid Plants. EPA 450/3-79-013.
Environmental Protection Agency (EPA). 1979. A Review of Standards of Performance for New
Stationary Sources—Sulfuric Acid Plants. EPA 450/3-79-003.
Environmental Protection Agency (EPA). 1977. New Source Performance Standards Inspection
Manual for Enforcement of Sulfuric Acid Plants. EPA 340/1-77-008.
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Lesson 14
Municipal Incinerators
Lesson Goal and Objectives
Goal
To familiarize you with the operation, emissions generated, and control equipment used at municipal
incinerators.
Objectives
At the end of this lesson you should be able to:
1. describe the six steps involved in the incineration of municipal waste.
2. name the three classes of pollutants emitted from municipal incinerators.
3. describe two control devices used to reduce pollutant emissions from municipal incinerators.
Introduction
One method of disposing of waste material is to burn or incinerate the waste. Incinerators are used
by a variety of sources (municipal, industrial, commercial, and residential) to dispose of solid, liquid,
or gaseous wastes. A variety of pollutant emissions are created by incineration. Control devices are
required to reduce emissions from large incinerators.
Municipal incinerators are large, centrally located facilities used to dispose of solid wastes. These
incinerators can burn from 50 to over 3000 tons of waste per day. However, small modular units are
designed to burn solid wastes of approximately 10 tons per day. The combustible solid waste is
transported by truck from the surrounding area to the facility. Generally, municipal wastes are com-
posed of combustible materials such as paper, wood, rags, food, yard clippings, and plastic and
rubber; and noncombustible materials such as rocks and glass. Municipal incinerators are used to
reduce the volume and the weight of the disposable materials.
Incinerator Operation
A typical municipal incinerator operation involves the following six steps:
1. delivering and weighing the solid waste,
2. storing the waste,
3. preparing the waste,
4. charging the incinerator,
5. burning the waste, and
6. disposing of residue.
Figure 14-1 illustrates a typical municipal incineration process.
14-1
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Primary
combustion
chamber
Boiler
area
Clamshell
bucket
Storage and
preparation pit
Underfire Refuse-
air fan burning area
Figure 14-1. Typical municipal incinerator.
Delivering and Weighing
Refuse is delivered in trucks to the storage pit at the incinerator. Before dumping the refuse, the
trucks pass over a scale so that the weight of refuse entering the plant can be measured and
recorded.
Storage
After the trucks are weighed, they dump the refuse into a storage pit. Usually the storage pit is
located below ground. An overhead crane with a clamshell bucket is used to move the refuse from
the storage pit to the charging chute. Small to moderate-sized incinerators may use a front end
loader rather than an overhead crane with a clam shell bucket.
Preparation
Preparation involves shredding any oversize refuse and/or removing any noncombustible material.
Metal can be removed by magnets, and other noncombustible material can be removed by screening
and sorting. These practices help achieve more efficient combustion in the furnace, since the waste is
made more homogenous (uniform). However, since the operating cost increases when waste is
prepared, this step is not always used.
14-2
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Charging
The prepared solid waste can be charged (dumped) into the furnace either continuously or in
batches. When batch feeding is used, solid waste is fed directly to the furnace by opening the
charging gate and dropping in the waste. However, large quantities of cool air, that reduce combus-
tion efficiency, can also enter the furnace. In continuous feeding, the waste is fed into a chute
leading to the incinerator grate. The waste fills the chute at all times providing an air seal that
prevents smoke or heat from escaping from the furnace chamber back into the charging area.
Therefore, continuous feeding also reduces the amount of cool air entering the furnace, minimizing
the irregularities in the combustion process. Most municipal incinerators constructed recently are
continuous feeding systems.
Burning
Combustion of the solid wastes occurs in the furnace. The furnace consists of a grate system and
combustion chambers. The grate system transports the solid waste and residue through the combus-
tion chambers, while agitating the solids to enhance combustion. The grates are also designed to
allow part of the combustion air (called under/ire air) to pass up through the grates. Additional com-
bustion air can be blown in over top (overfire car) of the burning refuse. Combustion chambers pro-
vide a time and a place for the mixing of hot gases to occur which ensures complete combustion.
Numerous designs and configurations of both grate systems and combustion chambers are used for
municipal incinerators, but no one design is considered the best (EPA, 1979). The most widely used
grate systems are the traveling, reciprocating, rocking, and circular grates. Commonly used furnace
designs are the rectangular, the multicell rectangular, the vertical circular, and the rotary kiln (EPA,
January 1977). ~
Grates
Traveling grates are continuous, belt-like conveyors (Figure 14-2). Usually two or more grates are
positioned at different levels. The solid waste and residue are transported and dropped from one level
to the next to provide agitation as the waste moves through the furnace.
Figure 14-2. Traveling grates.
14-3
-------
Reciprocating grates resemble steps (Figure 14-3). Every other "step" (grate) is designed so that it
can slide back and forth while the other "steps" remain fixed. The refuse is moved forward and
agitated as the movable grates slide across the fixed grates.
Fixed grates
Moving grates
Figure 14-3. Reciprocating grates.
Rocking grates typically slope downward and are arranged in rows across the width of the furnace
(Figure 14-4). Each row is capable of being rocked to produce an upward and forward motion. Alter-
nate rows are rocked to move and agitate the wastes.
Figure 14-4. Rocking grates.
14-4
-------
Circular grates are used in vertical, circular furnaces. Circular grates consist of a central rotating
cone and a number of rabble (mixing) arms (Figure 14-5). As the cone and arms rotate, the solid
waste and residue are agitated while they burn. This agitation moves the waste to the chamber's side
for disposal.
Grate area
Top view
Stoking or rabble arm
Figure 14-5. Circular grate.
Side view
Combustion Chamber
Most large municipal incinerators (capacity larger than 50 tons per day) have furnaces composed of
at least two combustion chambers: an ignition (or primary) chamber and a secondary chamber. Most
municipal solid wastes contain substantial amounts of surface and internal moisture and therefore
require a drying process before complete combustion can occur. Waste drying, ignition, and burning
occur in the ignition (primary) chamber. The secondary chamber is used to ensure complete combus-
tion of the gases and paniculate matter produced in the primary combustion chamber.
14-5
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Furnace Designs
The rectangular furnace is most commonly used in municipal incinerators (Figure 14-6). Waste is
charged through a single chute and travels through the furnace on a grate system. Several grate
systems can be used with this furnace. Commonly, two or more grates are arranged in tiers so that
the moving solid waste is agitated as it drops from one level to the next. Secondary combustion
occurs in the back end (opposite the charging chute) of the furnace. The back half of the fur-
nace is separated from the front half by a curtain wall. The curtain wall helps radiate heat back
toward the charging grate to promote drying and igniting the waste as well as increasing combustion
gas velocity and the level of turbulence.
Charging chute
Primary combustion chamber
Ash and cinder
discharge
Figure 14-6, Rectangular furnace.
14-6
-------
The multicell rectangular furnace, or mutual assistance furnace, contains two or more cells that sit
side-by-side (Figure 14-7). Each cell has its own rectangular grate. Solid waste is usually charged
through a door in the top of each cell. Generally, the cells of the furnace have a common secondary
combustion chamber and share a residue-disposal hopper.
Charging chutes
Overfire air inlet
To secondary
combustion
chamber
Stoking
door
Figure 14-7. Multicell rectangular furnace.
14-7
-------
In the vertical circular furnace (Figure 14-8), solid waste is charged through a door or lid in the ceil-
ing and is dropped onto a central cone grate. As the cone and rabble arms slowly rotate, the waste
bed is agitated and the residue is pushed to the chamber's sides where it is discharged through a
dumping grate. Stoking doors are provided to allow manual agitation and assistance in residue
dumping when necessary. A secondary combustion chamber is adjacent to the circular chamber.
Charging chutes
Primary
combustion
chamber
Secondary
combustion
chamber
Figure 14-8. Vertical circular furnace.
14-8
-------
A rotary kiln furnace consists of a rectangular furnace followed by a refractory-lined cylinder (kiln).
The cylinder is mounted at a slight incline downward from the charging ports (Figure 14-9). Waste is
charged into the furnace where it is dried and partially burned. The partially burned waste is then
fed (by grates) into the kiln, where the rotating action exposes unburned material for combustion.
The cylinder rotates slowly to evenly distribute the solid waste. Secondary combustion of the gases
and any suspended paniculate matter occurs in the mixing chamber. Residue from the end of the
kiln usually falls into a quenching trough.
i Charging
chute
Overfire air
duct
Drying
grate
Underfire
air fan
To mixing
chamber
Quenching*
trough
Figure 14-9. Combination traveling grate and rotary kiln.
14-9
-------
Residue Removal
The residue, all of the solid material that remains after burning, is discharged from the end of the
grate into ash hoppers. The hoppers are usually filled with water (referred to as quench tanks) to
control dust and to reduce the fire hazard in handling the hot residue. A slow-moving drag conveyor
is used to remove the wet residue from the quench tanks. The residue is then loaded into a holding
hopper or loaded directly into trucks.
In addition to the residue that comes off the end of the grate, fine materials that fall through the
grate openings must also be collected and disposed of. These fine materials are referred to as
shifting* and are removed manually from beneath the grates or collected in troughs and conveyed to
the residue hopper.
Review Exercise
1.
2.
3.
4.
5.
6.
7.
True or False? Municipal incinerators are large facilities
that burn from 50 to over 1000 tons of waste per day.
Tnrinerators are used to reduce the , . and the
. . , , of the solid wastes.
List the six steps involved in incineration of municipal waste.
involves the shredding of any oversize refuse
and/or the removing of any noncombustibles to enhance
the combustion of the waste.
a. Weighing
b. Storing
c. Preparing
d. Charging
Solid waste can be charged into the furnace either con-
tinuously or in batches. Most recently constructed
incinerators are feeciinjr systems heraiise they
provide more uniform control of the combustion process.
The furnace of a municipal incinerator consists of
a. a grate system
b. combustion chambers
c. weigh hoppers
d. a grate system and combustion chambers
e. none of the above
The system transports the solid waste and
residue through the furnace, while agitating the solids to
enhance combustion.
1. True
2. volume,
weight
3. a. delivering and
weighing
b. storing
c. preparing
d. charging
e. burning
f . disposing of the
residue
4. c. Preparing
5. continuous-
6. d. a grate system and
combustion chambers
7. grate
14-10
-------
8.
9.
10.
11.
12.
13.
Combustion air that passes up through the grates is termed
a. underfire
b. excess
c. overfire
d. theoretical
List four common types of grate systems.
Most large municipal incinerators have at least two com-
bustinn chambers; a(n) chamber and a(n)
., chamber.
Solid-waste drying, ignition, and burning occur in the
„,„ combustion chamber.
a. primary (or ignition)
b. secondary
c. grate
d. top
List four commonly used furnace designs.
Residue from the furnace is discharged from the end of
the grate into ash hoppers. The ash hoppers are normally
filled with water to ..,_.
a. control dust
b. reduce any potential fire hazard
c. act as an air tight seal
d. a and b
e. all the above
8. a. underfire
9. a. traveling
b. reciprocating
c. rocking
d. circular
10. ignition (or primary)
and secondary
11. a. primary (or ignition)
12. a. rectangular
b. multicell rectangular
c. vertical circular
d. rotary kiln
13. d. a and b
14-11
-------
Air Pollution Emissions
Municipal incinerators can emit four classes of air pollutants:
• paniculate matter,
• combustible gases (carbon monoxide, hydrocarbons, and odors),
• noncombustible gases (nitrogen oxides, sulfur oxides, hydrogen chloride and hydrogen fluoride),
• occasionally metal emissions (lead, cadmium, and chromium), and
• odors.
All of the pollutants, except odors, are emitted from the furnace exhaust stack (Figure 14-10). The
rate of emissions from an incinerator varies widely and depends on the
• composition of the waste,
• design of the incinerator (i.e., preparation system, charging system, and combustion chamber),
and
• method of operation (air flow rates, chamber temperature, etc.).
In general, paniculate matter is the main pollutant of concern. Paniculate matter emitted from
municipal incinerators is the only pollutant subject to the New Source Performance Standards
(NSPS). However, HC1 emissions are becomming of greater concern. Table 14-1 lists the estimates of
uncontrolled emission rates for the various pollutants (EPA, December 1977).
Table 14-1. Emission factors for municipal solid wane incinerators without
controls. Emission factor rating: A (excellent).
Pollutant
Paniculate matter
Sulfur dioxide
Nitrogen oxides
Carbon monoxide
Organic]
Emission factor
kg/Mg charged
into incinerator
15.00
1.25
1.50
17.50
0.75
Ib/ton charged
into incinerator
30.0
2.5
3.0
35.0
1.5
Source: EPA. December 1977.
Particulate Matter
The actual emission rate of paniculate matter is a function of many design and operating variables.
Generally, the most important variable affecting the paniculate emission rate is the combustion air
flow rate in the furnace. Excessive air flow can cause higher paniculate emissions in two ways —first,
a large amount of underfire air increases the air velocity through the grate, which increases particle
entrainment from the burning waste on the grate; second, excessive air flow decreases both furnace
temperature and residence time, which in turn reduces the completeness of combustion and results in
combustible particles in the exhaust gas.
Combustible Gases
When burning any combustible solid material, combustible gases (hydrocarbons and carbon
monoxide) are produced. If the incinerator is properly designed and operated, these gases are easily
converted to carbon dioxide and water vapor. As can be seen from Table 14-1, the emission factor
for organic vapors from a typical incinerator is very low. However, the emission factor for carbon
monoxide (CO) is high. In fact, more pounds per hour of CO than other air pollutants can be
emitted from municipal incinerators.
14-12
-------
One explanation for the high CO emissions is that the combustion of organics occurs in two stages:
first, rapid oxidation of all carbon to CO, followed by slow oxidation of the CO to CO*. In addition,
CO is converted to COS only at temperatures above 700 °C (1300°F). Proper design and operation
(high temperature, good mixing, and long residence time) of the incinerator can reduce the amount
of CO that is emitted.
Noncombustible Gases
Both nitrogen oxide and sulfur dioxide emissions occur in solid waste incineration. However, the
amounts per ton of fuel burned are very low compared to the quantity emitted by other combustion
sources (EPA, January 1977). For example, these emissions are approximately one-tenth the amount
emitted from fossil-fuel-fired boilers. Sulfur dioxide emissions are low since most solid wastes contain
only 0.1 to 0.2% sulfur. Nitrogen oxide emissions are low because the nitrogen content of most
wastes is low and incinerators operate at temperatures [below 1030°C (1900°F)] that do not promote
extensive nitrogen oxide formation.
Also there is some concern about the hydrogen chloride (HC1) emission resulting from the
increased incineration of plastics made of polyvinyl chloride. Several municipal incinerators have
been studied, with mixed results reported as to the actual emission levels of HC1 (Jahnke, 1977).
Air Pollution Control Equipment
The two most widely used control devices used on municipal incinerators are electrostatic precipitators
(ESPs) and wet scrubbers (Venturis). Mechanical collectors (cyclones and settling chambers) are occa-
sionally used as precleaning devices, but cannot achieve particle removal efficiencies that meet current
regulations. Fabric filters have been used on very few installations because of the high temperature and
moisture content of the exhaust stream. However, some new installations are using baghouses for reduc-
ing paniculate emissions. In addition to these "add on" control devices, improving the combustion
process can also reduce emissions. Improving the combustion process is especially effective in reducing
the amount of combustible paniculate matter and gases emitted.
Electrostatic Precipitators
Electrostatic precipitators (ESPs) have been successfully used to control paniculate emissions from
municipal incinerators for many years. In fact, the ESP has become the control system in the majority
of plants subject to current, stringent air pollution regulations (EPA, 1979). Paniculate matter removal
efficiencies in the range of 96 to 99.6% have been reported (Corey, 1977).
ESP collection efficiency is a function of the resistivity of the particles in the exhaust stream. Three
prime factors affecting panicle resistivity (hence collection efficiency) are the properties of the refuse,
the operating temperature, and the humidity of the exhaust stream (see Lesson 4). The properties of
the refuse determine the electrical resistivity of the particles formed. For example, burning large quan-
tities of paper products produces carbon panicles having a very low resistivity. Low resistivity causes the
particles to rapidly lose their charge, making them difficult to ultimately collect in the ESP. In addi-
tion, poor combustion in the incinerator also results in the generation of large quantities of car-
bonaceous particles.
The temperature of the exhaust stream entering the ESP is monitored to ensure proper operation.
The temperature must be within certain recommended operating limits. Operating temperatures below
120°C (250°F) would be ideal. However, since incinerator flue gas is exhausted at temperatures in
excess of 540°C (1000°F), cooling the flue gas to low temperatures is not feasible. Most ESPs on
municipal incinerators operate at between 205 and 315 °C (400 and 600 °F).
14-13
-------
Humidity of the exhaust gas stream also affects particle resistivity. Because of the variability of the
moisture content of most solid wastes, the humidity of the exhaust gas stream is monitored and
adjusted to ensure effective ESP operation. Generally, increasing the humidity increases particle col-
lection efficiency; however, too much moisture can adversely affect ESP operation.
Wet Scrubbers
Many different scrubber designs have been used on incinerators; however only high-energy, venturi
scrubbers have been capable of meeting current regulations (EPA, 1975). And even these high-energy
scrubbers have achieved only limited success in constantly meeting these regulations (EPA, 1979).
The paniculate matter removal efficiency of wet scrubbers is proportional to the power or energy
input to the system. For a venturi, the power input is indicated mainly by the pressure drop. To
achieve high efficiencies, Venturis installed on municipal incinerators operate in the range of 38 to
127 cm H2O (15 to 50 in. H2O). Operating a system at these high energy levels is very costly com-
pared to ESPs [that have a pressure drop of only 2.5 to 5 cm H2O (1 to 2 in. H2O)j. However, wet
scrubbers are capable of removing acid gases, whereas ESPs are not.
New Source Performance Standards
The Federal government has promulgated New Source Performance Standards (NSPS) for municipal
incinerators. The standards affect any facility under construction or modification as of August 17,
1971, and that is designed to have a capacity of 45 Mg per day (50 tons per day) or greater.
The only pollutant emitted from municipal incinerators that is regulated by an NSPS is paniculate
matter. The NSPS for municipal incinerators limits the paniculate emissions to less than 0.18 grams
per dscm (0.08 grains per dscf) corrected to 12% CO*. In order to establish a consistant basis for
comparing emission rates, all concentrations are adjusted to the arbitrary value of 12% CO2.
Review Exercise
1 . Municipal incinerators can emit ....
a. paniculate
b. combustible gaseous
c. noncombustible gaseous
d. all the above
2. Almost all the pollutants generated
incinerator are emitted from the
pollutants.
at a municipal
3. True or False? Generally, the most important variable
affecting the particulate emission rate is the combustion air
flow rate in the furnace.
4. Excessive air flow rates result in higher /lower particulate
emissions.
1. d. all the above
2. furnace exhaust stack
3. True
4. higher
14-14
-------
5.
6.
7.
8.
9.
10.
11.
12.
IS.
The main pollutants emitted from municipal
incinerators are
a. paniculate matter
b. CO
c. organics
d. all the above
True or False? Nitrogen oxide and sulfur dioxide are
emitted in large quantities from most municipal incinerators.
^missions can result from burning plastics made
of polyvinyl chloride.
The two most popular control devices used on municipal
incinerators are
a. baghouses (fabric filters).
b. electrostatic precipitators.
c. wet scrubbers ( Venturis).
d. cyclones.
Because of the high temperatures and high moisture content
of the exhaust stream, . _ have not been used to
control emissions from municipal incinerators.
a. baghouses (fabric filters)
b. electrostatic precipitators
c. wet scrubbers (Venturis)
d. cyclones
In the majority of plants subject to current air pollution
regulation, the _ , _ has proven to be the most
effective control device.
a. baghouse (fabric filter)
b. electrostatic precipitator
c. wet scrubber (venturi)
d. cyclone
The prime factors which determine panicle resistivity
(hence collection efficiency) in an ESP are
a. propenies of the refuse.
b. operating temperature.
c. humidity of the exhaust stream.
d. all the above
e. propenies of the collection plates.
Carbon panicles are very easy/ difficult to collect in an ESP
because they have a very low resistivity.
True or False? Within certain limits, increasing both the
humidity and the temperature of the exhaust stream will
increase particle collection in an ESP.
5. d. all the above
6. False
7. Hydrogen chloride
(HC1)
8. b. electrostatic
precipitators and
c. wet scrubbers
(venturis).
9. a. baghouses (fabric
filters)
10. b. electrostatic
precipitator
11. d. all the above
12. difficult
13. False— decrease
temperature and
increase humidity.
14-15
-------
14. The Federal New Source Performance Standards (NSPS)
for municipal incinerators define an affected facility as any
municipal incinerator with a design capacity of
or greater.
a. 5 Mg per day (6 tons per day)
b. 25 Mg per day (28 tons per day)
c. 45 Mg per day (50 tons per day)
d. 150 Mg per day (165 tons per day)
15.
is the only pollutant regulated by the NSPS
for municipal incinerators.
a. Particulate matter
b. Carbon monoxide
c. SO,
d. NO,
14. c. 45 Mg per day (50
tons per day)
15. a. Particulate matter
References
Corey, R. C. 1977. Incineration. In Air Pollution Vol. VI Engineering Control of Air Pollution,
A. C. Stern, ed. New York: Academic Press.
Environmental Protection Agency (EPA). June 1973. Field Surveillance and Enforcement Guide:
Combustion and Incinerator Sources. APTD-1449.
Environmental Protection Agency (EPA). January 1975. Inspection Manual for Enforcement
of New Source Performance Standards: Municipal Incinerators. EPA 340/1-75-003.
Environmental Protection Agency (EPA). January 1977. Municipal Incinerator Enforcement
Manual. EPA 340/1-76-013.
Environmental Protection Agency (EPA). March 1979. A Review of Standards of Performance
for New Stationary Sources—Incinerators. EPA 450/3-79-009.
Environmental Protection Agency (EPA). December 1977. Compilation of Air Pollution Emission
Factors. Supplement No. 8. AP-42.
Jahnke, J. A., Cheney, J. L., Rollins, R., and Fortune, C. R. 1977. A Research Study of Gaseous
Emissions from Municipal Incinerators. Air Pollution Control Association. 27:747-753.
14-16
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Lesson 15
Kraft Pulp Mills
Lesson Goal and Objectives
Goal
To familiarize you with the pulp production and chemical-recovery processes used in kraft pulp mills,
the air emissions generated, and the methods used to reduce pollutant emissions.
Objectives
At the end of this lesson, you should be able to:
1. describe the kraft pulp production process and the chemical-recovery methods used in kraft
pulp mills.
2. list at least three pollutants emitted as a result of producing kraft pulp.
3. identify major emission points in the kraft pulp and chemical-recovery process.
4. identify the control equipment or process changes used to reduce emissions at kraft pulp mills.
Introduction
Kraft pulp mills produce the dark-colored wood pulp used in the manufacture of a variety of paper
products. Pulp is a viscous slurry of fibrous materials. Liquid is removed from the pulp by passing
the pulp over screens and processing it to leave a dried mat of cellulose fibers, or paper. Paper made
from kraft pulp is quite strong. The word "kraft" is actually the German word for strong. This paper
is used for grocery bags, multiwall sacks, and corrugated cartons. Bleached pulp is used to make
printing paper.
Pulp can be made in a number of ways. One of the simplest is to mechanically grind wood in the
presence of water to make the slurry. This method, however, produces a weak paper that rapidly
deteriorates. Approximately 10% of pulp produced in the United States is made by this method.
Better quality papers can be made from wood pulps produced by chemical methods or a combina-
tion of chemical and mechanical methods. Two principal components of wood are cellulose fibers
and a material called lignin. Lignin is a complex chemical compound which binds the cellulose fibers
together. If the hold of the lignin can be weakened, or the lignin eliminated, the cellulose fibers can
become free to make strong, durable paper.
A number of processes use mild chemical action to soften wood chips by loosening the hold of the
lignin. After softening, the chips are broken apart mechanically to produce the pulp. These processes
are termed "semichemical" and "chemimechanical" methods and account for approximately another
10% of the U.S. pulp production.
Full chemical processes actually dissolve the lignin binding the cellulose. The kraft pulping process
is a full chemical method which releases the cellulose fibers by dissolving much of the lignin. The
good quality papers that result from this method and the efficiency of the chemical recovery methods
used in the process have led to its dominance in pulp production. Nearly 80% of the pulps produced
in the United States are made by the kraft method.
15-1
-------
Kraft Method
The kraft pulping method uses a solution of sodium hydroxide and sodium sulfide to dissolve lignin.
This is a fairly straightforward operation. Basically, wood chips are mixed with the chemical solution
and cooked in a large pressure cooker (digester) until the cellulose fibers are loosened. Because of the
high cost and large amount of chemicals used, the kraft method has been designed to recover and
regenerate as many materials as possible. The kraft method can be more easily understood if it is
separated into two processes as shown in Figure 15-1, pulping and chemical recovery.
The sodium sulfide (Na2S) which is used in conjunction with sodium hydroxide (NaOH) to dissolve
the lignin leads to a variety of chemical compounds which are quite odorous. These are produced in
the pulping process and can carry through in the various recovery processes. The recovery methods
can also emit significant amounts of particulate matter. Therefore, methods for controlling both
gaseous and particulate matter emissions are needed in this industry so that air pollution standards
can be met.
Wood chips
Pulping process
Chemical
recovery process
White liquor
(NaOH + Na,S)
Recovered
chemicals
Figure 15-1. Simplified kraft pulping and recovery process.
Pulping Process
A number of different steps are involved in the production of wood pulp. The principal devices used
in these steps and shown in Figure 15-2 are
1. the digester,
2. the blow tank, and
3. the brown stock washer.
Initially, pulpwood — which can be obtained from either hardwood or softwood trees —is debarked
and chipped. The chips are fed into the digester along with a chemical solution of sodium hydroxide
and sodium sulfide in water. This solution is referred to as white liquor.
The digesters are tall, cylindrical vessels that cook the mixture of wood and chemicals at
temperatures near 180°C (350°F) and at pressures of 760 kPa (110 Ibs per square inch). The cooking
takes from three to four hours and can be done either continuously or as a batch process.
15-2
-------
At the end of the cooking period, the contents of the digester are discharged into a tank. In the
blow tank of a batch system, the pressurized mixture is reduced, suddenly, to atmospheric pressure.
This change in pressure helps to break up the woody materials and produce the pulp.
Wood chips
Digester
White liquor
Brown stock
washer
o
) \ Weak black liquor
(to chemical recovery process)
Paper
Figure 15-2. Pulping process.
The wood pulp produced is then screened to remove knots and other undissolved materials,
diluted, and sent to the brown stock washer (Figure 15-3). Here, the pulp is washed with clean water
and separated from the digester chemicals. The cleaned pulp then enters the paper making process
where it may be treated and bleached. A continuous mat of cellulose fibers is eventually formed on
large screens and then dried and collected as large rolls of paper (Figure 15-4).
15-3
-------
Screen
Pulp from blow tank
Pulp
Cleaned pulp
Weak black liquor
Figure 15-3. Brown stock washer.
Pulp entry from
brown stock washer
Wire screen
Dryers
Finished roll
Figure 15-4. Paper making.
The solution of chemicals filtered and washed from the pulp contains complex compounds of
dissolved and reacted lignin, organic and inorganic compounds that contain sulfur, and unreacted
sodium hydroxide and sodium sulfide. This solution is known as weak black liquor because of its
15-4
-------
color. A principal feature of the kraft pulping process is that most of these materials can be recycled
back into the process. Efficient techniques have been developed to recover or regenerate as many
chemicals as possible.
Chemical Recovery Process
A larger number of steps are involved in the recovery of chemicals than in the actual pulping process
itself. In general, these steps are:
1. evaporation of the weak black liquor,
2. burning of the black liquor concentrate,
3. recovery of sodium sulfide, and
4. recovery of sodium hydroxide.
Kraft pulp mills incorporate each of the four steps listed above. However, the type of equipment used
and details of the many recovery and regeneration cycles will vary from plant to plant. Figure 15-5
shows how these steps lead to the recovery of the white liquor solution which is recycled back into the
digester with the wood chips. The principal process systems used to perform this recovery are also
shown. Each will be discussed in turn with the appropriate recovery step.
Multiple effect
evaporators
White liquor
Causticizing
tank
(sodium
hydroxide)
Direct contact evaporator
(black liquor concentrate)
Flue gas
I Smelt dissolving
tank
(sodium sulfide)
Figure 15-5. Kraft chemical recovery process.
15-5
-------
Step 1. Evaporating the Weak Black Liquor
The liquid material collected after filtering and washing the pulp contains reacted or "spent"
chemicals. This weak black liquor carries about 15% solid material and a high percentage of dis-
solved and reacted lignins. Since there is such a high percentage of organic material in the liquor,
burning it can provide large amounts of heat for plant operations. Before this can be done, however,
much of the water must be removed. This is partially achieved in the multiple effect evaporator
system (Figure 15-6), usually consisting of 3 to 8 steam heated condensers. The weak black liquor is
reduced in water content as it is passed from one condenser to the other. This produces an
increasingly viscous material called black liquor having a solids content of 45 to 60%.
Weak black
liquor
Black liquor
Figure 15-6. Step 1—Multiple effect evaporator system used
to produce black liquor.
15-6
-------
Because the liquor from the multiple effect evaporator system still has too low a solids content, two
concentration methods can be used. In one, a direct contact evaporator is used. Here, hot flue gas
emitted from the heat recovery furnace is passed directly through the lower solids liquor to produce
the combustible material (Figure 15-7). The idea here is to evaporate the 40% solids black liquor to
the point where it will have a solids content near 70%. At this level the concentration of the organic
materials will be so high that the liquor will burn. The other method uses one or two additional
multiple effect evaporators to concentrate the black liquor solids content, discussed later in this
lesson.
Figure 15-7. Step 1—Direct contact evaporator used to produce
black liquor concentrate.
15-7
-------
Step 2. Burning the Black Liquor Concentrate
The black liquor concentrate produced in the first recovery step is sprayed into the combustion
chamber of the boiler system that is called the black liquor recovery furnace (Figure 15-8). Here, con-
centrate bums to provide heat and steam for evaporators, digester, driers, and other plant processes.
Since many sodium and sulfur compounds are present in the black liquor along with the combus-
tible lignins, a number of other chemical reactions take place. A molten liquid of inorganic salts,
called smelt, is produced as a result of burning the black viscous liquor. The smelt contains both
sodium sulfide and sodium carbonate.
The black liquor recovery furnace is the central process system of the whole kraft recovery method.
The heat, steam, and chemicals resulting from the combustion of the black liquor cycle back into
practically every step of the plant operation.
Black liquor
concentrate
Green liquor
to causticizing tank I
Smelt dissolving tank
Figure 15-8. Steps 2 and 3—Combustion of black liquor concentrate and
recovery of sodium sulfide.
15-8
-------
Step 3. Recovering the Sodium Sulfide
The molten salts, or smelt, from the recovery furnace are dissolved in water in the smelt dissolving
tank to produce green liquor. When the molten salts hit the water in this process, large amounts of
steam and paniculate matter can be emitted.
The green liquor is principally a solution of sodium sulfide and sodium carbonate (Na»COs). We
now have essentially recovered the sodium sulfide (a component of the white liquor) that will be
recycled back into the digester. The remaining step is to regenerate sodium hydroxide from the
sodium carbonate in the green liquor.
Step 4. Recovering the Sodium Hydroxide
The sodium hydroxide is recovered by treating the green liquor with lime in a causticizing tank.
Lime (calcium oxide, CaO) reacts with the sodium carbonate in the green liquor and produces the
sodium hydroxide (the other component of white liquor) and also a sludge of calcium carbonate.
After this reaction occurs, the resulting solution is clarified and white liquor is obtained.
The white liquor solution of sodium sulfide and sodium hydroxide is then sent back to the digester
and the whole process can begin again.
Since the kraft method attempts to recover as many materials as possible, the sludge from the
causticizing tank is not discarded today as it was in the past. The calcium carbonate sludge is heated
in a lime kiln to regenerate the lime (calcium oxide) used in the causticizing tank. Temperatures vary
from 150 to 1300°C (-300 to 2400°F) along the length of the kiln. Figure 15-9 represents this
pan of the process. The kiln rotates and can be from 30 to 120 m (100 to 400 ft) in length.
jl Green liquor (Na,S + Na,CO,)
/ \ from smelt dissolving tank
White liquor
(Na.S + NaOH)
to digester
Causticizing tank
(Na.COj-NaOH)
CaCO, sludge
Figure 15-9. Step 4—Causticizer and lime regeneration system.
15-9
-------
Other Processes
Other recovery processes are used in kraft pulp mills in addition to those discussed here. For example,
turpentine can be condensed from the digester relief gases when pine wood chips are used. Soaps
and lignin can be recovered from the weak black liquor for eventual use in a variety of industries.
In order to power the plant operations, boilers that burn gas, oil, coal, bark and wood waste are
used. These can supply either steam or electricity to the plant. The black liquor recovery furnace
contributes to the power requirements of the plant.
Summary
Figure 15-10 combines the many separate processes that have been discussed here. For review, note
that the kraft method can be divided into the pulping process and the chemical recovery process. In
the pulping process, wood chips are digested with white liquor—a sodium hydroxide and sodium
sulfide solution. In the chemical recovery process, the spent chemicals from the digester are
recovered. The black liquor recovery furnace burns the evaporated liquor from the digester to pro-
duce both heat and the first component of the white liquor, sodium sulfide. The sodium sulfide is
recovered in the smelt dissolving tank, and the second component, sodium hydroxide, is regenerated
in the causticizing tank. As a consequence, white liquor is regenerated and sent back to the digester
to react with more wood chips.
15-10
-------
Wood chips «'««>*
"ir
White
liquor
Digester
Brown
stock
washer
Multiple
effect
evaporators
Paper
Lime for
causticizer
Salt
cake
makeup
(Na,SO,)
Direct contact
evaporator
/ or additional \
/ multiple effect \
J evaporators directly I
\ into recovery furnace/
\ (see Figure 15-12) /
Figure 15-10. Overall kraft proceu.
15-11
-------
Review Exercise
Wood pulp is
a. a suspension of wood chips in water.
b. the material burned in the recovery furnace.
c. a suspension of wood fibers in water.
d. the final product of the kraft chemical recovery process.
2. The main purpose of the kraft pulping process is to
a. make fine stationery.
b. recover chemicals.
c. separate lignin from wood fibers.
d. make lime.
1. c. a suspension of wood
fibers in water.
3. What two compounds are used in the digester to react
with and dissolve lignin?
a. sodium carbonate and sodium sulfide
b. sodium hydroxide and sodium sulfide
c. sodium carbonate and lime
d. sulfur dioxide and hydrogen sulfide
2. c. separate lignin
from wood fibers.
4. Place the following kraft process equipment in proper
sequence:
blow lank -
recovery furnace
digester
smelt dissolving tank
multiple effect evaporator
causticizing tank
brown stock washer
direct contact evaporator
3. b. sodium hydroxide
and sodium sulfide
5. The purpose of the multiple effect evaporator and
direct contact evaporator is to
a. produce paper.
b. produce black liquor concentrate.
c. produce calcium carbonate.
d. reduce sulfide emissions.
4. digester
blow tank
brown stock washer
multiple effect
evaporator
direct contact
evaporator
recovery furnace
smelt dissolving tank
causticizing tank
Black liquor burned in the recovery furnace produces
a. heat and smelt.
b. lime and pulp.
c. heat and sweat.
d. pulp only.
5. b. produce black
liquor concentrate.
6. a. heat and smelt.
15-12
-------
7.
8.
9.
10.
What is obtained from smelt?
a. white liquor
b. brown liquor
c. black liquor
d. green liquor
When in the kraft chemical recovery process is white
liquor regenerated?
a. while in the lime kiln
b. after the green liquor is causticized
c. while in the direct contact evaporator
d. before it is introduced into the multiple effect evaporator
The lime kiln
a. produces the lime used in the causticizer.
b. produces wood pulp.
c. produces sodium sulfide.
What are the two principal processes occurring at a
kraft pulp mill?
7. d. green liquor
8. b. after the green
liquor is causticized
9. a. produces the lime
used in the causticizer.
10. 1. the pulping process
2. the chemical
recovery process
Air Pollution Emissions
The introduction of sodium hydroxide and sodium sulfide to dissolve lignin in the kraft pulping
process leads to the production of many odorous substances. Among these are hydrogen sulfide, mer-
captans (simple organic compounds of carbon, hydrogen, and sulfur), dimethyl sulfide, dimethyl
disulfide, and sulfur dioxide. These compounds of sulfur are often measured and described in com-
bination as total reduced sulfur, or TRS. The odor of many of these compounds can be detected
even at the pan per billion concentration level.
Emissions of sulfur compounds occur in both the pulping and recovery processes. In addition to
these gaseous emissions, paniculate matter can be released from the black liquor recovery furnace,
smelt dissolving tank, lime kiln, and plant power boilers.
The amount and types of pollutants emitted from a kraft pulp mill depend very heavily on how
the plant is designed and how it is actually operated. Small changes in temperature, pressure, or
chemical composition of separate process steps can have a significant effect on emission levels. In
many cases, modifications to the basic process outlined in Figure 15-10 can reduce actual emissions.
For example, continuous rather than batch digestion can give better control over the gases emitted.
Air or pure oxygen bubbled into the black liquor before it reaches the direct contact evaporator can
reduce sulfide emissions. Also, cycling the exhaust from one part of the process into another part can
eliminate pollutant gases. For example, organic materials contained in exhaust gases can be
incinerated in the lime kiln.
15-13
-------
A kraft pulp mill has many possible emission points which are summarized here. Table 15-1 lists
the major process systems and types of emissions from each.
Table 15.1. Typical uncontrolled emissions from kraft pulping
and recovery processes.
Process
Digester and blow tank
Brown stock washer
Multiple effect
evaporator
Recovery furnace and
direct contact evaporator
system
Smelt dissolving tank
Lime kiln
Gas and level
Hydrogen sulfide )
Mercaptans J Hl8h
Hydrogen sulfide )
Mercaptans ! Low
Hydrogen sulfide »
Mercaptans J Hl«h
Hydrogen sulfide 1
Sulfur dioxide 1
Hydrogen sulfide 1 ,
Mercaptans I
Hydrogen sulfide 1 .
Mercaptans I
Paniculate matter
-
—
Sodium sulfate
Sodium carbonate
Smelt
Sodium salts
Calcium carbonate
Calcium oxide
and level
( High
I Low
( High
The emission levels of gases can vary from a few parts per million to tens of thousands of parts per
million. References given at the end of this lesson should be consulted for specific data.
Air Pollution Control Methods
Emissions can be reduced at kraft pulp mills by two primary means:
1. modifying plant design and operation, and
2. adding air pollution control equipment.
The most common add-on control devices used in this industry are electrostatic precipitators, wet
scrubbers, and incineration systems. In this section, examples of design modifications and applica-
tions of add-on equipment will be given.
Modifying Plant Design and Operation
Example 1
Figure 15-8 showed how the hot exhaust resulting from the combustion of organic matter in black
liquor can be used to "concentrate" the organic materials before injection into the direct contact
evaporator. Direct contact evaporators are merely venturi scrubbers or cascade evaporators. In both
systems, the black liquor acts as the scrubbing liquid to remove particles from the recovery furnace
exhaust. A 50% removal efficiency can be attained in this manner for paniculate matter.
However, a problem arises in using this technique because the carbon dioxide and sulfur dioxide in
the flue gas of the recovery furnace can react with chemicals in the black liquor to form hydrogen
sulfide. This would result in high sulfur emissions and a loss of sulfur that could otherwise be used in
the process. To avoid this, oxygen can be injected into either the weak or the strong black liquor to
form compounds that will not convert to hydrogen sulfide. Oxygen can be introduced by using black
liquor oxidation systems. These systems are used to treat the black liquor before it goes to the direct
contact evaporator (Figure 15-11).
15-14
-------
Black liquor
Figure 15-11. Black liquor oxidation system.
Example 2
In many new plants, in order to avoid the problem of hydrogen sulfide formation, black liquor is
concentrated without using a direct contact evaporator. One or two multiple effect evaporators are
used to concentrate the black liquor, thereby eliminating the need for the direct contact evaporator.
In some systems there may be as many as six or seven multiple effect evaporators in series.
These kraft pulp plants do not require the use of oxidation systems. The flue gas from the recovery
furnace is usually not scrubbed; the paniculate matter is usually removed by using electrostatic
precipitators.
15-15
-------
Multiple
effect
evaporators
To an electrostatic
precipitator
Figure 15-12. Concentrating black liquor using additional
multiple effect evaporators.
Example 3
An example of the odorous gas incineration technique is the practice in some plants of collecting
the mercaptan and sulfide vapors emitted from the digester, blow tank, and multiple effect
evaporators and routing them to the lime kiln, power boilers, or a separate incinerator where they
are incinerated.
Another practice is to use the exhaust from the brown stock washers as combustion air for the
black liquor recovery furnace.
Adding Air Pollution Control Equipment
Equipment designed specifically for the control of air pollutants is also used at kraft pulp mills.
Typical control devices are listed below in Table 15-2.
Table 15-2. Add-on control methods.
System
Digester
Multiple effect evaporator
Brown stock washer
Recovery furnace
Smelt tanks
Lime kiln
Control devices
Incinerators, wet scrubbers
Incinerators
Incinerators
Electrostatic precipitators, wet scrubbers,
black liquor oxidation systems
Wet scrubbers (venturi, impingement)
Demisters (packed tower)
Wet scrubbers (venturi, impingement),
electrostatic precipitators
15-16
-------
Electrostatic precipitators are used on kraft recovery systems having direct contact evaporators as
well as those using indirect contact evaporators. All of the particulate matter is not removed from the
recovery furnace exhaust in the direct contact scrubber, so it is often necessary to install precipitators.
The chemical composition of the liquid used in kraft mill wet scrubbers is often chosen to help in
the removal of sulfide gases. Although sized primarily to remove particles emitted from lime kilns or
smelt tanks, the absorption that occurs in the scrubbers is important in reducing the emission of
sulfur-containing compounds.
New Source Performance Standards
Kraft pulp mills are regulated by States and by the Federal EPA. The Federal standards apply to new
or modified kraft pulp mills constructed after September 24, 1976. The emission standards apply to
measured total reduced sulfur (TRS) and particulate matter released from specific units of the kraft
process. For example, the NSPS regulation limits paniculate matter emissions to a level of 0.10
g/dscm (0.044 gr/dscf) from the recovery furnace and 0.1 g/kg of black liquor solids (0.2 Ib/ton
solids) for emissions from the smelt dissolving tank. Lime kiln paniculate emissions are also
regulated.
TRS, as measured by the EPA Reference Test Method number 16, is limited to emission levels of
from 5 to 25 ppm for the various process units. Table 15-3 summarizes these NSPS emission
standards. Depending on plant design, both State and Federal standards might apply to other process
units not listed in this table.
Table 15-3. NSPS emission levels.
Process unit
Recovery furnace
Smelt tanks
Lime kilns
Digester, brown stock washer,
evaporator or black
liquor oxidation systems
Recovery furnaces
Cross recovery furnaces
Smelt tanks
Lime kilns
Pollutant
Particulate*
Paniculate
Paniculate
Total reduced sulfur (TRS)
TRS
TRS
TRS
TRS
Emission level
0.10 g/dscm (0.044 gr/dscf),
corrected to 8% oxygen
0.1 g/kg black liquor solids (dry
weight) [0.2 Ib/ton black liquor
solids (dry weight)]
0.15 g/dscm (0.067 gr/dscf).
corrected to 10% oxygen when
gaseous fossil fuel is burned
0.30 g/dscm (0.13 gr/dscf).
corrected to 10% oxygen when
liquid fossil fuel is burned
5 ppm by volume on a dry
basis, corrected to a
specific oxygen content
5 ppm by volume on a dry basis
corrected to 8% oxygen
25 ppm by volume on a dry basis
corrected to 8% oxygen
0.0084 g/kg black liquor solids
(dry weight) [0.0168 Ib/ton black
liquor solids (dry weight)]
8 ppm by volume on a dry basis,
corrected to 10% oxygen
'Opacity must not exceed 35%.
15-17
-------
Review Exercise
1.
2.
3.
4.
5.
6.
7.
8.
Pulp mill emissions depend on
a. operating variables.
b. plant design.
c. effectiveness of control equipment.
d. all the above
What does TRS stand for?
a. Total Recovery System
b. Typical Recovery Solvent
c. Total Reduced Sulfur
d. Tough Research Students
Paniculate emissions come predominantly from what
three processes in a kraft pulp mill?
What are the principal pollutant gases emitted from kraft
pulp mills?
What two types of pollution control techniques are common
in the kraft pulping industry?
Choose two.
a. plant design modification
b. hiring private vendors for pollutant removal
c. use of bubbles and emission trade-offs
d. use of add-on control equipment
Black liquor oxidation systems are used to
a. prevent the formation of paniculate matter in direct
contact evaporators.
b. prevent the formation of hydrogen sulfide in the
recovery furnace.
c. convert black liquor to smelt.
d. concentrate black liquor.
The direct contact evaporator can do which of the following?
a. dewater pulp
b. concentrate black liquor
c. control paniculate matter in the recovery furnace exhaust
d. all the above
Match up the control device(s) with the process.
a. recovery furnace w. incinerator
b. lime kiln x. baghouse
c. multiple effect evaporator y. electrostatic precipitator
z. wet scrubber
1. all the above
2. c. Total Reduced Sulfur
3. recovery furnace
smelt dissolving tank
lime kiln
4. hydrogen sulfide
mercaptans
sulfur dioxide
dimethyl sulfides
dimethyl disulfides
5. a. plant design
modification
d. use of add-on control
equipment
6. b. prevent the formation
of hydrogen sulfide in
the recovery
furnace.
7. b. concentrate black
liquor
c. control paniculate
matter in the recovery
furnace exhaust
8. a. y, z
b. z, y
c. w
15-18
-------
References
Environmental Protection Agency (EPA). September 1976. Standards Support and Environmental
Impact Statement, Volume 7. Proposed Standards for Performance for Kraft Pulp Mills. EPA
450/2-76-014a.
Environmental Protection Agency (EPA). April 1979. Guidance for Lowest Achievable Emission
Rates from 18 Major Stationary Sources. EPA 450/3-79-024.
Environmental Protection Agency (EPA). October 1976. Environmental Pollution Control Pulp and
Paper Industry. Part I—Air. EPA 625/7-76-001.
Hendrickson, E. R., Roberson, J. E., Koogler, J. B. 1970. Control of Atmospheric Emissions in the
Wood Pulping Industry—Final Report. Volumes 1,2, and 3. Department of Health, Education
and Welfare. National Air Pollution Control Administration. EPA-APTD-1234, 1235, and 1236.
U.S. Public Health Service, 1966. Proceedings from the International Conference on Atmospheric
Emissions from Sulfate Pulping. E. O. Painter Printing Co., DeLand, Florida.
15-19
-------
Lesson 16
Nonferrous Smelters
Lesson Goal and Objectives
Goal
The purpose of this lesson is to introduce you to the metallurgical processes used to obtain zinc, lead,
and copper from sulfide ores, the types of pollutants emitted in these processes, and the control
techniques applied to reduce emissions from nonferrous smelters.
Objectives
At the end of this lesson, you should be able to:
1. list the four basic metallurgical operations used to extract nonferrous metals from sulfide ores.
2. identify the purpose of sintering and roasting.
3. identify each pan of the process that emits sulfur dioxide in extracting lead, zinc, and copper
from ore.
4. describe two techniques used to control air pollutant emissions from nonferrous smelters.
5. recognize two types of emission regulations that apply to nonferrous smelters.
Introduction
Modern industry makes wide use of the metals lead, zinc, and copper. Lead batteries, copper wire,
and galvanized metal are all familiar and essential products used in the industrial world. The ores in
which these metals are found differ from the ferrous, or iron, ores that we discussed in Lesson 10.
Most of the iron produced today is extracted from ores containing the oxides of iron. In contrast,
lead, zinc, and copper are commonly found as compounds of sulfur.
Copper, zinc, lead, and nickel all occur in sulfide ore deposits. However, only copper, zinc, and
lead are mined in the United States. Large deposits of nickel and copper occurring in an ore body in
Ontario, Canada have led to the development of one of the world's largest smelters there. The
smelting processes for these nonferrous ores produce large amounts of sulfur dioxide. The sulfur diox-
ide can be converted into sulfuric acid in contact acid plants (see Lesson 13) located at the smelting
facilities. However, economically removing all of the sulfur dioxide from the smelter exhaust is dif-
ficult. The subsequent emissions are of concern because of their impact on the ambient air and the
worsening environmental problems of acid deposition.
This lesson will review the basic operations involved in producing nonferrous metal, and then it
will examine specific methods used to extract each metal —lead, zinc, and copper. Emission control
methods and emission standards will also be discussed in this lesson.
16-1
-------
Basic Operations of Nonferrous Metal Production
Four basic operations used to produce pure metals from sulfide ores are:
• concentration,
• preparation,
• smelting, and
• refining.
Zinc, lead, and copper metal are all obtained in this manner. However, the actual methods and
devices used to prepare and to smelt an ore differ depending on the specific metal to be obtained.
Therefore, this section will review the overall operations, methods, and equipment used to produce
pure metals from sulfide ores. The next section will discuss the methods used specifically to extract
either zinc, lead, or copper from their sulfide ores.
Concentration
Ores being mined in the U.S. today generally contain only small amounts of metallic elements. To
increase efficiency in transporting and smelting, the ores first need to be concentrated by separating
them from earthy and other unwanted materials. Zinc, lead, and copper are all concentrated by
froth flotation. Ores obtained from either open-pit or subsurface mines are crushed and ground into
a fine powder. This finely pulverized material is agitated in a mixture of oil, water, and selected
chemical reagents in a froth flotation tank (Figure 16-1). The chemical reagents selectively attach to
the powder that contains the metallic ore. Because of the reagents, the metallic ore powder selectively
attaches to air bubbles and rises to the tank surface. Powders containing only silicates or other earthy
materials will sink to the bottom of the tank. Air is introduced into the tank to create a froth that
keeps the water repellant materials floating on the surface. This froth is collected, then dried to pro-
duce ore concentrate.
Froth
Ore powder and
chemical reagents
Sludge
Figure 16-1. Froth flotation tank.
16-2
-------
Preparation Methods
Ore concentrates are often treated, or prepared, before they are smelted. Preparation methods can
change the physical form of the concentrate, the chemical form, or both. Two common preparation
methods are sintering and roasting.
Sintering changes the physical form of a material. The dried concentrate obtained by froth flota-
tion concentration is often so fine that it can't be efficiently treated in smelting devices such as a
blast furnace. Sintering fuses the fine concentrate into strong, porous products (like charcoal bri-
quettes) that are then crushed and sent to the smelting operations. Figure 16-2 shows a sintering
machine. In this device, ore concentrate, coke, water, and other materials are fed onto a traveling
grate. The combustion of the coke produces heat to fuse the concentrate.
Dried ore concentrate
Flue gas
Sinter
Figure 16-2. Sintering process.
16-3
-------
The roasting preparation method is specifically used to convert metallic sulfides into their oxides or
into sulfates. In this chemical process, sulfur dioxide is liberated. Metal impurities, such as arsenic
and cadmium, can also be removed by roasting. The product of a roasting operation is often called
calcine. Figure 16-3 shows two roasting devices: a multiple-hearth furnace and a fluidized-bed
roaster. Ore concentrate is fed into the roasters.
In the hearth furnace, rotating arms move the ore concentrate over grates, or hearths. Fuel
introduced into the furnace burns to heat the ore concentrate. Heat is also liberated as the sulfides
convert to oxides, and in some applications the process can become virtually self-sustaining. The ore
concentrate roasts as it falls from hearth to hearth, being pushed by the rotating arms. Air enters
from the bottom and is heated by contact with the roasting material.
In the fluidized-bed roaster, finely ground ore is kept in suspension by the roasting air. Material
may be entrained in the exhaust gas, so cyclones and other paniculate collection devices are used to
remove the product.
-
- Ore concentrate
Ore concentrate
Air Q
Roasted ore
concentrate
Roasted ore
concentrate
Figure 16-3a. Multiple-hearth furnace.
Figure 16-Sb. Fluidized-bed roaster.
16-4
-------
Smelting
Smelting takes a solid material such as ore or prepared sinter, heats it, and produces a molten
material. When smelting sulfide ores, different products can result. One is liquid metal and the other
is matte —a. liquid solution of metallic sulfides. The smelting process begins by mixing materials such
as carbon, limestone, and silica with the ore. This mixture is then fed into a furnace. The burning
carbon provides heat and produces carbon monoxide which can reduce the metallic sulfides to metal.
The limestone or silica (fluxing agents) fuse with the impurities in the ore, forming a molten slag that
can then be separated from liquid metal or matte.
Many different types of furnaces are used in smelting. In Lesson 10 we have already discussed the
blast furnace and electric furnace used to make steel. Blast furnaces are also used to produce copper
and lead. A lead blast furnace is shown in Figure 16-4. Feed materials enter from the top, with air
being supplied from the bottom. Molten metal is removed from the bottom. The slag formed by the
reactions between the fluxing agents and the impurities is removed from the top of the molten lead.
Feed materials
Water wall
Air supply
Figure 16-4. Smelting—lead blait furnace.
16-5
-------
Another furnace used in smelting is the reverberatory furnace (Figure 16-5). Used to produce
matte, this furnace is particularly important in the copper-smelting process. A fuel is chosen that will
give a long, luminous flame to extend over the material on the hearth. Reverberatory furnaces used
in copper smelting can be up to 120 feet long. The fuel and products of combustion do not react
with the material charged in the furnace. This furnace's primary function is to melt the charge and
produce the solution of liquid metal sulfides. Slag forms on top of the matte and is skimmed off.
Matte is drained off from the bottom at the end of the furnace. The matte is transferred to other
operations to be processed into liquid metal.
Charge . :y:;.^
Figure 16-5. Smelting—reverberatory furnace used to produce matte.
Refining Techniques
The last basic operation performed in extracting metals from sulfide ores is refining. The purpose of
refining is to improve the purity of the metal by removing any undesirable materials or elements.
Depending on the level of purity desired, the refining process may consist of just one or a number of
steps.
16-6
-------
Since most of the sulfur is removed by the time ore is refined, sulfur dioxide emissions at this point
are minimal. There are a variety of refining techniques. Among the most important are:
• fire refining,
• distillation refining, and
• electrolytic refining.
Fire Refining
Fire refining removes impurities by forming a slag. By blowing oxygen through the molten metal and
providing the proper fluxing agents, impurities in the form of slag will float to the surface. Slag is
easily removed by skimming into slag pots.
Distillation Refining
Metals can also be separated from each other using distillation refining techniques similar to those
used to separate petroleum products. The metal is melted, evaporated, and then condensed in frac-
tional distillation columns.
Electrolytic Refining
Electrolytic refining produces very pure metals. For example, in the electrolytic refining of copper,
impure copper plates are submerged in a special solution in an electrolytic cell. A carefully controlled
electric current is passed between the impure copper plate and a thin plate made of high purity
copper. The electric current causes the copper from the impure plate to electrolyze in the solution.
The electric current causes an equivalent amount of copper to deposit from the solution onto the
pure plate..Conditions are chosen so that impurities don't plate out onto the pure copper plate. Once
the deposits on the plate reach a certain weight, the plate is removed and sold as pure metal.
Impurities such as gold and silver are valuable in their own right and are collected in the sludge at
the bottom of the cell and subsequently recovered. Figure 16-6 illustrates an electrolytic cell used in
this type of refining operation.
Pure copper
plate
Impure copper
plate
Figure 16-6. Electrolytic cell.
16-7
-------
Summary
The metallurgical operations discussed in this section are all used in the extraction of lead, zinc, and
copper metals from their ores. Each type of ore and each metal has its own chemical characteristics.
Because of this, the actual metallurgical methods used will vary, depending on the metal being
extracted. For example, lead and zinc sulfides are converted to their oxides before smelting. Copper,
however, is smelted from a mixture of copper and iron sulfides.
The next section will discuss how concentration, sintering, roasting, smelting, and refining are
applied in practice to obtain nonferrous metals.
Review Exercise
1. Metals such as lead, zinc, and copper
a. are only found in the earth's crust as pure metals.
b. can be found in nature as oxides and sulfides.
c. are not found in the earth's crust.
2. List the four basic metallurgical operations used to produce
pure metals from sulfide ores.
1. b. can be found in
nature as oxides and
sulfides.
3. In the froth flotation process, air is used to
a. oxidize sulfides to oxides.
b. produce slag to remove impurities.
c. create bubbles to float mineral concentrates.
d. sinter fine concentrates.
2. • concentration
• preparation
• smelting
• refining
4. Sintering can be used to
a. agglomerate fine materials.
b. convert sulfides to oxides.
c. convert oxides to molten metal.
d. refine metals.
e. both a and b
3. c. create bubbles to
float mineral
concentrates.
5. A fluidized-bed roaster is used to
a. convert sinter into matte.
b. convert oxide ores into sulfide concentrates.
c. concentrate sulfide ores.
d. convert sulfide concentrates into oxides.
4. e. both a and b
6. What does a lead blast furnace produce?
a. sinter
b. matte
c. molten metal and slag
d. calcine
e. both matte and molten metal
5. d. convert sulfide
concentrates into oxides.
6. c. molten metal and
slag
16-8
-------
What does a reverberatory furnace used in the smelting
operation produce?
a. calcine
b. sinter
c. concentrate
d. matte and slag
8. In electrolytic refining,
a. metals are evaporated and condensed in tall columns.
b. slag is skimmed off liquid metal.
c. electricity is used to deposit pure metals.
7. d. matte and slag
8. c. electricity is used to
deposit pure metals.
Extracting Metals from Sulfide Ore
The extraction of lead, zinc, and copper from their ores requires concentration, preparation,
smelting, and refining. All three of these ores are concentrated by froth flotation; however, the
preparation and smelting method will vary depending on the specific metal. Electrolytic refining is
commonly-used-to purify (refine) these metals, although other techniques are used depending upon
the purity desired for the final product.
Sintering, roasting, smelting, and electrolytic reduction are used in various ways to extract the
metals lead, zinc and copper from their respective ores. Table 16-1 lists, in order, the methods com-
monly used in each extraction process.
Table 16-1. Preparation and smelting methods for lead, zinc, and copper.
Lead
Sintering
Smelting
Zinc
Roasting
Sintering Electrolytic
Smelting or reduction
Copper
Roasting
Matte smelting
Copper smelting
The application of these methods to the extraction of these specific metals will be discussed further in
this section.
16-9
-------
Lead
The principal lead ore smelted in the United States is lead sulfide, or galena. It can also occur
together with the ores of zinc, iron, or copper. Lead sulfide must first be concentrated before
undergoing the pyrometallurgical (heat using) operations such as sintering and smelting. Crushing,
grinding, and concentrating lead ore are generally conducted at the mine. Once concentrated, the 3
ores are sent to the smelting facility.
Concentration
Lead sulfide is concentrated by froth flotation. Compounds called xanthates attach to the lead sulfide
ore, causing it to float to the top. This selective flotation process produces a lead concentrate con-
taining from 55 to 70% lead from an ore containing only 3 to 10% lead.
Preparation and Smelting
Figure 16-7 illustrates a typical lead extraction process. Dried lead sulfide concentrate is fed into a
sinter machine where, in this case, both sintering and roasting occur. Air passing through the con-
centrate converts the lead sulfide into lead oxide. Heat for this roasting process comes from burning
the sulfur contained in the ore which is then emitted as sulfur dioxide. Other materials are added to
the feed of the sinter machine to create a hard, porous sinter suitable for smelting in the blast
furnace.
Sintering eliminates approximately 85% of the sulfur contained in the concentrate. The sinter
product of lead oxide and remaining sulfides is crushed, mixed with coke and slag-forming materials,
and sent to the blast furnace. The feed enters the top of the blast furnace while oxygen is blown
through pipes located at the bottom of the furnace. The coke burns to form carbon monoxide. Car-
bon monoxide reduces the lead oxide to form molten elemental lead. Remaining lead sulfides and
sulfates in the sinter are also reduced in the furnace. This generates sulfur dioxide exhaust concentra-
tions of between 500 and 2500 ppm. The elemental lead is separated from the slag and poured into
large kettles.
16-10
-------
, Ore bins
Lead blast
furnace
Crossing kettles
Figure 16-7. Extraction of lead from sulfide ore.
16-11
-------
Refining
The methods used to refine impure lead depend on the impurities contained within the original ore.
If a significant amount of copper impurities are present in the ore, they will also be present in the
molten lead coming from the smelter. The copper can be made to float on top of the molten lead.
This floating material, known as dross, is skimmed off and sent to a reverberatory furnace to produce
matte for copper recovery.
The majority of refined lead is produced by dressing and other thermal techniques. Approximately
20% of the world's lead is electrolytically refined.
Zinc
The most prevalent zinc ore is zinc sulfide, which is known as zinc blende or sphalerite.
Concentration
Zinc minerals are first separated from lead minerals by adding special reagents in the flotation tank.
The zinc sulfides settle to the bottom as tailings. The lead sulfides float on the froth and are col-
lected as discussed in the preceding section. In the second step, reagents are chosen so that the zinc
sulfides float on the froth and the earthy materials and other minerals drop to the bottom.
A typical dried zinc concentrate will contain approximately 60% zinc, 30% sulfur, and 5 to 10%
iron by weight.
Preparation and Smelting
Zinc sulfide must first be converted to zinc oxide before it can be reduced to metal. Figure 16-8
shows the recovery sequence. Zinc sulfide is prepared for smelting by roasting and sintering. Different
roasting devices can be used to convert the sulfide to the oxide. In the fluidized-bed roaster, shown in
Figure 16-3, air suspends the concentrate as the sulfur burns to form sulfur dioxide. The calcine pro-
duced will contain about 2.5% remaining sulfur and the exhaust gas will have a sulfur dioxide con-
centration of nearly 10%. An appreciable carryover of calcine from the roaster is entrained in the
exhaust stream. This finely divided material is then collected in cyclones and electrostatic
precipitators and either recycled back to the roaster or added to the product coming from it.
Next, the zinc oxide product coming from the roaster can be reduced to metallic zinc by two prin-
cipal methods: pyroreduction, also called thermal reduction; or hydrometallurgy, also called elec-
trolytic reduction. If a pyroreduction method is used, the zinc oxide calcine must first be sintered so
that it won't be blown away during smelting. (In addition to agglomerating the calcine, cadmium
and lead impurities can be volatilized and removed in the sintering process.) An example of thermal
reduction using a vertical retort furnace to smelt zinc oxide is illustrated in Figure 16-8. Coke and the
sinter material are fed in at the top of the retort furnace. Carbon monoxide produced by the burning
coke reacts with the zinc oxide to generate zinc vapor and carbon dioxide. Since the temperature
needed for reduction is 1200 to 1300°C (2192 to 2372°F), and since the boiling point of zinc metal is
907 °C (1665°F), zinc is released as vapor. This vapor is condensed and collected at the top of the
retort furnace.
Zinc can also be smelted using other thermal reduction furnaces such as a horizontal retort fur-
nace, an electrothermic furnace, or a blast furnace. The purity of the smelted zinc will vary,
depending on the furnace and operating procedure used.
16-12
-------
Calcine bins
Sinter machine
Vertical
retort
furnace
Residue
Figure 16-8. Extraction of zinc from sulfide ore.
16-13
-------
Most zinc plants in the U.S. now use electrolytic reduction to extract zinc metal from the oxide.
Here the calcine from the roaster is not sintered, but is treated with sulfuric acid. The sulfuric acid
leaches out the zinc from the zinc oxide produced in the roaster to form zinc sulfate. Impurities such
as iron and manganese are removed, and the resulting zinc sulfate solution is pumped to electrolytic
cells. By passing an electric current through the cell, zinc ions are reduced to zinc metal on
aluminum cathodes. The zinc is periodically stripped from the cathodes, melted, and cast into ingots.
High purity zinc metal can be obtained using electrolytic reduction. Any further refining is not
necessary.
Refining
Zinc metal produced by thermal reduction must be upgraded to an acceptable purity. This can be
accomplished by fractional distillation, where the zinc is condensed at lower temperatures than are
impurities such as lead, iron, and cadmium.
Copper
Copper exists in nature as "native" metal, as oxide, and as sulfide. Most of the world's copper comes
from the mineral chalcopyrite—a sulfide of iron and copper. Most of the sulfide ores mined are low
grade—some containing less than 1% copper. Low grade ores are concentrated using the froth flota-
tion method. Concentrates containing from 20 to 40% copper are commonly obtained.
Figure 16-9 illustrates the remaining extraction processes in the production of copper metal. The
one preparation operation and the two smelting operations include: roasting to remove excess sulfur
(preparation), producing a matte in a reverberatory furnace (smelting), and converting the matte to
copper metal (smelting).
Preparation (Roasting)
The ore concentrate, skimmed as froth from the flotation tanks, is sent to the roasters where it is
heated to about 800°C (1472°F). Moisture and oxides of antimony, arsenic and bismuth are driven
off. The main purpose of roasting, however, is to remove excess sulfur—as sulfur dioxide —from the
concentrate. Very specific sulfur concentrations are required for the reactions that occur in pro-
ducing matte. In cases where the concentrate is low in excess sulfur, roasting is omitted.
Roasting in copper extraction is performed in multiple-hearth roasters or fluidized-bed roasters
(shown in Figure 16-3). The fluidized-bed roasters will generate higher sulfur dioxide outlet concen-
trations (12 to 18% vs. 4 to 5% generated in the multiple-hearth roasters). Cyclones follow the
fluidized-bed roasters to remove calcine carried with the gas stream. Collected calcine is then sent to
the reverberatory furnace.
16-14
-------
Flotation
tank
Matte
Air
Copper
converter
Fire
refining
Electrolytic
refining
Copper I;
plate
Figure 16-9. Extraction of copper from sulfide ore.
Smelting
1. Production of matte
The reverberatory furnace does not produce copper metal, but a molten mixture of copper and
iron sulfides—matte. In this smelting process, the roaster calcine and/or untreated concentrates and
silica are heated in the furnace at temperatures from 1400 to 1500°C (2552 to 2732°F). The
reverberatory furnace can smelt finely divided material which has not been sintered. The hot flame
of the reverberatory furnace—fueled by oil, gas, or pulverized coal—passes over the charged material
and produces two liquid layers. The lower layer is the copper-iron matte and the upper one is a
silicate slag containing iron, aluminum, calcium, and other elements. The actual purposes of the
reverberatory furnace are to remove impurities by forming the slag and to produce a matte with the
appropriate amounts of copper, iron, and sulfur necessary for the conversion step. The slag is,
therefore, drawn off and the matte is tapped and sent in a molten state to the converters.
16-15
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2. Conversion to Copper Metal
Converters are essentially large cylindrical furnaces. The converter is charged by pouring in matte
from the reverberatory furnace and adding more silica (Figure 16-10). Three steps convert the matte
into molten copper metal. First, air blown in from the sides of the converter changes the iron sulfides
into iron oxides. The iron oxides combine with the silica to form a slag. In the second step, the air is
stopped from blowing and the slag is skimmed off. In the third step, air is again blown into the con-
verter, converting the copper sulfide into molten copper metal. These three steps are illustrated in
Figure 16-11. Heat is released in these reactions to produce temperatures of between 1200 and
1350°C in the converter. The molten copper produced by this process is sent to the fire refining fur-
nace and then cast into blocks or plates which are then sent to the refinery.
Hood
Silica
Figure 16-10. Charging the copper converter.
16-16
-------
Stepl.
Converting iron
sulfides to
iron oxides
forming slag.
Step 2.
Skimming slag.
StepS.
Converting copper
sulfide to
copper metal.
Figure 16-11. Action of the copper converter.
Refining
Copper is refined primarily by electrolytic refining, but is also purified by fire refining. In fire refin-
ing, air is blown through the molten metal to convert remaining metal impurities to oxides to form a
slag. Oil, natural gas, or poles of green wood are injected into the copper to reduce copper oxides
into metal. If the original ore contains little silver or gold, the fire-refined copper is cast and used for
industrial purposes. If enough precious metals are present, the cast metal is further purified by elec-
trolytic refining.
16-17
-------
Review Exercise
1. When extracting lead from galena, which of the following is
true?
a. The ore is concentrated, sintered and roasted, and then
smelted.
b. The ore is concentrated and then smelted in the blast
furnace.
c. The ore is smelted in a reverberatory furnace and reduced
to molten metal in a converter.
The feed to a lead blast furnace consists of
a. lead oxide.
b. lead sulfide and coke.
c. lead oxide, lead sulfide, and coke.
d. lead oxide, lead sulfide, coke, and slag-forming
compounds.
1. a. The ore is concen-
trated, sintered and
roasted, and then
smelted.
3. Zinc and lead sulfides can be separated by
a. sintering.
b. selective flotation.
c. roasting.
d. dressing.
2. d. lead oxide, lead
sulfide, coke, and slag-
forming compounds.
4. Zinc oxide coming from a roaster must be sintered if" it is to be
produced by
a. pyroreduction (thermal reduction).
b. hydrometallurgy (electrolytic reduction).
3. b. selective flotation.
5. True or False? A vertical retort furnace can be used for
smelting zinc.
4. a. pyroreduction
(thermal reduction).
6. Copper ore concentrates are roasted to
a. convert the copper sulfide to copper oxides.
b. adjust the sulfur content of the material sent to the
reverberatory furnace.
c. remove antimony, arsenic, and bismuth impurities.
d. both b and c
5. True
7. In copper smelting, the matte produced in the reverberatory
furnace is composed of
a. a mixture of copper and iron oxides.
b. pure copper metal.
c. a mixture of copper and iron sulfides.
d. a molten mixture of copper and iron.
6. d. both b and c
8. In which device, used in the extraction of copper from ore
concentrate, is molten copper obtained?
a. reverberatory furnace
b. copper converter
c. roaster
d. froth flotation tank
7. a. a mixture of copper
and iron sulfides.
8. b. copper converter
16-18
-------
For each metal extracted, list in sequential order the
appropriate preparation and smelting operations (sintering,
roasting, and smelting).
Lead
Zinc
Copper
10. What is the most common refining method used to obtain
copper of high purity?
a. dressing
b. fire refining
c. electrolytic refining
d. fractional distillation
9.
Sintering
Smelling
Zinc
Sintering Electrolytic
Smelting reduction
Copper
(touting
Mute imelting
Copper smelting
10. c. electrolytic refining
Pollution Emissions from Extraction Operations
Paniculate matter and sulfur dioxide are the major air pollutants emitted from nonferrous smelters.
The extraction of metal from sulfide ores produces large amounts of sulfur dioxide. This occurs
primarily in operations where ore concentrates are sintered, roasted, or smelted.
Lead Extraction Emissions
In extracting lead, the major emission points for paniculate matter are the sintering machine and the
blast furnace. Since losing material in the flue gas can mean losing raw material or the product, par-
ticulate matter is generally collected and recycled into the plant operation. The largest source of
sulfur dioxide emissions is the sintering-roasting process. Most of the lead sulfide concentrate should
be convened to lead oxide prior to entering the blast furnace. As a result, the exhaust gas from the
sintering step will have a sulfur concentration of up to 7%, depending on the type of sintering
machine. In contrast, SO* emissions from the blast furnace average about 1.5% or less.
Zinc Extraction Emissions
In roasting zinc concentrates, significant amounts of roasted material are entrained with the flue gas,
particularly when fluidized beds are used. This material is collected in cyclones and electrostatic
precipitators and returned to the process. Sintering of the calcine coming from the zinc roaster pro-
duces both fugitive and exhaust emissions of paniculate matter. Small amounts of zinc oxide (par-
ticulate matter) can be emitted from the horizontal and vertical retort furnaces (smelting).
Sulfur dioxide emissions occur principally in roasting, as was mentioned earlier. Since all of the
sulfide concentrate may not be oxidized during roasting, sulfur dioxide emissions can also occur in
the sintering operation.
16-19
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Copper Extraction Emissions
Emissions from copper extraction operations occur primarily from roasting and smelting. Sulfur diox-
ide concentrations from the roaster exhaust average about 12% for fluidized-bed roasters. Large
amounts of sulfur dioxide are emitted from the convenor, where air is blown through a molten matte
of iron and copper sulfides. During the blowing cycles, SO2 exhaust concentrations can range from
12 to 18% at the mouth of the converter; however, due to dilution, this becomes approximately 4 to
6% in the exhaust stream. The reverberatory furnace produces relatively low exhaust concentrations
of SO2 —varying from 0.5 to 3.5%.
Air Pollution Control Methods
Particulate Emission Control
Paniculate emissions are controlled by a number of methods at nonferrous smelters. Since in many of
the process steps the paniculate matter is an actual product or feed material, considerable effort is
expended in recovery for economic reasons. For example, cyclones and electrostatic precipitators are
used to recover and recycle calcine produced by fluidized-bed roasters.
Settling chambers, cyclones, electrostatic precipitators, and baghouses are all used to control par-
ticulate emissions from smelting operations. Often the hot exhaust gases coming from the pyroreduc-
tion processes must be cooled before entering control devices such as baghouses. Wet scrubbers are
also used in some applications. However, recovery of wet collected materials is somewhat more dif-
ficult, and the collected material must be dried before it can be recycled into the smelter.
Sulfur Dioxide Control
The high levels of sulfur dioxide in smelter waste gas streams make it economical to recover the gas
to produce either sulfuric acid or elemental sulfur. Contact sulfuric acid plants are commonly used to
treat the exhaust gas. However, the gas must be generally free of materials that might foul or poison
the catalysts used in the contact acid plant. The concentration of the sulfur dioxide should be near
4.0% in order for a double-contact acid plant to operate at its most efficient level (see Lesson 13).
One of the major technical problems here is in dealing with gas streams which are too dilute in sulfur
dioxide to make recovery affordable. In some cases, the smelting process can be modified to increase
SO2 levels, such as by changing from multiple-hearth furnaces to fluidized-bed roasters or by using
lesser amounts of dilution air. In cases where processes cannot be modified, a number of SO2 concen-
tration methods can increase weak streams to higher levels. In these systems, the SO2 is absorbed by
chemical solutions and then regenerated.
The major emission points, pollutants, and control methods occurring in the nonferrous smelting
industry are summarized in Table 16-1.
Major process changes —such as the use of hydrometallurgical (electrolytic reduction) methods
instead of the pyroreduction techniques — can also reduce emissions. When plant capacity is
expanded, the advantages of leaching ores with chemicals and then electrolyzing the metal-ion solu-
tions are often considered. Currently available hydrometallurgical methods are more costly than are
pyroreduction techniques, because energy requirements and corrosion of equipment can result in
excessive operating and maintenance costs.
16-20
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Table 16-1. Common air pollution control method*
in the nonferroui smelting indtutry.
Metal
Lead
Zinc
Copper
Operation
Sinter plant
Roaster
Sinter plant
Roaster
Reverberatory furnace
Convener
Pollutants
Paniculate matter
SO,
Paniculate matter
SO,
Paniculate matter
Paniculate matter
SO,
Control method
Mechanical collectors
ESPs
Baghouses
Sulfuric acid plants
Cyclones
ESPs
Sulfuric acid plants
Baghouses
ESPs
Mechanical collectors
ESPs
Sulfuric acid plants
New Source Performance Standards
Nonferrous smelters are regulated by States and by the U.S. EPA. The federal standards apply to
new or modified facilities constructed after October 16, 1974. The NSPS for smelters are summarized
in Table 16-2.
Table 16-2. New Source Performance Standards.
Type of
smelter
Lead
Zinc
Copper
Process unit
Sintering machine
Blast furnace
Sintering machine
Roaster
Concentrate dryer
Roaster
Smelting furnace
(reverberatory
furnace)
Converter
Pollutant
and opacity
Paniculate matter
Opacity
SO,
Paniculate matter
Opacity
Paniculate matter
Opacity
Opacity
SO,
Paniculate matter
Opacity
SO,
Emission standards
50 mg/dscm (0.022 gr/dscf)
20%
0.065%
50 mg/dscm (0.022 gr/dscf)
20%
50 mg/dscm (0.022 gr/dscf)
20%
20%
0.065%
50 mg/dscm (0.022 gr/dscf)
20%
0.065%
Existing smelters can qualify for special orders if ambient air quality standards can be maintained.
National rules for primary nonferrous smelter orders appeared in the Federal Register June 24, 1980
(45 FR 42514). These allow the use of supplemental control systems. By monitoring ambient air con-
centrations and using dispersion modeling techniques, production levels may be defined so that the
emission levels can be reduced to meet ambient standards.
16-21
-------
Review Exercise
At a lead smelter, sulfur dioxide is emitted primarily from the
a. blast furnace.
b. sintering machine.
c. dressing operations.
d. froth flotation tanks.
2. The primary emission point for sulfur dioxide at a zinc smelter
is the
a. sintering machine.
b. retort furnace.
c. roaster.
d. electrolytic process.
1. b. sintering machine.
3. Which of the following devices at a copper smelter will
emit sulfur dioxide?
a. froth flotation tank
b. roaster
c. reverberatory furnace
d. converter furnace
e. fire refining furnace
f. electrolytic refining tanks
2. c. roaster.
4. What is the principal device used to control sulfur dioxide
emissions at hbnferrous smelters?
a. limestone flue gas desulfurization systems
b. contact sulfuric acid plant
c. electrostatic precipitator
d. converter
3. b. roaster
c. reverberatory furnace
d. converter furnace
5. Paniculate matter emitted from smelting is
a. often collected to recover the product.
b. never collected, but dispersed from tall stacks.
c. used in the production of sulfuric acid.
4. b. contact sulfuric acid
plant
New Source Performance Standards for nonferrous smelters
address which of the following?
a. sulfur dioxide
b. plume opacity
c. nitrogen oxide
d. particulate matter
e. total reduced sulfur
5. a. often collected to
recover the product.
6. a. sulfur dioxide
b. plume opacity
d. particulate matter
16-22
-------
References
Burckle, J. O. and Worrell, A. C. Unpublished paper: Comparison of Environmental Aspects of
Selected Nonferrous Metals Protection Technologies.
Environmental Protection Agency. 1980. Industrial Process Profiles for Environmental Use:
Chapter 27, Primary Lead Industry. EPA 600/2-80-168.
Environmental Protection Agency. 1980. Industrial Process Profiles for Environmental Use:
Chapter 28, Primary Zinc Industry. EPA 600/2-80-169.
Environmental Protection Agency. 1980. Industrial Process Profiles for Environmental Use:
Chapter 29, Primary Copper Industry. EPA 600/2-80-170.
Environmental Protection Agency (EPA). 1974. Background Information for New Source Perfor-
mance Standards: Primary Copper, Zinc, and Lead Smelters. EPA 450/2-74-002a.
Environmental Protection Agency (EPA). 1973. Field Surveillance and Enforcement Guide for
Primary Metallurgical Industries. EPA 450/3-73-002.
Environmental Protection Agency (EPA). 1972. Field Operations and Enforcement Manual for
Air Pollution Control. Volume III: Inspection Procedures for Specific Industries. APTD-1102.
Newton, J. N. 1959. Extractive Metallurgy. New York: John Wiley and Sons, Inc.
Cotterill, C. H. and Ligan, J. M., editors. 1970. AIME World Symposium on Mining and
Metallurgy of Lead and Zinc. Volumes I and II. American Inst. of Mining, Metallurgical
and Petroleum Engineers, Inc. Baltimore, Port City Press, Inc.
Semrau, K. T. 1971. Control of Sulfur Oxide Emissions from Primary Copper, Lead and Zinc
Smelters—A Critical Review,/. Air Poll. Control Assoc. 21:185-194.
McMahon, A. D., Cotterill, C. H., Dunham, J. T., and Rice, W. L. 1974. The U.S. Zinc
Industry: A Historical Perspective. U. S. Dept. of the Interior—Bureau of Mines. IC8629.
Bureau of Mines. U.S. Department of the Interior. 1971. Control of Sulfur Oxide Emissions
in Copper, Lead, and Zinc Smelting. 1C 8527.
McKee, A. G. and Co. 1969. Systems Study for Control of Emissions—Primary Non-ferrous
Smelting Industry. Volumes I, II, and III. Nat. Air Poll. Control Admin. Contract No.
PHS 86-65-85.
16-23
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Lesson 17
Asphalt Concrete Plants
Lesson Goal and Objectives
Goal
To familiarize you with processes at asphalt (concrete) plants, the pollutant emissions produced by
them, and the devices used to reduce these emissions.
Objectives
At the end of this lesson, you should be able to:
1. describe the process steps involved in the production of asphalt.
2. recall the pollutant emissions resulting from the production of asphalt.
3. list the major pollutant emission points (if any) in each process step involved in asphalt
production.
4. recall the control device used to reduce pollutant emissions at a given emission point.
Introduction
Asphalt concrete is a paving material used for surfacing roads, airport runways, parking lots, and
driveways. Asphalt concrete (a mixture) is produced by mixing aggregate that is dried and heated
with hot asphaltic cement. Aggregate is any combination of the various crushed rock, gravel, sand,
or other solid materials used to make concrete. Asphalt cement is a tarlike substance that is refined
from crude oil. It is a solid at ambient temperatures and must be heated to between 135 and 163°C
(275 and 325 °F) before it is sprayed onto the hot aggregate.
There are three major types of asphalt concrete plants: batch, continuous mix, and dryer-drum
mix. The most common, the batch mix plant, is shown in Figure 17-1. The three processes differ in
the manner by which the hot liquid asphalt is mixed with the aggregate. In a batch process, the
mixing takes place in batches and each batch requires about one minute. In the continuous mix
process, aggregate and liquid asphalt are mixed on a continuous basis. In both of these processes,
heating of the aggregate and mixing (with asphaltic cement) occur in separate vessels. However, in
the dryer-drum mix process, the aggregate and liquid asphalt are dried and mixed in the same vessel.
Of the plants presently operating, batch plants are the most common (91%), followed by continuous
mix (6.5%) and dryer-drum mix (2.5%) (EPA, 1979). However, the majority of new plants now
being built are dryer-drum mix plants.
17-1
-------
Screening, weighing,
. \ and mixing operations
Figure 17-1. Typical asphalt batch mix plant.
Producing Asphalt
Batch Plant
The batch mix process begins with the loading of the aggregate from stockpiles into bins (usually
four) referred to as cold bins (Figure 17-2). From these cold bins, vibrating feeders control the
amount of aggregate falling onto a conveyor. The conveyor leads to the inlet of the dryer.
The function of the dryer is to remove any surface moisture and to heat the aggregate to a
temperature of 120 to 180°C (250 to 350°F). Asphalt cement can be coated onto the aggregate at
these temperatures. As shown in Figure 17-3, the dryer is an inclined cylinder or drum equipped with
an oil or gas burner. The cylinder rotates as the aggregate travels through it. To ensure maximum
drying and heating, the cylinder is equipped with longitudinal troughs, called flights, which lift and
tumble the aggregate, increasing its exposure to the hot gases. Dryer capacity is a function of two
17-2
-------
properties:^ aggregate-surface moisture and the percent of small particles, or fines, in the aggregate.
Increasing either of these requires either increasing the fuel usage or decreasing aggregate feed rates
to ensure proper drying.
Cold bins containing
the aggregate
Feeders
Figure 17-2. Cold him.
Dryer inlet
Exhaust from dryer
Flights
Figure 17-3. Rotary dryer.
17-3
Burner
Dried aggregate outlet
-------
From the discharge end of the dryer, the heated aggregate is transported by an elevator to a set of
vibrating screens located over bins referred to as hot bins. As shown in Figure 17-4, these screens sort
the aggregate according to size and drop them into the appropriate hot bin. Any oversized aggregate
is discharged. From these hot bins, the appropriate amount of each size aggregate is dropped into the
weigh hopper. The weigh hopper temporarily holds the hot aggregate until the pugmill (mixer) is
ready for the next batch. When the aggregate drops into the pugmill, it is mixed dry for a few
seconds before a fixed percentage of heated asphalt cement is sprayed into the pugmill. Mixing then
lasts for an additional thirty to forty seconds to ensure a homogeneous mixture of asphalt. The
finished batch of asphalt is then dumped into waiting trucks.
Bucket elevator
Liquid asphalt
Vibrating screens
Hot bins
Weigh hopper
Pug mill mixers
Figure 17-4. Aggregate sizing and asphalt mixing.
17-4
-------
Continuous Mix Plant
The basic operation of a continuous mix plant (Figure 17-5) is very similar to that of a batch plant,
except for the manner in which aggregate is fed to the pugmill. As the name implies, asphalt cement
and hot aggregate are continuously mixed in the pugmill. Hot aggregate from the dryer is
transported to sorting screens that size the aggregate and drop it into the appropriate hot bin. From
these hot bins, a continuous flow of aggregate is metered to the pugmill. In the pugmill, hot asphalt
is continuously sprayed and mixed with the aggregate. The feeder systems for the aggregate and hot
asphalt are mechanically interconnected to ensure proper proportions in the mix. The mixture is
then conveyed by mixing paddles to the outlet of the pugmill and continuously discharged into a
holding hopper. From the holding hopper, the asphalt is dumped into trucks.
Liquid asphalt "*** Paddle!l
Vibrating
screens
Holding hopper
Figure 17-5. Continuous mix asphalt plant.
Dryer-Drum Mix Plant
The dryer-drum mix process differs from batch and continuous mix processes (referred to as conven-
tional plants). Here, aggregate is dried, heated and mixed with the hot asphalt in the same vessel.
Figure 17-6 depicts a typical dryer-drum mix plant.
Aggregate is fed into the dryer at the burner flame. Further in the dryer, sprays coat the aggregate
with hot asphalt. The finished mix is discharged at the end of the dryer onto a conveyor where it is
transferred to a heated storage silo for loading into trucks.
The dryer-drum mix process is an improvement over the conventional processes since it eliminates
the need for hot aggregate screens, hot bins and a pugmill mixer. Eliminating these steps reduces the
capital cost of a new plant. It is estimated that in the next few years, dryer-drum mix plants will
represent up to 99% of all plants under construction (EPA, 1979).
17-5
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Asphalt storage tank
work
Asphalt discharge
Figure 17-6. Dryer-drum mix plant.
Cutback and Emulsion Asphalt
Another way to make asphalt concrete is to use a cutback or emulsion asphalt. Since asphalt cement
is solid at ambient temperatures, it must be heated or blended with a solvent to produce a liquid that
will coat aggregate. Cutback asphalt is made by blending asphalt cement with gasoline, naptha,
kerosene, or fuel oil. The proportions of asphalt cement and solvent are approximately 70% cement
to 30% organic solvent depending on the type of cutback desired. Emulsion asphalt is similar to cut-
back, except that asphalt cement is blended with water and emulsifying agents such as soaps, tall
oils, or rosin. Both cutback and emulsion asphalts are used for small paving projects such as
secondary roads, parking lots, and driveways. They are also used to patch potholes, for tack coats,
and to stabilize soil. Cutback and emulsion asphalts are blended at the asphalt-concrete plant and
then mixed with aggregate, either in a pugmill or at the site, in a traveling grate or blade mixer.
Cutback and emulsion asphalts are sprayed as liquids (without aggregate) when used for tack coats or
soil stabilizers.
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Review Exercise
Asphalt concrete is produced by mixing .
dried and heated with hot
that is
2. Asphalt cement is a
a. solid
b. liquid
c. gas
d. mixture of all three states
at ambient temperatures.
1. aggregate,
asphalt cement
3. List the three types of asphalt concrete plants.
2. a. solid
4. The function of the rotary dryer in a batch plant is to
remove and heat the aggregate to between 120
and 180°C.
3. batch,
continuous mix,
dryer-drum mix
5. True or False? The capacity of a batch asphalt plant depends
upon the moisture content and percent of fines in the feed
aggregate.
4. surface moisture
6. In a bateh plant, the aggregate and hot asphaltic cement are
mixed together in the
a. rotary dryer.
b. hot elevators.
c. pugmill.
d. truck.
5. True
7. In a
asphalt concrete plant, aggregate and hot
asphalt cement are continuously fed to the pugmill.
6. c. pugmill.
8. In a dryer-drum mix plant, the asphalt and aggregate are
both mixed in the
a. rotary dryer.
b. hot elevators.
c. pugmill.
d. truck.
7. continuous mix
9. Dryer-drum mix plants eliminate the need for the
a. hot bins.
b. hot screens.
c. pugmill.
d. all the above
8. a. rotary dryer.
9. d. all the above
17-7
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Air Pollution Emissions
The major pollutant emitted from an asphalt concrete plant is paniculate matter. Paniculate matter
is emitted from handling and drying the aggregate. The primary source of paniculate emissions is the
dryer; however, fugitive particulate emissions are also generated from screening, conveying, aggregate
handling, storage piles, and truck traffic (Figure 17-7). Emission factors for asphalt plants can be
found in Compilation of Air Pollutant Emission Factors, AP-42.
Clean exhaust
Baghouse
Dust pickup
from conveying
and screening
Dust pickup
from conveying
and handling
Conveying
emissions
Figure 17-7. Potential emission points.
17-8
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During the drying process, fine particles are entrained in the hot combustion stream as these gases
move through the dryer. The paniculate emission rate from the dryer depends on:
* the percent of fines added in the aggregate mixture,
• gas velocity in the dryer,
• rate of rotation of the dryer, and
• feed rate of aggregate to the dryer.
The rate of uncontrolled paniculate emissions will generally increase with an increase in any of the
above variables.
The points and magnitude of paniculate emissions from a dryer-drum mix plant are somewhat dif-
ferent from those of a conventional asphalt plant (even though the dryer is the major emission point
in both). Emissions from the hot screens, hot elevator, and hot bins are eliminated from the dryer-
drum mix plants. Test data show that uncontrolled paniculate emissions from the dryer in a dryer-
drum mix plant are significantly less than those from a conventional plant (EPA, March 1976; EPA
340/1-77-004). Dryer-drum mix plants do have a greater potential to emit volatile organic com-
pounds (VOCs) since the flame and asphalt may be in close contact (EPA, 1981).
VOC emissions occur when cutback asphalt is used. After the cutback asphalt has been paved or
sprayed onto a surface, the asphalt begins to dry, or cure. As the cutback asphalt cures, the solvent
evaporates, releasing volatile organic compounds into the atmosphere. VOC emissions can be
eliminated by switching from a cutback asphalt to an emulsion-based asphalt (that contains virtually
no organic solvents). For more information concerning cutback and emulsion asphalts, refer to APTI
Course 482, Sources and Control of Volatile Organic Air Pollutants.
Review Exercise
l.
is the major pollutant emitted from asphalt
concrete plants.
2. The
is/ are the primary source of emissions in
1. Paniculate matter
conventional asphalt plants.
a. trucks
b. screens
c. pugmill
d. dryer
3. Emission rates from the rotary dryer are proportional to
increase in
a. gas velocity.
b. feed rate.
c. rate of rotation of the dryer.
d. all the above
2. d. dryer
3. d. all the above
17-9
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4. Generally, the greater the percentage of fines in the aggregate,
the greater/smaller the uncontrolled emission rate.
5. Fugitive emissions from asphalt plants are generated from
a. stockpiles.
b. truck traffic.
c. conveying, screening, or handling the aggregate.
d. all the above
4. greater
6. The
is/are the major source of emissions for
a dryer-drum mix asphalt plant
a. trucks
b. screens
c. pugmill
d. dryer
5. d. all the above
7. Uncontrolled paniculate emissions are generally much
less from the dryer of a asphalt concrete plant
than from a asphalt concrete plant.
6. d. dryer
8. Uncontrolled VOC emissions are generally higher from the
dryer of a asphalt concrete plant than from a
- asphalt concrete plant.
7. dryer-drum mix,
conventional
8. dryer-drum mix,
conventional
Air Pollution Control Methods
Cyclones, wet scrubbers, or baghouses are all used at asphalt concrete plants to control paniculate
emissions. In a typical plant, emissions from the dryer and the scavenger (fugitive dust) system are
vented to the inlet of the primary collector (see Figure 17-7). The exhaust from the primary collector,
or precleaner, then goes to a secondary collector, which is generally a high-efficiency scrubber or
baghouse.
Primary Collectors
The functions of the primary dust collector are to remove the larger particles and to recycle the
material collected. The most common primary collection device is a large-diameter cyclone. Large-
diameter cyclones are effective for removing particles larger than 20 fan in diameter. These devices
are therefore not capable of meeting most air pollution regulations. The actual efficiency of the
cyclone depends on the particle size distribution of the dust and on the pressure drop across the
collector. Large-diameter cyclones used as primary collectors generally have a pressure drop across
the device of 5 to 10 cm (2 to 4 in.) of water. Occasionally, high-efficiency multicyclones are used.
Multicyclones can achieve 90% removal efficiencies in the particle size range of 5 to 10
17-10
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with a pressure drop of 10 to 15 cm (4 to 6 in.) of water (EPA, 1972). Since as much as 89% of the
dust from the dryer can be smaller than 10 /on in diameter, an additional high efficiency collection
device is required.
Secondary Collectors
In order to meet the Federal New Source Performance Standard for Asphalt Concrete Plants (90
mg/dscm or 0.04 gr/dscf), a collector capable of achieving an efficiency of 99% or better is required.
The two most commonly used control devices are high-energy scrubbers (venturis) and baghouses.
Wet Collectors
Wet collectors, or scrubbers, are available in a wide variety of configurations, water spray pattern
designs, and pollutant gas flow designs (see Lesson 5). Because of their versatility and ability to
achieve high removal efficiencies of small micron-sized panicles, venturi scrubbers are the most often
used type of wet collector. The paniculate removal efficiency of a wet collector is a function of
several variables; however, hi general, the higher the pressure drop the more efficient the wet collec-
tor. Venturi scrubbers used at asphalt concrete plants normally operate with a pressure drop of about
25 to 51 cm (10 to 20 in.) of water.
Advantages of a venturi scrubber (compared to a baghouse) are their relatively low initial cost,
ability to partially remove hydrocarbons which may be present, and they do not require pre-
conditioning of the incoming gas stream. The disadvantages of using a venturi are high operating
costs associated with producing high pressure drops, a need for large quantities of water, and a water
disposal problem. Most asphalt plants are relatively small in size and do not have the area required
for a settling lagoon.
Baghouses
A fabric filter, or baghouse, (see Lesson 3) is generally regarded as the most efficient control device
available for controlling paniculate emissions at asphalt concrete plants (EPA, 1979). An important
variable in the design and operation of baghouses is the air-to-cloth ratio, which is the ratio of the
volume of gases treated to the square foot of cloth used in the baghouse (see Lesson 3). In asphalt
concrete plants, baghouses normally are designed to operate with an air-to-cloth ratio of 6:1 (6 acfm
per square foot of filter area); however, air-to-cloth ratios ranging from 9:1 to 4:1 have also been used
(EPA, 1979).
The advantages of using a baghouse (compared to a venturi scrubber) are higher collection effi-
ciencies, capability of recycling the collected material, no water needed, and lower power usage. The
disadvantage is that pretreatment of the dryer exhaust stream may be required to avoid operating
problems caused by excessive temperature or moisture. The temperature must be kept below the
limits specified for the fabric used and yet high enough so no moisture condenses on the fabric.
Process Changes to Reduce VOC Emissions
VOC emissions that result from using a cutback asphalt can be reduced by using an emulsion
asphalt. VOCs are not emitted during the curing process because the emulsion asphalt does not con-
tain any solvent. EPA has defined RACT guidelines for reducing VOCs from cutback asphalt opera-
tions in the Control Technique Guideline (CTG) entitled Control of Volatile Organic Compounds
From Use of Cutback Asphalt (EPA, 1977). Most States have adopted regulations in their SIPs for
nonattainment areas that are similar to those regulations suggested in the CTG. The CTG suggests
using emulsion instead of cutback asphalt.
17-11
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New Source Performance Standards
The Federal government has promulgated New Source Performance Standards regulating particulate
matter and opacity emissions from asphalt concrete plants. The regulations apply to any asphalt con-
crete plant under construction or modification on or after June 1, 1973. Each source at the asphalt
concrete plant (especially the dryer exhaust) cannot emit an exhaust stream that
• contains particulate matter in excess of 90 mg/dscm (0.04 gr/dscf).
• exhibits 20% opacity or greater.
Review Exercise
1. True or False? Large-diameter cyclones are more effective
control devices than are small-diameter multicyclones.
2. True or False? Since cyclones are not capable of removing
submicrometer particles, additional collection equipment is
required for asphalt plants to meet Federal emission standards.
1. False
3. The two most commonly used secondary control devices are
and
2. True
4. In general for wet collectors, the higher the
higher the particle collection efficiency.
., the
3. baghouses,
wet scrubbers (venturis)
5. Venturi scrubbers used on asphalt concrete plants generally
operate with a pressure drop of about inches of
water.
a. 1
b. 10 to 20
c. 100
d. 200
4. pressure drop
6. One advantage in using a venturi scrubber is that it
a. has a low initial cost.
b. can reduce VOC emissions from dryer-drum mix plants.
c. has low operating costs.
d. all the above
e. only a and b
5. b. 10 to 20
6. e. only a and b
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7.
8.
The air-to-cloth ratio in baghouses used to control
paniculate emission in asphalt plants is generally
a. 6:1.
b. 1:6.
c. 1:1.
d. 6:6.
or
baghouses used
._,_. ... can cause operating problems in
on asphalt concrete plants.
7. a. 6:1.
8. High temperature,
moisture
References
Environmental Protection Agency (EPA). April, 1981. Compilation of Air Pollutant Emission
Factors. Supplement No. 12. AP-42.
Environmental Protection Agency (EPA). April, 1981. Emission of Volatile Organic Compounds
from Drum-Mix Asphalt Plants. EPA 600/S2-81-026.
Environmental Protection Agency (EPA). August, 1972. Field Operation and Enforcement Manual
for Air Pollution Control Vol III: Inspection Procedures for Specific Industries. APTD-1102.
Environmental Protection Agency (EPA). March, 1976. Inspection Manual for Enforcement of New
Source Performance Standards: Asphalt Concrete Plants. EPA 340/1-76-003.
Environmental Protection Agency (EPA). March, 1976. Preliminary Evaluation of Air Pollution
Aspects of the Drum-Mix Process. EPA 340/1-77-004.
Environmental Protection Agency (EPA). June, 1979. A Review of Standard of Performance
for New Stationary Sources—Asphalt Concrete Plants. EPA 450/3-79-014.
Environmental Protection Agency (EPA). May, 1981. Sources and Control of Volatile Organic
Air Pollutants. APTI Course 482. EPA 450/2-81-010.
Environmental Protection Agency (EPA). December, 1977. Control of Volatile Organic Compounds
From Use of Cutback Asphalt. EPA 450/2-77-037.
17-13
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TECHNICAL REPORT DATA
(Please read Inunctions on the reverse before completing)
1. REPORT NO.
EPA 450/2-82-006
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
APTI Course SI:431
Air Pollution Control Systems for Selected Industries
Self-instructional Guidebook
5. REPORT DATE
June. 1983
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
David S. Beachler, James A. Jahnke,
Gerald T. Joseph and Marilyn Peterson
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Northrop Environmental Training
Northrop Services Inc.
P.O. Box 12313
Research Triangle Park, NC 27709
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
68-02-3573
12. SPONSORING AGENCY NAME AND ADDRESS
U.S. Environmental Protection Agency
Manpower and Technical Information Branch
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Self-instructional Guidebook
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
EPA Project Officer for this Self-instructional Guidebook is R. E. Townsend,
EPA-ERC, MD 20, Research Triangle Park, NC 27711
16. ABSTRACT
This Self-instructional Guidebook is a self-instructional course, APTI Course
SI:431 "Air Pollution Control Systems for Selected Industries." This course is
an introduction to the fundamental operating characteristics of particulate
and gaseous emission control systems. The course reviews the physical,
chemical, and engineering principles of control systems and how they are
applied to several industrial processes. Major topics include: principles of
particulate and gaseous emission control equipment including cyclones,
electrostatic precipitators, baghouses, scrubbers, adsorbers, ccmbustors,
condensers, and the application of control equipment to selected industries—
power plants, municipal incinerators, asphalt batch plants, cement plants,
acid plants, steel mills, petroleum refineries, kraft pulp mills, and smelters.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution Control
Particulate Emission Control
Gaseous Emission Control
Self-training Manual
Self-instructional
Guidebook for Air
Pollution Control Systems
13B
51
68A
3. DISTRIBUTION STATEMENT IT - ,
unlimited
National Audio Visual Center
National Archives and Records Service,
-- ' ("IT-HOT- gQT-irifO HH '
19. SECURITY CLASS /This Report)
unclassified
21. NO. OF PAGES
269
20. SECURITY CLASS (This page)
unclassified
22. PRICE
EPA Form 2220-1 (9-73) Washington, DC 20409
17-15
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