syEPA
United States
Environmental Protection
Agency
EPA-600/8-80-029
August 1980
Office of Research and Development
Research
Summary
Controlling
Sulfur Oxides
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Coal is formed from the highly compressed remains
of the abundant plant and animal life that existed
some 200 to 250 million years ago. Thus it is not
surprising that the five most abundant elements
in coal—carbon, oxygen, hydrogen, sulfur, and
nitrogen—are also five of the principal elements of
life. Two of these elements—sulfur and nitrogen-
present major combustion emission problems.
About 65 million metric tons of sulfur oxide
pollutants are annually emitted into the atmo-
sphere by the world's industrialized societies. The
central and northeastern United States, southern
Canada, and northern Europe account for three
quarters of this total, but represent only one per-
cent of the global surface area. The resulting
concentration of sulfur oxides cause adverse
human health effects, are a principal contribution
to acid precipitation, and lead to significant
reductions in visibility. Fortunately, over the past
decade, the industrialized nations have made
excellent progress in developing sulfur control
technologies for new power plants.
This Research Summary describes EPA's program
to develop new and improve existing technologies
for sulfur oxides control. As we increasingly turn
to coal as the primary utility and industrial fuel,
while trying to deal with the problems of acid pre-
cipitation, visibility degradation, and unhealthy air,
we will have to confront the fact that most of the
sulfur oxides which will be emitted over the next
two decades will come from plants existing today. If
we vigorously pursue the successful demonstra-
tion of control technologies and then take advan-
tage of them, especially those which can reduce
emissions from existing plants, the adverse health
and environmental effects of the troublesome
sulfur oxides can be significantly reduced.
Stephen J. Gage
Assistant Administrator
for Research and Development
Cover Photo: EPA Documerica
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sulfur oxides problem
sources of SO_
More than 25 million metric tons of sulfur oxides (SOX) are
emitted annually in the United States. This is about a
quarter of the yearly total of sulfur oxides released from
human activities and natural sources throughout the world.
Sulfur oxides account for approximately 14 percent of the total
estimated national air pollutant emissions. Released primarily
in the form of sulfur dioxide, they are converted by atmo-
spheric processes to sulfates which interfere with normal
breathing patterns, reduce visibility, and contribute to the
formation of acid rain.
major air pollutants
SUSPENDED
PARTICLES
' 6% -
National Air Quality, Monitoring, and Emissions Trends Report, 1977,
EPA, December 1978.
More than two thirds of all national sulfur oxide emissions
result from fuel combustion in electric generating stations.
An additional 8 percent of SOX emissions result from fuel
combustion in industrial boilers. Copper smelters are the
largest noncombustion source of SOX emissions, followed
by petroleum refineries. Additional SOX pollutants are
released by furnaces used to heat homes, businesses, and
public institutions. A small amount, less than 5 percent, is
derived from the exhaust of cars, trucks, aircraft, and other
vehicles.
The largest single source of SOX is coal combustion. Sulfur
is a natural contaminant of coal, and is almost completely
1
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increased use
of coal
converted to sulfur oxide when coal is burned. In the United
States during the 1950's and 1960's, many industries turned
from coal to cleaner fuels such as oil and gas to avoid
polluting the atmosphere. Now that supplies of oil and gas
are becoming scarce, industries are compelled to use coal
once again.
sulfur oxides emissions sources
INDUSTRIAL
BOILERS 8%
Jt,'lrV',' '
COPPER1 isl-, .
SMELTERS'8%,,H^ >' '-
' J-JPETRO'LEUM >
TRANSPORTATION
~ AND OTHERS 5%
RESIDENTIAL,
COMMERCIAL,
INSTITUTIONAL 5%
EPA Environmental Outlook, 1980.
Annual consumption of coal by the electric power utilities is
expected to increase from 405 million metric tons per year in
1975 to between 600 million and 1 billion metric tons in 2000.
As a result, the amount of SOX generated by these utilities
could increase from about 20 million to 41 million metric
tons per year during the same period. If SOX control
systems are used, however, future emissions can be kept at
approximately the current annual level.
About 95 percent of all sulfur oxides are in the form of
sulfur dioxide (S02), a colorless gas that when cooled and
liquified can be used as a bleach, disinfectant, refrigerant,
or preservative. In the atmosphere, however, S02 is a pre-
cursor of highly destructive sulfates (S04), which are
formed by the chemical addition of oxygen (02). S04 is not
a stable compound, however, and in the presence of water
(H20) it frequently forms sulfuric acid (H2S04), a component of
acid rain.
health effects
As the concentration of sulfur oxides in the air increases,
breathing becomes more difficult, resulting in a choking
effect known as pulmonary flow resistance. The degree of
breathing difficulty is directly related to the amount of sulfur
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visibility
acid rain
compounds in the air. The young, the elderly, and individ-
uals with chronic lung or heart disease, are most susceptible
to the adverse effects of sulfur oxides.
Sulfates and sulfur acids are more toxic than sulfur dioxide
gas. They interfere with normal functioning of the mucous
membrane within the respiratory passages, increasing vul-
nerability to infection. The toxicity of these compounds
varies according to the nature of the metals and other
chemicals that combine with sulfur oxides in the
atmosphere.
Small particles suspended in a humid atmosphere are the
major cause of reduced visibility in the eastern United
States. Over the past 25 years, visibility on an average
summer day has decreased from 15 to 8 miles. Sulfates
constitute 30 to 50 percent of the suspended particles.
Acid rain is composed primarily of two acids: sulfuric
(H2S04) and nitric (HN03). Sulfuric acid, resulting from
sulfur oxide emissions, comprises from 40 to 60 percent of
acid rain depending on regional emission patterns. Acid rain
is a major problem throughout the world, especially in
Scandanavia, Canada, and the eastern United States.
Rain in the northeastern United States averages 10 to 100
times the acidity of normal rainwater. More than 90 lakes in
the Adirondack Mountains of New York State no longer
contain fish because the increased acidity of lake water has
caused toxic metals in the lake beds and surrounding soils
to be released into the lakes. Similar effects are beginning
to occur in other areas of the United States such as northern
Minnesota and Wisconsin.
materials
deterioration
Preliminary studies indicate that the direct effects of acids
on foliage and the indirect effects resulting from the leaching
of minerals from the soil can reduce the yield from some
agricultural crops.
Sulfur acids corrode materials normally considered to be
durable, such as metals, limestone, marble, mortar, and
roofing slate. As a result, acidic sulfates are destroying
statuary and other archeological treasures that have resisted
deterioration for thousands of years. These include such
well-known structures as the Parthenon in Greece and the
Taj Mahal in India, as well as lesser known bronze and
stone statuary in U.S. cities.
Corrosive destruction of statuary is most severe in areas
where droplets of moisture collect. Sulfates and other atmo-
spheric particles form a crust on the statuary that retains
moisture and promotes the formation of sulfuric acid. This
acid destroys the surface of the statuary, causing smooth
metal sculptures to become pitted, and resulting in such
severe spelling of stone figures that the outlines of the
features become blurred.
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SOX emissions
standards
Center for Archaeometry, Washington University
In accordance with the 1977 Clean Air Act Amendments,
the EPA has established regulations that require electric
power companies and industries to take steps to reduce
SOX emissions.
National Ambient Air Quality standards for sulfur oxides
establish a maximum safe level of the pollutant in the atmo-
spere. According to these standards, atmospheric concentra-
tions of SOX should not exceed 0.5 parts per million (ppm)
during a 3-hour period, or 0.14 ppm during a 24-hour
period. The annual mean concentration should not exceed
0.03 ppm.
In addition, EPA requires most new fossil-fuel fired steam
generators to meet its SOX New Source Performance Stan-
dards. These standards apply to all boilers built after August
1971 that are capable of producing more than 250 million
British Thermal Units (Btu) of heat per hour. The standards
limit emissions from both industrial and electric utility boilers
to no more than 0.8 pounds of S02 per million Btu of heat
input, if oil is burned. If coal or a coal-derived fuel is burned,
emissions must not exceed 1.2 pounds of S02 per million
Btu. Utility boilers built after September 1978 have an addi-
tional requirement to install some form of SOX control equip-
ment to achieve a reduction in potential S02 emissions. For
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EPA's research
program
coal-fired units, the required level of reduction varies from
70 percent for low-sulfur coals to 90 percent for high-sulfur
coals. Oil-fired utility units are required to achieve a 90 per-
cent reduction in potential S02 emissions.
The most prevalent means of SOX control are the flue gas
desulfurization (FGD) systems, which remove sulfur dioxide
from exhaust gases before they are emitted into the air.
Numerous public and private organizations are conducting
research to both improve existing control technologies and
develop new ones.
EPA's Office of Research and Development (ORD) is devel-
oping improved technologies for sulfur oxides control in four
major areas:
• fuel cleaning
• flue gas desulfurization
• combustion of coal-limestone mixtures
• coal liquefaction and gasification.
Research is being performed by EPA laboratories and
various public and private institutions. The Office of
Research and Development provides assistance in conduct-
ing research and demonstration projects through grants,
contracts, and cooperative agreements.
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fuel cleaning
coal cleaning
physical cleaning
About 450 to 500 coal cleaning plants are operating in the
U.S. Fifty percent of all domestically consumed coal is
mechanically cleaned to remove dirt and other impurities.
Some coal used for refining metals is cleaned to remove
sulfur, but coal destined to be burned to generate steam for
power plants and industries is usually not processed to
remove sulfur.
Coal contains sulfur in two forms: mineral sulfur, in the
form of pyrite, and organic sulfur that is chemically bound
in the coal. Most mineral sulfur can be removed by mechan-
ical coal cleaning processes, but removing organic sulfur
requires chemical processing.
To remove mineral sulfur, the coal is crushed, washed, and
then separated from impurities during a settling process.
From 40 to 90 percent of the total sulfur content in coal can
be removed by this physical cleaning process. Cleaning effec-
tiveness depends on the size of pyritic sulfur particles and
the proportion of sulfur in pyritic form.
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chemical
cleaning
microwave
desulfurization
Nearly half of the sulfur in coals from eastern Kentucky,
Tennessee, Georgia, and Alabama, and most of the sulfur in
coal from the western mountain states is in the pyritic form
and is relatively easy to remove by mechanical cleaning. The
combination of physical coal cleaning and partial flue gas
desulfurization enables many generating stations to meet
SOX emission standards at less expense than using flue gas
desulfurization alone.
U.S. coal deposits
ORD's Industrial Environmental Research Laboratory in
Research Triangle Park, North Carolina (IERL-RTP) is involved
in a program to advance two important chemical coal clean-
ing technologies: microwave desulfurization and hydrothermal
desulfurization.
One innovative technique for removing as much as 70 percent
of the sulfur in coal involves the use of microwave energy.
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hydrothermal
desulfurization
vehicular fuel cleaning
The coal is crushed, then heated for 30 to 60 seconds by ex-
posure to microwaves. Mineral sulfur selectively absorbs this
energy and escapes from the coal in the form of hydrogen
sulfide gas (H2S). Adding calcium hydroxide (Ca(OH)2) to
the crushed coal causes the organic sulfur in the coal to
convert to calcium sulfite (CaSO3) when it is exposed to the
microwave energy.
The coal is then washed with water to remove the calcium
sulfite and other impurities. The hydrogen sulfide gas (H2S)
resulting from microwave desulfurization can be collected
and processed to recover marketable elemental sulfur (S).
Hydrothermal desulfurization, developed by Battelle Labora-
tories in Columbus, Ohio, is undergoing further refinement
through experiments sponsored by the Office of Research
and Development. In this process, the coal is crushed to a
fine particle size and mixed with a solution of sodium and
calcium hydroxides (NaOH and Ca(OH)2). When this mixture
is heated to 275° C under pressure, nearly all of the inor-
ganic sulfur and 20 to 50 percent of the organic sulfur is
converted to sodium and calcium sulfites (Na2S03 and
CaS03). The hydrothermally treated coal is then rinsed to
remove the converted sulfur compounds, and the liquid
derived from the washing can be processed to recycle the
sodium and calcium hydroxides.
Although research remains in the experimental stages,
hydrothermal desulfurization appears to be an effective
means of removing sulfur from coal. ORD is directing cur-
rent efforts towards reducing the high cost of this process
by developing alternative methods of drying the coal and
recovering the sodium and calcium hydroxides.
Gasoline, diesel fuel, and jet fuel all contain sulfur which is
emitted in the form of sulfur oxides after combustion.
Although motor vehicle emissions currently account for only
about 3 percent of total national sulfur oxide emissions,
EPA is concerned about them for two reasons.
First, the catalytic converter being installed in cars to con-
trol hydrocarbon and carbon monoxide emissions can con-
vert exhaust sulfur dioxide to the more toxic compound,
sulfuric acid. These acid fumes could adversely affect the
health of people driving in heavy traffic.
Second, the production of diesel-powered vehicles is expected
to increase; diesel engines may be installed in 25 percent of
all new cars by 1985. Because diesel fuel is high in sulfur
content, its combustion results in emissions of large
amounts of sulfur dioxide and sulfuric acid. EPA is investi-
gating the feasibility and cost of reducing the sulfur content
of diesel fuel from its current average of 2,000 ppm to
200 ppm.
Gasolines vary widely in sulfur content; gasoline produced
in the Northeast has roughly two times the sulfur content of
gasoline produced in the Pacific Northwest. Regional differ-
ences are due to the type of crude oil processed, variations
in refinery processes, and the grades of gasoline produced.
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For example, unleaded and premium grade gasolines contain
less sulfur content than regular grade. EPA is examining the
possibility of reducing the sulfur content of all grades of
gasoline from the current range of 350-400 ppm to 100 ppm.
Commercial jet fuels used in the U.S. have an average
sulfur content of about 600 ppm. Some jet fuels contain up
to 2,000 ppm sulfur, and airline specifications allow a maxi-
mum of 3,000 ppm. Researchers believe that the sulfur con-
tent of jet fuel can be reduced to 200 ppm.
Sulfur can be removed from diesel fuel, gasoline, and jet
fuels during the refining process. When the sulfur in
petroleum is exposed to hydrogen in the presence of a
catalyst, hydrogen sulfide gas is formed. This compound
can be commercially marketed.
potential reduction of sulfur in fuels by 1990
gasoline
diesel fuel
jet fuel
current sulfur
content
(ppm)
350-400
2000
600-3000
potential sulfur
content
(ppm)
100
200
200
cost per
gallon
(cents)
1.9
3.4
2.5
Researchers at ORD's Environmental Sciences Research
Laboratory in Research Triangle Park, North Carolina have
been studying the costs and benefits of reducing the sulfur
content of vehicular fuel. They have found that added costs
for desulfurizing gasoline to meet a 100 ppm standard by
1990 would average 1.9 cents per gallon. Reducing the
sulfur content to 200 ppm would cost 3.4 cents per gallon
for diesel fuel, and 2.5 cents per gallon for jet fuel. Further
research is needed to develop more economical methods of
removing sulfur from vehicular fuels.
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flue gas desulfurization
Flue gas desulfurization (FGD) is the most commonly used
method of removing sulfur oxides resulting from the com-
bustion of fossil fuels. It is also the method that is best
suited to control SOX emissions from copper smelters. FGD
processes result in SOX removal by inducing exhaust gases
to react with a chemical absorbent as they move through a
long vertical or horizontal chamber. The absorbent is dis-
solved or suspended in water, forming a solution or slurry
that can be sprayed or otherwise forced into contact with
the escaping gases. The chamber is known as a scrubber,
and the process is often referred to as wet scrubbing.
More than 50 different flue gas desulfurization processes
have been developed, but only a few have received wide-
spread use. Of the systems currently in operation, 90 per-
cent use lime or limestone as the chemical absorbent.
In a lime slurry system, the sulfur dioxide reacts with lime to
form calcium sulfite and water.
SOX + CaO + H20 *- CaS03 + H20
sulfur lime water calcium water
dioxide sulfite
The use of limestone results in a similar reaction, but also
yields carbon dioxide.
S02 + CaC03 + H20 *- CaS03 + H20 + C02 j
sulfur limestone water calcium water carbon!
dioxide sulfite dioxide I
A few problems have arisen in the operation of the lime and
limestone FGD systems, and EPA's Industrial Environmental
Research Laboratory in Research Triangle Park, North
Carolina, has been successful in developing solutions. Cur-
rent efforts are directed towards using the limestone more
efficiently, removing more S02 from the exhaust gases,
improving equipment reliability, and altering the composition
of the waste sludge so that it can be more easily disposed
of in landfills.
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adipic acid additive
limestone utilization
LIME (CaO3)
OR
LIMESTONE
(OC03)
SLURRY
The recent discovery that the addition of adipic acid to FGD
limestone can increase the level of S02 removal from 85 per-
cent to 95-97 percent represented a major breakthrough in
S02 removal technology. Adipic acid, a crystalline powder
derived from petroleum, is available in large quantities.
EPA experiments have shown that when a limestone slurry
reacts with S02 in the scrubber, the slurry becomes very
acidic. This acidity limits S02 absorption. Adding adipic acid
to the slurry slightly increases the slurry's initial acidity, but
prevents it from becoming highly acidic during the absorp-
tion of S02. The net result is an improvement in scrubbing
efficiency.
Researchers have shown that adipic acid can reduce total
limestone consumption by as much as 15 percent. Further-
more, the additive is nontoxic (it is used as a food additive),
and does not degrade calcium sulfite sludge (CaS03) and
gypsum (CaS04), the FGD wastes.
Adipic acid is not currently being used in commercial FGD
systems. Full scale tests at an operating electrical generating
station are in the planning stage.
Adding adipic acid is one way to increase limestone utiliza-
tion in the scrubber system. Researchers are studying other
factors that affect S02 absorption and limestone utilization,
including the limestone's particle size, impurities, and
geological structure.
Limestone used in a scrubber system is crushed into small
particles to allow more calcium carbonate (CaC03)
molecules on the surface of the particles to react with the
sulfur dioxide (S02) gas. ORD scientists are testing two
sizes of limestone particles: a coarse grind, similar to that of
sugar or salt; and a fine grind, similar in consistency to
11
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forced oxidation
flour. Various types of limestone, crushed to the same parti-
cle size, are currently being compared for their effectiveness
in removing sulfur oxides from exhaust gases.
These tests have shown that different limestones of equal
particle size vary in their absorption effectiveness. Impurities
in the limestone account for part of this difference. Recent
experiments have shown that the presence of magnesium
carbonate, the main impurity in limestone, inhibits calcium
carbonate,from reacting with the sulfur dioxide.
The presence of such impurities, however, cannot fully
account for variations in the efficiencies of various
limestones. Researchers are investigating such geological
factors as crystal size and pore size to determine why some
kinds of limestone work better than others. These data can
then be used to improve the utilization of all limestones
employed in FGD systems.
Calcium sulfite that is formed during the scrubbing process
presents another important problem. This substance settles
and filters poorly, and can be removed from the scrubber
slurry only in a semiliquid, or paste-like, form which must be
stored in lined ponds. IERL-RTP is developing a way to solve
this problem through a process called forced oxidation.
Forced oxidation requires air to be blown into the tank that
holds the used scrubber slurry, composed primarily of
calcium sulfite and water. The air oxidizes the calcium
sulfite to calcium sulfate.
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CaS03
calcium
sulfite
H20
1/20
water Oxygen
H»- CaS04
calcium
sulfate
H2O
water
shawnee test facility
The calcium sulfate formed by this reaction grows to a
larger crystal size than does calcium sulfite. As a result, the
calcium sulfate can easily be filtered to a much drier and
more stable material which can be disposed of as landfill. In
some areas, the material may be useful for cement or
wallboard manufacture or as a fertilizer additive.
Another problem associated with limestone scrubbers is the
clogging of equipment due to calcium sulfate scale. Forced
oxidation can help control scale by removing calcium sulfite
from the slurry and by providing an abundance of pure gyp-
sum (calcium sulfate) to rapidly dissipate the supersaturation
normally present. The process also requires less fresh water,
which is scarce in many western locations, for scrubber
operation.
Current experiments at Research Triangle Park are directed
toward testing various forced oxidation designs to find the
best oxidation system using the least energy.
Since early 1972, EPA's Industrial Environmental Research
Laboratory, Research Triangle Park, has been conducting
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dry scrubbing
flue gas desulfurization tests at the Shawnee Lime and
Limestone Wet Scrubbing Test Facility near Paducah, Ken-
tucky. This test facility uses the Shawnee Power Station, a
coal-fired plant owned and operated by the Tennessee
Valley Authority (TVA).
The facility, shown in the photo on the previous page, was
built with three different FGD scrubber systems. Each sys-
tem treats part oft the exhaust gases from the generating
station. Emissions from the facility are monitored and scrubber
equipment is modified to find ways of solving operating
problems and reducing costs. Recent tests are focusing on
different methods of forced oxidation of lime and limestone
slurries. Researchers are also examining the effectiveness of
various slurry additives for increasing the efficiency of S02
removal systems.
Results of tests at the Shawnee prototype FGD facility are
presented to commercial utility operators and FGD system
manufacturers through workshops, conferences, and EPA
publications.
Dry scrubbing is a modification of wet scrubbing flue gas
desulfurization technology. As in other FGD systems, the
exhaust gases combine with a fine slurry mist of lime or
sodium carbonate. This system, however, takes advantage
of the heat of the exhaust gases to dry the reacted slurry
into particles of calcium sulfite and sodium sulfite.
CaO
lime
I S02
1 sulfur +
' dioxide
S02
sulfur + sodium
dioxide carbonate
CaSOa
calcium
sulfite
Na2S03 C02
sodium + carbon
sulfite dioxide
copper oxide
adsorption
The particles generated by this dry scrubbing process are
then collected along with other particles from coal combus-
tion in a baghouse collector. This collector uses fabric bags
that function similar to those in a vacuum cleaner, which
collect particles while permitting cleaned gases to escape.
Dry scrubbing typically removes 70 percent of the sulfur
dioxide in a waste gas stream. It is 15 to 30 percent cheaper
to install and operate than a conventional wet scrubbing
system. However, because dry scrubbing is less efficient
than wet scrubbing, the technology has been limited to use
with low sulfur coal.
Plans for future research include evaluating the performance
and reliability of a full scale utility boiler equipped with a
spray-dryer S02 control system. Improvements could make
these dry scrubbing systems acceptable for general use by
the late 1980's.
An experimental method of removing 90 percent of both
sulfur and nitrogen oxides is being developed by IERL-RTP.
In this process, combustion exhaust gases are moved over a
14
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bed of copper oxide. Sulfur oxides combine with the copper
oxide, forming copper sulfate.
r SOX + CuO ^ CuS04 j
f sulfur oxide copper oxide copper sulfate j
Periodically, the copper oxide is regenerated with diluted
hydrogen to produce a gas containing concentrated amounts
of sulfur dioxide. Two or more copper oxide reactors are
used to achieve continuous desulfurization.
The copper sulfate formed by this reaction serves as a
catalyst, causing ammonia injected into the system to react
with nitrogen oxides (NOX) forming nitrogen gas and water.
Researchers are conducting experiments to determine the
optimum method of removing both sulfur and nitrogen oxides
under various operating conditions. ORD is sponsoring the
demonstration of copper oxide absorption technology at
Tampa Electric Company's Big Ben Station near North
Ruskin, Florida. It is expected that copper oxide absorption
technology will soon be applicable to large coal-fired elec-
trical generating stations.
smelter scrubbers Copper ore contains large amounts of sulfur that are con-
verted to sulfur oxides when the ore is processed. About
two tons of sulfur dioxide (S02) are generated for each ton
of copper produced.
Smelters produce two streams of gases containing sulfur
oxides, a strong stream containing a 4 percent or greater
concentration of SOX, and a weak stream normally with less
than a 2 percent concentration of SOX. The strong stream is
usually treated by a chemical process that converts S02 to
sulfuric acid (H2S04). In this process, S02 is cleaned and
converted to S03. Then the S03 reacts with water, produc-
ing H2S04. Thirteen of the sixteen copper smelters in the
United States operate sulfuric acid plants. The sulfuric acid
can be used in ore processing operations or sold to other
industries.
Most of the S02 emissions from copper smelters come from
reverberatory furnaces. Eleven of the sixteen copper
smelters operating in the United States use this type of fur-
nace which burns gas, oil, or coal. When copper is heated,
sulfur is released and mixes with gases from the burning
fuel and with large quantities of air and is converted to S02.
The concentration of S02 ranges from 0.5 to 3.5 percent,
but rarely exceeds 2.5 percent. This level of S02 concentra-
tion is lower than the 4 percent or more required to process
S02 into sulfuric acid (H2S04), so the furnace exhaust gases
are vented to the atmosphere. None of the reverberatory
furnaces operating in this country are equipped with con-
trols for S02 emissions.
A project is underway at the Industrial Environmental
Research Laboratory in Cincinnati, Ohio, to identify SOX
emission control systems that are appropriate for use with
copper smelters. Two of the most promising systems are
15
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citrate process
magnesium oxide
process
the citrate and magnesium oxide processes. Both processes
concentrate S02 gas from the smelter furnace to allow the
production of sulfuric acid.
In the citrate process, sulfur dioxide is dissolved in water
and thus removed from the exhaust system.
S02 H
sulfur
dioxide
l- H20
water
-*- HS03~
bisulfite
ion
+ H+
hydrogen
ion
Adding citrate to the water increases the amount of S02
that the water will absorb because the citrate ion (CIT)
chemically bonds with the hydrogen ions (H+). Sulfur can
then be removed from the citrate solution in the form of an
S02 stream strong enough to be used in the acid plant and
converted to marketable sulfuric acid.
In the second SOX control process, magnesium oxide is
mixed with water to form a slurry. Washing the smelter
gases with this slurry causes the SO2 in the gases to com-
bine with the magnesium and form magnesium sulfite.
The magnesium sulfite is collected, dried, and heated to
temperatures of from 670° to 1000° C (1250° to 1800° F).
The heat causes the magnesium sulfite molecules to break
apart, regenerating magnesium oxide that can be used
again, and a highly concentrated S02 gas that can be con-
verted to sulfuric acid.
Both the citrate and the magnesium oxide processes are
being tested for their effectiveness in removing S02 from
the exhaust gases of industrial boilers and electric generat-
ing plants. The citrate process is being demonstrated in a
copper smelter in Sweden and in a zinc smelter in Pennsyl-
vania. The magnesium oxide process has been demon-
strated in a smelter in Japan. The demonstrations have
shown these processes to be at least 90 percent effective in
removing S02 from exhaust systems. Adapting them to the
U.S. smelting industry would be a major step in reducing
national sulfur oxide emissions.
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coal-limestone combustion
fluidized bed
combustion
Since sulfur oxides are emitted from the stacks of electrical
generating stations and industries, the initial approach to
SOX control concentrated on treatment of the waste gases
in the stack. Recently, however, scientists and engineers at
the IERL-RTP have been working with the U.S. Department
of Energy and the electrical power industry to develop
methods of removing sulfur oxides in the combustion area.
Two promising burning techniques are currently receiving
attention: fluidized bed combustion and the use of lime-
stone coal pellets as fuel.
In the fluidized bed combustion process, a grid supporting a
bed of crushed limestone or dolomite is set in the firebox.
Air forced upward through the grid creates turbulence,
causing the bed of limestone or dolomite to become
suspended and move in a fluid-like motion. Natural gas is
injected into the firebox and ignited, then pulverized coal is
pushed into the combustion area and burned. Once the coal
has started to burn well, the natural gas is shut off, and the
fire is maintained by burning coal. Sulfur oxidized during
combustion reacts with the limestone or dolomite in the
firebox forming calcium sulfate. Calcium sulfate and residual
17
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limestone or dolomite from fluidized bed combustion can be
disposed of in landfills or used in construction materials.
Fluidized bed combustion eliminates the need for flue gas
desulfurization since the bed of limestone or dolomite can
remove more than 90 percent of the sulfur oxides created
during combustion. Furthermore, it is expected that fluidized
bed combustion systems will cost less than conventional
boilers with flue gas desulfurization systems.
Demonstrations of fluidized bed combustion technology are
being conducted at several sites, including one at Rivesville,
West Virginia. This technology is expected to be available
for commercial application in the early 1980's.
limestone coal pellets Burning pellets composed of a limestone and coal mixture is
another way of eliminating the need for flue gas desulfuriza-
tion. ORD research has shown that the combustion of these
pellets in conventional stoker boilers not only reduces sulfur
oxide emissions, but also enhances boiler performance.
The pellets are made by pulverizing coal and limestone and
adding a binder material to form small cylinders. As the
pellet burns, the calcium in the limestone absorbs the S02
generated from burning the coal, resulting in the formation
of calcium sulfate (CaS04).
Ken Altshuler
The ability of the pellet to control sulfur emissions depends
on the ratio of limestone to coal, pellet size, binder material,
and types of coal and limestone used. For example, ORD has
developed a binder material that enables as much as 87 per-
cent of the S02 to be absorbed by the limestone when a
pellet composed of two-thirds coal and one-third limestone
is used.
The expense of preparing fuel with pellets would add about
$15 per ton to the cost of coal, which is substantially less
than the cost of installing and operating wet scrubber
systems for industrial boilers. In the future, fuel pellets will
be developed for a greater range of coal and boiler types.
This research could enable users of high sulfur coal from
eastern U.S. mines to meet SOX pollution control
requirements.
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coal liquefaction and gasification
The Industrial Environmental Research Laboratory in
Research Triangle Park is engaged in a program to develop
and evaluate techniques of controlling pollution from coal
liquefaction and gasification processes. Synthetic gas
derived from coal will be commercially available by about
1985 to 1990. Liquid fuels from coal are expected to be
available in the 1990's.
These fuels are not only far more useful than coal, but are
also cleaner to burn. With the impetus of the 1973 oil
embargo, consequent increases in the price of imported oil,
and our abundant U.S. coal resources, the synthetic fuel
industry may grow rapidly in the years ahead. The produc-
tion of synthetic gas and oil is expected to consume 120
million tons of coal by 1990 and 300 million tons by 2000.
While prime responsibility for developing synthetic fuel pro-
cesses lies with the Department of Energy, the Environ-
mental Protection Agency must ensure that these processes
do not create adverse health and ecological effects.
coal gasification A very elementary process of coal gasification was designed
in the late 1700's to fuel the gas lights that illuminated
cities. Since that time, approximately 70 different coal
gasification processes have been used commercially or are
currently under development.
Three basic steps are common to all coal gasification pro-
cesses: coal pretreatment, gasification, and gas cleaning.
Coal pretreatment includes various stages of coal washing
and pulverization. Gasification produces either a low- or
high-heat content gas by applying heat and pressure, or
using a catalyst to break down the components of coal.
Coal is gasified in an atmosphere of limited oxygen. General-
ly, oxidation of the coal provides a gas containing carbon
monoxide (CO), hydrogen (H2), carbon dioxide (C02), water
(H20), methane (CH4), and contaminants such as hydrogen
sulfide (H2S), and char.
Synthetic gas is composed primarily of carbon monoxide
and hydrogen. Variations in the process may increase the
quantity of methane formed, producing a gas that releases
more heat when it is burned.
The sulfur in coal is converted primarily to hydrogen sulfide
(H2S) during the gasification process. It exits from the
gasifier with the methane and synthetic gas, and is subse-
quently removed during the gas cleaning process. After
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coal liquefaction
removal, the hydrogen sulfide is then converted to elemental
sulfur (S) through partial oxidation and catalytic conversion.
An estimated 900 tons of sulfur flow daily through a plant
using high sulfur Eastern coals. Thus, the collection, con-
version, and removal of gaseous sulfur compounds is essen-
tial to prevent health and environmental damage. After the
sulfur compounds have been removed, the synthetic gas
can be burned without releasing dangerous emissions.
SYNTHETIC GAS
HYDROGEN (H2I
CARBON
MONOXIDE ICO)
METHANE (CH4)
CARBON
DIOXIDE (C02)
A process for converting carbon monoxide to liquid
hydrocarbons was developed by Fischer and Tropsch in
Europe in the 1930's, and tests and demonstrations of
processes for producing synthetic oil from coal were
initiated in the U.S. in the early 1960's.
There are two basic approaches in converting coal to oil.
One involves using a gasifier to convert coal to carbon
monoxide, hydrogen, and methane; followed by a conden-
sation process that converts the gases to oils. The second
approach involves using a solvent or slurry to liquefy
pulverized coal and processing this liquid into a fuel similar
to heavy oil. Solvents and slurries used in these processes
are usually produced from the coal and recycled in the sys-
tem. Recently developed liquefaction processes have com-
bined the use of solvents and distillation techniques to pro-
duce hydrocarbon gas and various hydrocarbon liquids.
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Processes involving solvents and slurries commonly remove
sulfur from the liquified coal by using hydrogen (H2) to
convert the sulfur to hydrogen sulfide gas.
As in the gasification processes, this hydrogen sulfide is
then partially oxidized to form elemental sulfur and water.
HYDROGEN
LIQUEFIER SULFIDE
IHjSI
More than 85 percent of the sulfur in coal is removed during
the liquefaction process. EPA research efforts currently
focus on determining the sulfur content in synthetic oils
produced by different liquefaction processes, and identifying
ways to improve their sulfur removal efficiencies. EPA is
also initiating programs to develop improved systems for
preventing the escape of hydrogen sulfide (H2S) and sulfur
dioxide (S02) from the gas converter into the atmosphere.
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costs of SOX control
By 1990, electrical utilities will have invested between $10
and $20 billion for the construction and operation of flue
gas desulfurization systems. The Office of Environmental
Engineering and Technology at EPA Headquarters in
Washington, D.C. has initiated several efforts to reduce
the costs of sulfur oxide control technology.
One of these efforts involves searching for new methods of
sulfur oxide control that will cost less than the wet
scrubbing systems currently available. EPA is sponsoring
workshop sessions with members of the electric utility
industry and other groups to keep them informed of new
cost-saving technological improvements of available
systems, and to encourage their cooperation in building and
operating test facilities to demonstrate new SOX control
technologies.
EPA is also trying to reduce costs by developing commercial
markets for the waste products of sulfur oxide removal pro-
cesses. Such products include gypsum, sulfur, and sulfuric
acid. Gypsum is a valuable commodity used in making
wallboard and other building materials, and sulfur and
sulfuric acid are used extensively in the chemical industry.
However, the cost of shipping these waste products to
existing markets has often made them too expensive to be
competitive with local resources. EPA is investigating the
potential use of SOX control waste products in the fertilizer
industry. Sulfur is a plant nutrient, and since fertilizer is
made locally in a large number of areas, shipping costs
should not be a major deterrent to the use of the sulfur
wastes. Developing other markets for these products will
both reduce the net costs associated with SOX removal and
alleviate some of the problems associated with the disposal
of wastes generated from SOX control systems.
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individual research projects
Selected research projects underway by or through the
various ORD laboratories or offices are listed below.
industrial environmental
research laboratory
research triangle park,
north Carolina
Sulfur Dioxide Oxidation in Scrubber Systems
Miniplant Studies in Support of Fluidized-Bed Com-
bustion Program
EPA Shawnee Alkali Scrubbing Test Facility—Advanced
Testing and Support Studies for Transfer of Tech-
nology to Full-Scale Operating Plants
Investigation of NOX, Nitrate, and Sulfate Formation in
Laboratory Flames
Emissions Assessment of Conventional Combustion
Systems
Coal Cleaning Technology Evaluation and Development
Pilot-Scale Evaluation of Simultaneous NOX/SOX Flue
Gas Treatment Technology
Develop Comparative Economics of SOX Control Processes
Marketing of By-Products from S0y Control Processes
industrial environmental
research laboratory
Cincinnati, ohio
• Feasibility of Smelter Weak S02 Stream Control
• Evaluation of Baghouse and Optional FGD Demonstra-
tion on Smelter Weak SO,
environmental sciences
research laboratory
research triangle park,
north Carolina
• Mesoscale Sulfur Balance Studies
• Sulfur Budget in Large Plumes
• Kinetic Study of Simulated S02, NOX, Reactive
Hydrocarbon-Polluted Atmospheres
• Continuous Monitoring of Particulate Sulfur Com-
pounds by Flame Photometry
• Remote Atmospheric Measurement of S02 and CH4
Using
a LiNb03 Tunable Laser Source
• S02 and SO4 Measurement Methods Evaluation
• Measurement of S02, S03, and H2S04 from Fossil-
Fueled Combustion Sources
• Measurement of H2S04 Emissions from Selected
Sources
• Develop and Evaluate Monitors for Detection of Sulfur
Containing Gaseous Compounds
• Outdoor Smog Chamber Studies of Sulfur Emissions
from Fuel Conversion Facilities
• Determination of S02 Mass Emission Rates by Remote
Sensing
• Characterization of Primary Sulfate Emissions from
Industrial/Residential Sources
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office of environmental
engineering and
technology
Washington, d.c.
Cost of Sulfur Reduction in Mobile-Source Fuels to the
Petroleum Refining Industry
Sulfur Dioxide and Sulfates Material Damage Study-
Part 4: Distribution in Cities
Atmospheric Transport and Transformation from Coal-
Fired Power Plants
A Study to Support the Development of New
Source Performance Standards for Control
of S02, NOX, and Particulates from Combustion Boilers
Deposition, Retention and Dosimetry of Inhaled Reac-
tion Products Which May Result from SO2 Particulate
Interactions
Atmospheric Interactions in Scrubber Plumes
Control Technology Assessment for Energy
Morbidity and Industrial Hygiene Study—Worker Expo-
sure to Sulfur and Nitrogen Oxides
Cost of Control for Precursors of Acid Deposition
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for further information
publications • Sulfur Emission: Control Technology and Waste
Management. EPA Decision Series. May 1979.
EPA-600/9-79-019. 33 pages.
A nontechnical examination of sulfur emission control
technology and waste management issues.
• EPA Research Outlook. February 1980.
EPA-600/9-80-006. 224 pages.
A description of EPA's plans for future environmental
research.
• EPA Research Highlights. January 1980.
EPA-600/9-80-005. 100 pages.
Highlights of the EPA research and development pro-
gram of 1979.
• EPA/ORD Program Guide. October 1979.
EPA-600/9-79-038. 85 pages.
A guide to the Office of Research and Development-
its organizational structure, program managers, and
funds available for contracts, grants, and cooperative
agreements.
• Energy/Environment IV. Proceedings of the Fourth
National Conference of the Interagency Energy/
Environment, Research and Development Program.
October 1979. EPA-600/9-79-040. 330 pages.
The proceedings of an annual conference discussing
energy/environment issues, sponsored by the Federal
Interagency Energy/Environment Research and Devel-
opment Program.
• Who's Who V in the Interagency Energy/Environment
R&D Program. January 1980. EPA-600/9-79-017. 72 pages.
• Sulfur Oxides Control in Japan. EPA Decision Series.
November 1979. EPA-600/9-79-043. 24 pages.
• Coal Cleaning with Scrubbing for Sulfur Control: An
Engineering/Economic Summary. EPA Decision Series.
August 1977 EPA-600/9-77-017. 16 pages.
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other research
summaries
EPA Research Summary: Industrial Wastewater.
June 1980. EPA-600/8-80-026. 32 pages.
EPA Research Summary: Chesapeake Bay. May 1980.
EPA-600/8-80-019. 32 pages.
EPA Research Summary: Controlling Hazardous
Wastes. May 1980. EPA-600/8-80-017. 24 Pages.
EPA Research Summary: Controlling Nitrogen Oxides.
February 1980. EPA-600/8-80-004. 24 pages.
EPA Research Summary: Acid Rain. October 1979.
EPA-600/8-79-028. 24 pages.
EPA Research Summary: Oil Spills. February 1979.
EPA-600/8-79-007. 16 pages.
Information on the availability of these publications may
be obtained by writing to:
Publications
Center for Environmental Research Information
US EPA
Cincinnati, OH 45268
technical reports
and manuals
EPA Utility FGD Survey: October - December 1979.
January 1980. EPA-600/7-80-029a. (PB 80-176811,
$27.00)
• Survey of Dry S02 Control Systems. February 1980.
EPA-600/7-80-030 (PB 80-166853, $9.00)
• Field Tests of Industrial Stoker Coal-Fired Boilers for
Emissions Control and Efficiency Improvement. March
1980. EPA-600/7-80-065a. (PB 80-183023, $9.00)
• Environmental Considerations of Energy-Conserving
Industrial Process Changes. Executive Briefing:
EPA-625/9-77-001. 25 pages. (PB 264 216, $7.50)
Methods Development for Assessing Air Pollution Con-
trol Benefits. Volume V, Executive Summary:
February 1979. EPA-600/5-79-001e. 22 pages.
(PB 293 619, $4.50)
26
Review of New Source Performance Standards for
Coal-Fired Utility Boilers. Volume II, Economic and
Financial Impacts. EPA-600/7-78-155b. 165 pages
(PB 285 855, $9.00)
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• Sammis Generating Station: Meeting SC>2 and Par-
ticulate Standards with Cleaned Ohio Coals.
EPA-600/7-80-009. January 1980. (PB 80-147077, $9.00)
EPA Technical Reports may be obtained by writing to:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
or by calling (703) 557-4650
conferences and
workshops
The Office of Research and Development periodically
sponsors various conferences, workshops and seminars
to inform environmental scientists, engineers, policymakers,
and the interested public of the latest research and develop-
ment accomplishments. Individuals interested in information
about upcoming conferences should write to:
ORD Conference Coordinator
Center for Environmental Research Information
US EPA
Cincinnati, OH 45268
questions or
comments
The Office of Research and Development invites you to
address any questions or comments regarding the EPA
sulfur oxide control research program to the appropriate
individuals listed below:
Topic
Contacts
Fuel Cleaning
Fuel Gas Desulfurization
& Coal-Limestone
Combustion
Coal Liquefaction
and Gasification
James Kilgroe
Industrial Environmental
Research Laboratory, MD-61
Research Triangle Park,
NC 27711
Robert Statnick
Office of Research &
Development, RD-681
US EPA
Washington, DC 20460
Morris Altschuler
Office of Research &
Development, RD-681
US EPA
Washington, DC 20460
EPA's sulfur oxides control research program is administered
by Dr. Steven Reznek, Deputy Assistant Administrator for
Environmental Engineering and Technology.
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