United States
Environmental
Protection
Agency
Office of
Research and
Development
Washington DC 20460
National Risk
Management
Research Laboratory
Cincinnati OH 45268
Air Pollution
Prevention and Control
Division
Research Triangle Park NC 27711
EPA/600/F-95/013
August 1995
4>EPA Flue Gas Desulfurization
Technologies for Control of
Sulfur Oxides
Research, Development, and
Demonstration
Sulfur oxide emissions (hundreds of short tons)
S Operational H Under Construction H Planned
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Improved Technology for Environmental Protection
Flue gas desulfurization (FGD) technologies have been
applied in the United States during the past two decades
to help reduce emissions of sulfur dioxide (SO2) and,
consequently, improve ambient air quality in response to
clean air legislation. While the burning of coal, a primary
source of SO2 emissions in this country, has increased
during this period, SO2 emissions have been reduced by
about 8 million tons, annually. The workhorses of these
control technologies, wet lime and limestone systems,
better known as "scrubbers," have been, to a great
extent, pioneered, developed, and demonstrated by
EPA's Air Pollution Prevention and Control Division
(APPCD) [formerly known as the Air and Energy Engi-
neering Research Laboratory (AEERL)].
BACKGROUND
SO2 in the atmosphere has been recognized as a major
air pollution problem in the U.S. since the inception of
clean air legislation. The Air Quality Act of 1967 required
that states develop ambient air quality standards for
SO2. The Clean Air Act (CAA) of 1970 mandated perfor-
mance standards for new and significantly modified
sources of SO2. In 1971, the Environmental Protection
Agency (EPA) issued the first such standards for fossil-
fuel-fired boilers greater than 25 MWe.1 The new source
performance standard (NSPS) limiting allowable emis-
sions to 1.2 Ib of SO2 per million Btu of heat input to the
boiler, promulgated by EPA in 1971, essentially limited
operators of these boilers to two choices: use low-sulfur
coal, or apply FGD technology.
In 1979 the NSPS were revised for power plants,
requiring a percentage reduction of SO2.2 This mandate
was intended to be technology forcing, essentially
requiring all new power plants to add SO2 removal
equipment to the base design.
In the 1980s Congress began debating the need for
additional SO2 control as a means of reducing damage
from acid rain, culminating in the Clean Air Act Amend-
ments of 1990. Under Title IV of the Act, three distinct
phases of SO2 control are mandated:
• Phase I targets specific large sources to reduce SO2
emissions 5 million tons by January 1, 1995.
• Phase II reduces all power plants to a nationwide
emission level of 1.2 Ib SO./106 Btu by January 1
2000.
• Phase III requires that SO2 emissions be capped
beyond the year 2000.
As shown in Figure 1, U.S. SO2 emissions have de-
creased from about 31 million tons in 1970 to about 23
Coai consumotion (thousands of short tonsi
Sulfur oxide emissions (hundreds of short tons)
Figure 1. U.S. annual sulfur oxide emissions and coal
consumption.
million tons in 1992, in spite of the increase in coal
consumption from about 560 million tons to about 890
million tons over the same period.34
At the end of the decade of the 80s, the U.S. utility
industry was controlling about 68,000 MWe of electric
generating capacity with FGD at an estimated installed
cost of $10 billion. At that time, another 29,000 MWe of
electric generating capacity had FGD systems under
construction or in the planning stages. If the Clean Air
Act Amendment goals are met, an additional 10 million
tons of SO2 emissions, annually, will be eliminated by
the year 2000, such that future SO2 levels will be
stabilized at less than half the level of the early 1970s 5
(Figure 2)
Q Other Sources [3 Electric Utilities
Non-utility Point Sources
1980
1990
Figure 2. Past and projected trend in sulfur oxide emissions
1980 to 2010.
Printed on Recycled Paper
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Improved Technology for Environmental Protection
EPA's research program has played an important role in
limiting SO2 emissions growth in the U.S. The interna-
tional community followed EPA's lead as evidenced by
the aggressive SO2 controls mandated in Japan and
Europe.
EPA'S SULFUR OXIDES RESEARCH PROGRAM
In the early 1970s, the viability of wet lime and limestone
scrubbing was controversial. EPA argued for accep-
tance and application of this FGD technology, while
utility companies argued that the technology was not
adequately demonstrated. At that time, predecessors of
EPA's APPCD forged an interagency agreement with the
Tennessee Valley Authority (TVA) to cooperatively
evaluate and improve wet lime and limestone FGD
technology at TVAs Shawnee Station in Paducah, KY,
on three parallel 10 MWe prototype scrubbers. To
support the Shawnee program, APPCD constructed a
0.1 MWe wet scrubbing pilot plant at the EPA facility in
Research Triangle Park (RTP), NC, to solve some of the
problems being experienced by the few commercial
attempts at FGD. These problems included severe
corrosion of scrubber components, plugging of the
scrubber by solids, and poor SO2 removal.6
Through the early 1980s this cooperative effort demon-
strated a number of FGD improvements which are in
commercial practice today. Important work was also
conducted on evaluating scrubber waste disposal
options.
By the mid 1980s, wet FGD had become commercially
established and accepted by the U.S. utility industry-a
complete turnaround from the perception just one
decade earlier. At that time, APPCD focused SO2
research on lower cost retrofit technologies such as dry
scrubbing (spray dryer absorption), limestone injection
with multistage burners (LIMB), calcium silicate injection
(ADVACATE), and combined spray dryer/electrostatic
precipitation (E-SOx), in anticipation of a major U.S. acid
rain retrofit program being considered by Congress.
RESEARCH ACTIVITIES
Lime and Limestone FGD
In the early 1970s when FGD was in its infancy, wet lime
or limestone slurry scrubbing was the system of choice.
A typical, no frills FGD system is shown in Figure 3.
These systems were fraught with operating problems.
The efforts of APPCD to bring wet FGD to commercial
acceptance resulted in the following innovations re-
searched and developed at the RTP pilot plant:
Flue gas from
participate |Scrubbe
collector
LJ
Limestone
I Scrubber
effluent
hold tank
_^ Waste to
disposal
Figure 3. Wet FGD technology for SO2 control.
• Use of high liquid-to-gas ratios (enhanced scrubber
internal recirculation) to prevent scaling.
• Use of forced oxidation to avoid scaling and improve
disposal/salability of solids.
• Use of thiosulfate-forming additives to inhibit scaling.
• Use of organic acid buffers to increase SO2 removal
and improve sorbent utilization.
Power Plant and FGD Waste Disposal
As a natural outgrowth of the research and development
of FGD technologies, the predecessors of APPCD
conducted a research and development program to deal
with disposal of wastes from coal-fired power plants,
including fly ash, bottom ash, and wet and dry FGD
wastes, in environmentally acceptable ways. In this
program, FGD and ash wastes were chemically and
physically characterized. Methods of physically stabiliz-
ing wet FGD wastes and minimizing their permeability
and teachability were investigated, and tested in both
the laboratory and the field. The use of forced oxidation
in wet FGD lime and limestone scrubbers also improved
the stability of these wet FGD wastes. Procedures to
determine the toxicity of trace metals leached from fly
ash at disposal sites were investigated in support of
Resource Conservation and Recovery Act (RCRA)
regulation development.
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Improved Technology for Environmental Protection
Emerging Technologies
During more recent efforts to develop lower-cost alter-
natives to the standard wet FGD, more suited to retrofit
of existing facilities, APPCD has fostered the develop-
ment of:
• Spray dryer absorption.
• Furnace injection of calcium sorbent (LIMB).
• Calcium silicate injection (ADVACATE).
• Combined spray drying and electrostatic precipitator
(E-SOJ.
• Use of organic acid buffers.
• Dual alkali technology from concept to full-scale
application.
Technology Transfer
Over a 20-year period, APPCD has established FGD as
a commercially accepted technology, through dissemi-
nation of program results at regularly sponsored sympo-
sia, sponsoring a number of commercial-scale demon-
strations, publishing numerous journal articles, and
holding industry seminars at the conclusion of success-
ful demonstrations to ensure that vendors are able to
offer FGD innovations, commercially. Also, the regula-
tory development has been greatly assisted by the
APPCD program results, most notably in the 1979
NSPS for utility boilers, which was based largely upon
FGD process improvements developed or sponsored by
APPCD.
During this period, APPCD has co-sponsored SO2
control symposia at intervals of about 1-1/2 years which
have grown from about 100 attendees in the early
1970s to nearly 800 in the 1990s. The international
audience for these symposia has gradually grown to
where nearly one-fourth of the papers and attendees
are from outside the United States, despite their being
held in the U.S.
ACCOMPLISHMENTS
To foster the development and implementation of cost-
effective SO2 control technology, APPCD has:
• Conducted 15+ years of pilot wet lime and limestone
FGD tests at RTP and TVA to improve the technol-
ogy to a universal acceptance.
• Sponsored a number of commercial demonstrations
to show high reliability, 90 percent SO2 control wet
FGD operation.
• Sponsored laboratory and field evaluation studies of
power plant and FGD waste disposal.
• Sponsored SO2 control technology symposia on a
regular basis since 1971; conducted industry brief-
ings to transfer successful technology demonstra-
tions to the private sector.
• Published over 100 reports and hundreds of journal
articles on FGD performance and economics.
• Published an economic model for evaluation of
alternative SO2, NOx, and PM control technologies.
• Received 11 patents on SO2 control technology with
several more pending.
• During the 1970s and early 1980s, provided leader-
ship through international forums such as NATO -
Critical Challenges to Modern Society (NATO-CCMS)
to transfer FGD technology to Europe.
Role of Other Non-EPA Research Organizations
The application of FGD control technology has bur-
geoned over the past two decades. In addition to the
role played by EPA, FGD commercialization has been
strongly influenced by the efforts of other federal agen-
cies, including the Department of Energy, research
organizations such as the Electric Power Research
Institute, and a host of progressive-thinking, environ-
mentally conscious innovators, private sector compa-
nies, FGD vendors, and the electric utility industry.
These organizations have been instrumental in pushing
FGD technology to its current level of high removal
efficiency and high reliability. This successful implemen-
tation, and continuing improvement of FGD systems,
attests to the accomplishments that can be made
through worldwide collaboration and cooperation
between regulators, research, and private industry.
Figure 4 shows the number of U.S. operational, under
construction, and planned utility FGD systems as a
function of time.6 Note that the effect of the 1990 Clean
Air Act Amendments (Acid Rain) is not depicted here,
but is expected to add many more applications of FGD
technology in the post-2000 time frame.
IMPACTS
The major impacts of APPCD's SO2 Control Technology
research program are:
• Development of wet FGD technology which is
reflected in the worldwide application of FGD by
commercial vendors.
• Support of the landmark 1979 NSPS which required
70 to 90 percent reduction of SO2 on a continuous
basis.
• Development of several new SO2 control technolo-
gies to enable cost-effective retrofit of existing power
plants.
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Improved Technology for Environmental Protection
125i
H Operational H Under Construction • Planned
Figure 4. History of utility FGD status, December 1970
through December 1988.
• Development of process and economic models which
enable the private sector to predict performance and
costs of FGD technology.
Influencing FGD Technology Abroad
Of the 347 FGD units installed outside the USA, nearly
65 percent (223) are lime/limestone FGD units using
technology first piloted and field tested under APPCD
sponsorship in the 1970s.7 In Japan 46 of 47 FGD units
are wet lime/limestone units designed by five major
Japanese vendors. During the 1970s these vendors
attended FGD symposia in the U.S. cosponsored by
APPCD and visited the FGD pilot facilities at RTP and
TVA's Shawnee unit. Information on FGD design and
operation was also exchanged freely during a number of
visits made under a Japan/U.S. environmental agree-
ment.
In Germany a similar situation to Japan exists in that
136 of the 205 FGD units are lime/limestone wet scrub-
bers, the majority designed by six German vendors.
Most German scrubbers were installed in the mid-1980s
as part of a massive acid rain mitigation program and
had the benefit of the complete EPA/TVA pilot experi-
ence that ended in the early 1980s. The German
vendors, too, were attendees at EPA-sponsored confer-
ences on FGD, and the German government acquired
additional information through NATO-CCMS activities
chaired by the APPCD Director.
In summary, worldwide FGD use, most notably in
Germany and Japan, is dominated by the lime/limestone
wet scrubber where basic design evolved from the EPA-
TVA pilot FGD evaluations.
Early participation by Japanese vendors, and later
German vendors, in EPA-sponsored information ex-
changes, visits, and symposia promoted the rapid
diffusion of FGD technology worldwide.
RECENT RESEARCH
DEVELOPMENT OF NEXT GENERATION SO2
RETROFIT CONTROL TECHNOLOGIES
In the 1980s international focus on acid rain and the
perceived need for low capital cost retrofit SO2 technol-
ogy altered APPCD's focus from wet FGD improve-
ments toward development of lower cost dry SO2
technologies. APPCD initially fostered the development
of spray dryer FGD technology which quickly achieved
commercial acceptance. During the latter half of this
decade, APPCD developed three related technologies-
lime/limestone injection with multistage burners (LIMB),
advanced calcium silicate injection (ADVACATE), and
electrostatic precipitator sulfur oxides removal (E-SOx).
LIMB
LIMB technology (shown in Figure 5) was demonstrated
at 50 to 60 percent SO2 removal in two demonstrations
sponsored by APPCD. A wall-fired demonstration at
Ohio Edison's Edgewater Station was completed in
1989.8 This was followed by a tangentially fired LIMB
demonstration at Virginia Power's Yorktown Station.9
Hydrated Lime
Coal
Figure 5. LIMB technology for SO2 control.
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Improved Technology for Environmental Protection
Based on these two demonstrations, LIMB technology
appears to be cost-effective for lower SO2 control
requirements compared to conventional wet FGD with
decreasing coal sulfur, boiler size, and plant life expect-
ancy. Figures 6 and 7 show the capital and annualized
costs of a 300 MWe LIMB retrofit system firing 1.7
percent sulfur coal contrasted with the cost of some
competing technologies.10
Technology
Figure 6. Capital cost of SO, control.
Technology
Figure 7. Annualized costs of SO2 control technology.
ADVACATE
The ADVACATE technology (Figure 8) is perhaps the
most competitive with conventional technology, offering
comparable (90+ percent) SO2 control at lower capital
and annualized costs, also shown in Figures 6 and 7. To
date, ADVACATE has been evaluated on a 10 MWe
prototype, and demonstrations on a commercial scale
are planned in the U.S. and overseas. The ADVACATE
Figure 8. ADVACATE process for SO2 control.
process was co-developed by APPCD with the Univer-
sity of Texas and is currently licensed for worldwide
use.11'12
E-SOx
The E-SOx technology (Figure 9) combines improved
electrostatic precipitation technology with conventional
spray drying FGD techniques to provide SO2 and dust
capture in one unit. E-SO has been field evaluated on a
Water
Electrostatic Precipitator
Coal
Figure 9. E-SO technology for SO. control.
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Improved Technology for Environmental Protection
5 MWe basis and is currently scheduled for installation
on two commercial-scale power plants in Russia in
1994-97.13 Figures 6 and 7 illustrate the low capital and
annualized costs of E-SOx. While capable of 50-60
percent SO2 control on U.S. precipitators, the larger
space available in eastern European electrostatic
precipitators affords the chance for 70 percent and
greater SO2 removal using E-SOx.
FUTURE PLANS
With the decreased emphasis of SO2 control and more
emphasis on control of toxic pollutants, acid gases, and
nitrogen oxides, APPCD is focusing the experience,
facilities, and resources acquired through two decades
of SO2 control research toward multipollutant control
technologies. As a cooperative effort, the Gas Cleaning
Technology and Combustion Research Branches of
APPCD are jointly pursuing a number of interrelated
control technology research activities including:
• Polychlorinated dibenzo-dioxin and -furan
(PCDD/PCDF) control by sorbent injection.
• Mercury control by sorbent injection.
• NOx absorption mechanisms.
• Metals control in combustion and post-combustion
flue gases.
• Hybrid SOx/NOx control system development.
• Fine particle control.
These activities are being pursued through a combina-
tion of Federal, State, and private funding with the goal
of reducing the overall cost of emission control for major
combustion sources by customizing sorbent materials
used for gas absorption and optimizing the absorption
process such that the majority of pollutant gases,
vapors, and particles are removed in integrated pro-
cesses.
Several demonstrations of low cost retrofit SO2 control
concepts are still to be performed in the 1990s--most
notably E-SOx and ADVACATE in Third World
countries-through the sponsorship of agencies such as
The World Bank and U.S. Agency for International
Development and new EPA programs such as the
Environmental Technology Initiative.
For more information contact:
David G. Lachapelle
APPCD (MD-4)
U.S. EPA
Research Triangle Park, NC 27711
Phone: 919-541 -3444 Fax: 919-541 -2382
REFERENCES
1. Federal Register. Standards of Performance for
New Stationary Sources, 36:247, Part II, December
23, 1971.
2. Federal Register. Standards of Performance for
New Stationary Sources, 44:133, Part II, June 11,
1979.
3. "Monthly Energy Review," Energy Information
Administration, DOE/EIA-0035(94/01), January
1994.
4. Curran, T et al., "National Air Quality and
Emissions Trends Report, 1992," Office of Air
Quality Planning and Standards, U.S. Environmen-
tal Protection Agency, EPA-454/R-93-031 (NTIS
PB94-146669), October 1993.
5. Nizich, S., "National Air Pollutant Emission Trends,
1900-1992," Office of Air Quality Planning and
Standards, U.S. Environmental Protection Agency,
EPA-454/R-93-032 (NTIS PB94-152097), October
1993.
6. Hance, S.L., R.S. McKibben, and F.M. Jones, "Utility
FGD Survey January-December 1988," ORNL/Sub/
86-57949, U.S. Department of Energy, September
1991.
7. FGD Installations on Coal-fired Plants, IEA Coal
Research Limited, London, June 1994, pp. 36-42.
8. Nolan, P.S., T.W. Becker, P.P. Rodemeyer, and E.J.
Prodesky, "Demonstration of Sorbent Injection
Technology on a Wall-Fired Utility Boiler (Edgewater
LIMB Demonstration)," EPA-600/R-92-115 (NTIS PB
92-201136), June 1992.
9. Clark, J.P., et al., "Performance Results from the
Tangentially Fired LIMB Demonstration Program at
Yorktown Unit No. 2," in Proceedings: 1993 SO2
Control Symposium, Vol. 2, EPA-600/R-9-95-015b,
(NTIS PB95-179230), pp. 44-1-12, February 1995.
10. Princiotta, FT and C.B. Sedman, "Technological
Options for Acid Rain Control," presented at the
Electric Utility Business Environment Conference,
Denver, CO, March 17,1993.
11. Hall, B.W. etal., "Current Status of the ADVACATE
Process for Flue Gas Desulfurization." JAWMA 42.
103(1992).
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Improved Technology for Environmental Protection
12. Sedman, C.B., B.K. Gullett, W.P. Linak, and N.
Plaks, "EPA's New Clean Air Technologies and
Opportunities for Cooperative Development,"
presented at the Conference on Environmental
Commerce, CONEC '93, Chattanooga, TN, October
17-20, 1993.
13. Redinger, K.E. et al., "Results from the E-SOx
5-MWe Pilot Demonstration," in Proceedings: 1990
SO2 Control Symposium, Vol. 4, EPA-600/9-91-015d
(NTIS PB91-197244), pp. 7A-71-89, May 1991.
METRIC EQUIVALENTS
For the reader's convenience, two nonmetric units are
used in this document, short tons and pounds, per
million British thermal units.
To convert to the metric system, readers more familiar
with that system should use:
Ib/mm Btu x 0.43 = kg/GJ, and
short ton x 0.907 = metric ton.
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