&EPA
United States
Environmental Protection
Agency
Industrial Environmental Researc
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-81-090 July 1981
Project Summary
Evaluation of Emissions and
Control Technology for
Industrial Stoker Boilers
Robert D. Giammar, Russell H. Barnes, David R. Hopper, Paul R. Webb, and
Albert E. Weller
This report presents the results of a
3-phase program to evaluate
emissions and control technology for
industrial stoker boilers. In Phase I,
emission characteristics were deter-
mined for a variety of coals fired in a
200-kW stoker boiler. It was observed
that significant amounts of sulfur
were retained in the lignite and
western subbituminous coals. Fuel
nitrogen conversion to NO was found
to be between 10 and 20 percent. In
addition, a limestone/coal fuel pellet
was developed and found effective in
capturing 80 percent of the fuel sulfur.
Phase II focused on identifying and
evaluating potential control concepts.
An 8-MW spreader stoker boiler was
used. It was found that improved
control of combustion air, that is
underfire and overfire air, resulted in
lower excess air operation (improved
efficiency), reduction in particulate
loading, smoke, CO and NO
emissions, and had no effect on SOz
levels. The limestone/coal pellet
(Ca/S = 7) was successfully fired
achieving 75 percent SOz reduction.
In Phase III, the limestone/coal fuel
pellet was refined. A pellet was pro-
duced that had physical properties
that could survive an industrial coal-
handling system. This pellet with a
Ca/S molar ratio of 3'/2 was fired in
the 8-MW boiler achieving sulfur
captures of 50 percent. The cost of
this pellet would add approximately
one dollar per million Btu to the cost of
the raw high-sulfur coal.
This Pro/act Summary was develop-
ed by EPA's Industrial Environmental
Research Laboratory, Research Tri-
angle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
The coal-fired stoker boiler provides
an option for industry to meet its energy
needs. This option has not been
exercised by a significant number of
industries primarily because oil- and
gas-fired equipment have been, and still
are, more environmentally and econom-
ically attractive. However, with the
dwindling supplies of oil and gas, the
rising costs of these fuels, and in-
creased attention given to coal utiliza-
tion, industry once again is considering
the coal-fired stoker boiler.
In support of our nation's
commitments to maintain a clean envi-
ronment and to utilize coal, EPA funded
a research and development program to
identify and demonstrate improvements
in stoker-coal firing that can provide an
incentive for greater industrial use of
coal. The overall objectives of this
program were to:
• Characterize the spectrum of
emissions from industrial coal-
fired stoker boilers using several
types ot coai under various stoker-
firing conditions
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• Investigate control methods to
reduce these emissions
• Determine the effect of these
control methods and variations in
stoker-boiler operation on the
overall performance of the stoker
boiler, and
• Assess the environmental impact
of new technology on the future
acceptability of stoker boilers.
This program was divided into three
phases. In Phase I, Alternative Fuels
Evaluation, emission characteristics
were determined for a variety of coals
fired in a 200-kW stoker boiler.
Emphasis was focused on identifying
coals with low pollutant potential, in-
cluding both physically and chemically
treated coals. In Phase II, Control Tech-
nology, potential concepts for control of
emissions for full-scale industrial
stokers were evaluated. In Phase III,
Limestone/Coal Pellet Development, a
limestone/coal fuel pellet was develop-
ed and evaluated as a viable SOaControl
for industrial stoker boilers.
This report presents the results of the
three phases of work. The report is
organized into the following three parts
corresponding to the three phases of"
work:
Phase I. Alternative Fuels Evalua-
tion
Phase II. Control Technology Evalu-
ation
Phase III. Limestone/Coal Pellet
Development
These parts actually represent separate
reports but are included under one
cover.
Phase I.
Alternative Fuels Evaluation
A 200-kW stoker-boiler facility was
used to evaluate characteristics of
emissions from combustion of a variety
of coals, including coals that could not
be conveniently or economically evalu-
ated in larger industrial systems. The
stoker was initially operated in an
underfeed mode to expand the data
base developed in an earlier EPA pro-
gram
111 This facility was modified to
"Giammar, R. D.,etal., Emissions from Residential
and Small Commercial Stoker-Coal-Fired Boilers
Under Smokeless Operation. EPA-600/7-76-029,
USEPA, Washington, DC 20460, October 1976.
accommodate a model spreader stoker
more typical of an industrial boiler.
Rawcoals with low pollution potential
and treated coals were evaluated.
Because there was only one treated coal
available during the time framework of
the program, Battelle developed, as part
of this program, a limestone/high-
sulfur coal fuel pellet.
Results of the Phase I emission
characterization were as follows:
NO
For the underfeed stoker, less than 10
percent of the fuel nitrogen was con-
verted to NO, assuming no thermal NO.
For the model spreader-stoker, between
10 and 20 percent of the fuel nitrogen
was converted to NO.
SO*
Coals naturally high in calcium and
sodium and those treated with these
elements retained significant
percentages of the sulfur in the ash. For
the eastern bituminous coals, with
relatively small amounts of calcium and
sodium but significant amounts of iron,
sulfur retention in the ash was as high
as 20 percent. Note that bed
temperatures in these laboratory
stokers are significantly lower than
those measured in an industrial stoker.
CO
CO levels can be controlled by the use
of overfire air and were generally less
than 100 ppm.
Paniculate Loading
Particulate loadings did not correlate
consistently with either ash content of
the coal or its size prior to feeding. It
appears that the friability and inherent
moisture content of the coal may affect
particulate loading since these proper-
ties influence the amount of fines
generated.
POM Loadings
POM loadings for continuous opera-
tion of the underfeed stoker were signif-
icantly less than those reported earlier'1'
for intermittent operations.
Particle-Size Distribution
For the model spreader, the average
stack particle size ranged between 15
and 30 micrometers.
Treated Coals
No commercially available, chemica
ly treated coals were identified. Treate
coals required palletization for firing i
stokers.
The Battelle Hydrothermally Treate
(HTT) coal was available for laboratoi
evaluation. The treatment reduced th
fuel sulfur from 2.6 percent to 1.1 pei
cent. Because of the relatively hig
calcium and sodium residual from th
treatment, only 28 percent of th
remaining sulfur was emitted as SO2
Also, the limestone/coal fuel pelle
with a Ca/S molar ratio of 7, reduce
SOa emissions by over 70 percent. Eve
at the elevated fuel-bed temperature
(> 1100°C), the calcium reacts with th
coal sulfur and retains it as a sulfide
sulfate as part of the fuel ash.
Phase II.
Control Technology Evaluatioi
Potential control concepts wer
identified and evaluated in the BattelleI
MWth (25,000 Ib steam/hr) spreade
stoker boiler. Control strategies wen
limited to:
• Use of compliance coals
• Combustion-system operational
modifications
• Minor combustion-system design
modification
• Use of treated coal (limestone/
coal fuel pellet)
Flue-gas clean-up techniques were not
considered. Criteria pollutants were
used as the basis for evaluation.
The Phase II experiments havt
demonstrated that emission levels car
be reduced by proper control of the
stoker operating variables. In addition
the limestone/coal pellets have beer
demonstrated to offer potential for SO:
control. In summary, the major findings
are:
• The limestone/high-sulfur coa
pellet showed a sulfur capture o
about 75 percent for a Ca/S molai
ratio of 7.
• Sulfur capture efficiencies ol
around 25 percent were noticec
with some eastern bituminous
coals.
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High excess air rates at low loads
result in increased sulfur
retention in the bed ash.
Co and smoke levels can be con-
trolled by providing adequate
excess air. CO levels were low for
all fuels tested except the lime-*
stone/coal pellet.
Clinker formation may be a limit-
ing factor in determining the mini-
mum excess air rate.
NO levels increase slightly with
increase in excess air.
Conversion of fuel nitrogen to NO
was between 12 to 20 percent,
assuming no thermal NO.
An increase in overfire air/total
air flow rate ratio reduced CO and
smoke, the latter more signifi-
cantly. Paniculate loadings are
also reduced with increased over-
fire air.
NO is lower for inactive overfire air
jets.
Clinker formation occurs readily if
bed depths become excessive,
while the danger of burning the
grates exists for operation with
very shallow beds. Bed depths
around 6.3 to 7.6 cm appear to be
optimum for low ash coals.
POM levels ranged from 13 to 24
/t/g/Nm3. They were stfmewhat
lower than those of the model
spreader and only slightly higher
than those from a 500 kW
packaged boiler firing natural gas
and fuel oil.
A higher excess air rate is required
for low-load than for partial- or
full-load operation. A greater per-
centage of overfire air is required
at low-load. Low-load smoke can
be reduced by a reduction in
underfire air, coupled with atten-
tive boiler operation.
At full-load, fly-ash reinjection
increased boiler efficiency by 1.5
percent. However, particulate
loadings were reduced by 10 to 25
percent by operating without fly-
ash reinjection.
The high-sulfur Ohio coals had to
be fired at higher excess air rates
than did the low-sulfur Ohio and
Kentucky coals. The high-ash
unwashed stoker coal, and high
moisture Illinois No. 6 coal could
not be fired satisfactorily.
Phase III.
Limestone/Coal Pellet
Development
The Phase III program focused on
refinement of the limestone/coal fuel
pellet and evaluation of its suitability as
an industrial stoker-boiler fuel. This
program consisted of four major tasks.
1. Pellet Development aimed at
developing a fuel pellet with
mechanical strength characteris-
tics that can withstand weather-
ing and the severe stresses of an
industrial stoker coal-handling
and feeding system, burns at
reasonable rates, and captures
sufficient sulfur to be competitive
with other control strategies.
Mechanical strength characteris-
tics were evaluated with standard
laboratory tests. Burning charac-
teristics and sulfur capture were
determined in a fixed-bed reactor
simulating the fuel bed of a
spreader stoker.
2. Process Variables Selection com-
bining a mathematical model
analysis with a series of
experimental studies to develop a
more comprehensive understand-
ing of the processes that influence
the combustion of the fuel pellet
and control the capture of sulfur.
3. Laboratory Evaluations conducted
in both the 200 kWth model -
spreader stoker and the 8 MWth
Battelle steam plant boiler to
evaluate the most promising
candidate pellets.
4. Economic Analysis aimed at
developing pellet process costs.
The major results and conclusions of
the four tasks are:
Pellet Development
• A fuel pellet was produced that,
according to laboratory tests, has
mechanical strength and durabil-
ity characteristics similar to those
of conventional coals.
• Pellets produced by auger extru-
sion or pellet mill processes had
better mechanical strength than
those produced by disc pelleting
or briquetting.
• Binders that provide some resis-
tance to the weather were identi-
fied. However, no binder was
identified that provided complete
weather proofing.
• The fixed-bed reactor experi-
ments indicated a weak depend-
ency between Ca/S' ratio and
sulfur capture for Ca/S ratios
above 2.
• Calcium oxide is a superior
absorbent to limestone, but is not
economically competitive with
limestone.
• Additives do not appear to
enhance sulfur capture.
Process Variables Selection
• The mathematical model predicts
an optimum coal size (35-40 mm
diameter) for maximum sulfur
retention.
• the model indicates a weak
dependence on the calcium/sul-
fur ratio.
• Scanning electron microscopy
and x-ray diffusion are powerful
tools for the study of solid-state
reactions in the pellets. Results
indicate that sulfur is retained
predominantly as CaSO«.
• Sulfur may react directly with
limestone by solid-state
processes without involving the
formation of SO2.
Laboratory Evaluations
• Auger-extruded and milled pellets
burned better than briquets and
disc-agglomerated pellets.
• Sulfur capture of about 65 percent
was achieved at Ca/S molar
ratios of 3.5.
• Sulfur capture of about 50 percent
was achieved in the steam-plant
stoker. In comparison to the model
spreader, this lower S02 capture
was attributed to higher tempera-
tures (in excess of 1300°C).
IU. OOVBMMBIT PHNT1NQ OFFICE: «1 -757-01Z/ 7214
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Sulfur capture appeared to be
weakly dependent on fuel-bed
temperature.
In the Battelle steam power plant,
the fuel pellets burned as well as
low-sulfur coal.
Economic Analysis
• It is estimated that limestone/
coal fuel pellets can be produced
for about $ 15.40/Mg ($14/ton) of
pellets above the costs of the
high-sulfur coal.
Robert D. Giammar, Russell H. Barnes, David R. Hopper. Paul R. Webb, and
Albert E. Weller are with Battelle-Columbus Laboratories, Columbus, OH
43201.
John H. Wasser is the EPA Project Officer (see below).
The complete report, entitled " Evaluation of Emissions and Control Technol-
ogy for Industrial Stoker Boilers," (Order No. PB 81-197 873; Cost: $20.00,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U. S. Environmental Protection Agency
Research Triangle Park. NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
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