&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
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