United States
Environmental Protection
Agency	
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-89/003 Oct. 1989
 Project Summary

 Development  of a  Vortex
 Containment Combustor for
 Coal  Combustion Systems

 J. F. La Fond, M. P. Heap, W. R. Seeker, and T. J. Tyson
  Major problems facing the  con-
version of oil - and gas-fired boilers
to coal are derating, inorganic impur-
ities in coal, and excessive pollutant
formation (NOX and SOX). To alleviate
these problems a combustion system
is desired that  has  a high firing
density,  separates and  retains fly
ash,  and is adaptable  to viable
pollution control technologies. The
Vortex Containment Combustor
(VCC) has been designed and tested
with these objectives in mind.
  An extensive literature  review and
the testing  of  two candidate iso-
thermal systems preceded the design
and construction  of a bench-scale
VCC.
  Coal combustion tests were per-
formed on the VCC to evaluate its
performance  in  terms  of  ash
retention  efficiency,  coal  burnout,
combustion  stability,  and slag and
ash deposition.  Results  were very
promising for  both  retention
efficiency and combustion stability.
Fuel injector modifications improved
internal slag deposition  conditions
while maintaining acceptable carbon
burnout levels.
  NOX  control  by  staging  and
reburning technologies was evalu-
ated In the VCC, along with sorbent
injection  for the  control of SO2
emissions. Both staging and reburn-
ing were shown to be effective tech-
niques for reducing NOX emissions in
the VCC.  Improvements in the sor-
bent injection approach are required
to obtain  an  acceptable  degree  of
SO2 reduction.
  Based  on particle  force balance
expressions,  scaling criteria  have
been established for the VCC. Also,
the effect of scaling on system pres-
sure drop and heat release on reten-
tion efficiency has been evaluated.
  The  VCC has performed success-
fully at bench-scale. Evaluation at a
larger scale would  be the next step
toward bringing the VCC concept to
fruition.
  This Project Summary  was devel-
oped by EPA's Air  and Energy Engi-
neering Research  Laboratory, Re-
search Triangle Park, NC, to announce
key findings 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  reduction of U.S dependence on
foreign petroleum products can  be ac-
complished in the near future  only by
increasing the use of coal in existing and
new boilers and furnaces.  Major prob-
lems facing the conversion of oil - and
gas-fired boilers to coal are  coal storage,
boiler derating, inorganic impurities in the
coal, and excessive atmospheric pollutant
formation (NOX and SOX). Derating repre-
sents a particularly imposing obstacle to
retrofitting with coal. To avoid derating a
converted boiler or furnace, a combustion
system is needed which will burn coal in
such a manner that only hot clean gas
will be  introduced  into the  furnace
volume. Such a system will also reduce
the frequency of convective pass soot
blowing and gas house cleaning. To meet
the needs of  such  a coal combustion
system, the following characteristics are
desired: high firing density,  the ability to
separate and retain  fly ash,  and an
adaptability to  viable pollution  control

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technologies. The Vortex Containment
Combustor (VCC)  has been  developed
with the objective of providing a system
which possesses these characteristics.

Research Approach
  The research approach to the develop-
ment  of  advanced chambers for coal
combustion has been carried out in three
phases: (1)  a  literature  review to deter-
mine the status of conventional cyclone
combustion  and  advanced  vortex
systems  not yet applied to combustion;
(2)  isothermal  testing  of combustor con-
cepts to determine potential candidate
systems and to investigate critical design
parameters; and (3) bench-scale combus-
tion and  pollution control testing to
determine the  feasibility of an advanced
vortex  coal  combustor. In  addition to
these three major areas of effort, scaling
criteria have been developed in terms of
ash retention  efficiency  performance,
allowable  pressure  drop,  and effect of
heat release in preparation  for scale-up to
a larger system.

Concept Screening
  The  literature  review revealed  that
relatively little  fundamental research has
been  reported  on the  development of
cyclone combustors. Based on available
literature,  a classification scheme has
been  proposed based  on  chamber
geometry, aerodynamic  flow pattern, and
the dominant mode of burning. There are
four generic classes of cyclone  com-
bustors:  conventional, reversed-flow,
symmetric  double  vortex,  and  free-
burning. Over  a decade of research has
been  carried  out  on  advanced  vortex
devices by the Department of Defense at
Wright-Patterson  Air  Force Base.  The
application of the Air Force work was on
low-pressure particle  separators  for use
on  turbine-powered vehicles, high pres-
sure systems  for colloidal core nuclear
reactors, and  vortex mixing for thrust
augmentation.  Performance criteria for
these  vortex  systems  were similar to
those  of  a  Vortex Containment Com-
bustor; i.e., separate and retain fine par-
ticulate matter; avoid  wall deposition;
obtain  high  vortex  efficiencies  with
minimal  input kinetic  energy;  and
maintain a  uniform  cloud of  particles
within  a volume. Guided by the work
done at Wright-Patterson,  two candidate
systems were  conceived  and proposed
as  potentially  successful  coal combus-
tors: a double-exhaust symmetric device,
and a reversed-flow chamber.
  Isothermal testing of the two candidate
systems in plexiglas prototypes led to the
adoption of  the  reversed-flow vortex
chamber for further investigation. Partic-
ulate retention efficiency was extremely
high (96-97% for  coal  smaller than 30
urn),  and  particle  residence times
appeared to be sufficient to support free-
flight  coal  combustion. A  parametric
study revealed that  the distribution of
inlet air, exhaust extension  into  the
chamber, and the location and method of
coal injection  are crucial to obtaining  high
retention efficiencies.

Bench-Scale VCC Design
  The development of  the reversed-flow
vortex chamber into   a  Vortex Con-
tainment Combustor next involved the
design and  construction  of a bench-scale
coal combustor.  Coal is injected into an
"active zone" within the combustor and is
held there by centrifugal forces until the
coal particles devolatilize and burn.  The
resulting fly ash  particles are selectively
removed aerodynamically and deposited
in a slag cone. The centrifugal force  field
holds the  large  coal  particles in  the
combustion zone until combustion is
complete.
  The VCC was designed as a low-heat-
loss system  at firing  densities com-
parable  to  conventional cyclone com-
bustors. Multiple layers of refractory were
used to  maximize inner wall temperatures
for  slag removal.  The design is modular
to  facilitate  internal  inspection  and
system repairs or design modifications.

Test Results
  To  establish  the performance of the
VCC,  the following were measured: ash
retention efficiency based on the amount
of escaping ash;  system pressure drop;
exhaust CO  concentration; carbon  con-
tent of the slag and exhaust paniculate
matter;  exhaust  gas  and refractory
temperature;  and ash/slag deposition on
the refractory walls within the combustor.
The retention efficiency  results for a
variety of coal types, sizes,  and coal-to-
natural-gas firing ratios were determined.
For all  cases when  the total firing  load
exceeded 150,000 Btu/hr, the ash reten-
tion efficiency  was  over 94%.  The
system  pressure drop was held under 3
in.  H2O. The carbon burnout  was  very
high, with zero carbon content in the slag
and typically  less than  3% carbon in the
fly  ash  (>99.9%  combustion efficiency).
Some undesirable wall deposition  was
observed initially,  but  was  substantially
reduced by a few simple injector modi-
fications.
  The VCC  was also tested for its
adaptability  to  conventional  pollution
control methods.  Both  staging  an
rebuming  combustion techniques wei
tested  and shown to be effective  mear
of reducing NOX emissions from the VC<
At a stoichiometric ration of 0.8, tr
reduction  of NOX  was  30 and   40")
respectively, for staged  and reburnir
conditions. Attempts to reduce S02 wei
also undertaken  by  injecting  sorbei
materials  into  both the  exhaust  streai
and the combustion region. The  result
however, were not as successful  as tt
NOX reduction techniques, with  typic
reduction  of  only  10% S02  whe
injecting at a Ca/S of 2. It is felt that, wi
conscientious design of the VCC exhaus
successful sorbent  injection  can t
achieved to reach acceptable S02  redu
tion levels.

System  Scaling
  Scaling  criteria  have been establish*
for  the VCC  based on  desired  partic
retention  efficiency  and acceptab
system pressure drop. When scaling up
system by a volumetric factor, S, partic
force balances reveal  that the volumeti
firing rate  must be held in proportion
the volumetric scale  factor.  In  Stok<
regime, the critical  particle  cutoff
defined by:
where:
   rp = particle radius,
   li = viscosity,
   r = radial position of particle.
   V,  = radial velocity of air flow,
   Pp = particle density, and
   YO = tangential velocity of particle.

   When  all  linear dimensions
increased by a factor of S1/3:
               Ğr
where:
   rp,s = critical particle padius of seal
   up system,
   S =  volumetric scaling factor, and
   QI = inlet volumetric rate.

  When scaling  up  in  this  fashio
however, the system pressure drop
expected to rise by a factor of S273. Thi
available  static  pressure will  be  t
limiting factor to system  scale-up. T
effect of heat release on the VCC wh

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considering  these same particle  force
balances is to lower the particle retention
efficiency. Typically, the critical partide
size cutoff will increase  by a  factor  of
approximately 8 compared to an isother-
mal system.

Summary
  Test results  of the VCC at the bench-
scale are encouraging.  Future  work on
the VCC  concept could include  more
detailed isothermal  testing  to determine
the accuracy of  the  proposed scaling
criteria. Also,  more extensive sorbent
injection  tests  are  required  to
successfully reduce S02  emissions from
the VCC. Scale-up of the VCC to a larger
system and engineering/economic  anal-
ysis could help determine the feasibility
of the VCC as an oil- and natural gas-to-
coal conversion device.

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J. F. La Fond, M. P. Heap,  W.  R. Seeker,  and T. J.  Tyson are with Energy and
 Environmental Research Corp., Irvine, CA 92718-2706.
W. Steven Lanier is the EPA Project Officer (see below).
The complete report, entitled "Development of a Vortex Containment Combustor
 for Coal Combustion Systems," (Order No. PB 89-180 921/AS;  Cost: $21.95,
 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:
        Air and Energy Engineering 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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