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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