United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-81-150 Oct. 1981
Project Summary
Feasibility of Coal Burning in
Catalytic Combustors
P. M. Goldberg, E. K. Chu, and J. T. Kelly
The results of this study show that
pulverized coal can be burned in a
catalytic combustor. Pulverized coal
combustion in catalytic beds is
markedly different from gaseous fuel
combustion. Gas combustion gives
uniform bed temperatures and reaction
rates over the entire bed length and,
depending on flow conditions and bed
geometry, little combustion may
occur downstream of the bed. For the
bed configurations, fuel supplies, and
test conditions studied, pulverized
coal combustion results in significant
temperature and reaction gradients
over the bed length and substantial
combustion downstream of the bed.
Thus, for pulverized coal combustion,
the bed acts mainly as an initiator and
stabilizer of combustion. A significant
portion of the combustion process,
primarily that associated with char
burnout, occurs downstream of the
bed.
This Project Summary was devel-
oped by EPA's Industrial Environ-
mental Research Laboratory. Research
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).
Test results show that the coal was
substantially devolatilized by the com-
bustor for all air/fuel ratios tested. Exit
gas composition results show that the
extent of reaction of the coal is near the
theoretical limit, given the available
oxygen. Increased bed velocity brings
the extent of reaction closer to the
theoretical limit.
Catalytic bed operating limits are
bounded by bed material upper temper-
ature limits, lower temperature volatili-
zation limits, thermal mass air/fuel
ratio limits, and particle deposition
preheat limits. Presently available bed
materials limit the maximum bed
operating temperature to 1900K. Based
on the lower temperature results of this
study and known relationships between
temperature and coal volatilization
histories, it appears that 1300K is the
minimum bed operating temperature
needed to sustain steady combustion.
Above an air/fuel ratio of 6.0, bed
temperatures decrease with time. This
air/fuel ratio roughly corresponds with
the stoichiometric air needs for only the
coal volatiles. Increases of air flow
above this level probably yield thermal
mass loads too high for maintenance of
steady state combustion conditions.
Front-end bed and air preheat temper-
atures must be less than 500K to avoid
coal deposition on the front of the bed.
Above 500K, char deposits rapidly build
up on the front of the bed and plug
channels. These deposits can be burned
off by operating the bed on propane fuel
under excess air conditions. However,
after several bed cleaning cycles,
sufficient ash builds up to plug bed
channels. This deposition of char and
ash in the bed was the major problem
identified in burning pulverized coal in a
catalytic combustor. Therefore, low air
preheat temperatures and cooled pre-
bed sections which prevent prebed
combustion are required to reduce the
potential for bed char deposition. During
this study, bed channel plugging reduced
the useful life of the beds to about 6
hours.
Degradation of the catalyst by chemi-
cal attack was not as severe a problem
as deposition. This was confirmed by
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propane tests before and after coal
combustion. These tests showed that
previously attained gas combustion
conditions could be nearly duplicated
following a significant amount of coal
combustion testing.
Comparison of catalytic coal combus-
tion with conventional premixed coal
combustion indicates that the presence
of the catalytic bed significantly improves
system volumetric heat release based
on volatiles. This characteristic of the
catalytic combustor could be beneficially
used to reduce combustor size for a
given volatiles heat release. As indicated
previously, coal combustion is only
initiated in the limited lengths of the
catalytic combustors tested. Therefore,
provision for downstream-of-bed coal
burnout must be incorporated into any
system design. This additional coal
burnout volume will be a significant
fraction of the total combustion volume.
NO emissions for the catalytic com-
bustor are high even under fuel-rich
conditions. The NO formed early under
locally lean conditions does not have
sufficient time within the combustor to
decay under oxygen deficient conditions.
No clear trend of NO emissions with
extent of prebed combustion could be
observed from the data. The effect of the
extent of combustion before the bed
versus combustion within the bed is
within the fairly large data scatter for
these tests. Subsequent stages with
longer residence times are needed to
decay NO concentrations to low levels.
As expected, NO levels increase with
air/fuel ratio and decrease with in-
creases in residence time.
As indicated previously, the coal-fired
catalytic combustor may have some
benefit as a high heat release rate, very
fuel-rich, initial flameholder which is
followed by a non-catalytic coal burnout
section. This type of system could have
heat release, stability, and emission
benefits. However, for this system to be
viable, the bed deposition and channel
plugging problems must be solved.
Recommendations to help solve these
problems include:
• Use microsized coal supplies (~1
fjm size range) to minimize bed face
impact and sticking. The smaller
particles would tend to follow gas
streamlines and thereby avoid
impacts with solid surfaces over
which the gas is flowing. Coal
supplies could also be blended with
oils to widen the operating range of
the system. The coal could be
ground with the oil and fed into the
system as a slurry. This would
eliminate grinding and feed prob-
lems associated with particles of
this size range.
• Employ longer beds with actively
cooled front sections to minimize
front-end deposition. With a longer
bed, the front end could remain
cool while the back end is hot for a
considerable distance. This should
improve extent of reaction and
stability characteristics.
• Use thin webbed beds to minimize
bed front-surface area off of which
coal can be scattered. Also, use
round channels to minimize slow
moving zones where coal particles
can burn and then stick to adjacent
surfaces.
These actions might improve operating
characteristics and deposition problems
considerably. Once this problem is
alleviated, a more extensive test program
could be undertaken to make a more
complete assessment of the perform-
ance and emissions benefits of burning
coal in a catalytic combustor. For
example, complete test systems could
be constructed which would consist of a
rich-burn catalytic flameholder, similar
to those tested herein, plus a conven-
tional burnout section in which addi-
tional combustion air is added and heat
is extracted. Results achieved with this
system could then be compared to
existing staged and unstaged premixed
pulverized coal results and conventional
diffusion flame pulverized coal burners
to determine more completely the
performance and emissions benefits, if
any, of this type of system.
P. M. Goldberg, E. K. Chu, andJ. T. Kelly are with Acurex Corporation, 485 Clyde
Avenue, Mountain View, CA 94042.
G. Blair Martin is the EPA Project Officer (see below).
The complete report, entitled "Feasibility of Coal Burning in Catalytic Com-
bustors," (Order No. PB 82-102 328; Cost: $9.50. 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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