United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-86/021 Aug. 1986
4>EPA Project Summary
Catalytic Combustion
Component and System
Prototype Development
J. P. Kesselring and W. V. Krill
The report gives results of a project
to develop the components required for
catalytic combustion system operation
and evaluation. The systems investi-
gated (firetube boiler, watertube boiler,
and gas turbine), when integrated with
the catalytic combustor, have potential
for both significant reductions in NOX
emissions and increases in system ther-
mal efficiency. The model gas turbine
combustor incorporated a multiple
spray nozzle fuel injector, an opposed
jet igniter, and optical pyrometer for
temperature control, and a graded cell
catalytic combustor. The firetube
boiler burner used a matrix of ceramic
fibers vacuum-formed into a cylinder,
and was successfully retrofitted into a
25-hp boiler. The radiative catalyst/
watertube boiler was tested as a proto-
type for small industrial boilers with the
catalyst tube either concentric with and
surrounding the watertube, or sur-
rounded by external watertubes. Sig-
nificant reductions in NOX emissions
were noted in all three systems. Addi-
tional work included both catalyst and
substrate development for increased
durability in reactors, as well as com-
pletion of a successful 1000-hour cata-
lytic combustor test under lean com-
bustion conditions comparable to gas
turbine combustor operation.
This Project Summary was devel-
oped by EPA's Air and Energy Engineer-
ing Laboratory, Research Triangle Park,
NC, to announce key findings of the re-
search project that is fully documented
in a separate report of the same title
(see Project Report ordering informa-
tion at back).
Introduction
The U.S. EPA has sponsored research
and development work in the area of
catalytic combustion for over 8 years.
This work has focused on the develop-
ment of catalytic combustion technol-
ogy by developing a thorough under-
standing of the operation of catalytic
combustors, screening many catalyst
and substrate materials to determine
performance limitations, and then
combining the basic understanding of
the catalytic combustor with catalyst
performance data and heat transfer
considerations to design practical com-
bustion systems. These systems have
the potential benefits of lowered emis-
sions of NOX and CO and increased ther-
mal efficiency. Composite systems
tested and described here include a
model gas turbine combustor, a fiber
burner installed for retrofit or new fire-
tube boiler applications, and a model
watertube boiler. The testing of these
systems was accompanied by durability
testing of the burners.
Catalyst Development
Catalyst development is one of the
keys to the commercial acceptability of
the catalytic combustor. Prior to the de-
velopment of the catalytic combustor
there was little need for catalysts capa-
ble of operation at temperatures above
1400K for extended periods of time, and
little information was available on the
composition of high temperature cata-
lysts or on suitable substrate carriers for
the catalysts. A total of 12 noble metal
and 20 metal oxide catalysts were
tested to determine catalyst lightoff
temperature, preheat temperature re-
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quired for sustained catalyst operation,
maximum throughput for the combus-
tor without extinguishing the catalysts,
and emissions performance of the com-
bustors. The typical test time for each
catalyst was 50 hours, and all of the
noble metal catalysts tested showed
significant decreases in activity during
this period. However, high heat release
rates were still achievable at the end of
the test period.
Two of the noble metal catalysts were
tested to evaluate the performance of
the graded cell catalytic combustor con-
cept, developed here and described in
U.S. Patent 4,154,568. As predicted, the
graded cell reactor was able to sustain
stable combustion at volumetric heat
release rates 2.7 times greater than
those achieved with a single cell reac-
tor. In addition, the graded cell reactor
had a much more uniform axial temper-
ature profile through the honeycomb
monolith than the single cell reactor.
While metal oxide catalysts generally
have lower activity than noble metal
catalysts at temperatures of 1400K, they
should have greater durability than no-
bile metals and good activity at temper-
atures above 1600K. In fact, at high
enough temperatures uncoated ceramic
monoliths should have catalytic proper-
ties. Test times for the oxide catalysts
investigated were again on the order of
50 hours, and high heat release rates
were generally obtained at test condi-
tions of 1700K bed temperature. How-
ever, interactions between the coated
metal oxide catalysts and the substrate
ceramic materials usually led to poor
thermal shock characteristics and frac-
ture of the substrate material. Since the
weakened substrate was a result of cat-
alyst/substrate interaction, it appeared
that testing of substrate materials with
an active component as an integral part
of the substrate would be of interest.
Eight of these "active monolith" materi-
als were prepared as either bundled
tubes or drilled disks and combustion
tested. All test specimens showed ac-
ceptable catalytic activity, with combus-
tion data similar to the coated oxide sys-
tems tested. Thermostructural promise
was also shown by these materials, but
better fabrication techniques (such as
extrusion) are required before futher
testing should be done.
Fundamental Studies
Fundamental studies were initiated
under this program to provide greater
understanding of the chemical proc-
esses of catalytic combustion. Develop-
ment of this information increases the
ability to predict combustor perform-
ance and allows analyses leading to
system optimization. The phenomena
studied here include a determination of
heterogeneous hydrocarbon and nitro-
gen oxidation rates, a comparison of the
extent of reaction with predicted mass
transfer rates, and a determination of
heterogeneous oxidation rate con-
stants.
In order to isolate surface reaction
events from the simultaneous gas-
phase reactions that occur in practical
high-temperature catalytic combustors,
the test setup incorporated catalyst-
coated cylinders in crossflow. Exhaust
gas was analyzed with continuous ana-
lyzers for CO, CO2, O2, NOX, and HC.
Tests were run over a range of stoi-
chiometries from fuel-rich to fuel-lean.
Test results showed that the use of
cylinders in crossflow was an effective
technique for studying the heteroge-
neous portion of catalytic combustion.
In the higher temperature ranges of op-
eration (near stoichiometric), the sur-
face reaction rate on the cylinder in
crossflow configuration is limited by
lean reactant diffusion. In this regime,
the combustion of methane was pro-
moted to completion by the catalyst
with C02 as the major product. These
data are most useful for understanding
normal, steady state operation encoun-
tered in honeycomb monolith catalysts.
They will allow further optimization in
the amount of surface area required for
a specific energy release rate.
The measured conversion rates were
significantly underpredicted by calcu-
lated lean reactant mass transfer limita-
tions. This may be due to the use of a
mass transfer coefficient which does
not fully account for freestream turbu-
lence levels or high surface tempera-
tures. Regions of flow stagnation or re-
circulation, possibly attached to the
cylinders, may also promote higher
conversion rates. However, other data,
including CO formation behavior, do
not clearly support this concept.
Small amounts of hydrogen in the re-
actant flow gave precise catalyst sur-
face temperature control over ranges in
which methane and propane heteroge-
neous kinetics could be quantified. The
experimentally measured activation en-
ergies were 7.54 x 107 J/kg-mole for
methane and 7.42 x 107 J/kg-mole for
propane. These values compare favor-
ably with those reported by others.
Also, the relative magnitudes agree
with experience in which propane igni-
tion occurs earlier and at a lower tem-
perature and is more difficult to blow
out than methane on a given catalyst.
This surface reaction rate information is
required for efficient design of catalyst
entry regions. The Nusselt number
(Sherwood number for mass transfer) is
large in these or other zones where flow
stagnation may occur, giving high mass
transfer rates. Additionally, kinetic in-
formation is necessry to predict lightoff
characteristics and maximum through-
put capabilities before catalyst blowout.
Gas Turbine Combustor
The model gas turbine combustor in-
corporated a multiple spray nozzle fuel
injector for No. 2 diesel fuel, an opposed
jet igniter, an optical pyrometer for tem-
perature control, and a graded cell hon-
eycomb catalytic combustor. The sys-
tem was successfully tested at
pressures from 1 x 105 to 5 x 105 Pa (1
to 5 atm), a combustor outlet tempera-
ture of 1700K, combustor inlet tempera-
tures of 589 to 672K, and a maximum
heat release rate of 4.78 x 105 J/hr. NOX
emissions obtained with this system
were typically less than 15 ppm and re-
sulted primarily from the nearly 100 per-
cent conversion of the small amount of
nitrogen in the liquid fuel. Combustion
efficiencies above 99.985 percent were
obtained at all test conditions.
Separate durability testing of a
terbium-cerium-thorium (TCT) honey-
comb catalyst which used a noble metal
entrance segment for low temperature
lightoff on propane showed successful
performance for over 1000 hours at a
pressure of 10s Pa (1 atm) and an inlet
velocity of 13.4 m/sec. Emissions at the
end of the test period were 2 ppm NOX,
17 ppm CO, and < 1 ppm HC. The test
was concluded with five successful on/
off cycling tests.
Firetube Boiler Burner
A 1055-MJ/hr fiber burner was in-
stalled in a York-Shipley 245 kW (25 hp)
firetube boiler. The fiber burner, a ma-
trix of ceramic fibers vacuum-formed
into a cylinder, radiates flamelessly to
the firetube wall during operation. Fol-
lowing preliminary tests of the boiler
with conventional burner and paramet-
ric tests of the fiber burner, the system
was operated for over 1000 hours at 100
percent load and 15 percent excess air.
At these conditions, emissions were 13
ppm NOX, 29 ppm CO, and 10 ppm HC.
This represents a decrease in NOX emis-
sions of almost 80 percent and is cou-
pled with an efficiency increase of
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nearly 2 percent. In addition, because
the fiber burner operates at lower ex-
cess air than the conventional burner
and is capable of more uniform radiant
energy transfer, the boiler with fiber
burner could be successfully operated
at 120 percent of load and operated in
an extremely quiet manner.
Radiative Watertube Boiler
A radiative catalyst/watertube boiler
was developed and tested as a proto-
type for small industrial boilers. Sys-
tems, both with the catalyst tube con-
centric with and surrounding the
watertube, and surrounded by external
nonconcentric watertubes, were tested.
The catalyst tube consisted of a catalyst-
coated ceramic cylinder, while the
watertube was a concentric cylinder ar-
rangement that operated on natural cir-
culation of the water and steam.
The most successful system tested
used concentric catalyst and watertubes
and was capable of operation to 1319
MJ/hr with thermal NOX emissions less
than 20 ppm at excess air levels greater
than 25 percent. A problem arose when
both concentric and nonconcentric sys-
tems had ceramic tube fractures after
several hours of operation. Further
work to minimize this problem will be
required.
Conclusions
The results of this program as de-
scribed above, including a large body of
data on individual catalyst performance
and fundamental behavior of catalytic
combustors, have brought the commer-
cial application of catalytic combustors
to certain systems much closer. The use
of ceramic fiber burners in gas-fired
equipment with a water-backed heat re-
ceiver, as for domestic water heaters
and firetube boilers, appears to require
only successful field demonstration be-
fore full-scale commercialization efforts
can begin. Further laboratory demon-
stration of honeycomb catalysts used in
gas turbine systems is required, but
major advances have been made under
the current program. The use of cata-
lytic combustors in watertube boilers
requires significant effort to eliminate
ceramic tube failure and flameholding
problems prior to field demonstration.
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J. P. Kesselringand W. V. Krill'(bothpresently withAlzeta Corp.) were withAcurex
Corp., Mountain View, CA 94042.
Jon E. Haebig is the EPA Project Officer (see below).
The complete report, entitled "Catalytic Combustion Component and System
Prototype Development," (Order No. PB86-211 380/AS; Cost: $28.95, 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 f
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S7-86/021
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