NASA Technical Memorandum 104423
AIAA - 91- 2463
Fuel-Rich, Catalytic Reaction
Experimental Results
R. James Rollbuhler
Lewis Research Center
Cleveland, Ohio
Prepared for the
27th Joint Propulsion Conference
cosponsored by the AIAA, SAE, ASME, and ASEE
Sacramento, California, June 24-27, 1991
NASA
FUEL-RICH, CATALYTIC REACTION EXPERIMENTAL RESULTS
Jim Rollbuhler
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
SUMMARY
An experimental investigation has been conducted involving the partial
oxidation reaction of a fuel-rich gaseous mixture of fuel and air in a cata-
lyst containing reactor. The fuel used was Jet-A and it was vaporized in a
hot air stream before being introduced into the catalytic reactor. There was
limited air in the mixture, the fuel to air equivalence ratio was 3.5 to 7.5,
so that only a partial breakdown and oxidation reaction of the fuel could
occur. The reactor discharge gases were at a maximum temperature of about
1375 K and contained high concentrations of hydrogen (5 to 10 vol %), carbon
monoxide (10 to 15 vol %), and light and heavy end hydrocarbons. The nitrogen
oxide concentration was very low, possibly because of hydrogen reduction.
Various ceramic catalyst mounting techniques were developed to maintain
physical integrity of the monolith pieces over the sequence of test cycles.
INTRODUCTION
A key issue in the development of the next generation high-speed civil
transport is environmental acceptability. Of particular concern is that the
nitrogen oxide (NO x ) emissions, from the aircraft gas turbine engines oper-
ating at high altitudes do not have detrimental effects on the Earth shielding
stratospheric ozone layer. This concern has led to significant development
efforts in atmospheric modeling, combustion analysis, and testing of various
combustor concepts that favor low NO combustion.
X
A concept of interest is staged combustion. A simplified explanation of
this is to have two distinct reaction regions in a combustor, one to initiate
a fuel-air reaction and a second to complete the combustion process. The goal
is to achieve the overall process with only minimum reaction occurring at
stoichiometric conditions where the maximum concentration of NO would be
x
formed. This is shown in figure 1, which is a generalized plot of reaction
temperature and NO
x
concentration as a function of the fuel to air or equiv-
alence ratio (ER). Ideally, the fuel and input air begin reacting in a
fuel-rich concentration, i.e., to the right of the fuel-air stoichiometric
concentration point in figure 1. Because of insufficient combustion air at
the fuel-rich ER, the vaporized fuel will only partially react. The partially
oxidized fuel-air mixture is then quickly mixed with additional air such that
a fuel-lean concentration condition results, at which condition the combustion
of the fuel is carried to completion. The key factor is quick and complete
mixing of the partially reacted fuel-air mixture with more air so that there
is not enough dwell time for stoichiometric combustion to occur.
The program described in this report concerns the first stage reaction
mechanisms required to vaporize and partially react a liquid hydrocarbon fuel
(i.e., Jet-A) with hot incoming combustion air. The technique used to bring
about the partial reaction was exposure of the fuel-air mix to a catalyst.
The catalyst will initiate a controlled reaction at low threshold temperatures
and in theory it has an unlimited life. Previous work involving partial
oxidation using catalysts here at NASA Lewis Research Center is reported in
reference 1.
This program tested a number of different catalyst types and material
configurations. This report details efforts using ceramic, monolith, sub-
strate material that has been commercially coated with platinum or palladium
active compounds. Steady-state testing was done with these catalysts over a
range of combustion air flows, incoming air temperatures, and fuel to air
ratios (ER); all in a flame tube type test reactor. Detailed diagnostic meas-
urements were made of the overall catalyst reactor performance, including the
product discharge gas composition and changing gas temperatures.
In the near future the results of this program will be incorporated into
a two stage combustor to be tested here at NASA Lewis.
TEST FACILITY
This program was carried out in the combustion research laboratory, cell
23, at NASA Lewis. A schematic of the test facility is shown in figure 2.
In order to simulate compressor or ram combustion air coming into a
combustor at high temperatures, the rig input air was passed through a
counter-flow, high temperature heat exchanger. The exchanger was designed to
heat up to 1.0 kg/sec of air to a maximum temperature of 1250 K. This is
accomplished with a single pass of the air through a slowly rotating, porous
ceramic wheel. As the wheel rotates, it passes through a burner exhaust gas
stream in an adjoining chamber separated from the air flow passage chamber by
gas seals on the rotating wheel surfaces. The portion of the wheel heated in
the exhaust gas stream proceeds to rotate into the air passage chamber where
the flow-through air removes the wheel heat as the wheel again enters the hot
exhaust gas chamber. By varying the hot exhaust gas temperature and/or the
wheel rotation speed, different air flow rates can be heated to different
temperatures.
The rig test section is immediately downstream of the heat exchanger hot
combustor air outlet. The liquid test fuel is injected into the hot air
stream using a fuel-air multi-orifice atomizing injector unit. The introduced
fuel was at ambient temperature and its flowrate was determined by the fuel
pump output pressure selected.
The fuel-air mixture then flowed through the
and was then cooled and discharged from the system
only interested in investigating the first-stage,
the product gases were vented to the atmosphere.
high concentrations of hydrocarbons, hydrogen, and
completely combusted in a flare burner at the end
the atmosphere downwind of the discharge stack did
level of pollution.
instrumented test reactor
. Since this program was
fuel-rich reaction process,
Because the gases contained
carbon monoxide they were
of the vent stack. Tests of
not indicate any measurable
2
TEST SECTION
The test section used in this program consist of a number of internally
insulated stainless steel pipe sections bolted together in series. Easy
buildup and teardown of the pieces simplified the installation of catalyst
pieces and post-test inspection of the internal components. The test section
is shown schematically in figure 3.
The hot combustion air, flowing out of the facility heat exchanger,
enters the test section at the upper left in figure 3. The air then flows
through a 19 port, air blast, nozzle unit where the liquid hydrocarbon test
fuel is introduced. The air flowrate has been set at a value between 0.2 and
0.5 kg/sec and is at a temperature of approximately 820, 930, or 1050 K. The
fuel is sprayed through 1 mm orifices, one at each of the 19 ports in the
nozzle unit. The fuel flowrate is varied in discrete quantities between 0.03
and 0.30 kg/sec to obtain the desired test fuel to air ratio. Each of the
fuel injection orifices is surrounded by a space through which cooling air is
injected to prevent the fuel from carbonizing in the orifice passages. This
is a concern because the entire nozzle unit is being heated by the combustion
air flow up to 1050 K. The cooling air flow is about 25 g/sec and this quan-
tity is included in the overall air mass being used to calculate the ER. The
fuel may enter the nozzle unit as a liquid but at injection into the hot air,
its state is a function of the heat transfer from the hot air to the nozzle
mass to the fuel stream. The fuel pressure drop is monitored as a function of
flow rate to determine if passage blockage is occurring and during this
program this did not occur.
After the fuel and air mix in the nozzle unit, there is a residence
volume about three pipe diameters long before the gas mixture enters the
catalytic reactor section. In this residence volume the fuel droplets have a
chance to absorb heat from the air and vaporize. The residence or dwell time
in the vaporization section is 12 to 19 msec depending on the mass flowrate.
By measuring the temperature drop of the fuel-air mixture in this volume, it
is possible to determine the extent of fuel vaporization and subsequent
heatup. The temperatures are measured at several locations in this residence
volume using shielded thermocouple rakes.
The catalyst reaction portion of the test section can accommodate up to
about 25 cm of 15 cm diameter catalyst material. The material has been in the
form of monolithic ceramic or metallic discs which are between 25 and 75 mm in
thickness. The discs look like honeycomb material with from 4 to 100 axial
flow-through passages per square centimeter of disc face surface. When the
reaction section was made with discs with space between them, thermocouples
were inserted into the void gas space.
After the reactor section, about two pipe diameters further downstream,
is a cross-sectional area containing 12 thermocouples and a gas sampling
probe. The thermocouples are equally spaced around the circumference for
determining the reactor product gas temperature pattern.
Downstream of the thermocouple/gas sample probe area, are two observation
windows on either side of the exhaust gas duct. A video camera televises the
gas flow and a digital temperature meter visible through the opposite window.
The temperature reading is that of the gases flowing by the windows. The view
gives an indication of any burning carbon particles in the gas stream.
3
Further downstream is a flow restriction that is used to maintain a
desired back pressure in the test section. The reactor pressure is 125 to
175 kPa depending on the gas mass flowrate. The gas pressure drop through
the reactor is between 4 and 8 percent. The gases after flowing through the
restriction are essentially at ambient pressure. At this point water spray is
used to reduce the exhaust stream gas temperature before the flow is vented.
TEST CATALYSTS
The catalysts tested
were obtained from commercial sources in the form of
15 cm diameter monolithic discs. Specified were the disc thickness, disc
diameter, number of flow-through passages per square centimeter of disc sur-
face, but not the catalyst formulation or technique of application to the
ceramic surfaces. These latter details are usually proprietary to each ven-
dor. The catalyst pieces reported on in this paper were obtained from the
Allied-Signal Company, Industrial Catalyst Division.
Previous testing had resulted in success in using reactor disc pieces
made from nickel foil. The foil had been crimped and then rolled into a disc
shape such that gases would flow between the crimps and adjacent wrapped
around layers. One such disc was used in conjunction with some of the ceramic
discs - either upstream or downstream of the ceramic pieces.
Six configurations were tested and are shown in a schematic axial cross-
sectional view in figure 4. Configurations 3 and 5 used platinum based cata-
lyst applied to the ceramic surfaces; the other configurations had palladium
based catalyst on the surfaces. The nickel foil disc was installed 1.2 cm
after the ceramic pieces for configurations 3 and 15 cm before the ceramic
pieces in configuration 5. Configurations 4 and 6 were tested with only cer-
amic pieces. Configuration 4 had 2.5 cm spaces or voids between the pieces
and configuration 6 had the pieces pressed together. Configurations 12 and 14
made use of discs twice as thick as the previously used pieces. In addition
configuration 12 had a nickel foil disc 5.5 cm downstream of the ceramic.
Configuration 14 consisted of three double thick pieces made of a foamed
ceramic material.
Initially the catalyst discs were sized to slide into the reactor section
with a tight fit. Metallic clamping rings were installed upstream and down-
stream to make sure the pieces did not move. After suffering physical deg-
radation of the ceramic pieces using this technique, a better method was
eventually determined for cushioning the pieces from the reactor wall and from
each other. This made use of high temperature ceramic cloth tape and roping.
An example of this latter mounting technique is shown in figure 5. The tape
has been wrapped around each disc outer edge several times to fill any void
space between the disc and the reactor wall. The ceramic roping is placed in
a single wrap on the outer edge to insure the ceramic pieces do not touch each
other when pressed into the reactor.
TEST INSTRUMENTATION
Gas temperatures and pressures were measured before and after the fuel-
air mixing nozzle; before, in, and after the catalyst reactor; and in the
downstream exhaust and vent system. These temperatures and pressures, along
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