Transient control of carbon monoxide with staged PrOx reactors Page: 2 of 31
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Transient Control of Carbon Monoxide
with Staged PrOx Reactors
Michael A. Inbody, Rodney L. Borup,
and Josd L Tafoya
Fuel Cell Team
Los Alamos National Laboratory
MS J-576, P.O. Box 1663
Los Alamos, NM 87545
Introduction
Fuel Processor systems generate hydrogen for fuel cell systems
from hydrocarbon fuels such as gasoline for automotive fuel cell
systems and natural gas for stationary fuel cell systems. These fuel
processor systems must remove any contaminants to levels that won't
poison the fuel cell before the outlet hydrogen-rich gas stream can be
used by the fuel cell to generate electricity.
Carbon monoxide is a contaminant that must be removed to
levels of < 100 ppm or < 10 ppm depending on the CO tolerance of
the fuel cell. Typically, the last unit operation in a fuel processor is a
preferential oxidation reactor or a selective oxidation reactor, which
removes CO by oxidizing it to form CO2. These are catalytic reactors
where the catalyst and operating conditions are selected so that the
oxidation rate of the carbon monoxide is higher than the oxidation
rate of hydrogen, even though the hydrogen is present at much higher
concentrations (> 30%) than carbon monoxide which is present at
trace concentrations (< 1%).
Multiple stages of preferential oxidation are used for removal of
CO concentrations from 1-2% to below 10 ppm. Because the CO
and H2 oxidation reactions are exothermic and selectivity for CO
decreases with increasing temperature, achieving high CO
conversions can increase the parasitic loss of hydrogen. Multiple
stages with lower CO conversion per stage can be used to achieve a
higher overall conversion with reduced parasitic loss of hydrogen by
maintaining the catalyst in each stage in a temperature range where it
is more selective for CO oxidation.
Transient control of the fuel processor outlet CO concentration
also is critical for the fuel cell system to generate electric power in
response to changing load demands. Both automotive and stationary
power fuel cell systems will require transient CO control, although
the characteristics of those transients will differ. A power transient is
a change in the total flow through the fuel processor as it responds to
changes in the hydrogen demand of the fuel cell. A composition
transient is a change in the gas composition such as variations in the
CO concentration caused by instabilities or variations in the fuel
processor inlet flows. A key transient for automotive applications is
the startup transient.
The Fuel Cell Team at Los Alamos National Laboratory has
been researching and developing preferential oxidation (PrOx)
technology for the removal of CO for automotive fuel processor
systems. Previous work focused on developing laboratory and
demonstration PrOx reactor hardware for gasoline fuel processing
systems. Recent research has focused on expanding the fundamental
knowledge of the CO removal process through steady-state and
transient experiments conducted on well-characterized laboratory
PrOx reactor hardware. We report here on the response and control
of PrOx reactors to simulated power transients and to a simulation of
a fuel processor startup.
Experimental Approach
PrOx Reactor. The PrOx reactor used in these experiments is
based on a laboratory PrOx reactor design incorporating staged
catalytic adiabatic reactors with interstage heat exchange. In eachstage, air is metered and injected into the primary gas stream from
either a low-temperature shift reactor or a previous PrOx stage. The
main gas stream then passes through a heat exchanger to control the
inlet temperature to the catalyst volume. Gas distribution elements
such as porous foams or frits are used to distribute the flow evenly
across the catalyst inlet. Catalysts are selected based on a desired
operating temperature and inlet CO concentration. This scheme was
implemented in a modular laboratory reactor with interchangeable
catalyst holders so that various catalysts and catalyst supports could
be tested. Lightweight internal components were used to enhance its
transient response.
PrOx Reactor Test Facility. PrOx reactor components were
tested in a facility capable of simulating the outlet stream and
conditions from a fuel processor. The major components of
reformate, hydrogen, nitrogen, carbon dioxide, and water (as steam)
along with carbon monoxide as a trace component, were metered
with mass flow controllers. The reformate flow was heated with
inline gas heaters to simulate the outlet temperatures from a fuel
processor. Fuel processor operating pressures were obtained using a
back pressure regulator. Computer control and measurement of these
functions allowed for simulation of a variety of fuel processor
configurations and transient operating conditions. CO, C02, and CH4
concentrations were measured with NDIR analyzers and 02
concentrations were measured with a paramagnetic 02 analyzer.
Power Transient Experiments. The response of PrOx reactor
components to a simulated fuel processor transient was measured in
both a 4-stage PrOx reactor and in a single-stage PrOx reactor. In the
4-stage reactor, the power transient response and CO control were
complicated by interactions between the stages. To better
characterize the response of PrOx components to power transients, a
PrOx single-stage reactor was subjected to step transients in total
reformate flow. These step transients were between 10 kW and 30
kW (based on the LHV of the H2 flow) in a simulated gasoline
reformate with 37% H2, 28% N2, 17% CO2, 17% H20 and 2000 ppm
CO. Air injection and its timing was varied to investigate the
conversion and control of CO through the transient.
Startup Transient Experiments. A 4-stage PrOx reactor was
used in a set of experiments to investigate the feasibility of using a
PrOx reactor to reduce system startup time by removing high CO
concentrations. A 10 kW (LHV H2) simulated gasoline reformate
flow with 5% CO was heated to 200 "C in bypass around the PrOx
reactor. The flow was then switched to flow through the PrOx. Air
injection flows were started at the same time and were set to achieve
a maximum setpoint temperature at the outlet of each stage. CO
concentrations at the outlet of each stage were monitored by NDIR
analyzers.
Results and Discussion
Power Transient Experiments. Figure 1 shows the CO flow
and air injection flow into the PrOx single-stage reactor through two
cycles of the step transient between 10 kW and 30 kW total flow.
The air injection is programmed to step between the flows that give
the desired CO outlet concentration at the steady-state 10 kW and 30
kW conditions. In this case, the air injection is programmed to lead
the up transient by 1 second and then lag by 1 second on the down
transient. Figure 2 shows the outlet CO concentration for the two
cycles of the step transient. The outlet CO concentration is
maintained below 100 ppm through the transient, which is the current
specification for an automotive fuel processor.
When the air injection is programmed to step coincident with
the step transient, the outlet CO concentration shows peaks above
400 ppm corresponding to the down transient. These peaks probably
result from formation of CO through reverse water-gas shift reaction.Fuel Chemistry Division Preprints 2002, 47(1), xxxx
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Inbody, M. A. (Michael A.); Borup, R. L. (Rodney L.) & Tafoya, J. (Jose I.). Transient control of carbon monoxide with staged PrOx reactors, article, January 1, 2002; United States. (https://digital.library.unt.edu/ark:/67531/metadc933045/m1/2/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.