U.S. patent application number 10/382446 was filed with the patent office on 2004-09-09 for selective methanation reactor for reducing carbon monoxide in a reformate stream.
Invention is credited to Feaviour, Mark Robert, Rowe, Julia Margaret.
Application Number | 20040175310 10/382446 |
Document ID | / |
Family ID | 31978205 |
Filed Date | 2004-09-09 |
United States Patent
Application |
20040175310 |
Kind Code |
A1 |
Feaviour, Mark Robert ; et
al. |
September 9, 2004 |
Selective methanation reactor for reducing carbon monoxide in a
reformate stream
Abstract
A methanation reactor to reduce carbon monoxide concentration in
a reformate stream. The reactor includes a noble metal catalyst
supported by a ceramic support such that the reactor preferentially
converts of carbon monoxide via methanation over that of carbon
dioxide. In one embodiment, the ceramic support is alumina with a
coating of silica deposited on the alumina to increase the support
surface acidity and consequent carbon monoxide conversion. The
purpose of the abstract is to enable the United States Patent and
Trademark Office and the public generally to determine from a
cursory inspection the nature and gist of the technical disclosure,
and is not to be used for interpreting the scope of the claims.
Inventors: |
Feaviour, Mark Robert;
(Reading, GB) ; Rowe, Julia Margaret;
(Wallingford, GB) |
Correspondence
Address: |
Killworth, Gottman, Hagan & Schaeff, L.L.P.
Suite 500
One Dayton Centre
Dayton
OH
45402-2023
US
|
Family ID: |
31978205 |
Appl. No.: |
10/382446 |
Filed: |
March 6, 2003 |
Current U.S.
Class: |
422/211 ;
422/600; 423/247 |
Current CPC
Class: |
C01B 2203/0445 20130101;
C01B 2203/0405 20130101; B01J 23/464 20130101; B01J 31/0272
20130101; B01J 19/2485 20130101; Y02E 60/50 20130101; B01D 53/864
20130101; C01B 3/501 20130101; C01B 2203/066 20130101; H01M 8/0612
20130101; C01B 2203/0283 20130101; C01B 2203/047 20130101; B01J
21/12 20130101; B01J 23/462 20130101; H01M 8/0662 20130101; C01B
3/586 20130101; C01B 2203/146 20130101 |
Class at
Publication: |
422/211 ;
422/190; 423/247 |
International
Class: |
B01J 008/02 |
Claims
What is claimed is:
1. A methanation reactor comprising: a reformate stream inlet; a
reformate stream outlet in fluid communication with said inlet; a
flowpath disposed between said inlet and outlet; and a
rhodium-based catalyst disposed on a silica-coated alumina support,
said support disposed within said flowpath.
2. A device for the removal of carbon monoxide from a reformate
stream, said device comprising a reactor, said reactor comprising:
a porous ceramic support defining a surface acidity; a coating
configured to increase said surface acidity; a noble metal catalyst
coupled to at least one of said support or said coating; and a
flowpath configured to place said reformate stream in fluid
communication with at least said noble metal catalyst such that
carbon monoxide conversion in said reactor is greater than if said
coating were not present.
3. A device according to claim 2, wherein said reactor is
configured to operate in a temperature regime such that selectivity
for carbon monoxide in said reactor is at least 70 percent and
conversion of carbon monoxide is at least 30 percent.
4. A device according to claim 2, wherein said noble metal
comprises rhodium.
5. A device according to claim 2, wherein said rhodium is present
in said reactor in a concentration of up to approximately two
percent.
6. A device according to claim 5, wherein said rhodium
concentration is approximately one percent.
7. A device according to claim 2, wherein said support is
alumina.
8. A device according to claim 7 wherein said coating is
silica.
9. A device according to claim 2, wherein said reactor is
configured to operate in a temperature regime such that said
selectivity is at least 80 percent and said conversion is at least
30 percent.
10. A device according to claim 2, wherein said noble metal
comprises ruthenium.
11. A device according to claim 2, further comprising a reformer in
fluid communication with said reactor, said reformer configured to
convert raw fuel into said reformate stream.
12. A device according to claim 11, further comprising: a fuel
supply and an oxygen supply, together configured to provide fuel
and oxygen to said reformer; and a fuel cell configured to receive
a hydrogen-rich portion of said reformats stream that has passed
through said reactor.
13. A device according to claim 12, wherein said fuel cell is a
proton exchange membrane fuel cell.
14. A device according to claim 12, wherein said fuel cell system
is part of a vehicle such that said fuel cell is a source of motive
power.
15. A device according to claim 14, wherein said vehicle comprises:
a platform configured to carry said source of motive power; a
drivetrain rotatably responsive to output from said source of
motive power, said drivetrain connected to said platform; and a
plurality of wheels connected to said drivetrain.
16. A device according to claim 2, further comprising a
preferential oxidation system placed in fluid communication with
said reactor, thereby effecting addition carbon monoxide removal
from said reformate stream.
17. A device according to claim 2, further comprising a
palladium-based permeation membrane placed in fluid communication
with said reactor, thereby effecting addition carbon monoxide
removal from said reformate stream.
18. A device for the removal of carbon monoxide from a reformate
stream, said device comprising: a ceramic support; a noble metal
catalyst coupled to said support; and a flowpath configured to
place said reformate stream in fluid communication with at least
said noble metal catalyst, wherein said support, noble metal
catalyst and said flowpath are configured such that while operating
in a temperature regime not in excess of 260 degrees Celsius,
selectivity for carbon monoxide in said reactor is at least
approximately 60 percent.
19. A device according to claim 18, wherein said ceramic is
zirconia.
20. A device according to claim 19, wherein said noble metal
comprises rhodium.
21. A device according to claim 19, wherein said noble metal
comprises ruthenium.
22. A method of delivering fuel to a fuel cell system, said method
comprising: configuring a fuel delivery system to include a fuel
supply and oxygen supply; fluidly connecting a fuel processing
system to said fuel delivery system, said fuel processing system
comprising: a reformer to evaporate a mixture of fuel and oxygen
coming from said fuel delivery system; and a methanation reactor
for the removal of carbon monoxide from a reformate stream produced
by said reformer, said reactor comprising: a porous ceramic support
defining a surface acidity; a coating configured to increase said
surface acidity; a noble metal catalyst coupled to at least one of
said support or said coating; and a flowpath configured to place
said reformate stream in fluid communication with at least said
noble metal catalyst such that carbon monoxide conversion in said
reactor is greater than if said coating were not present;
introducing fuel and oxygen to create a fuel-oxygen mixture;
heating said fuel-oxygen mixture in said reformer such that said
reformate stream is produced; purifying said reformate stream in
said reactor; and transporting a hydrogen-rich portion of said
reformate stream to said fuel cell.
23. A method according to claim 22, wherein said reactor is
configured to operate in a temperature regime such that selectivity
for carbon monoxide in said reactor is at least 70 percent and
conversion of carbon monoxide is at least 30 percent.
24. A method according to claim 22, wherein said noble metal
comprises rhodium.
25. A method according to claim 24, wherein said support is
alumina.
26. A method according to claim 25 wherein said coating is
silica.
27. A method of purifying a methanol reformate stream, said method
comprising: configuring a reformer to evaporate a mixture of fuel
and oxygen, said evaporated mixture defining said reformate stream;
fluidly connecting a methanation reactor to said reformer for the
removal of carbon monoxide from said reformate stream, said reactor
comprising: a porous ceramic support defining a surface acidity; a
coating configured to increase said surface acidity; a noble metal
catalyst coupled to at least one of said support or said coating;
and a flowpath configured to place said reformate stream in fluid
communication with at least said noble metal catalyst such that
carbon monoxide conversion in said reactor is greater than if said
coating were not present; evaporating said mixture of fuel and
oxygen such that said reformate stream is produced; and exposing at
least a portion of said reformate stream to said reactor such that
at least a portion of carbon monoxide is removed from said
reformate stream.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to a device for
purifying feedstock to be used in fuel cells, and more particularly
to material and configuration improvements in a methanation reactor
to facilitate carbon monoxide removal from a reformate stream.
[0002] While conventional power source devices (such as internal
combustion engines, including piston and gas turbine-based
platforms) are well-known as ways to produce, among other things,
motive, heat and electric power, recent concerns about the effects
they and their fuel sources have on the environment have led to the
development of alternative means of producing such power. The
interest in fuel cells is in response to these and other concerns.
One form of fuel cell, called the proton exchange membrane (PEM)
fuel cell, has shown particular promise for vehicular and related
mobile applications. A typical PEM construction includes an anode
and a cathode, with a solid polymer electrolyte membrane spaced
between them such that protons generated at the anode can travel
through the electrolyte and to the cathode. In PEM fuel cells,
hydrogen or a hydrogen-rich gas is supplied to the anode side of a
fuel cell while oxygen (such as in the form of atmospheric oxygen)
is supplied to the cathode side of the fuel cell. Catalysts,
typically in the form of a noble metal (such as platinum), are
placed at the anode and cathode to facilitate the ionization of
hydrogen and subsequent reaction between it and oxygen.
[0003] In an ideal fuel supply situation, pure hydrogen (H.sub.2)
gas is used as a direct fuel source. This is impractical in many
vehicle-based fuel cell systems, as the amount of gaseous hydrogen
required to be carried in order to achieve adequate vehicle range
between refueling stops would be prohibitively large. A promising
alternative to the direct feeding of H.sub.2 is the reformation of
on-board liquid hydrocarbons through a fuel processing system
upstream of the fuel cell such that the liquid hydrocarbons are
converted into H.sub.2-rich feedstock. Methanol (CH.sub.3OH) is an
example of a readily-available hydrocarbon fuel, and accordingly
has become one of the preferred H.sub.2 precursors, especially for
volume-constrained mobile fuel cell applications. Its relative low
cost and liquid state at practical temperatures of interest make it
compatible with existing fuel delivery infrastructure.
Unfortunately, during the conversion of CH.sub.3OH to H.sub.2,
carbon monoxide (CO) is also produced, of which even minute amounts
can poison the noble metal catalyst on the downstream fuel cell
anode and cathode. While reformation of CH.sub.3OH often produces
only small amounts of CO, even such small quantities have a
deleterious effect on fuel cell power output and life. Hence, the
CO concentration needs to be attenuated to very low levels
(preferably below 10 ppm) for the H.sub.2-rich reformate to be
suitable as fuel cell feedstock.
[0004] A typical fuel processing system incorporating CH.sub.3OH
includes a reformer and one or more purification (or clean-up)
stages. There have emerged three general types of reformers that
can be used on CH.sub.3OH and related liquid hydrocarbons: (1)
steam reforming; (2) partial oxidation reforming; and (3)
autothermal reforming. In the first variant, a pre-heated mixture
of fuel and steam is reacted, while in the second variant, a
pre-heated mixture of fuel and air is reacted. The third variant
combines elements of both processes in a single reactor, and using
a specially designed catalyst, enables balancing of the endothermic
first and exothermic second variants. In all three cases, a
reformate containing the desired end product, gaseous H.sub.2, as
well as undesirable CO, is produced. A shift reactor may be
employed to convert the CO in the reformate with water into
CO.sub.2 and H.sub.2 in what is called a water-gas shift reaction.
Since the water-gas shift reaction is reversible, it has been found
that to promote the formation of CO.sub.2 and H.sub.2, the
reformate should be cooled. Serially connected shift reactors of
successively lower temperatures may be used to further reduce the
CO concentration. While this level of CO cleanup could be
sufficient for certain types of fuel cells, it is still not
adequate for others, such as PEM fuel cells. While much of the
present disclosure is in the context of PEM fuel cells, it will be
appreciated by those skilled in the art that the invention
disclosed herein has utility in other forms of fuel cells, where
clean-up of fuel precursors can be used for improved fuel cell
system operability, as well as for other processes where highly
purified H.sub.2 feedstock is necessary. Accordingly, at least for
PEM fuel cells, additional steps must be taken to ensure that the
concentration of CO in the reformate is further reduced. There are
numerous ways to provide such reduction, including preferential
oxidation of CO, the use of diffusion membranes to separate H.sub.2
from the CO, and catalytic methanation reactions. Often, two or
more of these methods can be used in combination to achieve the
exceptionally low CO concentrations necessary for proper PEM
operation. Of these, the methanation reaction is achieved by
reacting CO with some of the just-produced H.sub.2, typically in
the presence of a catalyst, to produce methane (CH.sub.4) and water
according to the following:
CO+3H.sub.2.fwdarw.CH.sub.4+H.sub.2O (1)
[0005] Typically, the reformate stream contains (in addition to the
CO) other by-products, most notably CO.sub.2. Accordingly,
methanation reaction (1) above must compete with the following:
CO.sub.2+4H.sub.2.fwdarw.CH.sub.4+2H.sub.2O (2)
[0006] Typical reformate streams, including those produced by the
reformation of CH.sub.3OH, can possess considerably higher
concentrations of CO.sub.2 than CO, often more than an order of
magnitude higher. As a potential methanation reaction, reaction (2)
is particularly undesirable because, given the relatively high
concentration of CO.sub.2 in a CH.sub.3OH reformate stream, it
rapidly reduces the amount of H.sub.2 available to the fuel cell
anode (by consuming four H.sub.2 molecules for every CO.sub.2
molecule), thereby leaving the lower concentration and more
poisonous CO relatively unreacted. In addition, if the
concentration of CO.sub.2 is relatively high (on the order of a
percent or more), the strong affinity for H.sub.2 cuts into the
available H.sub.2 fuel supply, significantly reducing fuel
efficiency. Thus, to minimize both H.sub.2 losses and the presence
of poisons being delivered to the fuel cell anode, methanation
reactions should be used when the concentration of both of the
aforementioned carbon oxides is relatively low. Such a situation is
most readily achieved when the methantion reactor works in
conjunction with one or more of the previously indicated other
clean-up devices.
[0007] One way to promote the reaction (1) to the exclusion (or
near exclusion) of reaction (2) is to employ some means for
selective methanation. Selective methanation involves the
preferential reaction of one reactant species in lieu of others
when others are also present. In the present case, it is desirable
to achieve the selective methanation of CO over CO.sub.2 to remove
the more harmful former from the reformate stream that will
eventually find its way to the fuel cell anode. Previous attempts
at selective methanation of CO over CO.sub.2 have proven to be too
difficult, and hence expensive, to be viable for large-scale
commercial use. One reason is that the disproportionate
concentration of CO.sub.2 to CO, in addition to leading to the
aforementioned inordinate consumption of H.sub.2, also leaves some
of the CO unreacted. While some of this preferential reaction with
CO.sub.2 can be meliorated with proper methanation catalyst choice,
such choices are limited because of the second reason: both
reactions (1) and (2) are exothermic (heat-producing) in nature,
thereby producing higher temperatures in the region around the
methanation catalyst. These high temperatures remove from
consideration the relatively limited number of catalysts (for
example, rhodium and ruthenium) that are good at promoting
selective methanation of CO over CO.sub.2, as these catalyst
function best at low temperatures. In addition, high methanation
temperatures can facilitate the aforementioned reverse water gas
shift reaction, which by producing more CO, is undesirable.
Supplemental thermal management schemes, such as intrusive heat
exchangers and coolant injection can be used, but such schemes
increase system weight, volume, cost and complexity, especially
those which are untenable in configuration where space and weight
come at a premium, such as in vehicular and related mobile
applications. On the other hand, the temperature must also be high
enough to promote adequate levels of methanation activity. This is
important, as CO conversion (i.e., activity) is strongly
temperature-dependent, where higher temperature regimes tend to
promote CO methanation better than lower temperature regimes.
Accordingly, existing methanation reactors, typically with the
aforementioned rhodium or ruthenium catalysts on an alumina
(Al.sub.2O.sub.3) support, are neither sufficiently reactive at
lower temperatures nor sufficiently selective at higher
temperatures to maximize conversion of CO to CH.sub.4.
[0008] Accordingly, there exists a need for a methantion reactor
that can achieve selective methanation of CO over CO.sub.2 (and
other competing species) without having to resort to approaches
that require significant increases in weight, volume or complexity.
There also exists a need for a methanation reactor that is
compatible with other reformate stream CO reduction approaches.
BRIEF SUMMARY OF THE INVENTION
[0009] These needs are met by the present invention, wherein a
methanation reactor with improved features is disclosed. According
to a first aspect of the invention, a methanation reactor includes
a rhodium-based catalyst disposed on a silica (SiO.sub.2)-coated
Al.sub.2O.sub.3 support.
[0010] According to another aspect of the invention, a device
comprising a methanation reactor is disclosed. The methanation
reactor includes a porous ceramic support defining a surface
acidity, a coating configured to increase the surface acidity, a
noble metal catalyst coupled to at least one of the support or the
coating, and a flowpath configured to place the reformate stream in
fluid communication with at least the noble metal catalyst such
that carbon monoxide conversion in the reactor is greater than if
the coating were not present. In the present context, two
components are "coupled" when through their cooperation they affect
a common purpose, even absent direct connection between the two.
Thus, the noble metal catalyst is considered "coupled" to the
support, even if through a layer of the silica coating, so long as
the structure of the catalyst and support cooperate to facilitate
the methanation of the carbon monoxide.
[0011] Optionally, the reactor is configured to operate in a
temperature regime such that selectivity for carbon monoxide in the
reactor is at least 70 percent while conversion of carbon monoxide
is at least 30 percent. More particularly, the reactor is
configured to operate in a temperature regime such that the
selectivity is at least 80 percent and conversion of carbon
monoxide is at least 30 percent. Preferably, the noble metal is
rhodium, which can exist in a concentration of up to approximately
two percent, and more particularly approximately one percent.
Moreover, the support is preferably Al.sub.2O.sub.3, while the
coating is SiO.sub.2. As an alternative to rhodium, the noble metal
could be ruthenium. The device additionally comprises a reformer
configured to convert raw fuel into the reformate stream. More
particularly, the device additionally comprises a fuel supply and
an oxygen supply, and a fuel cell configured to receive a
hydrogen-rich portion of the reformate stream that has passed
through the reactor. Preferably, the fuel cell is a PEM fuel cell.
Furthermore, the device preferably comprises a vehicle (which could
be for example, a car, truck, aircraft, spacecraft, watercraft or
motorcycle) such that the fuel cell is a source of motive power.
The source of motive power may provide direct or indirect
propulsive force, the latter through (by way of example) a
mechanical coupling to one or more wheels or fluid-engaging means,
such as a propeller. Particularly, the vehicle comprises a platform
configured to carry the source of motive power, a drivetrain
rotatably responsive to output from the source of motive power, and
a plurality of wheels connected to the drivetrain. The device may
further comprise one or both of a preferential oxidation system or
palladium-based permeation membrane placed in fluid communication
with the reactor, thereby effecting addition carbon monoxide
removal from the reformate stream.
[0012] According to still another aspect of the invention, a
methanation reactor for the removal of carbon monoxide from a
reformate stream is disclosed. The methanation reactor comprises a
ceramic support, a noble metal catalyst coupled to the support and
a flowpath configured to place the reformate stream in fluid
communication with at least the noble metal catalyst. The support,
noble metal catalyst and the flowpath are configured such that
while operating in a temperature regime not in excess of 260
degrees Celsius, selectivity for carbon monoxide in the reactor is
at least approximately 60 percent. Preferably, the ceramic is
zirconia, while the noble metal comprises rhodium or ruthenium.
[0013] According to another aspect of the invention, a method of
delivering fuel to a fuel cell system is disclosed. The method
comprises the steps of configuring a fuel delivery system to
include a fuel supply and oxygen supply, fluidly connecting a fuel
processing system to the fuel delivery system, introducing fuel and
oxygen to create a fuel-oxygen mixture, heating the fuel-oxygen
mixture in a reformer such that a reformats stream is produced,
purifying the reformate stream in a methanation reactor and
transporting a hydrogen-rich portion of the reformate stream to a
fuel cell. The fuel processing system comprises the reformer to
evaporate a mixture of fuel and oxygen coming from the fuel
delivery system and the methanation reactor, the latter for the
removal of carbon monoxide from the reformate stream produced by
the reformer. The reactor is similar in configuration to at least
one of the ones discussed in the previous aspects. Optionally, the
reactor is configured to operate in a temperature regime such that
selectivity for carbon monoxide in the reactor is at least 70
percent while conversion of carbon monoxide is at least 30 percent.
Preferably, the noble metal is rhodium, while the support is
Al.sub.2O.sub.3 and the coating is SiO.sub.2.
[0014] According to yet another aspect of the invention, a method
of purifying a CH.sub.3OH reformate stream is disclosed. The method
includes configuring a reformer to evaporate a mixture of fuel and
oxygen, the evaporated mixture defining the reformate stream;
fluidly connecting a methanation reactor to the reformer for the
removal of carbon monoxide from the reformate stream, the reactor
comprising: a porous ceramic support defining a surface acidity; a
coating configured to increase the surface acidity; a noble metal
catalyst coupled to at least one of the support or the coating; and
a flowpath configured to place the reformate stream in fluid
communication with at least the noble metal catalyst such that
carbon monoxide conversion in the reactor is greater than if the
coating were not present; evaporating the mixture of fuel and
oxygen such that the reformate stream is produced; and exposing at
least a portion of the reformate stream to the reactor such that at
least a portion of carbon monoxide is removed from the reformate
stream.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0015] The following detailed description of the present invention
can be best understood when read in conjunction with the following
drawings, where like structure is indicated with like reference
numerals and in which:
[0016] FIG. 1 shows a block diagram of a generalized mobile fuel
cell system, including a fuel processing subsystem embodying the
present invention;
[0017] FIG. 2 shows a simplified methanation reactor according the
present invention;
[0018] FIG. 3 shows a graph representative of CO selectivity,
H.sub.2 consumption and CO conversion according to the prior
art;
[0019] FIG. 4 shows a graph representative of CO selectivity,
H.sub.2 consumption and CO conversion according to an embodiment of
the present invention;
[0020] FIG. 5 shows a graph comparing the catalyst loading and
inlet CO concentration on the outlet CO concentration; and
[0021] FIG. 6 shows the placement of a fuel cell, as well as a fuel
processing system containing a methanation reactor according to an
aspect of the present invention into a vehicle.
DETAILED DESCRIPTION
[0022] Referring initially to FIG. 1, a block diagram highlights
the major components of a mobile fuel cell system 1 according to
the present invention. The system includes a fuel delivery system
100 (made up of fuel supply 100A and oxygen supply 100B), reformer
200, supplemental CO clean-up device 300, methanation reactor 400
for cleaning up reformate coming out of supplemental CO clean-up
device 300, fuel cell 500, one or more energy storage devices 600,
a drivetrain 700 and one or more wheels 800. Together, reformer
200, supplemental CO clean-up device 300 and methanation reactor
400 make up fuel processing system 900. While the present system 1
is shown for mobile (such as vehicular) applications, it will be
appreciated by those skilled in the art that the use of the fuel
cell 500 and its ancillary equipment is equally applicable to
stationary applications. The energy storage devices 600 can be in
the form of one or more batteries, capacitors, electricity
converters, or even a motor to convert the electric current coming
from the fuel cell 500 into mechanical power such as rotating shaft
power that can be used to operate drivetrain 700 and wheels 800.
The fuel supply 100A takes a raw fuel, such as CH.sub.3OH, and
after combining it with an oxidant, such as oxygen from oxygen
supply 100B, sends the mixture to reformer 200, where the mixture
is converted to H.sub.2 fuel and various byproducts in the
following reactions:
CH.sub.3OH+H.sub.2O.fwdarw.3H.sub.2+CO.sub.2
CH.sub.3OH+2O.sub.2.fwdarw.2H.sub.2O+CO.sub.2
CH.sub.3OH.fwdarw.2H.sub.2+CO
[0023] The last of these reactions is a CH.sub.3OH decomposition
reaction, and produces CO. Since many of the aforementioned
reactions are reversible, heat exchangers (not shown) can be added
in one or more of the locations to create temperature regimes in
the reformate flowpath that will facilitate the reaction in the
direction most beneficial to the fuel cell system 1. Similar heat
exchangers may be installed to protect the various catalysts in the
fuel cell 500 from damage due to excessive heat. By way of example,
a set of heat exchangers can be disposed between the reformer 200,
the supplemental CO clean-up device 300 and the methanation reactor
400 to promote various reactions, such as the water-gas shift
reaction shown by the following formula:
CO+H.sub.2O.fwdarw.CO.sub.2+H.sub.2.
[0024] The water-gas shift reaction takes place in a shift reactor
and, under the proper temperature, pressure, steam ratio and
reformate composition, can proceed in the direction shown to reduce
the concentration of CO in the reformate stream, as well as to
increase the H.sub.2 yield. If the environmental temperature
surrounding the CO, H.sub.2O, CO.sub.2 and H.sub.2 is too high, the
reaction (which is reversible) will favor the formation of the
products on the left-hand side of the above equation, in what is
termed the reverse water-gas shift reaction. Since CO acts as a
poison to the catalysts in fuel cell 500, it is desirable to avoid
operating in temperature regimes that facilitate reverse water gas
shift reaction formation. Accordingly, to promote the reaction in
the direction shown above, temperatures should be kept fairly
low.
[0025] In any event, some of the various by-products, if left
untreated, would be in far too great a concentration for long-term
viability of the fuel cell 500. When the fuel cell 500 is
configured as a PEM fuel cell, which typically includes a polymer
membrane 515 disposed between an anode 505 and cathode 515, both of
which can be coated with a platinum catalyst adjacent the membrane
515, the most notable poison from the reformation reactions is the
CO produced in the CH.sub.3OH decomposition reaction, which
preferentially reacts with and consumes the platinum. The design of
a properly-functioning system that attempts to exploit both a
water-gas shift reaction and a methanation catalyst produces a
tension between having a high enough temperature to promote the
methanation of CO while simultaneously being below that conducive
for the formation of the reverse shift reaction. This bifurcated
requirement, by limiting the temperature range over which the
system operates, places restrictions on the class of potential
catalysts in the methanation reactor. Once within the temperature
range conducive for methanation, further striations are encouraged
to promote the selective methanation of CO over CO.sub.2.
[0026] Referring next to FIG. 3, the results of a prior art
methanation reactor are shown. The catalyst is rhodium-based, while
the support is made from Al.sub.2O.sub.3. The graph indicates that
the selectivity of this particular catalyst-support combination for
CO is very temperature-dependent, going from virtually one hundred
percent at approximately 265 degrees Celsius to about twenty five
percent at approximately 340 degrees Celsius. Such dependency makes
it difficult to integrate the methantion reactor with other CO
clean-up devices, as the temperature environment they operate in
may be dramatically different than that of the methanation reactor.
In order to ensure proper temperature regimes for each device,
supplemental thermal management schemes (such as controlled feed
inlet temperatures or the aforementioned heat exchangers) must be
adopted, adding complexity to the system. As can be seen from the
graph, since the preferential selectivity of CO over CO.sub.2 is
higher at low temperatures, it is important to operate a selective
methanation reactor at the lower end of the temperature spectrum to
keep CO.sub.2 methanation in check. Unfortunately, the lower
temperatures also inhibit CO conversion in the methanation
reaction, as the conversion rate is below ten percent at 250
degrees Celsius, and doesn't approach thirty percent until almost
300 degrees Celsius. This relatively narrow operating window makes
it more difficult to design a fuel processing system that can react
away the existing CO without forming more under the reverse water
gas shift reaction or consuming an inordinate amount of hydrogen in
the process.
[0027] Referring next to FIGS. 2 and 6, a simplified methanation
reactor 400 according to an aspect of the present invention, and an
exemplary placement of a fuel processing system 900 incorporating
the reactor 400 in a vehicle 1000, is shown. It will be appreciated
by those skilled in the art that, depending on the allowable cost
and CO reduction requirements, additional components can be coupled
to methanation reactor 400 to further reduce the CO levels in the
reformate. For example, a preferential oxidation reactor (not
shown) can be incorporated to react small quantities of oxygen with
the carbon monoxide in the presence of an oxidation catalyst to
convert the carbon monoxide into carbon dioxide, all the while
keeping the amount of H.sub.2 that reacts with the oxygen to a
minimum. Another component, in the form of a permeation device
(such as a palladium-based membrane, not shown), allows only
hydrogen to permeate through, thereby removing unwanted carbon
oxides from the permeate.
[0028] The major components of the reactor 400 include a ceramic
support 420 that defines a flowpath 430 therethrough and a noble
metal catalyst 440. While the support 420 is shown notionally as a
solid cylindrical rod, it will be appreciated by those skilled in
the art that other configurations, including tubular and planar
supports (neither of which are shown) could be employed. Similarly,
while the flowpath 430 is shown as extending axially through the
reactor 400, it will be appreciated that it could also flow
radially (either inward or outward, especially if the reactor 400
is tubular). The preferred noble metal for the catalyst is rhodium
or ruthenium, either of which facilitate high selectivity for CO at
relatively low temperatures. This low temperature performance is
important in methanation reactors, as the reactor often cooperates
with other devices, such as one or more supplemental CO clean-up
device 300 (which could be in the form of a diffusion membrane or
selective oxidation reactor, among others) to achieve very
aggressive CO reduction goals. In addition, by operating in a lower
temperature regime, the danger of promoting the aforementioned
reverse water gas shift reaction is reduced. Configurationally, the
catalyst 440 can be deposited directly onto the support 420, or
through a carrier (not shown). The combination of the support 420
and the catalyst 440 can be effected numerous ways. For example, a
rhodium solution can be used to impregnate the porous ceramic of
the support 420, then dried and reduced to yield a loaded rhodium
reactor 400. Impregnation is one particularly suitable technique
for depositing the catalyst 440 on the support 420, and includes
bringing a rhodium or ruthenium based solution into contact with
the support 420, after which the reactor 400 is dried and calcined.
Alternatively the catalyst 440 can be applied as part of a
ceramic-based washcoat (not shown), such that the catalyst 440 is
supported on or dispersed within the washcoat, which is in turn
deposited on the ceramic support 420.
[0029] While Al.sub.2O.sub.3 has been the ceramic material of
choice for methanation reactors according to the prior art, the
present inventors have discovered that by adding SiO.sub.2 to the
Al.sub.2O.sub.3 support 420 improved selectivity and higher CO
conversion rates at more compatible temperature regimes result. It
will be appreciated that the addition of SiO.sub.2 to the
Al.sub.2O.sub.3 support of the present invention is not the same as
an aluminosilicate (Al.sub.2O.sub.3 SiO.sub.2) support, where in
the latter, both Al.sub.2O.sub.3 and SiO.sub.2 are present in the
precursor material in a predetermined ratio. The configuration of
the present support is such that the SiO.sub.2 is dispersed over
the surface of the Al.sub.2O.sub.3 to increase the acidity of the
surface. The increase in surface acidity appears to increase CO
activity, thus promoting more active CO methanation than pure
Al.sub.2O.sub.3. The concentration of added SiO.sub.2 (for improved
acidity) has to be balanced against the superior catalyst
coatability of Al.sub.2O.sub.3. The coated support configuration
disclosed herein is beneficial, in that by improving the activity
of the CO coming into contact with the catalyst, the temperature
window over which the methanation reaction remains viable can be
made larger. This allows the otherwise stringent temperature
controls associated with the prior art devices to be relaxed,
resulting in a simpler, lower-cost fuel processing system 900.
[0030] Referring next to FIG. 4, the results of methanation reactor
400 according to an aspect of the present invention are shown. As
with the prior art device, the catalyst is rhodium-based. In
contrast to the prior art Al.sub.2O.sub.3 device, the support 420
of the present invention is made from Al.sub.2O.sub.3 with
SiO.sub.2 added which, as previously mentioned, produces
significant improvements in both the activity between the catalyst
and the CO, as well as the temperature range where such selectivity
is viable. As shown in the graph, the selectivity remains higher
over a considerably broader temperature range than the support of
FIG. 3. By way of example, the methanation reactor 400 of the
present invention has a selectivity of over seventy percent up to
temperatures of approximately 303 degrees Celsius, compared to only
292 degrees Celsius for the system of FIG. 3, and a selectivity
over sixty percent up to approximately 322 degrees Celsius,
compared to only 304 degrees Celsius for the system of FIG. 3, and
a selectivity of over fifty percent up to temperatures up to 340
degrees Celsius compared to only up to approximately 312 degrees
Celsius for the system of FIG. 3. This broader operating range is
beneficial is when considered in light of the CO conversion shown
on the graph, where dramatic improvements are shown relative to the
prior art of FIG. 3. While the conversion rate of CO for the
systems of FIGS. 3 and 4 are somewhat comparable at the higher
support temperatures, the selectivity is higher for the support 420
of the present invention, especially in the temperature range where
viable amounts of CO are being converted. Moreover, in the
temperature regimes where the support of FIG. 3 does exhibit strong
selectivity, the CO conversion is relatively poor compared to the
support 420 of the present invention. For example, CO conversion
rates are twice as high at the lower temperatures (250 degrees
Celsius-260 degrees Celsius), and approach comparable equilibrium
values (of around thirty percent) at considerably lower temperature
(approximately 275 degrees Celsius compared to approximately 300
degrees Celsius for the system of FIG. 3). As an additional
improvement, the equilibrium CO conversion rate plateaus rather
than peaks, thus evidencing substantially temperature-independent
behavior, thus affording a more steady, predictable performance
with further ease of system integration. The higher CO conversion
at the lower temperatures can be especially important in situations
where the methanation reactor 400 is connected to the output of a
water-gas shift reactor (not shown), where the desirable operating
temperature is low to prevent the reverse water gas shift reaction.
By having a catalyst in the methanation reactor 400 that is capable
of operating at these lower water-gas shift temperatures,
additional thermal matching devices and methods are avoided. Not
only is the temperature environment better-suited to integration
with other clean-up equipment, its higher level of selectivity
ensures that less H.sub.2 is being consumed during the methanation
process.
[0031] Referring next to FIG. 5, the effects of reformate stream
space velocity and rhodium loading are compared for the
SiO.sub.2-loaded Al.sub.2O.sub.3 support 420 of the present
invention and the Al.sub.2O.sub.3 support of the prior art. As
previously mentioned, too high of a CO methanation reaction
temperature should be avoided, as such temperature can promote the
water-gas shift reaction in the reverse direction such that H.sub.2
is consumed while CO is produced. To that end, the space velocity
can be tailored to the methanation reactor 400 to ensure
temperature compatibility. As can be seen from the graph, the
higher space velocities result in higher CO outlet concentrations.
In situations involving catalysts deposited onto the support
through a washcoat, it is preferable to avoid highly loaded (i.e.,
thicker) washcoat configurations. This is especially so where the
catalyst is to be used under higher gaseous hourly space velocities
(GHSVs) per washcoat loading (WCL), which is effectively the
equivalent flowrate per unit weight of catalyst. Higher washcoat
loadings inhibit reactant and product diffusion and consequent CO
conversion; thinner washcoats, while conducive to lower pressure
drop and consequent improved diffusion through the catalyst, must
not be so lightly loaded that insufficient catalyst activity
arises. By using the GHSV against the WCL, tests done with
different flow rates of gas and different amounts of catalyst on
the substrate can be normalized. The graph shows that under all
sets of conditions the rhodium catalyst deposited on the
SiO.sub.2-coated Al.sub.2O.sub.3 support gives lower CO outlet
concentration than the rhodium catalyst deposited on the
Al.sub.2O.sub.3 support. As the GHSV rises, the CO outlet
concentration increases, but less so for the rhodium catalyst
deposited on the SiO.sub.2-coated Al.sub.2O.sub.3 support.
[0032] Referring next to FIG. 6 in conjunction with FIG. 1, a
vehicle 1000 incorporating a fuel cell system according to the
present invention is shown. Fuel cell 500 is fluidly coupled to a
fuel cell processing system 900 that includes reformer 200,
supplemental CO clean-up device 300 and reactor 400. Fuel
processing system 900 accepts fuel and an oxidant (such as oxygen)
from respective fuel and oxygen supplies 100A and 100B and prepares
the fuel for consumption in fuel cell 500. While the vehicle 1000
is shown notionally as a car, it will be appreciated by those
skilled in the art that the use of fuel cell systems in other
vehicular forms is also within the scope of the present
invention.
[0033] While certain representative embodiments and details have
been shown for purposes of illustrating the invention, it will be
apparent to those skilled in the art that various changes may be
made without departing from the scope of the invention, which is
defined in the appended claims.
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