U.S. patent application number 10/224806 was filed with the patent office on 2004-02-26 for sulfur control for fuel processing system for fuel cell power plant.
Invention is credited to Cocolicchio, Brian A., Silver, Ronald G., Zhu, Tianli.
Application Number | 20040035055 10/224806 |
Document ID | / |
Family ID | 31886880 |
Filed Date | 2004-02-26 |
United States Patent
Application |
20040035055 |
Kind Code |
A1 |
Zhu, Tianli ; et
al. |
February 26, 2004 |
Sulfur control for fuel processing system for fuel cell power
plant
Abstract
A fuel cell power plant (110) has a fuel cell stack assembly
(CSA) (16) including an anode (18), and a fuel processing system
(FPS) (120) providing a hydrogen-rich reformate/fuel stream (34,
134, 62) for the anode (18) of the CSA (16). A relatively active
metal catalyst is associated with one or both of the anode (18) and
the FPS (120), and is subject to degradation by the presence of
even low levels, e.g. 100 ppb to 5 ppb-wt. reformate, of sulfur in
the fuel stream. A guard bed (70) containing a guard material (72)
is provided in the FPS (120) for protecting the relatively active
metal catalysts by adsorbing, and further reducing the level of,
sulfur in the fuel stream. The guard material (72) is a metal or
metal oxide capable of forming a stable sulfide in the presence of
low levels of H.sub.2S in the fuel stream (34), and is preferably
selected from the group consisting of: ZnO, CuO on CeO.sub.2-based
support, NiO on CeO.sub.2-based support, and Cu/ZnO. Provision is
also made (80, 74, 75, 76, 78, 82) for regenerating the guard
material (72) in situ.
Inventors: |
Zhu, Tianli; (Vernon,
CT) ; Silver, Ronald G.; (Tolland, CT) ;
Cocolicchio, Brian A.; (Danbury, CT) |
Correspondence
Address: |
Stephen A. Schneeberger
49 Arlington Road
West Hartford
CT
06107
US
|
Family ID: |
31886880 |
Appl. No.: |
10/224806 |
Filed: |
August 21, 2002 |
Current U.S.
Class: |
48/127.9 ;
422/187; 422/223; 422/24; 422/600; 48/128; 48/198.3; 48/198.7 |
Current CPC
Class: |
C10K 3/00 20130101 |
Class at
Publication: |
48/127.9 ;
48/128; 48/198.3; 48/198.7; 422/187; 422/189; 422/190; 422/24;
422/223 |
International
Class: |
C10K 003/00 |
Claims
What is claimed is:
1. In a fuel cell power plant (110) having a fuel cell stack
assembly (CSA)(16), including an anode (18), and a fuel processing
system (FPS) (120) for converting a hydrocarbon feedstock fuel (22)
to a hydrogen-rich fuel stream (34, 134, 62) for the anode (18) of
the CSA (16), the FPS (120) including at least a shift reactor
(150, 152, 154) having a shift catalyst for converting CO in the
fuel stream (34, 134) to CO.sub.2 and increasing the yield of
H.sub.2, the improvement comprising: a) the shift catalyst
comprising a relatively active metal; and b) the FPS (120)
including a guard bed (70) to protect at least the relatively
active metal shift catalyst of the shift reactor (150, 152, 154),
the guard bed (70) being positioned upstream of the relatively
active metal catalyst of the shift reactor (150, 152, 154) and
comprising a guard material (72) for at least adsorbing sulfur from
the fuel stream (34).
2. The fuel cell power plant (110) of claim 1 wherein the guard
material (72) is a metal or metal oxide capable of forming a stable
sulfide in the presence of low levels of H.sub.2S in the fuel
stream (34).
3. The fuel cell power plant (110) of claim 2 wherein the guard
material (72) is selected from the group consisting of: ZnO, CuO,
NiO, Cu/ZnO, Ce oxides, metal-doped Ce oxides, Mn oxide, Mg oxide,
Mo oxide, Zr oxide, Co oxide, Fe oxide, Sn oxide and Zn/Ti oxide,
either alone or in combination with a CeO.sub.2-based support.
4. The fuel cell power plant (110) of claim 3 wherein the guard
material is selected from the group consisting of: ZnO, CuO on
CeO.sub.2-based support, NiO on CeO.sub.2-based support, and
Cu/ZnO.
5. The fuel cell power plant (110) of claim 2 wherein the
relatively active metal of the shift catalyst comprises a noble
metal, and the guard material (72) is other than a noble metal.
6. The fuel cell power plant (110) of claim 1 wherein the FPS (120)
includes a desulfurizer (26) upstream of the guard bed (70), the
desulfurizer (26) being capable of removing high levels of sulfur
from the hydrocarbon feedstock (22).
7. The fuel cell power plant (110) of claim 6 wherein the FPS (120)
includes a reformer (30) upstream of the guard bed (70) for
reforming the hydrocarbon feedstock (22) to a reformate (34)
comprising a H.sub.2-rich fuel stream.
8. The fuel cell power plant (110) of claim 1 further including
means (80, 74, 75, 76, 78, 82) operatively connected to the guard
bed (70) for regenerating the guard material (72) in situ.
9. The fuel cell power plant (110) of claim 8 wherein the means
(80, 74, 75, 76, 78, 82) for regenerating the guard material (72)
comprises first means (80, 74, 75) for selectively admitting an
O.sub.2-containing fluid to the guard bed (70) to oxidize the
adsorbed sulfur as SO.sub.2 and second means (76, 78, 82) for
discharging the SO.sub.2 from the guard bed (70).
10. The fuel cell power plant (110) of claim 1 wherein the shift
reactor (150, 152, 154) comprises a high temperature shift reactor
(152) and a low temperature shift reactor (154) relatively
downstream of the high temperature shift reactor (152), each of
said high temperature and said low temperature shift reactors (152,
154) having a relatively active metal shift catalyst, and the guard
bed (70) being upstream of the high temperature shift reactor
(152).
11. In a fuel cell power plant (110) having a fuel cell stack
assembly (CSA) (16) including an anode (18), and a fuel processing
system (FPS) (120) providing a hydrogen-rich fuel stream (34, 134,
62) for the anode (18) of the CSA (16), and a relatively active
metal catalyst in at least one of the anode (18) and the FPS (120),
the improvement comprising: the FPS (120) including a guard bed
(70) for said relatively active metal catalyst, the guard bed (70)
being positioned upstream of said relatively active metal catalyst
and comprising a guard material (72) for at least adsorbing sulfur
from the fuel stream (34).
12. The fuel cell power plant (110) of claim 11 wherein the guard
material (72) is a metal or metal oxide capable of forming a stable
sulfide in the presence of low levels of H.sub.2S in the fuel
stream (34).
13. The fuel cell power plant (110) of claim 12 wherein the fuel
processing system (FPS) (120) includes a reformer (30) to provide a
reformate as the hydrogen-rich fuel stream (34) and the level of
sulfur in the reformate stream (34) just prior to the guard bed
(70) is in the range of 100 ppb to 5 ppb-wt. reformate
14. The fuel cell power plant (110) of claim 11 wherein the
relatively active metal of the shift catalyst comprises a noble
metal, and the guard material (72) is other than a noble metal.
15. The fuel cell power plant (110) of claim 13 wherein the
relatively active metal of the shift catalyst comprises a noble
metal, and the guard material (72) is other than a noble metal.
Description
TECHNICAL FIELD
[0001] This invention relates to fuel processing for fuel cells,
and more particularly to the provision of a low-sulfur,
hydrogen-rich fuel stream for a fuel cell power plant. More
particularly still, the invention relates to sulfur control in a
fuel cell power plant fuel processing system, for catalysts having
relatively high susceptibility to sulfur in a fuel.
BACKGROUND ART
[0002] Fuel cell power plants that utilize a fuel cell stack for
producing electricity from a hydrocarbon fuel source are well
known. The raw hydrocarbon fuel may be natural gas, gasoline,
diesel fuel, naptha, fuel oil, or the like. In order for the
hydrocarbon fuel to be useful in the fuel cell stack's operation,
it must first be converted to a hydrogen-rich fuel stream through
use of a fuel processing system. Such hydrocarbon fuels are
typically passed through a reforming process (reformer) to create a
process fuel (reformate) having an increased hydrogen content that
is introduced into the fuel cell stack. The resultant process fuel
contains primarily water, hydrogen, carbon dioxide, and carbon
monoxide. The process fuel has about 10% carbon monoxide (CO) upon
exit from the reformer as reformate.
[0003] Anode electrodes, which form part of the fuel cell stack,
can be "poisoned" by a high level of carbon monoxide. Thus, it is
necessary to reduce the level of CO in the process fuel, prior to
flowing the process fuel to the fuel cell stack. This is typically
done by passing the process fuel through one or more water gas
shift (WGS) converters, or shift reactors, and possibly additional
reactors, such as one or more selective oxidizers, prior to flowing
the process fuel to the fuel cell stack. The shift reactor also
increases the yield of hydrogen in the process fuel stream.
[0004] However, the raw hydrocarbon fuel source may also contain
sulfur or sulfur compounds, and hydrogen generation in the presence
of sulfur results in a poisoning effect on all of the catalysts
used in the hydrogen generation system, as well as the fuel cell
anode catalyst itself. To mitigate this problem, the hydrocarbon
fuel source is typically passed through a desulfurizer, either
prior to or following the reforming process, to remove in a known
manner, as by converting sulfur from the gaseous form to a solid,
substantial quantities of sulfur prior to the fuel entering the
sulfur-sensitive components of the fuel processing system and fuel
cell. Examples of such desulfurizers and descriptions of the
associated process may be found in U.S. Pat. Nos. 5,769,909 and
6,159,256. Additionally, a U.S. Pat. No. 6,299,994 discloses the
use of desulfurizers and other components of various fuel
processing systems with the goal of providing a "pure" hydrogen
stream for the fuel cell.
[0005] In a typical example, natural gas feedstock may have a
sulfur content of 6 ppm-wt. fuel Though substantial sulfur is
removed by the desulfurizer from the hydrocarbon fuel stream being
processed, nevertheless sulfur levels of 25 ppb-500 ppb-wt. fuel or
greater, typically remain. Such diminished levels of sulfur in the
fuel may be tolerated by the catalysts in the reformer, in part due
to higher operating temperatures. The reformation process dilutes
the fuel stream such that the reformate issuing from the reformer
may typically have sulfur levels in the range of 5 ppb-100 ppb wt.
reformate. While the catalysts used in the prior art in the
remaining elements of the fuel processing system and the fuel cell
itself may have tolerated such sulfur levels in the reformate, the
present more active catalysts tend to result in increased
sensitivity to sulfur, even at the reduced sulfur levels in the
reformate, due to their ability to be used in smaller
quantities.
[0006] Referring to FIG. 1, there is depicted, in simplified
functional schematic diagram form, the fuel cell stack assembly
(CSA) 16 and fuel processing system (FPS) 20 of a fuel cell power
plant 10 in accordance with the Prior Art as described above.
Briefly, a sulfur-containing hydrocarbon fuel feedstock,
represented by supply line 22, is delivered by a pump or blower 24
to a desulfurizer 26 at the input, or upstream end, of FPS 20. The
sulfur may be present in the form of hydrogen sulfide (H.sub.2S),
as well as mercaptans, sulfur oxides, etc. Following high-level
desulfurization, the hydrocarbon fuel feedstock is admitted to a
reformer 30 where, in the presence of steam, and possibly air,
supplied on line 32, it is reformed in a well known manner to
provide a hydrogen-rich reformate on line 34. The reformate, in
addition to containing H.sub.2 and CO, also contains any residual
low level sulfur not removed by the desulfurizer 26. That sulfur
may be present at the level of about 5 ppb-100 ppb-wt. reformate,
or greater. The result is substantially the same if the high-level
desulfurization occurs immediately after the reformer 30 rather
than before.
[0007] The reformate on line 34 is supplied to a water gas shift
reaction section 50 that typically contains a first high
temperature shift reactor 52 connected by line 53 to a second low
temperature shift reactor 54. The shift reaction section 50 serves
in a known manner to shift CO in the reformate to become CO.sub.2
and to increase the yield of H.sub.2. In the main, the prior art
shift reactors 52 and 54 have employed catalysts such as Cu/ZnO,
Fe/Cr oxide, and the like, with noble metals occasionally being
used in the low temperature shift reactor 54. The presence of the
non-noble-metal catalysts, such as Fe/Cr oxide in the high
temperature shift reactor 52 and Cu/ZnO in the low temperature
shift reactor 54, has provided sufficient additional sulfur sorbing
action with respect to the residual low level sulfur to further
decrease the sulfur levels such that they would not poison the
more-sulfur-sensitive catalysts downstream thereof. Following
passage through the shift reaction section 50, the hydrogen-rich
reformate may then pass through a selective oxidizer (SOX) 60
connected through line 56 from low temperature shift reactor 54,
and thence to the anode 18 of CSA 16 connected through line 62 from
the selective oxidizer 60. Partially-spent hydrogen is discharged
from anode 18 via discharge line 19, and may be recycled and/or may
be combusted to provide a source of heat.
[0008] Heretofore, the water gas shift catalysts of the shift
converter portion of the fuel processing system have typically been
Cu/ZnO and/or Fe/Cr oxide, and have incidentally served to adsorb
the residual sulfur sufficiently to prevent poisoning of the system
there and downstream thereof. This is due partly to the fact that
they are/were/used in relatively large quantities due to their
limited catalytic activity. Recently, however, there has been a
change in the type of shift catalyst used in the shift conversion
process, from Cu/ZnO and/or Fe/Cr oxide to relatively more active
catalysts, such as noble metal-based catalysts and/or some active
base metal catalysts. These more active catalysts offer advantages
in the shift conversion reaction process and elsewhere in the
system, principally by requiring smaller quantities than
heretofore. However, these very attributes may increase the
potential for sulfur poisoning, particularly in the absence of the
sorbing action of the Cu/ZnO. This is so, even at the low levels of
sulfur in the range of 5 ppb-100 ppb-wt. reformate, and may be
particularly a problem during surges, or upsets, when the sulfur
levels go higher.
[0009] Accordingly, it is an object of the present invention to
provide, in a fuel cell power plant having a fuel processing system
and employing relatively active metal catalyst(s), means for
guarding those catalyst(s) against sulfur in the
fuel/reformate.
[0010] It is a further object of the invention to provide such
means for guarding relatively active metal catalyst(s) against
sulfur in fuel in fuel cell fuel processing systems that include a
shift converter with relatively active metal shift catalyst(s).
[0011] It is a still further object of the invention to provide an
effective and economic means for removing, or reducing, low,
objectionable levels of sulfur from a hydrocarbon fuel process
stream for a fuel cell.
DISCLOSURE OF INVENTION
[0012] The present invention relates to an improved fuel processing
system (FPS) for a fuel cell stack assembly (CSA) in a fuel cell
power plant, which is operative to protect relatively active metal
catalysts in the FPS and/or CSA from the poisoning effects of even
low levels of sulfur (S) in a hydrogen (H.sub.2) fuel stream. As
used herein, the phrase "relatively active metal . . . catalyst(s)"
is intended to mean those noble metal catalysts and base metal
catalysts having a relatively greater catalytic activity than the
Cu/ZnO and/or Fe/Cr oxide catalysts of the prior art. A guard bed
of "guard material" is included in the FPS, upstream of the one or
more components that contain relatively active metal catalysts that
require protection against sulfur poisoning. The guard material is
material capable of adsorbing or removing sulfur, and is a metal or
metal oxide capable of forming a stable sulfide in the presence of
low levels of H.sub.2S in the process fuel stream, and is
preferably selected from the group consisting of ZnO, CuO, NiO,
Cu/ZnO, Ce oxides, metal-doped Ce oxides, Mn oxide, Mg oxide, Mo
oxide, Zr oxide, Co oxide, Fe oxide, Sn oxide and combined Zn/Ti,
either alone or in combination with a CeO.sub.2 support. Preferred
within that group is a guard material from the group consisting of
ZnO, CuO on CeO.sub.2-based support, NiO on CeO.sub.2-based
support, and Cu/ZnO.
[0013] In a representative fuel cell power plant, gross high level
sulfur removal, to levels in the range of 25 ppb-500 ppb-wt. fuel,
or greater is performed by a desulfurizer located upstream of a
reformer. Reformate from the reformer may have sulfur levels
further diluted to levels in the range of 5 ppb-100 ppb wt.
reformate, and is supplied to a guard bed having a guard material
as mentioned above, for removal of even low levels of sulfur prior
to the process fuel stream entering relatively-active metal
catalyst-containing, lower-temperature portions of the FPS and/or
the CSA, as for instance the shift reactor(s). Typically, there is
a high temperature shift reactor and a low temperature shift
reactor, each employing a relatively active metal catalyst, such as
a noble metal or base metal, with the guard bed being located prior
to (i.e., upstream of) the high temperature shift reactor. Other
elements of the FPS, such as one or more selective oxidizers, may
be included, and the resulting H.sub.2-rich fuel stream from the
FPS is provided to the anode of a CSA, relatively free of H.sub.2S,
for use as a fuel reactant. The sulfur content after the low-level
removal by the guard bed is typically less than about 20 ppb-wt.
reformate, and preferably less than about 5 ppb-wt. reformate.
[0014] Provision is made for regenerating the guard material in the
guard bed and discharging the resulting SO.sub.2. In this way, the
FPS and the CSA anode may use preferred relatively active metal
catalysts, some of which, such as the noble metals, being
relatively expensive, while, in an extended and economical manner,
minimizing concern for the possibility of sulfur poisoning.
[0015] The foregoing features and advantages of the present
invention will become more apparent in light of the following
detailed description of exemplary embodiments thereof as
illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
[0016] FIG. 1 is a simplified functional schematic diagram of a
fuel cell power plant having a fuel cell stack assembly and a fuel
processing system in accordance with the prior art; and
[0017] FIG. 2 is simplified functional schematic diagram of a fuel
cell power plant similar to FIG. 1, but showing an improved fuel
processing system having a guard bed to control sulfur in
accordance with the invention
BEST MODE FOR CARRYING OUT THE INVENTION
[0018] Referring to FIG. 2, there is illustrated a fuel cell power
plant 110 similar to that depicted in FIG. 1 with respect to the
prior art, but differing principally in that it includes an
improved fuel processing system (FPS) 120 in accordance with the
invention. The elements of FIG. 2 that are essentially the same as
their counterparts in FIG. 1 are given the same reference numeral
as in FIG. 1, whereas those elements that are functionally similar
but include some change in accordance with the invention, are
similarly numbered but with a "1" prefix. Added elements are given
new numbers. The CSA 16 is typically of the proton exchange
membrane (PEM) type, operating at temperatures less than
100.degree. C. and pressures less than 1 atmosphere gauge, for
example at 5 psig. It will be understood that the power plant 110
includes various elements and sub-systems that are well understood
and a part of the normal functioning of the system, but which are
not described herein because they are not essential to an
understanding of the invention and its benefit to the system.
[0019] As noted previously, a sulfur-containing hydrocarbon fuel
feedstock, represented by supply line 22, is delivered by a pump or
blower 24 to a desulfurizer 26 at the input, or upstream end, of
FPS 120. The hydrocarbon feedstock 22 may typically be natural gas,
gasoline, propane, diesel fuel, naptha, fuel oil, or the like, and
is likely to contain various forms of sulfur at levels sufficient
to pose a poisoning potential for the various noble metal catalysts
in the system. Moreover, the term "hydrocarbons", as used herein,
should be viewed as including not only the heavier C-H-only
hydrocarbons, but also the alcohols and other oxygen-containing
hydrocarbons, at least to the extent they contain the presence of
objectionable levels of sulfur. The hydrocarbon fuel feedstock is
delivered to the FPS 120, and specifically a desulfurizer 26, by
means of a pump, blower, or the like. The desulfurizer 26 is
generally capable of reducing sulfur levels in the hydrocarbon
feedstock 22 to levels of about 500 ppb-25 ppb-wt. fuel, following
which the feedstock is supplied to a reformer 30, for conversion or
reformation at high temperature, e.g., 600.degree.-800.degree. C.,
through the addition of steam (and possibly air) 32, to form a
hydrogen-rich reformate that also includes significant CO. That
reformate is provided on output line 34 from the reformer 30, and
continues to contain residual sulfur at or below the levels
provided by the desulfurizer 26, typically diluted by the
reformation process, such that sulfur levels of 100 ppb-5 ppb wt.
reformate remain. It should be understood that the relative
locations of the desulfurizer 26 and the reformer 30 may be
reversed, with a similar result occurring, because of the
reformer's higher operating temperature-tolerance of sulfur and/or
if possibly lower levels of sulfur are present in the hydrocarbon
fuel feedstock.
[0020] To reduce the level of CO in the reformate 34, the reformate
undergoes a shift reaction in the water gas shift (WGS) section 150
to shift CO to CO.sub.2 and to further enrich the H.sub.2 in the
process fuel stream. The WGS section 150 consists, in this
embodiment, of a high temperature shift reactor 152 as a first
stage, typically operating at 300.degree.-450.degree. C., and a low
temperature shift reactor 154, typically operating at
200.degree.-300.degree. C., as a second stage. Importantly, the
traditional Fe/Cr oxide and/or Cu/ZnO shift catalyst used in prior
art shift reactors has been replaced, instead, with a relatively
active metal shift catalyst (not separately shown) in the high
temperature shift reactor 152. A similar, though not necessarily
the same, relatively active metal shift catalyst is present in the
low temperature shift reactor 154.
[0021] The relatively active metal shift catalyst consists of noble
metal catalysts and/or base metal catalysts having a relatively
greater catalytic activity than Fe/Cr oxide and Cu/ZnO. That
activity, is 2 times, and preferably 5 or more times, greater at
300.degree. C. than the activity of Fe/Cr oxide or Cu/ZnO at that
temperature. As the temperature increases, the activity of the
relatively active metal catalyst, relative to those prior art
catalysts, is even greater, and vice versa. Because of the
foregoing, smaller WGS reactors and/or less WGS catalyst is
required relative to the prior art.
[0022] These relatively-active metal shift catalysts are chosen
from the group consisting of the noble metals rhenium, platinum,
palladium, rhodium, ruthenium, osmium, iridium, silver, and gold,
and the base metals having activities comparable to the noble metal
catalysts. Examples of such base metals include Cu on ceria and Cu
on perovskites. Preferred amongst the noble metal catalysts are
platinum, palladium, rhodium and/or gold, alone or in combination,
with platinum being particularly preferred because of a desirable
level of activity per volume. The relatively active metal shift
catalysts may be advantageously supported by, or on, a metal oxide
promoted support, in which the metal oxide may be an oxide of
cerium (ceria), zirconium (zirconia), titanium (titania), yttrium
(yttria), vanadium (vanadia), lanthanum (lanthania), and neodymium
(neodymia), with ceria and/or zirconia being generally preferred,
and a combination of the two being particularly preferred. The
shift catalysts may take the form of coated beads or pellets and be
arranged in a reactor bed, or they may constitute a coating on a
honeycomb-type structure, or various other forms known for use in
shift reactors. Because of the greater activities of these
catalysts relative to Fe/Cr oxide and Cu/ZnO, smaller quantities
may be used to get similar results.
[0023] Because of the susceptibility of the smaller quantities of
the relatively active metal shift catalysts, as well any relatively
active metal catalysts of the selective oxidizer 60 and fuel cell
anode 18, to sulfur poisoning by even low levels of sulfur at their
respective relatively low operating temperatures, the invention
provides a guard bed 70 to remove sufficient sulfur from the
reformate/process fuel stream 34 to allow safe and effective
processing/utilization of that stream downstream thereof. The
selective oxidizer 60 typically operates at 100.degree.-150.degree.
C., and the temperatures in the CSA 16 are typically less than
100.degree. C. The guard bed 70 is preferably located immediately
prior to (i.e., upstream of) the high temperature shift reactor
152, although additional such guard beds may be included elsewhere
in the fuel-processing stream if required. The guard bed 70 is
represented here as a chamber, or enclosure, containing a "bed" of
guard material 72. The guard material 72 may be in the form of
tablets, or pellets, or may be wash-coated onto a monolith or a
foam, or extruded, and is disposed in the bed chamber in a manner
for fluid flow of the reformate 34 thereover and therethrough to
facilitate sulfur adsorption. In the illustrated example, a bed of
pellets is supported by a porous screen or plate, though other
structural support arrangements are also well known.
[0024] Reformate 34 is supplied to the guard bed 70 via a multi-way
inlet valve 74 and inlet conduit 75. Effluent processed by the
guard material 72 exits the guard bed 70 via outlet conduit 76 and
a further multi-way valve 78, and is supplied to the high
temperature shift reactor 152 via conduit 134 as processed
reformate having any sulfur content reduced to an acceptable level,
normally below about 20 ppb wt. reformate, and preferably below
about 5 ppb-wt. reformate.
[0025] Referring to the guard material 72 in greater detail, it is
required to be a material, such as a metal or metal oxide, that can
adsorb or remove sulfur and form stable sulfides, from levels of
H.sub.2S in the process fuel stream temporarily as high as 1
ppm-fuel wt., such as during upsets, or the more usual lower levels
of between 100 ppb to 5 ppb-wt. reformate downstream of the
desulfurizer 26 and reformer 30 during normal operation. Moreover,
the guard material 72 must be capable of durable and satisfactory
operation at the temperatures and flow environment encountered at
its selected location in the fuel-processing stream. In this
regard, the guard material is selected from the group of materials
consisting of ZnO, CuO, NiO, Cu/ZnO, Ce oxides, metal-doped Ce
oxides typically of Ce/Zr or Ce/Pr, Mn oxide, Mg oxide, Mo oxide,
Zr oxide, and Co oxide, either alone or in combination with a
CeO.sub.2-based support. Other metal oxides that may be used
include oxides of Fe, Sn, and a combination of Zn/Ti. Preferred
from this group are ZnO, CuO on CeO.sub.2-based support, NiO.sub.2
on CeO.sub.2-based support, and Cu/ZnO, with ZnO being particularly
preferred, assuming the temperature of the fuel flow stream at that
location is below the ZnO decomposition temperature and the water
(H.sub.2O) level in the reformate is not so high as to result in an
equilibrium level of H.sub.2S that is unacceptable. The relevant
reaction is: ZnO+H.sub.2S.fwdarw.ZnS+H.sub.2O.
[0026] Cu/ZnO is a suitable alternative to ZnO as a preferred guard
material 72. Either ZnO or Cu/ZnO is preferred because of cost
and/or chemical activity considerations. Still further, copper
oxide (Cuo) or nickel oxide (NiO) on a ceria-based support is an
equally acceptable alternative to ZnO or Cu/ZnO as the guard
material 72. The ceria has been found to provide a support that
acts chemically, cooperatively with the CuO or NiO coating or
deposition thereon, to enhance the adsorbant characteristics of the
supported material. The ceria supports CuO or NiO, but adsorbs S
itself. When ceria is reduced, Cu or Ni help to keep it reduced,
and reduced CeO.sub.2 has oxygen vacancies that can be sulfur
adsorbers. The Ni or Cu, at sufficiently low levels, will not
generate a high enough exotherm to damage the adsorbant
material.
[0027] The above-mentioned guard materials can operate, in the
main, over a temperature range between about 20.degree. C. and
650.degree. C., with each of CuO-on-ceria and NiO-on-ceria having
the upper end of the range at about 600.degree. C. and
Fe.sub.2O.sub.3 having the upper end of the range at about
480.degree. C. These operating temperature ranges are generally
compatible with the temperatures of reformate 34 at that stage in
the FPS 120. The principal mode of sulfur removal is through the
action of surface adsorption by the guard material 72, which serves
to capture the sulfur in the passing H.sub.2S and convert it to a
sulfide of the guard material. In instances where the mass flow
rate of the reformate 34 by, over, and/or through the guard
material 72 is relatively slow, the guard material may additionally
act to remove the sulfur via absorption. As noted earlier, the use
of these guard materials 72 at this stage in the fuel processing
operation is capable of removing even low levels of sulfur from the
reformate/processed fuel stream to attain levels of 5 ppb-wt.
reformate and below, with a range of 20 ppb to less than 5 ppb-wt.
reformate being most typical. These "cleansed" levels of sulfur
allow the reformate/processed fuel stream to be further processed
and utilized by the shift reactors 150, the SOX 60 and the fuel
cell anode 18 with little or no poisoning of the associated
relatively active metal catalysts.
[0028] After extended usage of the guard bed 70 to remove sulfur,
the effectiveness of the guard material 72 is degraded by the
accumulation of sulfide at the surface. The rate of degradation is
a function of the size of the guard bed 70, the concentration of
sulfur in the fuel (reformate) stream entering the guard bed 70,
etc. While it is possible to avoid loss of the guarding effect
provided by the guard bed 70 by periodically replacing the degraded
guard material 72 with fresh guard material, such action may be
both expensive and time consuming. In accordance with a further
aspect of the invention, provision is made for regenerating the
guard material 72 in situ within the guard bed 70. An oxidant, such
as air, is admitted to the guard bed 70, either directly or
preferably via an inlet 80 to the multi-way valve 74. This is
typically done while the FPS 120 is otherwise inactive, as for
instance during shutdown of a vehicle in which the power plant 110
may be located. The introduction of oxidant through inlet 80 may be
facilitated by a flow-assisting pump or blower (not shown), to
assure that adequate oxidant is provided in a given period. The
oxidant reacts with the sulfide formed at the surface (at least) of
the guard material 72 to readily form sulfur dioxide, SO.sub.2,
which then may be discharged as a gas, either directly or
preferably via a further discharge outlet 82 from the multi-way
valve 78. The discharged SO.sub.2 may be further cleansed or used
elsewhere in the power plant system, or may simply be discharged
from the system all together.
[0029] Although the invention has been described and illustrated
with respect to the exemplary embodiments thereof, it should be
understood by those skilled in the art that the foregoing and
various other changes, omissions and additions may be made without
departing from the spirit and scope of the invention. For instance,
although the FPS of the fuel cell power plant has been described as
including a desulfurizer upstream of a reformer, it will be
understood that their relative positions may be reversed. Still
further, the guard bed might be positioned immediately upstream of
only the CSA, the SOX, or a LTS if the catalysts of the FPS
components upstream thereof are tolerant of the particular sulfur
levels.
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