U.S. patent application number 13/938504 was filed with the patent office on 2013-11-14 for method for adding sulfur to a fuel cell stack system for improved fuel cell stability.
The applicant listed for this patent is DELPHI TECHNOLOGIES, INC.. Invention is credited to KARL JACOB HALTINER,, JR., SUBHASISH MUKERJEE, JEFFREY G. WIESSMAN.
Application Number | 20130302709 13/938504 |
Document ID | / |
Family ID | 44858486 |
Filed Date | 2013-11-14 |
United States Patent
Application |
20130302709 |
Kind Code |
A1 |
MUKERJEE; SUBHASISH ; et
al. |
November 14, 2013 |
METHOD FOR ADDING SULFUR TO A FUEL CELL STACK SYSTEM FOR IMPROVED
FUEL CELL STABILITY
Abstract
A method is provided for adding sulfur to a solid oxide fuel
cell (SOFC) stack having a Ni--YSZ anode to prolong the life of the
SOFC stack. The method includes the steps of providing a reformate
stream essentially free of sulfur compounds, feeding the reformate
stream to the SOFC stack, and adding a predetermined amount of a
sulfur compound into the reformate stream upstream of the SOFC
stack. The predetermined amount of the sulfur compound is effective
to prolong the life of the Ni--YSZ anode by retarding the formation
of carbon onto the Ni--YSZ anode and the coarsening of the granular
microstructure of the Ni--YSZ anode, while minimizing the
degradation of power output of the SOFC stack within a
predetermined limit.
Inventors: |
MUKERJEE; SUBHASISH;
(HORSHAM, GB) ; HALTINER,, JR.; KARL JACOB;
(FAIRPORT, NY) ; WIESSMAN; JEFFREY G.; (GUILFORD,
CT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DELPHI TECHNOLOGIES, INC. |
Troy |
MI |
US |
|
|
Family ID: |
44858486 |
Appl. No.: |
13/938504 |
Filed: |
July 10, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13074589 |
Mar 29, 2011 |
8129054 |
|
|
13938504 |
|
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Current U.S.
Class: |
429/425 ;
429/423 |
Current CPC
Class: |
Y02E 60/50 20130101;
H01M 8/2425 20130101; H01M 8/04225 20160201; H01M 8/04798 20130101;
H01M 8/0612 20130101; H01M 8/0675 20130101; H01M 8/04776 20130101;
H01M 8/04223 20130101; H01M 8/0662 20130101; H01M 2008/1293
20130101; H01M 8/04447 20130101 |
Class at
Publication: |
429/425 ;
429/423 |
International
Class: |
H01M 8/06 20060101
H01M008/06 |
Goverment Interests
GOVERNMENT-SPONSORED STATEMENT
[0002] This invention was made with the United States Government
support under Contract DE-FC26-02NT41246 awarded by the U.S.
Department of Energy. The Government has certain rights in this
invention.
Claims
1. A method for adding sulfur to a solid oxide fuel cell (SOFC)
stack having a Ni--YSZ anode, comprising: providing a reformate
stream essentially free of sulfur compounds; feeding said reformate
stream to said SOFC stack; and adding a predetermined amount of a
sulfur compound into said reformate stream upstream of said SOFC
stack, wherein said predetermined amount of said sulfur compound is
effective to prolong the life of said Ni--YSZ anode, while
minimizing the degradation of power output of the SOFC stack within
a predetermined limit.
2. The method of claim 1, wherein said predetermined amount of said
sulfur compound is effective to retard either the formation of
carbon onto the Ni--YSZ anode or the coarsening of the granular
microstructure of the Ni--YSZ anode.
3. The method of claim 1, wherein said predetermined amount of said
sulfur compound is effective to retard both the formation of carbon
onto the Ni--YSZ anode and the coarsening of the granular
microstructure of the Ni--YSZ anode.
4. The method of claim 1, wherein said step of adding a
predetermined amount of a sulfur compound into said reformate
stream is performed during the initial start-up of said SOFC
stack.
5. The method of claim 1, wherein said step of adding a
predetermined amount of a sulfur compound into said reformate
stream is performed continuously during the operational life of the
SOFC stack.
6. The method of claim 1, wherein said step of adding a
predetermined amount of said sulfur compound into said reformate
stream is performed during the initial start-up of said SOFC stack
and periodically during the operational life of the SOFC stack.
7. The method of claim 1, wherein said sulfur compound comprises
H.sub.25.
8. The method of claim 7, wherein said H.sub.25 is added into said
reformate stream to provide a sulfur concentration of 0.01 to 2.50
ppmv.
9. The method of claim 7, wherein said H.sub.25 is added into said
reformate stream to provide a sulfur concentration of 0.1 to 2.50
ppmv
10. A method for adding sulfur to a solid oxide fuel cell (SOFC)
stack having a Ni--YSZ anode, comprising: feeding a hydrocarbon
fuel containing sulfur to a reformer configured to catalyze said
hydrocarbon fuel containing sulfur into a reformate stream
comprising H.sub.25; removing or adding H.sub.25 into said
reformation stream to provide a predetermined concentration of
H.sub.25 in said reformate stream to prolong the life of said
Ni--YSZ anode, while minimizing the degradation of power output of
the SOFC stack within a predetermined limit.
11. The method of claim 10, wherein said step removing or adding
H.sub.25 into said reformate stream is performed during the initial
start-up of said SOFC stack.
12. The method of claim 11, wherein said step removing or adding
H.sub.25 into said reformate stream is performed periodically
during the operational life of the SOFC stack.
13. The method of claim 11, wherein said step removing or adding
H.sub.25 into said reformate stream is performed continuously
during the operational life of the SOFC stack.
14. The method of claim 10, wherein predetermined concentration of
H.sub.25 is 0.01 to 2.50 ppmv in said reformate stream.
15. The method of claim 10, wherein predetermined concentration of
H.sub.25 is 0.1 to 2.50 ppmv in said reformate stream.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of co-pending U.S. patent
application Ser. No. 13/074,589 filed on Jan. 25, 2012, which is a
divisional of issued U.S. Pat. No. 8,129,054, filed on Mar. 29,
2011. Both, U.S. patent application Ser. No. 13/074,589 and U.S.
Pat. No. 8,129,054 are hereby incorporated by reference in their
entireties.
TECHNICAL FIELD OF INVENTION
[0003] The present disclosure is related to a method of adding
sulfur to a fuel cell stack system; more particularly, a method of
adding sulfur to the reformate stream that feeds a fuel cell
stack.
BACKGROUND OF INVENTION
[0004] Fuel cells are used to produce electricity when supplied
with fuels containing hydrogen and an oxidant such as air. A
typical fuel cell includes an ion conductive electrolyte layer
sandwiched between a cathode layer and an anode layer. There are
several different types of fuel cells known in the art, one of
which is a solid oxide fuel cell (SOFC). A SOFC is regarded as a
highly efficient electrical power generator that produces high
power density with fuel flexibility.
[0005] In a typical SOFC, air is passed over the surface of the
cathode layer and a reformate hydrocarbon fuel is passed over the
surface of the anode layer opposite that of the cathode layer.
Oxygen ions from the air migrate from the cathode layer through the
dense electrolyte to the anode layer in which the oxygen ions
reacts with the hydrogen and carbon monoxide in the fuel, forming
water and carbon dioxide; thereby, creating an electrical potential
between the anode layer and the cathode layer. The electrical
potential between the anode layer and the cathode layer is
typically about 1 volt and power around 1 W/cm.sup.2. Multiple
SOFCs are stacked in series to form a SOFC stack having sufficient
power output for commercial applications.
[0006] The anode acts as a catalyst for the oxidation of
hydrocarbon fuels and has sufficient porosity to allow the
transportation of the fuel to and the products of fuel oxidation
away from the anode/electrolyte interface, where the fuel oxidation
reaction takes place. The anode of a typical SOFC is typically
formed of a nickel/yttria-stabilized zirconia (Ni/YSZ) composition.
The use of nickel in the anode is desirable for its abilities to be
a catalyst for fuel oxidation and current conductor.
[0007] Although nickel is a desirable hydrogen oxidation catalyst,
nickel also catalyzes the formation of carbon from hydrocarbons
under reducing conditions. Over time, the carbons atoms are
deposited onto the surface of the Ni/YSZ based anode. As the number
of carbon atoms deposited on the surface of the anode increases,
the level of damage and deactivation of the anode from carbon
formation increases dramatically. Also, prolonged steady state
operation at elevated temperatures, which is between typically
between 600.degree. C. to 900.degree. C. for a SOFC stack, causes
the nickel in the Ni/YSZ composition to coarsen due to grain
growth. The coarsening of the granular microstructure of the anode
further reduces the efficiency of the anode for fuel oxidation.
Furthermore, the Ni/YSZ anode is susceptible to contaminates, such
as sulfur, in the fuel stream; sulfur compounds are known to poison
the Ni/YSZ based anodes, thereby deactivating the SOFC stack.
[0008] There is a long felt need for a SOFC stack that has anodes
that are minimally susceptible to degradation due to carbon
deposits, Ni grain growth, and sulfur poisoning. There is also a
long felt need to be able to treat the Ni/YSZ anodes of an existing
SOFC stack in situ to reduce the susceptibility to carbon deposits
and Ni/YSZ substrate grain growth.
SUMMARY OF THE INVENTION
[0009] Contrary to the recognition by one of ordinary skill in the
art that the presence of sulfur is detriment to the performance of
a Solid Oxide Fuel Cell (SOFC) stack, it was surprisingly
discovered that a controlled diminutive amount of sulfur added to
the reformate stream feeding the SOFC stack significantly prolonged
the operational life of the SOFC stack, while only minimally
degrading the voltage and power output of the SOFC stack. It is
suspected that this diminutive amount of sulfur in the reformate
stream poisons the Ni--YSZ based anode enough to retard both the
catalyzing of carbon and the coarsening of the granular
microstructure of the nickel/YSZ substrate, but not enough to
continually degrade the voltage and power density output of the
SOFC stack.
[0010] An embodiment of the present invention provides a method of
adding sulfur to a fuel cell stack having a Ni--YSZ anode to
prolong the life of the SOFC stack. The method includes the steps
of providing a reformate stream essentially free of sulfur
compounds, feeding the reformate stream to the SOFC stack, and
adding a predetermined amount of a sulfur compound into the
reformate stream upstream of the SOFC stack. The predetermined
amount of the sulfur compound is effective to prolong the life of
the Ni--YSZ anode by retarding the formation of carbon onto the
Ni--YSZ anode and the coarsening of the granular microstructure of
the Ni--YSZ anode, while minimizing the degradation of power output
of the SOFC stack within a predetermined limit.
[0011] The method may include feeding a hydrocarbon fuel containing
sulfur to a reformer configured to catalyze the hydrocarbon fuel
containing sulfur into a reformate stream having H.sub.2S, removing
or adding H.sub.25 into the reformation stream to provide a
predetermined desired concentration of H.sub.25 in the reformate
stream, and then feeding the reformate stream to the SOFC stack.
The desired concentration of sulfur in the reformate stream may be
ascertained by optimizing the sulfur levels in the reformate stream
for a given SOFC stack and system configuration to strike the
balance of the desired longevity of the operational life of the
SOFC stack with the acceptable degradation in performance.
[0012] An advantage to this invention is that it offers an
effective low cost solution for significantly reducing two known
primary SOFC degradation mechanisms: carbon attack of the nickel in
the anode and coarsening of nickel particles in the anode. Another
advantage is that diminutive amount of sulfur added to the
desulfurized fuel stream feeding the SOFC stack significantly
prolonged the operational life and minimized performance
degradation of the SOFC stack. Still, another advantage is that the
invention improves the longevity of the SOFC stack without having
to perform extensive modification to the SOFC stack.
[0013] Further features and advantages of the invention will appear
more clearly on a reading of the following detailed description of
an embodiment of the invention, which is given by way of
non-limiting example only and with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF DRAWINGS
[0014] The present invention may best be understood from the
following detailed description of the preferred embodiments
illustrated in the drawings, wherein:
[0015] FIG. 1 shows a prior art SOFC system having a reformer, a
sulfur trap, and a SOFC stack.
[0016] FIG. 2 is a graph showing the voltage and power density
outputs of a typical SOFC stack operating on a desulfurized
reformate stream.
[0017] FIG. 3 is a graph showing the voltage and power density
outputs of a SOFC stack operating on a reformate stream containing
2.5 ppm by weight of H.sub.2S.
[0018] FIG. 4 is a graph showing the voltage and power density
outputs of a SOFC stack operating on reformate stream containing
0.1 ppm by weight of H.sub.2S.
[0019] FIG. 5a shows a system for adding sulfur to a SOFC stack
system, in which a sulfur free hydrocarbon fuel is fed to the
reformer.
[0020] FIG. 5b shows a system for adding sulfur to a SOFC stack
system, in which a hydrocarbon fuel containing sulfur contaminants
is fed to the reformer.
[0021] FIG. 6 shows an alternative embodiment of a system for
adding sulfur to a SOFC stack system, which the system includes a
bypass of a portion of reformate stream around the sulfur trap.
DETAILED DESCRIPTION OF INVENTION
[0022] Shown in FIG. 1 is a typical SOFC system 1 known in the art.
The SOFC system 1 includes a reformer 10, a sulfur trap 12, and a
SOFC stack 14. The reformer 10 is typically of that of a catalytic
hydrocarbon reformer that receives a hydrocarbon fuel stream 5. The
hydrocarbon fuel stream 5 may be that of gasoline, diesel, ethanol,
kerosene, or the likes and may contain impurities such as sulfur,
which is typically present in the forms of sulfur compounds such as
carbonyl sulfides, disulfides, and mercaptans. The three types of
reformer technologies that are typically employed in conjunction
with the SOFC stack system 1 are stream reformers, dry reformers,
and partial oxidation reformers. The reformer 10 produces a
reformate stream 11 by converting the hydrocarbon fuel stream 5 to
typically methane, hydrogen, and by-products that includes carbon
dioxide, carbon monoxide, hydrogen sulfides (H.sub.25), and sulfur
dioxide (SO.sub.2).
[0023] Sulfur is known to poison the catalytic activity of many
metals, including the nickel in the Ni--YSZ based anode of a SOFC.
To prevent sulfur poisoning of the SOFC stack 14, a sulfur trap 12
is typically placed downstream of the reformer 10 to receive the
reformate stream 11. The sulfur trap 12 contains suitable materials
to remove and trap sulfur compounds, including H.sub.25 and
SO.sub.2, typically found in the reformate steam 11. Exiting the
sulfur trap 12 is a desulfurized reformate stream 13 that is
directed to the SOFC stack 14.
[0024] Contrary to the recognition by others in the industry that
the presence of sulfur is detriment to the performance of a SOFC
stack 14, it was surprisingly discovered that a diminutive amount
of sulfur remaining in the desulfurized reformate stream 13 feeding
the SOFC stack 14 significantly prolonged the operational life,
while only minimally degrading the voltage and power output of the
SOFC stack 14. It is suspected that this diminutive amount of
sulfur in the reformate stream poisoned the Ni--YSZ based anode
enough to retard both the catalyzing of carbon and the coarsening
of the granular microstructure of the nickel/YSZ substrate, but not
enough to continually degrade the voltage and power density output
of the SOFC stack 14.
[0025] FIGS. 2, 3, and 4 are graphs showing the voltage and power
density outputs for three SOFC stacks. Each of the SOFC stacks was
fed a reformate stream containing a different concentration of
hydrogen sulfide (H.sub.25). The y-axis on the left side of each of
the graphs shows the stack voltage output (V) and the y-axis on the
right side of each of the graph shows the power density output
(mW/cm2). The x-axis shows the length of time (Hrs) that the SOFC
stack was tested at steady state. Referring to FIG. 2, the lower
set of data points represents the stack voltage output and the
upper set of data represents the power density output. Referring to
FIGS. 3 and 4, the upper set of data points shown on each graph
represents the stack voltage output and the lower set of data
points represents the power density output.
[0026] With reference to FIG. 2, a SOFC stack was supplied with a
desulfurized reformate stream. Even with a reformate stream free of
sulfur, the SOFC stack exhibited a performance degradation of 0.5
to 2% per 500 hours of operating time. After 1300 hours of
operating at the steady state, the performance degradation trend of
the SOFC stack continued. The decrease in performance of the SOFC
stack was attributed to carbon disposition on the surface of the
anode and the coarsening of the granular microstructure of the
nickel/YSZ anode substrate.
[0027] With reference to FIG. 3, a SOFC stack was supplied with a
reformate fuel stream having sulfur in the form of H.sub.25 at a
concentration of 2.5 parts per million by volume (ppmv). Stacks of
5 cells were operated under normal operating conditions (at
constant current with initial stack voltage at V=0.8 Volts per
cell, T=750.degree. C., fuel=28% H.sub.2, 30% CO, 6% H.sub.2O, 2.5
ppmv H.sub.2S) for 3453 hours. At constant current, the stack
voltage dropped from 0.8V per cell (power density of 450 mW per
cm.sup.2) to 0.6 Volts per cell as the 2.5 ppmv of H.sub.25 was
added to the reformate. The current was lowered to adjust the
voltage back up to 0.76 V (power density of 145 mW per cm.sup.2).
After the initial lowering of power due to H.sub.2S, the stack
showed minimal or no degradation during the course of this
long-term durability test (3453 hours).
[0028] Surprisingly, it was found that after the initial voltage
and power density drop, the SOFC stack did not exhibited any
further significant performance degradation over 3,000 hours of
continuous steady state operation. Transmission electron microscopy
(TEM) analysis of the anode did not show any damage in the Ni--YSZ
structure. The nickel in the anode was unaffected by carbon present
in the reformate fuel stream, and the nickel particles exhibited
very little, if any, coarsening of the nickel particle
microstructure
[0029] With reference to FIG. 4, a SOFC stack was supplied with a
reformate fuel having sulfur in the form of H.sub.25 at a minimal
concentration of 0.10 ppmv. This continual addition of sulfur in
the reformate fuel to the SOFC stack caused a slight degradation in
performance of approximately 0.5 V and 25 mW/cm.sup.2 within
approximately 175 hours of steady state operation. Again,
surprisingly, it was found that the rate of performance degradation
was significantly reduced thereafter. In other words, by adding a
small amount sulfur to the reformate stream to the SOFC stack
caused a slight initial degradation in performance, but in return,
retarded the long term degradation of the performance of the SOFC
stack. Even after 550 hours of steady state operation, the SOFC
stack did not exhibit any measurable degradation in
performance.
[0030] If a sulfur free hydrocarbon fuel or pure hydrogen is
supplied to the SOFC stack, sulfur in the form of H.sub.25 may be
added to the fuel stream during the start-up of the SOFC stack and
periodically thereafter during steady state operations to increase
the operating life of the SOFC stack. The concentration of sulfur
required in the reformate stream may vary depending on the nature
of the Ni particles in the anode of the SOFC stack. The desired
concentration may be ascertained by optimizing the H.sub.25 levels
in the fuel stream for a given stack and system configuration to
strike the balance of the desired longevity of the operational life
of the SOFC stack with the acceptable degradation in performance.
The goal is to obtain maximum stability of operation over prolonged
periods while minimalizing drop in initial power due to the sulfur
poisoning of the anode. With reference to FIGS. 3 and 4, the
addition of approximately 0.10 to 2.5 ppmv of H.sub.25 minimally
poisoned the anode of the SOFC stack, but yet provided stability of
performance over thousands of hours. It is believed that as little
as 0.010 ppmv of H.sub.25 may be beneficial to the longevity of a
SOFC stack.
[0031] Each of FIGS. 5a, 5b, and 6 shows a SOFC system 100 having a
hydrocarbon reformer 110, a SOFC stack 114, and a system for adding
sulfur to the SOFC stack 114. The reformer 110 produces a typical
reformate stream 111 by converting a sulfur free hydrocarbon fuel
stream 105 to methane, hydrogen, and by-products that includes
carbon dioxide, and carbon monoxide. Sulfur free hydrocarbon fuels
may include hydrocarbon fuels that have been processed to remove
sulfur or non-hydrocarbon fuels such as hydrogen. If the
hydrocarbon fuel stream 105 contains sulfur contaminants, then
sulfur containing by-products such as hydrogen sulfides (H.sub.2S)
and sulfur dioxide (SO.sub.2) are also included in the reformate
stream 111.
[0032] Shown in FIG. 5a, if the hydrocarbon fuel steam 105 to the
reformer 110 is sulfur free, then a metering device 206, such as a
variable pump or a metering valve, may be provided to inject sulfur
from a sulfur source 208 in the form of H.sub.25 directly into the
sulfur free reformate stream 111 at a predetermined flow rate to
achieve the desired concentration of sulfur in the conditioned
reformate stream 115 to the SOFC stack 114.
[0033] Shown in FIG. 5b, if the hydrocarbon fuel 105 fed to the
reformer 110 contains sulfur contaminants, then a sulfur trap 112
is provided downstream of the reformer 110 to remove sulfur from
the reformate stream 111 to produce a desulfurized reformate stream
113. A metering device 206 may be provided to inject sulfur from a
sulfur source 208 in the form of H.sub.25 directly into the
desulfurized reformate stream 113 producing a conditioned reformate
stream 115. A sulfur sensor 202 in communication with a controller
204 may be positioned in the conditioned reformate stream 115 to
control the injection rate of sulfur into the desulfurized
reformate stream 113 to maintain a desired predetermined
concentration of sulfur in the conditioned reformate stream 115. As
an alternative embodiment, the sulfur sensor 202 may be positioned
upstream in the desulfurized reformate stream 113 (not shown).
[0034] Shown in FIG. 6 is an alternative embodiment of the
invention for use with a SOFC stack system 100 that accepts a
hydrocarbon fuel stream 105 containing sulfur contaminants for the
reformer 110. The system shown in FIG. 6 maintains a predetermined
level of sulfur concentration in the conditioned reformate stream
115 to the SOFC 114 stack by bypassing a portion 209 of the
reformate stream 111 around the sulfur trap 112 and combines the
bypassed portion 209 of reformate stream 111 with the desulfurized
reformate stream 113 producing the conditioned reformate stream
115. The sulfur addition system shown in FIG. 6 includes a sulfur
sensor 202 positioned in the conditioned reformate stream 115. An
alternative embodiment is to position the sulfur sensor 202 in the
desulfurized reformate stream 113 upstream of the conditioned
reformate stream 115 (not shown).
[0035] The sulfur sensor 202 works in conjunction with the
controller 204 to detects and monitor the concentration of sulfur
in the conditioned reformate stream 115 to the SOFC stack 114. If
the concentration of sulfur is below a predetermined level, the
controller activates the metering device 206 to bypass a larger
portion of the reformate stream 111 containing sulfur around the
sulfur trap 112 to combined with the desulfurized reformate stream
113. If the concentration of sulfur is above a predetermined level,
the controller 204 reduces or eliminate the bypass portion 209 and
direct a greater portion of the reformate stream 111 through the
sulfur trap 112.
[0036] An advantage to this invention is that it offers an
effective low cost solution for significantly reducing carbon
attack of the nickel in the anode. Another advantage to this
invention is that it offers an effective low cost solution for
significantly reducing coarsening of nickel particles in the anode.
Still, another advantage is that diminutive amount of sulfur added
to desulfurized reformate stream feeding the SOFC stack
significantly prolonged the operational life and minimized
performance degradation of the SOFC stack. Yet, still another
advantage is that the invention can improve the longevity of the
SOFC stack system without having to perform extensive modification
to the SOFC stack system.
[0037] While this invention has been described in terms of the
preferred embodiments thereof, it is not intended to be so limited,
but rather only to the extent set forth in the claims that
follow.
* * * * *