U.S. patent application number 10/189696 was filed with the patent office on 2003-07-10 for procedure for starting up a fuel cell system having an anode exhaust recycle loop.
Invention is credited to Reiser, Carl A., Sawyer, Richard D., Steinbugler, Margaret M., Van Dine, Leslie L., Yang, Deliang.
Application Number | 20030129462 10/189696 |
Document ID | / |
Family ID | 21743188 |
Filed Date | 2003-07-10 |
United States Patent
Application |
20030129462 |
Kind Code |
A1 |
Yang, Deliang ; et
al. |
July 10, 2003 |
Procedure for starting up a fuel cell system having an anode
exhaust recycle loop
Abstract
A procedure for starting up a fuel cell system that is
disconnected from its primary load and that has air in both its
cathode and anode flow fields includes a) connecting an auxiliary
resistive load across the cell to reduce the cell voltage; b)
initiating a recirculation of the anode flow field exhaust through
a recycle loop and providing a limited flow of hydrogen fuel into
that recirculating exhaust; c) catalytically reacting the added
fuel with oxygen present in the recirculating gases until
substantially no oxygen remains within the recycle loop;
disconnecting the auxiliary load; and then d) providing normal
operating flow rates of fuel and air into respective anode and
cathode flow fields and connecting the primary load across the
cell. The catalytic reaction may take place on the anode or within
a catalytic burner disposed within the recycle loop. The procedure
allows start-up of the fuel cell system without the use of an inert
gas purge while minimizing dissolution of the catalyst and
corrosion of the catalyst support during the start-up process.
Inventors: |
Yang, Deliang; (Houston,
TX) ; Steinbugler, Margaret M.; (East Windsor,
CT) ; Sawyer, Richard D.; (Groveton, NH) ; Van
Dine, Leslie L.; (Manchester, CT) ; Reiser, Carl
A.; (Stonington, CT) |
Correspondence
Address: |
Stephen E. Revis
11 Brenthaven
Avon
CT
06001-3941
US
|
Family ID: |
21743188 |
Appl. No.: |
10/189696 |
Filed: |
July 3, 2002 |
Current U.S.
Class: |
429/415 ;
429/429; 429/432; 429/441; 429/444 |
Current CPC
Class: |
H01M 8/04097 20130101;
H01M 8/04238 20130101; H01M 8/04022 20130101; H01M 8/0258 20130101;
Y02E 60/50 20130101; H01M 8/04302 20160201; H01M 8/04007 20130101;
H01M 8/2457 20160201; H01M 16/003 20130101; H01M 8/04231 20130101;
H01M 8/04225 20160201; H01M 8/241 20130101 |
Class at
Publication: |
429/17 ;
429/22 |
International
Class: |
H01M 008/06; H01M
008/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 4, 2002 |
PCT/US02/00078 |
Claims
What is claimed is:
1. A procedure for starting-up a fuel cell system that is shut
down, the shut down system comprising a source of hydrogen
containing fuel, a fuel cell including a cathode flow field
adjacent the cathode of the cell on one side of the cell
electrolyte and an anode flow field adjacent the anode of the cell
on the other side of the cell electrolyte, an anode recycle loop
for recirculating at least a portion of the anode flow field
exhaust through the anode flow field, wherein both the anode flow
field and cathode flow field are filled with air, and the primary
electricity using device is disconnected from the fuel cell
external circuit, the start-up procedure comprising the steps of:
(A) initiating a recirculation, through the recycle loop, of the
anode flow field exhaust, which is initially 100% air, and then
providing a limited flow of hydrogen containing fuel into the
recirculating anode exhaust to create a mixture of hydrogen and air
within the recycle loop; (B) catalytically reacting hydrogen and
oxygen within the mixture on a catalyst within the recycle loop as
the hydrogen and oxygen circulates through the recycle loop in
contact with a catalyst to form water, and continuing to add
hydrogen containing fuel to the recirculating mixture until
substantially no oxygen remains in the recycle loop, including
regulating the rate at which fuel is added such that the
recirculating gases do not contain a flammable ratio of hydrogen
and oxygen when they contact the catalyst or when the enter the
anode flow field; and, (C) after substantially no oxygen remains in
the recycle loop, (i) increasing the rate of fuel flow into the
anode flow field to a normal operating flow rate, (ii) providing an
oxidant into the cathode flow field at a normal operating flow
rate, and, (iii) connecting the primary electricity using device to
the external circuit.
2. The start-up procedure according to claim 1, wherein, in step
(B), catalytically reacting hydrogen and oxygen includes passing
the recirculating anode exhaust and hydrogen containing fuel into a
burner disposed within the recycle loop, the burner including a
catalytic element therein, and catalytically reacting hydrogen and
oxygen on the catalytic element.
3. The start-up procedure according to claim 1, wherein, throughout
step (B), the hydrogen containing fuel introduced into the recycle
loop is mixed with the recirculating anode flow field exhaust, and
that mixture is introduced into the anode flow field, whereupon
oxygen is catalytically consumed at the cell anode in the presence
of the hydrogen.
4. The start-up procedure according to claim 1, wherein prior to
said step of providing a flow of hydrogen containing fuel in step
(A), connecting an auxiliary resistive load across the cell to
lower the cell voltage to a preselected value, and maintaining the
cell voltage at or below the preselected value at least until
substantially no oxygen remains in the recycle loop.
5. The shut-down procedure according to claim 4, wherein the
auxiliary resistive load reduces the cell voltage to 0.2 volts per
cell or less, and the cell voltage is maintained at or below 0.2
volts per cell until substantially no oxygen remains in the recycle
loop.
6. The start-up procedure according to claim 5, wherein a diode is
in series with the auxiliary load and limits the cell voltage to
0.2 volts per cell or less.
7. The start-up procedure according to claim 4, wherein during step
(B) no air flow is provided to the cathode flow field while the
auxiliary load is connected.
8. The start-up procedure according to claim 5, wherein the voltage
is maintained at or below 0.2 volts per cell by a diode in series
with the auxiliary load.
9. The start-up procedure according to claim 2, wherein prior to
said step of providing a flow of hydrogen containing fuel in step
(A), connecting an auxiliary resistive load across the cell to
lower the cell voltage to a preselected value, and maintaining the
cell voltage at or below the preselected value at least until
substantially no oxygen remains in the recycle loop.
10. The start-up procedure according to claim 9, wherein said
preselected value is 0.2 volts per cell.
11. The start-up procedure according to claim 9, wherein a diode is
in series with the auxiliary load and limits the cell voltage to
said preselected value throughout step (B).
12. The start-up procedure according to claim 4, wherein the amount
of hydrogen containing fuel added in step (B) is regulated to
maintain the amounts of hydrogen and oxygen entering the inlet of
the anode flow field below the flammability limit.
13. The start-up procedure according to claim 1, wherein the
recycle loop includes a plurality of burners disposed in series,
and wherein in step (A), providing a flow of fuel includes
providing a separate flow of fuel to each burner, wherein hydrogen
and oxygen are catalytically consumed within each of the
burners.
14. The start-up procedure according to claim 4, wherein said step
(B) of catalytically reacting hydrogen and oxygen includes passing
the recirculating anode exhaust through a plurality of burners
arranged in series within the recycle loop, each burner including a
catalytic element therein, and delivering a separate controlled
flow of hydrogen containing fuel into each of the catalytic
burners, and catalytically reacting said hydrogen and oxygen on
each of the catalytic elements to produce water.
15. The start-up procedure according to claim 14, wherein the
amount of reaction air added in step (B) is regulated to maintain
the amounts of hydrogen and oxygen mixture below the flammability
limit before it comes into contact with the burner catalytic
element.
16. The start-up procedure according to claim 14, wherein the cell
voltage is limited to 0.2 volts per cell or less throughout step
(B).
17. The start-up procedure according to claim 16, wherein a diode
is in series with the auxiliary load and limits the cell voltage to
0.2 volts per cell or less throughout step (B).
18. The start-up procedure according to claim 8, wherein in step
(B) the controlled amount of hydrogen added into the recirculating
anode exhaust is burned, in a difussion burning zone, with oxygen
in said exhaust, upstream of said catalytic element, and the
exhaust from said zone is directed over said catalytic element,
wherein oxygen and hydrogen remaining in said diffusion burning
zone exhaust catalytically react to form water.
19. The start-up procedure according to claim 17, wherein the
hydrogen to oxygen mixture of said diffusion burning zone exhaust
is below the flammability limit.
20. A procedure for starting-up a fuel cell system that is shut
down, the shut down system comprising a hydrogen containing fuel
source, a fuel cell including a cathode flow field filled with air
adjacent the cathode of the cell on one side of the cell
electrolyte and an anode flow field filled with air adjacent the
anode of the cell on the other side of the cell electrolyte, and an
anode recycle loop for recirculating anode flow field exhaust
through the anode flow field, the recycle loop including one or
more burners, wherein during shut-down the primary electricity
using device is disconnected from the fuel cell external circuit,
the start-up procedure comprising the steps of: (A) connecting an
auxiliary resistive load across the cell; (B) initiating a
recirculation of anode flow field exhaust through the recycle loop,
including through the burners; (C) providing a controlled flow of
hydrogen containing fuel into the burners and consuming, in each
burner, (i) at least some of the oxygen in the anode exhaust and
(ii) at least some of the fuel provided to the burners; and (D)
continuing the controlled flow of fuel into the burners and the
recirculation of the anode flow field exhaust through the burners
at least until there is substantially no oxygen remaining in the
anode exhaust, and, thereafter, (i) providing a fuel flow to the
anode flow field and air flow to the cathode flow field at normal
operating flow rates, and (ii) connecting the primary electricity
using device to the external circuit, wherein the auxiliary load
remains connected at least until there is substantially no oxygen
remaining in the anode exhaust to limit the cell voltage during the
start-up process.
21. The start-up procedure according to claim 19, wherein the
burners each have a catalytic element therein, and hydrogen and
oxygen are catalytically converted to water on the elements.
22. The start-up procedure according to claim 20, wherein there is
a diffusion burning zone within each burner upstream of the
catalytic element of each burner, and the fuel added to each burner
in step (C) is added to the diffusion zone where it is ignited and
diffusion burns with oxygen in the recirculating anode exhaust
gases passing through the burner.
23. The start-up procedure according to claim 21, wherein the rate
of fuel flow into the burner is regulated to assure that the
unburned gases leaving the diffusion zone of each burner, as well
as the recirculating gases entering the anode flow field are always
below the flammability limits.
24. The start-up procedure according to claim 12, wherein, in
step(B), burning hydrogen and oxygen in a catalytic burner
comprises passing the anode exhaust through a plurality of burners
arranged in series, each burner including a catalytic element, and
said step of providing fuel includes providing a separate flow of
fuel for each catalytic burner in the series.
25. The start-up procedure according to claim 18, wherein the
auxiliary resistive load reduces the cell voltage to 0.2 volts per
cell or less, and the cell voltage is maintained at or below 0.2
volts per cell until substantially no oxygen remains in the recycle
loop.
26. The start-up procedure according to claim 25, wherein a diode
is in series with the auxiliary load and limits the cell voltage to
0.2 volts per cell or less.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field
[0002] This invention relates to fuel cell systems and, more
particularly, to procedures for starting up a fuel cell system.
[0003] 2. Background Information
[0004] It is well known in the fuel cell art that, when the
electrical circuit is opened and there is no longer a load across
the cell, such as upon and during shut-down of the cell, the
presence of air on the cathode, coupled with hydrogen fuel
remaining on the anode, often cause unacceptable anode and cathode
potentials, resulting in catalyst and catalyst support oxidation
and corrosion and attendant cell performance degradation. It was
thought that inert gas needed to be used to purge both the anode
flow field and the cathode flow field immediately upon cell
shut-down to passivate the anode and cathode so as to minimize or
prevent such cell performance degradation. Further, the use of an
inert gas purge avoided the possible occurrence of a flammable
mixture of hydrogen and air, which is a safety issue. While the use
of 100% inert gas as the purge gas is most common in the prior art,
commonly owned U.S. Pat. Nos. 5,013,617 and 5,045,414 describe
using 100% nitrogen as the anode side purge gas, and a cathode side
purging mixture comprising a very small percentage of oxygen (e.g.
less than 1%) with a balance of nitrogen. Both of these patents
also discuss the option of connecting a dummy electrical load
across the cell during the start of purge to lower the cathode
potential rapidly to between the acceptable limits of 0.3-0.7
volt.
[0005] It is undesirable to use nitrogen or other inert gas as a
shut-down or start-up purge gas for fuel cells where compactness
and service interval of the fuel cell powerplant is important, such
as for automotive applications. Additionally, it is desired to
avoid the costs associated with storing and delivering inert gas to
the cells. Therefore, safe, cost effective shut-down and start-up
procedures are needed that do not cause significant performance
degradation and do not require the use of inert gases, or any other
gases not otherwise required for normal fuel cell operation.
BRIEF SUMMARY OF THE INVENTION
[0006] In accordance with the present invention, a procedure for
starting up a fuel cell system that is disconnected from its
primary load and has air in both its cathode and anode flow fields
includes a) initiating a recirculation of the anode flow field
exhaust through a recycle loop and providing a limited flow of fuel
into the recirculating exhaust; b) catalytically reacting the
hydrogen and oxygen in the fuel and air mixture as it recirculates
until substantially no oxygen remains in the recycle loop; and then
c) increasing the fuel flow rate into the anode flow field to
normal operating levels and thereafter connecting the primary load
across the cell.
[0007] Preferably an auxiliary resistive load is connected across
the cell as the oxygen is being consumed to reduce the cell voltage
during the process. The use of a diffusion burner and separate
catalytic burner in the recycle loop, or a catalytic burner alone,
is also advantageous to speed up the removal of oxygen.
[0008] In one experiment using a stack of PEM fuel cells of the
general type described in commonly owned U.S. Pat. No. 5,503,944,
the primary electricity using device was disconnected, and the flow
of fuel (hydrogen) to the anode and the flow of air to the cathode
were shut off. No attempt was made to purge the anode flow field of
residual fuel or to purge the cathode flow field of air, such as by
using an inert gas purge. To restart the cell, fuel and oxidant
were flowed directly into their respective flow fields. (The
foregoing procedure is hereinafter referred to as an "uncontrolled"
start/stop cycle.) It was found that a cell stack assembly operated
in this manner experienced rapid performance decay which had not
previously been observed. Further, it was discovered that a large
number of start/stop cycles were more detrimental to cell
performance than were a large number of normal operating hours
under load. It was eventually determined, through experimentation,
that both the shut-down and start-up procedures were contributing
to the rapid performance decay being experienced by the cell; and
it was known that such rapid decay did not occur when, in
accordance with prior art techniques, inert gas was used to
passivate the cell at each shut down. Examination of used cells
that experienced only a few dozen uncontrolled start/stop cycles
showed that 25% to 50% of the high surface area carbon black
cathode catalyst support was corroded away, which had not
previously been reported in the prior art.
[0009] Further testing and analysis of results led to the belief
that the following mechanism caused the performance decay
experienced in the foregoing experiment: With reference to FIG. 2,
a diagrammatic depiction of a PEM fuel cell is shown. (Note that
the mechanism to be described is also applicable to cells using
other electrolytes, such as phosphoric acid or potassium hydroxide
with appropriate changes in ion fluxes.) In FIG. 2, M represents a
proton exchange membrane (PEM) having a cathode catalyst layer C on
one side and an anode catalyst layer A on the other side. The
cathode air flow field carrying air to the cathode catalyst is
divided into air zones 1 and 2 by a vertical dotted line that
represents the location of a moving hydrogen front through the
anode flow field, as further described below. The anode fuel flow
field that normally carries hydrogen over the anode catalyst from
an inlet I to an exit E is also divided into two zones by the same
dotted line. The zone to the left of the dotted line and adjacent
the inlet I is filled with hydrogen and labeled with the symbol
H.sub.2. The zone to the right of the dotted line and adjacent the
exit E is zone 3 and is filled with air.
[0010] Upon an uncontrolled shut-down (i.e. a shut-down without
taking any special steps to limit performance decay) some of the
residual hydrogen and some of the oxygen in their respective anode
and cathode flow fields diffuse across the PEM (each to the
opposite side of the cell) and react on the catalyst (with either
oxygen or hydrogen, as the case may be) to form water. The
consumption of hydrogen on the anode lowers the pressure in the
anode flow field to below ambient pressure, resulting in external
air being drawn into the anode flow field at exit E creating a
hydrogen/air front (the dotted line in FIG. 2) that moves slowly
through the anode flow field from the fuel exit E to the fuel inlet
I. Eventually the anode flow field (and the cathode flow field)
fills entirely with air. Upon start-up of the cell, a flow of air
is directed into and through the cathode flow field and a flow of
hydrogen is introduced into the anode flow field inlet I. On the
anode side of the cell this results in the creation of a
hydrogen/air front (which is also represented by the dotted line in
FIG. 2) that moves across the anode through the anode flow field,
displacing the air in front of it, which is pushed out of the cell.
In either case, (i.e. upon shut-down and upon start-up) a
hydrogen/air front moves through the cell. On one side of the
moving front (in the zone H.sub.2 in FIG. 2) the anode is exposed
substantially only to fuel (i.e. hydrogen); and in zone 1 of the
cathode flow field, opposite zone H.sub.2, the cathode is exposed
only to air. That region of the cell is hereinafter referred to as
the H.sub.2/air region: i.e. hydrogen on the anode and air on the
cathode. On the other side of the moving front the anode is exposed
essentially only to air; and zone 2 of the cathode flow field,
opposite zone 3, is also exposed to air. That region of the cell is
hereinafter referred to as the air/air region: i.e. air on both the
anode and cathode.
[0011] The presence of both hydrogen and air within the anode flow
field results in a shorted cell between the portion of the anode
that sees hydrogen and the portion of the anode that sees air. This
results in small in-plane flow of protons (H.sup.+) within the
membrane M and a more significant through-plane flow of protons
across the membrane, in the direction of the arrows labeled
H.sup.+, as well as an in-plane flow of electrons (e.sup.-) on each
side of the cell, as depicted by the arrows so labeled. The
electrons travel through the conductive catalyst layers and other
conductive cell elements that may contact the catalyst layer. On
the anode side the electrons travel from the portion of the anode
that sees hydrogen to the portion that sees air; and on the cathode
side they travel in the opposite direction.
[0012] The flow of electrons from the portion of the anode that
sees hydrogen to the portion of the anode that sees air results in
a small change in the potential of the electron conductor. On the
other hand, electrolytes in the membrane are relatively poor
in-plane proton conductors, and the flow of protons results in a
very significant drop in the electrolyte potential between zones
H.sub.2 and 3.
[0013] It is estimated that the reduction in electrolyte potential
between zones H.sub.2 and 3 is on the order of the typical cell
open circuit voltage of about 0.9-1.0 volts. This drop in potential
results in a proton flow across the PEM, M, from the cathode side,
zone 2, to the anode side, zone 3, which is the reverse direction
from what occurs under normal cell operating conditions. It is also
estimated that the reduction in electrolyte potential in the
portion of the anode that sees air (in zone 3) results in a cathode
potential in zone 2 of approximately 1.5 to 1.8 volts, versus the
normal cathode potential of 0.9 to 1.0 volts. (Note: These
potentials are relative to the hydrogen potential at the same
operating conditions.) This elevated cathode potential results in
rapid corrosion of the carbon support material and the cathode
catalyst, causing significant cell performance decay.
[0014] One object of the present invention is to minimize fuel cell
catalyst and catalyst support corrosion occurring during start-up
of the fuel cell, and to do it without purging air from the cells
with inert gas upon start-up.
[0015] In accordance with one embodiment of the start-up procedure
of the present invention, a recirculation of the gases within the
anode flow field, which is initially 100% air, is initiated within
a recycle loop, and a limited flow of hydrogen containing fuel is
provided into the recycle loop upstream of the inlet to the anode
flow field to create a recirculating mixture that includes hydrogen
and air. As the gas mixture circulates through the loop, hydrogen
and oxygen within the mixture come into contact with the anode
catalyst and react to form water, thereby depleting the
recirculating stream of oxygen. When substantially all the oxygen
within the recycle loop is gone, the fuel flow rate into the anode
flow field is increased to normal operating levels and the primary
load is connected across the cell.
[0016] Since only a mixture of air and a limited amount of hydrogen
enter the anode flow field during the start-up procedure described
above, there is no distinct hydrogen/air front traversing the anode
flow field; and there is never a time when one region of the anode
sees only hydrogen and the other sees only air. Thus, the high
cathode potentials that cause catalyst and catalyst support
corrosion are avoided.
[0017] Except in certain specific instances which are described
later with respect to certain embodiments of the present invention,
for safety reasons, the amount of hydrogen added into the recycle
loop while air is present should be less than an amount that would
result in a flammable mixture of hydrogen and oxygen. More than
about 4% oxygen (equivalent to about 20% air), by volume, in
hydrogen is considered in excess of the flammability limit; and
more than about 4%, by volume, hydrogen in air is considered in
excess of the flammability limit. Thus, if the recycle loop
contains 100% air, the rate of hydrogen flow into the recycle loop
should initially not exceed about 20% of the total recycle loop
flow rate, and is preferably lower than 20% to allow a safety
margin. A device for measuring the ratio of oxygen to hydrogen in
the circulating gases may be placed in the recycle loop and used to
control valves or other devices used to feed gases into the recycle
loop.
[0018] During start-up the recirculating gas stream is partially
vented (e.g. a small portion of the recirculating stream leaves the
system through a vent valve) to maintain the stream at
substantially ambient pressure. This also keeps the nitrogen
concentration low, and eliminates any need to purge the recycle
loop with hydrogen before the primary load can be connected across
the cell.
[0019] In another embodiment of the start-up procedure of the
present invention, the air that is present within the anode flow
field at the initiation of the start-up procedure is recirculated
within a recycle loop that includes at least one burner (and
preferably a plurality of burners, in series) having a
catalytically coated element therein. A limited amount of hydrogen
containing fuel is injected directly into the burner or into the
recirculating gas stream upstream of the burner; and that fuel
catalytically reacts, on the catalytic element, with the oxygen in
the recirculating stream to produce water and heat. (If a plurality
of burners are used, it is preferable that each burner be provided
with its own separate fuel flow.) The amount of hydrogen added to
the burner (or to each of the series of burners) is regulated so as
to minimize the amount of unconsumed hydrogen entering the anode
flow field while oxygen is still present within the recycle loop,
and to avoid creating a flammable mixture of hydrogen and oxygen,
as discussed above. The exhaust from the burner, significantly
depleted of oxygen, is circulated through the anode flow field and
through the burner until the recycle loop is substantially depleted
of oxygen. As in the preceding embodiment, the recycle loop is
preferably purged with hydrogen before the primary load is
reconnected and normal flow rates of air and fuel are delivered
into the cathode and anode flow fields, respectively.
[0020] Preferably, each burner includes a diffusion burner upstream
of and in series with the catalytic burner element, and preferably
integrated within the same housing. The diffusion burner includes
an igniter that is used to initiate the diffusion burning of the
air and hydrogen entering the diffusion burner. The diffusion
burning process speeds up the shut-down process by more quickly
consuming the hydrogen in the recycle stream (as compared to
catalytic burning alone); however, diffusion burning alone is not
as effective as catalytic burning for removing the hydrogen to the
levels required of the present invention. The combination of the
two provide the desired speed and substantially complete removal of
the hydrogen. The flammability limits, which, as discussed above,
should be observed for safety within the fuel cell system,
obviously do not apply to the diffusion burner; however, the
flammability limits should be observed with regard to the gas
composition leaving the diffusion burning zone.
[0021] To insure that the cathode potential does not rise to levels
that can cause rapid rates of cathode catalyst and catalyst support
corrosion during start-up, in each of the foregoing embodiments it
is preferred to reduce the cell voltage to a preselected low value
and to maintain or limit the cell voltage to no more than that low
value throughout the start-up procedure. This may be accomplished
by connecting and maintaining a small auxiliary resistive load
across the cell throughout the time hydrogen is delivered into the
recycle loop or into the burners during the start-up procedure. A
diode in series with the auxiliary load may be used to limit the
cell voltage to the preselected value. For best results, the cell
voltage is limited to approximately 0.2 volts per cell, or
less.
[0022] The following commonly owned U.S. non-provisional patent
applications, filed on Dec. 20, 2000, describe and claim inventions
related to the subject matter of this application: U.S. Ser. No.
742,497 "Procedure for Shutting Down a Fuel Cell System Using Air
Purge", invented by Carl Reiser, Richard Sawyer and Deliang Yang;
and U.S. Ser. No 742,481 "Procedure for Starting Up a Fuel Cell
System Using a Fuel Purge", invented by Carl Reiser, Richard
Sawyer, and Deliang Yang. The following commonly owned U.S.
non-provisional patent application, filed on Jan. 25, 2001,
describes and claims an invention related to the subject matter of
this application: U.S. Ser. No. 09/770,042 "Procedure for Shutting
Down a Fuel Cell System Having an Anode Exhaust Recycle Loop",
invented by Leslie Van Dine, Margaret Steinbugler, Carl Reiser, and
Glenn Scheffler.
[0023] 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 THE DRAWINGS
[0024] FIG. 1 is a schematic depiction of a fuel cell system that
may be operated in accordance with the start-up procedures of the
present invention.
[0025] FIG. 2 is a diagrammatic view of a fuel cell cross-section
used to explain a mechanism that may cause cell performance
degradation during start-up and shut-down.
DETAILED DESCRIPTION OF THE INVENTION
[0026] In FIG. 1, a fuel cell system 100 is shown. The system
includes a fuel cell 102 comprising an anode 104, a cathode 106,
and an electrolyte layer 108 disposed between the anode and
cathode. The anode includes an anode substrate 110 having an anode
catalyst layer 112 disposed thereon on the side of the substrate
facing the electrolyte layer 108. The cathode includes a cathode
substrate 114, having a cathode catalyst layer 116 disposed thereon
on the side of the substrate facing the electrolyte layer 108. The
cell also includes an anode flow field plate 118 adjacent the anode
substrate 110 and a cathode flow field plate 120 adjacent the
cathode substrate 114.
[0027] The cathode flow field plate 120 has a plurality of channels
122 extending thereacross adjacent the cathode substrate forming a
cathode flow field for carrying an oxidant, preferably air, across
the cathode from an inlet 124 to an outlet 126. The anode flow
field plate 118 has a plurality of channels 128 extending
thereacross adjacent the anode substrate forming an anode flow
field for carrying a hydrogen containing fuel across the anode from
an inlet 130 to an outlet 132. Each cell also includes a cooler 131
adjacent the cathode flow field plate 120 for removing heat from
the cell, such as by using a water pump 134 to circulate water
through a loop 132 that passes through the cooler 131, a radiator
136 for rejecting the heat, and a flow control valve or orifice
138.
[0028] Although only a single cell 120 is shown, in actuality a
fuel cell system would comprise a plurality of adjacent cells (i.e.
a stack of cells) connected electrically in series, each having a
cooler separating the cathode flow field plate of one cell from an
anode flow field plate of the adjacent cell. For more detailed
information regarding fuel cells like the one represented in FIG.
1, the reader is directed to commonly owned U.S. Pat. Nos.
5,503,944 and 4,115,627, both of which are incorporated herein by
reference. The '944 patent describes a solid polymer electrolyte
fuel cell wherein the electrolyte layer is a proton exchange
membrane (PEM). The '627 patent describes a phosphoric acid
electrolyte fuel cell wherein the electrolyte layer is a porous
silicon carbide matrix layer containing liquid electrolyte within
its pores.
[0029] Referring, again, to FIG. 1, the fuel cell system includes a
source 140 of hydrogen containing fuel, under pressure, a source
142 of air, an air blower 144, a primary electricity using device
referred to herein as the primary load 146, a diode 147, an
auxiliary resistive load 148 in series with the diode, and an anode
exhaust recycle loop 149. For purposes of this application and the
appended claims, the recycle loop 149 is considered to include the
anode catalyst layer 112, the porous anode substrate 110, as well
as the channels 128 that define the anode flow field.
[0030] Disposed in the recycle loop is a recycle loop blower 150, a
plurality of burners 151a, 151b, 151c, and a recycle loop heat
exchanger 152. Each burner includes an electrically heated,
catalytically coated burner element therewithin designated 153a,
153b, 153c, respectively. Within each burner, upstream of each
catalytic element, is a diffusion burning zone 155a, 155b, and
155c, respectively. Extending into each such zone is an igniter
157a, 157b, and 157c, respectively. A branch 164a of a conduit 164
from the air source 142 carries air to separate air feed valves
162a, 162b, 162c, which control the flow of air into each diffusion
burning zone. (If desired, the air feed valves 162a, 162b, and 162c
could be fixed orifices, in which case there would be a single air
shut-off valve (not shown) in the conduit 164 upstream of all the
air feed orifices.) A branch 164b carries air into the recycle
loop, preferably upstream of the blower 150. The branch 164b
includes a restricting orifice 180 and an air bleed control valve
182 for controlling the flow. A conduit 192 from the fuel source
140 carries fuel to separate fuel feed valves 190a, 190b, 190c,
which control the flow of fuel into each diffusion burner zone. (If
desired, the fuel feed valves 190a, 190b, and 190c could be fixed
orifices, in which case there would be a single fuel shut-off valve
(not shown) in the conduit 192 upstream of all the fuel feed
orifices.)
[0031] Although only a single heat exchanger 152 is shown located
immediately downstream of the burners, there are other possible
locations for the heat exchanger, and more than one may be used.
For example, there may be a heat exchanger located immediately
downstream of each burner; or each burner may have a heat exchanger
integral therewith. The type of heat exchanger used and its
location are not considered part of the present invention.
[0032] Normal Fuel Cell Operation
[0033] During normal fuel cell operation, when the cell is
providing electricity to the primary load 146, a primary load
switch 154 is closed (it is shown open in the drawing), and an
auxiliary load switch 156 is open. The air blower 144, the anode
flow field exhaust recycle blower 150 and the coolant pump 134 are
all on. The valve 182 is closed. A fuel feed valve 166 in a fuel
feed conduit 168 to the anode flow field is open, as is an anode
exhaust vent valve 172 in an anode exhaust conduit 174. The coolant
loop flow control valve 138 is also open. The air feed valves 162a,
162b, and 162c and the fuel feed valves 190a, 190b, and 190c are
closed; and the catalytic burner elements 153a, 153b, 153c are
off.
[0034] Thus, during normal operation, air from the source 142 is
continuously delivered into the cathode flow field inlet 124 via
the conduit 160 and leaves the outlet 126 via a conduit 176. A
hydrogen containing fuel from the pressurized source 140 is
continuously delivered into the anode flow field via the conduit
168. A portion of the anode exhaust containing depleted hydrogen
fuel leaves the anode flow field through the vent valve 172 via the
conduit 174, while the recycle blower 150 recirculates the balance
of the anode exhaust through the anode flow field via the recycle
loop in a manner well know in the prior art. Recycling a portion of
the anode exhaust helps maintain a relatively uniform gas
composition from the inlet 130 to the outlet 132 of the anode flow
field. As the hydrogen in the circulating gases passes through the
anode flow field, it electrochemically reacts on the anode catalyst
layer in a well-known manner to produce protons (hydrogen ions) and
electrons. The electrons flow from the anode 104 to the cathode 106
through an external circuit 178 to power the load 146.
[0035] Start-Up Procedure
[0036] Assuming the fuel cell system described above has been
shut-down and has 100% air within both the anode and cathode flow
fields. The following procedures may be used to restart that system
or any idle fuel cell system with air in the anode and cathode flow
fields.
[0037] In a preferred start-up procedure according to the present
invention, the valve 138 is opened and the coolant pump 134 is
turned on. Before initiating any hydrogen flow into the system, the
auxiliary resistive load 148 is optionally, but preferably,
connected across the cell by closing the switch 156 in the external
circuit. Application of the auxiliary load lowers cell voltage and
cathode potential during the start-up process to minimize cell
performance decay during the start-up procedure. Preferably the
cell voltage is maintained below 0.2 volts per cell throughout
start-up. Therefore, the diode 147 is selected to allow current to
pass through the auxiliary load whenever the cell voltage rises
above 0.2 volts per cell. In that way the cell voltage is limited
to 0.2 volts per cell or less during start-up.
[0038] With the air blower 144 off and the anode exhaust vent valve
172 partially opened, the recycle loop blower 150 is turned on to
initiate a recirculation of a portion of the anode flow field
exhaust gases through the recycle loop 150. The primary fuel flow
valve 166 remains closed. The igniters 157a, 157b, and 157c are
activated; and the fuel flow valves 190a, 190b, and 190c in the
conduit 192 are opened to allow a regulated, limited flow of
hydrogen containing fuel into each of the diffusion zones 155a,
155b, and 155c of the burners 151a, 151b, and 151c, respectively.
The hydrogen feed rate is limited such that the burner catalytic
elements do not exceed 700-800.degree. F. during operation. A
hydrogen feed rate of about 18% of the recycle gas flow rate has
been found to be acceptable.
[0039] Tn the first burner 151a, the added hydrogen and the
recirculating anode exhaust enters the preferred, but optional,
diffusion burning zone 155a where it is ignited by the igniter
157a. Some of the oxygen in the recirculating anode exhaust not
consumed in the diffusion burning zone 155a catalytically reacts
with unconsumed hydrogen on the catalytic burner element to produce
water. The exhaust from the burner 151a enters the next burner in
the series, and oxygen in that stream is consumed in the same
manner, and so forth, until the recirculating exhaust has passed
through all the burners and again passes through the anode flow
field. The recirculation of the anode flow field exhaust, as well
as the diffusion and catalytic burning are continued until
substantially no oxygen remains within the recycle loop. As a final
step in the process, a hydrogen purge of the anode flow field may
be conducted.
[0040] Normal fuel cell operation may now commence by closing the
valves 190a, 190b, and 190c; opening the switch 156 to disconnect
the auxiliary load; opening the vent valve 172 to a normal
operating position; opening the primary fuel flow valve 166 and
turning on the air blower 144 so as to deliver both air from the
source 142 and fresh fuel from the source 140 into their respective
cathode and anode flow fields at normal operating flow rates; and
closing the switch 154 to connect the primary load.
[0041] Although this preferred embodiment uses diffusion burning in
conjunction with a catalytic "burning", start-up may be
accomplished using catalytic burning, alone. The advantage of
diffusion burning is the significantly faster speed of the start-up
procedure by the more rapid burning of the oxygen. The small
amounts of oxygen not consumed by the diffusion burning are
consumed by catalytic reaction on the catalytic elements downstream
of each diffusion burning zone. In that manner, substantially all
the oxygen is removed.
[0042] Although not recommended due to the long start-up time, the
fuel cell system may also be started without the use or presence of
a burner in the recycle loop by simply reacting the hydrogen and
oxygen on the anode catalyst. In that procedure, with the anode
exhaust vent valve 172 at least partially opened, the blower 150 is
turned on to initiate a recirculation of anode flow field exhaust
gases (initially 100% air) through the recycle loop 150. As in
other embodiments, the auxiliary load is connected to limit the
cell voltage. The fuel flow valve 166 is opened to allow a limited
continuous flow of hydrogen containing fuel to mix with the
recirculating gases upstream of the anode flow field inlet 130.
(The maximum amount of hydrogen that may be added is controlled by
the flammability limits, as discussed above, and may be monitored
by a sensor in the recycle loop.) As the mixture circulates through
the recycle loop, the hydrogen and the oxygen in the circulating
gases catalytically react within the cell on the anode in the
presence of the anode catalyst to produce water. This is continued
until substantially no oxygen remains in the circulating gases, at
which time the auxiliary load is disconnected, the primary load is
connected, and the fuel cell system may resume normal operation, as
described above.
[0043] In the foregoing embodiments the auxiliary load is applied
throughout the entire time that hydrogen is delivered into the
recycle loop during start-up. It should be noted that benefits will
be obtained if the auxiliary load is applied from any time after
the hydrogen is first introduced until any time prior to
substantially all the oxygen having been removed.
[0044] 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.
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