Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2002-04-03
2004-09-14
Alejandro, Raymond (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C429S010000, C429S006000, C251S129040, C251S210000, C204S229400, C204S228300
Reexamination Certificate
active
06790548
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a fuel cell system and more particularly to a system having a plurality of cells which consume an H
2
-rich gas to produce power.
BACKGROUND OF THE INVENTION
Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are commonly bundled together to form a fuel cell stack and are commonly arranged in series. Each cell within the stack comprises the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. A group of adjacent cells within the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
In PEM fuel cells, hydrogen (H
2
) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O
2
), or air (a mixture of O
2
and N
2
). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies are relatively expensive to manufacture and require certain conditions, including proper water management and humidification, and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
For vehicular applications, it is desirable to use a liquid fuel such as an alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished within a chemical fuel processor or reformer. The fuel processor contains one or more reactors wherein the fuel reacts with steam and sometimes air, to yield a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide. In reality, carbon monoxide and water are also produced. In a gasoline reformation process, steam, air and gasoline are reacted in a fuel processor which contains two sections. One is primarily a partial oxidation reactor (POX) and the other is primarily a steam reformer (SR). The fuel processor produces hydrogen, carbon dioxide, carbon monoxide and water. Downstream reactors may include a water/gas shift (WGS) and preferential oxidizer (PROX) reactors. In the PROX carbon dioxide (CO
2
) is produced from carbon monoxide (CO) using oxygen from air as an oxidant. Here, control of air feed is important to selectively oxidize CO to CO
2
.
Fuel cell systems which process a hydrocarbon fuel to produce a hydrogen-rich reformate for consumption by PEM fuel cells are known and are described in co-pending U.S. patent application Ser. Nos. 08/975,422 and 08/980,087, filed in November, 1997 now U.S. Pat. Nos. 6,332,005 and 6,077,620, respectively, and U.S. Ser. No. 09/187,125, filed in November, 1998 now U.S. Pat. No. 6,238,815 and each assigned to General Motors Corporation, assignee of the present invention; and in International Application Publication Number WO 98/08771, published Mar. 5, 1998. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994, and assigned to General Motors Corporation.
Efficient operation of a fuel cell system depends on the ability to effectively control gas flows (H
2
reformate and air/oxygen) to the fuel cell stack not only during start-up and normal system operation, but also during system shutdown. During the shutdown of a fuel cell system that generates hydrogen from liquid fuel, the anode CO emissions increase and can degrade the stack. Accordingly, a primary concern during shutdown is diverting the gas flows of H
2
and air/oxygen around or away from the fuel cell stack and disposing of the excess H
2
. The H
2
and air flows being diverted from the stack during shutdown must also be kept separate to avoid creating a combustible mixture in the system. The stack must also be protected from prolonged (e.g., greater than five seconds) pressure differentials which could rupture the thin membranes in the membrane electrode assembly (MEA) separating the anode and cathode gases.
Fuel cell systems, in particular those used in vehicular applications, are often used to generate start-up and transient heat for the fuel processor. The combustor is fueled by the anode and cathode effluents, supplemental hydrocarbon fuel for start-up and high demand situations, and excess H
2
from the fuel processor. The combustor is also useful for burning off residual stack effluents and processor H
2
during system shut-down. During normal system operation, the combustor typically runs at a constant temperature, for example around 600° Celsius in an exemplary vehicle propulsion system application. It is important at all times to prevent the combustor from overheating, as the resulting degradation would require an expensive replacement and would interfere with the operation of the system as a whole. The combustor therefore generally receives a continuous air flow from the system air supply. Air flow to the combustor must be maintained during shutdown to prevent overheating as the combustor burns off residual gases.
The cooling of the combustor therefore competes with the shutdown objectives of gas flow diversion and residual H
2
combustion. Especially where the air supply to the system generally supplies both the combustor and the cathode inlet of the fuel cell stack, the diversion and venting of air from the cathode inlet must not even temporarily deprive the combustor of sufficient airflow for cooldown.
During normal shutdown of the system in which time is not a factor, the competing demands of gas flow diversion and combustor cooldown are relatively easy to offset and satisfy. However, during rapid shutdown, carbon monoxide emissions at the stack anode and pressure differentials at the cathode need to be dissipated in a few seconds. At the same time, sufficient air flow must be maintained to the combustor for the lengthier cooldown period. The coordinated diversion and venting of the gas flows with respect to both the fuel cell stack and combustor becomes difficult.
SUMMARY OF THE INVENTION
In one aspect, the invention provides a venting methodology for staging the diversion and venting of reformate H
2
and air relative to the fuel cell stack, the combustor, and one or more vents. This staged venting protects the stack from degradation due to CO and due to high pressure differentials, and protects the combustor from overheating. In ano
Clingerman Bruce J.
Doan Tien M.
Keskula Donald H.
Alejandro Raymond
Brooks, Esq. Cary W.
Deschere, Esq. Linda M.
General Motors Corporation
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