Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
1999-07-29
2001-08-28
Brouillette, Gabrielle (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S010000, C429S010000, C429S006000, C429S006000, C429S057000
Reexamination Certificate
active
06280869
ABSTRACT:
BACKGROUND OF THE INVENTION
A fuel cell is an electric cell that converts the chemical energy of a fuel, typically hydrogen, directly into electric energy in a continuous process. Although fuel cells can be used with a variety of fuels and oxidants, they almost exclusively combine hydrogen and oxygen to form water vapor. Fuel cells include an anode in contact with the fuel, a cathode in contact with the oxygen and an electrolyte between the anode and cathode. Each cell typically creates less than one volt so that a series or stack of fuel cells are used to convert fuel into usable energy. Interconnect plates are used between each cell to keep the fuel and oxygen separated and to electrically connect the anode of one cell to the cathode of an adjacent cell.
One source of hydrogen is natural gas. A common way to obtain hydrogen from natural gas is by using a reformer which combines natural gas and steam at a high temperature, such as 760° C. Some fuel cells operate using a separate, external reformer to create the hydrogen; other fuel cells combine the function of a reformer into the fuel cell itself by operating the fuel cell at a high enough temperature, as well as other appropriate design considerations.
One type of fuel cell uses radial flow configurations. In one solid oxide fuel cell design, disclosed in M. Petrik et al., “Stack Development Status of the Interscience Radial Flow (IRF) SOFC”, An EPRI/GRI Fuel Cell Workshop on Fuel Cell Technology Research and Development, Atlanta, Georgia, Mar. 22-23, 1994, the fuel and air are fed to each cell through a pair of holes at the center region of the cell. The fuel and air then flow radially outwardly to the edge of the cell. This flow configuration requires seals to segregate the fuel and air at the feed points and also runs the risk of temperature excursions at the center of the cell where both rich fuel and rich oxygen exist. In another configuration, disclosed in M. Prica et al., “Contoured PEN Plates for Improved Thermomechanical Performance in SOFCs”, Proceedings of the Second European Fuel Cell Forum, Vol. 1, pp. 393-402, Oslo, Norway, May 6-10, 1996, the fuel and air are fed to the center of each cell through a pair of needles. These gases then flow radially to the cell edge. This flow configuration eliminates the gas seal requirement but still has problems with regard to temperature excursion. In another configuration, disclosed in European Patent 0,635,896 A1, the fuel is fed to the center of the cell by a feed needle while air is fed to the entire cathode area by distribution nozzles. The spent fuel and spent air are collected at the cell edge. This configuration eliminates the need for a gas seal and does not have temperature excursion problems. It does, however, require a complex gas nozzle distribution system.
U.S. Pat. No. 5,851,689 discloses a radial flow fuel cell stack assembly in which the fuel gas is fed to the anode of each cell at positions located about a half way between the periphery and center of the cell by a pair of feed needles. From the needle tips, the fuel gas is split along circular paths by deflectors to flow circumferentially and both radially inwardly and outwardly. The air feed flows radially inwardly from the cell edge to the center of the cathode where the spent air is collected and discharged through a collection needle. This radial flow configuration eliminates the need for a gas seal. As no rich fuel and rich oxidant coexist at any point, it also eliminates the temperature excursion problem without the need for complex gas distribution nozzles. Due to the split fuel flow, portion of the spent fuel (which includes cell reaction product water and residual fuel) can be collected by a collection needle at the anode center to recycle the cell reaction product water as the steam source for reforming through an external ejector. Thus, no external steam generation and boiler feed water treatment are required once the fuel cell stack has reached operating temperature. The split fuel flow also distributes the fuel quickly to the entire cell area.
In the '689 patent, the radially inwardly moving air flow passes through a metal sponge ring around the stack to pick up heat radiated from the stack before it flows into the cells. This removes the waste heat generated in the stack and meanwhile preheats the air feed. The final heating of the air to the cell operating temperature is provided by direct combustion of spent fuel at the cell edge. The radial air flow toward the stack eliminates the need for a heat exchanger to preheat the air feed and helps to confine the stack heat to a hot core inside the metal sponge ring. Due to the heat confinement, the stack can be housed in a relatively low temperature enclosure with no significant heat losses.
SUMMARY OF THE INVENTION
The present invention is directed to a design of a fuel cell stack and system and also a method for operating the fuel cell stack and system. The fuel cell stack includes at least first and second cells and a separator assembly. Each cell includes an anode, a cathode and electrolyte between the anode and cathode. The separator assembly has a fuel side and an oxidant side. The separator assembly also includes an anode spacer element between the fuel side and the anode of the first cell and a cathode spacer element between the oxidant side and the cathode of the second cell. The separator assembly further includes a cathode exhaust passageway between the oxidant side and the cathode of the second cell, and an anode feed passageway and an anode exhaust passageway between the fuel side and the anode of the first cell.
According to one aspect of the invention, a gas deflector, which is spaced apart from and overlying the anode separator element, is used to help prevent generally radially inwardly moving oxygen-containing gas, typically air, from contacting the anode while permitting the oxygen-containing-gas access to the peripheral edge of the cathode spacer element. This aspect of the invention helps to eliminate one of the potential problems with the system describe in the '689 patent. That is, the system of the '689 patent may require relatively precise manufacturing of the fuel cell stacks to help ensure that the fuel gas and air are distributed uniformly in all radial directions; without such uniformity, air could intrude to the anode side to oxidize the anode and spent fuel, including residual fuel, could intrude to the cathode side to reduce the cathode. This undesirable electrode oxidation or reduction can quickly cause the stack to fail. However, with this aspect of the present invention, an air deflector forms a shield or barrier to help prevent undesired gas intrusion into the electrodes and thus helps reduce the need for high manufacturing precision in the manufacture of the stack.
Another aspect of the invention involves the use of a generally continuous loop (typically circular) anode feed tube spaced apart from the cell peripheral edge, the feed tube having a plurality of fuel outlets therealong. Fuel outlets are preferably oriented to be radially inwardly and radially outwardly directed. This aspect of the invention provides an advantage over the invention of the '689 patent because it eliminates the need to use corrugated separator plates to form the necessary flow channels for the fuel gas. This aspect of the invention also eliminates the relatively large clamping forces needed to tightly press corrugated separator plates against the cells to create the desired fuel paths, such clamping force having the potential to cause the cells to crack. This is particularly true for solid oxide cells made of brittle ceramic materials.
A third aspect of the invention is directed to the use of an anode recycle passageway having a recycle inlet fluidly coupled to spent fuel, including residual fuel and cell reaction product water, at the anode central region and a recycle outlet located at a recycle position within the anode feed passageway so that the cell reaction product water can be used as a steam source for ref
Brouillette Gabrielle
Nexant, Inc.
Townsend and Townsend / and Crew LLP
Yuang Dah-Wei D.
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