Solid oxide fuel cell and a carbon direct-oxidizing-type...

Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation

Reexamination Certificate

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C429S010000

Reexamination Certificate

active

06183896

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a solid oxide fuel cell (solid electrolyte-type fuel cell) wherein a carbon-series fuel, such as coal and charcoal, is partially oxidized without requiring any external gasifying process or an external fuel-reforming process, which produces free energy that can be converted to electrical energy. The present invention further relates to a fuel electrode that has a catalytic activity for directly oxidizing carbon-series fuels in solid oxide fuel cells.
BACKGROUND OF THE INVENTION
In a conventional fuel cell system, for example, of a phosphoric acid electrolyte-type, an alkali electrolyte-type, or a solid polymer electrolyte-type, natural gas, petroleum, or coal, which serves as a raw fuel, must first be gasified and reformed in an external gasifying-and-reforming reaction step by combining reforming reactions (I) and (II), which are endothermic reactions, with an exothermic reaction (III), and by converting all of the mixed gases of carbon monoxide and water by the shift reaction (IV).
C
m
H
n
+mH
2
O→mCO+(m+n/2)H
2
  (I)
C
m
H
n
+mCO
2
→2mCO+n/2H
2
  (II)
C
m
H
n
+m/2O
2
→mCO+n/2H
2
  (III)
CO+H
2
O→H
2
+CO
2
  (IV)
In molten carbonate fuel cell systems and oxide fuel cell systems, which are high-temperature fuel cell systems, the shift reaction (IV) can be omitted. Nevertheless, in these fuel cell systems raw fuel is first converted to a mixed gas of hydrogen and carbon monoxide, by the reactions (I) to (III), which is then introduced into the fuel cell. Since in these conventional fuel cell systems, the step of gasifying and reforming the raw fuel and the fuel cell step are separate steps, the whole system is complicated. Therefore, there is a disadvantage in that the efficiency, for example, of heat recovery is low.
Further, when coal is used as a fuel, the operating conditions of the conventional coal gasifying-and-reforming step require a pressure and a temperature (above 1473 K) that are far higher than even those of solid oxide fuel cells, which operate at higher temperature than other types of fuel cells. Therefore, it is necessary that the coal gasifying-and-reforming step and the fuel cell step be separated into separate plants when making up a power-generation system. As a result, for example, it is not possible for the heat generated by solid oxide fuel cells to be recycled for effective utilization in the coal gasification process. Therefore, when coal is used as fuel, only a lower generation efficiency can be obtained in comparison with the use of natural gas.
Recently, a direct internal reforming method has been studied, wherein a separate gasifying-and-reforming reaction step is not required by incorporating the fuel gasifying-and-reforming step, which is an endothermic reaction, in the fuel cell stack in which an exothermic reaction takes place. This enables recovery of at least the heat loss that occurs with the reaction (III). In this direct internal reforming method, the activity of a conventional electrode, made up mainly of nickel, decreases significantly. In order to restrict this decrease in activity, it is necessary to add a large amount of water. However, because heat flows in and out with the evaporation and condensation of this added water, the efficiency of the whole system, including the efficiency of heat exchange, is considerably reduced.
Although natural gas has been studied with respect to the direct internal reforming method, hydrocarbon fuel, naphtha, and coal, which have higher molecular weights, have not yet been studied. This is because with the use of long-chain hydrocarbons, such as naphtha, in the direct internal reforming method, deposition of carbon is significant. As a result, a conventional nickel fuel electrode is “poisoned” and deactivated even if a large amount of steam is introduced. Therefore, since the carbon ratio of coal is even higher than that of naphtha, the direct internal reforming method using coal as a raw fuel has been considered impossible to pursue.
As discussed above, a coal gasifying apparatus and a fuel cell are combined into a complex system, which requires the use of a fuel cell into which coal can be directly introduced or a fuel cell that can be installed in a coal gasifying apparatus. Further, as a fuel electrode for such a fuel cell, a fuel electrode made of a new material is required. This material must not be “poisoned” or deactivated, like the above-mentioned nickel electrode, even when it comes in direct contact with coal. However, there are no conventional fuel electrodes that satisfy this requirement.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a material for an electrode that will not be poisoned with carbon, and an electrode comprising that material.
Further, another object of the present invention is to provide a solid oxide fuel cell that is high in efficiency, wherein even a carbon-series fuel, such as coal, may be used. Conventionally, it is difficult to attain high efficiency of generation of electric power using solid state materials such as coal when such materials are subjected to a direct oxidation, i.e., combustion, reaction. Typically, a gasifying-and-reforming step is separately required to get an active fuel gas and electric power can be generated from the gasifying-and-reforming step itself.
Another object of the present invention is to provide a direct internal-reforming-type fuel cell wherein a small amount of steam may or may not be required.
Other and further objects, features, and advantages of the invention will appear more evident from the following description, taken in connection with the accompanying drawings.


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Fickett, “General Characteristics [of fuel cells]”, Chapter 41 of “Handbook of Batteries and Fuel Cells”, Linden, editor, McGraw Hill, publisher (no month), 1984.
Linden, editior, “Handbook of Batteries, 2nd edition”, McGraw-Hill, pp. 2.19-2.22. (no month), 1995.
Nobuyoshi Nakagawa et al, Performance of an Internal Direct Oxidation Carbon Fuel . . . , Americal Chemical Society, pp. 1181-1185 (1988).
Leo Dubal, Recommended Practice for Solid Oxide Fuel Cell Products and Systems Evaluation, Swiss Federal Office of Energy, pp. 17-19, (1992) (no month).

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