1. Field of the Invention
The present invention relates to an internal reforming type molten carbonate fuel cell, more particularly to an improved fuel cell which improves a method of supplying fuel gas to a cell stack.
2. Description of the Related Art
A fuel cell is a cell which is capable of providing direct current produced by the reaction of a readily oxidized gas such as hydrogen with an oxidizing gas such as oxygen through an electrochemical process.
The fuel cells are classified broadly according to their electrolyte into a phosphoric acid type, a molten carbonate type and a solid electrolytic type.
An outline of the constitution of the molten carbonate type fuel cell is described below:
The molten carbonate fuel cell has a fuel cell stack in which a plurality of single cells are piled up with separators interposed respectively therebetween, the separators being formed with fuel gas passages and oxidant gas passages, respectively crossing each other at right angles on the respective surfaces thereof. The single cell is comprised of a pair of gas diffusion electrodes and a molten carbonate electrolytic layer interposed between the electrodes. In this way in the molten carbonate fuel cell, applying the conventional process, H.sub.2 and CO are produced from raw fuel (for example, from various hydrocarbons, primarily methane), and become activated through an electrochemical reaction at the anode. The conventional types are generally classified into two types of the fuel cells, namely, an internal reforming type and an external or outer reforming type. In the external reforming type cell, hydrogen to be consumed as raw fuel in the fuel cell is generated from a hydrocarbon outside the fuel cell stack.
The internal reforming type cell is a type of the fuel cell which is capable of performing a reforming reaction and an electrochemical reaction simultaneously inside the fuel cell stack. The reforming reaction is to generate hydrogen and carbon monoxide from the hydrocarbon, to be consumed as raw fuel in the fuel cell. The internal reforming type cell is characterized in that a combination of an endothermic reforming reaction and an exothermic fuel cell reaction is utilized to compensate for excess or deficiency in the amount of heat. Accordingly, a fuel cell power generating system provided with the internal reforming type cell can readily attain higher efficiency of power generation as well as more effective use of heat, compared to the conventional power generating system provided in the outer reforming type fuel cell. The internal reforming type cells are mainly classified into a direct internal reforming type and an indirect internal reforming type according to their structure and characteristics. In the direct internal reforming type cell, reforming catalyst is placed in a gas channel at a fuel gas side electrode (anode) and a reforming reaction and an electrode reaction proceed simultaneously.
On the other hand, the indirect internal reforming type cell is provided with a reforming section at an area which is separated from and thermally adjacent to the fuel gas channel. Such arrangement can allow the heat of formation at the fuel cell to be utilized as reaction heat for reforming, so that reforming and electrode reactions can proceed independently.
In the direct internal reforming type cell, a fuel gas channel holds a reforming catalyst, and a reforming reaction taken place directly in the cell. The most severe problem inherent in a cell structure of this type is the poisoning of the reforming catalyst caused by an electrolyte held in the fuel gas electrode. For eliminating such a defect, there is provided an indirect internal reforming type cell in which the reforming reaction is separated from the electrode reaction. In this type of the cell, indirect reformers are piled up with cell units of several single cells interposed respectively therebetween to form a cell stack.
FIGS. 8 to 10 show a conventional indirect internal reforming type molten carbonate fuel cell disclosed in Japanese Patent Application No.01-185256 by the same applicant. There is shown in FIG. 8 a partial cutaway view of a manifold specifically used for supplying raw fuel 6 (for example, natural gas such as methane). In this type of the cell, an indirect reformer 8 is interposed between the several single cells 1a and 1b to form a fuel cell stack. FIG. 9 shows a schematic perspective view of the fuel cell stack 3 with manifolds 11 and 12 removed, the manifolds being used for supplying raw fuel gases 6a to 6d or an oxidant or oxidizing gas 9 to the fuel cell body as illustrated in FIG. 8. In this view, the fuel side (anode side) of the separator 2 incorporated with an indirect reformer 8 is partially cut away. The plane indirect reformer 8 is disposed in the separator 2 and it is piled with single cells 1a to 1c and 1d to 1e, on opposite surfaces, respectively to form the fuel cell stack 3. The single cells include components such as electrodes and the like. As is shown in FIGS. 8 and 9, gases from the oxidant side (cathode side) and the fuel gas side (anode side) are supplied to the cell body through the manifold 11 (cathode side) and the manifold 12 (anode side). These manifolds 11 and 12 are mounted on the cell body with a coil spring 13. The cell body is maintained at constant temperature with a flat heater 14. Reforming catalyst in the indirect reformer 8 is held with a wave form board, namely a corrugated fin. Raw fuel gas, or carbohydrate gas 6 such as methane is introduced into the reformer 8 through an opening 80a formed at one side of a cell stack 3. The raw fuel gas 6, supplied from a channel of a raw gas 8a is reformed at a reforming reaction section 8b and released from an opening (not shown) formed at the same plane where the opening 80a is formed. FIG. 10 shows a horizontal sectional view of the indirect reformer 8 illustrated in FIGS. 8 to 9. In this figure, the reforming reaction section 8b is provided with an opening 80b which is opposite to the opening 80a in the raw fuel gas channel 8a. The section 8b is also provided with a manifold 12a which supplies the raw fuel gas 6 to the raw fuel gas channel 8a. Fuel gas 7 released from the reforming reaction section 8b is distributed through a distributing manifold 12b into the fuel gas channels of the single cells respectively. The reformer 8 is separated into the raw fuel gas channel 8a and the reforming reaction section 8b by a partition plate 16. The reformer 8 is also provided with a raw fuel gas return section 17 to introduce raw fuel gas 6 into a section filled with catalyst for formation of hydrogen-rich reforming gas. Numerals, 18a and 18b, illustrated with hatching denote wet seals which are gas seals interposed between the cell layers. Numeral 11 denotes a fuel gas after a fuel cell reaction. A raw fuel manifold 12 is made up of a raw fuel gas supply manifold 12a and a fuel gas distributing manifold 12b which are mounted on one side of the cell stack across a plurality of single cells.
With reference to FIG. 10, gas flow of the conventional indirect internal reforming type molten carbonate fuel cell will be explained. The raw fuel gas 6 to be supplied to a cell stack from outside is supplied from the manifold 12a mounted on the the side of the cell body to the indirect reformer 8. In the interior of the reformer 8, the raw fuel gas 6 flows to the gas return section 17 through the raw fuel gas channel 8a having no reforming catalyst 5. The direction of the raw fuel gas 6 flow is then changed and the gas 6 is introduced into the reforming reaction section 8b having the reforming catalyst 5. Then a hydrogen-rich reforming gas, namely, a fuel gas 7 is generated. The fuel gas formed 7 is supplied to the respective channels in the anode side of the cells of the cell body through the fuel gas distributing manifold 12b, and the cell reaction occurs when an oxidant gas 9 such as air is supplied to the channels in the cathode side. The indirect internal reforming type molten carbonate fuel cell has a structure in which the reaction proceeding in the direction of the flow of the raw fuel gas 6 in the reforming reaction surface is a cross-flow to the flow of the oxidant gas 9 in the area of the cell reaction surface respectively.
FIGS. 11 and 12 show respectively experimental measurements of temperature distribution of the interiors of the indirect reformer and the fuel cell in the conventional indirect internal reforming type molten carbonate fuel cells during steady-state operation. In the interior of the indirect reformer, average temperature is 622.degree. C., maximum temperature 678.degree. C., and minimum temperature 493.degree. C. In the interior of the cell, average temperature is 643.degree. C., maximum temperature 682.degree. C. and minimum temperature 566.degree. C. The reforming reaction to form hydrogen from raw fuel gas (methane) by the use of the reforming catalyst is an endothermic reaction, whereas the cell reaction in the cell body is an exothermic reaction. The oxidant gas generally employs air which is allowed to include approximately 15% of the oxidant gas with its temperature cooled to approximately 550.degree. C., which is lower than that of the cell interior. Accordingly, the temperature distribution of the cell interior is as follows: approximately 200.degree. C. at the reformer, approximately 120.degree. C. at the cell body.
The conventional internal reforming type molten carbonate fuel cells have a construction as described above, the process direction of the endothermic reforming reaction crosses the direction of flow of the oxidant gas at a right angle for effectively cooling the cell reaction surface, causing a large temperature difference at the cell reaction surface and thereby exerting adverse effect upon the stability of cell operation.