In a typical solid electrolyte fuel cell, power generation occurs as a result of the reactions set forth below in which the fuel cell utilizes hydrogen or a gaseous hydrocarbon as the fuel and air as the oxidant, while operating the fuel cell at high temperatures of about 1,000.degree. C.
When using methane as the fuel, power generation occurs as the result of the following reactions: EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2 (1) EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 (2) EQU H.sub.2 +1/2O.sub.2 O.fwdarw.H.sub.2 O (3)
Reaction (1) is typically referred to as a fuel reforming reaction. Reaction (2) is typically referred to as a CO shift reaction. Reaction (3) is a redox reaction which results in the generation of power.
If the fuel is hydrogen, only Reaction (3) is applicable. If a gaseous hydrocarbon such as methane is used as the fuel, it is partially or completely converted to a hydrogen-containing gas in accordance with Reactions (1) and (2) before being supplied to the fuel cell for power generation. Since H.sub.2 O must be supplied for Reaction (1) and since H.sub.2 O is produced in Reaction (3), it is seen that recycling of the exhaust gas containing H.sub.2 O is desirable.
In considering the possibility of recycling the exhaust gas which emanates from the SOFC operated in accordance with Reactions (1), (2) and (3), the following factors are applicable:
(a) utilization of a gas in the system for supplying H.sub.2 O; PA1 (b) improving energy efficiency by recovering the exhaust heat of the exhaust gas at a high temperature; and PA1 (c) improving energy efficiency by recovering and recycling fuel not used for power generation. PA1 1) There is a limitation on the amount of a gas to be to be recycled to the entrance to the fuel cell anode such that the exhaust gas includes a significant amount of unreacted hydrogen or gaseous hydrocarbon fuel, thus lowering the power generation efficiency. PA1 2) The partial vapor pressure in the anode increase during the course of exhaust gas recycling, thereby lowering the electromotive force and reducing the power generation performance of the fuel cell.
However, it should be noted that in the course of recycling of the exhaust gas which emanates from the SOFC, the electromotive voltage decreases at the anode as the vapor partial pressure increases such that power generation efficiency decreases. The problem therefore is to provide a method for recycling the exhaust gas emanating from the SOFC operating with hydrogen or a gaseous hydrocarbon as the fuel and air as the oxidant at a high temperature of about 1,000.degree. C. without any diminution in power generating efficiency.
Methods are known in the prior art for recycling the exhaust gas, e.g. ejector method, high temperature blower method and turbo compressor method, etc. However, these methods suffer from several drawbacks. Two main disadvantages of the prior art methods are:
The particular disadvantages of the ejector method of recycling the exhaust gas are discussed below with reference to FIG. 3. The use of a circulating blower or turbo compressor has been suggested to overcome these disadvantages. However, these latter two devices are not applicable to a SOFC which operates at a high temperature of about 1,000.degree. C.