The present invention relates to a fuel cell power generation system including a reformer and a method of operating the same system. More particularly, it relates to a solid-oxide fuel cell power generation system where a hydrocarbon-based fuel is used as an anode gas, and a method of operating the same system.
A fuel cell is a kind of a power generation device, wherein an anode is provided on one side of the fuel cell, and a cathode is provided on the other side thereof with an electrolyte set up therebetween. Then, a fuel gas is supplied to the anode side, and an oxidant gas, which is mainly air, is supplied to the cathode side. Next, the fuel and the oxidant are electrochemically reacted with each other through the electrolyte, thereby generating electric power. In particular, researches are now being conducted concerning a solid-oxide fuel cell, which is one type of fuel cell. This is because, in this fuel cell, the operation temperature is high, i.e., 700 to 1000° C., the power generation efficiency is high, and the exhaust heat is easy reusable.
Usually, a hydrocarbon-based fuel, such as town gas, LNG, or kerosene, is supplied to the anode side of this solid-oxide fuel cell together with water vapor. CH4 is regarded as the most common fuel of the hydrocarbon-based fuels. Then, as indicated by a chemical formula (1), CH4 is reformed by reacting with the water vapor on the anode surface of the solid-oxide fuel cell. This reformation reaction, which is an endothermic reaction, is referred to as “internal reformation scheme”, since CH4 is reformed inside the solid-oxide fuel cell. It is also possible to perform temperature control over the fuel cell by taking advantage of the endothermic reaction of this internal reformation.CH4+H2O=3H2+CO  (1)
Then, H2 and CO, which are acquired by the reformation reaction indicated by the formula (1), react respectively with O2− from the cathode side as are indicated by chemical formulas (2) and (3). These processes result in acquisition of electricity and heat output.H2+O2−═H2O+2e−  (2)CO+O2−═CO2+2e−  (3)
The hydrocarbon-based fuel, however, also contains higher-order hydrocarbons having larger carbon numbers than CH4, such as C2H6, C3H8, and C4H10. If these higher-order hydrocarbons are supplied to the anode electrode with no reformation made thereto, the C component turns out to deposit. Accordingly, there exists a possibility of causing a performance degradation of the fuel cell to occur. On account of this, usually, the higher-order hydrocarbons are partially reformed up to CH4, or are reformed in total amount up to H2 and CO indicated by the formula (1), then being supplied to the anode in this reformed state. At this time, this reformation is performed using an external configuration appliance referred to as “reformer” which is different from the fuel cell. In this way, performing the reformation of hydrocarbon using the reformer composed of the external configuration appliance which is apart from the fuel cell is referred to as “external reformation scheme”.
Concerning the reformer based on the external reformation scheme, as is described in, e.g., JP-A-2003-109639 (Abstract), an innovation is devised that a heat source for the reformation reaction is ensured by setting up the reformer in a combustion chamber where unused fuel and unused oxidant from the fuel cell are combusted.