A known fuel cell system has a fuel cell for generating electric power by consuming hydrogen-rich reformed gas (fuel gas) at a fuel electrode and oxygen gas at an oxidant electrode; a blower for supplying oxygen gas to the oxidant electrode; a hydrogen generator for generating the reformed gas from a material gas (e.g., city gas or natural gas) and vapor through the steam reforming reaction; a water removing device for condensing vapor contained in the reformed gas (off gas) that has not been consumed at the fuel cell electrode; and a burner (heating combustor) for heating the reforming catalyst of the fuel generator through heat exchange with fuel gas obtained by combusting the off gas.
Generally, during shutdown of the fuel cell system, occurrence of abnormal combustion of a mixed gas comprising air and the fuel gas is properly restrained by purging the inside of the fuel cell system with an inert gas (e.g., nitrogen gas).
The purge process by use of nitrogen gas, however, requires provision of a special nitrogen system such as a nitrogen cylinder or nitrogen segregation/generation system. This nitrogen system reveals disadvantages in terms of both cost reduction and size reduction of the fuel cell system, especially in cases where the fuel cell system is used as a fixed-type distributed power generation system for household use or a power source for electric vehicles.
As attempts to solve the above problem, there have been proposed several techniques according to which the inside of the fuel cell system is purged with a material gas instead of nitrogen gas upon shutdown of the fuel cell system.
One known purging technique is such that desulfurized gas (a material gas from which sulfur is removed) that has passed through a bypass passage is guided into the fuel cell to purge the fuel cell with the material gas (see Patent Document 1 that is a prior art technique).
In another known purging technique, after vapor is guided into the inflammable gas system of a fuel cell system to thereby purge inflammable gas dwelling therein, a material gas is guided into the inflammable gas system to purge vapor upon a drop in the temperature of the catalyst of the hydrogen generator (fuel reformer) to a sufficient degree (see Patent Document 2 that is a prior art technique).
Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2003-229149
Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2003-282114