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A method of operating a fuel cell system including stopping power generation of a fuel cell which generates electric power using a fuel gas and an oxidizing gas, filling and keeping a combustible gas in a cathode of the fuel cell after said step, supplying the oxidizing gas to the cathode, supplying

A method of operating a fuel cell system including stopping power generation of a fuel cell which generates electric power using a fuel gas and an oxidizing gas, filling and keeping a combustible gas in a cathode of the fuel cell after said step, supplying the oxidizing gas to the cathode, supplying the combustible gas discharged from the cathode in response to the previous step to a combustor capable of heating a fuel generator for generating the fuel gas or an exhaust pipe connected to the combustor via a branch passage branching from an oxidizing gas passage located downstream of the cathode, diluting the combustible gas supplied to the combustor or the exhaust pipe with air supplied to the combustor or exhaust gas supplied to the exhaust pipe such that the combustible gas has a concentration lower than a combustion lower limit, and discharging the diluted combustible gas to atmosphere.

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1. A method of operating a fuel cell system, the method comprising steps of: (a) stopping power generation of a fuel cell which generates electric power using a fuel gas and an oxidizing gas;(b) filling a cathode of the fuel cell with a combustible gas in a stop operation of the fuel cell system aft

1. A method of operating a fuel cell system, the method comprising steps of: (a) stopping power generation of a fuel cell which generates electric power using a fuel gas and an oxidizing gas;(b) filling a cathode of the fuel cell with a combustible gas in a stop operation of the fuel cell system after step (a) and keeping the filled combustible gas in the cathode until starting a start-up operation of the fuel cell system;(c) supplying the oxidizing gas to the cathode and discharging the combustible gas filled in the cathode with the oxidizing gas after step (b);(d) supplying the combustible gas discharged from the cathode in response to step (c) to a combustor configured to heat a fuel generator for generating the fuel gas or to an exhaust pipe connected to the combustor via a branch passage branching from an oxidizing gas passage located downstream of the cathode;(e) diluting the combustible gas supplied to the combustor with air supplied to the combustor or the combustible gas supplied to the exhaust pipe with exhaust gas supplied to the exhaust pipe, such that the combustible gas has a concentration lower than a combustion lower limit; and(f) discharging the diluted combustible gas to atmosphere. 2. The method according to claim 1, wherein the fuel generator is configured to generate the fuel gas from a feed gas and the combustible gas is one of the fuel gas and the feed gas. 3. The method according to claim 1, further comprising the step of: (c-1) opening a first oxidizing gas passage valve provided at the oxidizing gas passage located upstream of the cathode and opening an on-off valve provided at the branch passage before step (c). 4. The method according to claim 1, further comprising the step of: (g) supplying the oxidizing gas to the cathode in the state of opening the oxidizing gas passage and closing the branch passage during power generation of the fuel cell after step (f). 5. The method according to claim 4, further comprising the step of: (f-1) opening a second oxidizing gas passage valve provided at the oxidizing gas passage located downstream of the cathode and closing an on-off valve provided at the branch passage in the period between step (f) and step (g). 6. The method according to claim 1, wherein the combustible gas of step (d) is supplied to a flame burner of the combustor. 7. The method according to claim 6, wherein the combustible gas is supplied to the flame burner during a stop period of combustion in the combustor. 8. The method according to claim 1, wherein the exhaust gas generated by combustion in the combustor is guided through the exhaust pipe to atmosphere. 9. The method according to claim 8, wherein the combustible gas is supplied to the exhaust pipe during a combustion period of the combustor. 10. The method according to claim 1, wherein the combustible gas is supplied during a stop period of combustion in the combustor in step (d).