Fuel cell power generation system with valve on raw material gas supply passage and valve downstream of carbon monoxide decreasing unit, and method for operating fuel cell power generation system

An operating method of a fuel cell power generation system, comprising the steps of:

• closing, when a process for stopping the system is performed, (i) a first valve provided on a first raw material gas supply passage for supplying a raw material gas, which contains an organic compound formed of at least carbon and hydrogen, to a reformer that generates a hydrogen rich fuel gas from the organic compound and water vapor, and (ii) a second valve provided downstream of a carbon monoxide decreasing unit that decreases a carbon monoxide in the hydrogen rich fuel gas generated by the reformer to supply the hydrogen rich fuel gas to a fuel cell;
• and
• supplying, with the carbon monoxide decreasing unit communicating with the reformer, the raw material gas to a closed space, which includes a fuel treatment device that comprises the reformer and the carbon monoxide decreasing unit, through a second raw material gas supply passage for supplying the raw material gas to an inside of a flow passage downstream of the carbon monoxide decreasing unit.