PURPOSE: To prevent the generation of gas in an expansion area by a method wherein during the starting of the generation of gas, during a normal stop, and during an emergency stop, a system containing a gas circulating system is filled with inactive gas, an enough sponge layer is formed by inactive gas between an air layer and a generating combustible gas layer.

CONSTITUTION: At a point of starting time, transfer to mono-fuel combustion of coal through multi-fuel combustion of oil-coal is effected. After a system is filled with inactive gas by discharging nitrogen gas in a dead space in a state that an amount of oxygen in exhaust gas from a coal partial burning furnace 1 is reduced to zero, inactive gas in the system is gradually replaced with combustible gas, and hazardous combustion is prevented from occurring or gas in an expansion area is prevented from generation. During an ordinary stop, only the feed of pulverized coal supplied to a coal partial burning furnace 1 is reduced, nitrogen gas is discharged in a dead space in the system, and gas in the system is replaced with inactive gas. After an enough sponge layer is formed, the feed of fuel is completed reduced and inactive gas is replaced with air. During an emergency stop, nitrogen gas is forcibly discharged, combustible gas in the system is diluted and simultaneously it is rapidly discharged to the outside of the system.