PURPOSE: To permit the promotion of gasification of solid fuel and the reduction of the amount of unburnt constituent, by a method wherein solid fuel is reacted with primary air to gasify the solid fuel partially, then, secondary air is supplied into a fuel to effect low NOx combustion and, thereafter, third air is supplied into a main combustion furnace to burn the fuel perfectly.

CONSTITUTION: In a fixed floor type gasifying furnace 1, solid fuel 9 is supplied onto a rotary fire grate 2 through a supplying line 6 while ash 10 is accumulated below the rotary fire grate 2 and is discharged through another line 8. A fuel 3, communicating the gasifying furnace 1 with a main combustion furnace 5, a is kept warm sufficiently by refractory materials and a duct burner 13 is provided near the entrance of the flue 3. The outlet port of the flue 3 is connected to a gas burner 4 for the main combustion furnace 5. Primary air 7 is supplied into the gasifying furnace 1 through the rotary fire grate 2. Secondary air 7, whose amount is less than the theoretical amount of air, is supplied into the fuel 3 through the duct burner 13 while third air 15, whose amount is more than the theoretical amount of air, is supplied into the main combustion furnace through the gas burner 4.