PURPOSE: To improve the rate of ash removal and to separate satisfactorily the gas and slag, by throwing at a high speed high temperature, gas and molten slag generated in a horizontal type furnace for partial combustion of coal into a vertical type furnace for partial combustion of coal in the tangential direction to it and making the molten slag flow down from an overflow port.

CONSTITUTION: Into a horizontal type furnace 10 for partial combustion of coal primary air and pulverized coal are blown from the nozzle 12a of a duct 12 which is in the direction tangential to the furnace 10 and secondary air from the nozzle 12b of the duct 12. The pulver ized coal is partially burned as it is swirled at a high speed in the furnace 10 which has been heated to high temperature by a preheating burner 13 at starting. The generated high temperature gas is blown into a vertical type furnace 15 for partial combustion of coal at a high speed in the direction tangential to the furnace 15 and the mist of the molten slag in the high temperature gas hits the circumferential wall and descends along a refractory material 26 to settle into a slag reservoir 16, and as the gas that contains a little amount of the mist swirls by a large swirling energy the gas flows out of a gas duct 25 at the center of the upper face. The slag that melted in the furnace 10 flows in the bottom of the furnace accompanied by the high temperature gas and flows into the furnace 15 tangentially to it from an outlet opening 14 and flows down to a slag reservoir 16 at the middle high section and is cooled and soldified in a water tank 29 and carried out by a conveyor 28.