PURPOSE: To prevent inclusion of unburnt lump-type materials in ash and facilitate the disposal of abandonment by a method wherein an unburnt lump-type material discharging pipe is attached to the surface of a hearth contacting to the protruding rim of a dropping cylinder.

CONSTITUTION: A rotary mixing and venting section 3 is provided with a plurality of radial branching tubes 5 on the top of a rotary venting pipe 4 established vertically at the center of the furnace while multiple mixing rods 6 are hung from respective branching tubes 5 toward the surface of the hearth 2. The upper rim of the ash dropping cylinder 11, provided at the center of the hearth 2, is protruded slightly from the surface of the hearth to form a protruded rim 12. The unburnt lump-type material discharging pipe 13, attached to the hearth 2 so as to be brought into contact with or closed to the upper rim 12 of the dropping pipe, is opened at the surface of the hearth and a ventilation intercepting type discharging section 14 is provided at one part of the pipe body thereof. According to this method, the ash, having light weight, drops into the dropping cylinder 11 under overriding the protruded rim 12 and is discharged to the outside of the furnace while the unburnt lump-type material, having comparatively heavy weight, collides against the protruding rim 12 and drops into the discharging pipe 13 together with the ash, thereafter, the material is separated from the combustion ash after passing through the discharging section 14.