PURPOSE: To predry a raw material chaff in the path of a combustion gas, completely remove the moisture in the chaff and smooth the feeding operation thereof, by forming a heating chamber of specific structure for utilizing the combustion gas in a feed hopper of a chaff combustion furnace.

CONSTITUTION: A chaff fuel is made to flow from a feed hopper 12 through a flow path 14 into a chaff press-sending cylinder 13 and then gradually sent under pressure from a chaff feed inlet 10 upward in the interior of smoking combustion chamber 4. The chaff begins to burn from the bottom part and is smoked while moving upward to give a smoked coal. The resultant smoked coal is turned and swept by whirlwinds from a blower 26 and then introduced from a smoked coal outlet 21 into a cyclone collector 20 provided in a heating chamber 18 and accumulated in a smoked coal collecting box 22. On the other hand, the unburned gas generated in the chaff smoking combustion chamber 4 is collected by a wind flowing through a ventilating chamber 17 and both cylinders 3 and 1 with perforated walls into the unburned gas chamber and burned and then completely burned in a gas combustion chamber 16 provided at the top of a flame hot air port 5.