An apparatus is disclosed for generating combustible gas from cellulosic fiber material. A gas production chamber contains an inclined support chute having a grate perforated with alternately angled ports for admission of a controlled amount of air through the grate portion for performing first a lifting function and then a propelling function on cellulosic material entering at the top and undergoing progressive destructive to form combustible gases in the chamber. The chamber atmosphere has an oxygen content insufficient for complete combustion. The combustible gases are withdrawn, additional secondary air is blended in the gas, and complete combustion of the blend is effected in a combustion chamber. The heated gases formed are then forced by a recirculation blower to a point of use, such as the interior atmosphere of a lumber drying kiln.