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 the combustible gases are withdrawn , additional secondary air is blended in the gas , and complete combustion of the blend is effected in a combustion chamber .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 .