Abstract: A process and apparatus for the thermal destruction of solid fuel material in a gravity-type multiple hearth furnace having groups of two or more superimposed hearths. Solid fuel is fed to the upper hearth of each group of hearths. Auxiliary fuel is selectively burned in the lower hearth of each group of hearths to maintain the discharge temperature of the exhaust gas flowing from each said upper hearth above the ignition temperature of said gases so that each group of hearths is operated as a combined drying and burning zone. Preheated air for combustion is introduced into the lower hearth of the lowermost group of hearths so that oxygen available for combustion increases as the solid fuel moves downwardly. Ash is removed from the lower hearth of at least the lowermost group of hearths.