Abstract: A new furnace is provided which employs a forced draft to efficiently and cleanly burn a wide variety of fuels such as wood and refuse. The furnace is designed to internally and continuously pyrolyze (gasify) fuel prior to the burning phase which is conducted in a lower chamber of the furnace where air is admitted at a controlled rate. Combustion temperature is maintained at 1800.degree. F. or higher to completely burn gasified fuel by enclosing the burning zone with refractory material and by not extracting sensible heat from the combustion region until after exhaust gases exit the lower chamber. Ash is produced in a fine powder as a consequence of its being removed from the combustion effluent by gravity and cyclonic (i.e. turbulent) separation.