A primary combustion chamber is located above a secondary combustion chamber with a support grate between the two for supporting particulate solid waste fuel to be burned efficiently. Primary air is introduced to the primary combustion chamber with secondary air introduced below the grate into a combustion gas flow to cause complete burning in the secondary combustion chamber. A brick wall screen means of high temperature, corrosion-resistant brick forms the secondary combustion chamber to allow exhaust gaseous product to exit therethrough while screening out ash which can be removed from the combustion area. The gaseous product passed through the screen is then passed to a heat exchanger to utilize part of the heat produced for heating primary and secondary air used in the combustor. The exhaust gas from the heat exchanger can then be put to use in a boiler or other energy producing device.