Abstract: A furnace includes a downdraft combustion chamber, a burner mounted at the top of a combustion chamber for burning particulate ash-containing fuel, and a generally horizontal duct near the lower end of the combustion chamber for withdrawing hot gases rsulting from the combustion. The combustion chamber is downwardly tapered to a relatively small conduit, downward into which molten slag from the ash flows. The slag, upon passing through the conduit, passes into and through a downwardly open slag-receiving chamber below the combustion chamber. The slag falls through out the bottom of the slag-receiving chamber into a volume of water immediately below. A portion of the hot gases is diverted through the slag-receiving chamber in order to maintain the slag-receiving chamber at a sufficiently high temperature so that the slag passing therethrough is prevented from solidifying. The portion of hot gas is then directed to a mill for drying fuel therein prior to combustion.