Abstract: A first-stage furnace for partial combustion of solid fuel and oxidizer gas to generate inflammable exhaust gases which are passed to a secondary-stage furnace is shown. The first-stage furnace comprises a vertical pre-combustion chamber and a likewise cylindrical main combustion chamber mounted in horizontal position, connected downstream of the pre-combustion chamber through a tangential connecting passage. The air-fuel mixture introduced into the pre-combustion chamber is given swirling motion and burned at a temperature that converts the mixture to a mix of incompletely burned fuel particles, exhaust gases and non-combustible products in molten state. The mix stream into the tangential passage into the main combustion chamber develops into a high-velocity vortex, with the molten slag being centrifuged onto the inner wall of the main combustion chamber to form a film which is extracted out through a tapping port.