Abstract: Waste material heated in the bottom of a thermal decomposition furnace is vaporized to create gas flow upwardly within the furnace. Cooling means located to have the upwardly flowing gas pass thereacross operates to control the temperature of the gas leaving the furnace through the upper end thereof so that gaseous product having a boiling point above the boiling point induced by the cooling means will condense and fall within the furnace onto tray means located below the cooling means and adapted to receive therein condensed liquid from the cooling means. The tray means include orifice means for enabling upwardly flowing gas to come into contact with the liquid received in the tray means, and downflow tubes for causing liquid within the tray means to flow downwardly to the bottom of the furnace into contact with heating means. Gas flowing upwardly from the heating means passes through the orifice means and into contact with the liquid in the tray means.