NOVELTY - Biomass of timber and sewage sludge cake are supplied to pre-chamber (4), and thermally decomposed using recycled coolant gas. The surplus combustible gas as fuel gas is recovered, when combustible gas produced by decomposition is used as coolant gas. Air for burning one portion of combustible gas is introduced to pre-chamber for partial combustion, and temperature of pre-chamber is favorably adjusted for thermal decomposition.    USE - For recovering combustible gas used as fuel gas and chemical substitutes for petroleum and liquefied natural gas.    ADVANTAGE - The method efficiently enables the recovery of combustible gas, without performing thermal decomposition of biomass. The temperature of pre-chamber is favorably adjusted for thermal decomposition of biomass.    DETAILED DESCRIPTION - Red-hot coke (1) is supplied to a cooling chamber (5) of main portion (2) of coke dry quencher. Coolant gas is introduced from the lower portion of cooling chamber. The coolant gas and red-hot coke are contacted, and temperature of coolant gas is raised. The temperature-raised gas is conveyed to a boiler (13), via cyclic duct (6) and flue (10). The heat of temperature-raised gas is collected, and gas is utilized as coolant gas. Biomass of timber and sewage sludge cake are supplied to pre-chamber (4) of the upper portion of cooling chamber. The recycled coolant gas is supplied to the lower portion of cooling chamber. The biomass is thermally decomposed using the sensible heat of red-hot coke. The surplus combustible gas as fuel gas is recovered, when combustible gas produced by thermal decomposition is used as coolant gas. Air for burning one portion of combustible gas is introduced to pre-chamber for partial combustion, and temperature of pre-chamber is favorably adjusted for thermal decomposition.    DESCRIPTION OF DRAWING(S) - The figure shows a block diagram of coke dry quencher illustrating the recovery of combustible gas. (Drawing includes non-English language text).    red hot coke (1)    main portion of coke dry quencher (2)    pre-chamber (4)    cooling chamber (5)    cyclic duct (6)    coke discharge port (7)    flue (10)    boiler (13)