Combustible gases produced by pyrolysis of combustible material undergo partial combustion in a combustion chamber to which air is admitted in substoichiometric quantity ratio, at a temperature of about 800.degree. C. A gas duct leading out of the combustion chamber and passing partly or all the way across the combustion chamber with one or more openings for entrance of the combustible gas is supplied with additional air in at least stoichiometric quantity ratio, with the result that complete combustion of the combustible gas takes place within the duct as the gases pass out of the combustion chamber, at a temperature of about 1100.degree. C., producing a flame jet that can be used to heat an adjacent boiler, heat exchanger, or the like.