Part of the generated gas is diverted for use in the generator. The diverted gas is highly preheated in 1 of 2 heat accumulators, the hot gas is mixed with steam, and the mixture fed into the generator. Meanwhile the other accumulator is reheated. For this purpose is used an extraneous gas supplied through the top and part of the originally diverted generator gas fed through the bottom. The volume of the latter is such as to supply enough pressure to prevent the combustion gases from entering the generator.