Abstract: In case of a steam generator having a circulating atmospheric or pressurized fluidized-bed combustion system, normally a fluidized-bed combustor, at least one separator, at least one fluidized-bed cooler connected to said separator and adapted to be fed by a part stream of the solids separated from the combustion and fluidization gases, and a waste-heat boiler pass are provided. In these elements feed water, evaporator and reheating surfaces are arranged. Together with a reheater heating surface at least one superheater heating surface is arranged in the fluidized-bed cooler and an adjustable by-pass line is associated to the reheater heating surface to allow a simple and quick control of the combustion while simultaneously offering a simple control of the outlet temperature of the reheater.