bundle or the like not described in detail the flue gas is conveyed to a stack 10 a part of this gas at high temperature is withdrawn through a pipe 11 and fed to the nozzle 4 by means of a compressor 12 along the pipe 11 there are provided a settling chamber 13 for depositing unburnt solid particles present in the flue gas and a heat exchanger 14 which cools the flue gas before it reaches the compressor 12 should its temperature be too high for the compressor the throughput of the gaseous combustion products fed by the compressor 12 can be adjusted by a bypass valve 12a to attain the optimum ratio of fuel to fed gaseous combustion products for best atomisation the valve 12a can be controlled automatically to adapt to different burner operating loads passage through the heat exchanger 14 is controlled by an automatically operated damper 15 controlled by a gas temperature sensor 40 the cooling fluid of the heat exchanger 14 is the actual intake air of the fan 7 so that the heat removed from the flue gas is recovered in the form of heat content in the combustion air and is therefore added to the heat produced.