In order to provide a rotary-furnace burner in which the quantity fraction of cost-effective particulate secondary fuels to be used as energy carriers can be increased and the configuration of the burner flame can be influenced, even during the operation of the burner, it is proposed, according to the invention, to arrange an expansion chamber open towards the burner mouth and having a widened cross section, as compared with the tube or individual tubes in the burner in front of the issue of the tube or tubes for blowing out the secondary fuels, and to make the axial length and the volume of the expansion chamber variable, during the operation of the burner, by means of the axial displacement of the secondary-fuel tubes, so that the particulate secondary fuel particles blown out at the burner mouth with a considerably reduced velocity do not fly past the burner flame, but, instead, burn out in the flame.