extended position in operation when the chemical recovery boiler is at rated capacity a fully open air nozzle is required to supply the combustion air for this condition the burner diffuser is in a retracted position and the air velocity discharged into the furnace is at the optimum value for good penetration of the air stream from the nozzle into the interior of the furnace at lower loads when less combustion air is required the diffuser on the end of the burner gun is advanced into the tapered air nozzle opening to the point where the open nozzle area will again result in the optimum air velocity into the furnace the burner gun can also function in a chemical recovery boiler as a starting burner or as a load carrying burner capable of providing the heat to generate steam in its fully extended position a fuel control system not shown will allow fuel to flow into the inner tube 40 the apparatus in accordance with the invention includes an igniter 46 to light the oil air required for combustion of the fuel passes through the air nozzle and around the diffuser to provide the necessary recirculation zone for a clean flame a typical embodiment of the invention will include several rodding ports 48 to allow insertion of rods to break up material that may accumulate and harden and several sight glasses 50 that permit visual observation of the operation it will be understood that in the operation of a.