PURPOSE: To restrain a sharp increase in NOX at change-over of combustion state by a method wherein a fuel nozzle is arranged at the center of a first stage combustion chamber, an air rotating device is concentrically arranged around the periphery, and fuel is supplied at a position in the first stage combustion chamber which is downstream an air back-flow region formed in the first stage combustion chamber.
CONSTITUTION: First stage fuel 201 is supplied into a combustion chamber through a first stage flame stabilizing nozzle 24, and ignited at an air back-flow region which is formed in a first combustion drum by flame stabilizing air 106 supplied from a rotating device 2, so that a stable flame is formed. To achieve a complete combustion and a low NOS generation, excess air is fed from first stage combustion air nozzles 20 and a clearance between an auxiliary fuel nozzle 25 and the combustion drum wall. The first stage fuel 201 is rapidly decreased and second stage fuel 203 is increased fast so that a diffusion combustion takes place in the first stage combustion drum and pre-mix combustion in the second combustion drum. The fuel 201 is decreased and the corresponding amount of fuel is supplied from the auxiliary fuel nozzle 25. And finally, the fuel 201 is stopped and pre-mix combustion is carried out in both first and second combustion drums. Thereby, a low NOX combustion can be achieved.