To reduce the production of NOx in the ignition stage of coal dust by a method wherein an average ratio, consisting of the rate of oxygen in primary gas and a necessary amount of oxygen for burning combustible volatile constituents, is reduced.

A ratio, based on the rate of oxygen in primary gas and the necessary amount of oxygen for burning voltage constituents in the primary gas, is reduced. For this purpose, a swirler 24 is arranged at the vicinity of entrance of a burner in a dust pipeline 7 while the swirler 24 divides the flow of mixture, consisting of the primary air and coal dust, into a dust-rich branch flow at the outside and a dust-poor branch flow at the inside of the pipeline 7. Thereafter, the dust-poor branch flow is received from the divided flow of mixture through a predetermined pipeline constitution and is supplied into a primary gas pipe 9. On the other hand, the dust-rich branch flow or the branch flow including little air arrives at a primary dust pipe 6. Thus, these branch flows are supplied into the ignition area of the burner through mutually separated passages under different loads of dust.