Abstract: A method for reducing the NO.sub.x content of effluent gas in a coal-dust-fired combustion system comprising establishing at least two vertically adjacent burner planes, in which the burner flame cores are fed with primary air in the core zones in a substoichiometric ratio to the coal dust feed stream and are additionally fed with secondary air comprising recycled flue gas in optional combination with preheated fresh air, and in which the burner mantle zones are fed with tertiary air comprising recycle flue gas and optional fresh air, so that, overall, a substantially stoichiometric combustion air ratio is established in the region of the lower burner plane and, overall, a highly substoichiometric combustion air ratio is established in the region of the upper burner plane. The apparatus employed is an economical primary measure requiring little space and minimal loss of energy, and is easily retrofitted in existing plants.

Abstract: A method and apparatus for reducing the NO.sub.x content of hot stack (exhaust) gas produced by a furnace burning a fossil fuel, comprising recycling part of said NO.sub.x -containing stack gas, in admixture with cold fresh air, back into the combustion chamber of the furnace.