PURPOSE: To reeuce NOx present in combusion exhaust gas without generating soot and smoke and with high denitrification rate by the procedure in which a reducing agent consisting of an oxygen-containing hydrocarbon containing a fixed proportion of an oxygen-free hydrocarbon or H2 is added to the high temperature portion of combustion exhaust gas and then air is added to the rear flow of the gas.

CONSTITUTION: A fuel and primary air are charged into the surface 1d of the boiler 1 for power generation for combustion, and then the reducing agent 3 consisting of an oxygen-free hydrocarbon (A), e.g., methane, kerosine, etc., supplied from the line 5 and an oxygen-containing hydrocarbon (B), e.g., alcohols, or H2, supplied from the line 4 is added to the combustion exhaust gas to convert most of NOx into N2, HCN, NH3, etc., and at the same time to generate CO. In this case, the proportion of the reducting agent B to be added is regulated to 0.3 to 0.95 by weight on the whole (A + B) amount basis. Then, air is added to the gas to completely burn off a considerable amount of CO and a small amount of unburned substances HCN, NH3, etc., formed at the precombustion stage to CO2, H2O, N2, etc., which are released through the air preheater 7 and chimney 8 to the air.