Abstract: In a method of removing nitrogen oxides non-catalytically from an exhaust combustion gas by maintaining said combustion gas at a temperature of from 700.degree. to 1300.degree. C in the presence of oxygen and an ammonia source in a combustion apparatus which has heat exchange surfaces in a zone downstream of the position where the ammonia source is added, the improvement which comprises feeding said ammonia source to the exhaust combustion gas at a position where the heat exchange surface area in the space downstream of the position of ammonia source addition which has a combustion gas residence time of up to 1 second is less than 0.5 .times. 10.sup.-2 m.sup.2 per 1 Nm.sup.3 /H of said combustion gas flow; and the temperature of said combustion gas at the outlet of said space is above 700.degree. C.

Abstract: A method of treating a combustion exhaust gas containing nitrogen oxides comprises:A first step of removing nitrogen oxides by treating a mixture of the exhaust gas, oxygen and ammonia in the absence of hydrogen or in the presence of lower than 10 mole ratio of hydrogen to ammonia at a temperature from 700.degree. C to 1300.degree. C andA second step of adjusting a mole ratio of hydrogen to ammonia in the pretreated exhaust gas to higher than 3 and treating the mixed gas at a temperature from 490.degree. C to 700.degree. C.