Widening the temperature range of the thermal DeNO x process. An experimental investigation

Abstract

To obtain a high efficiency of selective non-catalytic reduction of NO over a wide temperature range the reactions of NO reduction by NH3 and NH3-derivatives in the presence of excess O2 and the influence of additives have been studied. Laboratory experiments were performed in an isothermal plug flow quartz reactor, at residence times varying from about 0.07 to 0.13 s, and mostly over a temperature range between 450 and 1000°C. It was observed that the reduction of NO by NH3 occurs within a temperature window with an optimum temperature of about 960°C. The effects of additives on reduction of NO and conversion of NH3 were examined. Combustible additives can cause a downward shift of the reduction temperature window and a slight decrease of the maximum reduction degree, the effect depending on the species and concentration of additives. Additives will shift NO reduction temperature and NH3 conversion temperature downwards to the same extent. Selective reduction of NO in a low temperature range around 530°C was achieved by injection of methylamine, CH3NH2, and in a middle temperature range around 840°C by injection of ethylenediamine, (CH2NH2)2. As an additive, CH3NH2 has an unusual effect on the reduction of NO with NH3. An inhibiting effect of NH3 on NO reduction by CH3NH2 was observed. It is proposed that the additives can be ignited in oxygen-rich gases at lower temperatures than NH3, and hence NH3 is more readily converted to NH2 by radicals formed in combustion of the additives. The different influences of alkanes and the complexity of reactions in the presence of amines are interpreted in terms of the bond strengths of molecules.