A study of NO x emission characteristics in two stage combustion

Abstract

To suppress the emission of NOx a new technique for two stage combustion has been developed and the formation of NOx by this combustion process examined by changing the combustion mode. The combustion is divided into two parts, a primary and a secondary stage. In the primary stage, partial combustion is carried out for a much higher fuel-air equivalence ratio than that of usual staged combustion. After promoting partial combustion, secondary air is supplied to complete combustion in the secondary combustion chamber. Nitrogen compounds such as HCN, NH3, and NO are formed in the primary stage, and these are converted to NOx in the exhaust gas at a high conversion rate. When NO is added to a fuel, the greater part is converted to HCN in the primary fuel-rich combustion, and a relatively small quantity of NH3 is also formed. Interactions between NO, HCN, NH3 and other species, including carbon compounds, are strongly suggested in fuel-rich flames. HCN and NH3 may be formed through several different reactions. These nitrogen compounds can be reduced by carrying out the primary combustion at a high fuel-air equivalence ratio and premixed rate. The rate of conversion of the nitrogen compounds to NOx is not affected but Thermal NO is significantly suppressed by the combustion modifications in the secondary stage. By applying this combustion technique to a practical boiler, successful reduction of NOx is obtained.