Reduction of N 2O emissions from a coal-fired circulating fluidised bed combustor by afterburning

Abstract

This paper presents experimental results of afterburning for the reduction of N 2O emissions from a pilot-scale, coal-fired, circulating fluidised bed combustion (CFBC) system with a riser of diameter 161 mm and length 6.2 m. The primary cyclone and the secondary cyclone have been used as the main part of the afterburning zone, and propane has been used as the afterburning fuel. Two different afterburning configurations, one a commercial gas-fired burner and the other direct fuel injection via a simple fuel injector, have been investigated. Up to 80% N 2O reduction has been achieved experimentally. Experimental results show that propane afterburning by direct fuel injection is more effective in reducing N 2O emission than afterburning with the burner. It is also shown that when afterburning is conducted through the burner, a higher N 2O reduction can be achieved with a lower air flow rate to the burner, especially if the thermal input to the burner is limited. Little adverse effect of afterburning on sulfur capture by limestone addition has been found, although NO x emissions increase slightly with the introduction of an afterburning fuel and CO emissions may also increase if excessive afterburning fuel is introduced. Analysis of experimental results and simple kinetic estimates suggest that radical reactions probably contribute more than N 2O thermal decomposition in reducing N 2O emissions in the afterburning zone under the conditions of this study (temperature from 1073 to 1173 K, residence time about 0.3 s).