N2O reduction by afterburning in a coal-fired circulating fluidised-bed combustor

This paper presents experimental results of afterburning for the abatement of N2O 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 pa rt of the afterburning zone. Two different afterburning configurations, one a commercial gas-fired burner and the other direct fuel injection via a si mple fuel injector, have been investigated. Propane has been tested with bo th afterburning configurations and ethane and methane have been tested usin g direct fuel injection. Up to 80% N2O reduction has been achieved experime ntally with either propane afterburning or ethane injection. However, when methane was injected, only about 30% N2O reduction could be obtained due to incomplete combustion at higher thermal input levels within the available residence time (similar to0.3 s). From the thermal input point of view, met hane was also less effective in reducing N2O than propane and ethane. Exper imental results show that propane afterburning by direct fuel injection is more effective in reducing N2O emission than propane afterburning with the burner. It is also shown that when afterburning is conducted through the bu rner, a higher N2O reduction can be achieved when the air flow rate to the burner is lower, especially if the thermal input to the burner is limited. NOX emissions usually increase slightly with the introduction of an afterbu rning fuel and CO emissions may also increase if excessive afterburning fue l is introduced. Gas phase modelling using the detailed reaction scheme, GR I-Mechanism 2.11, shows that thermal decomposition of N2O plays only a mino r role in the reduction of N2O emissions compared with N2O reduction by rad icals reactions. In addition, the modelling also confirms that it is more d ifficult to achieve complete combustion of methane within the available res idence time (0.3 s) in the afterburning zone and methane is less efficient in reducing N2O than ethane.