Reduction of N2O Emissions from a Coal-Fired Circulating Fluidised Bed Combustor by Afterburning

This paper presents experimental results of afterburning for the reduction of N20 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. Two different afterburning configurations, one a commercial gas-fired burner and the other direct fuel injection via a simple fuel injector, have been investigated. Propane has been tested with both afterburning configurations, while ethane and methane have also been tested using direct fuel injection. Up to 80% N20 reduction has been experimentally achieved with either propane afterburning or ethane injection. However, when methane was injected, only about 30% N20 reduction could be obtained due to incomplete combustion at higher thermal input levels within the available residence time (~0.3 s). From the thermal input point of view, methane was also less effective in reducing N20 than propane and ethane. Experimental results show that propane afterburning by direct fuel injection is more effective in reducing N20 emission than propane afterburning with the burner. It is also shown that when afterburning is conducted through the burner, a higher N20 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 afterburning fuel and CO emissions may also increase if excessive afterburning fuel is introduced. Gas phase modelling using the detailed reaction scheme, GRI-Mechanism 2.11, shows that thermal decomposition of N20 plays only a minor role in the reduction of N20 emissions in comparison to N20 reduction by radicals reactions. In addition, the modeling also confirms that it is more difficult to achieve complete combustion of methane within the available residence time (0.3 s) in the afterburning zone and methane is less efficient in reducing N20 than ethane.