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Show REDUCTION OF N20 EMISSIONS FROM A COAL-FIRED CIRCULATING FLUIDISED BED COMBUSTOR BY AFTERBURNING Hao Liu, Bernard M. Gibbs Department of Fuel & Energy The University of Leeds, Leeds LS2 9JT, U K Telephone: 44-113-2332492 Fax 44-113-2440572 ABSTRACT This paper presents experimental results of afterburning for the reduction of N 2 0 emissions from a pilot-scale, coal-fired, circulating fluidised bed combustion (CFBC) system with a riser of diameter 161 m m and length 6.2 m. The primary cyclone and the secondary cyclone have been used as the main part of the afterburning zone. T w o 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. U p to 8 0 % N 2 0 reduction has been experimentally achieved with either propane afterburning or ethane injection. However, when methane was injected, only about 3 0 % N 2 0 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 N 2 0 than propane and ethane. Experimental results show that propane afterburning by direct fuel injection is more effective in reducing N 2 0 emission than propane afterburning with the burner. It is also shown that when afterburning is conducted through the burner, a higher N 2 0 reduction can be achieved when the air flow rate to the burner is lower, especially if the thermal input to the burner is limited. N O x emissions usually increase slightly with the introduction of an afterburning fuel and C O 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 N 2 0 plays only a minor role in the reduction of N 2 0 emissions in comparison to N 2 0 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 N 2 0 than ethane. Key words: N20 reduction, afterburning, secondary fuel injection, circulating fluidised bed combustor Short title: Reduction of N 2 0 emissions from C F B C by afterburning |