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Show --••-• initial 02=6% -•-initial 02=4% CH4=10.07 kW, T=0.3 s 1120 1140 1160 1180 1200 1220 1240 Temperature, K Figure 11 Modeling results: Percentage of methane reacted versus reaction temperature Modeling conditions same as Figure 10 The thick broken line shown in Fig. 10 represents the N20 reduction resulting from thermal decomposition alone. Obviously thermal decomposition does not contribute significantly to the total N 2 0 reduction, as was indicated by methane injection tests. Prompt-NOx formation due to secondary fuel injection can be one of the reasons for the observation of increases in N O x emissions shown in Figs. 2(b), 4(b) and 7. However, the gas phase modeling predicts only a very small amount of increase in N O x emissions which accounts for less than a quarter of the additional N O x generated, shown in Fig. 7. The char burnout efficiency in the afterburning zone of this study could be improved after a secondary fuel injection due to the temperature increase in the afterburning zone, which in turn resulted in an increase in N O x emissions. Due to low levels of gas temperature in the afterburning zone (ca. 1123 K ) , N O x reduction due to the reburning mechanism is expected to be small. CONCLUSIONS Experimental results of afterburning to reduce N 2 0 emissions from a pilot-scale coal-fired circulating fluidised bed combustor have been presented and analysed. T w o different afterburning configurations, one through an externally accommodated commercial gas-fired burner and the other by direct fuel injection through a simple fuel injector, have been investigated. Propane has been tested with both afterburning configurations while ethane and methane have also been tested with direct fuel injection for comparison. U p to 8 0 % N 2 0 reduction has been achieved with propane afterburning and ethane injection. Gas-phase modeling using a detailed reaction scheme, GRI-Mechanism Version 2.11, qualitatively confirms experimental observations and determines the role of N 2 0 thermal decomposition in reducing N 2 0 in the afterburning zone. The following conclusions can be drawn: (1) Operational considerations may favour the afterburning configuration via an externally accommodated, commercial gas-fired burner. However, on the basis of thermal input from the afterburning fuel, propane afterburning by direct fuel injection was more efficient in reducing N 2 0 than propane afterburning through the burner. X o |