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Show input from the burner. This implies that excessive addition of afterburning fuel would result in an increase in N O and N O x emissions although N 2 0 emission could be reduced to a lower level. Figure 2(c) shows the emissions of S 0 2 and C O as a function of thermal input from the burner. N o limestone had been used to control S 0 2 emissions in the experiments reported in this paper. It can be seen, from Fig. 2(c), that S 0 2 emission increases slightly with the introduction of afterburning fuel, possibly because of additional coal bumout in the cyclones, while C O emission remains relatively constant unless excessive afterburning fuel is introduced. Experimental conditions for the results shown in Fig. 2(a) - (c), were the same as those shown in Table 2 and hence the afterburner was operated at a fixed air flow rate of 0.2 m /rnin at STP. However, the afterburner can also be operated at other air flow rates which may have an influence on the reduction of N 2 0 . Fig. 3 shows the effect of thermal input from the burner on N 2 0 emissions at three different burner air flow rates (0.20, 0.15, 0.10 m3/min at STP) while other air flow rates to the C F B C system were fixed at the values shown in Table 2. From Fig. 3, it can be seen that a lower air flow rate to the burner results in greater N 2 0 reductions, especially when the thermal input to the burner is lower than about 10 k W . This indicates that more N 2 0 is reduced per k W of thermal input from the burner for a lower burner air flow rate. For each of the burner air flow rates investigated, the effect of the afterburner thermal input on N O x or S 0 2 emissions was relatively small and of comparable to the data shown in Figs. 2(b) and 2(c), respectively. O n the other hand, C O emissions, as expected, increased more dramatically with thermal input from the burner when the burner air flow rate decreased. 180 160 140 120- 100- 80- 60 40 20 • 3rd air: 0.20 rrfVmin at STP • 3rd air: 0.15 m3/min at STP A 3rdair:0.10m3/minatSTP 2nd stage excess air: 19% A A - I - 12 15 Thermal input by propane burner, k W Figure 3 The effect of thermal input from the propane afterburner on N 2 0 emission at three different burner air flow rates Propane afterburning via the fuel injector: In this series of experiments, all the air flow rates to the combustor were kept constant as shown in Table 2. Hence a fixed amount of tertiary air (0.2 m3/min at STP) was always supplied to the exit of riser through the afterburner flame tube. The afterburning fuel, propane, was directly injected into the top of the riser, entirely through the afterburning fuel injector without the use of any carrier gas. The results of this series of experiments are shown in Figs. 4(a) - (c). From Fig. 4(a), it can be seen that N 2 0 |