OCR Text |
Show input from the secondary fuel injected for three secondary fuels under the same initial operating conditions (initial 0 2=6%). From Fig. 6, it can be clearly seen that N 2 0 reduction always increases with the thermal input of a secondary fuel. From the thermal input point of view, methane is the least effective fuel in reducing N 2 0 , while ethane and propane have similar abilities to reduce N 2 0 . The relatively small range of thermal input tested for methane injection, shown in Fig. 6, was not caused by any flow rate restrictions but as a result of incomplete combustion of methane in the afterburning zone, the latter was indicated by a dramatic increase of C O concentration in the exhaust when the thermal input of methane exceeded 11 k W . This difficulty in complete combustion in the afterburning zone showed that methane was a poor afterburning fuel for direct injection to control N 2 0 emissions under the conditions of this study. If methane is to be used as an afterburning fuel, it would be better to introduce methane through a burner so that both low N 2 0 emissions and complete combustion of methane can be obtained. O f course, this still remains to be experimentally confirmed. w • 1 • 1 ' 1 ' 1 • 1 0 5 10 15 20 25 Thermal input by the burner or fuel injection, k W Figure 5 Comparison of the effects of thermal input from afterburning fuel on the N 2 0 reduction efficiency between different afterburning configurations Figure 7 shows comparisons of N20 reductions and NOx reductions due to methane, ethane and propane injections. Due to different values of N 2 0 and N O x concentrations, before fuel injection (ca. 200 ppmv for N 2 0 and ca. 100 ppmv for N O x ) , absolute values of N 20 reductions and N O x reductions, rather than the percentage reductions, have been used in Fig. 7 so that the comparison between N 2 0 reductions and N O x reductions can be made on a comparative basis. These results clearly demonstrated that N 2 0 reduction, due to the injection of any of the three secondary fuels, was always accompanied by an increase in N O x emissions, although the increase in N O x emissions was relatively small compared to the decrease in N 2 0 emissions. From Fig. 7 it can also be seen that methane injection resulted in the highest N O x increase compared with either ethane or propane injection. These results again indicated that methane was not a good choice as a secondary fuel for direct injection to control N 20 emissions. |