OCR Text |
Show quantitatively. The main purposes of gas phase modeling carried out in this study were twofold: (1) to determine the possible role of thermal decomposition in the N 2 0 reduction by secondary fuel injection and, (2) to study the influences, on the N 2 0 reduction, of secondary fuel, initial oxygen level in the afterburning zone before the secondary fuel injection and gas temperature in the afterburning zone. A detailed reaction scheme, the GRI-Mechanism Version 2.11 , has been chosen as the homogenous chemical kinetic model due to its up to date, optimised reaction constants. Since only C\- and C2- hydrocarbons were included in the GRI-Mechanism Version 2.11, the N 2 0 reduction by propane afterburning was not modelled in this study. Nevertheless, general conclusions drawn from the current modeling work should also applicable to propane afterburning. Although the actual flow regime in the afterburning zone is complex, for simplicity, a plug flow reactor, with specified temperatures and residence times, was assumed in the modeling calculations. The S E N K I N programme 31, in conjunction with the C H E M K I N programme32, was used to calculate exit mole fractions of species with given inlet conditions and reaction temperatures and residence times. Figure 10 shows the modeling results of N20 reduction by methane and ethane injection at two initial oxygen levels over a range of reaction temperatures. The thermal input from either secondary fuel was fixed at 10.07 k W , while the residence time in the afterburning zone was assumed to be 0.3 s. The gas phase modeling results, shown in Fig. 10, clearly demonstrate that the N 2 0 reduction depends on not only the reaction temperature but also the initial oxygen level. Higher reaction temperatures always reduce more nitrous oxides if other conditions are kept constant. A lower initial oxygen level results in a higher N 2 0 reduction for ethane injection. However, if the reaction temperature is below 1200 K, a decrease in the initial oxygen level from 6 % to 4 % will result in a decrease in the N 2 0 reduction for methane injection due to incomplete consumption of the injected methane, as shown by Fig. 11. Modeling results, shown in Fig. 11, demonstrate that the percentage of methane reacted after a residence time of 0.3 s depends on not only the reaction temperature but also the initial oxygen concentration. In contrast to methane, ethane is always completely reacted under all modeling conditions of Fig. 11. W h e n both methane and ethane are completely consumed in the afterburning zone, modeling results, shown in Fig. 10, indicate that ethane injection is slightly more effective than methane injection under the modeling conditions, which is consistent with the trend shown in Fig. 6. The N 2 0 reductions achieved by experiments with 10.07 k W methane and 10.07 k W ethane injection were about 3 0 % and 4 0 % respectively, as shown in Fig. 6. If the reaction temperature and the initial oxygen level, in the modeling, were set as 1173 K and 6 % respectively, the predicted N 2 0 reduction by 10.07 k W methane and 10.07 k W ethane injection would be about 3 6 % and 4 0 % respectively, and these values are consistent with the experimental results. |