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Show this effect for a simple axial injector in the test facility. The high, medium and low quench rates corresponded to residence times of 69, 170 and 400 msec respectively, between 1267 and 1100 K (i.e., the quench rates are 2420, 982 and 418 K/sec). Over the injection temperature range investigated, the NO reduction efficiency decreased as the injection temperature decreased and there was almost no difference in measured reduction efficiencies with the high and medium quench rates. Mixing and Dispersion In practice the selective reducing agent must be dispersed throughout the product stream and well mixed to provide a uniform reagentINO ratio while the combustion products are within the desired temperature window for effective NO reduction. In this experiment dispersion and mixing occur simultaneously as the reagent was introduced through a single injector. The momentum of the reagent jet was altered by varying the amount of diluent, in this case nitrogen, that was added with the ammonia. This not only increased the jet momentum but also affected the heat up of the reagent jet. The change in NO reduction for a constant NH3INO ratio as a function of the momentum ratio of the reagent jet and the product stream is shown in Figure 6 for two injection temperatures and the high and low quench rates. For both higher quench rates and lower injection temperatures the NO reduction efficiency decreases with increasing jetJ product stream momentum ratio due to a less favorable temperature history (which will be discussed later). At the higher injection temperature and low quench rate increasing the jet momentum had no effect on the NO reduction efficiency. . , Influence of Other Species - CO Several studies have shown the influence of CO on the selective reduction process8,9,lO showing that the presence of CO shifts the selectivity by increasing the rate of NH2 formation and the rate ofNH3 oxidation to NO. Figure 7 presents data obtained at the medium quench rate showing the influence of added CO on NO reduction at two injection temperatures. In these experiments the CO was added to the NH31N2 reagent jet. The addition of CO has little beneficial impact on NO reduction at the optimum injection temperature (1303 K not shown) but increases the reduction efficiency to a maximum depending on the amount of CO added for the lower injection temperatures. Figure 8 shows how the presence of CO affects NO reduction at the high and low quench rates as a function of the reagentjetJproduct stream momentum ratio. Increasing the momentum ratio has a beneficial effect on NO reduction at both quench rates in the presence of CO. The opposite effect was observed in the absence of CO. Modeling and Discussion The presence of CO effectively increases the rates of SNCR reactions by two mechanisms: increasing local temperatures and increasing net production of OH radical which is necessary for the initiation of SNCR chemistry. The first effect is due to the exothermicity of the main CO burnout reaction, CO + OH -> CO2 + H, while the second effect is due to chain branching reactions that result from the H produced in this reaction. This enhancement of SNCR chemical rates leads to the shifting and narrowing of the temperature window for SNCR and also explains why increased jet momentum leads to higher reductions in the presence of CO (Figure 8). The Page 6 |