OCR Text |
Show literature 19,20, were applied in SENKIN21 to model homogeneous SNCR chemistry under quenched or isothermal conditions. A simplified, two-equation model for SNCR chemistry was derived from this same database through sensitivity analyses and curve fitting. This reduced SNCR chemistry model was used in JASPER to model the coupled turbulent mixing and chemical kinetic problem. Simplified SNCR Chemistry The simplified SNCR chemistry model consists of two irreversible reactions which compete for consumption of ammonia. The first reaction is the desired NO reduction reaction which dominates at lower temperatures in the temperature window. The second reaction results in NO production from reaction ofNH3 with oxygen which dominates at higher temperatures in the window. Rate parameters for these equations were detennined through detailed sensitivity analyses performed with the full chemical kinetic model for conditions that encompassed typical SNCR operation. Parameters that were varied included initial NO, reagent nitrogen to NO ratio, injection temperature, and quench rate in the SNCR zone. The rate parameters that resulted from the sensitivituy analyses were fit to a modified Arrhenius form which contains a pre-exponential factor, a temperature exponent and an activation energy. Figure 2 presents a comparison of this reduced . mechanism afflied in JASPER for homogeneous conditions to the full mechanism and to the data of Lyon . Initial conditions for this data and the model are 225 ppm NO, 1.23% 02, 450 ppm NH3, in a balance of helium. Notice the two-step mechanism predicts the data very well and approximates the fully detailed chemical kinetic solution. ModeJing the Effects of CO and InitiaJ NO Leyels In order to model the SNCR process in a reacting flow model, one must incorporate the effects of local CO concentrations and initial NO levels on the effectiveness of selective NOx reduction. Figure 3 shows the effect of adding various amounts of CO to a homogeneous mixture of combustion products, NO, and NH3 which undergoes reaction in a thermally quenched reactor. These calculations were performed using SENKIN and the fully detailed chemical kinetic mechanism described above. As' the injected CO level increases the optimal temperature for NO reduction shifts to lower temperatures, and, the width of the temperature window is reduced. Although the calculations presented in Figure 3 are for the specific case of 400 ppm initial NO, NH3INOi = 1.5, and a quench rate of 260C/sec, they can be generalized. Whenever CO is added to the reactants in SNCR both a shifting and a narrowing of the temperature window is observed. Additional calculations using the fully detailed chemical kinetic set were performed for variations in NOi from 100 to 1000 ppm, NH31N0i ranging from 0.5 to 3.0, and quench rates between 0 and 2500 Klsec in order to generalize the effect of CO. The general concept of a shifting and narrowing of the effective temperature window for SNCR with increasing levels of CO was incorporated into the simplified chemical kinetic model by a curve fit of this information. Therefore, in the reacting flow code, SN CR rates are modified depending upon local CO concentrations to account for the influence of CO on SNCR chemistry. Page 4 |