OCR Text |
Show Figure 6 shows the solution for NO and CO in the homogeneous reactor for all three regions of the simulation: the individual jet, the merged jet, and the plug-flow. The CO fraction increases rapidly as fuel is consumed in the individual jet, then begins decaying after the merged jet entrains excess air. Further CO burnout continues in the plug-flow, after the mixing is finished. The model predicts a significant amount of CO remaining, approximately 30 ppm; in contrast, the experiments do not measure CO above the detection limit of 10 ppm. The reason for this discrepancy is most likely that the entrainment rate for the merged jet is not correct for the confined jet, leading to a residence time that is too small. The NO fraction shown in Fig. 6 indicates that NO is formed rapidly during the individual jet, then continues more gradually in the merged jet. Beyond the flame tip, when the temperature begins to drop due to further entrainment and heat transfer, the NO formation essentially freezes, and the NO level becomes constant (the mole fractions in these figures are corrected to a fixed dilution). The emissivity used in the radiation model for these computations was adjusted once, to obtain a maximum temperature of approximately 2000 K, as observed in the experiments. The final NO level of 114 ppm is in excellent agreement with the measured value of 101 ppm. A model prediction of major importance to the refinery research forum is the air toxic species nick-named "BTEX": benzene, toluene, ethylbenzene, and xylene. The model predicts formation of these species to the ppm levels during the fuel-rich portion of the individual jet, but these species are completely oxidized even before the entrainment takes the composition to stoichiometric. To emphasize this point, Fig. 7 shows the BTEX species plot ted against excess air (negative means less air than stoichiometric requires), instead of distance as in the previous figures. This prediction is in general agreement with the experiments, which measured these species at or below the detection limits of about 1 ppb. To examine the behavior of the blended mixture used to represent the refinery fuel gas, Fig. 8 shows the fractions of the fuel components in the homogeneous reactor. The ethane and propane break apart first in the simulation, followed closely by methane. Hydrogen appears to be the last to burn, but in fact it is being generated as the hydrocarbon species oxidize, so this impression is slightly misleading. As an example of the effect of the composition of the fuel, the results of similar computations for fuel mixtures with varying hydrogen content are presented in Fig. 9. The NO emission increases with hydrogen in the fuel, as evidenced by both the model and experimental data. SUMMARY Application of the Two-Stage Lagrangian model for jet mixing with a complete chemical kinetic mechanism demonstrates a capability for predicting NO and air toxic emissions from a conventional diffusion-flame burner. Computations predict that NO emissions increase slightly with hydrogen content in the refinery gas fuel. The air toxic species included in the current chemical mechanism are completely oxidized, even prior to the flame tip in the simulation. The remaining discrepancy between the model predictions and measure- 4 |