OCR Text |
Show than other boiler characteristics. Consequently, it is probably more difficult to predict the effect of changes in operating conditions on NO emission that other variables such as burnout, major species concentrations, etc. Since operation with over-fire air had the largest effect on effluent NO emissions, and the NO model seemed to correctly predict this trend, Figure 4 and 5 are shown to illustrate some of the differences between these two cases. Figure 4 shows the predicted average NO concentration as a function of burner height for the base line test and the test with over-fire air. The measured exit values are also shown for the two cases. This figure shows that in the case with over-fire air, the NO formation is delayed by 1-2 meters. Once NO formation begins, it occurs at approximately the same rate, but does not achieve the same peak value as the base line case. Figure 5 shows the predicted average temperatures as a function of burner height for the base line test and the test with over-fire air. The bottom of the burners is located at a height of approximately 8.5 m, and the top of the third level of burners occurs at approximately 11.5 m. The top of the fourth level of burners occurs at approximately 12.5 m. These burner locations correlate directly with the different locations of the rapid increase in temperature and NO concentrations. For the case with over-fire air, the region below the top lovel of burners (below 11 .5 m) will be more fuel-rich than in the base line case. Probably the major factor contributing to the lower NO formation in the case with over-fire air is that the nitrogen is released from the coal in a more fuel-rich and lower temperature region. This environment favors reduction of the devolatilized nitrogen to N2, and results in less NO production. It is doubtful that thermal NO is much of a factor in the differences in NO levels for these two cases, because of the low temperatures. 8 |