OCR Text |
Show The primary interest is in the effect of fuel on the N0X levels achievable under staged combustion conditions related to the low N0X burner. The N0X emissions as a function of nitrogen content are shown in Figure 8 for a fuel-rich premixed primary at the optimum stoichiometry. The overall stoichiometry is brought to 125 percent of theoretical air by the addition of secondary air at a fixed axial location. Under staged conditions the differences between coals are much smaller and N0X emissions are between 300 and 400 ppm almost independent of the fuel nitrogen content. Both the two lignite (around 1.2 percent nitrogen) and the two bituminous coals (1.4 to 1.6 percent nitrogen) have retained their relative position although the absolute difference is less than 100 ppm. The absolute levels are also greater than those observed with the low N0X burner (Figure 4). There are a number of factors that may account for this including: (a) effects of scale; (b) the premixed primary provides a less rich primary for the same burner stoichiometry due to mixing effects in the large burner; (c) the premixed primary staged does not have the distribution of stoichiometries which may lead to lower levels in the burner; and, (d) the higher temperatures in the refractory lined system may affect both fuel devolatilization history and thermal N0X formation. Limited tests show that a three-stage system with a very rich first stage (40 percent theoretical air), a less rich second stage (60 to 90 percent theoretical air) and a lean third stage (125 percent theoretical air) gives lower emissions than the two-stage system.' It has also been shown that interstage cooling can decrease N0X levels even further. Overall, the fuel screening studies have been very valuable in identifying fuels with different emission potentials. From these results a smaller number of fuels are being selected for testing in the larger scale burners (3 to 35 thermal MW). If similar trends are shown to exist, the screening procedure will be established as a viable technique for low cost testing of specific fuels. 3-23 23 |