OCR Text |
Show ° + N2 -+ NO + N N + 02 -+ NO + ° The first step is rate-limiting and occurs at elevated temperatures (>2800 0 F) (5). At equilibrium, very high levels of NOx can be formed under oxidizing conditions. Figure 3 shows equilibrium NOx formation for a mixture of methane and air at various temperatures and equivalence ratios. At an equivalence ratio of 0.87 (approximately 3% 02)' NOx levels in the range of 1000 to 4000 ppmv are possible at temperatures above 2400 o F. However, due to the high activation energy and long residence times required for Zeldovich reactions to go to completion, only a small fraction of the equilibrium levels of NOx are realized. Figure 2 shows that at an equivalence ratio of 0.87, only 30 to 35 ppmv of NOx was formed in the PM burner due to the low residence time in the matrix and the cooling effect of radiant heat transfer. In fuel-rich flames, equivalence ratios of 1.0 to 1.5, NOx is formed from HCN which is produced by a reaction between the excess hydrocarbon radicals and elemental nitrogen. Under most conditions, the dominant path from HCN to NO is the sequence initiated by the reaction of HCN with atomic oxygen: CH2 + N2 -+ HCN + NH HCN + ° -+ NO + HC (5) Equilibrium NOx formation, under fuel-rich conditions, is in the range of 10 to 200 ppm, dry at temperatures above 2800°F. The single-stage data presented in Figure 2 indicates that, under actual firing conditions, the PM burner will generate 25 to 50% of the equilibrium NOx levels. The conclusion which can be drawn from these data is that, under oxidizing conditions, NO forma- x tion is rate limited. Whereas, at conditions of excess fuel, NO x formation may approach equilibrium conversions, which is the limiting factor for levels of NOx that are formed. Note that at temperatures below 2400 oF, equilibrium NOx formation for fuel-rich combustion conditions approaches zero. This points out the need for maintaining reduced temperatures in the PM burner for operation under reducing as well as oxidizing conditions. -9- |