OCR Text |
Show be greater than heterogeneous nitrogen conversion (5), the latter effect could also increase NO emissions. It would appear that with the Douglas Fir test (Figure 5) bed stoichiometry had virtually no effect on NO emissions at a given firing x rate; however, it should be noted that the two values of bed stoichiometry, 0.5 and 1.0, lie on each side of the optimum SR, . Thus, the levels of NO D emissions at the two chosen bed stoichiometries were nearly equal, but the NO emissions would have been significantly lower at bed stoichiometries between those chosen, as is indicated by Figure 4. Inlet Particle Size Distribution As noted previously, suspension zone combustion conditions apparently have a significant effect on NO emissions; this agrees with the findings of Starley et al. (6), in which fuel nitrogen conversions approaching 40 percent were found in the suspension phase. In order to evaluate the total impact of suspension zone burning on NO emissions, several series of experiments were conducted with fuel particle size distributions PSD-2 and PSD-3, which had been passed over 8 and 4 mesh screens, respectively, to reduce the fines content of the coal feed. These two feed size distributions, PSD-2 and PSD-3, represented 85 and 80 percent, respectively, of the normal feed distribution. In the first series of experiments, the furnace was operated under conditions typical of full-scale units. Figure 6A shows the NO emissions from the MSF under field-operating conditions for the three different feed particle size distributions. The uppermost set of points represents a duplication of the data presented in Figure 2. By reducing the mass of coal which burns in the suspension phase, the NO emissions were reduced as much as 20%. In all cases the total burning rates were 470,000 Btu/hr. In typical field-operating conditions, both the fuel bed and the suspension zone are under excess-air conditions; a separate series of experiments was conducted to show the effect of suspension-zone stoichiometry on NO formation with the coal feed most deficient in fines (PSD-3). 25-14 |