| OCR Text |
Show 13 difference may be related to the very high ash content and high density of this fuel compared to the others. Discussion The trend in NOx emission with equivalence ratio is consistent with other fuels and with much of the NOx coming from fuel nitrogen. For hydrocarbon fuels, NO formation from fuel nitrogen occurs on time scales comparable to the hydrocarbon oxidation, and is known to be sensitive to equivalence ratio, with fuel lean conditions producing high yields and fuel rich conditions producing low yields (Seinfeld, 1986; Bowman, 1991). Under fuel rich conditions, the relatively fast conversion of fuel carbon to CO competes for oxygen, leading to a reduced availability of oxygen for NOx production. The addition of sulfur has also been observed to decrease the formation of NO due to the competition for available OH radicals (Bowman, 1991). The results here show increasing concentrations of S02 with increasing equivalence ratio, which in some of the experiments might be important in the NOx reduction. However, the form of the sulfur not accounted for as S02 is unknown, and this mechanism cannot be specifically identified as contributing to the NOx decline at higher equivalence ratios. Reduction in N Ox concentration might also be related to increasing char particle concentration with increasing equivalence ratio, but this remains untested. The decrease in NOx with increasing equivalence ratio is consistent with the use of staged combustion in reducing total NOx emitted from combustion processes. The fractional conversion of fuel nitrogen to NOx has been shown to decrease with increasing fuel N concentration for hydrocarbon fuels and coal (Seinfeld, 1986; Miller and Bowman, 1989; Bowman, 1991). The declining N to NO conversion is postulated to be due to the formation of a nitrogen containing species important to both the production and removal of NO. Data obtained from commercial biomass-fueled fluidized bed combustors (Grass and Jenkins, 1994) and laboratory experiments with fIT and birch wood (Leckner and Karlsson, 1993) also suggested declining fuel N conversion with increasing fuel N concentration, at least to 1.1 % N. The MFC results are plotted in Figure 7 along with other biomass and coal data reported in the above references, and with results from MFC experiments on biomass-derived pyrolysis oils. The NOx emission is shown as a relative production rate, taken as the NOx concentration |
