OCR Text |
Show (3) where ~08 represents the overall equivalence ratio of the first and second stage combined. The second-stage fuel flow rate was derived as a function of second-stage air flow rate and equivalence ratio. Y· fuel = y. air (~) (~) 2 2 P fuel AFst (4) The overall equivalence ratio was maintained in the rich/lean two-stage configuration by setting a desired rich operating condition for the first stage (equivalence ratio and total flow rate of reactants), a lean equivalence ratio for the second stage and calculating the necessary fuel and air flow needed in the second stage to produce the desired overall equivalence ratio. The lean/rich configuration used to make the comparison was achieved by inverting the operating conditions obtained by the above analysis. EXPERIMENTAL RESULTS AND DISCUSSION The porous media burner was operated at 50 slpm in a singlestage configuration to determine the baseline NOx formation at various equivalence ratios which exhibited stable burning within the matrix. As shown in Figure 2, stable burning was achieved at equivalence ratios from 0.6 (67% excess air) to 1.5 (50% excess fuel). NOx levels at equivalence ratios of 0.6 to 0.8 were quite low, in the range of 5 to 15 ppmv, dry corrected to 3% 02. At high equivalence ratios, 1.0 to 1.5, NOx levels ranged from 25 to 50 ppmv, dry corrected to 3% 02. The reason for the higher NOx levels being formed under operating conditions having an excess of fuel compared to conditions having an excess of oxygen is not readily understood, but may be the results of two reaction paths that are taken. Under oxidizing conditions, most of the NOx is formed by Zeldovich reactions, consisting of the following: -7- |