| OCR Text |
Show AFRC - 1996 Int. Symposium Baltimore September 30 - October 2 1996 Page 8 of 14 flame tube from the NOx in the flue-gas downstream of the flame tube. Doing this, it should be considered that due to combustion, the "dry" flow rate (in m3(n)/s) upstream of the flame tube is larger than the flow rate downstream of it. In this study, the NOx production in the flame tube have been calculated using the measured (dry) oxygen concentrations upstream and downstream of the flame tube and known natural gas composition and the measured (dry) NOx concentrations at both locations. By this procedure, the effect of the potential inaccuracy of the mass flow meters in the system on the calculated NOx production has been eliminated . Experiments showed that the NOx production is not dependent on the cooling air flow around the flame tube. Furthermore, the NOx production was not dependent on the mixture velocity (thermal input). The absence of both effects on the NOx emission is a well known feature of premixed natural gas flames, and are theoretically reasonably well understood. The NOx production due to the combustion in the flame tube was also not dependent on the initial NOx concentration in the mixture. Figure 4 presents the measured NOx production as a function of the NOx concentration in the natural gas/air/flue-gas mixture for two conditions. 12 D 0 0 _10 .. ~ ..2: . • • . • . • • . to t5 20 25 NOx In unburnt mixture (ppm, dry) 1.02 : 14Sj 395'C .02 = t4lj 585 'C I Figure 4: Experimental data on HOx production in flame tube versus the (dry) HOx concentration in the natural gas/air/flue-gas mixture. The data shown in figure 4 are obtained by changing both the dilution air flow and the air flow through the burner in the precombustor. By doing this, the composition of the unbumt gas/air/flue-gas mixture, with exception of the NOx level, was unaltered. The results plotted in figure 4 suggest that it is unlikely that NOx reduction reactions playa significant role in the flames (with n > 1.2 and T < 1800 °C) under investigation. |
