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Show diluent case (Figure 10). This suggests that combustion is slower and staging is improved by nitrogen dilution. The average rate of reduction from the initial hydrocarbon concentration to a level of 1 % along the secondary jet path for the nitrogen staged flame is approximately 96 ppm/mm while for the air diluent flame, the average rate of HC reduction is 139 ppmlmm. The air diluent flame (Flame 3, Figure 9) has an apparent secondary jet combustion rate that is 45 percent faster than the nitrogen diluent flame (Flame A, Figure 10). Considering that jet velocity, entrainment of gases and all other process variables are essentially the same, the increased rate of combustion for the air diluent flame can be attributed to the higher stoichiometric ratio in the secondary jet. Comparison of in-flame temperatures substantiate the conclusion that the nitrogen diluent postponed combustion more than air diluent at a 2.6: 1 gas to fuel ratio. Flame A, the nitrogen diluent case, had the lower heat release rate as indicated in Figure 11 by the change in temperature along the radial secondary jet path which averaged 3.7 °F/mm across the entire radial distance of the furnace. In comparison temperatures for Flame 3, the air diluent case shown in Figure 12, had an average change of 6.4 °F/ mm, a rate 73 percent higher than for that of Flame A. The general outcome of faster combustion rates is increased NOx formation and not surprisingly, NOx emissions are 75 percent greater for the air diluent flame than the nitrogen diluent flame. 30 25 N 0 0~ M 20 E a. ~ Ul 15 c 0 .Ci) Ul ·we 10 )( 0 z 5 0 0 • Air Diluent (Temperature) • Nitrogen Diluent (Temperature) 6. Air Diluent (NOx Emissions) o Nitrogen Diluent (NOx Emissions) 2000 1950 1900 LL ~ 1850 -~ co ~ 1800 ~ E 1750 ~ Ul 1700 ~ Q) 1650 ~ c ~ 1600 ~ 1550 1500 20 40 60 80 100120140160180200220240 Secondary Jet Velocity (ft/s) I o 1.33 2.03 2.66 3.37 4.10 Secondary Gas/Fuel Ratio (v/v) Figure 7. NOx emissions and furnace gas temperatures at various secondary air and nitrogen to fuel ratios. By using a Single secondary nozzle size, the secondary jet velocity varied with changing gas to fuel ratio. All other process variables were held constant at baseline conditions. 20 __ --------------------------------------------------------. N 0 15 ~ 0 CO) e- ooS. S 0 (I) 10 c 0 ·in Ul ·e w )( 0 5 z o ~--~~~~~~~~~~~~~~ 0.5 1 1.5 2 2.5 Secondary AirlFuel Ratio (v/v) Figure 8. The effect of secondary air to fuel volume ratio on NOx emissions independent of the secondary jet velocity. The burner variables were maintained at baseline conditions. Secondary jet velocity is maintained by interchanging injector nozzles at different air to fuel ratios. 9 |