OCR Text |
Show 10/79 Figure 13 shows data collected with the combination kiln injector with and without flue gas recirculation. The effect of flue gas recirculation is dramatic, however, it is primarily because of reduction in flame temperature due to the dilution effect of the recirculated combustion products. The reduced flame temperatures result in reduced heat transfer rates. Figure 14 shows the effect of combustion air staging. The upper curve shows the NO emissions with moderate air preheat without staging. The middle curve shows the data for moderate air preheat with 25% of the stoichiometric air supplied premixed with the fuel as primary air. The reduction in NO emissions is significant. The lower curve gives the NO emissions for a staged burner with ambient combustion air. This data shows that the emissions with a staged burner are much less influenced by air preheat than an unstaged burner. Fuel N0y Formation The formation of N0X from fuel-nitrogen species is mainly dependent on the concentration of nitrogen-containing species in the fuel and the oxygen concentration in the flame. For residual oils the nitrogen content is 0.1 to 0.4 weight percent.12 Studies of low-boiling-point additives7 suggest that conversion of fuel-nitrogen to N0X is not dependent on the chemical structure although this may not be the case for high-boiling-point nitrogen compounds.10 Fuel N0X is relatively independent of peak flame temperatures. This is shown by the fact that fuel-NOx levels are little affected by air preheat10 or flue gas recirculation.10 Increasing excess air increases fuel N0X but not to the same degree as thermal N0X.9 Figure 15 shows the N0X emissions from an ammonia-doped medium-Btu gas as a function of fuel-nitrogen concentration. The N0X emissions at very low fuel-nitrogen concentrations (100 ppm NH3 in the fuel) represent approximately 50% conversion of the fuel-nitrogen to N0X. At high concentrations (1% NH3) approximately 7% of the fuel nitrogen is converted to N0X. 6-5 I N S T I T U T E OF G A S T E C H N O L O GY |