OCR Text |
Show firing; however, it is more costly to use. On a combined basis LEA firing with FGR does have an additive reduction effect on N O emissions. Arrhenius Correlation for Excess Air Levels Figure 9 is a plot of the £n N O x versus the inverse of the adiabatic flame temperature for 2.3%), 4.1%), and 7.5% oxygen in the flue. This plot of the Arrhenius equation illustrates NO dependency on the flame temperature and gives an approximation of the activation energy by the slope of the line. It appears that the energy of activation is the same for each O2 level. Arrhenius Correlation for FGR Figure 10 is a plot of £n N O versus the inverse of the adiabatic flame temperature for 0%, 5%, 10%, and 20% FGR at 10% excess air. As illustrated in the figure, the slope of the line for each rate of recirculation is different. It appears the energy of activation decreases as flue-gas recirculation increases. Since it is doubtful that the activation energy is affected by the level of FGR, apparently the adiabatic flame temperature is not an appropriate temperature for evaluating the data. The level of FGR would, however, effect the oxygen concentration in the flame zone and this should be a part of any explanation for the results seen. The Effect of Combustion Air Preheat on Pounds of NO Formed Per 106 Btu Absorbed Figure 11 illustrates the effect of combustion air preheat temperature and FGR on N O formation in terms of pounds of N O generated for each 10 Btu absorbed by the load. For this figure, it is assumed that all N O generated has been converted to NO^. This figure illustrates the effectiveness of FGR. The Effect of Substoichiometric Firing on NO Formation for 1200°F Combustion Air Preheat Figure 12 illustrates the effect of substoichiometric firing on N O formation using 1200°F combustion air preheat. Comparing the N O x results compiled for 1200°F substoichiometric conditions and those results compiled for any other preheat temperature at near stoichiometric conditions, the potential of two-stage combustion as a N O control technique for high air preheat levels can be seen. 4-5 |