OCR Text |
Show The Effect of Excess Air Firing on N O Formation Figure 4 illustrates the effect of excess air firing on N O formation at four different combustion air temperatures. Thermal NO is strongly dependent on the time/temperature history of flame gases and the availability of oxygen. Oxygen concentrations are affected by the availability of excess air and the rate of fuel/air mixing. Both of these parameters will raise the peak flame temperature up to a point that will, in turn, increase the thermal N O x levels. As seen in each of the four cases, increase oxygen availability results in increased N O x formation up to about 6% oxygen in the flue. Therefore, by firing at low oxygen levels, N O formation is minimized. Low excess air firing is especially effective at the higher air preheat levels. As mentioned previously, N O emissions increase with the amount of available oxygen. However, as also seen in Figure 4, N O x formation was the highest between 6% and 7 % excess oxygen in the flue, but beyond this oxygen level, N O x formation began to decrease. The reduction in N O formation is due mainly to the reduction in the peak flame temperature, which is reduced because of dilution. Large amounts of excess oxygen not only eventually reduce NO formation but obviously reduce furnace efficiency. The Effect of Flue-Gas Recirculation on N O x Formation Figure 5 illustrates the effect of flue-gas recirculation on N O x formation at three different combustion air preheat temperatures. As seen in each of the three curves, increasing the rate of recirculation decreases N O formation. Reduction in the amount of N O formed is due to the reduction of the peak flame temperature and the reduced availability of oxygen in the flame, which are caused by the recirculation of inert combustion products. Much like low excess air firing, FGR appears to be even more effective at the higher preheat temperatures than at the lower temperatures. Implementing 5% recirculation at 800°F results in a 70 ppm reduction in N O x while implementing 5% recirculation at 1200°F level results in a 320 ppm N O x reduction. It also appears that the effectiveness of FGR is greater in the first 5% of recirculation. The Effect of Excess Air Firing on NO Formation at Various Rates of Recirculation Figures 6, 7, and 8 illustrate the effect of excess air firing at 0%, 5%, 10%, and 20% recirculation for 800°, 1000°, and 1200°F combustion air preheat, respectively. In each of the three figures, the effect of excess air firing and flue gas recirculation on N O formation is illustrated on both an individual and combined basis. As seen in each of the figures, flue-gas reciruclation is a more effective N O control technique than LEA 4-4 |