OCR Text |
Show relatively insensitive to overall stoichiometry as long as the bed region stoichiometry is held constant. These data are consistent with similar pulverized coal results (3) and indicate a weak dependence on oxygen partial pressure under excess air conditions. In the case of the hogged wood fuel, when the overfire air was injected at 20 inches (below the spreader), increasing the excess oxygen from 2 to 10 percent increased the exhaust NO by approximately 35 percent. Bed-Region Stoichiometry The effect of bed-region stoichiometry was studied by firing the Utah coal at 470,000 Btu/hr under conditions such that the overall stoichiometry was held constant at 1.45. As the air/fuel ratio in the bed region was reduced by putting a larger fraction of the air through overfire air jets, the NO emissions decreased significantly as shown in Figure 4. When the bed was operated with substantial excess air (SR, > 1.3) the visual flame zone was confined to approximately 6 inches above the bed. However, as the bed stoichiometry was decreased, the height of the volatile combustion zone increased significantly and by SR = 0.65 was on the order of 3 feet. Prior to these tests there had been serious concern about the operability of a continuous feed system at sub-stoichiometric conditions with coal firing; however, no combustion problems were encountered. The extent of the reduction in NO emissions when the bed was operated under sub-stoichiometric conditions was also a function of the overfire air height (OF ht) above the bed. This difference is an indication of the effect that suspension zone oxygen levels exert on exhaust NO emissions. When the overfire air is injected at 20 inches above the grate, (i.e. below the level of fuel injection at the spreader), then much of the suspension zone combustion occurs under an excess air condition. Lower levels of NO are produced when the overfire air is injected at 48 inches above the grate, thus enclosing the suspension zone in a fuel-rich, oxygen deficient environment. Figure 4 also shows the results of similar experiments conducted with the hogged Douglas Fir. In these tests the overall level of excess air 25-10 |