OCR Text |
Show was maintained at 31% and the firing rate was 450,000 Btu/hr. The overfire air was injected at 48 inches above the grate. The emissions of NO were observed to pass through a minimum, similar to emissions during coal firing; however, the minimum was fairly shallow, with emissions ranging from 0.21 to 0.35 lb NO /10 Btu. Although under excess air conditions these emission levels were significantly lower than NO emissions obtained during analogous coal firing (0.35 lb NO /10 Btu versus 0.52 lb NO /10 Btu), when based on fuel nitrogen, they represent conversions of approximately 100 percent. Further, the minimum NO emissions were not nearly as different (0.21 lb NO /10 Btu for wood versus 0.27 lb NO /10 Btu for coal) as their respective fuel nitrogen contents (0.1 percent versus 1.1 percent) would suggest. These data indicate that under optimum staged combustion conditions, the TFN chemistry may be partially equilibrated which reduces the influence of fuel nitrogen content and speciation. The effect of overfire-air height on NO emissions is also shown x in Figure 4. Unlike similar experiments with coal as the fuel, only a very slight effect on NO emissions was found by changing the overfire-air height from 48 inches to 20 inches above the grate. This suggests that the concentrations of nitrogenous intermediates in the first stage had largely equilibrated before reaching the overfire-air jets at 20 inches, and that the amount of NO being formed in the suspension zone is not of disproportionate importance relative to the fixed-bed phase. Firing Rate The effect of boiler load was examined by conducting tests at several different grate heat-release rates. Figure 5 shows that NO emissions increased slightly with increasing firing rate for both excess air fuel beds and staged conditions with both fuels. These results agree qualitatively with the field data (coal) of Langsjoen et al. (2). The higher NO emissions at increased loads probably were a result of higher local temperatures in the burning fuel bed as well as in the suspension burning zone. Higher local temperatures increase the formation of thermal NO and, as noted by Pohl and Sarofim (4), enhance the evolution of volatile nitrogen from the solid phase. Since the homogeneous conversion of volatile nitrogen is thought to 25-12 |