The Trade-Off Between N2, NO, and N2O Under Fluidized Bed Combustor Conditions

Abstract

NO and N2O are harmful pollutants. Under fluidized bed combustor conditions, the nitrogen of the solid fuel is partly converted to these species. The trade-off between N2, NO, and N2O depends on the fuel and fuel characteristics, the complex homogeneous and heterogeneous formation and destruction paths, temperature and residence times, and so forth. Because of these complex interrelations, it is necessary to study these processes separately and to analyze their relative importance. To obtain a better understanding of the formation and destruction paths of NO and N2O, comprehensive studies have been performed in a laboratory-scale fluidized bed reactor optimized to obtain formation rates. The influence of the temperature and radicals on the NO and N2O formation from HCN and NH3 and destruction reactions were studied. The results show that N2O is formed only from HCN. Oxidation of NH3 forms NO and N2, HCN forms NO, N2O, and N2. Typically, 30 to 70 percent of NH3 are converted to N2, depending on bed temperature. In the case of HCN, only 5 to 25 percent are converted to N2. At temperatures below 800°C, NO reacts with CH4 oxidation products to NO2. Tests with HCN show that HCN conversion starts already at 700°C in the fluidized bed, N2O is formed in significant amounts only in the presence of CH4. The results of the NO and N2O destruction tests show that the thermal mechanism is of minor importance. At 900°C, N2O destruction with H radicals can be seen. N2O formation shows a maximum at 850°C. The gas reaction studies were used to understand the NH3, HCN, NO, and N2O single-particle formation characteristics of coke, bituminous coal, peat, and spruce wood under fluidized bed combustor conditions. [S0195-0738(00)00702-0]