Study on NO and N2O Formation and Destruction Mechanisms in a Laboratory-Scale Fluidized Bed

Abstract

In fluidized bed combustors, the harmful pollutants NO and N2O are formed from fuel-nitrogen (fuel-N). The complex homogeneous and heterogeneous reaction mechanisms determine the tradeoff between N2, NO, and N2O from fuel-N conversion affected by the bed temperature, fuel characteristics, residence time, and many more factors. To obtain a better understanding of these mechanisms and to study the relative importance of homogeneous and heterogeneous catalyzed reactions, a study on the gas reactions in a laboratory-scale fluidized bed reactor was performed. NH3 and HCN oxidation, as well as NO and N2O destruction, was studied simulating the conditions of devolatilization and char combustion stages. The experimentally obtained results were analyzed with a detailed chemical kinetic model considering the two-phase structure of a fluidized bed and the quenching of radicals on the solids' surface. The significance of homogeneous reactions depends on temperature and the presence of combustible gases. Heterogeneous catalyzed reactions oxidize HCN and NH3 to N2 and NO, while almost no N2O is formed. The CH4 addition increasing the radical level enhances the NO formation in NH3 oxidation and N2O formation in the case of HCN. The presence of NO increases the selectivity toward N2 and N2O for HCN and NH3 oxidation. NO and N2O destruction tests demonstrate that thermal reduction of NO and N2O is negligible under present conditions. The presence of CH4 reduces N2O emissions slightly due to reduction with the H radical. However, the presence of CH4 affects not only the formation and destruction paths of NO and N2O but NO also significantly influences CH4 combustion in fluidized beds by sensitizing its oxidation.