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 N 2 O are formed from fuel-nitrogen (fuel-N). The complex homogeneous and heterogeneous reaction mechanisms determine the tradeoff between N 2, NO, and N 2 O 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. NH 3 and HCN oxidation, as well as NO and N 2 O 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 NH 3 to N 2 and NO, while almost no N 2 O is formed. The CH 4 addition increasing the radical level enhances the NO formation in NH 3 oxidation and N 2 O formation in the case of HCN. The presence of NO increases the selectivity toward N 2 and N 2 O for HCN and NH 3 oxidation. NO and N 2 O destruction tests demonstrate that thermal reduction of NO and N 2 O is negligible under present conditions. The presence of CH 4 reduces N 2 O emissions slightly due to reduction with the H radical. However, the presence of CH 4 affects not only the formation and destruction paths of NO and N 2 O but NO also significantly influences CH 4 combustion in fluidized beds by sensitizing its oxidation.