Transformation Mechanisms of NO_x into N_2O and Its Depression in Coal Combustion and De-NO_x Process

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Nitrous oxide (N_2O) has been arrested the attention as one of environmental pollutants produced from coal combustion. In this study, characteristics of N_2O formation/destruction in both turbulent pulverized coal combustion (high temperature combustion) and bubbling fluidized bed coal combustion (low temperature combustion) are discussed experimentally and theoretically.In case of turbulent pulverized coal combustion, during the volatile matter combustion region, N_2O concentration increases. The maximum N_2O concentration becomes higher, if the temperature gradient gets gentler. Finally, at the exit of furnace, N_2O reaches quite low value. The influence of non-catalytic selective reduction (NCSR) of NO by NH_3 injection on N_2O formation/destruction depends on the combustion atmosphere where NH_3 is supplied.In the bubbling fluidized bed coal combustion, N_2O concentration increases with decreasing temperature and with increasing the combustion stoichiometry. The difference of N_2O … More concentration due to coal types can be explained by the difference of N-contained species evolved together with the volatile matter. HCN is found to contribute more to N_2O formation than NH_3. In case that the coal particles are directly supplied onto the bed surface, N_2O is mainly formed in the freeboard of combustor. In case of bottom feed experiments, on the other hand, N_2O is rapidly formed just above the air distributor. In the experiments of N_2O decomposition through the beds of several kinds of particles, char, CaO and oxidized stainless steel particles can catalytically contribute to the N_2O decomposition. In the char combustion experiments, the tendency of N_2O formation characteristics is similar to that of the combustion of volatile matter.From the numerical model for single coal particle combustion, N_2O is mainly produced during the volatile combustion. The reactions by way of NCO, NH and N_2 contribute to N_2O formation. On the other hand, H radical becomes one of the important species for N_2O reduction. The modeling could explain well the experimental results. Less