Influence of pressure on the heterogeneous formation and destruction of nitrogen oxides during char combustion

The influence of pressure on the heterogeneous formation and destruction of NO and N2O during the burning of char was studied in a pressurized fixed-bed reactor. The combustion experiments were performed at 0.2, 0.6, and 1.0 MPa for temperatures between 850 and 1200 K with a char whose parent coal is a high volatile bituminous coal (Westerholt). Nitrogen oxides originating from the fuel-N were assumed to be formed solely in surface reactions. Then a mechanism with eight reactions describing the heterogeneous formation and destruction of nitrogen oxides was considered. In this mechanism, NO is formed from nitrogen bound at active sites (-CNO), and N2O is formed by reactions between NO and (-CNO). The rate of each of the eight reactions was determined at different pressures between 0.2 and 1.0 MPa. It was found that the total pressure has no effect on the formation rates of NO and N2O. N2O production was characterized by a high activation energy of 120 kJ mol(-1). The present work also confirmed that a pressure increase accelerates the decomposition of NO and N2O on the char, and that N2O is more readily reduced on the char than NO. It was finally established that rates of reactions requiring only active sites were independent of pressure, whereas, except for N2O formation, rates of reactions involving gaseous species decrease when pressure increases. (C) 1998 by The Combustion Institute.