Kinetic relationship between NO/N2O reduction and O2 consumption during flue-gas recycling coal combustion in a bubbling fluidized-bed

Flue-gas recycling combustion of a sub-bituminous coal and its rapid pyrolysis char at 1120 K has been simulated experimentally in a bubbling fluidized-bed. O2, CO2 and H2O, and NO or N2O were pre-mixed and fed into the bed together with coal/char particles with the O2 concentration in the exit gas maintained at 3.5 vol%. Increasing the inlet O2 concentration, thus increasing the O2 consumption rate and decreasing the flue-gas recycling ratio, caused the once-through conversion of fuel-bound nitrogen into N2O to decrease while the conversion to NO to remain unchanged. The in-bed reductions of NO and N2O were both first order with respect to the respective nitrogen oxide, with the rate constants to increase linearly with the rate of O2 consumption in the bed and thus also with that of char/volatiles consumption. This finding, which indicated linear increase in the concentrations of reactive species involved in NO/N2O reduction with the rate of O2 consumption, enabled consideration that the homogeneous and heterogeneous reduction rates of NO and N2O were proportional to the consumption rates of O2 by the volatiles and char, respectively. The rate analysis of the kinetic data revealed the relative importance of burning volatiles and char as the agents for the reduction of NO and N2O. While the reduction in the gas phase was fully responsible for the NO-to-N2O conversion, the reactions over the char surface governed the NO-to-N2 reduction. The volatiles and char had comparable contributions to the reduction of N2O to N2. The NO-to-N2 and N2O-to-N2 reductions over the char surface were, respectively, accelerated and decelerated by increasing the H2O concentration.