NO AND N2O FORMATION FOR COAL COMBUSTION IN A FLUIDIZED-BED - EFFECT OF CARBON CONVERSION AND BED TEMPERATURE

The conversion of fuel nitrogen to NO and N2O have been determined in a small-scale fluidized bed. Small batches of coal particles were used , permitting the separation of the formation reactions within individu al particles from subsequent destruction or re-formation on other part icles. From time-resolved measurements of the concentrations of NO, N2 O, CO2, CO, and CH4, the instantaneous fractional conversions of coal nitrogen to NO and N2O as a function of fractional char burnout were o btained for bed temperatures between 975 and 1148 K. The conversion to N2O decreases with increasing temperature, whereas that to NO exhibit s a maximum between 1023 and 1095 K. As a particle bums out, the insta ntaneous conversion to N2O decreases, whereas the reversed trend is se en for NO. The cumulative fuel nitrogen conversion to NO is in the ran ge of 0.18-0.46, whereas the conversion to N2O is in the range of 0.04 -0.18. The NO and N2O emissions can be explained by a model in which t he nitrogen bound in the char is converted to NO and N2O on oxidation within pores. The split between the NO and N2O depend on the local NO concentration and the temperature. The NO and N2O formed are subsequen tly reduced as they diffuse out of the pores. This model explains the increase in fractional conversion to NO and decrease in fractional con version to N2O with increasing carbon conversion. The temperature depe ndence is a function of the activation energies of the governing react ions.