COAL AND CHAR NITROGEN CHEMISTRY DURING PRESSURIZED FLUIDIZED-BED COMBUSTION

Two programmes of work were undertaken to provide information on the fate of fuel nitrogen during pressurized fluidized bed combustion (PFBC), for the development of a predictive mathematical model to optimize conditions for low-NO(x) combustion. The distribution of coal nitrogen between the volatiles and residual char during devolatilization of coal under conditions pertinent to PFBC was investigated. The results show that at devolatilization temperatures > 850-degrees-C the coal nitrogen was distributed preferentially in the volatiles. The influence on the proportion of nitrogen remaining in the char of the residence time, coal particle size, pressure, oxygen concentration and coal type was small in the ranges studied. At a given temperature, the nitrogen:carbon ratios of the chars were comparable with those of the parent coals, so the coal nitrogen distribution could be predicted from the volatile and residual char yields. However, this distribution was dependent on a number of factors, including coal type, temperature and pressure. Kinetic information on the activities of PFBC char and bed material for the NO reduction reaction was also obtained. Both PFBC char and bed material were found to be important for the heterogeneous catalytic reduction of NO by CO, char being the more effective catalyst. The rate of the heterogeneous NO reduction reaction increased with temperature, pressure and NO and CO partial pressure. The catalytic activities of the four PFBC bed materials for the reduction of NO by CO were relatively comparable.