Effects of primary air velocity on co-combustion characteristics of bituminous coal and semicoke under reducing-to-oxidizing environment

This study evaluated the effect of primary air velocity (PAV) on the ignition characteristics of bituminous coal and semicoke mixture (1:1, wt%) in a reducing-to-oxidizing environment (R-O) in a 300-kW pilot-scale furnace that characterized the ignition behaviors of pulverized coal (PC) furnaces. The early ignition characteristics of the primary combustion zone (PCZ), including ignition distance, ignition mode, flame stability and kinetic parameters, were studied. The results showed that the difference in ignition delays obtained from the flame temperature and O-2 profiles indicated that convection heating was more important than radiant heating for fuel ignition. The transformation of the ignition mode from non-homogeneous to homogeneous with the increase in PAV was confirmed by the changes of flame temperature, residues, and flue gas composition. Flame stability was optimized by the earlier appearance of the continuous flame boundary and was the best at a PAV of 18 m/s. A simplified first-order kinetic model based on the CO2 evolution profile was established to calculate the reaction rate of the blended fuel during ignition, and the experimental char burnout ratio was consistent with the predicted result. For the potential application of co-firing bituminous coal with a large proportion of semicoke, the recommended PAV may be no more than 22 m/s.