Determining the optimum fuel concentration for ignition and combustion of semi-coke and bituminous coal blends with rich/lean burner

To burn semi-coke with ultra-low volatile content effectively at a large blending proportion with high volatile bituminous coal using a pulverized fuel (PF) rich/lean burner, the ignition and combustion behaviors of blended fuel of semi-coke and bituminous coal (1:1 by weight) in reducing to oxidizing atmosphere were investigated in a 300 kW pilot-scale test furnace with an arrangement of varying pulverized fuel-rich side stream concentration (P-fc,P-rich, kg/kg). The results showed that ignition distance of the fuel-rich stream decreased from 339 mm to 159 mm when P-fc,P-rich increased from 0.37 kg/kg to 0.58 kg/kg, which then increased again to 0.63 kg/kg. The ignition distance for the fuel-lean stream increased monotonically from 332 mm to 383 mm with an increase in the P-fc,P-rich. The intersection of two streams showed a positive effect on overall ignition when P-fc,P-rich was varied from 0.49 kg/kg to 0.58 kg/kg, which was mainly attributed to the mean volatile content of blended fuel. A parameter V-index was introduced to evaluate the effects of quantity of volatiles on the ignition behaviour of blended fuels, and it was found that the volatile effect was more sensitive to the ignition distance than the ignition temperature. The ignition of the fuel-rich stream transformed from the heterogeneous-homogeneous combined mode to the homogeneous mode at P-fc,P-rich of 0.49 kg/kg. The burnout ratio of blended fuel was highest at P-fc,P-rich = 0.52 kg/kg and 12.21% higher than that of the ordinary unbiased burner. NO emission was inhibited under rich/lean combustion conditions, and the existence of a strong reducing zone was the key factor for the low conversion ratio of fuel-N to NO. Based on the experimental results, P-fc,P-rich ranging from 0.52 kg/kg to 0.58 kg/kg was observed to be optimum for rich/lean co-combustion at large proportion of semi-coke in the blends.