Coal combustion emission and ash formation characteristics at high oxygen concentration in a 1Â MWth pilot-scale oxy-fuel circulating fluidized bed

Combustion experiments for one common Chinese bituminous coal (Datong) were conducted in air and in O2/dry recycle flue gas (RFG) mixtures with input O2 concentrations of 30%, 40% and 50% in a 1 MWth pilot-scale oxy-fuel circulating fluidized bed (CFB). The effect of combustion atmosphere and O2 concentration were investigated to determine their impact on coal combustion efficiency, gaseous pollutant emissions, limestone desulfurization efficiency and chemical composition of the fly ash. The experimental results showed that the unburned char content in fly ash for the 30% O2/RFG combustion mode is slightly higher than that obtained for the air combustion mode. However, when the O2 concentration reaches and exceeds 40%, the content of unburned char in fly ash is less than that obtained under the air firing mode. Compared with the air firing mode, the SO2 emissions under O2/RFG modes are notably higher, and the limestone desulfurization efficiency is lower. Under O2/RFG modes, the NO emissions are much lower, but the N2O emissions are higher than that under the air firing mode, with the level of N2O emission being nearly the same as the corresponding NO emission. There are no obvious differences in the chemical composition of fly ash under air and O2/RFG modes, except for a slightly lower percentage of K2O, Na2O and SO3 under O2/RFG modes at higher O2 concentrations.