Diffusional effects on differences of coal char reactivity between air and oxy-fuel combustion in thermogravimetric experiments

Oxy-fuel combustion can realize large-scale CO2 capture and low NOx emission from coal-fired power plants, while the evaluation of coal reactivity differences between air and oxy-fuel conditions is of importance for retrofitting existing conventional boilers to oxy-fuel ones. Here, two sets of specially designed experiments were contrastively conducted to assess the diffusional limitation effects on differences of coal char reactivity between air (O-2/N-2) and oxy-fuel (O-2/CO2) combustion in non-isothermal thermogravimetric (TG) experiments, which were seldom investigated previously. Experimental results show that the TG/DTG curves of char combustion present distinct differences before and after reducing diffusional limitation. The differences of char combustion measured in non-isothermal TG experiments between O-2/CO2 and O-2/N-2 conditions are shrinking with the reduction of diffusion resistance. Compared with conventional air combustion, change of combustion atmosphere in oxy-fuel condition mainly exhibits influence on diffusion process, while it has no observable effect on char-O-2 chemical reaction in TG experiments. Obviously different results between air and oxy-fuel combustion can be obtained when the diffusional limitation in TG tests is significant, while the change of combustion atmosphere has negligible influence on char reactivity under minimized inhibition of heat and mass transfer. Knowledge of the respective influence of intrinsic reaction and diffusion resistance on the measured TG experiments is of considerable significance for the exploration of reactivity and kinetics differences between air and oxy-fuel combustion.