Effect of CO₂ during coal pyrolysis and char burnout in oxy-coal combustion

One of the primary concerns in oxy-coal combustion is whether or not conventional coal pyrolysis and char burnout model can be used because of the presence of high CO₂ in gas medium around coal particles. This paper attempts to understand the effect of high concentration of CO₂ during coal pyrolysis and oxidation by conducting experiments in a drop-tube reactor and a thermogravimetric analyzer (TGA). Two ranks of bituminous coal particles were pyrolyzed in CO₂ and Ar atmosphere during the pyrolysis tests and in 21% O₂/79% CO₂ and air during combustion tests. The results showed that the differences observed during pyrolysis and combustion tests conducted with and without CO₂ can be attributed to the effects of char-CO₂ reaction. This result implies that char-CO₂ reaction should be included in model reactions while modeling coal pyrolysis or char oxidation in oxy-coal combustion. Furthermore, a faster increase in the rate of char burnout during combustion in O₂/CO₂ relative to that with air for an lvb coal was due to lower activation energy of C-O ₂ and higher activation value of C-CO₂ reaction compared to that of an hvCb coal. This result implies that some coals may be more suitable for oxy-coal combustion which can be identified based on the intrinsic activation energy value of char-CO₂ and char-O₂ reactions.