Oxy-fuel combustion: The effect of coal rank and the role of char-CO2 reaction

The role of coal rank during oxy-fuel (O2/CO2) combustion is investigated using a suite of four coals. Combustion tests carried out in a drop-tube reactor under air and oxy-fuel atmospheres showed that for the lower rank coals, Beulah (lignite) and Dietz (subbituminous coal), oxy-fuel combustion produced a higher carbon conversion compared to air combustion at both short and long residence times. For Pittsburgh (high volatile) coal, oxy-fuel combustion produced a higher carbon conversion compared to air combustion only at longer residence times. On the other hand, for the Pocahontas coal (highest rank coal used in this study), oxy-fuel combustion produced a lower conversion than air combustion at all residence times. The higher conversion in low-rank coals under oxy-fuel conditions was due to the exponential increase in the char-CO2 reaction rate occurring at high temperatures (> 1500 K). This was shown by theoretical rate extrapolation to high temperatures, using intrinsic rate parameters generated using thermogravimetric analyzer (TGA). The char particle temperatures were modeled under oxy-fuel atmospheres and the results showed a significant role of the char-CO2 reaction at high temperatures and high CO2 partial pressures. The role of ion-exchangeable cations during oxy-fuel combustion in catalyzing the reactions particularly in low rank coals was investigated. Ammonium acetate washed samples indicated a significant reactivity loss in the TGA but the conversion was unaffected in DTR at 1873 K, suggesting that catalytic activity is not significant under practical combustion conditions.