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

The role of coal rank during oxy-fuel (O-2/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 (>1500K). 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. (C) 2012 Elsevier B.V. All rights reserved.