Coal combustion with O-2/CO2 is promising because of its easy CO2 recovery, extremely low NOx emission and high sulfation efficiency. The mechanism of direct sulfation of limestone under high CO2 concentration, different from that of CaO-SO2 sulfation, is one of the factors to account for its high sulfation efficiency. However, many unknowns about the mechanism of direct sulfation exist. The mechanisms and kinetics of direct sulfation were examined. Both experiment and modeling suggested that sintering is much mitigated during direct sulfation of limestone. The rate of direct sulfation does not decrease so much with sulfation degree as CaO-SO2 sulfation. Direct sulfation is favorable for high sulfation degree. Furthermore, the diffusivity in the product layer demonstrates high temperature dependence and hardly changes with sulfation degree. This result is associated with the fact that the calcium sulfate layer produced is porous owing to CO2 formation. (C) 2000 Elsevier Science Ltd. All rights reserved.