Characteristics of the sulphation reaction between SO2 and CaO/fly ash sorbent were analyzed based on TGA results to develop a kinetic model for a dry moderate temperature (400-800 degrees C) FGD process. It was found that SO2 diffusion within sorbent particles involved three sub-processes: inter-particle diffusion, inter-grain diffusion and diffusion through product layers and the diffusion dominated the whole sulphation reaction process. The activation energy for product layer diffusion E-diff of 49.3 kJ mol(-1) being greater than the chemical reaction activation energy E-a of 13.9 kJ mol(-1) verified the importance of the diffusion. Predictions using the kinetic model in which k(0) varies with temperature agree well with the experimental data. (c) 2006 Elsevier Ltd. All rights reserved.