Solid-gas reactions of calcium compounds with hydrogen chloride (HCl) are used in dry scrubbing of HCl from hot gases. The experimental data in the literature were analysed using the shrinking unreacted core model. It has been shown that the rate limiting step for the reactions alters with the extent of solid conversion in this time sequence : chemical reaction control, combined control, product layer diffusion control, and chemical reaction control again. The extent of solid conversion with time could not be predicted by the classical correlations obtained by assuming a single rate limiting step. A modified correlation was presented to obtain product layer diffusion coefficients. A new model, the crystallisation and fracture model, was proposed to describe the mechanism of the formation of the solid product layer. Based on free energy-mechanical work analysis, the model can fundamentally explain the reasons for the change of the rate limiting step, the existence of the maximum conversions of sorbent, the variation of the structure of the solid product layer with temperature, and the effect of gaseous products. The model may also be applicable to other solid-gas reaction systems.