The variable diffusivity model was extended to include physically expanding product layer and two reacting zones, consisting of a high-reactivity outer shell and a low-reactivity inner core, to describe the gas/solid carbonation reaction involving carbon dioxide and a potassium-promoted half-calcined dolomite sorbent. The model provides an excellent fit to the TGA experimental data on sorbent carbonation at various operating conditions. The parameters in the model which include an initial product layer diffusivity and diffusivity decay rate can correctly capture the trends on the effects of operating conditions on the physical properties of the sorbent observed in the experiments. (C) 2012 Elsevier Ltd. All rights reserved.