A model based on the shrinking core model is proposed and evaluated to predict the behavior of limestones during the sulfation reaction under oxy-fuel firing mode in calcining conditions at typical time scales and particle sizes of circulating fluidized bed combustors (CFBCs). This model is chosen due to its simplicity in order to incorporate it in an easy way into available global CFBC models. It is observed that the model predicts properly the evolution of the sorbent sulfation conversion assuming that the first step of the sulfation reaction is controlled by gas diffusion through the porous system of the particle and the second step is controlled by gas diffusion through the CaSO4 product layer. The kinetic parameters needed in the model are determined for two limestones. For that, sulfation reaction tests were carried out in a thermogravimetric analyzer and in a batch fluidized bed reactor.