The calcination reaction of limestone is always companied by sintering of the calcined product. In addition, accelerated sintering rates and a reduced specific surface area are observed in the presence of steam and carbon dioxide. To simulate the change of surface area and the porosity of limestone samples in a simultaneous calcination and sintering process, a combined model based on both a sintering model and a calcination model is established. The calcination model, which predicts calcination conversion as a function of time, is based on the initial properties of the sorbent. The sintering model is according to the German and Munir model in which the main transport mechanism is supposed to be lattice diffusion. In a flow reactor, the surface area value and calcination rate of limestone in the presence of steam and CO₂ are also described by the combined model with modified parameters.