A new unidimensional computational model is developed to simulate a calcination reactor in a Calcium-looping process for thermochemical energy storage in concentrating solar power systems. The proposed reactor is an absorber tube exposed to concentrated solar radiation. This tube is also the riser of a circulating fluidised bed where the calcination reaction takes place. The proposed heat transfer process models are based on the core-annulus model and the hydrodynamic model is a modified version of the Kunii-Levenspiel model. The model considers the change in the mass flow rate of species and the density change of the phases in the axial direction of the reactor, usually considered constant in the models found in the literature. A higher calcination efficiency, up to 8 p.p., is obtained for the studied reference case when assuming constant density and mass flow rate. Simulations were performed by imposing a solar-driven non-uniform heat flux distribution on the reactor wall. The results show that a 6 m height reactor allows achieving a calcination efficiency of 66% for the reference conditions used. A sensitivity analysis shows that the solids mass flow rate and the inlet bed temperature are the parameters that most affect the calcination process efficiency.