In recent years several processes incorporating a carbonation-calcination loop in an interconnected fluidized bed reactor have been proposed as a way to capture CO2 from flue gases. This paper is a first approximation to the modelling of a fluidized bed carbonator reactor. In this reactor the flue gas comes into contact with an active bed composed of particles with very different activities, depending on their residence time in the bed and in the carbonation-calcination loop. The model combines the residence time distribution functions with existing knowledge about sorbent deactivation rates and sorbent reactivity. The fluid dynamics of the solids (CSTR) and gases (PF) in the carbonator are based on simple assumptions. The carbonation rates are modelled defining a characteristic time for the transition between a fast reaction regime to a regime with a zero reaction rate. On the basis of these assumptions the model is able to predict the CO2 capture efficiency for the flue gas depending on the operating and design conditions. Operating windows with high capture efficiencies are discussed, as well as those conditions where only modest capture efficiencies are possible. (C) 2008 Elsevier Ltd. All rights reserved