Postcombustion CO2 capture systems that use CaO as a regenerable CO2 sorbent in two interconnected circulating fluidized beds (CFBs) are rapidly developing at increasing scale. The properties of the circulating materials, the reactor geometries and operating conditions (gas velocities and temperature ranges) are remarkably similar to those present in existing large scale circulating fluidized bed combustors, CFBCs. In this work a fluid-dynamic model published for CFBCs is adopted and applied to the study of the interconnected twin reactor calcium looping (CaL) system. The model was used to find operating windows that will allow the CaL system to be applied on a large scale. It was found that when operating at high gas velocities (4-5 m/s) in the carbonator, unacceptably high solids circulation rates might be present when high bed inventories are in place to achieve large CO2 capture efficiencies. An additional loop to allow the internal circulation of the excess solids can be quantified with the help of the model used in this work. (c) 2012 Elsevier B.V. All rights reserved.