CO2 capture system based on the carbonation/calcination loop, still in its infancy, has gained rapid interest due to promising carbonator CO2 capture efficiency, low sorbent cost and the fact that no flue gases desulphurization unit is needed before entering the system. The sum of these features results in a competitively low Cost CO2 capture system. There are different options to design the carbonation loop. in this work, a basic configuration that makes use of two interconnected circulating fluidized beds (carbonator and calciner) has been studied. Among the key variables that influence the performance of these systems, the carbonation conversion of the sorbent and the heat requirement at calciner are the most relevant. Both variables are mainly influenced by CaO/CO2 ratio and make-up flow (purge) of solids. A purge is necessary in order to reduce the sorbent deactivation and to compensate the formation Of CaSO4 from the SOx content in the flue gas. Large CaO/CO2 ratios improve the carbonation conversion but also increase the cost of the system due to a more intensive solid circulation. High make-up flow also improves the carbonation conversion and hence the CO2 capture, but increases the heat demand at calciner and the fresh sorbent cost. The aim of this paper is to calculate the optimum make-up flow and CaO/CO2 ratio in order to minimize the capture cost of the system. Independent variables are make-up flow of fresh CaCO3 and CaO/CO2 ratio. The constraint equations are experimental data on carbonation reaction, mass and energy balances, oxygen requirement and fuel composition. (C) 2008 Elsevier B.V. All rights reserved.