Several systems for CO2 capture using CaO as regenerable sorbent are under development. In addition to a carbonation step, they all need a regeneration step (calcination of CaCO3) to produce a concentrated stream of CO2. Different options for calcination may be possible, but they all share common operating windows that appear when the mass and heat balances in the system are solved incorporating equilibrium data, sorbent performance information, and fuel composition (sulphur and ash content). These relatively narrow operating windows are calculated and discussed in this work. Due to sorbent performance limitations, low carbonation levels of the sorbent in the carbonator are expected and the heat demand in the calciner is dominated by the heating of inert solids flowing in the carbonation chemical loop. High make up flows of fresh limestone reduce this effect by increasing the average reactivity of the sorbent, but they also increase the heat demand in the calciner to calcine the fresh feed of limestone. Hence, an optimum level of sorbent activity appears under different operating conditions, processes and fuel characteristic, and these are discussed in this work. (C) 2007 Elsevier B.V. All rights reserved.