This paper examines the average carbonation conversion, CO2 capture efficiency and energy requirement for post-combustion CO2 capture system during the modified calcium-based sorbent looping cycle. The limestone modified with acetic acid solution, i.e. calcium acetate is taken as an example of the modified calcium-based sorbents. The modified limestone exhibits much higher average carbonation conversion than the natural sorbent under the same condition. The CO2 capture efficiency increases with the sorbent flow ratios. Compared with the natural limestone, much less makeup mass flow of the recycled and the fresh sorbent is needed for the system when using the modified limestone at the same CO2 capture efficiency. Achieving 0.95 of CO2 capture efficiency without sulfation, 272 kJ/mol CO2 is required in the calciner for the natural limestone, whereas only 223 kJ/mol CO2 for the modified sorbent. The modified limestone possesses greater advantages in CO2 capture efficiency and energy consumption than the natural sorbent. When the sulfation and carbonation of the sorbents take place simultaneously, more energy is required. It is significantly necessary to remove SO2 from the flue gas before it enters the carbonator in order to reduce energy consumption in the calciner. (C) 2011 Elsevier Ltd. All rights reserved.