CO2 separation is an energy intensive process, which plays an important role in both energy saving and CO2 capture and storage (CCS) implementation to deal with global warming. To quantitatively investigate the energy consumption of CO2 separation from different CO2 streams and analyze the effect of temperature, pressure and composition on energy consumption, in this work, the theoretical energy consumption of CO2 separation from flue gas, lime kiln gas, biogas and bio-syngas was calculated. The results show that the energy consumption of CO2 separation from flue gas is the highest and that from biogas is the lowest, and the concentration of CO2 is the most important factor affecting the energy consumption when the CO2 concentration is lower than 0.15 in mole fraction. Furthermore, if the CO2 captured from flue gases in CCS was replaced with that from biogases, i.e. bio-CO2, the energy saving would be equivalent to 7.31 million ton standard coal for China and 28.13 million ton standard coal globally, which corresponds to 0.30 billion US$ that can be saved for China and 1.36 billion US$ saved globally. This observation reveals the importance of trading fossil fuel-based CO2 with bio-CO2. (C) 2014 Published by Elsevier Ltd.