CO2 separation is an energy intensive process, and it plays an important role in both energy saving and CO2 capture and storage (CCS) to deal with global-warming. CO2 can be from different sources in a wide temperature, pressure and concentrations range. Meanwhile, new liquid absorbents are under-development to cost-effectively separate CO2 from gas mixtures. All this makes it crucial to analyze the energy consumption for CO2 separation from different streams and with different absorbents. In this work, the theoretical energy consumption of CO2 separation from flue gas (CO2/N2), lime kiln gas (CO2/N2), biogas (CO2/CH4) and bio-syngas (CO2/H2/CO) 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. If the CO2 captured from flue gases was substituted by that from biogases, the energy saving would be equivalent to 28.13 million ton standard coal globally. The energy consumption of CO2 separation from biogas using traditional absorbent of 30%MEA and new developed ionic liquids (ILs) was further studied, in which 1-ethyl-3-methy- limidazolium bis[(trifluoromethyl)sulfonyl]imide ([Emim][NTf2]), 1-butyl-3-methylimida- zolium tetrafluoroborate ([Bmim][BF4]), 1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] imide ([Hmim][Tf2N]) and 1-butyl-1-methylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide ([Bmpy][Tf2N]) were screened from 75 ILs. The energy consumptions of CO2 separation using ILs are lower than those of 30%MEA and that of [Bmim][BF4] is the lowest in the four screened ILs. With a very low vapor pressure and high CO2 solubility, it's promising to use ILs as absorbents for CO2 separation.