Our planet is warming up and it is most likely resulted from human activity in accordance with variety of data. We heat our planet by two ways: the emission of greenhouse gases (GHG) through the burning of fossil fuels and cutting off existing GHG sink by deforestation. The GHGs entrap extra amount of heat reflected by the earth surface and accumulate energy in the atmosphere. This heat-entrapping process will accelerate by itself because extra heat will in turn release more GHGs from the sea and land. Among the GHGs, CO2 is the most influential one considering its amount and lifetime in atmosphere.
Though large amount of money has been invested in renewable energy that is believed to have very little or zero CO2 emission, none of them is yet commercially competitive or industrially mature compared to conventional fossil fuel energy. As we cannot expect those coal-fired power plants to shut down in one night, it is more rational and feasible to modify the existing coal-fired power plants as a short-term solution. After all, coal-fired power plants generate more than 40% of US electricity and are responsible for about 35% of total CO2 emission in the US.
One strategy to modify the existing power plants is carbon capture and sequestration (CCS). For the “capture” part, CO2 is separated from the flue gas by physical/chemical absorption, membrane, adsorption, etc. For the “sequestration” part, CO2 is compressed, transported and injected into geological sites that are suitable for its long-term storage. There are already commercial-scale CCS power plants running for testing purpose .
There are three categories for the capture process: pre-combustion, post-combustion and oxy-fuel combustion . Their principal ideas and procedures are shown in figure 1. Among them post-combustion is the technique that is best understood and most suitable for our present power plants. A post-combustion capture unit can remove approximately 90% of the total CO2 emission and can be retrofitted directly at the end of a coal-fired power plant without involving any air separation unit or special unit for fuel-gasification and reforming like the other two techniques do. The air separation unit in oxy-fuel combustion and pre-combustion separate O2 from air through cryogenic process which requires lots of energy. Moreover, the existing gas turbines cannot be used for oxy-fuel combustion and new turbines need to be developed . Contrastively, the absorption and stripping process (see fig. 2) for post combustion is studied as early as the 1930s when flue gas desulfurization is first added to power plants. Therefore, post-combustion is believed to be an efficient way to capture CO2 because of its technique maturity, operational flexibility and significant development potential through process improvements and absorbents developments .
(ASU means air separation unit)
Fig. 1 Ideas and procedures for three types of capture approach 
Fig. 2 Schematic of absorption and stripping process for CO2 removal
According to thermodynamic data, the minimum energy requirement to separate CO2 from flue gas and compress CO2 to 150 bar is 0.11 megawatt-hours per metric ton of CO2. Process and solvent improvements should reduce the energy consumption to 0.2 megawatt-hour. Other capture techniques cannot compete with post-combustion in terms of energy-efficiency and timely solutions . Post combustion capture technique is currently used in other industrial applications, although not at the same scale as might be required in a commercial scale power station .
Some may ask why we don’t just capture CO2 from the ambient air instead of sticking to those coal-fired power plant sites. Indeed, if we can capture CO2 from the air, the large part of GHG from transportation will no longer be a headache. However, the CO2 concentration in the ambient air (vol. 0.04%) is way lower than that in flue gas of power plants (vol. 12%), and the minimum energy requirement for CO2 capture per mole increases rapidly with decreasing CO2 concentration (see fig. 3). Moreover, with lower CO2 concentration, more volume of gas has to be processed by the blower which serves as another addition to capture cost. Therefore, it is economically unrealistic to capture CO2 directly from ambient air.
Fig. 3 The minimum energy to separate the CO2 per mole from an ideal gas mixture at room temperature 
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