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Adsorption is a physical process that involves the attachment of a gas or liquid to a solid surface. The adsorbent is regenerated by the application of heat (temperature swing adsorption, TSA) or the reduction of pressure (pressure swing adsorption, PSA). Adsorbents which could be applied to CO2 capture include activated carbon, alumina, metallic oxides and zeolites. Current adsorption systems may not be suitable for application in large-scale power plant flue gas treatment. At such scale, the low adsorption capacity of most available adsorbents may pose significant challenges. In addition, the flue gas streams to be treated must have high CO2 concentrations because of the generally low selectivity of most available adsorbents. For instance, zeolites have a stronger affinity for water vapour.
This involves the physical absorption of CO2 into a solvent based on Henry’s law. Regeneration can be achieved using heat, pressure reduction or both. Absorption takes place at high CO2 partial pressures. As such, the main energy requirements originate from the flue gas pressurization. Physical absorption is therefore not economical for flue gas streams with CO2 partial pressures lower than 15 vol%. Typical solvents are Selexol (dimethyl ethers of polyethylene glycol) and Rectisol (methanol).
Cryogenics separation separates CO2 from the flue gas stream by condensation. At atmospheric pressure, CO2 condenses at −56.6 ◦C. This physical process is suitable for treating flue gas streams with high CO2 concentrations considering the costs of refrigeration. This is typically used for CO2 capture for oxyfuel process.
When membranes are used in gas absorption, membranes act as contacting devices between the gas stream and the liquid solvent. The membrane may or may not provide additional selectivity. These offer some advantages over the conventional contacting devices such as packed columns as they are more compact and are not susceptible to flooding, entrainment, channelling or foaming. They, however, require that the pressures on the liquid and gas sides are equal to enable CO2 transport across the membrane. Their separation efficiency such as flue gas streams from oxyfuel and IGCC processes.
In membrane-based separation, selectivity is provided by the membranes themselves. These usually consist of thin polymeric films and separate mixtures based on the relative rates at which constituent species permeate. Permeation rates would differ based on the relative sizes of the molecules or diffusion coefficients in the membrane material. The driving force for the permeation is the difference in partial pressure of the components at either side of the membrane. However, the selectivity of this separation process is low and thus a fraction of the CO2 is captured. In addition, the purity of the captured CO2 is low for the same reason. Multistage separation is employed to capture a higher proportion of CO2 incurring extra capital and operating cost.
Chemical absorption involves the reaction of CO2 with a chemical solvent to form a weakly bonded intermediate compound which may be regenerated with the application of heat producing the original solvent and a CO2 stream. The selectivity of this form of separation is relatively high. In addition, a relatively pure CO2 stream could be produced. These factors make chemical absorption well suited for CO2 capture for industrial flue gases.
M. Wang, A. Lawal, P. Stephenson, J. Sidders, C. Ramshaw. Post-combustion CO2 capture with chemical absorption: A state-of-the-art review. Chemical engineering research and design 89 ( 2011) 1609–1624