Absorbents allow a gas to permeate a solid or liquid under one set of conditions, and desorb under others. The rate of absorption or desorption is temperature and pressure dependent. Smaller differences in conditions require less energy, but require more absorbent to capture CO2 at an equivalent rate.
Absorption in most current physical solvent systems occurs at high partial pressure of CO2 and low temperatures. The solvents are then regenerated by either heating, pressure reduction, or a combination of both. The interaction between CO2 and the absorbent is weak relative to chemical solvents, decreasing the energy requirement for regeneration. Capacity can be higher than chemical solvents, since it is not limited by the stoichiometry of the chemical system. Physical solvent scrubbing of CO2 is well established. Selexol, a liquid glycol-based solvent, has been used for decades to process natural gas, both for bulk CO2 removal and H2S removal. Glycol is effective for capturing both CO2 and H2S at higher concentration. However, the CO2 is released at near atmospheric pressure, requiring recompression for transportation and geologic storage. The Rectisol process, based on low temperature methanol, is another physical solvent process that has been used for removing CO2. Glycerol carbonate is interesting because of its high selectivity for CO2, but it has a relatively low capacity.
A physical solvent selectively absorbs CO2 without a chemical reaction. The loading that can be achieved depends upon the solvent being used, the partial pressure of CO2 in the gas stream, and the temperature, with higher partial pressures and lower temperatures being more favorable. With physical solvents, capacity is generally proportional to CO2 partial pressure. R&D pathways to process improvements include modifying regeneration conditions to recover the CO2 at a higher pressure, improving selectivity to reduce H2 losses, and developing a solvent that has a high CO2 loading at a higher temperature. Commercial acid gas removal processes that use physical solvents, such as Selexol and Rectisol, have such properties, but are energy intensive due to their heat transfer requirements. Therefore, their commercial promise is likely to be in the near term until higher performance and less costly technologies are demonstrated.
Another common physical solvent that is commercially used is propylene carbonate (Fluor process). The weaker bonding between CO2 and this solvent allows the CO2 to be separated from the solvent in a stripper by reducing the total pressure. However, there is a need for higher efficiency gas–liquid contactors and lower energy requirements for regeneration.
Physical solvents, rather than chemical solvents, can be used in IGCC because of the relatively high partial pressure of CO2 in the syngas exiting the shift converter. A main benefit of a physical solvent is that it requires less energy for regeneration.
Since the main problem with physical solvents is that their capacity is best at low temperatures, it is necessary to cool the syngas before carbon capture. A physical solvent with acceptable capacity at a higher temperature would improve IGCC efficiency.