> CO₂ Capture - separation technologies

Absorption

Solvent absorption involves a cyclical process in which carbon dioxide is absorbed from a gas stream directed into a liquid, typically an amine. The gas stream, with most of the carbon dioxide removed, is then emitted to the atmosphere. The liquid is processed to remove the carbon dioxide, which is then compressed for storage. The resulting carbon-dioxide-free liquid is used again for absorption and the process continues. This technique is fairly widely used in a range of applications, but it needs a large amount of power to regenerate the solvent.

The absorption process is most commonly applied to post-combustion capture and in the main, the absorbents are liquid solvents.

In post-combustion systems, the flue gas needs to be cooled and impurities removed so that the solvent can efficiently absorb the CO₂. The flue gases containing CO₂ and N₂ are then fed into a tower called the absorber tower. The flue gas comes into the bottom of the tower while the solvent is fed into the top of the tower. There is packing material in the tower, and the flue gas flows up through the packing, making contact with the solvent as it falls down. The CO₂ is absorbed by the solvent as the flue gas rises so that the gas that comes out of the top of the tower contains very little CO₂. Modifying the packing material to improve the absorption of CO₂ is another area of research.

The solvent, with dissolved CO₂, is then removed from the chamber. The N₂ is released as it is not absorbed in the solvent. The recovery of CO₂ from the solvent is called desorption. The usual process for recovery of CO₂ from the solvent is temperature change. Other methods include pressure changes and the use of membranes with solvents.

Solvent – based absorption capture

>> View Flash animation of the absorption and desorption cycle

Temperature changes

The loaded solvent passes into another tower where is it heated. As it falls down through the tower, the CO₂ is desorpted (released) from the rich solvent at temperatures of 100^o+ C. Water-saturated CO₂ flows from the top of the tower into a tank which removes excess water. The solvent is recovered to absorb more CO₂. As all the solvent must be heated, not just the CO₂, solvent capture of CO₂ results in significant reductions in energy efficiency. Heat is necessary to regenerate the solvent and recover the CO₂. Energy is also required to operate pumps and fans and to compress the CO₂. This can amount to a significant energy penalty. Research aims to design systems which will re-use heat efficiently.

Types of solvents

A common solvent is monoethanolamine (MEA). Other solvents used are carbonates or bicarbonates. The volume of solvent is considerable. A 500MW generator would need approximately 6ML of solvent, about half in the absorption tower and the other half in the desorption tower.

Composition of flue gas

Black coal flue gas contains 71% N₂, 12.6% CO₂ and 11.1% water.
Brown coal flue gas contains 60% N₂, 12% CO₂ and 24% water.

Solvent recovery

Impurities such as nitrogen oxides (NO_x) and sulphur oxides (SO_x) can form heat stable salts and reduce the capacity of the solvent to absorb CO₂. In addition, fly ash and soot also need to be removed. These impurities can be removed prior to absorption and often in coal-fired power plants there is already equipment in place as part of environmental controls which will remove most of the impurities. More of the oxides can be removed in a scrubber which can also double as the cooling equipment. As recovery of the solvent for re-use is an important element in the cost of capture, researchers working with new solvents target the effects of SO_x and NO_x on the new solvents and processes that may remove SO_x and NO_x in the one step.

CO₂ solvent removal plant

Source: Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) in Australian

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