Depending on the type of plant, CO2 capture may use one of three different technologies: postcombustion, precombustion or oxycombustion.

Postcombustion

This is the technique that has been mastered to the greatest extent so far. It involves extracting diluted CO₂ from the smoke and fumes by means of a chemical solvent that reacts selectively on contact with CO₂. The CO₂ is then recovered from the solvent by means of heat regeneration. One advantage of this technique is that it can be adapted to existing industrial plant. On the other hand, it is expensive and also relatively energy-intensive.

Precombustion

The idea here is to remove carbon from the fuel before combustion. This is done by transforming the fuel into a synthetic gas comprising mainly carbon monoxide and hydrogen. Then water vapor is added, and this reacts with the carbon monoxide converting it into CO₂. The CO₂ and the hydrogen are then separated using an amine-type solvent. The hydrogen is used to produce the required energy, without any CO₂ emissions. This technique is already being used on an industrial scale. It is less energy intensive that postcombustion but requires specific equipment that is still being developed. This means that the technique must be selected right from the start and its reliability improved.

Oxycombustion

In a conventional combustion scheme air is used, but this generates a large volume of smoke and fumes, where the CO₂ is very diluted. This makes extracting, or capturing, the CO₂ more expensive. On the other hand, oxycombustion – which is still at the proving stage – generates smoke and fumes that are mainly made up of CO₂ and water (which is easy to separate by condensation). What is oxycombustion? Simply combustion in pure oxygen rather than air. This technique is better than the other two in terms of both cost and energy efficiency. It also has the advantage of being able to be used in existing facilities.

The upstream extraction of the oxygen from air is the most energy-intensive stage of the process. But this stage has considerable potential for improvement thanks to a new “chemical loop” process. This technology involves the use of a metal oxide to provide the oxygen needed for combustion. In concrete terms, the “chemical loop” is made up of two reactors connected together. In the first reactor is a metal oxide in contact with air. This is injected into the second reactor – the combustion chamber – in the presence of the combustible. The combustible then consumes the oxygen “carried” by the metal and transforms it into a mixture of CO₂ and water, which can be easily separated. The metal is then regenerated and re-injected into the first reactor so that the next cycle can begin. This technology uses less energy overall and reduces the cost of the capture phase. But it is still at the experimental stage and has only been demonstrated on a reduced scale under laboratory conditions.