CO₂ capture

Oxyfuel combustion plant at Schwarze Pumpe, Germany. Lignite and hard coal are combusted in a mixture of oxygen and re-circulated CO<sub>2</sub>, which also contains water vapour. The flue gas is treated and sulphur oxide particles and other contaminants are removed. Finally, the water is condensed and the concentrated CO<sub>2</sub> compressed into liquid. (Photo: BGS/M Stephenson)

Oxyfuel combustion plant at Schwarze Pumpe, Germany. Lignite and hard coal are combusted in a mixture of oxygen and re-circulated CO₂, which also contains water vapour. The flue gas is treated and sulphur oxide particles and other contaminants are removed. Finally, the water is condensed and the concentrated CO₂ compressed into liquid.

CO₂ can be captured from large point sources, such as large fossil fuel or biomass electricity power plants, industries with major CO₂ emissions, natural gas processing, synthetic fuel plants and fossil fuel-based hydrogen production plants. Capture from the open atmosphere is also possible.

Where fossil fuels are burnt for electricity, there are three techniques to remove or ‘scrub' CO₂ : post-combustion, pre-combustion, and oxyfuel combustion.

In post-combustion, the CO₂ is removed after burning the fossil fuel. This is the method that would be applied to most conventional power plants. Here, carbon dioxide is captured (‘scrubbed') from the exhaust (or ‘flue') gases.
The technology is well understood and is currently used in other industrial applications, although not at the same scale as might be required in a commercial scale CCS power station.
The technique of pre-combustion is used in the production of fertiliser, chemical gaseous fuel (hydrogen — H₂, methane — CH₄), and power production.
First the fossil fuel is partially oxidised in air in a ‘gasifier' to form synthetic gas or ‘syngas' and CO₂ can be captured from this relatively pure exhaust stream. The H₂ can also be separated and used as fuel, and the carbon dioxide is removed before combustion takes place.
In oxy-fuel combustion the fossil fuel is burned in oxygen instead of air. The resulting flue gas consists of mainly carbon dioxide and water vapour. The water is condensed through cooling and the result is an almost pure carbon dioxide stream that can be transported and stored.
Electricity plant processes based on oxyfuel combustion are sometimes referred to as ‘zero emission', because the CO₂ stored is not a fraction or a proportion removed from the flue gas (as in the cases of pre- and post-combustion capture) but the entire flue gas stream itself.

It is possible that some CO₂ from burning will dissolve in the condensed water, and so the water may have to be further treated. The initial oxygen burning process is also energy intensive.

Air capture

CO<sub>2</sub> can be captured directly from the open atmosphere, but this is still being researched. The image shows how carbon capture towers might look in the future. (Image: C.Wardle) CLICK TO ENLARGE.

CO₂ can be captured directly from the open atmosphere, but this is still being researched. The image shows how carbon capture towers might look in the future. CLICK TO ENLARGE.

It is possible to capture CO₂ directly from the open atmosphere, but this is still being researched.

Capture costs are estimated to be higher than from point sources, but may be feasible for dealing with emissions from diffuse sources like cars and aircraft.

The theoretically required energy for air capture is only slightly more than for capture from point sources.

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