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Read more: 鈥�Instant Expert: Carbon capture and storage鈥�

Carbon dioxide forms around 10 per cent of a typical coal power station鈥檚 exhaust gas, so before burial it must be separated from other gases. Burying all the exhaust would be infeasible, not least due to the huge volumes involved. However, separating CO₂ is currently the most expensive part of carbon capture and storage (CCS), so extensive research is under way to increase efficiency.

Three ways to carbon capture

POST-COMBUSTION captures CO₂ from the exhaust gas. It uses chemicals called amines that selectively react with CO₂ when exhaust gas is bubbled through. Heating this solvent releases the concentrated CO₂, allowing the solvent to be reused. Amine-based capture is now used in natural gas processing and at pilot carbon capture plants. It can be retrofitted to existing power plants.

PRE-COMBUSTION strips the CO₂ out of the fuel before it is burned, via a series of interlinked chemical reactions. The fossil fuel is converted to syngas (carbon monoxide and hydrogen), which is then reacted with water vapour (a 鈥渟hift reaction鈥�) to form an easily separated mixture of hydrogen and CO₂. The hydrogen is then burned. Many proposed CCS projects plan to use this method.

OXYFUEL COMBUSTION burns fuel in almost pure oxygen instead of air. This means that the exhaust gas is made up mostly of CO₂ and water, which are easily separated. However, producing pure oxygen is an expensive and energy-intensive process. A handful of pilot test facilities currently operate.

Smaller, better, cheaper

Present day CO₂ capture methods (left) use up a lot of energy, so a lot of effort is going into developing more efficient alternatives.

At present, chemicals called post-combustion amines are used to capture CO₂ by reacting with it. They are likely to be replaced in the near future by advanced amines, which require less heat to release the CO₂ they capture. They will also be more resistant to degradation by other chemicals in the flue gas, so will last longer. Upgrading won鈥檛 require any expensive major restructuring either.

In the medium term, 鈥減hysical鈥� adsorbents of CO₂ may be introduced. These materials stick to CO₂ rather than reacting with it, so less energy is needed to release the CO₂ and reuse the adsorbent. Initially, these physical adsorbents are likely to be liquids, but switching to solid materials known as molecular sieves could greatly reduce the physical size and operating costs of equipment.

Another technology under development uses crystalline compounds called metal organic frameworks, which have a vast internal surface area relative to their weight. Pressure changes can force these materials to selectively soak up or release CO₂ as required. The equipment would be minimal in terms of size and energy consumption compared with current methods.

Alternatively, an existing type of filter called a selectively porous membrane could capture CO₂ from exhaust gases. These 鈥渇ilms鈥� are already established in other industries, for separating CO₂ from hydrogen. However, to work with exhaust gas the whole membrane needs to be increased in size, adapted to hotter operating temperatures, and attain higher purity of separation.

Algae are widely considered for use in CO₂ separation. Exhaust gas could be pumped through tanks of algae, which would photosynthesise the CO₂. This would produce biomass that could then be reused as liquid or solid fuel. However, very large surface areas would be necessary to absorb enough sunlight and a lot of nutrient input would also be needed.

Ultimately, the most efficient solution could rest in an entirely different style of fuel combustion 鈥� a process known as chemical looping. Instead of using air to supply the oxygen necessary to burn fuel, the oxygen carrier would be a solid metal oxide. This reacts with the fossil fuel to form easily separated hydrogen and CO₂; the hydrogen is used as a fuel, while the CO₂ is captured. This requires energy input, but regenerating the metal oxide by exposing it to air releases energy 鈥� so the two processes almost cancel each other out. Overall, the energy needed to capture CO₂ this way could be as low as 2 per cent of the total energy generated, a fraction of the 25 per cent penalty of current systems.

鈥淯ltimately, the most efficient solution could rest in an entirely different style of fuel combustion鈥�