Capture

Post-combustion - separating the CO₂ from flue (exhaust) gases produced after burning fossil fuels in the air. The small volume of CO₂ in the flue gas (ranging from 3-15% by volume) is captured by dissolving the CO₂ into a liquid solvent such as amines, a class of organic chemical compounds. This technique is already applied to provide the CO₂ used in the food and beverage industry (for lemonade, beer and food preservation) and as a feedstock for fertilizer manufacturing, and can be retrofitted to existing power plants and industrial processes such as cement production, as well as integrated into new built facilities.

Pre-combustion - separating fossil fuels into hydrogen and carbon dioxide before they are burnt. For instance for coal, the process involves converting the coal into a synthetic gas comprising carbon monoxide and hydrogen.

This gasification process was invented in 1794 and was used to make the “town-gas” used to light cities before natural gas was discovered and electricity became preferred.

This “syngas” can be reacted again with steam to produce a mix of CO₂ and hydrogen - passing the mixed gas through a catalytic reactor under high pressure - in a process called “water-gas shifting”’. This process produces high concentrations of CO₂ (35-45%) that can then be captured.

The resulting hydrogen can then become the energy source used to generate CO₂-free electricity because when burnt, it produces only heat and water vapor. Pre-combustion capture technology is well established in the fertilizer industry and natural gas reforming, which uses similar technology, has been widely applied in the refining and chemicals industries. This approach, while more complex and consequently more expensive than those involved in post-combustion, are proved and effective at scale and and could be cost-effectively used to make large amounts of clean hydrogen for the power industry as well as in refineries and other chemical plants.

Oxy-fuel combustion - burning the fossil fuel in oxygen instead of air results in an exhaust gas consisting only of concentrated CO₂ and water vapor. The CO₂ - typically greater than 80% by volume - is then far more easily captured when the water vapor is removed by cooling and compressing the gas stream. Oxy-fuel combustion systems have already been commercially applied in the glass furnace industry, while application to CO2 capture is now approaching the next stage. They show interesting possibilities for boilers and gas turbines systems and would largely be applied in the power industry.

All of the three approaches to CO₂ capture in industrial processes rely on gas separation technologies. There are three basic methods of separating gases from each - solvent/sorbents, membranes and cryogenic removal - which can be applied as technologically appropriate to the above three capture approaches. Find out more about the three gas separation methods.