Explainer: How carbon is captured and stored

The three main techniques for preventing carbon dioxide from coal-fired power stations contributing to global warming
Alok Jha, green technology correspondent
guardian.co.uk,
Friday September 05 2008 17:16 BST

Carbon capture and storage (CCS) is a range of technologies that hold the promise of trapping up to 90% of the carbon dioxide emissions from power stations and industrial sites. It involves collecting, transporting and then burying the CO2 so that it does not escape into the atmosphere and contribute to climate change.
There are three main techniques: the post-combustion process involves scrubbing the power plant's exhaust gas using chemicals. Pre-combustion CCS takes place before the fuel is placed in the furnace by first converting coal into a clean-burning gas and stripping out the CO2 released by the process. The third method, oxyfuel, burns the coal in an atmosphere with a higher concentration of pure oxygen, resulting in an exhaust gas that is almost pure CO2. (See below for further details).
Once the CO2 has been trapped, it is liquefied, transported – sometimes for several hundred miles – and buried, either in suitable geological formations, deep underground saline aquifers or disused oil fields. The last method is often used in a process called "enhanced oil recovery", where CO2 is pumped into an oil field to force out the remaining pockets of oil that would otherwise prove difficult to extract.
The technology is not cheap – up to 40% of a power station's energy could end up being used to run the CCS scrubbing and transport systems and experts estimate the average cost of retro-fitting Britain's aged power stations at about £1bn each. Pipes to transport the CO2 to suitable burial sites could cost £1m per mile. The oldest power stations may end up being uneconomical to refit.
All the components of CCS have been in use by oil companies and chemical engineering plants for decades. But, so far, only a small pilot project at Vatenfall's Schwarze Pumpe power station in northern Germany has connected all the different stages of the CCS chain together. The pilot is an oxyfuel boiler that can generate 30MW of heat and around 12MW of electricity.
Trapping methods
Post-combustionIn the post-combustion method, CO2 is separated from the flue gas of the power station by bubbling the gas through an absorber column packed with liquid solvents (such as ammonia) that preferentially take out the CO2. In the most commonly-used techniques, once the chemicals in the absorber column become saturated, a stream of superheated steam at around 120C is passed through it. This releases the trapped CO2, which can then be transported for storage elsewhere.
More experimental techniques to scrub CO2 from flue gas without the two-step process include using seawater to absorb the gas and then returning the mixture back to the ocean for long-term storage. But, so far, these methods have proved less efficient and reliable.
OxyfuelWhen coal, oil or natural gas is burned in normal air, the amount of CO2 produced is between 3-15% of the waste gases, depending on the conditions. Separating the greenhouse gas out after combustion requires energy so an alternative CCS method is to burn the fossil fuel in an atmosphere of pure oxygen. In this environment, virtually all the waste gas will be composed CO2 and water vapour. The latter can be condensed out while the former can be piped or transported directly to a storage facility.
In the oxyfuel system, the air fed into the boiler has to be separated into liquid oxygen, gaseous nitrogen, argon and other trace gases and this process can use up to 15% of the power produced at the station.
Pre-combustionThis method is normally applied to coal-gasification combined cycle power plants. The coal is gasified to produce a synthetic gas made from carbon monoxide and hydrogen. The former is reacted with water to produce CO2, which is captured, and more hydrogen. The hydrogen can be diverted to a turbine where it can be burned to produce electricity. Alternatively, some of this gas can be bled off to feed hydrogen fuel cells for cars.
One disadvantage of the pre-combustion method is that it cannot be retro-fitted to the older pulversised coal power plants that make up much of the world's installed base of fossil fuel power. It could perhaps be used in natural gas stations, where a synthetic gas is first produced by reacting the methane with steam to produce carbon dioxide and hydrogen. But the economic advantage of this method over post-combustion is yet to be proven.