Carbon Storage

A number of options for the storage of CO2 are being researched at the present time, including geological storage and mineral carbonation.

Geological Storage – Injection of CO2 into the earth's subsurface offers potential for the permanent storage of very large quantities of CO2 and is the most comprehensively studied storage option.

The CO2 is compressed to a dense state, before being piped deep underground into natural geological 'reservoirs'. Provided the reservoir site is carefully chosen, the CO2 will remain stored (trapped in the bedrock or dissolved in solution) for very long periods of time and can be monitored.
The options for geological storage include:
• Deep saline aquifers
• Depleted oil and gas reservoirs
• Operational oil and gas reservoirs
• Unmineable coal seams


Deep Saline Aquifers – Storing large amounts of CO2 in deep water-saturated porous rock offers great potential. A number of projects are already being conducted including one major project by the Norwegian company Statoil. This project is at the Sleipner field in the Norwegian section of the North Sea, where about 1 million tonnes a year of CO2 are being injected into the Utisira Formation at a depth of about 800-1000 metres below the sea floor.

A comprehensive analysis of the storage potential of saline reservoirs in Australia was undertaken as part of the GEODISC (Geological Disposal of Carbon Dioxide) project. It indicated storage potential of Australian saline aquifers adequate to store that country's total CO2 emissions for many hundreds of years at current rates of emission.
Depleted oil and gas reservoirs – CO2 can be stored in oil and gas reservoirs that are no longer used. The benefit of this type of storage is that the same infrastructure used to extract oil and gas and be re-used for the storage of CO2. In the USA, it is estimated by the US Department of Energy (DOE) that the storage capacity of depleted gas reservoirs is about 80-100 Gigatonnes, or enough to store US emissions of CO2 from major stationary sources (e.g. power stations) for 50 years or more.

While the permanent storage of CO2 would be the primary goal of geological storage, the practice can have ancillary economic benefits, by enabling improved oil and coalbed methane extraction, which may aid its adoption by industry.

Enhanced Oil Recovery – CO2 is already widely used in the oil industry to increase oil production – the CO2 helps in the extraction of oil out of the underground strata, increasing the level of recovery from the field. Without such methods of enhanced production, many oil fields can only produce half or less of the original resource. The CO2 therefore has a positive commercial value in such situations.

Enhanced Coalbed Methane – is a potential opportunity for storing CO2 in unmineable coal seams and obtaining improved production of coalbed methane as a valuable by-product.

Mineral Carbonation is another option for permanent CO2 storage. It a process whereby CO2 is reacted with substances to create a product chemically equivalent to naturally occurring carbonate minerals. Mineral carbonation is still at the laboratory stage of development and research is focusing on how to accelerate reaction rates.

Carbon Capture

There are a number of options for the capture of CO2:
• post-combustion
• pre-combustion
• oxyfuel
• chemical looping
‘Post-combustion’ capture can be attached to a conventional combustion power plant and removes the CO2 from the flue gas stream. An amine solvent is used to remove the CO2 from the other flue gases – primarily air. The CO2 is then stripped of the solvent for storage and the re-generated solvent is re-used. A major benefit of post-combustion capture is that it may be possible to retrofit to existing power stations.

‘Pre-combustion' capture can be achieved via IGCC technology by adapting the process so that hydrogen is produced along with CO2, rather than carbon monoxide. The hydrogen is then combusted in a gas turbine and the CO2 is captured for storage or use. In the future, the hydrogen could be used in a fuel cell.

Oxyfuel combustion relies on the relatively simple principle of burning coal in an oxygen-rich atmosphere to produce a pure stream of CO2. Much the same technology is used in steel production and hence there may be no insurmountable technical barriers to CO2 capture linked to oxyfuel power generation in the future.

Chemical looping combusts coal indirectly in an air-fired boiler that uses a continuously looping solid oxygen-carrier, which oxidises the fuel into primarily water and carbon dioxide. Simple condensation of the water then yields a fairly pure stream of CO2 for compression and liquefaction.