Carbon capture and storage (CCS), or Carbon capture and sequestration, is a measure for mitigating the emission contribution of the combustion of fossil fuels to global warming, based on capturing carbon dioxide (CO2) from large point sources such as fossil fuel power plants, transporting and storing it in such a way that it does not enter the atmosphere, usually being injected into underground geological formations for long-term storage. It may be regarded as a bridging technology to sustain the existing quality of life whilst deferring/avoiding the climate change caused by the fossil fuels used, before the availability of future energy based on clean renewable sources.
As of 2007, there are four industrial-scale storage projects are in operation: Sleipner and Snøhvit in Norway, In Salah in Algeria, and Weyburn in Canada. The first three projects strip CO2 from natural gas and store it in deep aquifers. Weyburn uses CO2 captured in Great Plains Synfuels Plant for enhanced oil recovery with an injection rate of about 1.5 million tonnes per year.
There are three steps in CCS: capture, transport and storage. Three different types of technologies exist for carbon capture: post-combustion, pre-combustion, and oxyfuel combustion. Post combustion capture is commonly applied to fossil fuel burning power plants. The CO2 is removed from the flue gas after the combustion of the fossil fuel. Current methods include physical separation for CO2 concentrations over 10% and chemical separation for lower concentrations. Pre-combustion is widely used in fertilizer, chemical, gaseous fuel, and power production. In this case, the fossil fuel is partially oxidized and then shifted into CO2 and more H2. The H2 can be used as fuel and the resulting CO2 can be captured from a relatively pure exhaust stream. In oxy-fuel combustion, the fuel is burned in oxygen instead of air and the results flue gas consists of mainly carbon dioxide and water vapor. Power plant processes based on oxyfuel combustion are sometimes referred to as “zero emission” cycles because the flue gas stream itself is stored.
Pipeline is commonly used to transport of CO2 captured to suitable places. Piped carbon dioxide is kept in the supercritical state, at over 73 atmospheres and 31.1 ºC, in order to maintain high density and therefore high mass flow rates. Typically, pipelines are considered for use for distances up to approximately 500 km. For longer distances tanker ships are favored.
The storage of CO2 can be either geological storage, ocean storage or mineral storage. CO2 can be geologically stored in deep aquifers or depleted oil or gas fields. Sometimes, CO2 may be used in enhanced oil recovery (a relatively mature technology), enhanced gas recovery and enhanced coalbed methane recovery. In ocean storage, CO2 will be piped into a water column at a depth of 1000 m or more and dissolves subsequently, or deposited onto the sea floor as a CO2 ‘lake’ at a depth greater than 3000 m. In mineral storage, CO2 will be reacted with abundantly available metal oxides which produces stable carbonates. This process occurs naturally over many years and is responsible for much of the surface limestone. CO2 can be re-used as a feedstock for the production of oil-rich algae in solar membranes to produce plastics, transport fuel or animal foods, or as a feed stock in industry e.g. manufacture of carbonated beverage.
Current post-combustion and pre-combustion systems for power plants could capture 85–95% of the CO2 that is produced. However, due to the associated CO2 emissions from the capture and compression procedure (need roughly 10–40% more energy), the net amount of CO2 captured will decrease to approximately 80–90%. Higher capture efficiencies are possible, although separation devices become considerably larger, more energy intensive and more costly. Principally, Oxyfuel combustion systems are able to capture nearly all of the CO2 produced. However, the need for additional gas treatment systems to remove pollutants such as Sulphur and nitrogen oxides lowers the level of CO2 captured to slightly more than 90%.
The capture, transport and storage process would increase the energy requirement of a plant with CCS by about 25% for a coal-fired plant and about 15% for a gas-fired plant. It also involves additional operating costs and added investments or capital costs. Some new technologies are likely to be more expensive than mature CCS technologies, the IPCC estimates that a power plant equipped with CCS using mineral storage will need 60-180% more energy than a power plant without CCS.
Table 1. An estimate of costs of energy with and without CCS (2002 US$ per kWh)
|Natural gas combined cycle||Pulverized coal||Integrated gasification combined cycle|
|Without capture (reference plant)||0.03 – 0.05||0.04 – 0.05||0.04 – 0.06|
|With capture and geological storage||0.04 – 0.08||0.06 – 0.10||0.06 – 0.09|
|(Cost of capture and geological storage)||0.01 – 0.03||0.02 – 0.05||0.02 – 0.03|
|With capture and Enhanced oil recovery||0.04 – 0.07||0.05 – 0.08||0.04 – 0.08|
All costs refer to costs for energy from newly built, large-scale plants. Natural gas combined cycle costs are based on natural gas prices of US$2.80–4.40 per GJ (LHV based). Energy costs for PC and IGCC are based on bituminous coal costs of US$1.00–1.50 per GJ LHV. Note that the costs are very dependent on fuel prices (which change continuously), in addition to other factors such as capital costs. Also note that for EOR, the savings are greater for higher oil prices. Current gas and oil prices are substantially higher than the figures used here. All figures in the table are from [IPCC, 2005].
Another disadvantage of CCS is the risk of gradual or sudden CO2 leakage. In geological storage, some leakage occurs upwards through the soil slowly (over 99% of the injected CO2 are retained over 1000 years). Leakage through the injection pipe is a greater risk when the pipes wear or break due to the pressure. A natural catastrophe of CO2 leakage was sawn in 1986 from the CO2 deposit sequestered in Lake Nyos, which killed 1,700 people and is regarded as evidence for the potentially catastrophic effects of sequestering carbon. For ocean storage, large concentrations of CO2 can kill ocean organisms and the acidity of the ocean water increases.
A further consideration lies in the deferral of behavioural change. Thus, whilst CCS may temporarily deal with the carbon emissions associated with certain activities, it does not address the root cause of the emissions that lie with technological and behavioural choices in society.
International Energy Agency, Prospects for CO2 Capture and Storage, 2004.
IPCC, IPCC special report on Carbon Dioxide Capture and Storage. Prepared by working group III of the Intergovernmental Panel on Climate Change. Metz, B., O. Davidson, H. C. de Coninck, M. Loos, and L.A. Meyer (eds.). Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 442 pp, 2005.
SEI, Carbon Dioxide Capture and Storage in Ireland: Costs, Benefits and Future Potential, 2006. http://en.wikipedia.org/wiki/Carbon_capture_and_storage.