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CCS applied to a modern conventional power plant could reduce CO2 emissions to the atmosphere by approximately 80-90 % compared to a plant without CCS. Capturing and compressing CO2 requires much energy and would increase the energy needs of a plant with CCS by about 10-40 %. This and other system costs is estimated to increase the costs of energy from a power plant with CCS by 30-60% depending on the specific circumstances. Storage of the CO2 is envisaged either in deep geological formations, deep oceans, or in the form of mineral carbonates. Geological formations are currently considered the most promising, and these are estimated to have a storage capacity of at least 2000 Gt CO2. IPCC estimates that the economic potential of CCS could be between 10 % and 55% of the total carbon mitigation effort until year 2100. COST OF CCS Capturing and compressing CO2 requires much energy, significantly raising the costs of operation, apart from the added investment costs. It would increase the energy needs of a plant with CCS by about 10-40%. This and other system costs are estimated to increase the costs of energy from a power plant with CCS by 30-60%, depending on the specific circumstances. Costs of energy with and without CCS (2002 US$ per KWh ) The costs of CCS are dominated by costs of capture. The storage is relatively cheap, geological storage in saline formations or depleted oil or gas fields typically cost 0,5 - 8 US$ per tonne of CO2 injected, plus an additional 0,1 - 0,3 US$ for monitoring costs. However, when storage is combined with Enhanced Oil Recovery to extract extra oil from an oil field, the storage could yield net benefits of 10 - 16 US$ per tonne of CO2 injected (based on 2003 oil prices). However, as the table above shows, the benefits do not outweigh the extra costs of capture. CO<SUB>2</SUB> CAPTURE Capturing CO2 can be applied to large point sources, such as large fossil fuel or biomass energy facilities, major CO2 emitting industries, natural gas production, synthetic fuel plants and fossil fuel-based hydrogen production plants. Broadly, three different types of technologies exist: Post-combustion, pre-combustion, and oxyfuel combustion. In post-combustion, the CO2 is removed after combustion of the fossil fuel - this is the scheme that would be applied to conventional power plants. Here, carbon dioxide is captured from Flue Gas es at Power Station s (in the case of coal, this is known as " Clean Coal "). The technology is well understood and is currently used in niche markets. The technology for pre-combustion is widely applied in fertilizer production and hydrogen production. In case of the latter, the fossil fuel is first gasified, then CO2 is captured whereupon the produced H2 can be used for power generation. CO<SUB>2</SUB> TRANSPORT After capture, the CO2 must be transported to suitable storage sites. This is done by pipeline, which is generally the cheapest form of transport, or by ship when no pipelines are available. Both methods are currently used for transporting CO2. For other applications than storage. CO<SUB>2</SUB> STORAGE Various forms of more or less permanent storage of CO2 outside the atmosphere have been conceived. These are storage in various deep geological formations (including saline formations and exhausted gas fields), ocean storage, and reaction of CO2 with metal Oxide s to produce stable Carbonate s. Geological storage Also known as ''geo-sequestration'', this method involves injecting carbon dioxide directly into underground geological formations. Oil Field s, Gas Field s, saline formations, and unminable Coal Seam s have been suggested as storage sites. Here, various physical (e.g. highly impermeable caprock) and geochemical trapping mechanisms would prevent the CO2 from escaping to the surface. CO2 is sometimes injected into declining oil fields to increase oil recovery ( Enhanced Oil Recovery ). This option is attractive because the storage cost are offset by the sale of additional oil that is recovered. Disadvantages of old oil fields are their geographic distribution and their limited capacity. Unminable Coal Seam s can be used to store CO2, because CO2 adsorbs to the coal surface, but the technical feasibility depends on the permeability of the coal bed. In the process it releases methane, that was previously adsorbed to the coal surface, and that may be recovered ( Enhanced Coal Bed Methane Recovery ). Again the sale of the methane can be used to offset the cost of the CO2 storage. Saline formations contain highly mineralized brines, and have so far been considered of no benefit to humans. Saline aquifers have been used for storage of chemical waste in a few cases. The main advantage of saline aquifers is their large potential storage volume and their common occurrence. This will reduce the distances over which CO2 has to be transported. The major disadvantage of saline aquifers is that relatively little is known about them, compared to oil fields. To keep the cost of storage acceptable the geophysical exploration may be limited, resulting in larger uncertainty about the aquifer structure. Unlike storage in oil fields or coal beds no side product will offset the storage cost. Leakage of CO2 back into the atmosphere, may be a problem in saline aquifer storage. However, current research shows that several ''trapping mechanisms'' immobilize the CO2 underground, reducing the risk of leakage. As of 2005, three industrial-scale storage projects are in operation. One is at an oil field at Weyburn in southeastern Saskatchewan , Canada. In the North Sea, Norway's Statoil natural gas platform Sleipner strips carbon dioxide out of the natural gas with amine solvents and disposes of this carbon dioxide in a saline formation. Sleipner stores about one million tonnes CO2 a year. The third is the In Salah project in a Gas Field in Algeria . For well-selected, designed and managed geological storage sites, IPCC estimates that CO2 could be trapped for millions of years, and are likely to retain over 99% of the injected CO2 over 1000 years. Ocean storage Another proposed form of carbon storage is in the oceans. Two concepts exist. The 'dissolution' type injects CO2 by ship or pipeline into the water column at depths of 1000 m or more, and the CO2 subsequently dissolves. The 'lake' type deposits CO2 directly onto the sea floor at depths greater than 3000 m, where CO2 is denser than water and is expected to form a 'lake' that would delay dissolution of CO2 into the environment. The environmental effects are generally negative, but poorly understood. Large concentrations of CO2 kills ocean organisms, but another problem is that dissolved CO2 would eventually equilibrate with the atmosphere, so the storage would not be permanent. Also, as part of the CO2 reacts with the water to form Carbonic Acid , H2CO3, the acidity of the ocean water increases. The resulting environmental effects on Benthic life forms of the Bathypelagic , Abyssopelagic and Hadopelagic zones are poorly understood. Even though life appears to be rather sparse in the deep ocean basins, energy and chemical effects in these deep basins could have far reaching implications. Much more work is needed here to define the extent of the potential problems. Ocean storage is likely to be more 'leaky' than geological storage; IPCC estimates 30-85% of the injected CO2 would be retained after 500 years for depths 1000-3000 m. An additional method of long term ocean based sequestration is to gather crop residue such as corn stalks or excess hay into large weighted bales of biomass and deposit it in the Alluvial Fan areas of the deep ocean basin. Dropping these residues in alluvial fans would cause the residues to be quickly buried in silt on the sea floor, sequestering the biomass for very long time spans. Alluvial fans exist in all of the world's oceans and seas where river deltas fall off the edge of the continental shelf such as the Mississippi alluvial fan in the gulf of Mexico and the Nile alluvial fan in the Mediterranean Sea. Mineral storage Mineral storage aims to trap carbon in stable minerals, and CO2 would be forever trapped. In this process, CO2 is reacted with (abundantly available) metal oxides which produces stable carbonates. This process occurs naturally and is responsible for much of the surface Limestone . However, the natural reaction is very slow and has to be enhanced by pre-treatment of the minerals, which is very energy intensive. IPCC estimates that a power plant equipped with CCS using mineral storage will need 60-180 % more energy than a power plant without CCS. Leakage A major concern with CCS is whether leakage of stored CO2 will compromise CCS as a climate change mitigation option. For well-selected, designed and managed geological storage sites, IPCC estimates that CO2 could be trapped for millions of years, and are likely to retain over 99 % of the injected CO2 over 1000 years. For ocean storage, the retention of CO2 would depend on the depth; IPCC estimates 30-85% would be retained after 500 years for depths 1000-3000 m. Mineral storage is not regarded as having any risks of leakage. IPCC recommends that there must be set an upper limit to the amount of leakage than can take place. REFERENCES
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