CARBON CAPTURE & STORAGE

Carbon capture and storage is an approach to reduce global warming and climate change caused by greenhouse gas emissions. It aims to capture carbon dioxide from large point sources such as power plants and subsequently store it away safely instead of releasing it into the atmosphere. Technology for capturing of carbon dioxide is already commercially available for large carbon dioxide emitters, such as power plants.

Storage of Carbon dioxide, on the other hand, is a relatively untried concept and in 2006, no power plant operated with a full carbon capture and storage system. Currently, United States government has approved the construction of world’s first carbon capture and storage power plant, FutureGen.

Carbon capture and storage applied to a modern conventional power plant could reduce carbon dioxide emissions to the atmosphere by approximately 80-90 % compared to a plant without carbon capture and storage. Capturing and compressing carbon dioxide requires much energy and would increase the fuel needs of a plant with carbon capture and storage by about 10 to 40 %. These and other system costs are estimated to increase the cost of energy from a power plant with carbon capture and storage by 30-60% depending on the specific circumstances.

Storage of the carbon dioxide 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 Carbon dioxide. IPCC estimates that the economic potential of carbon capture and storage could be between 10 % and 55% of the total carbon mitigation effort until year 2100.

Cost of carbon capture and storage

Capturing and compressing carbon dioxide 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 carbon capture and storage by about 10 to 40%. This, the costs of storage, and other system costs are estimated to increase the costs of energy from a power plant with Carbon capture and storage by 30 to 60%, depending on the specific circumstances.

The costs of Carbon capture and storage are dominated by costs of capture. The storage is relatively cheap, geological storage in saline formations or depleted oil or gas fields typically cost between 0.5 to US$8 per tonne of carbon dioxide injected, plus an additional 0.1 to 0.3 US$ 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 to 16 US$ per tonne of carbon dioxide injected (based on 2003 oil prices). However, the benefits do not outweigh the extra costs of capture.

Environmental impacts of carbon capture and storage

The major merit of Carbon capture and storage systems is the reduction of carbon dioxide emissions, which is typically on the order of 90%, depending on plant type. Generally, environmental impacts from use of carbon capture and storage arise during power production, carbon dioxide transport and carbon dioxide storage. Problems with the latter are discussed in the sections on storage.

The substantial extra amounts of energy required for carbon dioxide capture means that more fuel has to be used; how much depends on the plant type. For new supercritical pulverized coal plants using current technology, the extra energy requirements range from 24 to 40%, while for natural gas combined cycle plants the range is 11 to 22% and for coal-based gasification combined cycle systems it is 14-25%. Obviously, fuel use and environmental problems arising from mining and extraction of coal or gas increase accordingly. Plants equipped with flue gas desulphurization systems for SO2 control require proportionally greater amounts of limestone, and systems equipped with SCR systems for NOx requires proportionally greater amounts of ammonia.

The Intergovernmental Panel on Climate Control (IPCC) has provided estimates of air emissions from various carbon capture and storage plant designs. While carbon dioxide is drastically reduced (though never completely captured), emissions of air pollutants increase significantly, generally due to the energy penalty of capture, hence the use of carbon capture and storage entails some sacrifice of air quality.

Carbon dioxide capture

Capturing carbon dioxide can be applied to large point sources, such as large fossil fuel or biomass energy facilities, major carbon dioxide-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 carbon dioxide 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 gases at power stations (in the case of coal, this is sometimes 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, chemical, gaseous fuel (H2, CH4), and power production. In these cases, the fossil fuel is gasified and the resulting carbon dioxide can be captured from a relatively pure exhaust stream.

An alternate method, which is under development, is chemical looping combustion. Chemical looping uses a metal oxide as a solid oxygen carrier. Metal oxide particles react with a solid, liquid or gaseous fuel in a fluidized bed combustor, producing solid metal particles and a mixture of carbon dioxide and water vapor. The water vapor is condensed, leaving pure carbon dioxide which can be sequestrated. The solid metal particles are circulated to another fluidized bed where they react with air, producing heat and regenerating metal oxide particles that are recirculated to the fluidized bed combustor.

Carbon dioxide transport

After capture, the carbon dioxide 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 carbon dioxide for other applications.

Carbon dioxide storage

Various forms of more or less permanent storage of carbon dioxide isolated from 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 carbon dioxide with metal oxides to produce stable carbonates.

Geological storage

Also known as geo-sequestration, this method involves injecting carbon dioxide directly into underground geological formations. Oil fields, gas fields, saline formations, and unminable coal seams have been suggested as storage sites. Here, various physical (e.g., highly impermeable caprock) and geochemical trapping mechanisms would prevent the carbon dioxide from escaping to the surface. Carbon dioxide is sometimes injected into declining oil fields to increase oil recovery. This option is attractive because the storage costs 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 seams can be used to store carbon dioxide, because carbon dioxide 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. Again the sale of the methane can be used to offset the cost of the carbon dioxide 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 carbon dioxide 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 carbon dioxide back into the atmosphere, may be a problem in saline aquifer storage. However, current research shows that several trapping mechanisms immobilize the carbon dioxide underground, reducing the risk of leakage.

For well-selected, designed and managed geological storage sites, IPCC estimates that carbon dioxide could be trapped for millions of years, and the sites are likely to retain over 99% of the injected carbon dioxide over 1,000 years.

Examples of carbon capture and storage projects

As of 2005, three industrial-scale storage projects were in operation. Sleipner is the oldest project (1996) and is located in the North Sea where Norway’s Statoil strips carbon dioxide from natural gas with amine solvents and disposes of this carbon dioxide in a saline formation. The carbon dioxide is a waste product of the field’s natural gas production and the gas contains more (9% carbon dioxide) than is allowed into the natural gas distribution network. Storing it underground avoids this problem and saves Statoil hundreds of millions of euro in avoided carbon taxes. Sleipner stores about one million tonnes carbon dioxide a year.

The Weyburn project started in 2000 and is located in an oil reservoir discovered in 1954 in Weyburn, Southeastern Saskatchewan, Canada. The carbon dioxide for this project is captured at the Great Plains Coal Gasification plant in Beulah, North Dakota which has produced methane from coal for more than 30 years. At Weyburn, the carbon dioxide will also be used for enhanced oil recovery with an injection rate of about 1.5 million tonnes per year.

The third site is In Salah, which, like Sleipner, is a natural gas reservoir located in In Salah, Algeria. The carbon dioxide will be separated from the natural gas and re-injected into the subsurface at a rate of about 1.2 million tonnes per year.

Ocean storage of captured carbon

Another proposed form of carbon storage is in the oceans. Two main concepts exist. The ‘dissolution’ type injects carbon dioxide by ship or pipeline into the water column at depths of 1000 meters or more, and the carbon dioxide subsequently dissolves. The ‘lake’ type deposits carbon dioxide directly onto the sea floor at depths greater than 3000 m, where carbon dioxide is denser than water and is expected to form a ‘lake’ that would delay dissolution of carbon dioxide into the environment. A third concept is to convert the carbon dioxide to bicarbonates (using limestone) or hydrates.

The environmental effects of ocean storage are generally negative, but poorly understood. Large concentrations of carbon dioxide kills ocean organisms, but another problem is that dissolved carbon dioxide would eventually equilibrate with the atmosphere, so the storage would not be permanent. Also, as part of the carbon dioxide 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.

The time it takes water in the deeper oceans to circulate to the surface has been estimated to be on the order of 1600 years, varying upon currents and other changing conditions. Costs for deep ocean disposal of liquid carbon dioxide are estimated at 40-80US$ per ton. This figure covers the cost of sequestration at the power plant and naval transport to the disposal site. The bicarbonate approach would reduce the pH effects and enhance the retention of carbon dioxide in the ocean, but this would also increase the costs and other environmental impacts.

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 of captured carbon

Mineral storage aims to trap carbon in stable minerals, and carbon dioxide would be forever trapped. In this process, carbon dioxide 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. The IPCC estimates that a power plant equipped with carbon capture and storage using mineral storage will need 60-180% more energy than a power plant without carbon capture and storage.

Possible Leakage of captured carbon

A major concern with Carbon capture and storage is whether leakage of stored carbon dioxide will compromise carbon capture and storage as a climate change mitigation option. For well-selected, designed and managed geological storage sites, IPCC estimates that carbon dioxide could be trapped for millions of years, and are likely to retain over 99% of the injected carbon dioxide over 1000 years. For ocean storage, the retention of carbon dioxide would depend on the depth; IPCC estimates 30 to 85% would be retained after 500 years for depths 1000-3000 m. Mineral storage is not regarded as having any risks of leakage. The IPCC recommends that limits are set to the amount of leakage than can take place.

It should also be noted that at the conditions of the deeper oceans, (~400 bar, 280K) water-carbon dioxide mixing is very low (where carbonate formation/acidification is the rate limiting step), but the formation of water-carbon dioxide hydrates is favorable.

To further investigate the safeness of Carbon dioxide sequestration, we can look into Norway’s Sleniper gas field, as it is the oldest plant that sequesters carbon dioxide in an industrial scale. According to an environmental assessment of the gas field conducted after ten years of its operation, the author affirmed that geographic sequestration of carbon dioxide was the most definite way to store Carbon dioxide permanently.

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