Carbon capture and storage

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Carbon capture and storage (CCSD) is an approach to mitigating climate change by capturing carbon dioxide (CO₂) from large point sources such as power plants and subsequently storing it away safely instead of releasing it into the atmosphere. Technology for capturing of CO₂ is already commercially available for large CO₂ emitters, such as power plants. Storage of CO₂, on the other hand, is a relatively untried concept and as yet (2007) no power plant operates with a full carbon capture and storage system. Currently, the United States government has approved the construction of the world's first CCS power plant, FutureGen, while BP has indicated that it intends to develop a 350 MW carbon capture and storage plant in Scotland, in which the carbon from a natural gas fired generator plant will be stripped out and pumped into the Miller field in the North Sea.

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 fuel needs of a plant with CCS by about 10-40%. These and other system costs are estimated to increase the cost 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. In the case of deep ocean storage, there is a risk of greatly increasing the problem of ocean acidification, a problem that also stems from the excess of carbon dioxide. Geological formations are currently considered the most promising sequestration sites, 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.

Contents 1 Cost of CCS 2 Environmental impacts 3 CO2 capture 4 CO2 transport 5 CO2 storage 5.1 Geological storage 5.1.1 Example CCS projects 5.2 Ocean storage 5.3 Mineral storage 5.4 Leakage 6 References 7 See also 8 External links

[edit] Cost of CCS

Capturing and compressing CO₂ requires much energy, significantly raising the running costs of CCS-equipped power plants. In addition there are added investment or capital costs. The process would increase the energy needs of a plant with CCS by about 10-40%. The costs of storage 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)

	Natural gas combined cycle	Pulverized coal	Integrated gasification combined cycle
Without capture (reference plant)	0,031 - 0,050	0,043 - 0,052	0,041 - 0,061
With capture and geological storage	0,043 - 0,077	0,063 - 0,099	0,055 - 0,091
With capture and Enhanced oil recovery	0,037 - 0,070	0,049 - 0,081	0,040 - 0,075
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 2,8 — 4,4 US$ per GJ (LHV based). Energy costs for PC and IGCC are based on bituminous coal costs of 1,0 - 1,5 US$ 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 Table 8.3a in [IPCC, 2005]

The costs of CCS are dominated by costs of capture. The storage cost 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.

[edit] Environmental impacts

The major merit of CCS systems is the reduction of CO₂ emissions by up to 90%, depending on plant type.

Generally, environmental impacts from use of CCS arise during power production, CO₂ transport and CO₂ storage. Problems with the latter are discussed in the sections on storage.

The substantial extra amounts of energy required for CO₂ capture means that more fuel has to be used, how much depends on the plant type. For new supercritical pulverized coal (PC) plants using current technology, the extra energy requirements range from 24-40%, while for natural gas combined cycle (NGCC) plants the range is 11-22% and for coal-based gasification combined cycle (IGCC) systems it is 14-25% [IPCC, 2005]. Obviously, fuel use and environmental problems arising from mining and extraction of coal or gas increase accordingly. Plants equipped with flue gas desulfurization (FGD) systems for SO₂ control require proportionally greater amounts of limestone, and systems equipped with SCR systems for NO_X requires proportionally greater amounts of ammonia.

IPCC has provided estimates of air emissions from various CCS plant designs (see table below). While CO2 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 CCS entails some sacrifice of air quality.

Emissions to air from plants with or without CCS (kg/MWh)

	Natural gas combined cycle	Pulverized coal	Integrated gasification combined cycle
CO2	43 (-89%)	107 (-87%)	97 (-88%)
NOX	0,11 (+22%)	0,77 (+31%)	0,1 (+11%)
SOX	-	0,001 (-99,7%)	0,33 (+17,9%)
Ammonia	0,002 (before: 0)	0,23 (+2200%)	-
Based on Table 3.5 in [IPCC, 2005]. Between brackets the increase or decrease compared to a similar plant without CCS.

[edit] CO₂ 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 processing, 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 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 [1]. In these cases, the fossil fuel is gasified and the resulting CO₂ can be captured from a relatively pure exhaust stream.

In oxyfuel combustion the lignite is burned in oxygen instead of air. This produces a flue gas consisting only carbon dioxide and water vapour, which is cooled and condensed. The result is an almost pure carbon dioxide stream that can be transported to the sequestration site and stored. The technique is promising, but the initial air separation step demands a lot of energy.

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 sequestered. 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.

A few engineering proposals have been made for the much more difficult task of capturing CO2 directly from the air, but work in this area is speculative and conceptual at this point. Capture costs are estimated to be much higher than from point sources, but may be feasible for dealing with emissions from diffuse sources like automobiles and aircraft [2].

[edit] CO₂ 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.

[edit] CO₂ storage

Various forms of more or less permanent storage of CO₂ 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 CO₂ with metal oxides to produce stable carbonates.

[edit] Geological storage

Also known as geo-sequestration, this method involves injecting carbon dioxide directly into underground geological formations. Oil fields, gas fields, saline formations, unminable coal seams, and saline-filled basalt formations 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 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 CO₂ because CO₂ adsorbs to the surface of coal. However, the technical feasibility depends on the permeability of the coal bed. In the process of absorption the coal releases previously absorbed methane, and the methane can be recovered (Enhanced Coal Bed Methane recovery). 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.

For well-selected, designed and managed geological storage sites, IPCC estimates that CO₂ could be trapped for millions of years, and the sites are likely to retain over 99% of the injected CO₂ over 1,000 years.

[edit] Example CCS projects

As of 2005, three industrial-scale storage projects are in operation. Sleipner [3] 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% CO₂) 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 CO₂ a year. The Weyburn project started in 2000 and is located in an oil reservoir discovered in 1954 in Weyburn, Southeastern Saskatchewan, Canada. The CO₂ 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 CO₂ 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 CO₂ will be separated from the natural gas and re-injected into the subsurface at a rate of about 1.2 million tonnes per year.

[edit] Ocean storage

Another proposed form of carbon storage is in the oceans. Two main 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. A third concept is to convert the CO₂ to bicarbonates (using limestone) or hydrates.

The environmental effects of ocean storage 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.

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 CO2 are estimated at 40-80USD/ton. (2002 USD) This figure covers the cost of sequestration at the powerplant and naval transport to the disposal site. [2]

The bicarbonate approach would reduce the pH effects and enhance the retention of CO₂ 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.

[edit] 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. 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.

[edit] 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. The IPCC recommends that limits be 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-CO₂(l) mixing is very low (where carbonate formation/acidification is the rate limiting step), but the formation of water-CO₂ hydrates is favorable. (a kind of solid water cage that surrounds the CO₂) [3]

To further investigate the safeness of CO₂ sequestration, we can look into Norway's Sleniper gas field, as it is the oldest plant that sequesters CO₂ in an industrial scale. According to an environmental assessment of the gas field which was conducted after ten years of operation, the author affirmed that geographic sequestration of CO₂ was the most definite way to store CO₂ permanently [4].

"Available geological information shows absence of major tectonic events after the deposition of the Utsira formation [saline reservoir]. This implies that the geological environment is tectonically stable and a site suitable for carbon dioxide storage. The solubility trapping [is] the most permanent and secure form of geological storage." [4]

[edit] References

[edit] See also

• Carbon dioxide sink
• Mitigation of global warming
• Low-carbon economy

[edit] External links

• Intergovernmental Panel on Climate Change IPCC Special Report on Carbon Dioxide Capture and Storage.
• Stanford University Collection of recent news articles on CO₂ capture and storage.
• Webcasts, Reports and Articles Relavent Climate Change Action Posts
• DOE Fossil Energy Department of Energy programs in carbon dioxide capture and storage.
• CO2 Capture and Storage International Energy Agency Greenhouse Gas Research Programme (includes CSS project summaries)
• CO2 Capture and Geologic Storage National Energy and Technology Laboratory summary of worldwide projects