Artificial sequestration - Carbon Dioxide Sink

For carbon to be sequestered artificially (i.e. not using the natural processes of the carbon cycle) it must first be captured, or it must be significantly delayed or prevented from being re-released into the atmosphere (by combustion, decay, etc.) from an existing carbon-rich material, by being incorporated into an enduring usage (such as in construction). Thereafter it can be passively stored or remain productively utilized over time in a variety of ways.

For example, upon harvesting, wood (as a carbon-rich material) can be immediately burned or otherwise serve as a fuel, returning its carbon to the atmosphere, or it can be incorporated into construction or a range of other durable products, thus sequestering its carbon over years or even centuries. One ton of dry wood is equivalent to 1.8 tons of carbon dioxide.

Indeed, a very carefully designed and durable, energy-efficient and energy-capturing building has the potential to sequester (in its carbon-rich construction materials), as much as or more carbon than was released by the acquisition and incorporation of all its materials and than will be released by building-function "energy-imports" during the structure's (potentially multi-century) existence. Such a structure might be termed "carbon neutral" or even "carbon negative". Building construction and operation (electricity usage, heating, etc.) are estimated to contribute nearly half of the annual human-caused carbon additions to the atmosphere.

Natural-gas purification plants often already have to remove carbon dioxide, either to avoid dry ice clogging gas tankers or to prevent carbon-dioxide concentrations exceeding the 3% maximum permitted on the natural-gas distribution grid.

Beyond this, one of the most likely early applications of carbon capture is the capture of carbon dioxide from flue gases at power stations (in the case of coal, this is known as "clean coal"). A typical new 1000 MW coal-fired power station produces around 6 million tons of carbon dioxide annually. Adding carbon capture to existing plants can add significantly to the costs of energy production; scrubbing costs aside, a 1000 MW coal plant will require the storage of about 50 million barrels of carbon dioxide a year. However, scrubbing is relatively affordable when added to new plants based on coal gasification technology, where it is estimated to raise energy costs for households in the United States using only coal-fired electricity sources from 10 cents per kW·h to 12 cents.

Carbon capture

Currently, capture of carbon dioxide is performed on a large scale by absorption of carbon dioxide onto various amine-based solvents. Other techniques are currently being investigated, such as pressure swing adsorption, temperature swing adsorption, gas separation membranes, and cryogenics. Recent pilot studies include flue capture and conversion to baking soda and use of algae for conversion to fuel or feed.

In coal-fired power stations, the main alternatives to retrofitting amine-based absorbers to existing power stations are two new technologies: coal gasification combined-cycle and oxy-fuel combustion. Gasification first produces a "syngas" primarily of hydrogen and carbon monoxide, which is burned, with carbon dioxide filtered from the flue gas. Oxy-fuel combustion burns the coal in oxygen instead of air, producing only carbon dioxide and water vapour, which are relatively easily separated. Some of the combustion products must be returned to the combustion chamber, either before or after separation, otherwise the temperatures would be too high for the turbine.

Another long-term option is carbon capture directly from the air using hydroxides. The air would literally be scrubbed of its CO₂ content. This idea offers an alternative to non-carbon-based fuels for the transportation sector.

Examples of carbon sequestration at coal plants include converting carbon from smokestacks into baking soda, and algae-based carbon capture, circumventing storage by converting algae into fuel or feed.

Oceans

Another proposed form of carbon sequestration in the ocean is direct injection. In this method, carbon dioxide is pumped directly into the water at depth, and expected to form "lakes" of liquid CO₂ at the bottom. Experiments carried out in moderate to deep waters (350–3600 m) indicate that the liquid CO₂ reacts to form solid CO₂ clathrate hydrates, which gradually dissolve in the surrounding waters.

This method, too, has potentially dangerous environmental consequences. The carbon dioxide does react with the water to form carbonic acid, H₂CO₃; however, most (as much as 99%) remains as dissolved molecular CO₂. The equilibrium would no doubt be quite different under the high pressure conditions in the deep ocean. In addition, if deep-sea bacterial methanogens that reduce carbon dioxide were to encounter the carbon dioxide sinks, levels of methane gas may increase, leading to the generation of an even worse greenhouse gas. The resulting environmental effects on benthic life forms of the bathypelagic, abyssopelagic and hadopelagic zones are unknown. 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.

Carbon storage in or under oceans may not be compatible with the Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter.

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. A downside, however, would be an increase in aerobic bacteria growth due to the introduction of biomass, leading to more competition for oxygen resources in the deep sea, similar to the oxygen minimum zone.

Geological sequestration

The method of geo-sequestration or geological storage involves injecting carbon dioxide directly into underground geological formations. Declining oil fields, saline aquifers, and unminable coal seams have been suggested as storage sites. Caverns and old mines that are commonly used to store natural gas are not considered, because of a lack of storage safety.

CO₂ has been injected into declining oil fields for more than 40 years, to increase oil recovery. This option is attractive because the storage costs are offset by the sale of additional oil that is recovered. Typically, 10-15% additional recovery of the original oil in place is possible. Further benefits are the existing infrastructure and the geophysical and geological information about the oil field that is available from the oil exploration. Another benefit of injecting CO₂ into Oil fields is that CO₂ is soluble in oil. Dissolving CO₂ in oil lowers the viscosity of the oil and reduces its interfacial tension which increases the oils mobility. All oil fields have a geological barrier preventing upward migration of oil. As most oil and gas has been in place for millions to tens of millions of years, depleted oil and gas reservoirs can contain carbon dioxide for millennia. Identified possible problems are the many 'leak' opportunities provided by old oil wells, the need for high injection pressures and acidification which can damage the geological barrier. Other disadvantages of old oil fields are their limited geographic distribution and depths, which require high injection pressures for sequestration. Below a depth of about 1000 m, carbon dioxide is injected as a supercritical fluid, a material with the density of a liquid, but the viscosity and diffusivity of a gas. Unminable coal seams can be used to store CO₂, because CO₂ absorbs to the coal surface, ensuring safe long-term storage. 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 CO₂ storage. Release or burning of methane would of course at least partially offset the obtained sequestration result – except when the gas is allowed to escape into the atmosphere in significant quantities: methane has a higher global warming potential than CO₂.

Saline aquifers contain highly mineralized brines and have so far been considered of no benefit to humans except in a few cases where they have been used for the storage of chemical waste. Their advantages include a large potential storage volume and relatively common occurrence reducing the distance 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. Another disadvantage of saline aquifers is that as the salinity of the water increases, less CO₂ can be dissolved into aqueous solution. To keep the cost of storage acceptable the geophysical exploration may be limited, resulting in larger uncertainty about the structure of a given aquifer. 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.

A major research project examining the geological sequestration of carbon dioxide is currently being performed at an oil field at Weyburn in south-eastern Saskatchewan. 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 by geological sequestration. Sleipner reduces emissions of carbon dioxide by approximately one million tonnes a year. The cost of geological sequestration is minor relative to the overall running costs. As of April 2005, BP is considering a trial of large-scale sequestration of carbon dioxide stripped from power plant emissions in the Miller oilfield as its reserves are depleted.

In October 2007, the Bureau of Economic Geology at The University of Texas at Austin received a 10-year, $38 million subcontract to conduct the first intensively monitored, long-term project in the United States studying the feasibility of injecting a large volume of CO₂ for underground storage. The project is a research program of the Southeast Regional Carbon Sequestration Partnership (SECARB), funded by the National Energy Technology Laboratory of the U.S. Department of Energy (DOE). The SECARB partnership will demonstrate CO₂ injection rate and storage capacity in the Tuscaloosa-Woodbine geologic system that stretches from Texas to Florida. Beginning in fall 2007, the project will inject CO₂ at the rate of one million tons per year, for up to 1.5 years, into brine up to 10,000 feet (3,000 m) below the land surface near the Cranfield oil field about 15 miles (24 km) east of Natchez, Mississippi. Experimental equipment will measure the ability of the subsurface to accept and retain CO₂.

Mineral sequestration

Mineral sequestration aims to trap carbon in the form of solid carbonate salts. This process occurs slowly in nature and is responsible for the deposition and accumulation of limestone (calcium carbonate) over geologic time. Carbonic acid in groundwater slowly reacts with complex silicates to dissolve calcium, magnesium, alkalis and silica and leave a residue of clay minerals. The dissolved calcium and magnesium react with bicarbonate to precipitate calcium and magnesium carbonates, a process that organisms use to make shells. When the organisms die, their shells are deposited as sediment and eventually turn into limestone. Limestones have accumulated over billions of years of geologic time and contain much of Earth's carbon. Ongoing research aims to speed up similar reactions involving alkali carbonates.

One proposed reaction is that of the olivine-rich rock dunite, or its hydrated equivalent serpentinite with carbon dioxide to form the carbonate mineral magnesite, plus silica and iron oxide (magnetite).

Serpentinite sequestration is favored because of the non-toxic and stable nature of magnesium carbonate. The ideal reactions involve the magnesium endmember components of the olivine (reaction 1) or serpentine (reaction 2), the latter derived from earlier olivine by hydration and silicification (reaction 3). The presence of iron in the olivine or serpentine reduces the efficiency of sequestration, since the iron components of these minerals break down to iron oxide and silica (reaction 4).

Serpentinite reactions

Reaction 1
Mg-Olivine + Carbon dioxide → Magnesite + Silica

Mg₂SiO₄ + 2CO₂ → 2MgCO₃ + SiO₂ + H₂O

Reaction 2
Serpentine + carbon dioxide → Magnesite + silica + water

Mg₃[Si₂O₅(OH)₄] + 3CO₂ → 3MgCO₃ + 2SiO₂ + 2H₂O

Reaction 3
Mg-Olivine + Water + Silica → Serpentine

3Mg₂SiO₄ + 2SiO₂ + 4H₂O → 2Mg₃[Si₂O₅(OH)₄

Reaction 4
Fe-Olivine + Water → Magnetite + Silica + Hydrogen

3Fe₂SiO₄ + 2H₂O → 2Fe₃O₄ + 3SiO₂ + 2H₂

Zeolitic imidazolate frameworks

Zeolitic imidazolate frameworks is a metal-organic framework carbon dioxide sink which could be used to keep industrial emissions of carbon dioxide out of the atmosphere.