Artificial Methods of Carbon Sequestration
For carbon to be sequestered artificially (i.e. not using the natural processes of the carbon cycle) it must first be captured. Thereafter it can be stored in a variety of ways.
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 6m 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 kWh to 12 .
Currently, carbon capture and storage 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 and temperature swing absorption, gas separation membranes and cryogenics.
In coal-fired power stations, the main alternatives to retro-fitting amine-based absorbers to existing power stations are two new technologies - coal gasification combined-cycle and oxyfuel combustion. Gasification first produces a "syngas" primarily of hydrogen and carbon monoxide, which is burned, with carbon dioxide filtered from the flue gas. Oxyfuel combustion burns the coal in oxygen instead of air, producing only carbon dioxide and water vapour, which are relatively easily separated. Oxyfuel combustion, however, produces very high temperatures, and the materials to withstand its temperatures are still being developed.
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.
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 meters) 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. 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.
It is not clear whether carbon storage in or under oceans is compatible with the London Convention (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.
Also known as geo-sequestration or geological storage, this method 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 30 years, to increase oil recovery. This option is attractive because the storage cost are offset by the sale of additional oil that is recovered. Further benefits are the existing infrastructure, and the geophysical and geological information about the oil field that is available from the oil exploration. All oil fields have a geological barrier preventing upward migration of buoyant fluids (oil in the past, CO2 in the future).
Mineral sequestration aims to trap carbon by placing it in its thermodynamics groundstate where it will be nonreactive. This occurs naturally and is responsible for much of the surface limestone. Acids are used to convert mineral silicates to mineral carbonates. Ongoing research aims to speed up the kinetics of the reactions.
One proposed reaction is that of the rock dunite, or serpentinite with carbon dioxide to form the carbonate mineral magnesite, plus some silica and magnetite. This is proposed by ZECA Corporation, a consortium aiming to produce a low-emission coal-fired power source.
Serpentinite sequestration is favored because of the non-toxic and predictable nature of magnesium carbonate. However, the ideal reaction (reaction 1) takes place only with extremely magnesium rich olivine or serpentine minerals. The presence of iron in the olivine or serpentine will reduce the efficiency of the circuit and reactions 2 and 3 must take place, producing a slag of silica and iron oxide (magnetite).
Reaction 1Mg-Olivine + Water + Carbon dioxide → Serpentine + Magnesite + Silica
Reaction 2Fe-Olivine + Water + Carbonic acid → Serpentine + Magnetite + Magnesite + Silica
Reaction 3Serpentine + carbon dioxide → Magnesite + silica + water