Sequestration, the capture and storage of carbon dioxide, CO2, and other greenhouse gases, is necessary to control the concentrations of greenhouse gases, that unless controlled will, according to most engineers and scientists, lead to global warming. CO2 constitutes 81% of green house gases, followed with methane which represents another 9%. When any fossil fuel is burned this results in production of CO2. Since 86% of our energy comes from fossil fuels they release such large quantities of CO2 without imposition of controls, that global warming is inevitable. The concentration of CO2 in our atmosphere has increased dramatically since the start of the industrial revolution, increasing its concentration to 380 ppm in 2004, accompanied by a corresponding increase in the temperature of the atmosphere. By 2020 our energy requirements are forecast to increase by 40% and 86% of the energy will still come from fossil fuels, increasing CO2 concentration to even more dangerous levels. The exact point at which global warming will become excessive is not known, but it according to one report, when the atmospheric temperature is increased by another 2oC (3.6oF) and the CO2 level reaches 400 ppm we will have reached conditions of unacceptable global warming.
About 40% of our energy comes from coal, oil and gas each supply 24% and the other 12% is split between nuclear and renewables. Electrical generation contributes 39% of the CO2, transportation 32% and other sectors 30%.
Carbon can be reduced by using more renewable and nuclear energy, improvements in efficiency or by sequestration. Sequestration can be accomplished by storing CO2 in underground reservoirs, in trees, plants or algae, converting it to soil materials or dissolved it in deep oceans.
The above items are further elaborated in a presentation by the National Energy Technological Laboratory (NETL)
Carbon Capture - Before CO2 can be sequestered it must be captured as relatively pure CO2 from power plants and other point sources. On a mass basis, CO2 is the largest commodity chemical in the US and industry regularly separates, captures and purifies CO2. These techniques are not economical when considered in light of sequestering CO2. Separation and capture concepts are being persued in the following areas: absorption, adsorption, low-temperature distillation, gas separation membranes, mineralization and biomineralization.
Geological Sequestration - Storage of greenhouse gases in geological formations such as oil and gas reservoirs, unmineable coal seams and deep saline reservoirs is a technique that can be implemented with relative ease, its main drawback being that the source of the gas must be relatively near the geological formation. The US is the world leader in enhanced oil recovery (EOR), using about 32 million tons of CO2 a year for this purpose. Thus the use of CO2 emissions is a natural extension of this technology. In the Permian Basin of west Texas and New Mexico this technique has been put into use. CO2 taken from factory and utility smokestacks, as well as commercial CO2, is pumped into the ground to stimulate the flow of oil once believed to be unrecoverable.
Coal beds beds typically contain large amounts of methane gas that is adsorbed on the surface of the coal. Tests have shown that the gas can be recovered by displacing the methane with CO2, but more work is required to understand and optimize the process. Large quantities of unmineable coal seams are located near the location of many coal fired power plants thus limited pipeline transport would be required.
Saline formations are widespread in the US and have the potential to store up to 550 million tons of CO2. Additionally most large CO2 point sources are within easy acess to saline formations. The sequestration of CO2 into deep water saline formations does not produce any byproducts, but they are very large, estimated to able to hold 500 billion tons of CO2, and are located near many power stations permitting injection to be done cost effectively.
The summer 2005 issue of Clean Coal Today, p8, has an article updating current geological sequestration methods being investigated in the US. The following is a brief summary:
- In work spearheaded by the IEA the Weyburn enhanced oil recovery test being conducted in Saskatchewan, Canada is the longest running program, being started in 2001. It is estimated that half the CO2 injected will remain sequestered. The CO2 is being transported by pipeline for 200 miles from the Dakata Gasification Plant in Beulah, ND. It is expected that half will remain sequestered and half will be recycled back into the reservoir. The test is targeted to run 15 years and inject a total of 20 million tons of C02.
- In the Frio, TX saline formation project 1,600 tons of CO2 was injected, in October of 2004 and various types of measuring tools are being evaluated as well as movement of the plume, which has stabilized much as predicted.
- Coalbed methane recovery has been combined with CO2 sequestration in some field projects. In a seven year project with CONSOl Energy R&D in Marshall County WV, both methane recovery and sequestration in an unmineable coal seam are being investigated. The project is currently in the pre-injection phase with over 26,000 tons of CO2 to be injected over a one year period. Another similar project is being undertaken in the San Juan Basin, New Mexico.
Ocean Sequestration - CO2 is soluble in ocean water and it is widely believed that the oceans will eventually absorb 80-90% of the CO2 in the atmosphere. However this absorption occurs at unacceptably slow rates. Using methodologies which modify regions of the ocean to increase absorption rates are being investigated. The effectiveness of these techniques as well as the environmental impact of these techniques is unknown at this time. CO2 could be injected into deep ocean strata with known techniques, but the impacts of this technique are also unknown.
Terrestrial Sequestration and Advanced Chemical and Biological Approaches are also being investigated. Vegetation and soils are carbon storage sinks. Among the areas being studied are determination of what crops and trees are most effective in storing carbon, with an emphasis on increasing long-lived soil carbon. Storing it as magnesium carbonate or as CO2 clathrate which are stable solid forms that are much more dense that gaseous CO2 would permit safe and compact storage. Either may also have some value as a commercial product.
Last modified 9/27/05