INTERMEDIATE/SECONDARY Article: Carbon Sequestration

Carbon dioxide (CO2) is a colorless, odorless gas that is produced naturally when humans and animals breathe. The primary source of man-made CO2 emissions, however, is the burning of fossil fuels (oil, natural gas and coal) for energy production. Carbon dioxide is essential to the photosynthesis process that sustains plant and animal life, but it can accumulate in the air and trap heat near the earth's surface, a phenomenon known as the greenhouse effect. Learn more about the greenhouse effect and greenhouse gases from the brochure Greenhouse Gases, Climate Change, and Energy.

Fossil fuels will be a major source of energy production for generations to come. The availability of these fuels to provide clean, affordable energy is essential for global prosperity and security. Nevertheless, atmospheric concentrations of CO2 due to carbon emissions will continue to rise unless major changes are made in the way we produce and use energy and, especially, how we manage carbon. One approach to controlling carbon emissions is carbon sequestration.

Carbon sequestration is the capture and long-term storage of carbon dioxide and other greenhouse gases that would otherwise be emitted into the atmosphere. The greenhouse gases can be captured at the point of emission (direct sequestration), or removed from the air (indirect sequestration). The captured gases can be stored in underground reservoirs (geological sequestration), injected deep into oceans (ocean sequestration), stored in vegetation and soils (terrestrial sequestration), or converted to rock-like solid materials (advanced concepts).

Geological Sequestration involves storing carbon dioxide in depleted oil and gas reservoirs, unmineable coal seams and underground saline formations.

Depleted Oil and Gas Reservoirs - Injecting compressed carbon dioxide into a depleted oilfield creates a CO2 “flood” that forces the remaining oil into a well where it can be captured; the CO2 remains behind, safely and permanently stored beneath the earth’s surface.

Unmineable Coal Seams - Coal beds typically contain large amounts of methane-rich gas. The current way to recover coal bed methane is to depressurize the bed, usually by pumping water out of the reservoir. An alternative approach is to inject pressurized carbon dioxide into the bed. The porous coal surfaces absorb CO2 more easily than methane, so the CO2 displaces the methane and remains sequestered in the bed.

Underground Saline Formations - Carbon dioxide can also be pumped into underground saline, or brine, formations. Underground brine formations are so common that geologists believe they could provide enough space to store all the CO2 captured from fossil fuels burned in the 21st century.

Ocean Sequestration involves enhancing the natural process of carbon sequestration in the ocean and directly injecting CO2 deep into the ocean. Carbon dioxide is soluble in sea water and oceans absorb and emit huge amounts of CO2 into the atmosphere naturally. Ocean sequestration, however, is considered controversial.

Enhancement of Natural Carbon Sequestration - To stimulate the growth of phytoplankton, which consume great amounts of CO2, scientists add nutrients to ocean surface waters. When phytoplankton use up the carbon in ocean surface waters, it is replaced naturally by CO2 drawn from the atmosphere.

Direct Injection of CO2 - Technology exists for the direct injection of CO2 into deep areas of the ocean, but scientists do not yet have enough knowledge to determine the effectiveness of the sequestration or understand potential changes it may cause to the ocean, such as the effect of CO2 on the acidity of the water.

Terrestrial Sequestration involves the removal of CO2 from the atmosphere in terrestrial ecosystems. Vegetation and soils are widely recognized as carbon storage sinks. The global biosphere absorbs about two billion tons of carbon annually, an amount equal to one third of all global carbon emissions from human activity. Significant amounts of this carbon remain stored in the roots of certain plants and in the soil. Ecosystems with significant opportunities for carbon sequestration include forests, agricultural lands such as crop land, grassland and range land, biomass crop lands, deserts and degraded lands, boreal wetlands and peat lands.

Advanced Sequestration Concepts seek to reduce the cost and energy required to chemically and/or biologically convert CO2 into commercial products and stable solid compounds.

Scientists are working with minerals that could change huge amounts of carbon dioxide gas into a compact, solid state. Two promising chemical pathways are magnesium carbonate and CO2 clathrate, an ice-like material. Scientists believe the global carbon emissions from an entire year could be contained as magnesium carbonate in a space 10 kilometers by 10 kilometers by 150 meters.

Biological systems research is under development to enhance the carbon uptake of photosynthetic systems. Also, harnessing naturally occurring, non-photosynthetic microbiological processes capable of converting CO2 into useful forms, such as methane and acetate, could represent a technology breakthrough.

Scientists continue to research and develop carbon sequestration technologies. It is important to make sure these new processes are environmentally acceptable and safe. For example, scientists must determine that CO2 will not escape from underground formations and migrate up to the earth's surface or contaminate drinking water supplies. Carbon capture and storage is an expanding area of research and development for today’s scientists.

Information from the U.S. Department of Energy - Office of Fossil Energy.