DOE Carbon Storage

What is carbon capture and storage? Often touted as a solution to the growing problem of irreversible climate change, carbon capture — also known as CCS or carbon sequestration — is a group of technologies that aims to capture carbon dioxide at the places where this greenhouse gas is most often produced: power stations, industrial sites or even directly from the air. The captured carbon is then stored underground indefinitely, preventing it from having an effect on climate.

According to the U.S. Department of Energy, which touts carbon sequestration as a potential way to continue utilizing fossil fuels for energy while dampening the effects of these fuels on climate, these technologies could help manage emission levels, enhance energy security and benefit the nation economically by developing the technologies here in the U.S. and then selling the equipment and knowledge to other countries. Other nations around the world, including the United Kingdom, agree.

Some environmentalists, however, argue that carbon sequestration is based upon a fallacy: the idea that coal can ever be 'clean'. Focusing on carbon sequestration, they say, would prolong reliance on fossil fuels and slow progress in renewable energy technologies. On the opposite end of the spectrum, critics who don't believe that global warming is really happening or caused by human activity worry about 'unnecessary' costs associated with the technologies.

How carbon capture works

The majority of man-made carbon dioxide emissions come from fossil fuel-powered energy plants, and it is these plants that are the primary focus of carbon capture and storage. Carbon dioxide is stripped from fossil fuels either before or after they're burned for energy. In post-combustion systems, the flue gas produced by combustion of the fuel is injected into a liquid that selectively absorbs carbon dioxide; nearly pure carbon dioxide can then be released from the liquid by heating it. In pre-combustion systems, the fuel is first heated to convert it to a gas containing mainly hydrogen and carbon dioxide, which can be separated out. The hydrogen can then be used for energy or heat production.

The carbon dioxide is then piped back into the depths of the Earth, at least 800 meters from the surface. At this depth, the pressure of the earth keeps the carbon dioxide in liquid form where it can remain for thousands or possibly millions of years.

While more research is needed to determine the safest places to store carbon dioxide, researchers believe that depleted oil and gas reservoirs are the most practical locations. Other possibilities include deep saline aquifers and deep coal seams.

Economic challenges

The processes needed to capture and store carbon dioxide have been in use for years, but to put them to work on a large enough scale to combat climate change, a significant investment must be made in both existing and new fossil fuel-powered energy plants. Depending on the plant's design, adding carbon capture systems can increase the cost of energy production by 35 to 85 percent; this cost would theoretically be passed on to consumers.

Various studies have disagreed on whether carbon capture and storage systems are cost-effective. Capturing and storing carbon increases the energy needs of power plants, raising operating costs. Some of these costs could be offset by finding a use for the stored carbon dioxide, but such options have not yet been explored.

Further advances in CCS technologies could potentially reduce these costs, however. A power plant in China equipped with a CCS system reportedly scrubs 120,000 tons of carbon dioxide a year from 3 percent of the facility's flue gases at a cost of U.S. $30-$35, compared to the $100 estimated for first-generation projects to retrofit existing power plants in the U.S. and Europe.

Health, safety and environmental issues

In addition to high costs, the safety and effectiveness of carbon capture and storage is called into question by many critics. If stored carbon dioxide leaks back to the surface, it could potentially put human and animal lives in danger and lead to delayed climate change. In Cameroon in 1986, a natural carbon dioxide leak killed 1,700 people and thousands of animals. Such leaks could become more common if power plants rely on carbon storage to continue burning fossil fuels. Many geologists and engineers, however, argue that leakage of properly stored and managed carbon dioxide is unlikely.

Carbon capture and storage plays a major role in "clean coal," a coal industry campaign to improve the fossil fuel's reputation as a major force behind anthropogenic climate change. Advocates claim that CCS technologies can make coal a viable, environmentally friendly energy option well into the future, but critics — including both environmentalists and scientists — point out other detrimental aspects of coal-fired power plants. Coal mining destroys mountains and damages ecosystems, requiring millions of gallons of water each day. In addition to CO2, coal-fired power plants emit other pollutants such as mercury and particulates that are linked to a host of health problems.

Image: U.S. Department of Energy

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