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Reducing carbon dioxide emissions from coal-fired power plants

The leading technology for significantly reducing the CO₂ emissions from coal-fired power plants is known as Carbon capture and sequestration (CCS). It is currently (2008) regarded as the technology which could significantly reduce coal-fired power plant carbon dioxide emissions while also allowing the use of the Earth's abundant coal resources to provide the increasing global need for energy. However, CCS technology is still in development and it is not expected to be ready for widespread commercial implementation on a large scale until about 2020.

It involves capturing the carbon dioxide produced by the combustion of coal and storing it in deep ocean areas or in underground geological structures deep within the Earth's upper crust. The capture of the carbon dioxide from the coal combustion flue gases can be accomplished by using absorbents such as amines (see Amine gas treating). The carbon dioxide is then compressed into a supercritical fluid at about 150 atmospheres (15 MPA), dehydrated and transported to the storage sites for injection into the underground or undersea reservoirs. Compressing the carbon dioxide into a supercritical fluid greatly increases its density which greatly reduces its volume as compared to transporting and storing the carbon dioxide as a gas.

Since the current global emissions of carbon dioxide from all energy supply sources is 28 Gt per year, the scale of carbon dioxide storage required to make a major difference in those emissions is massive. For example, based on a carbon dioxide emission factor of 1 kg per kWh, 570 coal-fired plants, each producing 1000 MW of electricity, would emit about 5 Gt per year of carbon dioxide into the atmosphere. Storing 5 Gt per year of carbon dioxide requires injection of about 65 million barrels per day (about 10 x 10⁶ cubic meters per day) of supercritical carbon dioxide.

The worldwide capacity for storing carbon dioxide in depleted natural gas and crude oil production fields and in unmineable deep coal seams has been estimated as about 1000 Gt which is equivalent to 140 years of the 7 Gt emissions (in 2005) from the worldwide total coal-fired power generation. In addition, the worldwide capacity in deep saline formations has been estimated as 1000 to 10,000 Gt. There are currently four commercial sequestration sites in operation:

• The Weyburn-Midale facilty in Saskatchewan, Canada constructed by a consortium of oil companies, research organizations and others
• The Sleipner facilty in offshore Norway constructed by StatoilHydro
• The Snøvit facility in the Barents Sea constructed by StatoilHydro
• The Salah facility in Algeria constructed by StatoilHydro

There are many other sequestration sites currently in planning, development or construction.

No matter what governmental regulations are eventually adopted to mitigate the carbon dioxide emissions from coal-powered power plants (or other processes involving the combustion of substances containing carbon), there must be successful, integrated large-scale demonstration of the technical, environmental and economic aspects of the major components of a CCS sytem, namely carbon dioxide capture, transportation and storage. Such an integrated demonstration must also provide a definition of regulatory protocols for sequestration projects including site selection, injection operation, and eventual transfer of custody to public authorities after a period of successful operation.