Schematic diagram of possible CCS systems (Image: IPCC)
The possibility of capturing carbon dioxide greenhouse gas (CO2) has become an increasingly attractive idea, especially as people realize that it is unlikely we will stop using fossil fuels entirely in the next hundred years. This approach, known as carbon capture and storage (CCS), could help mitigate global warming.
The strategy is to trap CO2 where it is produced at power plants that burn fossil fuels and at factories so that the greenhouse gas isn’t spewed into the air. The captured CO2 would then be transported and stored or used in industrial processes. In a 2005 report, the Intergovernmental Panel on Climate Change estimated that catching carbon at a modern conventional power plant could reduce CO2 emissions to the atmosphere by approximately 80-90% compared to a plant without any mechanism to catch carbon.
How do we catch carbon?
There are three main approaches to capturing CO2. You can catch it after burning fuel, you can catch it before the fuel is burned, or you can burn fuel in such a way that the carbon is easy to catch.
- Catch it after burning fuel: In a post-combustion method, the CO2 is removed after combustion of the fossil fuel - this is the scheme that would be applied to conventional power plants. Here, carbon dioxide is captured from exhaust gases at power stations. The exhaust (or flue) gases are mostly nitrogen and water vapor; only about 15% of the exhaust is CO2. To adapt current power plants to this technology, absorption towers would replace smokestacks where the CO2 would be taken out of the emissions by chemicals called amines. A second tower, known as a stripping tower, would collect the CO2 from the chemicals so that they could be used again.
- Catch it before burning fuel: In one of the pre-combustion methods, fossil fuel is oxidized before it’s burned. This produces “syngas” which is made of carbon oxides and hydrogen. The resulting carbon emissions can be pulled off in a relatively pure stream while the hydrogen is burned as fuel. The carbon is widely applied in fertilizer, chemical, and gaseous fuel (H2, CH4) production.
- Make carbon easy to catch: In Oxy-fuel combustion, the fuel is burned in pure oxygen instead of air. The exhaust is mostly CO2 and water vapor. The water vapor condenses through cooling leaving an almost pure stream of CO2 that can be transported to the sequestration site and stored.
Where do we put it?
Once the carbon has been captured it must be stored. A typical 1,000-megawatt coal-fired power plant will generate approximately six million tons of CO2 each year of operation. Various methods have been conceived for permanent storage of CO2. These forms include gaseous storage in various deep geological formations, liquid storage in the ocean, and solid storage by reaction of CO2 with metal oxides to produce stable carbonates.
Trap it in rocks:
Storage of CO2 in rocks deep underground uses many of the same technologies that have been developed by the oil and gas industry and has been proven to be economically feasible under certain conditions. Carbon dioxide is injected underground, often into the same porous rocks in which oil and gas is found or into underground salt deposits or basalt rocks. The rocks must be capped by an overlying layer of impermeable rock to prevent the CO2 from escaping to the surface and into the air.
Today, CO2 is often injected into oil fields to increase oil recovery (a process known as enhanced oil recovery). This option is attractive because the storage costs may be partly offset by the sale of additional oil that is recovered. Disadvantages of old oil fields are their geographic distribution and their limited capacity, as well as that the subsequent burning of the recovered oil would emit CO2.
The main advantage of storing carbon dioxide in salt rock formations and saline aquifers is that these salty places have a large volume for storage and are common. But relatively little is known about them compared to oilfields. Unlike storage in oil fields or coal beds no side product will offset the storage cost. Leakage of CO2 back into the atmosphere may also be a problem in saline aquifer storage.
Trap it in the ocean:
Carbon dioxide could be injected into the deep ocean, over 1000 meters below the surface. There, the pressure is high enough that CO2 would dissolve. Alternatively, CO2 could be injected directly onto the sea floor at depths greater than 3000 m, where CO2 is denser than water. It is expected to form a “lake” at the bottom that would delay dissolution of CO2 into the environment. A third concept is to convert the CO2 to bicarbonates (using limestone) or hydrates.
The environmental effects of oceanic storage are poorly understood. Large concentrations of CO2 kill ocean organisms, but another problem is that the storage would not be permanent. Also, as CO2 dissolved in water forms carbonic acid, H2CO3, the acidity of the ocean water would increase. The resulting environmental effects on seafloor life forms are also poorly understood. Even though life appears to be rather sparse in the deep ocean, 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.
Trap it in minerals:
Carbon dioxide and metal oxides create minerals like limestone through a chemical reaction. This process happens slowly in nature, but the reaction rate could be sped up by heating the ingredients or putting them under pressure. But the heat and pressure would require more energy, which might be a bit counterproductive if that energy comes from fossil fuels. A power plant set up with a way to store CO2 in minerals would need 60-180% more energy than a power plant without. Researchers are still trying to find a way to make this process efficient.
Can We Bury Global Warming?
R. Socolow, Scientific American, 2005.
Carbon Dioxide Capture and Storage
IPCC Special Reports, 2005.