Schematic diagram of possible CCS systems (Image: IPCC)
The possibility of capturing carbon dioxide greenhouse gas (CO2), an 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. 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 that doesn't have the technology to catch carbon.
How do we catch carbon?
Researrchers have found several ways to capturing CO2. We can catch it after burning fuel, we can catch it before the fuel is burned, or we can burn fuel in ways that make the carbon easy to catch.
- Catch it after burning fuel: In a post-combustion method, the CO2 is removed after the fossil fuel is burned. This is the method that would need to be used at power plants that burn natural gas or coal. About 15% of the gases produced as these fuels are burned is CO2. To adapt power plants to cartch carbon dioxide, absorption towers would need to replace smokestacks. The towers would take the CO2 out using chemicals called amines. Another tower would take the CO2 out of the chemicals so that they could be used again.
- Catch it before burning fuel: If fossil fuel is oxidized before it’s burned to make syngas, which is carbon oxides and hydrogen, then the carbon can be pulled off while the hydrogen is burned as fuel.
- Make carbon easy to catch: Fossil fuels burned in pure oxygen instead of air produce exhaust that is mostly CO2 and water vapor. The water vapor condenses leaving almost pure CO2 that can be 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. Researchers are developing methods for permanent storage of CO2 such as storing the gas deep underground, storing it as liquid in the ocean, and turrning CO2 into carbonate minerals throuugh chemical reactions with metal oxides.
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.