CO2 capture and storage (CCS) is a process whereby the CO2 produced by point sources, such as industry and power stations, is separated and transported to a storage point suitable to keep the CO2 out of the atmosphere in the long term (centuries or millennia). As a strategy, it has great potential to reduce global CO2 emissions, though realizing this potential depends on further development of CCS technologies and the transfer of this technology to developing economies such as China and India.
Assuming fossil fuels will continue to dominate our sources of energy over the coming decades, CCS provides a way in which such continued widespread utilization of fossil fuels can occur without a burgeoning growth in global CO2 emissions. The principle is an established one: CO2 is collected from a strong point source like a power station, separated, compressed and then stored in geological formations (in the oceans), fixed within inorganic carbonates by reaction with metal oxides or used in industrial processes as discussed by Aresta and Dibenedetto (Chapter 7, this volume).
There are several ways in which CO2 is separated, including postcombustion, precombustion and what is termed oxyfuel combustion. The capture of CO2 after burning of fossil fuels already occurs in a number of power stations - CO2 being collected from the flue gases before they are emitted to the atmosphere. Precombustion capture is used in the gas industry - separation of CO2from natural gas before being used. It is also used in the manufacture of fertilizers and in the production of hydrogen. Such precombus-tion capture is generally easier given the higher concentrations of CO2 involved compared to those in flue gases. The use of oxy-fuel combustion is still being developed, but in essence it involves the burning of fossil fuels in high-purity oxygen to produce CO2 concentrations in the exhaust gases that are much higher than normal, thus allowing easier CO2 separation.
Current technology is able to capture ~85-95% of the CO2 processed in the capture plant, though this process itself requires a significant amount of energy. A power plant using a CCS system would require 10-40% more energy than a plant without CCS, and so there is a considerable extra cost involved, in terms of both the money and the extra CO2 produced. Overall, a power plant with CCS can be expected to reduce emissions by ~80-90% compared to a non-CCS plant. Where the captured CO2 is stored in inorganic carbonates, rather than transported to a suitable geological or oceanic storage site, the plant would require 60-180% more energy than a non-CCS plant (IPCC, 2005).
The transport of captured CO2 to the storage sites is usually by a pipeline, especially where the distances involved are less than 1000 km. For greater distances, the transport of captured CO2 by ships has been suggested as a more cost-effective alternative.
Storage within geological formations can utilize depleted oil and gas fields, as well as unminable coal beds and deep saline aquifers. Much of the technology for using depleted oil and gas fields has already been developed by energy companies, who have used it as a strategy to increase yield rates from existing reservoirs - called enhanced recovery. Saline aquifers have also been shown to represent an economi cally viable 'sink' for captured CO2, which is injected into subterranean aquifers where it dissolves and is held in place by impermeable rock layers above. The full potential for storage in unminable coal beds (coal beds that are too deep or too thin) is still to be proven - in theory even relatively shallow coal beds could have CO2 pumped into them, which is then absorbed by the coal.
Clearly, a key property of any geological storage site is that it can contain a large amount of CO2 and keep it there without leakage for centuries or millennia. In general, the existence of an impermeable rock layer above the storage site is required, with the CO2 normally being injected to depths greater than 800 m. At these depths the pressure is great enough to compress the CO2 to an almost liquid state, thus helping to maximize storage and reduce chances of leakage.
Industrial use of CCS is already underway, with three large-scale storage projects in operation: (i) the Sleipner project making use of an offshore saline aquifer off the coast of Norway; (ii) the In Salah project, using a gas field in Algeria; and (iii) the Weyburn project in Canada. This latter CCS project - the Weyburn-Midale CO2 Monitoring and Storage Project (Fig. 16.1) - is located in Saskatchewan, and has been in operation since 2000. The project has successfully demonstrated that the site is suitable for long-term storage of CO2. Results from Weyburn-Midale indicate that trapped CO2 would not escape to groundwater or the surface in the next 5000 years.
The storage of CO2 in the oceans can be achieved in two ways. First, it can be done by injection into the water at depths greater than 1000 m, using a fixed pipeline or ship. This method is unlikely to result in truly long-term storage, as the CO2 will dissolve in the water (dissolution) and will eventually be released back into the atmosphere from the ocean surface. There are also concerns that such large and concentrated injections of CO2 would cause significant reductions in water pH and hence have detrimental impacts on marine life in the vicinity. Secondly, there is the option of injection right onto the seabed at depths greater than 3000 m. Using this strategy, the CO2 may form a lake on the seabed and this would help slow the rate at which the CO2 dissolves into the surrounding water. Consequently, such deep injection of CO2 may help to prolong the lifetime of this 'sink'.
The fixation of captured CO2 with metal oxides to produce stable carbonates is an option that is still being developed. Ordinarily, the reaction rates are very slow and so pretreat-ment of the metal oxides is required - currently a very energy-intensive process.
Overall, the potential of CCS as a strategy to mitigate climate change through stabilization of atmospheric CO2 concentrations is huge. In particular, geological storage appears to represent a great opportunity - many of the large point sources of CO2 around the world are located within 300 km of geological formations, which, in theory at least, have potential for CO2 storage. Projections indicate that by 2050 ~20-40% of global fossil fuel CO2 emissions could be suitable for CCS, covering up to 60% of CO2 emissions from electricity generation and up to 40% of those from industry. Given an aim of stabilizing CO2 concentrations in the atmosphere at 450-750 ppm during the 21st century, CCS may provide 15-55% of the global mitigation effort required to meet this aim.
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