CO2 Capture and Storage

Carbon dioxide (CO2) capture and storage (CCS) is a process consisting of the separation of CO2 from industrial and energy-related sources, transport to a storage location and long-term isolation from the atmosphere. This report considers CCS as an option in the portfolio of mitigation actions for stabilization of atmospheric greenhouse gas concentrations. CCS has the potential to reduce overall mitigation costs and increase flexibility in achieving greenhouse gas emission reductions. The widespread application of CCS would depend on technical maturity, costs, overall potential, diffusion and transfer of the technology to developing countries and their capacity to apply the technology, regulatory aspects, environmental issues and public perception. Most models also indicate that known technological options could achieve a broad range of atmospheric stabilization levels but that implementation would require socio-economic and institutional changes.

The net reduction of emissions to the atmosphere through CCS depends on the fraction of CO2 captured, the increased CO2 production resulting from loss in overall efficiency of power plants or industrial processes due to the additional energy required for capture, transport and storage, any leakage from transport and the fraction of CO2 retained in storage over the long term. Available technology captures about 85–95% of the CO2 processed in a capture plant. A power plant equipped with a CCS system (with access to geological or ocean storage) would need roughly 10–40% more energy than a plant of equivalent output without CCS, of which most is for capture and compression. For secure storage, the net result is that a power plant with CCS could reduce CO2 emissions to the atmosphere by approximately 80–90% compared to a plant without CCS (see Figure SPM.2). To the extent that leakage might occur from a storage reservoir, the fraction retained is defined as the fraction of the cumulative amount of injected CO2 that is retained over a specified period of time. CCS systems with storage as mineral carbonates would need 60– 180% more energy than a plant of equivalent output without CCS.

What is the current status of CCS technology?
There are different types of CO2 capture systems: postcombustion, pre-combustion and oxyfuel combustion. The concentration of CO2 in the gas stream, the pressure of the gas stream and the fuel type (solid or gas) are important factors in selecting the capture system. Post-combustion capture of CO2 in power plants is economically feasible under specific conditions. It is used to capture CO2 from part of the flue gases from a number of existing power plants. Separation of CO2 in the natural gas processing industry, which uses similar technology, operates in a mature market . The technology required for pre-combustion capture is widely applied in fertilizer manufacturing and in hydrogen production. Although the initial fuel conversion steps of pre-combustion are more elaborate and costly, the higher concentrations of CO2 in the gas stream and the higher pressure make the separation easier. Oxyfuel combustion is in the demonstration phase and uses high purity oxygen. This results in high CO2 concentrations in the gas stream and, hence, in easier separation of CO2 and in increased energy requirements in the separation of oxygen from air.

Pipelines are preferred for transporting large amounts of CO2 for distances up to around 1,000 km. For amounts smaller than a few million tonnes of CO2 per year or for larger distances overseas, the use of ships, where applicable, could be economically more attractive. Pipeline transport of CO2 operates as a mature market technology (in the USA, over 2,500 km of pipelines transport more than 40 MtCO2 per year). In most gas pipelines, compressors at the upstream end drive the flow, but some pipelines need intermediate compressor stations. Dry CO2 is not corrosive to pipelines, even if the CO2 contains contaminants. Where the CO2 contains moisture, it is removed from the CO2 stream to prevent corrosion and to avoid the costs of constructing pipelines of corrosion-resistant material. Shipping of CO2, analogous to shipping of liquefied petroleum gases, is economically feasible under specific conditions but is currently carried out on a small scale due to limited demand. CO2 can also be carried by rail and road tankers, but it is unlikely that these could be attractive options for large-scale CO2 transportation.

Storage of CO2 in deep, onshore or offshore geological formations 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 specific conditions for oil and gas fields and saline formations, but not yet for storage in unminable coal beds If CO2 is injected into suitable saline formations or oil or gas fields, at depths below 800 m , various physical and geochemical trapping mechanisms would prevent it from migrating to the surface. At depths below 800–1,000 m, CO2 becomes supercritical and has a liquid-like density (about 500–800 kg m-3) that provides the potential for efficient utilization of underground storage space and improves storage security.

In general, an essential physical trapping mechanism is the presence of a caprock10. Coal bed storage may take place at shallower depths and relies on the adsorption of CO2 on the coal, but the technical feasibility largely depends on the permeability of the coal bed. The combination of CO2 storage with Enhanced Oil Recovery (EOR11) or, potentially, Enhanced Coal Bed Methane recovery (ECBM) could lead to additional revenues from the oil or gas recovery. Well-drilling technology, injection technology, computer simulation of storage reservoir performance and monitoring methods from existing applications are being developed further for utilization in the design and operation of geological storage projects.
Ocean storage potentially could be done in two ways: by injecting and dissolving CO2 into the water column (typically below 1,000 meters) via a fixed pipeline or a moving ship, or by depositing it via a fixed pipeline or an offshore platform onto the sea floor at depths below 3,000 m, where CO2 is denser than water and is expected to form a “lake” that would delay dissolution of CO2 into the surrounding environment. Ocean storage and its ecological impacts are still in the research phase.

The reaction of CO2 with metal oxides, which are abundant in silicate minerals and available in small quantities in waste streams, produces stable carbonates. The technology is currently in the research stage, but certain applications in using waste streams are in the demonstration phase.

Industrial uses14 of captured CO2 as a gas or liquid or as a feedstock in chemical processes that produce valuable carbon-containing products are possible, but are not expected to contribute to significant abatement of CO2 emissions.

Components of CCS are in various stages of development. Complete CCS systems can be assembled from existing technologies that are mature or economically feasible under specific conditions, although the state of development of the overall system may be less than some of its separate components.


IPCC - http://www.ukcip.org.uk/climate_change
/natural_manmade.asp