27-11-2012, 01:29 PM
Carbon Capture and Storage
Carbon Capture.ppt (Size: 5.85 MB / Downloads: 355)
Carbon Sequestration
Carbon capture and storage (CCS) is an approach to mitigating global warming by capturing carbon dioxide (CO2) from large point sources such as power plants and subsequently storing it instead of releasing it into the atmosphere. Technology for capturing of CO2 is already commercially available for large CO2 emitters, such as power plants; however, capture is meaningless without storage. Storage of CO2, on the other hand, is a relatively untried concept and as yet (2007) no power plant operates with a full carbon capture and storage system.
CO2 storage
Various forms have been conceived for permanent storage of CO2. These forms include gaseous storage in various deep geological formations (including saline formations and exhausted gas fields), liquid storage in the ocean, and solid storage by reaction of CO2 with metal oxides to produce stable carbonates
Geological storage
Also known as geo-sequestration, this method involves injecting carbon dioxide directly into underground geological formations. Oil fields, gas fields, saline formations, unminable coal seams, and saline-filled basalt formations have been suggested as storage sites. Here, various physical (e.g., highly impermeable caprock) and geochemical trapping mechanisms would prevent the CO2 from escaping to the surface. CO2 is sometimes injected into declining oil fields to increase oil recovery (enhanced oil recovery). This option is attractive because the storage costs are offset by the sale of additional oil that is recovered. Disadvantages of old oil fields are their geographic distribution and their limited capacity.
Ocean storage
Two main concepts exist. The 'dissolution' type injects CO2 by ship or pipeline into the water column at depths of 1000 m or more, and the CO2 subsequently dissolves. The 'lake' type deposits CO2 directly onto the sea floor at depths greater than 3000 m, where CO2 is denser than water and is expected to form a 'lake' that would delay dissolution of CO2 into the environment.
CCS v non-CCS
CCS applied to a modern conventional power plant could reduce CO2 emissions to the atmosphere by approximately 80-90% compared to a plant without CCS. Capturing and compressing CO2 requires much energy and would increase the fuel needs of a plant with CCS by about 10-40%. These and other system costs are estimated to increase the cost of energy from a power plant with CCS by 30-60% depending on the specific circumstances.
Storage of the CO2 is envisaged either in deep geological formations, deep oceans, or in the form of mineral carbonates. In the case of deep ocean storage, there is a risk of greatly increasing the problem of ocean acidification, a problem that also stems from the excess of carbon dioxide already in the atmosphere and oceans.
Geological formations are currently considered the most promising sequestration sites, and these are estimated to have a storage capacity of at least 2000 Gt CO2. IPCC estimates that the economic potential of CCS could be between 10% and 55% of the total carbon mitigation effort until year 2100.
How does CO2 affect oceans?
About half of the extra CO2 from the atmosphere will dissolve in the oceans, making the water more acidic. The diagram shows how acidic the oceans will become in the future, upto the year 3000. To work this out, it was necessary to:
predict how CO2 emissions will change in the future (the top of the diagram)
calculate how this will change the amount of CO2 in the atmosphere (middle part of the diagram)
finally work out how acidic the oceans will become (bottom part of the diagram)
The acidity is shown as a change in pH units. The effects of this change on marine life is unknown, but could be disastrous.
Cost of CCS
Capturing and compressing CO2 requires much energy, significantly raising the running costs of CCS-equipped power plants. In addition there are added investment or capital costs. The process would increase the energy needs of a plant with CCS by about 10-40%. The costs of storage and other system costs are estimated to increase the costs of energy from a power plant with CCS by 30-60%, depending on the specific circumstances.
Safety smarts
The goal of carbon sequestration is to store the carbon dioxide permanently, i.e. very, very long-term, geological time periods.
The greatest concern surrounding carbon dioxide storage is the potential for it to leak. The obvious worry is that leakage would lead to more global warming, defeating the purpose of storage in the first place. But studies have shown that leakage, if it happened at all, would be insignificant. The IPCC reported that 99% retention of the carbon dioxide that is stored would be ''very likely'' over 100 years and ''likely'' over 1,000 years. If done right, selecting the site correctly and monitor, it can be near permanent.