कार्बन सीक्वेशट्रेशन (पृथक्‍करण) प्रक्रिया  

Carbon capture and storage (CCS) (or carbon capture and sequestration) is the process of capturing waste carbon dioxide (CO₂) from large point sources, such as fossil fuel power plants, transporting it to a storage site, and depositing it where it will not enter the atmosphere, normally an underground geological formation.

The aim is to prevent the release of large quantities of CO₂ into the atmosphere. It is a potential means of mitigating the contribution of fossil fuel emissions to global warming and ocean acidification. Although CO₂ has been injected into geological formations for several decades for various purposes, including enhanced oil recovery.

Capturing and compressing CO₂ may increase the fuel needs of a coalfired CCS plant by 25-40%. These and other system costs are estimated to increase the cost of the energy produced by 21-91% for purpose built plants. Applying the technology to existing plants would be more expensive especially if they are far from a sequestration site.

Capture CO₂

Capturing CO₂ is probably most effective at point sources, such as large fossil fuel or biomass energy facilities, industries with major CO₂ emissions, natural gas processing, synthetic fuel plants and fossil fuelbased hydrogen production plants. Extraction (recovery) from air is possible, but not very practical. The CO₂ concentration drops rapidly moving away from the point source.

The lower concentration increases the amount of mass flow that must be processed (per tone of carbon dioxide extracted). Concentrated CO₂ from the combustion of coal in oxygen is relatively pure, and could be directly processed.

Impurities in CO₂ streams could have a significant effect on their phase behavior and could pose a significant threat of increased corrosion of pipeline and well materials.

Post combustion capture

The CO₂ is removed after combustion of the fossil fuel this is the scheme that would be applied to fossil fuel burning power plants. Here, carbon dioxide is captured from flue gases at power stations or other large point sources.

The technology is well understood and is currently used in other industrial applications, although not at the same scale as might be required in a commercial scale power station.

Precombustion capture

It is widely applied in fertilizer, chemical, gaseous fuel (H₂,CH₄), and power production.In these cases, the fossil fuel is partially oxidized, for instance in a gasifier. The resulting syngas (CO and H₂) is shifted into CO₂ and H₂. The resulting CO₂ can becaptured from a relatively pure exhaust stream.

The H₂ can now be used as fuel; the carbon dioxide is removed before combustion takes place. There are several advantages and disadvantages when compared to conventional post combustion carbon dioxide capture. The CO₂ is removed after combustion of fossil fuels, but before the flue gas is expanded to atmospheric pressure. This scheme is applied to new fossil fuel burning power plants, or to existing plants where repowering is an option.

The capture before expansion, i.e. from pressurized gas, is standard in almost all industrial CO₂ capture processes, at the same scale aswill be required for utility power plants.

Oxyfuel combustion

The fuel is burned in oxygen instead of air. To limit the resulting flame temperatures to levels common during conventional combustion, cooled flue gas is re-circulated and injected into the combustion chamber. The flue gas consists of mainly carbon dioxide and water vapour, the latter of which is condensed through cooling.

The result is an almost pure carbon dioxide stream that can be transported to the sequestration site and stored. Power plant processes based on oxyfuel combustion are sometimes referred to as "zero emission" cycles, because the CO₂ stored is not a fraction removed from the flue gas stream (as in the cases of pre and post combustion capture) but the flue gas stream itself.

A certain fraction of the CO₂ generated during combustion will inevitably end up in the condensed water. To warrant the label "zero emission" the water would thus have to be treated or disposed of appropriately. The technique is promising, but the initial air separation step demands a lot of energy.

Transport CO₂

After capture, the CO₂ would have to be transported to suitable storage sites. This is done by pipeline, which is generally the cheapest form of transport. In 2008, there were approximately 5,800 km of CO₂ pipelines in the United States, used to transport CO₂ to oil production fields where it is then injected into older fields to extract oil.

The injection of CO₂ to produce oil is generally called Enhanced Oil Recovery or EOR. In addition, there are several pilot programs in various stages to test the longterm storage of CO₂ in nonoil producing geologic formations.

Ships could also be utilized for transport where pipelines are not feasible. These methods are currentlyused for transporting CO₂ for other applications.

Sequestration

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), 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, generally in supercritical form, directly into underground geological formations. Oil fields, gas fields, saline formations, un-mineable coal seams, and salinefilled basalt formations have been suggested as storage sites. 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. Approximately 30 to 50 million metric tonnes of CO₂ are injected annually in the United States into declining oil fields. This option is attractive because the geology of hydrocarbon reservoirs is generally well understood and 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 the fact that subsequent burning of the additional oil recovered will offset much or all of the reduction in CO₂ emissions.

Enhanced oil recovery

Enhanced oil recovery (EOR) is a generic term for techniques used to increase the amount of crude oil that can be extracted from an oil field. In Carbon Capture & Sequestration Enhanced Oil Recovery (CCS EOR), carbon dioxide is injected into an oil field to recover oil that is often never recovered using more traditional methods.

Crude oil development and production in U.S. oil reservoirs can include up to three distinct phases: primary, secondary, and tertiary (or enhanced) recovery. During primary recovery only about 10% of a reservoir's original oil in place is typically produced. Secondary recovery techniques extend a field's productive life generally by injecting water or gas to displace oil and drive it to a production well bore, resulting in the recovery of 20 to 40% of the original oil in place.

However, with much of the easy to produce oil already recovered from U.S. oil fields, producers have attempted several tertiary, or enhanced oil recovery (EOR), techniques that offer prospects for ultimately producing 30 to 60%, or more, of the reservoir's original oil in place.

Ocean storage

In the past, it was suggested that CO₂ could be stored in the oceans, but this would only exacerbate ocean acidification and has been made illegal under specific regulations. Ocean storage is no longer considered feasible.

Mineral storage

In this process, CO₂ is exothermically reacted with available metal oxides, which in turn produces stable carbonates (e.g. calcite, magnesite). This process occurs naturally over many years and is responsible for a great amount of surface limestone. The IPCC estimates that a power plant equipped with CCS using mineral storage will need 60–180% more energy than a power plant without CCS.

The economics of mineral carbonation at scale are now being tested in a worldfirst pilot plant project based in Newcastle, Australia. New techniques for mineral activation and reaction have been developed the GreenMag Group and the University of Newcastle and funded by the New South Wales and Australian Governments to be operational by 2013.

Leakage

A major concern with CCS is whether leakage of stored CO₂ will compromise CCS as a climate change mitigation option. For well selected, designed and managed geological storage sites, IPCC estimates that risks are comparable to those associated with current hydrocarbon activity. Although some question this assumption as arbitrary citing a lack of experience in such long term storage.

CO₂ could be trapped for millions of years, and although some leakage occurs upwards through the soil, well selected storage sites are likely to retain over 99% of the injected CO₂ over 1000 years. Leakage through the injection pipe is a greater risk. Although the injection pipe is usually protected with nonreturn valves to prevent release on a power outage, there is still a risk that the pipe itself could tear and leak due to the pressure.

The Berkel en Rodenrijs incident in December 2008 was an example, where a modest release of CO₂ from a pipeline under a bridge resulted in the deaths of some ducks sheltering there. In order to measure accidental carbon releases more accurately and decrease the risk of fatalities through this type of leakage, the implementation of CO₂ alert meters around the project perimeter has been proposed.

Malfunction of a carbon dioxide industrial fire suppression system in a large warehouse released CO₂ and 14 citizens collapsed on the nearby public road. A release of CO₂ from a salt mine killed a person at distance of 300 meters.

Carbon Capture and Storage and the Kyoto Protocol

One way to finance future CCS projects could be through the Clean Development Mechanism of the Kyoto Protocol. At COP16 in 2010, The Subsidiary Body for Scientific and Technological Advice, at its thirtythird session, issued a draft document recommending the inclusion of Carbon dioxide capture and storage in geological formations in Clean Development Mechanism project activities. At COP17 in Durban, a final agreement was reached enabling CCS projects to receive support through the Clean Development Mechanism.

References

1. IPCC Special Report Carbon Dioxide Capture and Storage Summary for Policymakers(PDF). Intergovernmental Panel on Climate Change. Retrieved 2011-10-05.
2. Introduction to Carbon Capture and Storage - Carbon storage and ocean acidification activity. Commonwealth Scientific and Industrial Research Organisation (CSIRO) and the Global CCS Institute. Retrieved 2013-07-03

Authors

Durga C^*

Department of Agronomy (Water Management) PJTSAU,

College of Agriculture, Rajendranagar,

Hyderabad–500 030, Telangana, India

* Email: This email address is being protected from spambots. You need JavaScript enabled to view it.