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Carbon Capture & Storage Capture Ready CCS Projects
How does CCS Work
CCS History
CCS Projects
 
How does CCS Work

Carbon dioxide, as the most important greenhouse gas, is produced by all animals, plants, microorganisms and fungi during respiration. It is generated as a by-product of the combustion of fossil fuels or the burning of vegetable matter among other chemical processes. A great amount of CO2 is also emitted from volcanoes and other geothermal processes such as hot springs and geysers.

Carbon Capture and Storage, also known as Carbon Capture and Sequestration, refers to a process involving the capture and separation of CO2 emitted from large-scale fixed sources, transport to a storage site and long-term isolation from the atmosphere. In general, CCS system consists of the following three stages:
■ Capture and separation
■ Transportation
■ Injection and storage

Carbon Dioxide Capture Technologies & Cost

There are four basic technologies for capturing man-made CO2, consisting of post-combustion capture, pre-combustion capture, oxy-fuel combustion capture, and industrial separation.

Pre-combustion capture is mainly used at power plants that employ IGCC (Integrated Gasification Combined Cycle) technology, involves reacting a fuel with oxygen or air (or steam) to give mainly a syngas or fuel gas composed of carbon monoxide and hydrogen. The carbon monoxide is reacted with steam in a catalytic reactor to give CO2 and more hydrogen. CO2 is then separated usually by a chemical or physical absorption process, resulting in a hydrogen-rich fuel that can be used in many applications. This technology, with a small capture system, low energy consumption, and huge potential in efficiency and pollutants control, has aroused wide concern. However, IGCC power technology is still faced with issues of particularly high cost of investment, insufficient liability, etc. 

Capture of carbon dioxide from flue gases produced by combustion of fossil fuels is referred to as post-combustion capture. Chemical absorption (absorb by acidity and alkaline) and physical absorption (temperature swing adsorption or pressure swing adsorption) processes are frequently used for CO2 separation, and membrane separation technique is still at its development stage, although widely accepted as a technique with hugh potential in the aspect of energy consumption and equipment compactedness. Theoretically speaking, post-combustion capture is applicable to all types of thermal power plants. However, ordinary flue gases have low pressure, large volume, low carbon dioxide concentration, as well as large amount of N2, therefore leading to huge capture system and great amount of energy consumption.

The oxy-fuel combustion process eliminates nitrogen from the flue gas by combusting a hydrocarbon or carbonaceous fuel in either pure oxygen or a mixture of pure oxygen and a CO2-rich recycled flue gas. Currentlyn oxy-fuel combustion capture has been applied to certain retrofitted small power plants in Europe. The toughest problem facing this technology lies in the high investment and large energy consumption of oxygen producing.

The capture costs of CO2 vary widely mainly because of different technical factors related to technology option, plant design and operation such as plant size, fuel property, net efficiency and load factor,  CO2 transport distance, final burial or end-use type, as well as economic factors such as fuel cost and interest rates.

Costs will vary with the choice of CO2 capture technology and the type of power plant or industrial process that generates the original CO2 emissions. The choice of technology, time frame and assumed rate of cost improvements can make a great difference in the estimation of future CO2 capture cost.

Transport of Carbon Dioxide

Except when plant is located directly above a geological storage site, collected CO2 must be transported from the point of capture to a storage site.

Pipelines currently operate as a mature market technology and act as the most common method for CO2 transport.

The use of ships for CO2 transport across the sea is currently at an embryonic stage. The low temperature type, pressurised type and semi-refrigerated type are three types of containment system for liquid CO2 transport ships.

Tanker Transport of CO2

Carbon Dioxide Storage

Various methods have been conceived for the permanent storage ('sequestration') of carbon dioxide, including gaseous storage in various deep porous geological formations, liquid storage in the deep ocean, and solid storage by reaction of CO2 with metal oxides to produce stable carbonates or conversion to solid charcoal. The gaseous storage methods can be combined with Enhanced Oil/Gas Recovery (EOR/EGR) in existing oil and gas fields to generate useful revenue to partly offset the CO2 burial costs. In fact, CO2 from a variety of sources has been routinely collected, transported and purchased for this purpose as a commercial commodity in several parts of the USA for about 20 years.

Geologic storage of carbon dioxide means the permanent deep underground disposal of CO2. There are a large number of sedimentary areas worldwide suitable for CO2 storage, and suitable sites are identified by proven methods similar to those used to find oil and gas fields. 

Ocean storage has not yet been implemented or demonstrated even at a pilot scale, and is still at research stage.

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