Capture

CO2 capture is a technology in which CO2 is separated from a mixture of several different gas components. Today there are thousands of large industrial plants worldwide emitting large volumes of CO2 into the atmosphere. If CO2 capture technology is used at these emission sources, then CO2 emissions will be almost eliminated.

CO₂ capture is a well known technology but no full-scale or industrial power plant has yet implemented the technology in order to reduce their CO₂ emissions. The reason for this is simple: CO₂ capture is expensive and it is cheaper to just continue emitting CO₂ into the atmosphere. Furthermore, CO₂ capture has only been demonstrated in small-scale test plants and not full-scale ones (i.e. the scale of a commercial coal power plant). Finally, infrastructure for transport and storage of CO₂ must be established prior to capture or there is no reason to capture CO₂.

In theory CO₂ can be captured from any source. However, it is only really practical at large stationary sources such as coal or gas power plants. CO₂ capture can also be used to remove CO₂ emissions from industrial plants, such as refineries and cement or steel factories.

The goal of CCS is to make sure that CO₂ is stored safely underground. However, it is not possible to take all of the flue gas from a coal power plant and store it underground. The CO₂ first needs to be separated from the flue gas; which is the process that we call CO₂ capture.

Flue gas from industrial sources such as coal power plants has a low CO₂ content, usually around 10 percent. The rest is composed of nitrogen, steam and small amounts of particles and other compounds. The total gas volume would be too large to inject underground and there are other components apart from CO₂ in the flue gas that are hazardous and therefore should not be stored underground. Therefore, CO₂ capture is absolutely necessary for storing CO₂ under the ground.

CO₂ capture is both technically and economically viable for large CO₂ sources. In general it is considered a good idea for plants emitting more than 100 000 tonnes of CO₂ annually. Globally, there are around 8000 industrial plants emitting above this level and in total these sources emit around half of all global man-made CO₂ emissions (reference 1).

The heaviest CO₂ emitters are mainly coal power plants, but gas power plants and industrial plants for production of steel, cement and ammonia are also large emitters.

There are a number of technologies available for capturing CO₂. The post-combustion CO₂ capture technology can be added to existing plants to clean CO₂ from their flue gas, but for new power plants, the cleaning process of the CO₂ could be integrated into the power production. The CO₂ capture technologies are usually divided into three main-categories:

• Post-combustion CO2 capture: CO₂ capture from the flue gas after combustion of the fossil fuel.
• Pre-combustion CO2 capture: Removal of CO₂ from the fossil fuel prior to combustion.
• Oxyfuel combustion with CO2 capture: Combustion of fossil fuel with pure oxygen rather than air.

Post-combustion means end-of-pipe separation of CO₂ from the flue gas. This requires chemical cleaning utilizing an absorbent, often in combination with mechanical cleaning. (An absorbent is a chemical substance, such as amines or carbonates that attracts CO₂). Approximately 80 to 90 percent of the CO₂ can be captured using post-combustion technologies.

Post combustion CO2 capture. Illustration: Prosjektlab

In pre-combustion CO₂ capture, the CO₂ is removed from the fuel prior to combustion. The process is carried out in a traditional steam reformer where the fuel is converted to hydrogen (H₂) and carbon monoxide (CO). The CO-gas and steam is then converted into H₂ and CO₂. Finally, the H2 and CO₂ gas is separated in the same way as in post combustion CO₂ capture. However, in pre-combustion capture a smaller installation than that required in post-combustion capture is sufficient due to the smaller volumes of gas. The separated H₂ gas can be used as fuel for power plants or vehicles.

Pre combustion CO2 capture. Illustration: Prosjektlab

A traditional fossil fuel power plant is operated by combusting fuel and air. A new technology called oxyfuel has one important difference from traditional plants, which is that the combustion is carried out with oxygen instead of air. This is very advantageous when it comes to CO₂ capture, as the flue gas is mainly composed of steam and CO₂, which can be very easily separated. With oxyfuel CO₂ capture it is possible to capture 100 percent of the CO₂, with the disadvantage being that production of pure oxygen makes oxyfuel very expensive.

Oxyful combustion with CO2 capture. Illustration: Prosjektlab

The three main categories of CO₂ capture mentioned above are considered to be 'close to market', meaning that they will soon be commercially available. There are other new novel technologies for CO₂ capture which are considered immature today, but which could become very important in the future.

No paradigm shift in CO₂ capture technology is expected in the near future. Short-term development is therefore expected to be simply a further development of existing technologies.

If you need a print-friendly version of how CO₂ capture works, please see our factsheet.

The idea of CO₂ capture is to produce a stream of pure CO₂ gas from a mixture of CO₂ and other gas components. There are many ways to perform this operation:

• Absorption or adsorption (separating CO₂ by using solvents)
• Membranes
• Distillation
• Mineralisation
• Thermal processes

All of these options for CO₂ capture are depicted in the figures below and detailed descriptions are given below each image.

Absorption and adsorption

Absorption or adsorption with solvents or sorbents

Absorption

Absorption is a common process for post-combustion CO₂ capture in which the challenge is to separate CO₂ and nitrogen.

The mixture of CO₂ and other gas components, such as nitrogen, is mixed with a liquid solvent in a large tank called an 'absorption column'. The solvent must be able to react with CO₂, but not with the other gas components. Using this method produces different streams coming out of the absorption column; one being a mixture of solvent and CO₂ and the other one a mixture of all of the other gas components.

The mixture of solvent and CO₂ enters a second tank where the solvent and the CO₂ is separated through heating. This results in a stream of pure CO₂ and a stream of pure solvent, which is then re-circulated to the absorption column.

A substantial amount of energy is needed to heat up the mixture of solvent and CO₂ and this energy requirement is the reason for the high cost of CO₂ capture. Worldwide research is currently taking place in order to find new solvents that require less energy.

Absorption is today the most mature method for CO₂ capture. The most typical solvents used in CO₂ capture are amines or ammonia, well-know chemicals that react with CO₂, but rarely with other gases. However, the emissions released by amines and ammonia are pollutive to the environment, which is therefore an incentive for the development of new solvents.

Adsorption
Adsorption is very similar to absorption. The only difference is that the CO₂ will bind to the surface of the solvent in adsorption, while there is a chemical binding between CO₂ molecules and solvent molecules in absorbtion.

Membranes

CO2 capture by membranes is performed using a membrane that can be penetrated by CO2, but not by other gas molecules.

Gas separation by membranes is well known but today the process is costly compared to absorption. Further research and development of membranes will probably improve their performance and reduce the cost. It is believed that membranes can be a very interesting alternative in the future.

Distillation

Distillation is a common process used to separated liquids. When a mixture comprised of two liquid components is heated, the component with the lowest boiling point will boil first. The mixture is thus separated into two components; a gas stream and a liquid.

CO₂ can be separated from other gas components by first cooling the mixture until it becomes a liquid. Very low tempertures are required for this and, due to the low temperature, the following distillation to separate the CO₂ is called 'cryogenic distillation'.

Mineralisation

CO2 capture by mineralisation is a process in which the CO2 is reacted with a mineral to form another solid mineral. Mineralisation is a well known technology and represents a viable alternative for CO2 capture.

However, there are some significant barriers to mineralisation. The process produces a by-product of solid material, in the form of rock. A solution to this cumbersome problem will need to be found if mineralisation is to be realistically used in the reduction of CO₂ emissions. Mineralisation is therefore unlikely to be one of the main technologies used in combatting global warming as long as this problem remains unresolved. It will rather be used as an alternative technology when there are large deposits of minerals that are suitable for the mineralisation process.

The material that is produced through the process of mineralisation has potential commercial value. Therefore this process has been put forward by industrial stakeholders. However, if CO₂ is captured in a mineralisation process and the product is then sold as an industrial product, what will happen in the future when the product is degraded or deposited as waste? Will mineralisation still be considered a strategy in reducing CO₂ emissons? If this is the case, then CO₂ could be released into the atmosphere, leaving us without a process for reducing CO₂ emissions.

If CO₂ capture by mineralisation is to be regarded as an emission-reducing strategy, the safe deposit of the mineralisation product must be fully documented.

Thermal processes

In a thermal CO2 capture process a gas mixture of CO2 and another component is cooled to ensure that one component becomes liquid.

Separating CO₂ and steam is cheap and easy and by cooling a CO₂ and steam mixture, the steam will eventually condense to liquid water, leaving pure CO₂ gas.

This is exactly what happens in oxyfuel combustion and in a possible future solution called 'chemical looping'.

• CO₂ capture factsheet
• Post combustion CO₂ capture
• Pre combustion CO₂ capture
• Oxyfuel with CO₂ capture
• Novel technologies

1. IPCC, Special Report on Carbon Dioxide Capture and Storage, http://www.ipcc.ch/ipccreports/srccs.htm

• Report on CO₂ capture from IEA GHG
• Read about CO₂ capture at the NETL home page