What is carbon capture and storage, and how does it work?

Carbon capture explained

Carbon capture and storage (CCS) technologies capture the greenhouse gas carbon dioxide (CO₂) and store it safely underground so that it does not contribute to climate change.

Many industrial processes emit significant amounts of CO₂ – for example, the cement, steel, pulp and paper, chemicals, and natural gas processing industries account for around 25% of global energy-related CO₂ emissions.

With the global pressure to reduce carbon emissions and operate sustainably, CCS is often the only option open to many industrial applications looking to secure a significant reduction in their emissions rapidly. 

The carbon capture process 

The first step in carbon capture is to separate out and capture the CO₂, which can be accomplished using several well-established and effective methods, including pre-combustion, post-combustion, and oxyfuel combustion capture technologies.

Once captured, the CO₂ must be transported – usually via pipeline and/or ship – to a storage site. This carbon capture and storage is sometimes called carbon sequestration. Usually, former oil and gas reservoirs are used for storage, as they have held their resources in place for millions of years.

Captured CO₂ can also be utilised in a range of industrial manufacturing processes, such as plastics production. However, current CO₂ emissions far exceed the demand for these applications, so storage remains more common than usage.

Carbon capture usage and storage

There are two ways to deal with captured CO₂ – either it is stored (for future use or, more commonly, to keep it out of the atmosphere), or it is put to use in another application.

What are the differences between carbon capture storage and carbon capture usage? 

Carbon capture and storage is a term that refers to capturing CO₂ and sequestering it safely, typically in underground facilities, which may be depleted oil and gas reservoirs, coalbeds, or deep saline aquifers. Carbon capture and usage (CCU) refers to capturing CO₂ for use in another process, such as the manufacture of plastics, concrete, or biofuel. Captured CO₂ is also used in enhanced oil recovery applications, where it is injected into oil and gas reservoirs to increase their extraction.

What are the main types of carbon capture technology? 

Carbon capture technologies take CO₂ from exhaust or reformed gases or stationary sources, using pre-combustion, post-combustion, or oxy-combustion techniques.

In the pre-combustion method, fuel is gasified rather than combusted, creating a synthesis gas (also known as syngas) mostly made up of carbon monoxide (CO) and hydrogen (H₂). A shift reaction converts the CO to CO₂, then a solvent is used to separate this CO₂ from the H₂. An integrated gasification combined cycle (IGCC) power plant can be combined with this technique, burning the H2 in a combustion turbine and using the exhaust heat to power a steam turbine.

Typically, post-combustion carbon capture uses chemical solvents to separate the CO₂ out of the flue gas resulting from fossil fuel combustion. This method is useful for existing power plants that are being retrofitted for carbon capture.

Oxyfuel capture requires utilises pure oxygen – rather than air – for fossil fuel combustion. This results in an exhaust gas which is CO₂-rich, making capture easier to achieve.

There are also inherent capture processes which capture CO₂ as part of the process design. In these instances, the CO₂ is not contaminated with other gases, so remains highly pure. While not currently in widespread use, direct capture mechanisms are also available to collect CO₂ directly from atmospheric air.

Finally, there are bioenergy-based CCS technologies that extract and store CO₂ from biomass, itself a renewable energy source. 

How does carbon capture and storage work?

Carbon capture has been in use dating back to the 1970s. However, its utilisation in power generation applications is a much more recent development.

There are many industrial processes where large-scale carbon capture has been demonstrated and is used commercially. These processes include coal gasification, ethanol production, fertiliser production, natural gas processing, refinery hydrogen production, and coal-fired power generation.

How can we capture carbon?

Carbon capture technology comes in a range of forms, including absorption, adsorption, calcium looping, chemical looping combustion, cryogenic, membrane, multiphase absorption, and oxyfuel combustion technologies

The carbon capture process accounts for around two-thirds of the total cost of CCS, as the transport and storage steps are already well-established. There are ongoing efforts to develop and improve capture technologies to make them more efficient and cost-effective.

Solvent-based carbon capture

A long-proven technique, solvent-based carbon capture is currently the most widely used method for capturing CO₂ from power plants and other industrial sources. In this process, the flue gas is passed through a liquid solvent, such as an amine solution, which absorbs the CO₂. The solvent is then separated from the flue gas.

Heating the solvent will release the CO₂ for reuse. However, this requires a significant amount of energy and can be expensive to implement on a large scale.

Solid sorbent carbon capture

An alternative approach to using solid solvents is to utilise solid materials, such as metal-organic frameworks (MOFs), zeolites, or activated carbon, to capture CO₂ from the flue gas. These materials have high surface areas and can adsorb CO₂ selectively from other gases.

Once the CO₂ is captured, the solid sorbent can be regenerated by heating it to release the CO₂, which can then be captured and stored. This approach has the potential to be more energy-efficient and cost-effective than solvent-based techniques.

Other alternative techniques

Aside from the traditional amine solvent approach and solid sorbent carbon capture, there are other technologies in the early stages of development which have the potential to be even more energy-efficient and cost-effective in reducing CO₂ emissions.

Membrane-based carbon capture uses a permeable membrane designed to selectively allow CO₂ to pass through while blocking other gases, thus separating out the CO₂.

Chemical looping combustion uses a solid oxygen carrier, such as iron oxide or copper oxide, to transfer oxygen from air to fuel. This produces a stream of CO₂ that is more concentrated than the flue gas produced by traditional combustion, making it easier to capture and store.

Step-by-step on the process 

Pre-combustion carbon capture

This technology captures CO₂ before the fuel is burned. It converts the fuel into a mixture of H₂ and CO₂, then separates the CO₂ from the H₂ using methods such as pressure swing adsorption or membrane separation.

Post-combustion carbon capture

This technology captures CO₂ from flue gases emitted by power plants and industrial facilities after the fuel is burned. The most common method involves the use of solvents that absorb the CO₂ from the flue gases.

Oxy-fuel combustion carbon capture

This technology involves burning the fuel in an oxygen-rich environment to produce flue gases that are mostly composed of CO₂ and water vapour. The CO₂ can then be captured from the flue gases using methods similar to post-combustion capture.

Other technologies

Carbon mineralisation involves reacting CO₂ with naturally occurring minerals, such as olivine or serpentine, to produce stable carbonates that can be stored underground. Direct air capture takes CO₂ directly from the air using chemical adsorbents or solvents. Membrane separation uses special membranes that selectively allow CO₂ to pass through while blocking other gases, such as nitrogen or oxygen.

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