Carbon capture and storage (CCS) is a set of technologies that capture carbon dioxide (CO₂), prepare and transport it, and place it in long-term storage rather than release it into the atmosphere. It is principally used to reduce emissions from industrial facilities and energy production. Storage usually involves injection into deep geological formations selected for their ability to contain CO₂. CCS addresses an important greenhouse gas responsible for climate change, but its effectiveness depends on the performance of the entire capture, transport, and storage chain. (iea.org)
Scope and related concepts
Carbon capture is only the separation stage: capturing CO₂ does not itself ensure that the gas remains outside the atmosphere. CCS combines capture with durable storage. The broader term carbon capture, utilisation and storage (CCUS) also includes uses of captured CO₂ in products or industrial processes. Utilisation is not necessarily permanent storage; fuels and many chemicals eventually release their carbon, whereas some mineral products can retain it much longer. (iea.org)
CCS applied to fossil-fuel emissions primarily prevents new atmospheric emissions; it does not remove CO₂ already present in the air. It can contribute to carbon dioxide removal when coupled with direct air capture or with processes using sustainably sourced biomass. Bioenergy with carbon capture and storage (BECCS) captures carbon previously absorbed by growing biomass. Whether such systems achieve net removal depends on their full life-cycle emissions and the durability of storage. (iea.org)
Capture technologies
Capture methods depend on the composition, pressure, and origin of the gas stream. Three major approaches used in energy systems are:
- Post-combustion capture: CO₂ is separated from exhaust gas after combustion. Separation technologies include liquid absorption, solid adsorption, and membranes.
- Pre-combustion capture: fuel is converted into a gas mixture from which CO₂ is separated before a hydrogen-rich stream is used for energy.
- Oxy-fuel combustion: fuel burns in an oxygen-rich environment rather than ordinary air, producing exhaust consisting largely of CO₂ and water. This simplifies CO₂ separation but requires oxygen production and additional purification. (netl.doe.gov)
Industrial capture also treats CO₂ generated by manufacturing processes, including cement production, rather than only by fuel combustion. The suitability and cost of capture vary substantially between concentrated process streams and dilute exhaust gases. (ipcc.ch)
Conditioning and transport
Captured CO₂ is conditioned and compressed before transport. Pipelines and ships can connect emission sources with storage sites; rail and trucks are also possible. Shared transport and storage infrastructure can serve several facilities within an industrial cluster, reducing the need for each emitter to develop an independent system. Transport capacity and access to suitable storage must therefore be developed alongside capture facilities. (iea.org)
Geological storage
The principal storage options include deep saline formations and depleted oil and gas reservoirs, both onshore and offshore. CO₂ occupies pore spaces in rock rather than a purpose-built underground tank. Suitable sites require adequate porosity, sufficient permeability for injection, and geological barriers that limit migration toward the surface. Conventional saline storage commonly targets formations deeper than about 800 metres, where pressure and temperature can maintain CO₂ in a dense state. (energy.gov)
Several mechanisms retain injected CO₂:
- Structural and stratigraphic trapping: low-permeability rock layers and geological structures prevent upward migration.
- Residual trapping: disconnected portions of CO₂ become immobilised within pore spaces.
- Dissolution trapping: CO₂ dissolves in formation fluids.
- Mineral trapping: reactions with rock constituents convert dissolved carbon into stable minerals.
Their relative importance changes with geology and time. Another approach injects CO₂ dissolved in water into reactive rocks, such as basalt, to promote mineral formation. Mineralisation rates are site-dependent and should not be assumed identical across storage formations. (energy.gov)
Monitoring and containment
Storage management includes site characterisation, well construction, monitoring during injection, and care after injection ends. Monitoring follows well integrity, the movement of the CO₂ plume, changes in formation pressure, and possible effects on groundwater. Existing wells and geological pathways require assessment because they can compromise containment. (epa.gov)
In the United States, the Environmental Protection Agency regulates dedicated geological CO₂ injection through its Class VI well programme, which includes requirements for site assessment, construction, testing, monitoring, financial responsibility, and closure. These requirements principally protect underground sources of drinking water. More generally, the Intergovernmental Panel on Climate Change concludes that appropriately selected and managed geological sites can permanently isolate CO₂ from the atmosphere. (epa.gov)
Emissions performance and economics
CO₂ captured is not the same quantity as CO₂ emissions avoided. Capture and compression consume energy, while transport and storage introduce additional requirements. For fossil-based systems, upstream emissions—including methane released during fuel production—also affect overall performance. A life-cycle assessment must account for these emissions, uncaptured CO₂, and any leakage, rather than treating the capture percentage alone as the reduction in total emissions. (ipcc.ch)
CCS costs depend on gas composition, facility scale, energy supply, transport distance, and storage characteristics. Economic assessments distinguish the cost per tonne captured from the cost per tonne avoided: the latter considers emissions associated with operating the system. Capital requirements, suitable storage access, coordinated infrastructure development, and policy arrangements can constrain deployment. CCS is consequently assessed as one component of emissions mitigation, especially for industrial sources that are difficult to decarbonise, rather than as a universal substitute for reducing fossil-fuel use or deploying other low-emission technologies. (ipcc.ch)
References
- Carbon capture, utilisation and storage - Fuels & Technologies - IEAiea.org
- About CCUS – Analysis - IEAiea.org
- CCUS in the transition to net-zero emissions – CCUS in Clean Energy Transitions – Analysis - IEAiea.org
- CCUS in Clean Energy Transitions – Analysis - IEAiea.org
- Carbon Capture | netl.doe.govnetl.doe.gov
- Advanced Combustion | netl.doe.govnetl.doe.gov
- DOE Explains...Carbon Sequestration | Department of Energyenergy.gov
- Report of the Interagency Task Force on Carbon Capture and Storage, August 2010energy.gov
- Class VI - Wells used for Geologic Sequestration of Carbon Dioxide | US EPAepa.gov
- Chapter 6: Energy systemsipcc.ch
- Chapter 11: Industryipcc.ch
- Summary for Policymakersipcc.ch