Carbon capture and storage

Carbon capture and storage is a process by which carbon dioxide from industrial installations or natural sources is separated before it is released into the atmosphere, then transported to a long-term storage location. The CO₂ is captured from a large point source, such as a natural gas processing plant and is typically stored in a deep geological formation. Around 80% of the CO₂ captured annually is used for enhanced oil recovery, a process by which CO₂ is injected into partially depleted oil reservoirs in order to extract more oil and then is largely left underground. Since EOR the CO₂ in addition to it, CCS is also known as carbon capture, utilization, and storage.
Oil and gas companies first used the processes involved in CCS in the mid-20th century. Early CCS technologies were mainly used to purify natural gas and increase oil production. Beginning in the 1980s and accelerating in the 2000s, CCS was discussed as a strategy to reduce greenhouse gas emissions. Around 70% of announced CCS projects have not materialized, with a failure rate above 98% in the electricity sector. As of 2024 CCS was in operation at 44 plants worldwide, collectively capturing about one-thousandth of global carbon dioxide emissions. 90% of CCS operations involve the oil and gas industry. Plants with CCS require more energy to operate, thus they typically burn additional fossil fuels and increase the pollution caused by extracting and transporting fuel.

CCS could have a critical but limited role in reducing greenhouse gas emissions. However, other emission-reduction options such as solar and wind energy, electrification, and public transit are less expensive than CCS and are much more effective at reducing air pollution. Given its cost and limitations, CCS is envisioned to be most useful in specific niches. These niches include heavy industry and plant retrofits. In the context of deep and sustained cuts in natural gas consumption, CCS can reduce emissions from natural gas processing. In electricity generation and hydrogen production, CCS is envisioned to complement a broader shift to renewable energy. CCS is a component of bioenergy with carbon capture and storage, which can under some conditions remove carbon from the atmosphere.
The effectiveness of CCS in reducing carbon emissions depends on the plant's capture efficiency, the additional energy used for CCS itself, leakage, and business and technical issues that can keep facilities from operating as designed. Some large CCS implementations have sequestered far less CO₂ than originally expected. Controversy remains over whether using captured CO₂ to extract more oil ultimately benefits the climate. Many environmental groups regard CCS as an unproven, expensive technology that perpetuates fossil fuel dependence. They believe other ways to reduce emissions are more effective and that CCS is a distraction.
Some international climate agreements refer to the concept of fossil fuel abatement, which is not defined in these agreements but is generally understood to mean use of CCS. Almost all CCS projects operating today have benefited from government financial support. Countries with programs to support or mandate CCS technologies include the US, Canada, Denmark, China, and the UK.

Terminology

The Intergovernmental Panel on Climate Change defines CCS as:

"A process in which a relatively pure stream of carbon dioxide from industrial and energy-related sources is separated, conditioned, compressed and transported to a storage location for long-term isolation from the atmosphere."

The terms carbon capture and storage and carbon capture, utilization, and storage are closely related and often used interchangeably. Both terms have been used predominantly to refer to enhanced oil recovery a process in which captured CO₂ is injected into partially depleted oil reservoirs in order to extract more oil. EOR is both "utilization" and "storage", as the CO₂ left underground is intended to be trapped indefinitely. Prior to 2013, the process was primarily called CCS. In 2013 the term CCUS was introduced to highlight its potential economic benefit, and this term subsequently gained popularity.
Around 1% of captured CO₂ is used as a feedstock for making products such as fertilizer, fuels, and plastics. These uses are forms of carbon capture and utilization. In some cases, the product durably stores the carbon from the CO₂ and thus is also considered to be a form of CCS. To qualify as CCS, carbon storage must be long-term, therefore utilization of CO₂ to produce fertilizer, fuel, or chemicals is not CCS because these products release CO₂ when burned or consumed.
Some sources use the term CCS, CCU, or CCUS more broadly, encompassing methods such as direct air capture or tree-planting which remove CO₂ from the air. In this article, the term CCS is used according to the IPCC's definition, which requires CO₂ to be captured from point-sources such as a natural gas processing plant.

History and current status

In the natural gas industry, technology to remove CO₂ from raw natural gas was patented in 1930. This processing is essential to make natural gas ready for commercial sale and distribution. Usually after CO₂ is removed, it is vented to the atmosphere. In 1972, American oil companies discovered that CO₂ could profitably be used for EOR. Subsequently, natural gas companies in Texas began capturing the CO₂ produced by their processing plants and selling it to local oil producers for EOR.
The use of CCS as a means of reducing human-caused CO₂ emissions is more recent. In 1977, the Italian physicist Cesare Marchetti proposed that CCS could be used to reduce emissions from coal power plants and fuel refineries. Small-scale implementations were first demonstrated in the early 1980s and an economic evaluation was published in 1991. The first large-scale CO₂ capture and injection project with dedicated CO₂ storage and monitoring was commissioned at the Sleipner gas field in Norway in 1996.
In 2005, the IPCC released a report highlighting CCS, leading to increased government support for CCS in several countries. Governments spent an estimated US$30 billion on subsidies for CCS and for fossil-fuel-based hydrogen. Globally, 149 projects to store 130 million tonnes of CO₂ annually were proposed to be operational by 2020. Of these, around 70% were not implemented. Limited one-off capital grants, the absence of measures to address long-term liability for stored CO₂, high operating costs, limited social acceptability and vulnerability of funding programmes to external budget pressures all contributed to project cancellations.
In 2020, the International Energy Agency stated, "The story of CCUS has largely been one of unmet expectations: its potential to mitigate climate change has been recognised for decades, but deployment has been slow and so has had only a limited impact on global CO₂ emissions."
By July 2024, commercial-scale CCS was in operation at 44 plants worldwide. Sixteen of these facilities were devoted to separating naturally occurring CO₂ from raw natural gas. Seven facilities were for hydrogen, ammonia, or fertilizer production, seven for chemical production, five for electricity and heat, and two for oil refining. CCS was also used in one iron and steel plant. Additionally, three facilities worldwide were devoted to CO₂ transport/storage. As of 2024, the oil and gas industry is involved in 90% of CCS capacity in operation around the world. Collectively, the facilities capture about one-thousandth of global greenhouse gas emissions.
Eighteen facilities were in the United States, fourteen in China, five in Canada, and two in Norway. Australia, Brazil, Qatar, Saudi Arabia, and the United Arab Emirates had one project each. As of 2020, North America has more than of CO₂ pipelines, and there are two CO₂ pipeline systems in Europe and two in the Middle East.

Process overview

CCS facilities capture carbon dioxide before it enters the atmosphere. Generally, a chemical solvent or a porous solid material is used to separate the CO₂ from other components of a plant's exhaust stream. Most commonly, the gas stream passes through an amine solvent, which binds the CO₂ molecule. This CO₂-rich solvent is heated in a regeneration unit to release the CO₂ from the solvent. The CO₂ stream then undergoes conditioning to remove impurities and bring the gas to an appropriate temperature for compression. The purified CO₂ stream is compressed and transported for storage or end-use and the released solvents are recycled to capture more CO₂ from the facility.
After the has been captured, it is usually compressed into a supercritical fluid and then injected underground. Pipelines are the cheapest way of transporting CO₂ in large quantities onshore and, depending on the distance and volumes, offshore. Transport via ship has been researched. CO₂ can also be transported by truck or rail, albeit at higher cost per tonne of CO₂.

Technical components

CCS processes involve several different technologies working together. Technological components are used to separate and treat CO₂ from a gas mixture, compress and transport the CO₂, inject it into the subsurface, and monitor the overall process.
There are three ways that CO₂ can be separated from a gas mixture: post-combustion capture, pre-combustion capture, and oxy-combustion:

In post combustion capture, the CO₂ is removed after combustion of the fossil fuel.
The technology for pre-combustion is widely applied in natural gas processing. In these cases, the fossil fuel is partially oxidized, for instance in a gasifier. The CO from the resulting syngas reacts with added steam and is shifted into CO₂ and H₂. The resulting CO₂ can be captured from a relatively pure exhaust stream. The H₂ can be used as fuel. Several advantages and disadvantages apply versus post combustion capture.
In oxy-fuel combustion the fuel is burned in pure oxygen instead of air. The gas that is released consists of mostly CO₂ and water vapor. After water vapor is condensed through cooling, the result is an almost pure CO₂ stream. A disadvantage of this technique is that it requires a relatively large amount of oxygen, which is expensive and energy-intensive to produce.

Absorption, or carbon scrubbing with amines is the dominant capture technology. Other technologies proposed for carbon capture are membrane gas separation, chemical looping combustion, calcium looping, and use of metal-organic frameworks and other solid sorbents.
Impurities in CO₂ streams, like sulfur dioxides and water vapor, can have a significant effect on their phase behavior and could cause increased pipeline and well corrosion. In instances where CO₂ impurities exist, a process is needed to remove them.