Global proposed (grey bars) vs. implemented (blue bars) annual CO₂ sequestration. More than 75% of proposed gas processing projects have been implemented, with corresponding figures for other industrial projects and power plant projects being about 60% and 10%, respectively.^[1]

Carbon capture and storage (CCS) or carbon capture and sequestration^[2] is the process of capturing carbon dioxide (CO₂) before it enters the atmosphere, transporting it, and storing it (carbon sequestration) for centuries or millennia. Usually the CO₂ is captured from large point sources, such as a chemical plant or biomass power plant, and then stored in an underground geological formation. The aim is to prevent the release of CO₂ from heavy industry with the intent of mitigating the effects of climate change.^[3] CO₂ has been injected into geological formations for several decades for enhanced oil recovery and after separation from natural gas, but this has been criticised for producing more emissions when the gas or oil is burned.^[4]

Carbon capture and utilization (CCU) and CCS are sometimes discussed collectively as carbon capture, utilization, and sequestration (CCUS). This is because CCS is a relatively expensive process yielding a product which is often too cheap.^[5] Hence, carbon capture makes economically more sense where the carbon price is high enough, such as in much of Europe,^[4] or when combined with a utilization process where the cheap CO₂ can be used to produce high-value chemicals to offset the high costs of capture operations.^[6]

CO₂ can be captured directly from an industrial source, such as a cement kiln, using a variety of technologies; including absorption, adsorption, chemical looping, membrane gas separation or gas hydration.^[7][8] As of 2022^[update], about one thousandth of global CO₂ emissions are captured by CCS, and most projects are for fossil gas processing.^[4]: 32

Storage of the CO₂ is either in deep geological formations, or in the form of mineral carbonates. Pyrogenic carbon capture and storage (PyCCS) is also being researched.^[9] Geological formations are currently considered the most promising sequestration sites. The US National Energy Technology Laboratory (NETL) reported that North America has enough storage capacity for more than 900 years worth of CO₂ at current production rates.^[10] A general problem is that long-term predictions about submarine or underground storage security are very difficult and uncertain, and there is still the risk that some CO₂ might leak into the atmosphere.^[11][12][13] Despite this, a recent evaluation estimates the risk of substantial leakage to be fairly low.^{[14][15][when?]}

Opponents point out that many CCS projects have failed to deliver on promised emissions reductions.^[16] Additionally, opponents argue that carbon capture and storage is only a justification for indefinite fossil fuel usage disguised as marginal emission reductions. One of the most well-known failures is the FutureGen program, partnerships between the US federal government and coal energy production companies which were intended to demonstrate ″clean coal″, but never succeeded in producing any carbon-free electricity from coal.

Capture

Capturing CO₂ is most cost-effective at point sources, such as large carbon-based energy facilities, industries with major CO₂ emissions (e.g. cement production, steelmaking^[17]), natural gas processing, synthetic fuel plants and fossil fuel-based hydrogen production plants. Extracting CO₂ from air is possible,^[18] although the lower concentration of CO₂ in air compared to combustion sources complicates the engineering and makes the process therefore more expensive.^[19]

Impurities in CO₂ streams, like sulfurs and water, can have a significant effect on their phase behavior and could pose a significant threat of increased pipeline and well corrosion. In instances where CO₂ impurities exist, especially with air capture, a scrubbing separation process is needed to initially clean the flue gas.^[20] It is possible to capture approximately 65% of CO₂ embedded in it and sequester it in a solid form.^[21]

Broadly, three different technologies exist: post-combustion, pre-combustion, and oxyfuel combustion:

• In post combustion capture, the CO₂ is removed after combustion of the fossil fuel—this is the scheme that would apply to fossil-fuel power plants. CO₂ is captured from flue gases at power stations or other point sources. The technology is well understood and is currently used in other industrial applications, although at smaller scale than required in a commercial scale station. Post combustion capture is most popular in research because fossil fuel power plants can be retrofitted to include CCS technology in this configuration.^[22]
• The technology for pre-combustion is widely applied in fertilizer, chemical, gaseous fuel (H₂, CH₄), and power production.^[23] In these cases, the fossil fuel is partially oxidized, for instance in a gasifier. The CO from the resulting syngas (CO and H₂) reacts with added steam (H₂O) 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; the CO₂ is removed before combustion. Several advantages and disadvantages apply versus post combustion capture.^[24][25] The CO₂ is removed after combustion, but before the flue gas expands to atmospheric pressure. The capture before expansion, i.e. from pressurized gas, is standard in almost all industrial CO₂ capture processes, at the same scale as required for power plants.^[26][27]
• In oxy-fuel combustion^[28] the fuel is burned in pure oxygen instead of air. To limit the resulting flame temperatures to levels common during conventional combustion, cooled flue gas is recirculated and injected into the combustion chamber. The flue gas consists of mainly CO₂ and water vapour, the latter of which is condensed through cooling. The result is an almost pure CO₂ stream. Power plant processes based on oxyfuel combustion are sometimes referred to as "zero emission" cycles, because the CO₂ stored is not a fraction removed from the flue gas stream (as in the cases of pre- and post-combustion capture) but the flue gas stream itself. A certain fraction of the CO₂ inevitably end up in the condensed water. To warrant the label "zero emission" the water would thus have to be treated or disposed of appropriately.

Separation technologies

The major technologies proposed for carbon capture are:^[7][29][30]

• Membrane
• Oxyfuel combustion
• Absorption
• Multiphase absorption
• Adsorption
• Chemical looping combustion
• Calcium looping
• Cryogenic

Absorption, or carbon scrubbing with amines is the dominant capture technology. It is the only carbon capture technology so far that has been used industrially.^[31]

CO₂ adsorbs to a MOF (Metal–organic framework) through physisorption or chemisorption based on the porosity and selectivity of the MOF leaving behind a CO₂ poor gas stream.^[32] The CO₂ is then stripped off the MOF using temperature swing adsorption (TSA) or pressure swing adsorption (PSA) so the MOF can be reused. Adsorbents and absorbents require regeneration steps where the CO₂ is removed from the sorbent or solution that collected it from the flue gas in order for the sorbent or solution to be reused. Monoethanolamine (MEA) solutions, the leading amine for capturing CO₂ , have a heat capacity between 3–4 J/g K since they are mostly water.^[33][34] Higher heat capacities add to the energy penalty in the solvent regeneration step. Thus, to optimize a MOF for carbon capture, low heat capacities and heats of adsorption are desired. Additionally, high working capacity and high selectivity are desirable in order to capture as much CO₂ as possible. However, an energy trade off complicates selectivity and energy expenditure.^[35] As the amount of CO₂ captured increases, the energy, and therefore cost, required to regenerate increases. A drawback of MOF/CCS is the limitation imposed by their chemical and thermal stability.^[22] Research is attempting to optimize MOF properties for CCS. Metal reservoirs are another limiting factor.^[36]

About two thirds of CCS cost is attributed to capture, making it the limit to CCS deployment. Optimizing capture would significantly increase CCS feasibility since the transport and storage steps of CCS are rather mature.^[37]

An alternate method is chemical looping combustion (CLC). Looping uses a metal oxide as a solid oxygen carrier. Metal oxide particles react with a solid, liquid or gaseous fuel in a fluidized bed combustor, producing solid metal particles and a mixture of CO₂ and water vapor. The water vapor is condensed, leaving pure CO₂ , which can then be sequestered. The solid metal particles are circulated to another fluidized bed where they react with air, producing heat and regenerating metal oxide particles for return to the combustor. A variant of chemical looping is calcium looping, which uses the alternating carbonation and then calcination of a calcium oxide based carrier.^[38]

A 2019 study found CCS plants to be less effective than renewable electricity. The electrical energy returned on energy invested (EROEI) ratios of both production methods were estimated, accounting for their operational and infrastructural energy costs. Renewable electricity production included solar and wind with sufficient energy storage, plus dispatchable electricity production. Thus, rapid expansion of scalable renewable electricity and storage would be preferable over fossil-fuel with CCS. The study did not consider whether both options could be pursued in parallel.^[39]

In 2021 High Hopes proposed using high-altitude balloons to capture CO₂ cryogenically, using hydrogen to lower the already low-temperature atmosphere sufficiently to produce dry ice that is returned to earth for sequestration.^[40]

In sorption enhanced water gas shift (SEWGS) technology a pre-combustion carbon capture process, based on solid adsorption, is combined with the water gas shift reaction (WGS) in order to produce a high pressure hydrogen stream.^[41] The CO₂ stream produced can be stored or used for other industrial processes.^[42]

Transport

After capture, the CO₂ must be transported to suitable storage sites. Pipelines are the cheapest form of transport. Ships can be utilized where pipelines are infeasible, and for long enough distances ships may be cheaper than a pipeline.^[43] These methods are used for transporting CO₂ for other applications. Rail and tanker truck cost about twice as much as pipelines or ships.^[43]

For example, approximately 5,800 km of CO₂ pipelines operated in the US in 2008, and a 160 km pipeline in Norway,^[44] used to transport CO₂ to oil production sites where it is injected into older fields to extract oil. This injection is called enhanced oil recovery. Pilot programs are in development to test long-term storage in non-oil producing geologic formations. In the United Kingdom, the Parliamentary Office of Science and Technology envisages pipelines as the main UK transport.^[44]

In 2021, two companies, namely Navigator CO₂ Ventures and Summit Carbon Solutions were planning pipelines through the Midwestern US from North Dakota to Illinois to connect ethanol companies to sites where liquefied CO₂ is injected into porous rock.^[45]

Sequestration (storage)

Various approaches have been conceived for permanent storage. These include gaseous storage in deep geological formations (including saline formations and exhausted gas fields), and solid storage by reaction of CO₂ with metal oxides to produce stable carbonates.

Geological storage

Geo-sequestration, involves injecting CO₂ , generally in supercritical form, into underground geological formations. Oil fields, gas fields, saline formations, unmineable coal seams, and saline-filled basalt formations have been suggested as alternatives. Physical (e.g., highly impermeable caprock) and geochemical trapping mechanisms prevent the CO₂ from escaping to the surface.^[46]

Unmineable coal seams can be used because CO₂ molecules attach to the coal surface. Technical feasibility depends on the coal bed's permeability. In the process of absorption the coal releases previously absorbed methane, and the methane can be recovered (enhanced coal bed methane recovery). Methane revenues can offset a portion of the cost, although burning the resultant methane, however, produces another stream of CO₂ to be sequestered.^{[citation needed]}

Saline formations contain mineralized brines and have yet to produce benefit to humans. Saline aquifers have occasionally been used for storage of chemical waste in a few cases. The main advantage of saline aquifers is their large potential storage volume and their ubiquity. The major disadvantage of saline aquifers is that relatively little is known about them. To keep the cost of storage acceptable, geophysical exploration may be limited, resulting in larger uncertainty about the aquifer structure. Unlike storage in oil fields or coal beds, no side product offsets the storage cost. Trapping mechanisms such as structural trapping, residual trapping, solubility trapping and mineral trapping may immobilize the CO₂ underground and reduce leakage risks.^{[46] [47]}

Enhanced oil recovery

CO₂ is occasionally injected into an oil field as an enhanced oil recovery technique,^[48] but because CO₂ is released when the oil is burned,^[49] it is not carbon neutral.^{[50][failed verification]}

Algae/bacteria

CO₂ can be physically supplied to algae or bacteria that could degrade the CO₂. It would ultimately be ideal to exploit CO₂ metabolizing bacterium Clostridium thermocellum.^[51][52]

Mineral storage

CO₂ can exothermically react with metal oxides, which in turn produce stable carbonates (e.g. calcite, magnesite). This process (CO₂-to-stone) occurs naturally over periods of years and is responsible for much surface limestone. Olivine is one such MOX.^{[53][self-published source?]} The reaction rate can be accelerated with a catalyst^[54] or by increasing temperatures and/or pressures, or by mineral pre-treatment, although this method can require additional energy. The IPCC estimates that a power plant equipped with CCS using mineral storage would need 60–180% more energy than one without.^[43] Theoretically, up to 22% of crustal mineral mass is able to form carbonates.

Ultramafic mine tailings are a readily available source of fine-grained metal oxides that can serve this purpose.^[55] Accelerating passive CO₂ sequestration via mineral carbonation may be achieved through microbial processes that enhance mineral dissolution and carbonate precipitation.^[56][57][58]

Cost

Cost is a significant factor affecting CCS. The cost of CCS, plus any subsidies, must be less than the expected cost of emitting CO₂ for a project to be considered economically favorable.

CCS technology is expected to use between 10 and 40 percent of the energy produced by a power station.^[59][60] Energy for CCS is called an energy penalty. It has been estimated that about 60% of the penalty originates from the capture process, 30% comes from compression of CO₂ , while the remaining 10% comes from pumps and fans.^[61] CCS would increase the fuel requirement of a plant with CCS by about 15% (gas plant).^[43] The cost of this extra fuel, as well as storage and other system costs, are estimated to increase the costs of energy from a power plant with CCS by 30–60%.

Constructing CCS units is capital intensive. The additional costs of a large-scale CCS demonstration project are estimated to be €0.5–1.1 billion per project over the project lifetime. Other applications are possible. CCS trials for coal-fired plants in the early 21st century were economically unviable in most countries,^[62] including China,^[63] in part because revenue from enhanced oil recovery collapsed with the 2020 oil price collapse.^[64] A carbon price of at least 100 euros per tonne CO₂ is estimated to be needed to make industrial CCS viable,^[65] together with carbon tariffs.^[66] But, as of mid-2022, the EU Allowance had never reached that price and the Carbon Border Adjustment Mechanism had not yet been implemented.^[67] However a company making small modules claims it can get well below that price by mass production by 2022.^[68]

According to UK government estimates made in the late 2010s, carbon capture (without storage) is estimated to add 7 GBP per MWh by 2025 to the cost of electricity from a gas-fired power plant: however most CO₂ will need to be stored so in total the increase in cost for gas or biomass generated electricity is around 50%.^[69]

Business models

Possible business models for industrial carbon capture include:^[70]

• Contract for Difference CfDC CO₂ certificate strike price
• Cost Plus open book
• Regulated Asset Base (RAB)
• Tradeable tax credits for CCS
• Tradeable CCS certificates + obligation
• Creation of low carbon market

Governments have provided various types of funding for CCS demonstration projects, including tax credits, allocations and grants.^[71]

Clean Development Mechanism

One alternative could be through the Clean Development Mechanism of the Kyoto Protocol. At COP16 in 2010, The Subsidiary Body for Scientific and Technological Advice, at its thirty-third session, issued a draft document recommending the inclusion of CCS in geological formations in Clean Development Mechanism project activities.^[72] At COP17 in Durban, a final agreement was reached enabling CCS projects to receive support through the Clean Development Mechanism.^[73]

Environmental effects

Alkaline solvents

CO₂ can be captured with alkaline solvents at low temperatures in the absorber and released CO₂ at higher temperatures in a desorber. Chilled ammonia CCS plants emit ammonia. "Functionalized Ammonia" emits less ammonia, but amines may form secondary amines that emit volatile nitrosamines^[74] by a side reaction with nitrogen dioxide, which is present in any flue gas. Alternative amines with little to no vapor pressure can avoid these emissions. Nevertheless, practically 100% of remaining sulfur dioxide from the plant is washed out of the flue gas, along with dust/ash.^{[clarification needed]}

Natural gas processing and enhanced oil recovery

The Institute for Energy Economics & Financial Analysis has criticised companies for not reporting greenhouse gas emissions from the use of their products.^[4]: 33 CO2 from natural-gas processing is often used for EOR.^[4] It has been suggested that enhanced oil recovery only be allowed to use anthropogenic CO2 and should only receive financial incentives such as tax credits when carbon negative, which is generally only in the early years of a project.^[75]

Gas and coal-fired power plants

Although the global total CO2 emitted by fossil fuel power plants is very large, coal plant flue gas typically only contains 10–14% CO2, and gas power plants only 4–5% CO2.^[4]: 37 Cost per tonne CO2 increases as the capacity factor decreases (the plant is used less - for example only for times of highest demand or in emergencies).^[4]: 42

The extra energy requirements deriving from CCS for natural gas combined cycle (NGCC) plants range from 11 to 22%.^[76] Fuel use and environmental problems (e.g., methane emissions) arising from gas extraction increase accordingly. Plants equipped with selective catalytic reduction systems for nitrogen oxides produced during combustion^[77] require proportionally greater amounts of ammonia.

A 2020 study concluded that half as much CCS might be installed in coal-fired plants as in gas-fired: these would be mainly in China and India.^[78] However a 2022 study concluded that it would be too expensive for coal power in China.^[79]

For super-critical pulverized coal (PC) plants, CCS' energy requirements range from 24 to 40%, while for coal-based gasification combined cycle (IGCC) systems it is 14–25%.^[76] Fuel use and environmental problems arising from coal extraction increase accordingly. Plants equipped with flue-gas desulfurization (FGD) systems for sulfur dioxide control require proportionally greater amounts of limestone, and systems equipped with selective catalytic reduction systems for nitrogen oxides produced during combustion require proportionally greater amounts of ammonia. As of 2022^[update] Boundary Dam is the only coal-fired power station which uses post-combustion CCS.^[4]: 42

Leakage

Long-term retention

IPCC estimates that leakage risks at properly managed sites are comparable to those associated with current hydrocarbon activity. It recommends that limits be set to the amount of leakage that can take place.^[80] However, this finding is contested given the lack of experience.^[81][82] CO₂ could be trapped for millions of years, and although some leakage may occur, appropriate storage sites are likely to retain over 99% for over 1000 years.^[83]

Mineral storage is not regarded as presenting any leakage risks.^[84]

Norway's Sleipner gas field is the oldest industrial scale retention project. An environmental assessment conducted after ten years of operation concluded that geosequestration was the most definite form of permanent geological storage method:

Available geological information shows absence of major tectonic events after the deposition of the Utsira formation [saline reservoir]. This implies that the geological environment is tectonically stable and a site suitable for CO₂ storage. The solubility trapping [is] the most permanent and secure form of geological storage.^[85]

In March 2009 StatoilHydro issued a study documenting the slow spread of CO₂ in the formation after more than 10 years operation.^[86]

Gas leakage into the atmosphere may be detected via atmospheric gas monitoring, and can be quantified directly via eddy covariance flux measurements.^[87][88][89]

Sudden leakage hazards

Transmission pipelines may leak or rupture. Pipelines can be fitted with remotely controlled valves that can limit the release quantity to one pipe section. For example, a severed 19" pipeline section 8 km long could release its 1,300 tonnes in about 3–4 min.^[90] At the storage site, the injection pipe can be fitted with non-return valves to prevent an uncontrolled release from the reservoir in case of upstream pipeline damage.

Large-scale releases present asphyxiation risk. In the 1953 Menzengraben mining accident, several thousand tonnes were released and asphyxiated a person 300 meters away.^{[90][better source needed]} Malfunction of a CO₂ industrial fire suppression system in a large warehouse released 50 t CO₂ after which 14 people collapsed on the nearby public road.^[90] In the Berkel en Rodenrijs incident in December 2008 a modest release from a pipeline under a bridge killed some ducks sheltering there.^[91]

Monitoring

Monitoring allows leak detection with enough warning to minimize the amount lost, and to quantify the leak size. Monitoring can be done at both the surface and subsurface levels.^[92]

Subsurface

Subsurface monitoring can directly and/or indirectly track the reservoir's status. One direct method involves drilling deep enough to collect a sample. This drilling can be expensive due to the rock's physical properties. It also provides data only at a specific location.

One indirect method sends sound or electromagnetic waves into the reservoir which reflects back for interpretation. This approach provides data over a much larger region; although with less precision.

Both direct and indirect monitoring can be done intermittently or continuously.^[92]

Seismic

Seismic monitoring is a type of indirect monitoring. It is done by creating seismic waves either at the surface using a seismic vibrator, or inside a well using a spinning eccentric mass. These waves propagate through geological layers and reflect back, creating patterns that are recorded by seismic sensors placed on the surface or in boreholes.^[93] It can identify migration pathways of the CO₂ plume.^[94]

Examples of seismic monitoring of geological sequestration are the Sleipner sequestration project, the Frio CO₂ injection test and the CO2CRC Otway Project.^[95] Seismic monitoring can confirm the presence of CO₂ in a given region and map its lateral distribution, but is not sensitive to the concentration.

Tracer

Organic chemical tracers, using no radioactive nor Cadmium components, can be used during the injection phase in a CCS project where CO₂ is injected into an existing oil or gas field, either for EOR, pressure support or storage. Tracers and methodologies are compatible with CO₂ – and at the same time unique and distinguishable from the CO₂ itself or other molecules present in the sub-surface. Using laboratory methodology with an extreme detectability for tracer, regular samples at the producing wells will detect if injected CO₂ has migrated from the injection point to the producing well. Therefore, a small tracer amount is sufficient to monitor large scale subsurface flow patterns. For this reason, tracer methodology is well-suited to monitor the state and possible movements of CO₂ in CCS projects. Tracers can therefore be an aid in CCS projects by acting as an assurance that CO₂ is contained in the desired location sub-surface. In the past, this technology has been used to monitor and study movements in CCS projects in Algeria (Mathieson et al. “In Salah CO 2 Storage JIP: CO 2 sequestration monitoring and verification technologies applied at Krechba, Algeria”, Energy Procedia 4:3596-3603), in the Netherlands (Vandeweijer et al. “Monitoring the CO₂ injection site: K12B”, Energy Procedia 4 (2011) 5471–5478) as well as in Norway (Snøhvit).

Surface

Eddy covariance is a surface monitoring technique that measures the flux of CO₂ from the ground's surface. It involves measuring CO₂ concentrations as well as vertical wind velocities using an anemometer.^[96] This provides a measure of the vertical CO₂ flux. Eddy covariance towers could potentially detect leaks, after accounting for the natural carbon cycle, such as photosynthesis and plant respiration. An example of eddy covariance techniques is the Shallow Release test.^[97] Another similar approach is to use accumulation chambers for spot monitoring. These chambers are sealed to the ground with an inlet and outlet flow stream connected to a gas analyzer.^[92] They also measure vertical flux. Monitoring a large site would require a network of chambers.

InSAR

InSAR monitoring involves a satellite sending signals down to the Earth's surface where it is reflected back to the satellite's receiver. The satellite is thereby able to measure the distance to that point.^[98] CO₂ injection into deep sublayers of geological sites creates high pressures. These layers affect layers above and below them, change the surface landscape. In areas of stored CO₂ , the ground's surface often rises due to the high pressures. These changes correspond to a measurable change in the distance from the satellite.^[98]

Carbon capture and utilization (CCU)

Comparison between sequestration and utilization of captured carbon dioxide

Carbon capture and utilization (CCU) is the process of capturing carbon dioxide (CO₂) to be recycled for further usage.^[99] Carbon capture and utilization may offer a response to the global challenge of significantly reducing greenhouse gas emissions from major stationary (industrial) emitters.^[100] CCU differs from carbon capture and storage (CCS) in that CCU does not aim nor result in permanent geological storage of carbon dioxide. Instead, CCU aims to convert the captured carbon dioxide into more valuable substances or products; such as plastics, concrete or biofuel; while retaining the carbon neutrality of the production processes.

Captured CO₂ can be converted to several products: one group being alcohols, such as methanol, to use as biofuels and other alternative and renewable sources of energy. Other commercial products include plastics, concrete and reactants for various chemical synthesis.^[101]

Although CCU does not result in a net carbon positive to the atmosphere, there are several important considerations to be taken into account. Because CO₂ is a thermodynamically stable form of carbon manufacturing products from it is energy intensive.^[102] The availability of other raw materials to create a product should also be considered before investing in CCU.

Considering the different potential options for capture and utilization, research suggests that those involving chemicals, fuels and microalgae have limited potential for CO₂ removal, while those that involve construction materials and agricultural use can be more effective.^[103]

The profitability of CCU depends partly on the carbon price of CO₂ being released into the atmosphere.

Social acceptance

Multiple studies indicate that risk and benefit perception are the most essential components of social acceptance.^[104]

Risk perception is mostly related to the concerns on its safety issues in terms of hazards from its operations and the possibility of CO₂ leakage which may endanger communities, commodities, and the environment in the vicinity of the infrastructure.^[105] Other perceived risks relate to tourism and property values.^[104]

People who are already affected by climate change, such as drought,^[106] tend to be more supportive of CCS. Locally, communities are sensitive to economic factors, including job creation, tourism or related investment.^[104]

Experience is another relevant feature. Several field studies concluded that people already involved or used to industry are likely to accept the technology. In the same way, communities who have been negatively affected by any industrial activity are also less supportive of CCS.^[104]

Few members of the public know about CCS. This can allow misconceptions that lead to less approval. No strong evidence links knowledge of CCS and public acceptance. However, one study found that communicating information about monitoring tends to have a negative impact on attitudes.^[107] Conversely, approval seems to be reinforced when CCS is compared to natural phenomena.^[104]

Due to the lack of knowledge, people rely on organizations that they trust.^{[citation needed]} In general, non-governmental organizations and researchers experience higher trust than stakeholders and governments. Opinions amongst NGOs are mixed.^[108][109] Moreover, the link between trust and acceptance is at best indirect. Instead, trust has an influence on the perception of risks and benefits.^[104]

CCS is embraced by the shallow ecology worldview,^[110] which promotes the search for solutions to the effects of climate change in lieu of/in addition to addressing the causes. This involves the use of advancing technology and CCS acceptance is common among techno-optimists. CCS is an "end-of-pipe" solution^[104] that reduces atmospheric CO₂, instead of minimizing the use of fossil fuel.^[104][110]

On 21 January 2021, Elon Musk announced he was donating $100m for a prize for best carbon capture technology.^[111]

Environmental justice

Carbon capture facilities are often designed to be located near existing oil and gas infrastructure.^[112]

A 2021 DeSmog Blog story highlighted, "CCS hubs are likely be sites in communities already being impacted by the climate crisis like Lake Charles and those along the Mississippi River corridor, where most of the state carbon pollution is emitted from fossil fuel power plants. Exxon, for example, is backing a carbon storage project in Houston's shipping channel, another environmental justice community."^[113]

Political debate

CCS has been discussed by political actors at least since the start of the UNFCCC^[114] negotiations in the beginning of the 1990s, and remains a very divisive issue.^{[citation needed]}

Some environmental groups raised concerns over leakage given the long storage time required, comparing CCS to storing radioactive waste from nuclear power stations.^[115]

Other controversies arose from the use of CCS by policy makers as a tool to fight climate change.^{[citation needed]} In the IPCC's Sixth Assessment Report in 2022, most pathways to keep the increase of global temperature below 2 °C include the use of negative emission technologies (NETs).^[116]

Carbon emission status-quo

Opponents claimed that CCS could legitimize the continued use of fossil fuels, as well obviate commitments on emission reduction.^{[citation needed]}

Some examples such as in Norway shows that CCS and other carbon removal technologies gained traction because it allowed the country to pursue its interests regarding the petroleum industry. Norway was a pioneer in emission mitigation, and established a CO₂ tax in 1991.^[117]

Environmental NGOs

Environmental NGOs are not in widespread agreement about CCS as a potential climate mitigation tool. The main disagreement amid NGOs is whether CCS will reduce CO₂ emissions or just perpetuate the use of fossil fuels.^{[118][better source needed]}

For instance, Greenpeace is strongly against CCS. According to the organization, the use of the technology will keep the world dependent on fossil fuels.^{[119][better source needed]}

On the other hand, BECCS is used in some IPCC scenarios to help meet mitigation targets.^[120] Adopting the IPCC argument that CO₂ emissions need to be reduced by 2050 to avoid dramatic consequences, the Bellona Foundation justified CCS as a mitigation action.^[119] They claimed fossil fuels are unavoidable for the near term and consequently, CCS is the quickest way to reduce CO₂ emissions.^[105]

Example projects

According to the Global CCS Institute, in 2020 there was about 40 million tons CO₂ per year capacity of CCS in operation and 50 million tons per year in development.^[121] In contrast, the world emits about 38 billion tonnes of CO₂ every year,^[122] so CCS captured about one thousandth of the 2020 CO₂ emissions. Iron and steel is expected to dominate industrial CCS in Europe,^[5] although there are alternative ways of decarbonizing steel.^[123]

CCS and climate change mitigation

CCS can be employed to achieve a number of goals regarding climate change mitigation, such as preventing average global temperature from reaching certain levels above the pre-industrial average. In December 2015, the Paris Agreement articulated a census to not exceed pre-industrial global temperatures by more than 2 °C and recognized that different countries would have different contributions to help realize this goal.^[124] Under the Paris Agreement, different scenarios and climate models were analyzed for different temperature goals considering a wide range of mitigation methods from a temperature goal of less than 2 °C to an upper limit of exactly 2 °C increase above the pre-industrial average.

The terms CCS and CCUS (Carbon Capture, Utilization, and Storage) are often used interchangeably. The difference between the two is the specified 'utilization' of the captured carbon and refers to its use for other applications, such as enhanced oil recovery (EOR), potentially making liquid fuel, or the manufacturing of useful consumer goods, such as plastics. Since both approaches capture emitted CO₂ and effectively store it, whether that be under-ground in geological formations or long-term trapping in material products, the two terms are often treated the same.

CCS is considered as a basis of one climate stabilization wedge, which is a proposed climate mitigation action to reduce approximately 1 billion tonnes of carbon emissions over 50 years.^[125]

CCS and different climate models

Carbon dioxide emissions and atmospheric concentrations over the 21st century

Large scale CCS plays a crucial role in reaching climate change stabilization. According to the IPCC, the carbon emission patterns can greatly vary based on the uncertainty of human power consumption. A file regarding the fluctuations of greenhouse gas emissions is shown to the right. However, CCS' primarily role is to delay the shift from fossil fuels and thereby reducing transition costs. The implementation of default technology assumptions would cost 29-297% more over the century than efforts without CCS for a 430-480 ppm CO₂/yr scenario.^{[126][unreliable source?][127]} The Paris agreement upholds a goal to reach no more than a 2.0 °C increase above pre-industrial temperatures. If the 2.0 °C goal is to be reached in time, CCS must be utilized to achieve net zero emissions by 2060-2070. After 2060-2070, negative emissions will need to be achieved to remain below the 2.0 °C target. The variations in methods depend heavily on the climate change model being used and the anticipated energy consumption patterns. It is widely agreed upon, however, that CCS would need to be utilized if there is to be any negative climate change mitigation.^[128]

CCS and 2.0°C target

The concept of a 2.0 °C came to light in the European Union of 1996 where the goal was to reduce the global temperature range relative pre-industrial levels. The decision of the 2 °C range was decided mostly on the evidence that many ecosystems are at risk if average global temperatures exceeded this limit. In order to limit the anthropogenic emissions such that there is no more than a 2 °C change relative to the periods between 1861 and 1880, carbon emissions would need to be limited to about 1000 GtC by 2100 since that period. However, by the end of 2011 about half of the budget was already released (445 GtC) indicating that a lower budget is necessary.^[129]

A distinctive path that aims for a 2.0 °C limit might have complications. The first complication involves the lack of positive feedback loops in IPCC climate models. These loops include reduction of ice sheet size, which would mean less sunlight is reflected and more is absorbed by the darker colored ground or water, and the potential release of greenhouse gases by thawing tundra. Since the lifetime of CO₂ in the climate atmosphere is so long, these feedback loops have to be taken into consideration. Another important factor to consider is that a 2.0 °C scenario necessitates tapping into alternative fossil fuels sources that are harder to obtain. Some examples of these methods are the exploitation of tar sands, tar shales, hydrofracking for oil and gas, coal mining, drilling in the Arctic, Amazon, and deep ocean. Therefore, 2.0 °C scenarios result in more CO₂ produced per unit of usable energy. Further, the danger of extra released CH₄ via mining processes must be taken into account.^[130]

Global greenhouse gas emissions by gas type

Different models are based on when the peak of carbon emissions happen on a global scale. In one article regarding the 2.0 °C scenario with respect to pre-industrial levels, possible approaches are short term and long term emission resolutions as well as the considering the cost effectiveness of different solutions to reduce carbon emissions. Short term goals are set to quantify progress towards the temperature goal. In a short term goal, looking ahead to the year 2020, the allowable carbon emissions must be between 41 and 55 GtCO₂ per year. The short term 2 °C scenario is not feasible without CCS.^[131]

Currently, greenhouse gas emissions would need to be reduced by 7 Gt of carbon equivalent each year by 2050 to achieve 2 °C stabilization. This requires power generation with CCS at 800 coal-fired power plants of 1 GW energy generation capacity, 180 coal-synfuel plants, or natural gas plants worth 1,600 GW.^[132] In this scenario, one of the wedges, or 1 Gt of carbon is accounted for by CCUS.^[133]

CCS and below 2.0°C target

Achieving below 2.0°C target

A change of temperature below 2 °C is, to certain extent, almost impossible to achieve due to the current carbon emission practices. The IPCC notes that it is difficult to assess a climate mitigation scenario that would limit average global temperature increase to only 1.5 °C above pre-industrial levels. This is mainly due to the fact that few reliable multi-model studies have been conducted to thoroughly explore this scenario. Nevertheless, what few studies that have been done agree that mitigation technologies must be implemented immediately and scaled up quickly and reflect energy demand decrease.^[134] A change below 1 °C with respect to pre-industrial era is now inconceivable because by 2017 there was already an increase of 1 °C.^[135]

Because of the immediate inability to control the temperature at the 1 °C target, the next realistic target is 1.5 °C. There is enough confidence that past emissions alone (pre-industrial time) will not be enough to go beyond the 1.5 °C target. In other words, if all anthropogenic emissions were stopped today (reduced to zero), any increase beyond the 1 °C change for more than half of a degree before 2100 is unlikely. If anthropogenic emission are considered, the probability for the planet increasing for more than 1.5 °C before 2100 are high. Then, scenarios where the degree change is maintain below 1.5 °C are very challenging to achieve but not impossible.^[136]

For a below 2.0 °C target, Shared socioeconomic pathways (SSPs) had been developed adding a socio-economic dimension to the integrative work started by RCPs models. The advantage of using SSPs is that they incorporate social standards, fossil fuel use, geographical development, and high energy demand. SSPs also incorporate the use of six other models such as GCAM4, IMAGE, MESSAGE-GLOBIOM, and REMIND-MAgPIE. The combination of models and scenarios concluded that by 2050, annual CO₂ emissions are in the range between 9 and 13 billion tons of CO₂. All of the scenarios estimated that temperature will remain below 2.0 °C change with a 66% probability of success. To do so, a 1.9 W/m² within the year 2100 is necessary. Net zero GHG emissions have to be achieved between 2055 and 2075, and CO₂ emissions have to be in a range between 175 and 475 GtCO₂ between the years 2016-2100. All SSPs scenarios show a shift away from unabated fossil fuels, that is process without CCS.^[136]

Assumptions for below 2.0°C target

To achieve a 1.5 °C target before 2100, the following assumptions have to be considered; emissions have to peak by 2020 and decline after that, it will be necessary to reduce net CO₂ emissions to zero and negative emissions have to be a reality by the second half of the 21st century. For this assumptions to take place, CCS has to be implemented in factories that accompany the use of fossil fuels. Because emissions reduction has to be implemented more rigorously for a 1.5 °C target, methods such as BEECS, and natural climate solutions such as afforestation can be used to aim in the reduction of global emissions.^[137] BECCS is necessary to achieve a 1.5 °C. It is estimated by the models that with the help of BECCS, between 150 and 12000 GtCO₂ still have to be removed from the atmosphere.^[136]

Another negative emission strategy which includes CCS can also be approached through DACCS. Direct Air Carbon Capture and Sequestration (DACCS) is a carbon negative technology that utilizes solid amine based capture and it has proven to capture carbon dioxide from the air even though content of the air is much lower than of a flue gas from a coal plant.^[138] However, it would require renewable energies to power since approximately 400kJ of work is needed per mole of CO₂ capture. Furthermore, it is estimated that the total system cost is $1,000 per tonne of CO₂, according to an economic and energetic analysis from 2011.^[139]

Going forward in the utilization of models such as SSPss and RCP, feasibility of the model has to be to take into consideration. Feasibility includes concerns in various fields, such as geophysics, technology, economics, social acceptance, and politics, all of which can serve to facilitate or obstruct the carbon capture and sequestration of emissions needed in order to achieve the global temperature targets. Uncertainty in feasibility is especially a problem with more strict temperatures limits such as 1.5 °C. Real world feasibility of SSPs models, or any other models, in general are coarse approximations of reality.^[136]