Carbon capture and storage (CCS) or carbon capture and sequestration 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. Although CO2 has been injected into geological formations for several decades for various purposes, including enhanced oil recovery, the long-term storage of CO2 is a relatively new concept. 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 with an intrinsic low value (i.e. CO2). Hence, carbon capture makes economically more sense when being 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.
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. As of 2020[update], about one thousandth of global CO2 emissions are captured by CCS. Most projects are industrial.
Storage of the CO2 is envisaged either in deep geological formations, or in the form of mineral carbonates. Pyrogenic carbon capture and storage (PyCCS) is also being researched. 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. 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. Despite this, a recent evaluation estimates the risk of substantial leakage to be fairly low.
Opponents point out that many CCS projects have failed to deliver on promised emissions reductions. 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.
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), natural gas processing, synthetic fuel plants and fossil fuel-based hydrogen production plants. Extracting CO2 from air is possible, although the lower concentration of CO2 in air compared to combustion sources complicates the engineering and makes the process therefore more expensive.
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. It is possible to capture approximately 65% of CO2 embedded in it and sequester it in a solid form.
Broadly, three different technologies exist: post-combustion, pre-combustion, and oxyfuel combustion:
The major technologies proposed for carbon capture are:
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.
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. 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. 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. 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. Research is attempting to optimize MOF properties for CCS. Metal reservoirs are another limiting factor.
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.
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.
CCS could reduce CO2 emissions from smokestacks by 85–90% or more, but it has no net effect on CO2 emissions due to the mining and transport of coal (something that is frequently overlooked when considering “green” alternatives such as batteries). It will actually "increase such emissions and of air pollutants per unit of net delivered power and will increase all ecological, land-use, air-pollution, and water-pollution impacts from coal mining, transport, and processing, because CCS requires 25% more energy, thus 25% more coal combustion, than does a system without CCS".
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.
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.
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. The CO2 stream produced can be stored or used for other industrial processes.
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. These methods are used for transporting CO2 for other applications. Rail and tanker truck cost about twice as much as pipelines or ships.
For example, approximately 5,800 km of CO2 pipelines operated in the US in 2008, and a 160 km pipeline in Norway, 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.
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.
Main article: Carbon sequestration
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. It was once suggested that CO2 could be stored in the oceans, but this would exacerbate ocean acidification and was banned under the London and OSPAR conventions.
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.
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.
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. 
CO2 is occasionally injected into an oil field as an enhanced oil recovery technique, but because CO2 is released when the oil is burned, it is not carbon neutral.
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.
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.[self-published source?] The reaction rate can be accelerated with a catalyst 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. Theoretically, up to 22% of crustal mineral mass is able to form carbonates.
|Earthen oxide||Percent of crust||Carbonate||Enthalpy change (kJ/mol)|
Ultramafic mine tailings are a readily available source of fine-grained metal oxides that can serve this purpose. Accelerating passive CO2 sequestration via mineral carbonation may be achieved through microbial processes that enhance mineral dissolution and carbonate precipitation.
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. 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. CCS would increase the fuel requirement of a plant with CCS by about 15% (gas plant). 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, including China, in part because revenue from enhanced oil recovery collapsed with the 2020 oil price collapse. A carbon price of at least 100 euros per tonne CO2 is estimated to be needed to make industrial CCS viable, together with carbon tariffs. But, as of mid-2022, the EU Allowance had never reached that price and the Carbon Border Adjustment Mechanism had not yet been implemented. However a company making small modules claims it can get well below that price by mass production by 2022.
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%.
Possible business models for industrial carbon capture include:
Governments have provided various types of funding for CCS demonstration projects, including tax credits, allocations and grants.
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. At COP17 in Durban, a final agreement was reached enabling CCS projects to receive support through the Clean Development Mechanism.
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 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]
The extra energy requirements deriving from CCS for natural gas combined cycle (NGCC) plants range from 11 to 22%. 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 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. However a 2022 study concluded that it would be too expensive for coal power in China.
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%. 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.
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. However, this finding is contested given the lack of experience. 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.
Mineral storage is not regarded as presenting any leakage risks.
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.
In March 2009 StatoilHydro issued a study documenting the slow spread of CO2 in the formation after more than 10 years operation.
Gas leakage into the atmosphere may be detected via atmospheric gas monitoring, and can be quantified directly via eddy covariance flux measurements.
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. 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.[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. In the Berkel en Rodenrijs incident in December 2008 a modest release from a pipeline under a bridge killed some ducks sheltering there.
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.
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.
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. It can identify migration pathways of the CO2 plume.
Examples of seismic monitoring of geological sequestration are the Sleipner sequestration project, the Frio CO2 injection test and the CO2CRC Otway Project. Seismic monitoring can confirm the presence of CO2 in a given region and map its lateral distribution, but is not sensitive to the concentration.
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).
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. 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. 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. They also measure vertical flux. Monitoring a large site would require a network of chambers.
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. 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.
Carbon capture and utilization (CCU) is the process of capturing carbon dioxide (CO2) to be recycled for further usage. Carbon capture and utilization may offer a response to the global challenge of significantly reducing greenhouse gas emissions from major stationary (industrial) emitters. 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.
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. 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.The profitability of CCU depends partly on the carbon price of CO2 being released into the atmosphere.
Multiple studies indicate that risk and benefit perception are the most essential components of social acceptance.
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. Other perceived risks relate to tourism and property values.
People who are already affected by climate change, such as drought, tend to be more supportive of CCS. Locally, communities are sensitive to economic factors, including job creation, tourism or related investment.
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.
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. Conversely, approval seems to be reinforced when CCS is compared to natural phenomena.
Due to the lack of knowledge, people rely on organizations that they trust. In general, non-governmental organizations and researchers experience higher trust than stakeholders and governments. Opinions amongst NGOs are mixed. Moreover, the link between trust and acceptance is at best indirect. Instead, trust has an influence on the perception of risks and benefits.
CCS is embraced by the shallow ecology worldview, 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 that reduces atmospheric CO2, instead of minimizing the use of fossil fuel.
On 21 January 2021, Elon Musk announced he was donating $100m for a prize for best carbon capture technology.
Carbon capture facilities are often designed to be located near existing oil and gas infrastructure.
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."
CCS has been discussed by political actors at least since the start of the UNFCCC negotiations in the beginning of the 1990s, and remains a very divisive issue. CCS was included in the Kyoto Protocol, and this inclusion was a precondition for the signing of the treaty by the United States, Norway, Russia and Canada.
Some environmental groups raised concerns over leakage given the long storage time required, comparing CCS to storing radioactive waste from nuclear power stations.
Other controversies arose from the use of CCS by policy makers as a tool to fight climate change. In the IPCC's Fourth Assessment Report in 2007, a possible pathway to keep the increase of global temperature below 2 °C included the use of negative emission technologies (NETs).
Opponents claimed that CCS could legitimize the continued use of fossil fuels, as well obviate commitments on emission reduction.
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. However, strong growth in Norway's petroleum sector made domestic emission cuts increasingly difficult throughout the 1990s. The country's successive governments struggled to pursue ambitious emission mitigation policies. The compromise was set to reach ambitious emission cuts targets without disrupting the economy, which was achieved by extensively relying on Kyoto Protocol's flexible mechanisms regarding carbon sinks, whose scope could extend beyond national borders.
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.
For instance, Greenpeace is strongly against CCS. According to the organization, the use of the technology will keep the world dependent on fossil fuels. In 2008, Greenpeace published "False hope: Why Carbon Capture and Storage Won't Save the Climate" to explain their posture. Their only solution is the reduction of fossil fuel usage. Greenpeace claimed that CCS could lead to a doubling of coal plant costs.
On the other hand, BECCS is used in some IPCC scenarios to help meet mitigation targets. 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. They claimed fossil fuels are unavoidable for the near term and consequently, CCS is the quickest way to reduce CO2 emissions.
Main article: List of carbon capture and storage 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. In contrast, the world emits about 38 billion tonnes of CO2 every year, so CCS captured about one thousandth of the 2020 CO2 emissions.
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Since 2008 Norway's Statoil has been transporting CO2 (obtained from natural gas extraction) through a 160 km seabed pipeline