The Intergovernmental Panel on Climate Change (IPCC) defines CCS as:
"A process in which a relatively pure stream of carbon dioxide (CO2) from industrial and energy-related sources is separated (captured), conditioned, compressed and transported to a storage location for long-term isolation from the atmosphere."[15]: 2221
Around 1% of captured CO2 is used as a feedstock for making products such as fertilizer, fuels, and plastics.[16] These uses are forms of carbon capture and utilization.[17] 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.[17]
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.[18][19][20] 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 the flue gas of a power plant.
The use of CCS as a means of reducing anthropogenic 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.[24][25] 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.[8]: 25
In 2020, the International Energy Agency (IEA) 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."[8]: 18
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.[5] As of 2020, North America has more than 8000 km of CO2 pipelines, and there are two CO2 pipeline systems in Europe and two in the Middle East.[8]: 103–104
After the CO2 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.[8]: 103–104 Transport via ship has been researched. CO2 can also be transported by truck or rail, albeit at higher cost per tonne of CO2.[8]: 103–104
CCS processes involve several different technologies working together. Technological components are used to separate and treat CO2 from a flue 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 flue gas mixture: post-combustion capture, pre-combustion capture, and oxy-combustion:[32]
Absorption, or carbon scrubbing with amines is the dominant capture technology.[8]: 98 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.[36][37][38]
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 scrubbing separation process is needed to initially clean the flue gas.[39]
Storing CO2 involves the injection of captured CO2 into a deep underground geological reservoir of porous rock overlaid by an impermeable layer of rocks, which seals the reservoir and prevents the upward migration of CO2 and escape into the atmosphere.[8]: 112 The gas is usually compressed first into a supercritical fluid. When the compressed CO2 is injected into a reservoir, it flows through it, filling the pore space. The reservoir must be at depths greater than 800 meters to retain the CO2 in a fluid state.[8]: 112
Around 20% of captured CO2 is injected into dedicated geological storage,[2] usually deep saline aquifers. These are layers of porous and permeable rocks saturated with salty water.[8]: 112 Worldwide, saline formations have higher potential storage capacity than depleted oil wells.[42] Dedicated geologic storage is generally less expensive than EOR because it does not require a high level of CO2 purity and because suitable sites are more numerous, which means pipelines can be shorter.[43]
In geologic storage, the CO2 is held within the reservoir through several trapping mechanisms: structural trapping by the caprock seal, solubility trapping in pore space water, residual trapping in individual or groups of pores, and mineral trapping by reacting with the reservoir rocks to form carbonate minerals.[8]: 112 Mineral trapping progresses over time but is extremely slow.[48]: 26
Once injected, the CO2 plume tends to rise since it is less dense than its surroundings. Once it encounters a caprock, it will spread laterally until it encounters a gap. If there are fault planes near the injection zone, CO2 could migrate along the fault to the surface, leaking into the atmosphere, which would be potentially dangerous to life in the surrounding area. If the injection of CO2 creates pressures underground that are too high, the formation will fracture, potentially causing an earthquake.[49] While research suggests that earthquakes from injected CO2 would be too small to endanger property, they could be large enough to cause a leak.[50]
The IPCC estimates that at appropriately-selected and well-managed storage sites, it is likely that over 99% of CO2 will remain in place for more than 1000 years, with "likely" meaning a probability of 66% to 90%.[47]: 14,12 Estimates of long-term leakage rates rely on complex simulations since field data is limited.[51] If very large amounts of CO2 are sequestered, even a 1% leakage rate over 1000 years could cause significant impact on the climate for future generations.[52]
Facilities with CCS use more energy than those without CCS. The energy consumed by CCS is called an "energy penalty". The energy penalty of CCS varies depending on the source of CO2. If the flue gas has a very high concentration of CO2, additional energy is needed only to dehydrate, compress, and pump the CO2.[8]: 101–102 If the flue gas has a lower concentration of CO2, as is the case for power plants, energy is also required to separate CO2 from other flue gas components.[8]: 101–102
Early studies indicated that to produce the same amount of electricity, a coal power plant would need to burn 14 - 40% more coal and a natural gas combined cycle power plant would need to burn 11 - 22% more gas.[47]: 27 When CCS is used in coal power plants, it has been estimated that about 60% of the energy penalty originates from the capture process, 30% comes from compression of the extracted CO2, and the remaining 10% comes from pumps and fans.[53]
Depending on the technology used, CCS can require large amounts of water. For instance, coal- fired power plants with CCS may need to use 50% more water.[54]: 668
Since plants with CCS require more fuel to produce the same amount of electricity or heat, the use of CCS increases the "upstream" environmental problems of fossil fuels. Upstream impacts include pollution caused by coal mining, emissions from the fuel used to transport coal and gas, emissions from gas flaring, and fugitive methane emissions.
Since CCS facilities require more fossil fuel to be burned, CCS can cause a net increase in air pollution from those facilities. This can be mitigated by pollution control equipment, however no equipment can eliminate all pollutants.[6] Since liquid amine solutions are used to capture CO2 in many CCS systems, these types of chemicals can also be released as air pollutants if not adequately controlled. Among the chemicals of concern are volatile nitrosamines which are carcinogenic when inhaled or drunk in water.[57][58]
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