Supercritical carbon dioxide (sCO
2[disputed –discuss]) is a fluid state ofcarbon dioxide where it is held at or above itscritical temperature andcritical pressure.
Carbon dioxide usually behaves as agas inair atstandard temperature and pressure (STP), or as asolid calleddry ice when cooled and/or pressurised sufficiently. If thetemperature andpressure are both increased from STP to be at or above thecritical point for carbon dioxide, it can adopt properties midway between a gas and aliquid. More specifically, it behaves as asupercritical fluid above its critical temperature (304.128 K, 30.9780 °C, 87.7604 °F)[1] and critical pressure (7.3773 MPa, 72.808 atm, 1,070.0 psi, 73.773 bar),[1] expanding to fill its container like a gas but with adensity like that of a liquid.
SupercriticalCO
2 is becoming an important commercial and industrialsolvent due to its role in chemicalextraction, in addition to its relatively lowtoxicity and environmental impact. The relatively low temperature of the process and the stability ofCO
2 also allows compounds to be extracted with little damage ordenaturing. In addition, the solubility of many extracted compounds inCO
2 varies with pressure,[2] permitting selective extractions.
Carbon dioxide is gaining popularity amongcoffee manufacturers looking to move away from classic decaffeinatingsolvents. sCO
2 is forced through green coffee beans which are then sprayed with water at high pressure to remove the caffeine. The caffeine can then be isolated for resale (e.g., to pharmaceutical or beverage manufacturers) by passing the water throughactivated charcoal filters or bydistillation,crystallization orreverse osmosis. Supercritical carbon dioxide is used to removeorganochloridepesticides and metals from agricultural crops without adulterating the desired constituents from plant matter in theherbal supplement industry.[3]
Supercritical carbon dioxide can be used as a solvent indry cleaning.[4]
Supercritical carbon dioxide is used as the extraction solvent for creation ofessential oils and otherherbal distillates.[5] Its main advantages over solvents such ashexane andacetone in this process are that it is non-flammable and does not leave toxic residue. Furthermore, separation of the reaction components from the starting material is much simpler than with traditionalorganic solvents. TheCO
2 can evaporate into the air or be recycled by condensation into a recovery vessel. Its advantage oversteam distillation is that it operates at a lower temperature, which can separate the plantwaxes from the oils.[6]
Inlaboratories, sCO
2 is used as an extraction solvent, for example for determining total recoverable hydrocarbons from soils, sediments, fly-ash, and other media,[7] and determination ofpolycyclic aromatic hydrocarbons in soil and solid wastes.[8] Supercritical fluid extraction has been used in determininghydrocarbon components in water.[9]
Processes that use sCO
2 to produce micro andnano scale particles, often forpharmaceutical uses, are under development. The gasantisolvent process, rapid expansion of supercritical solutions, and supercritical antisolventprecipitation (as well as several related methods) process a variety of substances into particles.[10]
Due to its ability to selectively dissolve organic compounds and assist enzyme functioning, sCO
2 has been suggested as a potential solvent to support biological activity onVenus- orsuper-Earth-type planets.[11]
Environmentally beneficial, low-cost substitutes for rigidthermoplastic and firedceramic are made using sCO
2 as achemical reagent. The sCO
2 in these processes is reacted with the alkaline components of fully hardenedhydraulic cement orgypsumplaster to form various carbonates.[12] The primary byproduct is water.
sCO
2 is used in the foaming ofpolymers. Supercritical carbon dioxide can saturate the polymer with solvent. Upon depressurization and heating, the carbon dioxide rapidly expands, causing voids within the polymer matrix, i.e., creating afoam. Research is ongoing on microcellular foams.
Anelectrochemicalcarboxylation of a para-isobutylbenzyl chloride toibuprofen is promoted under sCO
2.[13]
sCO
2 is chemically stable, reliable, low-cost, non-flammable and readily available, making it a desirable candidateworking fluid fortranscritical cycles.[14]
Supercritical CO2 is used as the working fluid in domestic waterheat pumps. Manufactured and widely used, heat pumps are available for domestic and business heating and cooling.[14] While some of the more common domestic water heat pumps remove heat from the space in which they are located, such as a basement or garage, CO2 heat pump water heaters are typically located outside, where they remove heat from the outside air.[14]
sCO
2 has also been considered as a working fluid for enhanced geothermal systems due to its aqueous and gas properties.[15] sCO
2 is also favorable for enhanced geothermal due to its high buoyancy, large expansivity and low viscosity.[16]
The unique properties of sCO
2 present advantages for closed-loop power generation and can be applied to power generation applications. Power generation systems that use traditional airBrayton and steamRankine cycles can use sCO
2 to increase efficiency and power output.
The relatively newAllam power cycle uses sCO2 as the working fluid in combination with fuel and pure oxygen. The CO2 produced by combustion mixes with the sCO2 working fluid. A corresponding amount of pure CO2 must be removed from the process (for industrial use or sequestration). This process reduces atmospheric emissions to zero.
sCO2 promises substantial efficiency improvements. Due to its high fluid density, sCO2 enables compact and efficientturbomachinery. It can use simpler, single casing body designs whilesteam turbines require multiple turbine stages and associated casings, as well as additional inlet and outlet piping. The high density allows more compact, microchannel-based heat exchanger technology.[17]
Forconcentrated solar power, carbon dioxidecritical temperature is not high enough to obtain the maximum energy conversion efficiency. Solar thermal plants are usually located in arid areas, so it is impossible to cool down the heat sink to sub-critical temperatures. Therefore,supercritical carbon dioxide blends, with higher critical temperatures, are in development to improve concentrated solar power electricity production.
Further, due to its superior thermal stability and non-flammability, direct heat exchange from high temperature sources is possible, permitting higher working fluid temperatures and therefore higher cycle efficiency. Unliketwo-phase flow, the single-phase nature of sCO
2 eliminates the necessity of a heat input for phase change that is required for the water to steam conversion, thereby also eliminating associatedthermal fatigue and corrosion.[18]
The use of sCO
2 presentscorrosion engineering, material selection and design issues. Materials in power generation components must display resistance to damage caused byhigh-temperature,oxidation andcreep. Candidate materials that meet these property and performance goals include incumbent alloys in power generation, such as nickel-basedsuperalloys for turbomachinery components andaustenitic stainless steels for piping. Components within sCO
2 Brayton loops suffer from corrosion and erosion, specifically erosion in turbomachinery and recuperative heat exchanger components andintergranular corrosion and pitting in the piping.[19]
Testing has been conducted on candidate Ni-based alloys, austenitic steels, ferritic steels and ceramics for corrosion resistance in sCO
2 cycles. The interest in these materials derive from their formation of protective surface oxide layers in the presence of carbon dioxide, however in most cases further evaluation of the reaction mechanics and corrosion/erosion kinetics and mechanisms is required, as none of the materials meet the necessary goals.[20][21]
In 2016, General Electric announced a sCO2-based turbine that enabled a 50% efficiency of converting heat energy to electrical energy. In it the CO2 is heated to 700 °C. It requires less compression and allows heat transfer. It reaches full power in 2 minutes, whereas steam turbines need at least 30 minutes. The prototype generated 10 MW and is approximately 10% the size of a comparable steam turbine.[22] The 10 MW US$155-million Supercritical Transformational Electric Power (STEP) pilot plant was completed in 2023 in San Antonio. It is the size of a desk and can power around 10,000 homes.[23]
Due to its low viscosity while retaining solvent capacities, supercritical carbon dioxide is also used for impregnation of different raw materials. The biggest industrial application in this field isimpregnation of wood with biocides. The big advantage is, that theCO
2 can penetrate the complete structure of the board, carrying the active ingredient and therefore fully impregnating it.[24]
Work is underway to develop a sCO
2closed-cycle gas turbine to operate at temperatures near 550 °C. This would have implications for bulk thermal and nuclear generation of electricity, because the supercritical properties of carbon dioxide at above 500 °C and 20 MPa enable thermal efficiencies approaching 45 percent. This could increase the electrical power produced per unit of fuel required by 40 percent or more. Given the volume of carbon fuels used in producing electricity, the environmental impact of cycle efficiency increases would be significant.[25]
SupercriticalCO
2 is an emerging natural refrigerant, used in new, low carbon solutions for domesticheat pumps. SupercriticalCO
2 heat pumps are commercially marketed in Asia. EcoCute systems from Japan, developed by Mayekawa, develop high temperature domestic water with small inputs of electric power by moving heat into the system from the surroundings.[26]
SupercriticalCO
2 has been used since the 1980s to enhance recovery in mature oil fields.
"Clean coal" technologies are emerging that could combine such enhanced recovery methods withcarbon sequestration. Usinggasifiers instead of conventional furnaces, coal and water is reduced to hydrogen gas, carbon dioxide and ash. This hydrogen gas can be used to produce electrical power Incombined cycle gas turbines,CO
2 is captured, compressed to the supercritical state and injected into geological storage, possibly into existing oil fields to improve yields.[27][28][29]
SupercriticalCO
2 can be used as a working fluid for geothermal electricity generation in bothenhanced geothermal systems[30][31][32][33] and sedimentary geothermal systems (so-calledCO
2 Plume Geothermal).[34][35] EGS systems utilize an artificially fractured reservoir in basement rock while CPG systems utilize shallower naturally-permeable sedimentary reservoirs. Possible advantages of usingCO
2 in a geologic reservoir, compared to water, include higher energy yield resulting from its lower viscosity, better chemical interaction, and permanentCO
2 storage as the reservoir must be filled with large masses ofCO
2. As of 2011, the concept had not been tested in the field.[36]
Supercritical carbon dioxide is used in the production of silica, carbon and metal basedaerogels. For example, silicon dioxide gel is formed and then exposed to sCO
2. When theCO
2 goes supercritical, all surface tension is removed, allowing the liquid to leave the aerogel and produce nanometer sized pores.[37]
SupercriticalCO
2 is an alternative for thermal sterilization of biological materials and medical devices with combination of the additiveperacetic acid (PAA). SupercriticalCO
2 does not sterilize the media, because it does not kill the spores of microorganisms. Moreover, this process is gentle, as the morphology, ultrastructure and protein profiles of inactivated microbes are preserved.[38]
Natural Cork is a suberin based natural polymer foam that can have unpleasant substances which are responsible for the loss of quality in wine. The leading substance is 2,4,6 trichloroanisole (TCA), which can be detected already at 2 ng/l in the wine. SupercriticalCO
2 is used at industrial scale to remove this substance and guarantee that the wine remains in its full quality.[39]
SupercriticalCO
2 is used in certain industrialcleaning processes.
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