A carbon dioxide scrubber is a piece of equipment that absorbs carbon dioxide (CO2). It is used to treat exhaust gases from industrial plants or from exhaled air in life support systems such as rebreathers or in spacecraft, submersible craft or airtight chambers. Carbon dioxide scrubbers are also used in controlled atmosphere (CA) storage. They have also been researched for carbon capture and storage as a means of combating climate change.
The primary application for CO2 scrubbing is for removal of CO2 from the exhaust of coal- and gas-fired power plants. Virtually the only technology being seriously evaluated involves the use of various amines, e.g. monoethanolamine. Cold solutions of these organic compounds bind CO2, but the binding is reversed at higher temperatures:
As of 2009 [update] , this technology has only been lightly implemented because of capital costs of installing the facility and the operating costs of utilizing it. [1]
Several minerals and mineral-like materials reversibly bind CO2. [2] Most often, these minerals are oxides or hydroxides, and often the CO2 is bound as carbonate. Carbon dioxide reacts with quicklime (calcium oxide) to form limestone (calcium carbonate), [3] in a process called carbonate looping. Other minerals include serpentinite, a magnesium silicate hydroxide, and olivine. [4] [5] Molecular sieves also function in this capacity.
Various (cyclical) scrubbing processes have been proposed to remove CO2 from the air or from flue gases and release them in a controlled environment, reverting the scrubbing agent. These usually involve using a variant of the Kraft process which may be based on sodium hydroxide. [6] [7] The CO2 is absorbed into such a solution, transfers to lime (via a process called causticization) and is released again through the use of a kiln. With some modifications to the existing processes (mainly changing to an oxygen-fired kiln) the resulting exhaust becomes a concentrated stream of CO2, ready for storage or use in fuels. An alternative to this thermo-chemical process is an electrical one which releases the CO2 through electrolyzing of the carbonate solution. [8] While simpler, this electrical process consumes more energy as electrolysis, also splits water. To prevent negating the environmental benefit of using electrolysis over the kiln method, the electricity should come from a renewable (or less emissive than the otherwise needed kiln) source. Early incarnations of environmentally motivated CO2 capture used electricity as the energy source and were therefore dependent on green energy. Some thermal CO2 capture systems use heat generated on-site, which reduces the inefficiencies resulting from off-site electricity production, but it still needs a source of (green) heat, which nuclear power or concentrated solar power could provide. [9]
Zeman and Lackner outlined a specific method of air capture. [10]
First, CO2 is absorbed by an alkaline NaOH solution to produce dissolved sodium carbonate. The absorption reaction is a gas liquid reaction, strongly exothermic, here:
Causticization is performed ubiquitously in the pulp and paper industry and readily transfers 94% of the carbonate ions from the sodium to the calcium cation. [10] Subsequently, the calcium carbonate precipitate is filtered from solution and thermally decomposed to produce gaseous CO2. The calcination reaction is the only endothermic reaction in the process and is shown here:
The thermal decomposition of calcite is performed in a lime kiln fired with oxygen in order to avoid an additional gas separation step. Hydration of the lime (CaO) completes the cycle. Lime hydration is an exothermic reaction that can be performed with water or steam. Using water, it is a liquid/solid reaction as shown here:
Other strong bases such as soda lime, sodium hydroxide, potassium hydroxide, and lithium hydroxide are able to remove carbon dioxide by chemically reacting with it. In particular, lithium hydroxide was used aboard spacecraft, such as in the Apollo program, to remove carbon dioxide from the atmosphere. It reacts with carbon dioxide to form lithium carbonate. [11] Recently lithium hydroxide absorbent technology has been adapted for use in anesthesia machines. Anesthesia machines which provide life support and inhaled agents during surgery typically employ a closed circuit necessitating the removal of carbon dioxide exhaled by the patient. Lithium hydroxide may offer some safety and convenience benefits over the older calcium based products.
The net reaction being:
Lithium peroxide can also be used as it absorbs more CO2 per unit weight with the added advantage of releasing oxygen. [12]
In recent years lithium orthosilicate has attracted much attention towards CO2capture, as well as energy storage. [8] This material offers considerable performance advantages although it requires high temperatures for the formation of carbonate to take place.
The regenerative carbon dioxide removal system (RCRS) on the Space Shuttle orbiter used a two-bed system that provided continuous removal of carbon dioxide without expendable products. Regenerable systems allowed a shuttle mission a longer stay in space without having to replenish its sorbent canisters. Older lithium hydroxide (LiOH)-based systems, which are non-regenerable, were replaced by regenerable metal-oxide-based systems. A system based on metal oxide primarily consisted of a metal oxide sorbent canister and a regenerator assembly. It worked by removing carbon dioxide using a sorbent material and then regenerating the sorbent material. The metal-oxide sorbent canister was regenerated by pumping air at approximately 200 °C (392 °F) through it at a standard flow rate of 3.5 L/s (7.4 cu ft/min) for 10 hours. [13]
Activated carbon can be used as a carbon dioxide scrubber. Air with high carbon dioxide content, such as air from fruit storage locations, can be blown through beds of activated carbon and the carbon dioxide will adhere to the activated carbon [adsorption]. Once the bed is saturated it must then be "regenerated" by blowing low carbon dioxide air, such as ambient air, through the bed. This will release the carbon dioxide from the bed, and it can then be used to scrub again, leaving the net amount of carbon dioxide in the air the same as when the process was started. [ citation needed ]
Metal-organic frameworks are one of the most promising new technologies for carbon dioxide capture and sequestration via adsorption. [14] Although no large-scale commercial technology exists nowadays, several research studies have indicated the great potential that MOFs have as a CO2 adsorbent. Its characteristics, such as pore structure and surface functions can be easily tuned to improve CO2 selectivity over other gases. [15]
A MOF could be specifically designed to act like a CO2 removal agent in post-combustion power plants. In this scenario, the flue gas would pass through a bed packed with a MOF material, where CO2 would be stripped. After saturation is reached, CO2 could be desorbed by doing a pressure or temperature swing. Carbon dioxide could then be compressed to supercritical conditions in order to be stored underground or utilized in enhanced oil recovery processes. However, this is not possible in large scale yet due to several difficulties, one of those being the production of MOFs in great quantities. [16]
Another problem is the availability of metals necessary to synthesize MOFs. In a hypothetical scenario where these materials are used to capture all CO2 needed to avoid global warming issues, such as maintaining a global temperature rise less than 2 °C above the pre-industrial average temperature, we would need more metals than are available on Earth. For example, to synthesize all MOFs that utilize vanadium, we would need 1620% of 2010 global reserves. Even if using magnesium-based MOFs, which have demonstrated a great capacity to adsorb CO2, we would need 14% of 2010 global reserves, which is a considerable amount. Also, extensive mining would be necessary, leading to more potential environmental problems. [16]
In a project sponsored by the DOE and operated by UOP LLC in collaboration with faculty from four different universities, MOFs were tested as possible carbon dioxide removal agents in post-combustion flue gas. They were able to separate 90% of the CO2 from the flue gas stream using a vacuum pressure swing process. Through extensive investigation, researchers found out that the best MOF to be used was Mg/DOBDC, which has a 21.7 wt% CO2 loading capacity. Estimations showed that, if a similar system were to be applied to a large scale power plant, the cost of energy would increase by 65%, while a NETL baseline amine based system would cause an increase of 81% (the DOE goal is 35%). Also, each ton of CO2 avoided would cost $57, while for the amine system this cost is estimated to be $72. The project ended in 2010, estimating that the total capital required to implement such a project in a 580 MW power plant was 354 million dollars. [17]
An Extend Air Cartridge (EAC) is a make or type of pre-loaded one-use absorbent canister that can be fitted into a recipient cavity in a suitably-designed rebreather. [18]
Many other methods and materials have been discussed for scrubbing carbon dioxide.
Carbonation is the chemical reaction of carbon dioxide to give carbonates, bicarbonates, and carbonic acid. In chemistry, the term is sometimes used in place of carboxylation, which refers to the formation of carboxylic acids.
Soda lime, a mixture of sodium hydroxide (NaOH) and calcium oxide (CaO), is used in granular form within recirculating breathing environments like general anesthesia and its breathing circuit, submarines, rebreathers, and hyperbaric chambers and underwater habitats. Its purpose is to eliminate carbon dioxide from breathing gases, preventing carbon dioxide retention and, eventually, carbon dioxide poisoning. The creation of soda lime involves treating slaked lime with a concentrated sodium hydroxide solution.
Lithium hydroxide is an inorganic compound with the formula LiOH. It can exist as anhydrous or hydrated, and both forms are white hygroscopic solids. They are soluble in water and slightly soluble in ethanol. Both are available commercially. While classified as a strong base, lithium hydroxide is the weakest known alkali metal hydroxide.
Flue-gas desulfurization (FGD) is a set of technologies used to remove sulfur dioxide from exhaust flue gases of fossil-fuel power plants, and from the emissions of other sulfur oxide emitting processes such as waste incineration, petroleum refineries, cement and lime kilns.
Amine gas treating, also known as amine scrubbing, gas sweetening and acid gas removal, refers to a group of processes that use aqueous solutions of various alkylamines (commonly referred to simply as amines) to remove hydrogen sulfide (H2S) and carbon dioxide (CO2) from gases. It is a common unit process used in refineries, and is also used in petrochemical plants, natural gas processing plants and other industries.
Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases, as from a fireplace, oven, furnace, boiler or steam generator. It often refers to the exhaust gas of combustion at power plants. Technology is available to remove pollutants from flue gas at power plants.
A direct combination reaction (also known as a synthesis reaction) is a reaction where two or more elements or compounds (reactants) combine to form a single compound (product). Such reactions are represented by equations of the following form: X + Y → XY (A+B → AB). The combination of two or more elements to form one compound is called a combination reaction. In other words, when two or more elements or compounds react so as to form one single compound, then the chemical reaction that takes place is called a combination reaction. | a)- Between elements | C + O2 → CO2 | Carbon completely burnt in oxygen yields carbon dioxide |- | b) Between compounds | CaO + H2O → Ca(OH)2 | Calcium oxide (lime) combined with water gives calcium hydroxide (slaked lime) |- | c) Between elements and compounds | 2CO + O2 → 2CO2 | Oxygen combines with carbon monoxide,And carbon dioxide is formed. |}
Carbon capture and storage (CCS) is a process in which a relatively pure stream of carbon dioxide (CO2) from industrial sources is separated, treated and transported to a long-term storage location. For example, the carbon dioxide stream that is to be captured can result from burning fossil fuels or biomass. Usually the CO2 is captured from large point sources, such as a chemical plant or biomass plant, and then stored in an underground geological formation. The aim is to reduce greenhouse gas emissions and thus mitigate climate change. The IPCC's most recent report on mitigating climate change describes CCS retrofits for existing power plants as one of the ways to limit emissions from the electricity sector and meet Paris Agreement goals.
Lithium oxide (Li
2O) or lithia is an inorganic chemical compound. It is a white solid. Although not specifically important, many materials are assessed on the basis of their Li2O content. For example, the Li2O content of the principal lithium mineral spodumene (LiAlSi2O6) is 8.03%.
Douglas Patrick Harrison is a professor emeritus of chemical engineering from Louisiana State University's Gordon A. and Mary Cain Department of Chemical Engineering, where he taught undergraduate and graduate classes and served as a dissertation advisor to Ph.D. and M.S. students.
Oxy-fuel combustion is the process of burning a fuel using pure oxygen, or a mixture of oxygen and recirculated flue gas, instead of air. Since the nitrogen component of air is not heated, fuel consumption is reduced, and higher flame temperatures are possible. Historically, the primary use of oxy-fuel combustion has been in welding and cutting of metals, especially steel, since oxy-fuel allows for higher flame temperatures than can be achieved with an air-fuel flame. It has also received a lot of attention in recent decades as a potential carbon capture and storage technology.
A primarylife support system (PLSS), is a device connected to an astronaut or cosmonaut's spacesuit, which allows extra-vehicular activity with maximum freedom, independent of a spacecraft's life support system. A PLSS is generally worn like a backpack. The functions performed by the PLSS include:
Eco-Cement is a brand-name for a type of cement which incorporates reactive magnesia, another hydraulic cement such as Portland cement, and optionally pozzolans and industrial by-products, to reduce the environmental impact relative to conventional cement. One problem with the commercialization of this cement, other than the conservatism of the building industry, is that the feedstock magnesite is rarely mined.
Zeolitic imidazolate frameworks (ZIFs) are a class of metal-organic frameworks (MOFs) that are topologically isomorphic with zeolites. ZIF glasses can be synthesized by the melt-quench method, and the first melt-quenched ZIF glass was firstly made and reported by Bennett et al. back in 2015. ZIFs are composed of tetrahedrally-coordinated transition metal ions connected by imidazolate linkers. Since the metal-imidazole-metal angle is similar to the 145° Si-O-Si angle in zeolites, ZIFs have zeolite-like topologies. As of 2010, 105 ZIF topologies have been reported in the literature. Due to their robust porosity, resistance to thermal changes, and chemical stability, ZIFs are being investigated for applications such as carbon dioxide capture.
Post-combustion capture refers to the removal of carbon dioxide (CO2) from a power station flue gas prior to its compression, transportation and storage in suitable geological formations, as part of carbon capture and storage. A number of different techniques are applicable, almost all of which are adaptations of acid gas removal processes used in the chemical and petrochemical industries. Many of these techniques existed before World War II and, consequently, post-combustion capture is the most developed of the various carbon-capture methodologies.
The carbonite ion is the double ionized ion of dihydroxymethylidene, with the chemical formula: CO2−
2. Alkali metal salts, Li
2CO
2, K
2CO
2, Rb
2CO
2 and Cs
2CO
2, have been observed at 15 K. Interestingly, sodium does not form a carbonite. Due to the lone pair on the carbon atom, salts of the carbonite ion would be protonated to form formate and formic acid, rather than the carbene.
Calcium looping (CaL), or the regenerative calcium cycle (RCC), is a second-generation carbon capture technology. It is the most developed form of carbonate looping, where a metal (M) is reversibly reacted between its carbonate form (MCO3) and its oxide form (MO) to separate carbon dioxide from other gases coming from either power generation or an industrial plant. In the calcium looping process, the two species are calcium carbonate (CaCO3) and calcium oxide (CaO). The captured carbon dioxide can then be transported to a storage site, used in enhanced oil recovery or used as a chemical feedstock. Calcium oxide is often referred to as the sorbent.
Solid sorbents for carbon capture include a diverse range of porous, solid-phase materials, including mesoporous silicas, zeolites, and metal-organic frameworks. These have the potential to function as more efficient alternatives to amine gas treating processes for selectively removing CO2 from large, stationary sources including power stations. While the technology readiness level of solid adsorbents for carbon capture varies between the research and demonstration levels, solid adsorbents have been demonstrated to be commercially viable for life-support and cryogenic distillation applications. While solid adsorbents suitable for carbon capture and storage are an active area of research within materials science, significant technological and policy obstacles limit the availability of such technologies.
Direct air capture (DAC) is the use of chemical or physical processes to extract carbon dioxide directly from the ambient air. If the extracted CO2 is then sequestered in safe long-term storage, the overall process will achieve carbon dioxide removal and be a "negative emissions technology" (NET).
Sorption enhanced water gas shift (SEWGS) is a technology that combines a pre-combustion carbon capture process with the water gas shift reaction (WGS) in order to produce a hydrogen rich stream from the syngas fed to the SEWGS reactor.
{{cite journal}}: CS1 maint: unfit URL (link)The new metal-oxide-based system replaces the existing non-regenerable lithium hydroxide (LiOH) carbon dioxide (CO2) removal system located in the EMU's Primary Life Support System.
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