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Acarbon dioxide scrubber is a piece of equipment that absorbscarbon dioxide (CO2). It is used to treatexhaust gases fromindustrial plants or from exhaled air inlife support systems such asrebreathers or inspacecraft,submersible craft orairtight chambers. Carbon dioxide scrubbers are also used incontrolled atmosphere (CA) storage andcarbon capture and storage processes. Some of them include anabsorbent canister.
The amine scrubbers use variousorganicamines, 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 in coal-fired power plants because of capital costs of installing the facility and the operating costs of utilizing it.[1] However, the technology has been utilized as a primary part of atmosphere control in nuclear submarines since the late 1950s.
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 withquicklime (calcium oxide) to formlimestone (calcium carbonate),[3] in a process calledcarbonate looping. Other minerals includeserpentinite, amagnesiumsilicatehydroxide, andolivine.[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 theKraft process which may be based onsodium 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 akiln. With some modifications to the existing processes (mainly changing to an oxygen-fired kiln[citation needed]) 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 throughelectrolyzing of the carbonate solution.[8] While simpler, this electrical process consumes more energy aselectrolysis also splits water. 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, whichnuclear power orconcentrated solar power could provide.[9]
Zeman and Lackner outlined a specific method of air capture.[10]
First, CO2 is absorbed by an alkalineNaOH solution to produce dissolvedsodium carbonate. The absorption reaction is a gas liquid reaction, strongly exothermic, here:
Causticization is performed ubiquitously in thepulp 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 strongbases such assoda lime,sodium hydroxide,potassium hydroxide, andlithium hydroxide are able to remove carbon dioxide bychemically reacting with it. In particular, lithium hydroxide was used aboardspacecraft, such as in theApollo program, to remove carbon dioxide from the atmosphere. It reacts with carbon dioxide to formlithium carbonate.[11] Recently lithium hydroxide absorbent technology has been adapted for use inanesthesia 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 yearslithium orthosilicate has attracted much attention towards CO2 capture, 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 regenerativecarbon dioxide removal system (RCRS) on theSpace 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 itssorbent canisters. Olderlithium hydroxide (LiOH)-based systems, which are non-regenerable, were replaced by regenerablemetal-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 issaturated 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 well-studied for carbon dioxide capture and sequestration viaadsorption.[14] No large-scale commercial technology exists.[15] In one set of tests MOFs were able to separate 90% of the CO2 from the flue gas stream using a vacuum pressure swing process. The cost of energy is estimated to increase by 65% if MOFs were used vs an increase of 81% for amines as the capturing agent.[16]
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.[17]
Many other methods and materials have been discussed for scrubbing carbon dioxide.
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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