View source for Carbon dioxide scrubber

{{short description|Device which absorbs carbon dioxide from circulated gas }}
[[File:Carbon Dioxide (CO2) Scrubber.jpg|thumb|A carbon dioxide scrubber]]
A '''carbon dioxide scrubber''' is a piece of equipment that absorbs [[carbon dioxide]] (CO<sub>2</sub>). It is used to treat [[exhaust gas]]es from [[industrial plant]]s or from exhaled air in [[life support system]]s such as [[rebreather]]s or in [[spacecraft]], [[submersible]] craft or [[airtight chamber]]s. 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]].

==Technologies==

===Amine scrubbing===
{{main|Amine gas treating}}
The primary application for CO<sub>2</sub> scrubbing is for removal of CO<sub>2</sub> from the exhaust of coal- and gas-fired [[power plant]]s.  Virtually the only technology being seriously evaluated involves the use of various [[amine]]s, e.g. [[monoethanolamine]].  Cold solutions of these organic compounds bind CO<sub>2</sub>, but the binding is reversed at higher temperatures:
:CO<sub>2</sub>  +  2 {{chem|HOCH|2|CH|2|NH|2}} ↔ {{chem|HOCH|2|CH|2|NH|3|+}} + {{chem|HOCH|2|CH|2|NHCO|2|−}}
{{asof|2009}}, this technology has only been lightly implemented because of capital costs of installing the facility and the operating costs of utilizing it.<ref>{{cite journal
|author=Gary T. Rochelle |title=Amine Scrubbing for CO<sub>2</sub> Capture |journal=Science |volume=325 |issue=5948 |pages=1652–4 |year=2009 |doi=10.1126/science.1176731|pmid=19779188 |bibcode=2009Sci...325.1652R |s2cid=206521374 }}</ref>

===Minerals and zeolites===
Several minerals and mineral-like materials reversibly bind CO<sub>2</sub>.<ref>{{cite journal |author=Sunho Choi |author2=Jeffrey H. Drese |author3=Christopher W. Jones |title=Adsorbent Materials for Carbon Dioxide Capture from Large Anthropogenic Point Sources |journal=ChemSusChem |year=2009 |volume=2 |issue=9 |pages=796–854 |doi=10.1002/cssc.200900036|pmid=19731282 |bibcode=2009ChSCh...2..796C }}</ref>  Most often, these minerals are oxides or hydroxides, and often the CO<sub>2</sub> is bound as carbonate. Carbon dioxide reacts with [[quicklime]] (calcium oxide) to form [[limestone]] ([[calcium carbonate]]),<ref>{{cite journal |url=https://www.sciencedaily.com/releases/2002/04/020412080812.htm |title=Imagine No Restrictions On Fossil-Fuel Usage And No Global Warming |journal=ScienceDaily |date=April 15, 2002}}</ref> in a process called [[Calcium looping|carbonate looping]]. Other minerals include [[serpentinite]], a [[magnesium]] [[silicate]] [[hydroxide]], and [[olivine]].<ref>{{cite journal |url=https://www.sciencedaily.com/releases/2004/09/040903084832.htm |title=Natural Mineral Locks Up Carbon Dioxide |journal=ScienceDaily |date=September 3, 2004 |access-date=2011-06-01}}</ref><ref>{{cite web|url=http://www.tececo.com/sustainability.tececo_kiln.php|title=Sustainability and the TecEco Kiln|url-status=dead|archive-url=https://web.archive.org/web/20051025083643/http://www.tececo.com/sustainability.tececo_kiln.php|archive-date=October 25, 2005|access-date=October 25, 2005}}</ref> [[Molecular sieve]]s also function in this capacity.

Various (cyclical) scrubbing processes have been proposed to remove CO<sub>2</sub> 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]].<ref name="ChangNYT2008">{{cite news | author=Kenneth Chang | url =https://www.nytimes.com/2008/02/19/science/19carb.html | title=Scientists would turn greenhouse gas into gasoline | work=[[The New York Times]] | date=February 19, 2008 | access-date = 2009-10-29}}</ref><ref>{{cite journal |first=Catherine |last=Brahic |url=https://www.newscientist.com/article/dn12727 |title=Chemical 'sponge' could filter CO2 from the air – environment |journal=[[New Scientist]] |date=October 3, 2007|access-date=2009-10-29}}<br/>{{cite journal |first=Frank |last=Zeman |title=Energy and Material Balance of CO<sub>2</sub> Capture from Ambient Air |journal=Environ. Sci. Technol. |volume=41 |issue=21 |pages=7558–63 |date=2007 |doi=10.1021/es070874m |pmid=18044541 |bibcode=2007EnST...41.7558Z |s2cid=27280943 |url= }}</ref>  The CO<sub>2</sub> 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 CO<sub>2</sub>, ready for storage or use in fuels. An alternative to this thermo-chemical process is an electrical one which releases the CO<sub>2</sub> through [[Electrolysis|electrolyzing]] of the carbonate solution.<ref name="A High Temperature Lithium Orthosil">{{cite journal |first1=R. |last1=Quinn |first2=R.J. |last2=Kitzhoffer |first3=J.R. |last3=Hufton |first4=T.C. |last4=Golden |title=A High Temperature Lithium Orthosilicate-Based Solid Absorbent for Post Combustion CO<sub>2</sub> Capture |journal=Ind. Eng. Chem. Res. |volume=51 |issue=27 |pages=9320–7 |date=2012 |doi=10.1021/ie300157m |url=}}</ref> While simpler, this electrical process consumes more energy as [[Electrolysis of water|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 CO<sub>2</sub> capture used electricity as the energy source and were therefore dependent on green energy. Some thermal<!-- *some
thermo chemical is also thermal --> CO<sub>2</sub> 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 energy|(green) heat]], which [[nuclear power]] or [[concentrated solar power]] could provide. <ref name="newscientist1">{{cite journal |first1=Robert |last1=Kunzig |first2=Wallace |last2=Broecker |url=https://www.newscientist.com/article/mg20126901.200-can-technology-clear-the-air.html?full=true |title=Can technology clear the air? |journal=[[New Scientist]] |date=January 12, 2009 |access-date=2009-10-29}}</ref>

====Sodium hydroxide====
Zeman and Lackner outlined a specific method of air capture.<ref name="naoh-zeman">{{cite journal |first1=F.S. |last1=Zeman  |first2=K.S. |last2=Lackner |title=Capturing carbon dioxide directly from the atmosphere |journal=World Resour. Rev. |volume=16 |pages=157–172 |year=2004}}</ref>

First, CO<sub>2</sub> is absorbed by an alkaline [[Sodium hydroxide|NaOH]] solution to produce dissolved [[sodium carbonate]]. The absorption reaction is a gas liquid reaction, strongly exothermic, here:

:2NaOH(aq) + CO<sub>2</sub>(g) → {{chem|Na|2|CO|3}}(aq) + {{H2O}}(l)

:{{chem|Na|2|CO|3}}(aq) + {{chem|Ca(OH)|2}}(s) → 2NaOH(aq) + {{chem|CaCO|3}}(s)

:ΔH° = -114.7 kJ/mol

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.<ref name="naoh-zeman"/><!-- is this it??? --> Subsequently, the calcium carbonate precipitate is filtered from solution and thermally decomposed to produce gaseous CO<sub>2</sub>. The calcination reaction is the only endothermic reaction in the process and is shown here:

:{{chem|CaCO|3}}(s) → CaO(s) + CO<sub>2</sub>(g)

:ΔH° = + 179.2 kJ/mol

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:

:CaO(s) + {{H2O}}(l) → {{chem|Ca(OH)|2}}(s)

:ΔH° = -64.5 kJ/mol

====Lithium hydroxide====
Other strong [[base (chemistry)|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]].<ref name=":0">{{cite journal |author=J.R. Jaunsen |title=The Behavior and Capabilities of Lithium Hydroxide Carbon Dioxide Scrubbers in a Deep Sea Environment |journal=US Naval Academy Technical Report |volume=USNA-TSPR-157 |year=1989 |url=http://archive.rubicon-foundation.org/4998 |access-date=2008-06-17 |archive-url=https://web.archive.org/web/20090824104846/http://archive.rubicon-foundation.org/4998 |archive-date=2009-08-24 |url-status=usurped }}</ref>  Recently lithium hydroxide absorbent technology has been adapted for use in [[Anaesthetic machine|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.

:2 LiOH(s) + 2 {{H2O}}(g) → 2 LiOH·{{H2O}}(s)
:2 LiOH·{{H2O}}(s) + CO<sub>2</sub>(g) → {{chem|Li|2|CO|3}}(s) + 3 {{H2O}}(g)

The net reaction being:

:2LiOH(s) + CO<sub>2</sub>(g) → {{chem|Li|2|CO|3}}(s) + {{H2O}}(g)

[[Lithium peroxide]] can also be used as it absorbs more CO<sub>2</sub> per unit weight with the added advantage of releasing oxygen.<ref>{{Cite book | last1 = Petzow | first1 = G. N. | last2 = Aldinger | first2 = F. | last3 = Jönsson | first3 = S. | last4 = Welge | first4 = P. | last5 = Van Kampen | first5 = V. | last6 = Mensing | first6 = T. | last7 = Brüning | first7 = T. | chapter = Beryllium and Beryllium Compounds | doi = 10.1002/14356007.a04_011.pub2 | title = Ullmann's Encyclopedia of Industrial Chemistry | year = 2005 | isbn = 978-3527306732 | s2cid = 262306041 }}</ref>

In recent years [[lithium orthosilicate]] has attracted much attention towards CO<sub>2</sub>capture, as well as energy storage.<ref name="A High Temperature Lithium Orthosil"/> This material offers considerable performance advantages although it requires high temperatures for the formation of carbonate to take place.

===Regenerative carbon dioxide removal system===
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 {{convert|200|°C|°F|abbr=on}} through it at a standard flow rate of {{convert|3.5|l/s|cuft/min|abbr=on}} for 10 hours.<ref>{{cite web |url=http://www.hamiltonsundstrand.com/hsc/proddesc_display/0,4494,CLI1_DIV25_ETI5338_PRD776,00.html |title=Carbon Dioxide Removal |publisher=Hamilton Sundstrand |access-date=2008-10-27 |quote=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. |archive-url=https://web.archive.org/web/20071031085108/http://www.hamiltonsundstrand.com/hsc/proddesc_display/0%2C4494%2CCLI1_DIV25_ETI5338_PRD776%2C00.html |archive-date=2007-10-31 |url-status=dead }}</ref>

===Activated carbon===
[[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 [[Binding site#Binding curves|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|date=March 2019}}

=== Metal-organic frameworks (MOFs) ===
[[Metal-organic framework]]s are one of the most promising new technologies for carbon dioxide capture and sequestration via [[adsorption]].<ref>{{cite web |title=MOFs for CO2 |url=https://www.moftechnologies.com/mofs-for-co2 |website=MOF Technologies |access-date=2021-04-07 |archive-date=2021-02-27 |archive-url=https://web.archive.org/web/20210227181026/https://www.moftechnologies.com/mofs-for-co2 |url-status=dead }}</ref> Although no large-scale commercial technology exists nowadays, several research studies have indicated the great potential that MOFs have as a CO<sub>2</sub> adsorbent. Its characteristics, such as pore structure and surface functions can be easily tuned to improve CO<sub>2</sub> selectivity over other gases.<ref>{{Cite journal|last=Li|first=Jian-Rong|date=2011|title=Carbon dioxide capture-related gas adsorption and separation in metal-organic frameworks.|url=http://www.chem.tamu.edu/rgroup/zhou/PDF/095.pdf|journal=Coordination Chemistry Reviews|volume=255|issue=15–16|pages=1791–1823|doi=10.1016/j.ccr.2011.02.012|archive-url=https://web.archive.org/web/20160909234606/http://www.chem.tamu.edu/rgroup/zhou/PDF/095.pdf|archive-date=2016-09-09|url-status=dead}}</ref>

A MOF could be specifically designed to act like a CO<sub>2</sub> removal agent in post-combustion power plants. In this scenario, the flue gas would pass through a bed packed with a MOF material, where CO<sub>2</sub> would be stripped. After saturation is reached, CO<sub>2</sub> could be desorbed by doing a [[Pressure swing adsorption|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.<ref name=Smit14>{{Cite book|title=Introduction to Carbon Capture and Sequestration|last1=Smit|first1=Berend|last2=Reimer|first2=Jeffrey R.|last3=Oldenburg|first3=Curtis M.|last4=Bourg|first4=Ian C.|publisher=Imperial College Press|year=2014|isbn=978-1-78326-327-1}}</ref>

Another problem is the availability of metals necessary to synthesize MOFs. In a hypothetical scenario where these materials are used to capture all CO<sub>2</sub> needed to avoid global warming issues, such as maintaining a global temperature rise less than 2&nbsp;°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 CO<sub>2</sub>, 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.<ref name=Smit14 />

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 CO<sub>2</sub> 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% CO<sub>2</sub> 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 [[National Energy Technology Laboratory|NETL]] baseline amine based system would cause an increase of 81% (the DOE goal is 35%). Also, each ton of CO<sub>2</sub> 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.<ref>{{Cite news|osti=1003992|title=Carbon Dioxide Removal from Flue Gas Using Microporous Metal Organic Frameworks|last1=Willis|first1=Richard|date=2010|work=Final Technical Report|last2=Lesch|first2=David A.|others=DOE Award Number: DE-FC26-07NT43092|doi=10.2172/1003992}}</ref>

===Extend Air Cartridge===
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.<ref>{{Cite web|title=Extend Air Cartridge|url=https://www.dykarna.nu/|access-date=2021-12-30|website=dykarna|language=sv-se}}</ref>

===Other methods===
Many other methods and materials have been discussed for scrubbing carbon dioxide.
* [[Adsorption]]<ref>{{cite web |last1=Siriwardane |first1=R. |last2=Shen |first2=M. |last3=Fisher |first3=E. |last4=Poston |first4=J. |last5=Shamsi |first5=A. |title=Adsorption and desorption of CO<sub>2</sub> on solid sorbents |date=2001 |id=DOE/NETL-2001/1144 |publisher=National Energy Technology Laboratory |url=https://www.osti.gov/servlets/purl/1207903 |citeseerx=10.1.1.205.844}}</ref>
* [[Regenerative carbon dioxide removal system]] (RCRS)
* Algae filled [[bioreactor]]s
* [[Membrane gas separation]]s
* Reversing [[heat exchanger]]s

==See also==
*{{annotated link|Carbon capture and storage}}
*{{annotated link|Carbon dioxide removal}}
*{{annotated link|Greenhouse gas}}
*{{annotated link|Rebreather}}
*{{annotated link|Sabatier reaction}}

==References==
{{Reflist}}

{{Breathing apparatus|general}}
{{Underwater diving|divequ}}

[[Category:Scrubbers]]
[[Category:Carbon dioxide]]
[[Category:Space suit components]]
[[Category:Spacecraft life support systems]]
[[Category:Rebreather components|*]]
[[Category:Gas technologies]]
[[Category:Carbon capture and storage]]