Carbon dioxide scrubber
Encyclopedia
A carbon dioxide scrubber is a device which absorbs carbon dioxide
(CO2). It is used to treat exhaust gas
es from industrial plants or from exhaled air in life support system
s such as rebreather
s or in spacecraft
, submersible
craft or airtight chambers. Carbon dioxide scrubbers are also used in controlled atmosphere
(CA) storage.
Various scrubbing
processes have been proposed to remove CO2 from the air, or from flue gases. These usually involve using a variant of the Kraft process
. Scrubbing processes may be based on sodium hydroxide. The CO2 is absorbed into solution, transferred to lime via a process called causticization and released in a kiln. With some modifications to the existing processes, mainly an oxygen-fired kiln, the end result is a concentrated stream of CO2 ready for storage or use in fuels. An alternative to this thermo-chemical process is an electrical one in which a nominal voltage is applied across the carbonate solution to release the CO2. While simpler, this electrical process consumes more energy as it splits water at the same time. Since it depends on electricity, the electricity needs to be renewable, like PV. Otherwise the CO2 produced during electricity production has to be taken into account. Early incarnations of air capture used electricity as the energy source; hence, were dependent on a carbon-free source. Thermal air capture systems use heat generated on-site, which reduces the inefficiencies associated with off-site electricity production, but of course it still needs a source of (carbon-free) heat. Concentrated solar power is an example of such a source.
Zeman and Lackner outlined a specific method of air capture.
First, CO2 is absorbed by an alkaline NaOH solution to produce dissolved sodium carbonate. The absorption reaction is a gas liquid reaction, strongly exothermic, (below)
The carbonate ion is removed from the solution by reaction with calcium hydroxide (Ca(OH)2), which results in the precipitation of calcite (CaCO3). The causticization reaction is a mildly exothermic, aqueous reaction that occurs in an emulsion of calcium hydroxide (below)
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 (below).
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 (below).
orbiter uses a two-bed system that provides continuous removal of carbon dioxide without expendable products. Regenerable systems allow 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, are being replaced by regenerable metal
-oxide
-based systems. A system based on metal oxide primarily consists of a metal oxide sorbent canister and a regenerator assembly. It works by removing carbon dioxide using a sorbent material and then regenerating the sorbent material. The metal-oxide sorbent is regenerated by pumping air heated to around 400 °F at 7.5 scfm through its canister for 10 hours.
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 adsorb onto the activated carbon. 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.
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
is used aboard space craft to remove carbon dioxide from the atmosphere. It reacts with carbon dioxide to make lithium carbonate
:
The net reaction being:
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
(CO2). It is used to treat exhaust gas
Exhaust gas
Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline/petrol, diesel fuel, fuel oil or coal. According to the type of engine, it is discharged into the atmosphere through an exhaust pipe, flue gas stack or propelling nozzle.It often disperses...
es from industrial plants or from exhaled air in life support system
Life support system
In human spaceflight, a life support system is a group of devices that allow a human being to survive in space.US government space agency NASA,and private spaceflight companies...
s such as rebreather
Rebreather
A rebreather is a type of breathing set that provides a breathing gas containing oxygen and recycled exhaled gas. This recycling reduces the volume of breathing gas used, making a rebreather lighter and more compact than an open-circuit breathing set for the same duration in environments where...
s or in spacecraft
Spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....
, submersible
Submersible
A submersible is a small vehicle designed to operate underwater. The term submersible is often used to differentiate from other underwater vehicles known as submarines, in that a submarine is a fully autonomous craft, capable of renewing its own power and breathing air, whereas a submersible is...
craft or airtight chambers. Carbon dioxide scrubbers are also used in controlled atmosphere
Controlled atmosphere
A controlled atmosphere is an agricultural storage method. An atmosphere in which oxygen, carbon dioxide and nitrogen concentrations as well as temperature and humidity are regulated.Two major classes of commodity can be stored in controlled atmosphere:...
(CA) storage.
Methods
Methods of carbon dioxide scrubbing include :- AdsorptionAdsorptionAdsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...
- AmineAmine gas treatingAmine gas treating, also known as gas sweetening and acid gas removal, refers to a group of processes that use aqueous solutions of various alkylamines to remove hydrogen sulfide and carbon dioxide from gases...
absorption - Calcium oxideCalcium oxideCalcium oxide , commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline crystalline solid at room temperature....
: Carbon dioxide reacts with quicklime (calcium oxide), to form limestoneLimestoneLimestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
(calcium carbonateCalcium carbonateCalcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
). The according process is called Carbonate Looping. - SerpentiniteSerpentiniteSerpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle...
: The metamorphicMetamorphic rockMetamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
mineralMineralA mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
serpentinite (magnesiumMagnesiumMagnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
silicateSilicateA silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
hydroxideHydroxideHydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...
), is composed of magnesium, silicon and oxygen. - Regenerative carbon dioxide removal system (RCRS)
- Algae based carbon sink
- Olivine:
- Molecular sieveMolecular sieveA molecular sieve is a material containing tiny pores of a precise and uniform size that is used as an adsorbent for gases and liquids.Molecules small enough to pass through the pores are adsorbed while larger molecules are not. It is different from a common filter in that it operates on a...
- Polymer membrane gas separators
- Activated carbonActivated carbonActivated carbon, also called activated charcoal, activated coal or carbo activatus, is a form of carbon that has been processed to make it extremely porous and thus to have a very large surface area available for adsorption or chemical reactions.The word activated in the name is sometimes replaced...
- Reversing heat exchangerHeat exchangerA heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
s - Monoethanolamine solutions absorb carbon dioxide when cold, and release it when warmed.
Various scrubbing
Carbon dioxide scrubber
A carbon dioxide scrubber is a device which absorbs carbon dioxide . 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...
processes have been proposed to remove CO2 from the air, or from flue gases. These usually involve using a variant of the Kraft process
Kraft process
The kraft process describes a technology for conversion of wood into wood pulp consisting of almost pure cellulose fibers...
. Scrubbing processes may be based on sodium hydroxide. The CO2 is absorbed into solution, transferred to lime via a process called causticization and released in a kiln. With some modifications to the existing processes, mainly an oxygen-fired kiln, the end result is a concentrated stream of CO2 ready for storage or use in fuels. An alternative to this thermo-chemical process is an electrical one in which a nominal voltage is applied across the carbonate solution to release the CO2. While simpler, this electrical process consumes more energy as it splits water at the same time. Since it depends on electricity, the electricity needs to be renewable, like PV. Otherwise the CO2 produced during electricity production has to be taken into account. Early incarnations of air capture used electricity as the energy source; hence, were dependent on a carbon-free source. Thermal air capture systems use heat generated on-site, which reduces the inefficiencies associated with off-site electricity production, but of course it still needs a source of (carbon-free) heat. Concentrated solar power is an example of such a source.
Zeman and Lackner outlined a specific method of air capture.
First, CO2 is absorbed by an alkaline NaOH solution to produce dissolved sodium carbonate. The absorption reaction is a gas liquid reaction, strongly exothermic, (below)
- 2NaOH(aq) + CO2(g) → Na2CO3(aq) + H2O(l)
- ΔH° = -109.4 kJ/mol
The carbonate ion is removed from the solution by reaction with calcium hydroxide (Ca(OH)2), which results in the precipitation of calcite (CaCO3). The causticization reaction is a mildly exothermic, aqueous reaction that occurs in an emulsion of calcium hydroxide (below)
- Na2CO3(aq) + Ca(OH)2(s) →-> 2NaOH(aq) + CaCO3(s)
- ΔH° = -5.3 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 (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 (below).
- CaCO3(s) → CaO(s) + CO2(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 (below).
- CaO(s) + H2O(l) → Ca(OH)2(s)
- ΔH° = -64.5 kJ/mol
Regenerative carbon dioxide removal system
The regenerative carbon dioxide removal system (RCRS) on the space shuttleSpace Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...
orbiter uses a two-bed system that provides continuous removal of carbon dioxide without expendable products. Regenerable systems allow a shuttle mission a longer stay in space without having to replenish its sorbent
Sorbent
A sorbent is a material used to absorb liquids or gases. Examples include:*A material similar to molecular sieve material. It has a large internal surface area and good thermal conductivity. It is typically supplied in pellets of 1 mm to 2 mm diameter and roughly 5 mm length or as...
canisters. Older lithium hydroxide
Lithium hydroxide
Lithium hydroxide is an inorganic compound with the formula LiOH. It is a white hygroscopic crystalline material. It is soluble in water and slightly soluble in ethanol...
(LiOH)-based systems, which are non-regenerable, are being replaced by regenerable metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
-oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
-based systems. A system based on metal oxide primarily consists of a metal oxide sorbent canister and a regenerator assembly. It works by removing carbon dioxide using a sorbent material and then regenerating the sorbent material. The metal-oxide sorbent is regenerated by pumping air heated to around 400 °F at 7.5 scfm through its canister for 10 hours.
Activated carbon
Activated carbonActivated carbon
Activated carbon, also called activated charcoal, activated coal or carbo activatus, is a form of carbon that has been processed to make it extremely porous and thus to have a very large surface area available for adsorption or chemical reactions.The word activated in the name is sometimes replaced...
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 adsorb onto the activated carbon. Once the bed is saturated
Saturation (chemistry)
In chemistry, saturation has six different meanings, all based on reaching a maximum capacity...
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.
Strong bases
Various strong basesBase (chemistry)
For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...
such as soda lime
Soda lime
Soda lime is a mixture of chemicals, used in granular form in closed breathing environments, such as general anaesthesia, submarines, rebreathers and recompression chambers, to remove carbon dioxide from breathing gases to prevent CO2 retention and carbon dioxide poisoning.It is made by treating...
, sodium hydroxide, potassium hydroxide
Potassium hydroxide
Potassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash.Along with sodium hydroxide , this colorless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive...
, and lithium hydroxide
Lithium hydroxide
Lithium hydroxide is an inorganic compound with the formula LiOH. It is a white hygroscopic crystalline material. It is soluble in water and slightly soluble in ethanol...
are able to remove carbon dioxide by chemically reacting with it. In particular, lithium hydroxide
Lithium hydroxide
Lithium hydroxide is an inorganic compound with the formula LiOH. It is a white hygroscopic crystalline material. It is soluble in water and slightly soluble in ethanol...
is used aboard space craft to remove carbon dioxide from the atmosphere. It reacts with carbon dioxide to make lithium carbonate
Lithium carbonate
Lithium carbonate is a chemical compound of lithium, carbon, and oxygen with the formula Li2CO3. This colorless salt is widely used in the processing of metal oxides and has received attention for its use in psychiatry. It is found in nature as the rare mineral zabuyelite.-Properties:Like almost...
:
- 2 LiOH(s) + 2 H2O(g) → 2 LiOH.H2O(s)
- 2 LiOH.H2O(s) + CO2(g) → Li2CO3(s) + 3 H2O(g)
The net reaction being:
- 2 LiOH(s) + CO2(g) → Li2CO3(s) + H2O(g)
See also
- Breath
- Carbon capture and storageCarbon capture and storageCarbon capture and storage , alternatively referred to as carbon capture and sequestration, is a technology to prevent large quantities of from being released into the atmosphere from the use of fossil fuel in power generation and other industries. It is often regarded as a means of mitigating...
- Carbon dioxide air captureCarbon dioxide air captureCarbon dioxide removal methods refers to a number of technologies which reduce the levels of carbon dioxide in the atmosphere. Among such technologies are bio-energy with carbon capture and storage, biochar, direct air capture, ocean fertilization and enhanced weathering.CDR is a different...
- Greenhouse gasGreenhouse gasA greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...