Direct air capture

Direct air capture (DAC) is a process of capturing carbon dioxide () directly from the ambient air (as opposed to capturing from point sources, such as a cement factory or biomass power plant) and generating a concentrated stream of for sequestration or utilization or production of carbon-neutral fuel and windgas. Carbon dioxide removal is achieved when ambient air makes contact with chemical media, typically an aqueous alkaline solvent or functionalized sorbents. These chemical media are subsequently stripped of CO2 through the application of energy (namely heat), resulting in a CO2 stream that can undergo dehydration and compression, while simultaneously regenerating the chemical media for reuse. 
DAC was suggestested in 1999 and is still in development, though several commercial plants are in operation or planning across Europe and the US. Large-scale DAC deployment may be accelerated when connected with economical use cases, or policy incentives. 
DAC is not an alternative to traditional, point-source carbon capture and storage (CCS), but can be used to manage emissions from distributed sources, like exhaust fumes from cars. When combined with long-term storage of , DAC can act as a carbon dioxide removal tool, although practicality of such approach is disputed among scientists.
The idea of using many small dispersed DAC scrubbers—analogous to live plants—to create environmentally significant reduction in levels, has earned the technology a name of "artificial trees" in popular media.
Methods of capture.
Commercial techniques require large fans to push ambient air through a filter. There, a liquid solvent—usually amine-based or caustic—absorbs from a gas. For example, a common caustic solvent: sodium hydroxide reacts with and precipitates a stable sodium carbonate. This carbonate is heated to produce a highly pure gaseous stream. sodium hydroxide can be recycled from sodium carbonate in a process of causticizing. Alternatively, the binds to solid sorbent in the process of chemisorption. Through heat and vacuum, the is then desorbed from the solid.
Among the specific chemical processes that are being explored, three stand out: causticization with alkali and alkali-earth hydroxides, carbonation, and organic−inorganic hybrid sorbents consisting of amines supported in porous adsorbents.
Other explored methods.
Moisture swing sorbent.
In cyclical a process designed in 2012 by professor Klaus Lackner, the director of the Center for Negative Carbon Emissions (CNCE), dilute can be efficiently separated using an anionic exchange polymer resin called Marathon MSA, which absorbs air when dry, and releases it when exposed to moisture. The technology requires further research to determine its cost-effectiveness.
Metal-organic frameworks.
Other substances which can be used are Metal-organic frameworks (or MOF's).
Membranes.
Membrane separation of rely on semi-permeable membranes. This method requires little water and has a smaller footprint.
Environmental impact.
Proponents of DAC argue that it is an essential component of climate change mitigation. Researchers posit that DAC could help contribute to the goals of the Paris Climate Agreement (namely limiting the increase in global average temperature to well below 2 °C above pre-industrial levels). However, others claim that relying on this technology is risky and might postpone emission reduction under the notion that it will be possible to fix the problem later, and suggest that reducing emissions may be a better solution.
DAC relying on amine-based absorption demands significant water input. It was estimated, that to capture 3.3 Gigatonnes of a year would require 300 km3 of water, or 4% of the water used for irrigation. On the other hand, using sodium hydroxide needs far less water, but the substance itself is highly caustic and dangerous.
DAC also requires much greater energy input in comparison to traditional capture from point sources, like flue gas, due to the low concentration of . The theoretical minimum energy required to extract from ambient air is about 250 kWh per tonne of , while capture from natural gas and coal power plants requires respectively about 100 and 65 kWh per tonne of . Because of this implied demand for energy, some geoengineering promoters have proposed using "small nuclear power plants" connected to DAC installations.
When DAC is combined with a carbon capture and storage (CCS) system, it can produce a negative emissions plant, but it would require a carbon-free electricity source. The use of any fossil-fuel-generated electricity would end up releasing more to the atmosphere than it would capture. Moreover, using DAC for enhanced oil recovery would cancel any supposed climate mitigation benefits.
Applications.
Practical applications of DAC include:
These applications require different concentrations of product formed from the captured gas. Forms of carbon sequestration such as geological storage require pure products (concentration > 99%), while other applications such as agriculture can function with more dilute products (~ 5%). Since the air that is processed through DAC originally contains 0.04% (or 400 ppm), creation of a pure product through DAC requires a large amount of thermal energy to facilitate bonding and thus is more expensive than a dilute product. 
DAC is not an alternative to traditional, point-source carbon capture and storage (CCS), rather it is a complementary technology that could be utilized to manage carbon emissions from distributed sources, fugitive emissions from the CCS network, and leakage from geological formations. Because DAC can be deployed far from the source of pollution, synthetic fuel produced with this method can use already existing fuel transport infrastructure.
Cost.
One of the largest hurdles to implementing DAC is a cost required to separate and air. A study from 2011 estimated that a plant designed to capture 1 megatonne of a year would cost $2.2billion. Other studies from the same period put the cost of DAC at $200–1000 per tonne of and $600 per tonne. 
An economic study of a pilot plant in British Columbia, Canada, conducted from 2015 to 2018, estimated the cost at $94–232 per tonne of atmospheric removed. It is worth noting that the study was done by Carbon Engineering, which has financial interest in commercializing DAC technology.
, capture costs for hydroxide-based solvents generally cost $150 per tonne . Current liquid amine-based separation is $10–35 per tonne . Adsorption based capture costs are between $30–200 per tonne . It is difficult to find a specific cost for DAC because each method has wide variation in sorbent regeneration and capital costs.
Large-scale DAC deployment can be accelerated by policy incentives such as 45Q or the California Low-carbon fuel standard.
Development.
Carbon Engineering.
It is a commercial DAC company founded in 2009 and backed, among others, by Bill Gates and Murray Edwards. , they run a pilot plant in British Columbia, Canada that has been in use since 2015 and is able to extract about a tonne of a day. An economic study of their pilot plant conducted from 2015 to 2018 estimated the cost at $94–232 per tonne of atmospheric removed.
While partnering with California energy company Greyrock, they convert a portion of its concentrated into synthetic fuel, including gasoline, diesel, and jet fuel.
The company uses a potassium hydroxide solution. It reacts with to form potassium carbonate, which removes a certain amount of from the air.
Climeworks.
Their first industrial scale DAC plant, which started operation in May, 2017 in Hinwil, in the canton of Zurich, Switzerland, is capable of capturing 900 tonnes of per year. To lower its energy requirements, the plant uses heat from a local waste incineration plant. The is used to increase vegetable yields in a nearby greenhouse.
The company stated that it costs around $600 to capture one tonne of from the air.
Climeworks partnered with Reykjavik Energy in CarbFix project launched in 2007. In 2017, CarbFix2 project was started and received funding from EuropeanUnion's Horizon2020 research program. The CarbFix2 pilot plant project runs alongside a geothermal power plant in Hellisheidi, Iceland. In this approach, is injected 700 meters under the ground and mineralizes into basaltic bedrock forming carbonate minerals. DAC plant uses low-grade waste heat from the plant, effectively eliminating more than they both produce. 
Global Thermostat.
It is private company founded in 2010, located in Manhattan, New York, with a plant in Huntsville, Alabama. Global Thermostat uses amine-based sorbents bound to carbon sponges to remove from the atmosphere. The company has projects ranging from 40 to 50,000 tonne/year.
The company claims to remove for a $120 per tonne at its facility in Huntsville.
Global Thermostat has closed deals with Coca-Cola (which aims to use DAC to source for its carbonated beverages) and ExxonMobil which intends to pioneer a DACtofuel business using Global Thermostat's technology.
Prometheus Fuels.
Is a start-up company based in Santa Cruz which launched out of Y Combinator in 2019 to remove CO2 from the air and turn it into zero-net-carbon gasoline and jet fuel. The company uses a DAC technology, adsorbing CO2 from the air directly into process electrolytes, where it is converted into alcohols by electrocatalysis. The alcohols are then separated from the electrolytes using carbon nanotube membranes, and upgraded to gasoline and jet fuels. Since the process uses only electricity from renewable sources, the fuels are carbon neutral when used, emitting no net CO2 to the atmosphere.