Direct air capture is the removal of carbon dioxide (CO2) from air for sequestering or conversion into oxygen.
Direct air capture has historically been used in closed, manned environments where oxygen is not available from the outside world, such as submarines and space craft, to remove CO2 from the air before concentrations become too high for humans. The technology can also be used to recycle breathable air; the collected carbon can be put to other uses, for example to create carbon nanotubes or a low-carbon alternative crude product that can be refined for fuel. Potential applications under study include capturing CO2 from large facilities, such as data centers, and even from ambient air.
Currently, direct air capture is being explored as a means of mitigating the greenhouse effect. CO2 is one of the main greenhouse gases; if it could be effectively and efficiently removed on a broad enough scale, it could reduce global warming. To that end, analysis has been done on existing processes such as the use of sodium hydroxide. Researchers are working on direct air capture with a sodium hydroxide (lye) solution and then heating it to release the CO2, allowing for continuous and lower cost large-scale operation.
The use of lithium hydroxide (LiOH) to absorb CO2 is one of the oldest methods. That process is very efficient on a scale of weight-to-CO2 absorption and has been used in space craft and submarines. However, LiOH methods have high power requirements and are non-renewable. Furthermore, the caustic and corrosive solution that typically comes in canisters must be replaced after saturation.
Soda lime, a mixture of mixture of calcium hydroxide, sodium and potassium hydroxides, removes CO2 from air, creating heat and water. Another method is molecular sieves; zeolite, a crystaline combination of aluminum and silicon, is suited to the task due to is porous nature and appropriate well-defined pore size. Another molecular sieve material is silver hydroxide (AgOH). While not as efficient as LiOH, AgOH is easily renewable up to 60 times before it requires replacement. Modern submarines use amine systems (monoethlamine) to capture CO2. Although it is effective, its very high power requirement can require a fallback for use in long dives, usually LiOH.
Harvard University scientist David Keith, along with Adrian Corless, formed the Carbon Engineering Company. They are also working with a sodium hydroxide (lye) solution and then using an amine solution with kilns and calcinators to remove the CO2.
See an introductory video about Climework's method of removing CO2 from ambient air: