Direct air capture
Direct air capture is the use of chemical or physical processes to extract carbon dioxide directly from the ambient air. If the extracted is then sequestered in safe long-term storage, the overall process is called direct air carbon capture and sequestration, achieving carbon dioxide removal. Systems that engage in such a process are referred to as negative emissions technologies.
DAC is in contrast to carbon capture and storage, which captures from point sources, such as a cement factory or a bioenergy plant. After the capture, DAC generates a concentrated stream of for sequestration or utilization. Carbon dioxide removal is achieved when ambient air makes contact with chemical media, typically an aqueous alkaline solvent or sorbents. These chemical media are subsequently stripped of CO2 through the application of energy, resulting in a CO2 stream that can undergo dehydration and compression, while simultaneously regenerating the chemical media for reuse.
As of 2023, DACCS has yet to be integrated into emissions trading because, at over US$1000, the cost per ton of carbon dioxide is many times the carbon price on those markets. The current high cost of DAC is driven by the scale of deployment and energy factors. It is reported that for DAC plant less than 50,000 tonnes CO2 per annum, like the current largest DAC plant, DAC costs would exceed $1000 per tonne. However, for plant scales of 1 Mtpa and above, DAC cost would generally be within $94–232 per tonne of atmospheric removed. Future innovations may reduce the energy intensity of this process.
DAC was suggested in 1999 and is still in development. Several commercial plants are planned or in operation in Europe and the US. Large-scale DAC deployment may be accelerated when connected with economical applications or policy incentives.
In contrast to carbon capture and storage which captures emissions from a point source such as a factory, DAC reduces the carbon dioxide concentration in the atmosphere as a whole. Thus, DAC can be used to capture emissions that originated in non-stationary sources such as airplanes.
Methods of capture
There are the three stages of capture in DAC: the contacting stage, the capture stage, and the separation stage. In the contacting stage, the DAC system transports atmospheric air containing to the equipment using large-scale fans. Subsequently, in the capture stage, rapidly and effectively binds with liquid solvents in chemical reactors or solid sorbents in filters, which must possess binding energies equivalent to that of. Later in the separation stage, external energy sources facilitate the separation of from the solvents or sorbents, yielding pure and regenerated solvents or sorbents. Following the completion of these three stages, the separated pure is either utilized or stored, while the recovered solvents or sorbents are recycled for reuse in the capture process.Generally, solid sorbents DAC uses low temperature process DAC, while liquid sorbents DAC uses low or high temperature process. S-DAC and L-DAC feature different properties in terms of kinetics and heat transfers. Currently, L-DAC and S-DAC represent two mature technologies for industrial deployment. Additionally, several emerging DAC technologies, including electro-swing adsorption, moisture-swing adsorption, and membrane-based DAC, are in different stages of development, testing, or limited practical application.
More recently, Ireland-based company Carbon Collect Limited has developed the MechanicalTree™ which simply stands in the wind to capture. The company claims this 'passive capture' of significantly reduces the energy cost of Direct Air Capture, and that its geometry lends itself to scaling for gigaton capture.
Most commercial techniques use a liquid solvent—usually amine-based or caustic—to absorb 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
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.Moisture swing sorbent
In a cyclical process designed in 2012 by professor Klaus Lackner, the director of the Center for Negative Carbon Emissions, 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. A large part of the energy for the process is supplied by the latent heat of phase change of water. The technology requires further research to determine its cost-effectiveness.Metal-organic frameworks
Other substances which can be used are metal–organic frameworks.Membranes
-based separation employs semi-permeable membranes. This method requires little water and has a smaller footprint. Typically polymeric membranes, either glassy or rubbery, are used for direct air capture. Glassy membranes typically exhibit high selectivity with respect to Carbon Dioxide; however, they also have low permeabilities. Membrane capture of carbon dioxide is still in development and needs further research before it can be implemented on a larger scale.Electro-Swing Adsorption
Electro-swing adsorption has also been proposed.Rock flour
, soil ground into nanoparticles by glacier ice, has potential both as a soil conditioner and for carbon capture. Glacier melting deposits one billion tons of rock flour annually, and one ton of Greenlandic rock flour can capture of carbon.Environmental impact
DAC is a carbon negative technology, with its greenhouse gas emissions estimated to range from 0.01 t emitted per t captured when renewable electricity is used to 0.65 t emitted per t captured when grid electricity and natural gas heating are used. The energy source emission factor of DAC is the primary driver of DAC's GHG emissions. The combination of renewable wind and grid electricity would also be carbon negative, providing high carbon removal benefits. The emissions factors for renewable wind and grid electricity are typically less than 0.1 t emitted per t captured if the wind plant supplies at least 50–80% of the plant capacity factor when grid electricity emission factors do not exceed 0.3077 kg/kWh. Higher grid electricity emission factors could still be used, but this would require their use to be less than 20% to achieve very high carbon removal.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 Agreement. The IEA estimates that capture of at least 85 million tonnes and 980 million tonnes of CO2 annually by 2030 and 2050, respectively, are needed to achieve net zero. 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. It is important to see DAC as a complementary solution that is necessary in helping to achieve climate targets.
Opponents of DAC argue that the resources required to operate DAC technologies, are an immense burden that may outweigh the goal of the technology itself. A 2020 analysis revealed that DAC 2 technology may be an unsuitable option to capture the projected 30 Gt- per year as it requires an enormous amount of materials The same study found that DAC 1 technology requires at least 8.4–13.1 TW-yr, an estimate that was calculated with the exclusion of the associated energy costs for carbon storage. However, the IEA net zero approaches require CO2 capture from DAC in the magnitude of 0.1 Gt- annually in 2050, which is significantly lower than 30 Gt- per year that opponents of DAC were assessing.
Energy cost concerns were explored in 2021 and found that in order for DAC technology to maintain a carbon removal of 73-86% per ton of captured, DAC would demand land occupation and renewable energy equivalent to what is needed for a global switch from gasoline to electric vehicles, with approximately five times higher material consumption. However, the material demand of DAC is mostly from common materials, such as steel, concrete and earth minerals. The use of electric vehicle may require substantial accessibility to critical materials, and this limited availability of critical materials may not be able to sustain the demand needed for net zero.
Some DAC technologies, especially liquid systems, require both high temperature heat and electricity. In these systems the electrical demand is made using natural gas, imported electricity from the grid, and oxyfuel combustion of natural gas. This means that many DAC technologies are powered by fossil fuels, the very thing the technology is meant to eliminate reliance on. However, from GHG emissions standpoint, DAC would generally be carbon-negative even if natural gas was used for heating, with emission factors of 0.3–0.65 t emitted per t captured. Thus, the aim of DAC of offsetting emissions could still be achieved.
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. Additionally, it is important to note that different carbon removal technologies could have their design and operational advantages, for example, while nature-based solutions are cheap, DAC plant that captures 1 MtCO2 per year using a land area of 0.4–1.5 km2 is equivalent to the CO2 capture rates of roughly 46 million trees, requiring approximately 3,098–4,647 km2 of land.
DAC also requires higher energy input in comparison to traditional capture from point sources, like flue gas, due to the low concentration of. Some authors give the theoretical minimum energy required to extract from ambient air as 250 kWh per tonne of, while capture from natural gas and coal power plants requires, respectively, about 100 and 65 kWh per tonne of. The additional penalty from the use of fans to pump air could add a 10% to 30% energy penalty if DAC energy demand is 10 to 4 MJ/t, respectively.