Direct air capture
Direct air capture (DAC) is a capture method of carbon capture and storage that separates carbon dioxide from air. While still in the early stages, proponents of direct air capture argue that it is an essential component of climate change mitigation.
- 1 Methods of capture
- 2 Energy and water use requirements for direct air capture
- 3 Direct air capture projects and companies around the world
- 4 Financial costs
- 5 References
Methods of capture
Like most forms of carbon capture and storage, direct air capture relies on three main technologies for separating the carbon dioxide from the air. Direct air capture of carbon dioxide (DAC) is a carbon capture technique which produces a concentrated CO
2 gas stream from ambient air. Generally ambient air is flows through a filter where either adsorption, absorption or mineralization removes the CO
2 from the air. This concentrated CO
2 stream may be sequestered or reused for enhanced oil recovery to produce low carbon fuel. When DAC is combined with a carbon capture system, it can produce a negative emissions plant. DAC is one tool used to mitigate climate change and reach the Paris Climate Agreement goals. Direct air capture have larger energy and water requirements, because CO
2 is removed from a very dilute stream. Current atmospheric CO
2 levels are 410 parts per million (ppm). There are currently, seven DAC projects worldwide. Most projects are in infancy and do have not been scaled up to meet climate mitigation goals.
Absorption in the context of carbon capture refers to the use of a liquid solvent absorbing carbon dioxide from a gas. Amine solvents are most commonly used. This is currently the most common technology for carbon capture. Absorption is the most common method of direct air capture. A fan blows air into a filter that is lined with a liquid solvent absorbent. Currently, the most common sorbents are caustic solution and amine based solvents. Absorption with a caustic solvent is an acid-base reaction in which CO
2 reacts with sodium hydroxide and precipitates a stable sodium carbonate. This sodium carbonate is heated to produce a highly pure gaseous CO
Adsorption differs from absorption in that the carbon dioxide is removed via a solid. Alternatively, a solid sorbent is used in adsorption. In adsorption, the CO
2 binds to a gas-solid contact or. Amine based solid sorbents are often proposed for DAC. CO
2 is removed from air by chemisorption, usually through primary amines. Through heat and vacuum the CO
2 is then desorbed from the solid sorbent. This results in a gaseous CO
2 stream that can be cooled and cycled back into the system.
Membrane separations of carbon dioxide rely on semi-permeable membranes to separate carbon dioxide from air. This method differs from the other two in that it requires little water and has a smaller footprint.
In mineral carbonation cations from chemically weathered minerals reaction with atmospheric CO
2. In the process mineral carbonates are formed that store the CO
2. There are large reserves of magnesium silicate minerals useful for this process. This method is researched because it provides storage for a geological time scale. Current technology requires 1.6 to 3.7 tonnes of rock to fix a tonne of CO
2. Since the reaction is exothermic, it produces energy that can be recycled back to source the reactor. The kinetics of this process are slower than adsorption and absorption processes, and storage of carbonated solids must be done with low environmental impact. There are currently no large scale implementation of mineral carbonation. One impedance to the implementation of mineralization is the large energetic costs of the preparation of the solid reactant and its additives.
Energy and water use requirements for direct air capture
Direct air capture of carbon dioxide requires water input. One estimate is that for Absorption, amine based technology, to capture 3.3 Gigatonnes of carbon dioxide a year, the water requirements would be 300km3 a year, or 4% of the water used for irrigation.
In addition to water consumption, direct air capture would require a large amount of energy. This is due to the low concentration of carbon dioxide in air, leading to more energy intensive capture.
Direct air capture requires more energy than carbon capture from a flue gas because the separation occurs from a highly dilute CO
2 stream.Estimates state that CO
2 separation requires 1.8 to 3.6 times more energy than flue gas CO
2 separation. A minimum of 20 kJ of energy is required to separate one mole of CO
2 from an air stream. Energy estimates for contact, separation, release and compression of CO
2 are 0.66 GJ/t CO
Direct air capture projects and companies around the world
Antecyis a Dutch company that has developed non-amine technology for both DAC (CO2-From-Air) and CC (CO2-From-Point Source). Antecy was founded in 2010 and has a "first of its kind" pilot plant. It unique design allows to operate at large scale considering minimal use of energy, materials and footprint while maximising capacity. Apposed to amine sorbents, Antecy's inorganic solid sorbent provides high stability, robustness and long life. The materials used are non-toxic, disposable and fully recyclable. The CO
2 produced does not contain any toxic contaminants, which makes this DAC technology environmentally friendly. Based on current achievements, the estimated cost of CO2 captured from air at large-scale is expected to be $80-100 USD/mt. Estimated costs for CO2 captured from point-sources with higher CO2 concentrations are lower.
Carbon Engineering (CE) is a commercial direct air capture company located in Calgary, Canada and founded in 2009. They currently have a direct air capture pilot plant in British Columbia, Canada that has been in use since 2015. As of 2017, the plant converts a portion of its concentrated CO
2 into fuel using enhanced oil recovery. By 2021, the project is expected to implement a full scale direct air capture to liquid fuel plant. The proposed scale of the project would be 2000 barrels a day of fuel that would be distributed to areas with Low Carbon Fuel Standards. Currently, the plant captures 1 ton of CO
2 a day and produces 1 barrel of fuel a day using its AIR to FUELS technology.
Climeworks has created a plant in Zurich, Switzerland that is capable of capturing 900 tonnes of CO2 in a year directly from the air. The $23 million plant with 18 capturing units traps enough CO2 to grow vegetables in a nearby greenhouse. Climeworks stated that it costs around $600 to capture one tonne of CO2 from the air.
Climeworks received funding from the European Union to partner with Reykjavik energy to design a pilot plant using direct air capture and carbfix technology. The large-scale plant is expected to remove CO
2 from air and sequester it in basalt in Hellisheidi, Iceland. The direct air capture project runs alongside a geothermal powerplant that currently injects and mineralizes CO
2 into basalt bedrock. By combining both technologies, Climeworks hopes to produce the first carbon negative power plant. The project is currently testing direct air capture and CO
2 injection 700 meters into the ground.
Global Thermostat is privately funded carbon capture company located in Manhattan, New York and founded in 2010. Global Thermostat uses amine based sorbents bound to carbon sponges to remove CO
2 from the atmosphere. The company has projects ranging from large scale capture at 50,000 tonne/year to small projects at 40 tonne/year. The company claims to produce a 98% pure stream of CO
2 from direct air capture. Global Thermostat differentiates itself from other CO
2 projects because produces its energy and utilities independently and can be transported to various locations. The company has no post-capture process and sells its CO
2 from its remote capture locations.
Center for Negative Emission
The Center for Negative Emission of Arizona State University is currently working on a DAC process using ion exchange resin. The estimated cost of capture is expected to be $30-200 USD/ton CO
2. Water is used to desorb the carbon dioxide with an energy requirement of 50 MJ/mol.
Due to high energy consumption, direct air capture is expected to cost around $600 a ton of captured carbon dioxide. A plant designed to capture 1 megatonne of carbon dioxide a year is estimated to cost $2.2 billion.
One of the largest hurdle to implementation of direct air carbon capture (DAC) are capital and energy costs required to separate CO
2 and air. Point source carbon dioxide levels (flue gas of a power plant) are 300 times more concentrated than air. From the Sherwood's Rule the separation costs varies linearly with dilution of the feed stream. CO
2 capture costs for hydroxide based solvents generally cost $150 USD/ tonne-CO2. Current liquid amine-based separation is $10-35 USD/ tonne CO
2. Adsorption based CO
2 capture costs are between $30-200 USD/ tonne CO
2. It is difficult to find a specific cost for DAC because each method has wide variation in sorbent regeneration and capital costs. These costs do not take into consideration future plans for the CO
2 stream generated. Enhanced oil recovery, urea yield boosting, beverage carbonation and food processing are all economically viable CO
2 demands.
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- "Direct Air Capture of CO2 with Chemicals: A Technology Assessment for the APS Panel on Public Affairs" (PDF). www.aps.org. 1 June 2011. Retrieved 2018-12-07.
- Ranjan, Manya; Herzog, Howard J. (2011). "Feasibility of air capture". Energy Procedia. 4: 2869–2876. doi:10.1016/j.egypro.2011.02.193. ISSN 1876-6102.
- Tollefson, Jeff. "Sucking carbon dioxide from air is cheaper than scientists thought". Nature. Retrieved 7 December 2018.
- "Global Thermostat". Global Thermostat. Retrieved 2018-12-07.
- "Cost plunges for capturing carbon dioxide from the air". Science | AAAS. 2018-06-07. Retrieved 2018-11-19.