Direct air capture
CO2 is removed from ambient air using large filter systems. Air flows through filters where the CO2 chemically absorbs onto the filter. The air that flows out of the filter contains a substantially reduced amount of CO2. Through heating the filters, the CO2 can be removed from them. The CO2 is then processed differently based on the type of long-term storage. For storage in geological formations, the CO2 is compressed and pumped deep into the ground, between 800 and 2500m deep. Here the CO2 reacts with the surrounding rock and mineralizes and/ or is turned into a stable form of carbon. Projects that employ this method, such as CarbFix in Iceland, report that the carbon turns into rock in as little as two years.
The captured carbon can also be stored in long-lived products. For example, captured CO2 can be embedded into concrete that is used as a building material or be used to make carbon fibres. Using captured carbon in long-lived products is known as Carbon Capture, Utilization and Storage (CCUS).
The reversal of carbon storage can occur if there is leakage from geological storage of carbon or issues within the processing of carbon releases the gas back into the atmosphere. If products containing captured carbon are not employed sufficiently or not used as long-lived products, CO2 may be released back into the atmosphere.
Readiness and scale
DACS is still a very nascent technology and there are currently only a small handful of companies that are developing the technology to remove carbon from the atmosphere. However, because carbon can be stored in geological formations and the relatively low land requirement per tonne of CO2 removed, the potential scale of DACS to remove carbon from the atmosphere is very large. Experts suggest that DACS could remove up to 5 Gt of CO2 out of the atmosphere every year by 2050. This technology will therefore be a vital method to remove enough CO2 to help the world reach net-zero emissions. In order to reach this scale, DACS projects have to be expanded and developed.
Co-benefits / limits
+ Because of the potential – and desperately needed- large scale of DACS in the future, many jobs will be created as this technology develops and reaches scale.
+ Filter systems for DACS can be stacked and organized in a manner that requires very little space per tonne of CO2. Because of this the land requirement for this technology is substantially lower compared to other NETs.
– A significant amount of electricity is needed to force ambient air through the filters and especially to heat the filters to release the CO2 again. Renewable electricity has to be used to power these processes to ensure that more CO2 is permanently removed from the atmosphere than is released through electricity production for the process.
Captured CO2 can also be used for short-lived products, such as carbon dioxide in beverages or synthetic fuels (e-fuels), this is called Carbon Capture and Utilzation (CCU) without CO2 storage. However, by using these products, the CO2 very quickly re-enters the atmosphere and thus there is no negative balance of CO2 and the positive effect on the climate is cancelled out. Only if the captured CO2 is stored long-lived under the ground or in long-lived products, one can speak of a real negative emission.
Support projects that use other methods with DACCS projects coming in the future and contribute to the removal of CO2 from the atmosphere.