Direct Air Capture

Direct air capture (DAC) technologies extract CO₂ directly from the atmosphere. The CO₂ can be permanently stored in deep geological formations, thereby achieving carbon dioxide removal (CDR). Benefits of DAC as a CDR option include high storage permanence when associated with geological storage and a limited land and water footprint. The captured CO₂ can also be used, for example in food processing or combined with hydrogen to produce synthetic fuels. In a transition to net zero emissions, the CO₂ used to produce synthetic fuels would increasingly need to be captured from sustainable bioenergy sources or from the atmosphere to avoid delayed emissions from fossil-based CO₂ when the fuel is combusted. DAC is therefore one option to achieve this.  

Eighteen DAC plants are currently operational in Europe, the United States and Canada. All of these plants are small scale, and the large majority of them capture CO2 for utilisation – for drinks carbonation, for instance – with only two plants storing the captured CO2 in geological formations for removal. Only a few commercial agreements are in place to sell or store the captured CO2, while the remaining plants are operated for testing and demonstration purposes. 

The first large-scale DAC plant of up to 1 Mt CO₂/year is in advanced development and is expected to be operating in the United States by the mid-2020s.  

An improved investment environment led to announcements of several new DAC projects in 2021, including the Storegga Dreamcatcher Project (United Kingdom; aimed at carbon removal) and the HIF Haru Oni eFuels Pilot Plant (Chile; producing synthetic fuels from electrolysis-based hydrogen and air-captured CO₂). Synthetic fuels (up to 3 million litres) are also set to be produced by the Norsk e-Fuel AS consortium in Norway by 2024, including (but not using exclusively) CO₂ captured from DAC. In June 2022 1PointFive and Carbon Engineering announced plans to deploy 70 large-scale DAC facilities by 2035 (each with a capture capacity of up to 1 million tonnes per year) under current policy and voluntary and compliance market conditions, while Climeworks announced the construction of their largest plant to date, Mammoth (capture capacity up to 36 000 t CO₂/year), which should become operational by 2024. 

Plans for a total of eleven DAC facilities are now in advanced development. If all of these planned projects were to go ahead, DAC deployment would reach around 5.5 Mt CO₂ by 2030; this is more than 700 times today’s capture rate, but less than 10% of the level of deployment needed to get on track with the Net Zero Scenario. 

Two technological approaches are currently being used to capture CO₂ from the air: solid and liquid DAC. Solid DAC (S-DAC) is based on solid adsorbents operating at ambient to low pressure (i.e. under a vacuum) and medium temperature (80-120°C). Liquid DAC (L-DAC) relies on an aqueous basic solution (such as potassium hydroxide), which releases the captured CO₂ through a series of units operating at high temperature (between 300°C and 900°C).

Capturing CO₂ from the air is more energy intensive and therefore expensive than capturing it from a point source. This is because the CO₂ in the atmosphere is much more dilute than, for example, in the flue gas of a power station or a cement plant. This contributes to the higher energy need and cost of DAC relative to other CO₂ capture technologies and applications. 

Capturing CO₂ from the air is more energy intensive and therefore expensive than capturing it from a point source. This is because the CO₂ in the atmosphere is much more dilute than, for example, in the flue gas of a power station or a cement plant. This contributes to the higher energy need and cost of DAC relative to other CO₂ capture technologies and applications. 

In current DAC plant configurations, the proportion of heat in the total energy needed is influenced by the operating temperature of the technologies. Both S-DAC and L-DAC were initially designed to operate using heat and electricity, with flexible configurations allowing for heat-only operation.

While S-DAC and L-DAC innovation efforts are mostly focused on innovative sorbents and solvents, and optimised processes and layouts, emerging DAC technologies (at a technology readiness level [TRL] below 6) include electro-swing adsorption (ESA) and membrane-based DAC (m-DAC). ESA is based on an electrochemical cell where a solid electrode adsorbs CO₂ when negatively charged and releases it when a positive charge is applied (swinging therefore the electric charge, rather than the operating temperature or pressure as happens in other physical separation techniques). m-DAC has been proposed as another feasible option for capturing CO₂ from the air; however, it is still in its infancy and major challenges are yet to be overcome (including the need for the expensive compression of a very large amount of ambient air to separate CO₂ efficiently).

While S-DAC could be powered by a variety of low-carbon energy sources (e.g. heat pumps, geothermal, solar thermal and biomass-based fuels), the current high-temperature needs of today’s L-DAC configuration does not allow that level of flexibility and could at best operate using low-carbon fuels such as biomethane or renewables-based electrolytic hydrogen. In the future L-DAC could shift to fully electric operation (currently only available for small-scale calcination). Large-scale L-DAC plants have been designed to use natural gas for heat and to co-capture the CO₂ produced during combustion of the gas without the need for additional capture equipment. This integration substantially reduces the L-DAC plant’s overall emissions and can still enable carbon removal. However, any future ability of renewable energy to supply high-temperature heat could reduce the process emissions to near zero, maximising the potential for carbon removal and associated revenue streams. Accelerating the commercial availability of large-scale electric calcination technology is considered a high priority to enable L-DAC plants to operate purely on renewable energy. 

A major advantage of DAC is its flexibility in siting: in theory, a DAC plant can be situated in any location that has low-carbon energy and a CO₂ storage resource or CO₂ use opportunity. Yet there may be limits to this siting flexibility. To date, DAC plants have been successfully operated in a range of climatic conditions in Europe and North America, but further testing is still needed in locations characterised, for instance, by extremely dry or humid climates, or polluted air.

The choice of location also needs to be based on the energy source needed to run the DAC plant. The energy used to capture the CO₂ will determine how net-negative the system is and can also be a significant determinant of the cost per tonne of CO₂ captured. For instance, both S-DAC and L-DAC technologies could be fuelled by renewable energy sources, while recovered low-grade waste heat could power an S-DAC system. 

Carbon removal requires the CO₂ to be permanently stored. While the overall technical capacity for storing CO₂ underground worldwide is understood to be vast, detailed site characterisation and assessment are still needed in many regions. An operating CO₂ storage site can take three to ten years to develop from project conception to CO₂ injection. This could become a bottleneck for DAC deployment (and CCUS deployment in general) without accelerated efforts to identify and develop CO₂ storage sites.

The choice of location also needs to be based on the energy source needed to run the DAC plant. The energy used to capture the CO₂ will determine if and how net-negative the system is and can also be a significant determinant of the cost per tonne of CO₂ captured. For instance, both S-DAC and L-DAC technologies could be fuelled by renewable energy sources, while recovered low-grade waste heat could power an S-DAC system. 

Carbon removal requires the CO₂ to be permanently stored. While the overall technical capacity for storing CO₂ underground worldwide is understood to be vast, detailed site characterisation and assessment to render potential storage sites operational are still needed in many regions. An operating CO₂ storage site can take three to ten years to develop from project conception to CO₂ injection. This could become a bottleneck for DAC deployment (and CCUS deployment in general) without accelerated efforts to identify and develop CO₂ storage sites.

Countries and regions that have taken an early lead in supporting DAC research, development, demonstration and deployment include the United States, the European Union, the United Kingdom, Canada and Japan.

The United States has established a number of policies and programmes to support DAC RD&D, including the 45Q tax credit (providing USD 35 per tonne of CO₂ used in enhanced oil recovery and USD 50 per tonne of CO₂ stored) and the California Low Carbon Fuel Standard credit (providing the DAC project meets the requirements of the Carbon Capture and Sequestration Protocol). Meanwhile, the Investment and Jobs Act (signed into law in November 2021) includes funding (USD 3.5 billion) to establish four large-scale DAC hubs and related transport and storage infrastructure. 

The European Commission has been supporting DAC through various research and innovation programmes (including the Horizon Europe programme and through the Innovation Fund, launched in 2020 for the decade 2020-2030 with an initial budget of around USD 11.8 billion) and more broadly carbon dioxide removal through its first Communication on Sustainable Carbon Cycles (published in December 2021), which suggests that 5 Mt of CO₂ should be removed annually by 2030 from the atmosphere through land- and technology-based approaches such as DAC. 

Private investors are also increasingly supporting DAC. It is one of four technologies being targeted for up to USD 1.5 billion in investment by Breakthrough Energy Catalyst, established by Bill Gates and a coalition of private investors, and it is an eligible technology for the USD 100 million Carbon Removal XPRIZE. 

Support for DAC has come from programmes such as X-Prize (offering up to USD 100 million for as many as four promising carbon removal proposals, including DAC) and Breakthrough Energy’s Catalyst Program (which raises money from philanthropists, governments and companies to invest in critical decarbonisation technologies, including DAC). Additionally, in April 2022 Lowercarbon Capital Fund announced the intention to invest USD 350 million in start-ups developing technology-based CDR solutions, while a number of businesses (including Stripe, Alphabet, Shopify, Meta and McKinsey) have launched Frontier Climate, a buyers group using advance market commitments to buy an initial USD 925 million of permanent carbon removal between 2022 and 2030.

Private investment rounds have also been successful: in 2022 Climeworks raised the largest-ever DAC investment, equivalent to USD 650 million. 

The development of agreed methodologies and accounting frameworks based on life cycle assessment (LCA) for DAC – alongside other CDR approaches – will be important to support its inclusion in regulated carbon markets and national inventories. Notably, the latest IPCC Guidelines for National Greenhouse Gas Inventories do not include an accounting methodology for DAC, meaning that CDR associated with DAC cannot be counted towards meeting international mitigation targets under the United Nations Framework Convention on Climate Change. 

Efforts to develop carbon removal certification, including for DAC-based CDR, have commenced in Europe and the United States, as well as through initiatives such as the Mission Innovation CDR Mission. These efforts should be coordinated with the aim of establishing internationally consistent approaches.

The voluntary market for DAC-based CO₂ removal has expanded, with companies such as Microsoft, Stripe, Shopify, Swiss Re and Airbus purchasing future DAC removal to offset their CO₂ emissions.  

Some of these agreements are hybrid, wherein the company purchasing the offsets is effectively supporting the capital investment to build the DAC plant that is eventually going to capture CO₂ from the atmosphere. For instance, United Airlines is directly investing in DAC in line with its commitment to become carbon neutral by 2050, while Microsoft is purchasing DAC removal from Climeworks and, through its climate innovation fund, has also invested in Orca, the largest operating DAC plant for carbon removal. 

On a smaller scale, some DAC companies are currently offering commercial offset services, to individuals as well as companies willing to pay a recurring subscription to have CO₂ removed from the atmosphere and stored underground on their behalf. The price of the subscription varies (depending on the amount of removal purchased) from USD 600/t CO₂ to USD 1 000/t CO₂. 

Carbon removal technologies such as DAC are not an alternative to cutting emissions or an excuse for delayed action, but they can be an important part of the suite of technology options used to achieve climate goals. 

For this reason, DAC needs to be demonstrated at scale, sooner rather than later, to reduce uncertainties regarding future deployment potential and costs, and to ensure that these technologies can be available to support the transition to net zero emissions and beyond. 

In the near term, large-scale demonstration of DAC technologies will require targeted government support, including through grants, tax credits and public procurement of CO₂ removal. 

Technology deployment is currently benefiting from corporate-sector initiatives and pledges to become carbon negative through the voluntary market.  

However, longer-term deployment opportunities will be closely linked to robust CO₂ pricing mechanisms and accounting frameworks that recognise and value the negative emissions associated with storing CO₂ captured from the atmosphere. 

As an increasing number of countries make net zero pledges, the focus of decision makers has shifted to how to turn these pledges into clear and credible policy actions and strategies. To date, very few countries and companies have developed detailed strategies or pathways to achieve their net zero goals, but a critical question for all will be the extent to which these strategies will need to rely on CDR approaches alongside direct emission reductions. DAC and other CDR approaches are part of the portfolio of technologies and measures needed in a comprehensive response to climate change. Promoting transparency and planning for the anticipated role of CDR in net zero strategies can support the identification of technology, policy and market needs within countries and regions while supporting public understanding of these approaches. 

Innovation will be central to reducing the cost of DAC technologies and supporting accelerated commercialisation. Priority innovation needs for DAC include: 

• Reducing the energy consumption needed to separate CO₂ through emerging separation technologies and innovative approaches able to regenerate the solvent at low to medium temperatures. 
• Advancing engineering maturity and market conditions to support the availability of renewables-based high-temperature heat to maximise the carbon removal potential and provide an alternative to current designs based on capture of CO₂ from natural gas. 
• Reducing the cost of large-scale opportunities to use air-captured CO₂, particularly for synthetic fuels. 

Increased RD&D spending to drive innovation in DAC technologies at a national and global level will be essential in the near term. 

For DAC technologies, international co-operation can drive faster deployment and accelerated cost reductions through shared knowledge and reduced duplication of research efforts. International co-operation can also support the development and harmonisation of LCA methodologies for DAC technologies. International organisations and initiatives such as the IEA, Mission Innovation CDR Mission, the Clean Energy Ministerial CCUS Initiative, and the Technology Collaboration Programme on Greenhouse Gas R&D (GHG TCP/IEAGHG) can provide important platforms for knowledge-sharing and collaboration.