How does direct air capture work, and how far has it got?

What direct air capture is and why it matters

Direct air capture (DAC) is a group of technologies that pull carbon dioxide (CO2) out of ordinary air. The basic idea is simple. A machine passes air over a material that binds CO2, then releases the gas in concentrated form so it can be stored away from the atmosphere.

Most work on climate technology has gone into cutting emissions. A 2019 US National Academies report notes that negative emissions technologies, which take CO2 out of the atmosphere, have received much less attention from researchers than conventional mitigation. Its preface lists DAC, described as using chemical processes to capture CO2 directly from the air, among four groups of these technologies that the committee was asked to cover (National Academies, 2019). Whether DAC can make a real contribution depends on the technology running at scale and at an acceptable cost. This article separates what has been measured from what has only been announced or proposed.

How it works

CO2 is dilute in air: the laboratory tests described below used simulated air containing 400 parts per million. A capture system therefore has to bring a large volume of air into contact with the capture material. The process has three main stages.

• Air contact. Air is passed over a sorbent, which is a material that takes up CO2 on its surface or within its pores.
• Regeneration. The sorbent is heated or otherwise treated so that it releases a concentrated stream of CO2. This step uses energy, and it allows the sorbent to be reused.
• Storage. The concentrated CO2 is stored, for example deep underground. The National Academies preface describes chemical DAC as capturing CO2 directly from the air and then sequestering it in geologic reservoirs (National Academies, 2019). Without a storage route, the captured CO2 does not leave the carbon cycle permanently.

What has been demonstrated

"Demonstrated" here means measured results from tests. The sources reviewed for this article do not describe operating commercial plants with verified capture totals, so this section covers laboratory work.

A 2024 paper in Nature reports "charged sorbents": activated carbon cloth loaded with hydroxide ions by electrochemical charging (Li and colleagues, 2024). The authors prepared the material by applying a voltage of +0.565 volts against the standard hydrogen electrode for four hours in 6 molar potassium hydroxide, then washing and drying the electrode. Their measurements include the following.

• The sorbent captured CO2 from air through (bi)carbonate formation. The authors report that it can be regenerated at 90 to 100 °C, including by direct Joule heating, where an electric current heats the conductive carbon itself.
• In laboratory tests in simulated dry air at 400 ppm CO2, it captured roughly 0.2 mmol of CO2 per gram of sorbent.
• The sorbent completed 150 adsorption and desorption cycles using a simulated gas with 30% CO2 rather than air. Adsorption ran at 30 °C and desorption at 100 °C.
• After 14 months of storage, the sorbent had lost about 10% of its capacity in tests with pure CO2. The authors report that it could still capture CO2 from air at comparable capacity after that time.

These are measurements on small laboratory samples. They show how one material behaves. They do not show a complete capture plant, and they do not show the energy use or cost of a full system.

What is only reported

"Reported" here means plans and claims that the sources reviewed do not verify.

• Government funding. The US Department of Energy issued a notice of intent on 20 September 2024 for its Regional Direct Air Capture Hubs programme. It anticipates up to US$1.8 billion, with awardees providing at least 50% cost sharing. The notice expects mid-scale facilities capturing 2,000 to 25,000 tons of CO2 per year and large-scale facilities capturing at least 25,000 tons per year (US Department of Energy, 2024). The notice states that it is for information only, that applications were not being accepted at the time and that its contents are subject to change. These are planned figures from 2024, not awarded projects or built plants. The sources reviewed here do not show whether the funding opportunity was later issued, changed or withdrawn.
• Company claims. Companies often announce capacity and cost targets. The sources reviewed for this article contain no verified cost-per-tonne figures, so any such claim should be treated as reported until independent data is published with its assumptions.

What is speculative

• Removal at very large scale. Whether DAC could remove carbon at the scale that climate targets may need is not shown by the sources reviewed here. The case rests on a hypothesis that depends on capture costs falling and storage routes scaling.
• Future costs. No reviewed source gives a verified cost trajectory. Any forecast of cost per tonne in future years would be speculation, and this article does not offer one.
• Uses for captured CO2. The Department of Energy notice says captured CO2 could be converted into products such as concrete instead of being stored. The sources reviewed here report no measured results for this route.

Limits and open problems

• Energy use. The authors estimate a minimum electrical energy for sorbent heating of 6.5 GJ per tonne of CO2 at 11% relative humidity, rising to 11.4 GJ per tonne at 38%. That figure covers heating the sorbent only, so the energy needed for the whole process is still unknown from these sources.
• Cost per tonne. This is the central barrier, and no verified figure is available from the sources reviewed.
• Humidity. In the laboratory tests, capacity fell from roughly 0.14 to 0.08 mmol of CO2 per gram as relative humidity rose from 11% to 38%. The authors suggest this is probably due to water filling the pores and blocking hydroxide sites, rather than to sorbent degradation. Any real air system would need to manage humidity.
• Speed and capacity. The authors report a lower saturation capacity than benchmark sorbents and say the material saturates quickly, so short adsorption cycles should be used. They say that increasing sorbent CO2 capacity is the most straightforward way to improve energy efficiency.
• Durability. The 150 cycles used a simulated gas with 30% CO2. The tests in ambient air with Joule heating ran for four cycles, so long-term performance in real air is not shown.
• Scale-up. The results come from small samples, such as 2 × 2 cm pieces of carbon cloth. The paper does not report performance in larger sheets or packed beds.
• Storage and permanence. Captured CO2 must be stored or used. Without storage, capture does not permanently remove carbon.
• Verification. The sources reviewed do not explain how removed CO2 would be measured and independently verified at plant scale.

What to watch next

• Whether the Department of Energy moves from its notice to awards, and whether awarded facilities publish measured capture totals and energy use.
• Independent cost-per-tonne figures, published with their assumptions.
• Long-running sorbent tests in real ambient air rather than simulated gas mixtures.
• Energy figures for the whole capture process, not only sorbent heating.
• Storage permits and the availability of geologic storage close to capture sites.

Direct air capture is at an early stage. Some laboratory results are measured. Plant capacity and cost claims are mostly reported. Removal at large scale remains speculative.