The innovators trying to bring down the sky-high cost of direct air capture
October 24, 2023
Industry Insight from Ethical Corporation Magazine, a part of Thomson Reuters
By Mike Scott | Although the fundamentals of direct air capture are fairly standard – huge facilities with massive fans drawing air in to remove the CO2, normally in a remote landscape – there are a number of technologies to do it. But all of them are trying to bring down the high energy needs, and hence cost, of DAC.
The key divide is between high-temperature and low-temperature DAC. Two of the three most established DAC companies, Swiss-based Climeworks and Global Thermostat, based in Colorado, use low-temperature technologies, with sorbents that attract CO2 molecules to stick to the sorbent surface.
“It acts like a sponge that soaks up CO2 from the atmosphere,” says Christoph Gebald, chief executive of Climeworks. “When it is saturated, we heat it to 100C, and the sponge releases the CO2 and we can suck it out of the system.”
British Columbia-based Carbon Engineering, by contrast, uses a liquid solvent, which absorbs the CO2 and is then turned into calcium carbonate. To release the CO2, this must be heated to 900C.
The advantage of the low-temperature process is that it requires less energy to provide heat and can use a wide range of heat sources, including waste heat from industrial facilities, power stations and elsewhere, as well as heat pumps and geothermal sources.
“Energy consumption can be make-or-break for any DAC technology,” says Gebald. “Any technology that is a couple of percentage points more energy-efficient is much more likely to be competitive in the long run.” He adds that with CO2 accounting for only 0.04% of the atmosphere, the capacity to cut energy use is limited. “Currently we are operating at low single-digit percentage energy efficiency, which is comparable to the early days of solar and wind.”
He believes that through research and development energy efficiency could realistically reach 20%, following a similar trajectory to solar, and through cost-savings from scaling the cost of Climeworks’ technology could come down from close to $1000 per ton of CO2 to $400-$500 by the end of the decade, and $300-400 by 2040.
Climeworks chose Iceland to build Orca, which opened in 2021, because of its geothermal energy, which provides the plentiful heat and electricity required to operate the capture plant. Orca has a nominal capture capacity of up to 4,000 metric tons of CO2 a year while its next Iceland plant, Mammoth, will be able to capture up to 36,000 tons a year when it opens, originally slated for 2024.
However, cheap geothermal energy isn’t available everywhere. In the Project Cypress project in Louisiana, for example, he said solar and wind energy would provide the green electricity to run the plant.
Another promising source of affordable energy is waste heat from nuclear power stations: in the UK, Sizewell C nuclear power station and Associated British Ports are building a demonstration DAC project that could lead to a scaled-up plant removing CO2 from the atmosphere using waste heat from Sizewell C, which is due to be fully operational by 2034.
Matt Kirley, manager of climate-aligned industries at Rocky Mountain Institute, says that while low-temperature materials do require less energy, they wear out and need to be replaced. High-temperature processes, need gas combustion to reach the required temperatures, but the materials being used are very inexpensive. The techniques are also mature and have been used for many decades in natural gas processing, refining, chemicals production and food and beverage industries.
There are a number of alternative technologies to capture the CO2, including membrane separation and cryogenic separation.
Norway’s Removr is partnering with Climeworks' storage partner Carbfix to launch a 2,000-ton commercial plant in Iceland in 2025, and to build a much bigger 100,000-ton plant in 2027.
It aims to halve the energy consumption of current DAC plants using materials known as zeolites to capture the carbon.
“Zeolites are materials that have an affinity for both water and CO2,” Kirley says. “They have tiny pores, meaning there is a much bigger surface area to store the carbon in and more carbon can be captured per unit of air processed. This is really important when you are trying to extract parts per million out of the air.”
Membrane separation techniques that use zeolites are seen as more energy-efficient than conventional separation techniques such as amine absorption, where amines absorb CO2 to form a soluble carbonate salt. Absorption techniques however, require higher temperatures to release the CO2 from the solution for capture. The company, which has had funding from the Norwegian government, has built four small-scale pilots of its technology in Norway.
Another promising option is electro-swing adsorption. This technique uses “materials whose affinity for CO2 depends on applying a voltage. When you hook them up to electricity, there is a chemical change in the material that causes it to trap CO2,” Kirley explains. “When you remove the voltage, they release the CO2.”
According to the Energy Transitions Commission, “though not yet proven at scale, there is widespread expectation that electro-swing adsorption (ESA) will become commercially viable in the coming decade.” By using power instead of heat to release the CO2, ESA should be able to reduce the energy requirements, and therefore the cost, significantly.
The Energy Transitions Commission says the energy required for both high and low-temperature processes “typically ranges up to 10 gigajoules per ton of carbon dioxide (GJ/tCO2) captured for technologies available today. Electro-swing adsorption eliminates thermal energy requirements entirely, potentially lowering total energy requirements as low as 2 GJ/tCO2, according to some industry projections.”
Verdox, a spin-out from MIT that is working on ESA, says that by using this electrochemistry approach it will be able to use up to 70% less energy San Francisco’s Heirloom, which has a goal of removing 1 billion tons of CO2 from the air, is using lime to absorb CO2 from the air to form limestone, accelerating the natural binding of CO2 and lime from a period of years to just three days. This year it signed a memorandum of understanding with Europe’s Leilac to integrate its renewably powered kiln technology to capture CO2 from limestone into its processes.
“Leilac’s unique indirect heating approach requires no additional chemicals or processes and can be directly powered by renewable electricity. By keeping the process CO2 emissions pure, Leilac’s technology removes the need to separate gases from gases, enabling it to target the lowest-cost solution for the capture of CO2 from limestone,” Heirloom said.
Meanwhile, Thalo Labs in New York City is attempting to build the case for “distributed DAC”. “We’re tackling the problem from a different angle,” says Dr Brendan Hermalyn, founder and chief executive. “The built environment is the second-largest source of emissions: in cities like New York, it’s over 70% of emissions.”
Hermalyn points out that while the global concentration of CO2 emissions is currently around 420 parts per million, in cities it is much higher, particularly indoors. The company has developed devices to capture carbon from indoor air, or from the flue exhausts of boilers and furnaces, which it says are very low power. The captured carbon can be used “to create circular low-carbon building materials”, Hermalyn adds.