Why United Airlines is investing in crushed rocks

The airline industry is notoriously hard to decarbonize: large jets traveling long distances can’t feasibly use batteries, and sustainable aviation fuel is still only produced in tiny volumes.

As airlines explore a range of options, United Airlines Ventures’ Sustainable Flight Fund just invested in one possible solution—a system that uses crushed rocks to capture CO2 for use in fuel or to store underground.

The fund announced today that it invested an unspecified amount in Heirloom, a company that uses a powder made from limestone to pull CO2 from the air, relying on the material’s natural ability to absorb the greenhouse gas. At a facility in California’s Central Valley, robots stack trays of the limestone powder into tall stacks exposed to outdoor air. Then the powder is heated in furnaces to release the CO2 so it can be used or stored.

United also now has an agreement with Heirloom that gives it the right to buy up to half a million tons of carbon dioxide removal from the startup. “We can either sequester it and track it as a carbon removal credit, or we can use it for [sustainable aviation fuel],” says Andrew Chang, managing director at United Airlines Ventures.

The investors liked the basic simplicity of the technology. “We understand how limestone can capture and release CO2,” Chang says. “It is not a novel, unproven technology or catalyst or chemical pathway. It works: Heat it and cool it and it’ll lock and unlock CO2.”

Several other companies are paying Heirloom for the service of carbon removal to offset emissions, including Microsoft, which has a long-term contract to buy as much as 315,000 metric tons of CO2 removal from the startup.

If the captured CO2 is combined with green hydrogen, it can be made into fuel that can be used on existing planes. Some other sustainable aviation fuel is limited because of the feedstock—making jet fuel from corn, for example, poses environmental challenges because of the amount of land that’s needed to grow it. (United has separately invested in biofuels made from corn, along with several other approaches.) But CO2 has a supply advantage: There’s more than enough extra CO2 in the atmosphere to meet the industry’s needs.

Some startups, including Twelve and Infinium, are now beginning to scale up production of CO2-based sustainable aviation fuel. The carbon footprint is as much as 94% less than conventional jet fuel. (If airlines also pay Heirloom to remove CO2 and store it, that can help offset the remaining carbon footprint.) Right now, this type of sustainable aviation fuel is two to four times as expensive as traditional fuel, though as airlines work on long-term plans to cut emissions by 2050, there’s a path to eventual price parity. The same approach could be used to cut emissions in other hard-to-decarbonize industries, like cargo shipping.

United’s investment will help Heirloom scale up production faster. “The funding will be used to continue to drive down the cost of the technology, develop additional projects, and provide the funding needed to subsequently access infrastructure capital,” says Heirloom spokesperson Scott Coriell. The startup is focused on what it calls “deployment-led innovation,” using real-world installations to help it iterate and reduce costs. “As we continue to build larger projects, costs will come down and the market will grow,” he says. “The critical objective to scale [direct air capture] is to repeat this cycle again and again.”

Ultimately, though huge volumes of captured CO2 could be used to make fuel, the carbon removal industry will have to grow even faster to deal with the problem of CO2 in the atmosphere; even as companies cut emissions, pulling CO2 out of the air is necessary to avoid the worst impacts of climate change. Around a trillion extra tons of CO2 have been added to the atmosphere since the Industrial Revolution, and the number keeps growing. As it’s captured, much of it will be stored.

“We believe that over time, the vast majority of CO2 will end up underground,” Coriell says. “Even in a world where aviation, and other hard-to-abate industries, transition to cleaner fuels, there will still be billions of tons of CO2 emissions that will need to be abated each year.”

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