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........

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