United Airlines bets on direct air capture to help decarbonize aviation

The venture arm of United Airlines has invested in a leading carbon dioxide removal (CDR) startup in a deal that will give the carrier new decarbonization and revenue options.

The agreement between United Airlines Ventures (UAV) and Heirloom is notable for its size: it includes an undisclosed investment in the startup and the option to buy 500,000 tons of carbon removal, which if exercised would be a significant purchase for the nascent removals sector. The deal’s structure is also innovative. Like other agreements in this space, UAV can sequester the carbon dioxide to offset emissions — or use the CO₂ as feedstock for the production of sustainable aviation fuel (SAF). 

“This is the first time we’ve done a deal that provides for either CDRs or sustainable aviation fuel production,” said Max Scholten, Heirloom’s head of commercialization. “It gives them the opportunity to meet their climate goals in a way that’s flexible.”

Carbon cycle

Heirloom’s technology captures carbon dioxide from the atmosphere by reacting it with calcium hydroxide and water to create limestone. After heating to release the CO₂, the limestone converts back to calcium hydroxide and the process is repeated. The company’s first facility, in Tracy, California, opened in 2023 and can capture 1,000 tons of carbon dioxide annually. Heirloom is working on two new facilities in Louisiana, which when fully operational will have a combined capacity of 320,000 tons per year. 

At Tracy, the CO₂ released from the limestone is embedded permanently in concrete. UAV’s purchases will be supplied by the Louisiana facilities, which will offer the option of permanent geological storage. The venture arm could then claim the sequestered carbon as an offset or sell an equivalent number of credits on carbon markets. 

If UAV instead chooses to create SAF, the carbon dioxide would form the input for a series of chemical reactions that use renewable energy to produce SAF. Because the SAF is created from carbon captured from the atmosphere, over its life cycle the fuel releases dramatically less CO₂ than jet fuel derived from crude oil. Several startups, including Twelve and Infinium, have developed “power-to-liquid” SAF technologies. Scholten said that Heirloom has not chosen a partner for future SAF work.

Many times the price

The decision on how to use the captured carbon will be determined by whichever option is most profitable, said Andrew Chang, UAV’s managing director. UAV is spearheaded by United but counts Air Canada, Google and others as partners. The investment comes from its $200 million Sustainable Flight Fund. Chang would not reveal the amount invested in Heirloom, but said that the fund typically invests in the $5 million to $15 million range. Heirloom’s last round was a $150 million Series B in December.

The little SAF that is burned today — it accounted for just 0.3 percent of global jet fuel production in 2024 — is mainly derived from biological sources. Regardless of source, it is much more expensive than the conventional option. SAF produced from waste cooking oil, for example, is typically twice the price of fossil jet fuel, said John Dees, a biofuels specialist at Carbon Direct, a carbon management company. The price differential of power-to-liquid SAF is more like fivefold, he added.

Dees pointed to several reasons why companies are pursuing the technology, despite the price. For one, the use of CO₂ as an input means the process has a “theoretically limitless feedstock.” By contrast, if biomass is used there is the potential for competition with food production. Because it is a relatively new technology, power-to-liquid SAF also has “a lot of runway” to get cheaper, added Dees — unlike biomass SAF, which is a much more mature process.