United Bets on Photosynthesis and Limestone to Fuel Net-Zero Flight Path

The Chicago-based carrier has partnered with a number of clean-tech startups in recent months in a renewed effort to meet its climate goals

A rendition of a JetZero blended-wing aircraft with United Airlines livery. The aircraft have the potential to reduce fuel burn by up to 50% per passenger mile compared with traditional planes, United said. Photo: United Airlines

Airlines are shifting their approach to a low-carbon future, seeking not only to offset the emissions they generate, but remove them altogether and then use the CO2 to make sustainable aviation fuel.

For United Airlines, this means teaming up with startups that are working to pull carbon dioxide out of the air, mimic photosynthesis to produce jet fuel and revolutionize aircraft design to slash emissions.

Earlier this year, the Chicago-based carrier joined forces with Heirloom, a company specializing in direct air capture of carbon dioxide, in a partnership that not only sees the companies sequester CO2 but also — potentially — use it to make sustainable aviation fuel, or SAF. In another deal announced Tuesday, United said it was working with Twelve, a startup that uses industrial photosynthesis to produce SAF from CO2 and water.

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Last month, United said it was investing in next-generation blended-wing startup JetZero in a deal that could see the airline buy up to 200 of the wide-bodied aircraft, which according to United, have the potential to reduce fuel burn by up to 50% per passenger mile compared with traditional planes.

Lauren Riley, the airline’s chief sustainability officer, sets the company’s decarbonization strategy. Her mandate includes reaching an interim sustainability target by 2035: a 50% reduction in carbon intensity compared with 2019, as verified by the Science Based Targets initiative.

At the center of the airline’s strategy is United Airlines Ventures’ Sustainable Flight Fund, a \$200 million investment vehicle backed by the carrier and 22 partners, including Air Canada, Air New Zealand, Boeing and Google.

Andrew Chang, head of corporate development at United, is the managing director of UAV and oversees its investments.

Carbon Removal Using Crushed Up Rocks

Startup Heirloom Carbon uses low-cost limestone to absorb carbon from the air.

Crushed up limestone is heated in a kiln.

That heat separates the limestone into carbon dioxide and calcium oxide powder. The carbon dioxide can then be stored permanently underground or in concrete.

The calcium oxide powder is mixed with water to form calcium hydroxide, which is spread out on large trays that are stacked vertically.

The calcium hydroxide absorbs carbon dioxide from the air. After about three days, it becomes limestone and the process is repeated.

Limestone naturally absorbs carbon. Heirloom accelerates the natural process so it takes days instead of years. To remove carbon from the air, the company warms up crushed limestone in a kiln heated to about 1,650 degrees Fahrenheit and powered by renewable electricity. That heat separates the carbon dioxide, which is stored underground or in concrete, and leaves a chemical powder called calcium oxide.

That powder is then combined with water to become calcium hydroxide and spread onto trays the size of picnic blankets. Outside, the calcium hydroxide puffs up like a cookie in the oven as it absorbs carbon dioxide over about three days. The resulting limestone can go back into the kiln to restart the cycle.

But what makes United’s deal with Heirloom unusual is its versatility: The captured CO2 can either be permanently sequestered—generating carbon-dioxide removal, or CDR, credits—or used in the production of power-to-liquid SAF.

United’s Sustainable Flight Fund has the option to buy 500,000 tons of CO2 from Heirloom. It said capturing that amount of CO2 from the air and sequestering it permanently underground is the equivalent to abating CO2 emissions from a 737 MAX flying about 33 million miles.

The airline acknowledged that SAF, like regular jet fuel, emits CO2 into the atmosphere when burned. But it pointed out the CO2 used to make SAF primarily comes from the atmosphere, whereas the CO2 used to make jet fuel comes from oil that was underground. The advantage of burning SAF is that it results in no net change to atmospheric CO2, the company says.

However, United added capturing and sequestering CO2 using Heirloom’s technology while continuing to use regular jet fuel is currently cheaper than using CO2 to make sustainable aviation fuel. That is because making SAF also requires making hydrogen and combining it with the captured CO2, which is expensive.

Riley said that the type of carbon removal Heirloom offers is preferable to traditional nature-based offsets, which she said lack permanence and are harder to measure.

Chang didn’t disclose exactly how much UAV invested in the three deals, saying only when it comes to investments, “our sweet spot is \$5 million to \$10 million, maybe sometimes up to \$15 million.”

How Twelve makes sustainable aviation fuel

In Twelve’s carbon dioxide electrolyzer, carbon dioxide flows in and comes into contact with a metal catalyst. One of the oxygen atoms in CO2 gets separated, leaving carbon monoxide (CO).

Water flows into a water electrolyzer and comes into contact with a metal catalyst, splitting into hydrogen and oxygen.

Carbon monoxide and hydrogen are combined to make synthesis gas, while oxygen is vented or captured and used in other industrial processes.

While the Heirloom deal is key to lowering emissions in the short term, United’s real decarbonization challenge lies in its long-term fuel strategy. In 2024, United sourced 14 million gallons of SAF—some 0.3% of its total fuel usage for the year. Yet, with SAF costing three to four times more than conventional jet fuel, increasing that share represents a challenge.

“Scaling the SAF industry is the major hurdle air travel needs to clear in order to increase the supply and reduce the price of lower carbon fuels,” said Chang.

United’s deal with Twelve goes some way to address that. It follows an \$83 million funding round secured by the startup for its first production facility in Moses Lake, Wash. The plant is expected to begin production this year and plans to produce 50,000 gallons of SAF annually, with the potential to reduce life cycle emissions by up to 90% compared with conventional jet fuel, according to United.

Riley and her team helped shape the SAF provisions in the Inflation Reduction Act, including the 45Z Clean Fuel Production Tax Credit, which provides a credit of 35 cents a gallon for sustainable aviation fuel. Today, following the election of President Trump, questions continue to hang over the future of the IRA, and the path ahead for the aviation industry appears turbulent.

Yet Riley remains bullish. She points to the SAF Coalition, a 60-member nonprofit advocacy group established last year including oil majors, startups, labor unions and agriculture stakeholders that she says are all pushing for the continuation of 45Z.

“What’s really interesting in the U.S. is that the states that are benefiting from new SAF production facilities coming online are largely in red states. It is largely in the geographies that you wouldn’t expect would be behind any kind of climate ambition. This is good for the economy. This is creating jobs in a transitioning industry,” she said.