In a narrow valley with steep sides near the ancient city of Cartagena, Spain, a team of 150 engineers has just finished building a plant that could be a game changer for Repsol, the Spanish energy company, and a bellwether for the transportation industry.

Emilio Mayoral, who manages the unit, said his colleagues were in the early days of brewing fuels for trucks and airplanes from what was formerly garbage. “It’s quite flexible,” he said. “We are currently using used cooking oil, but we can use other waste.”

Repsol says these alternative fuels will cut emissions by up to 90 percent compared with the petroleum-based products they will replace. The new fuels emit some carbon dioxide when consumed, but they are produced from plants and other organisms that absorbed carbon dioxide during their lifetimes, which is factored into the emissions calculation.

As an added benefit, this new generation of biofuel products performs as well as their fossil fuel counterparts, even in cold Northern European weather that creates problems for some fuels, Mr. Mayoral said.

Repsol, which is based in Madrid, is one of Europe’s largest energy companies, with 26,000 employees and more than 4,500 service stations as well as investments in renewable energy like wind and solar power. Repsol reported income of 1.6 billion euros for the first half of 2024.