Fulcrum said its technology can cut an airline’s carbon emissions by 80 percent compared with traditional jet fuel. “There is definitely a huge interest from airlines in this market,” said Angela Foster-Rice, United’s managing director for environmental affairs and sustainability.

United’s deal with Fulcrum is one of many that airlines have made in recent years.

Alaska Airlines aims to use biofuels at least at one of its airports by 2020. Southwest Airlines announced last year that it would purchase about three million gallons a year of jet fuel made from wood residues from Red Rock Biofuels. The first blend of this new fuel product, however, won’t be available until 2016.

Last year, British Airways joined with Solena Fuels to build a biofuel refinery near London’s Heathrow Airport, which will be completed by 2017.

United’s deal is the airline’s second major push toward alternative fuels. In 2013, the airline agreed to buy 15 million gallons of biofuels over three years from a California-based producer called AltAir Fuels, which makes biofuels out of nonedible natural oils and agricultural waste. United expects that the first five million gallons from AltAir will be delivered this summer at its Los Angeles International Airport hub to help power the flights to San Francisco.

For the first two weeks, four to five flights a day will carry a fuel mixture that is 30 percent biofuel and 70 percent traditional jet fuel; after that, the fuel will be blended into the overall supply, United said.

“The AltAir project serves as a catalyst intended to pave the way for the industry,” Ms. Foster-Rice said. By burning biofuel products like farm waste that have already absorbed carbon during their lifetime, jet engines avoid introducing into the atmosphere new carbon from a fossil fuel that has been locked away, underground, for millions of years.

And the airlines seem to have little choice. For example, airlines, unlike automakers, cannot turn to other options like electrification, said Ms. Hammel of the Natural Resources Defense Council, which is why it is important, she added, that the fuels be sustainably produced. But despite the airlines’ interest, there are still substantial hurdles to the large-scale development of biofuels — most notably reasonable cost and reliable supplies.