Without much in the way of fanfare, United Airlines began flying regularly scheduled flights using biofuels earlier this month. Following years of demonstration and test flights to prove the fuel's viability, it's the first instance of a US airline putting commercial-scale volumes of biofuel into passenger-carrying planes on an ongoing basis.

On March 11, the bio-fueled United flight 708 flew from Los Angeles International Airport (LAX) to San Francisco's SFO. It comes after United signed deal with LA-based refinery, AltaAir Paramount, to supply 15 million gallons (56.7 million liters) of biofuel to the airline at LAX over a three-year period, with the option to purchase more.

AltaAir converts sustainable feedstocks, including agricultural waste and non-edible natural oils, into drop-in jet fuel. The United planes will run a 30/70 mix of biofuel and regular petroleum-derived jet fuel. United says its biofuel mix will power the LAX to SFO flights for two weeks, while the fuel is integrated into its regular daily operations at the airport. The idea is to fuel the 12,500 flights between LA and San Francisco over the three years of the contract, or roughly 10 flights per day.

The cost of the biofuel is competitive with traditional jet fuel, though comes with the benefit of a 50 percent reduction in CO2 emissions on a life cycle basis compared to jet fuel. And while the biofuel accounts for just two percent of all the fuel used by United at LAX, it could signal the start of wider industry adoption of renewable-sourced fuels by the industry.

Airlines have been researching and experimenting with alternative fuels for some time. A project led by the University of British Columbia is studying the conversion of forestry waste into fuel for planes, while other airlines, including Air Canada, Virgin Atlantic, Continental, and KLM have all taken to the air in biofuel-powered test flights.

In 2015, United invested $30 million in Fulcrum BioEnergy, a California-based alternative fuels developer that converts municipal solid waste into cheap aviation biofuel.

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