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Alternative fuels aren't cheap, and there aren't many providers.

So the subsidiaries of United Continental Holdings are still powering their jetliners with conventional fuels, two years after first successfully testing biofuels on a flight.

For now, no provider has an ample supply of biofuel at the right price, said Jimmy Samartzis, the company's managing director of global environmental affairs and sustainability.

"We would be using it. It's just not there yet," he said.