That's what popped into my mind: the old waste fryer diesel fuel. A nice "gee whiz" science reporting that the MSM eats up, but not scalable in any way for any significant application.
Also not scalable because "waste" oil is, by & large, not usually wasted. Restaurants are often paid by collectors who repurpose it for a variety of applications, including biofuel. So the idea that massive quantities of fryer oil get wasted when it could be used is a myth.