This stuff is already recycled
Not sure what they do with it (I think it goes to animal feed, soaps, cosmetics, and other things), but most fast food places have a big container of old grease outside for some truck to haul off.
While it might as well be used for fuel as anything else, I doubt if the energy gained back by burning it would balance the energy required to produce it in the first place. At least, as with plant ethanol, it would be part of the "short term" carbon cycle (organic grease and fats), so it wouldn't contribute to overall CO2 accumulation.