Bio-fuel is a very hard problem regardless of what you grow ... because compared to soil and fresh water, any other medium to grow anything on is orders of magnitude worse.
Bioreactors have massive material requirements. Open pools in the desert need to get lots of fresh water for it from somewhere (can't use salt water, because eventually you'd have a 100% saturated brine and even the hardiest organisms won't grow in that). Open pools on land where there is plenty of fresh water, might as well skip the pool and use the land. Ocean have lousy nutrient density and no way to contain any fertilizer.
If you have a 250 micrometer flexible solar cell which costs next to nothing you can throw it in the desert and have some robot occasionally brush the sand off it ... with biofuel it doesn't work that way.
PS. crop waste is nice and all, but it would provide only a tiny contribution ... let the market figure out if it can make that profitable, not worth spending public money on.