[cesarb]

Playing devil's advocate, the parent comment didn't mention "synfuels" at all, it mentioned "renewable fuel", which includes things like sugarcane alcohol, which is very common here in Brazil as a car fuel; many cars come out of the factory with flex fuel engines which can run on either gasoline or alcohol, and every fueling station I've ever seen always has at least one pump for alcohol.

[adrianN]

That is of course a good point. You can make small amounts of those from agricultural waste. If demand outstrips supply from waste though, the area you need to farm to drive a couple of miles is really large. I didn't do the math, but I wouldn't be surprised if you got more energy out of agricultural waste if you turn it into electricity+heat in a biogas setup.