Here is the US Department of Energy definition of an alternative fuel:
Alternative Fuel Definition
The following fuels are defined as alternative fuels by the Energy Policy Act (EPAct) of 1992: pure methanol, ethanol, and other alcohols; blends of 85% or more of alcohol with gasoline; natural gas and liquid fuels domestically produced from natural gas; liquefied petroleum gas (propane); coal-derived liquid fuels; hydrogen; electricity; pure biodiesel (B100); fuels, other than alcohol, derived from biological materials; and P-Series fuels. In addition, the U.S. Department of Energy may designate other fuels as alternative fuels, provided that the fuel is substantially nonpetroleum, yields substantial energy security benefits, and offers substantial environmental benefits. For more information, see the EPAct website. (Reference 42 U.S. Code 13211)
The last I checked, none of the plant derived fuels were produced very efficiently. The above definition of 85% alcohol mixed with gasoline is deceptive because it does not take into account the other fuels used to produce the 85% alcohol part. Not only does planting, watering, weeding, and harvesting the plant require fuel, but processing it into alcohol takes even more.
I don't think plant derived fuels mixed with petrochemicals are much of a bargain as far as climate change goes, so far. Anything that uses pure plant alcohol looks better to me, but most of those are going to come from factory farms. Factory farms are not a real bargain for climate change either.
It is ironic that factory farms will have an increasingly difficult time staying in business because of climate change, since they are contributing to it so much.
It takes real work to get oil out of plants, especially in large enough amounts to do any good for fuel. It is a whole different scale from producing enough sunflower seed oil to cook with than it is to produce enough to run a truck or generator on.
Running a still to produce alcohol for fuel does not seem that efficient unless you use solar or wind power or hydroelectric power.
Converting plants into something like oil or alcohol does not seem like a good thing to use for fuel whether you mix it with oil or gas or not. It seems to take too much energy to make it for us to come out ahead by using it for fuel. It seems better to just skip all the processing and burn the plant in the first place. Instructables has some plans for running vehicles on trash or wood.
Some internet posts mentioned that during WWII people converted vehicles to run directly by burning plant materials. It didn't sound as if the WWII plant fuel vehicles ran great, but they did run. We might have to do that again soon, since we are choosing to use up the easily available oil and gas right now.
I am trying to picture city buses and garbage trucks running around with someone shoveling wood into a furnace. Maybe we won't have to picture it much longer, but will get to see it up close.