with regard to the energy yield for ethanol -- you're right that ethanol from corn starch (most of current US ethanol) isnt a reasonable long term proposition. The main hope for ethanol is to generate it from cellulosic biomass--all the stalks and husks and other agricultural waste (including...
an addendum to my previous post: it was pointed out to me that hydrocarbon fuels are accessible from biomass using gasification to a syn gas-like product (ie mostly H2 and CO and some other gaseous hydrocarbons). This "bio syn gas" can then be polymerized using a Fischer Tropsch process to give...
Greg -
biofeedstocks are quite different chemically from the hydrocarbon components of petroleum (for more detail, see below). The short answer is that if you're burning it for energy, the exact chemical structure isnt all that relevant as long as other properties (boiling point, flashpoint...