The energy cost to biomass --> ethanol is not in the distillation, it's in the enzymatic digestion of the biomass into fermentable sugars. Current processes require large amounts of enzymes made in a separate process. According to a recent talk at the annual meeting of the American Chemical Society in Boston, the smart money is in pairing the enzymatic digestion with microorganisms that exoress the enzymes. This compresses two steps into one ("process intensification") and as a side benefit, the enzymes and microbes work better together than individually.