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So I have read that sugar (cane) is way more efficient than corn, (and that Brazil leads the way in taking initiative to actually bother with this advance in biofuels sustainability), and that, based on multiple independent sources, they produce EIGHT times the energy than what they put in to produce it.

But could the little-mentioned hemp, (via any of its relevant ethanol production methods), be even better than sugar? Or could something else (let's limit it to a small scale if necessary) actually be the most sustainable feedstock for ethanol?

Provisos:

  • Assume there is no licence to grow particular crops like industrial hemp. (In your respective country, there may, but I want to take out money from the equation and the sustainability impact of that licensing factor per se.)

  • Assume one has no legal requirement to denature the ethanol (as in many countries this is the case), or filter it with 15% minimum regular gasoline (like in the USA). This is pure ethanol, only including and requiring what is needed to make 100% pure, and efficient, automobile burning fuel.

  • Assume scalability issues are not there. Maybe this is for the purposes of small-scale, personal production, where time isn't an issue, except when comparing time per energy output between different ethanol feedstocks - so economic viability needs not matter, except where the difference affects even small-scale sustainability comparison.

  • Also assume that all feedstocks can be grown effectively in your area. (So that they can be compared on an equal playing field.) You have a patch of suitable land, and are ready to grow any of them, requiring whatever inputs they respectively need, in the most sustainable way possible.

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    it would also be worth comparing with methanol for local use. Engine conversion is harder, energy density is lower, but production is much easier. It's also more poisonous. – Móż Jul 16 '14 at 2:30
  • Are you only interested in this from an EROI point of view? Or should other negative effects that are generally associated with biofuels (e.g. deforestation, increased water usage and rising food prices) also be included in the answer? – THelper Oct 22 '14 at 9:24
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    Both. Both the EROI, and then that combining with other factors (that you mentioned) to give a final general metric, of general ecological sustainability; EROI I suppose being one of the main contributor metrics, since it has a big impact on how much a crop you have to plant in the first place. (for the same amount of energy return) – user487 Oct 22 '14 at 9:42
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There isn't one general sustainability metric that can easily be calculated or derived to compare the various options, so this question is rather difficult to answer. The choice of feedstock for producing any type of product from biomass has an influence on:

  • GHG emissions (the method and amount of energy required for harvesting and processing)
  • use of resources (amount of arable land and fresh water required, use of fertilizer)
  • social effects (does the crop compete with food, effects on local agriculture)
  • ecological effects (effect on biodiversity, water and soil quality)

First generation feedstock such as sugar cane and corn are easy to process as they have readily available sugars and little (pre)processing is needed to create ethanol. As such they have a relatively high EROI, but a problem is that they compete with food. It’s been suggested that increases in food prices in the last decade are the result of government policy that a part of the gasoline mix has to be from biomass. Use of biomass and increasing food prices has even been linked to the Arab spring revolts.

Second generation feedstock such as switchgrass and hemp, residues from food crops and biomass from waste streams are more difficult to process compared to first generation feedstock. This is because more preprocessing is needed to release the sugars from the cellulose. AFAIK there aren't any realistic EROI calculations for ethanol from second generation biomass as this is mostly in the experimental or research phase, but EROI is likely to be lower than that of first generation feedstock. The big advantage of second generation feedstock however is that it does not directly compete with food crops.

So the real question here is what do you find more important; EROI or food?

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