Posted by Christopher Sanderson
Quick overview of fuels:
The main fuels worth considering in this context are ethanol, butanol, bio-diesel, which we can consider producing as bio-fuels, and for benchmark reference: petro-diesel and petroleum.
The first consideration to make is the energy density by volume of each fuel:
Ethanol has around 21.2MJ/L found http://en.wikipedia.org/wiki/Ethanol#As_a_fuel and cited http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/storage.pdf although the value appears to vary by around 2-3MJ/L depending on source.
Butanol has around 29.2MJ/L found http://petrofed.winwinhosting.net/upload/4_Verma.pdf, note that this source cites ethanol as 19.6MJ/L.
Bio-Diesel has around 37.27MJ/L found http://en.wikipedia.org/wiki/Biodiesel and cited from http://webarchive.nationalarchives.gov.uk/+/http://www.berr.gov.uk/files/file14925.pdf
When these are considered with respect to the current mineral derived fuels:
Petroleum with 34.8MJ/L found http://en.wikipedia.org/wiki/Gasoline
Diesel with 38.6MJ/L found at the same. Although it's wiki, the values seem reasonably consistant with the other values I've seen.
From this perspective it would appear that as practical replacements for fossil fuels the best candidates are butanol to replace petroleum and bio-diesel to replace petro-diesel.
Also of interest:
BP's perspective - http://www.technologyreview.com/energy/18443/
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