Ethanol has many benefits as a biofuel, with the most important being that it is a naturally occurring product of the metabolic processes of certain bacteria and yeasts. It is therefore relatively easy to produce. Massive research efforts into "cellulosic" ethanol aim to shift the viable feedstocks for ethanol production away from food crops and towards non-food biomass. However, the use of ethanol has already hit a technological ceiling. Most of the US supply of gasoline currently contains 10% ethanol (E10). Conventional vehicles cannot safely use fuels with significantly higher concentrations of ethanol (E15) without retrofits to replace gaskets and metal components susceptible to chemical attack by ethanol. New drop-in biofuels that are chemically indistinguishable from conventional gasoline would alleviate all of these problems and allow the unfettered growth of renewable transportation fuels.
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