The biggest benefits butanol offers as an alternative to ethanol in motor fuel derive directly from its chemical and physical properties. A little reference work reveals some useful comparisons:
- The vapor pressure of butanol is well below that of ethanol at room temperature. That means a gasoline-butanol blend would not suffer from the volatility concerns that plague gasoline-ethanol blends, thus reducing evaporative emissions from vehicles using it.
- Butanol's boiling point (between 181-243 deg. F, depending on which form of the compound you're looking at) is above ethanol's 173 deg. That puts it closer to or over the desired T50 (distillation curve midpoint) for reformulated gasoline. That could result in minor blending challenges for some refiners.
- The octane of all the butanol isomers is a good deal lower than that of ethanol, and the octane of n-butanol, which would presumably predominate in fermentation, is the lowest, falling right at the 87 R+M/2 of regular unleaded gasoline. Compared to ethanol's 110 or so, there's little or no octane benefit from adding butanol to a gasoline blend.
- As cited by DuPont, the combustion of butanol yields much more energy than ethanol, about 110,000 BTU/gallon, compared to 84,000. That puts butanol almost at a par with gasoline's 120,000 or so BTUs, so gasoline with a high butanol content wouldn't suffer the mileage loss experienced in ethanol-gasoline blends.
- Importantly, butanol is much less soluble in water than ethanol. That means it is much less prone to the kind of separation problems that have kept ethanol out of petroleum product pipelines. It also suggests it should be easier to separate from water after fermentation than ethanol, requiring less energy.
- A review of the Material Safety Data Sheet for butanol indicates it's in pretty much the same toxicity league with gasoline, and a good deal safer than methanol.
Overall, butanol--bio- or otherwise--compares well with ethanol and would eliminate many of the logistical constraints that add to the cost and complexity of using ethanol as a gasoline additive or extender. I don't see anything in the above to make me question BP's assertion that it can be used at higher concentrations than ethanol in unmodified cars. Even more intriguing is the possibility that butanol could be used as a diesel extender, too.
The big open questions about butanol hinge on the ease and economics of its manufacture in current ethanol facilities, using similar feedstocks. The BP/DuPont partnership also indicates that the detailed well-to-wheels comparison of energy and greenhouse gas efficiency versus other fuels hasn't been completed. I'll reserve judgment until I see whether it beats ethanol in these areas, as well as in its physical properties. I'd also want to be sure that it's been carefully scrutinized for the sort of unintended consequences that cropped up with MTBE. That means looking closely at its toxicity pathways and co-solvent properties in gasoline blends exposed to water.
Biobutanol raises some fascinating possibilities for our alternate fuels strategy. Its properties remind us that ethanol, which many regard as our most available and practical gasoline substitute, falls well short of a being an ideal motor fuel, particularly for use in a distribution system dominated by petroleum products. Whether or not butanol is the best alternative, it highlights the option of using traditional and biotech-based processes to produce fuels that are optimal for use in modern internal combustion engines, as well as highly compatible with conventional fuel distribution channels. An alternate fuel chosen for these properties would be immediately usable by most of the cars on the road, and it could be distributed to the largest number of service stations at the lowest possible cost. If it passes muster on all these criteria, butanol should create some very interesting choices for the owners of current and planned ethanol facilities.
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