November 4, 2010 § Leave a comment
A recent article in the Economist describes an important new direction for biofuels, namely the pursuit of drop-in fuels. These are synthesized hydrocarbons and can be used directly in any proportion for engines running on gasoline, diesel or jet fuel. The last two cannot be served by ethanol.
As we have discussed in the past, ethanol has about 33% less energy than the same quantity of gasoline. This calorific penalty decreases as we move to higher alcohols. The article discusses ongoing work on the production of butanol. It has 4 carbons compared to 2 in ethanol, so it has more calories. It is very similar to gasoline in calorific content and less corrosive and water absorbent than ethanol, and so a better substitute.
Most of the story is directed to the production of alkanes from sugar. These are straight chain compounds with the formula CnH2n+2. Conventional oil derived fuels have this formula as well. The number n is about 7 to 9 for gasoline and about 12 to 16 for diesel and a bit higher for jet fuel. So, alkanes with the right number are for all practical purposes direct drop-ins for these conventional fuels.
Herein lies the attraction. Also, being tailored, often through genetic engineering, the composition will be predictably uniform. This is not the case for the input to refineries from a variety of crude oil sources. In fact oil refineries today are forced to be very picky about the mix of crude they will accept. Seed based oils also suffer from this variability.
The reliance on sugar as feed stock is of note. Today, Brazil is the only source for economical sugar for this purpose. Tariffs apply only for ethanol; at least for now. So the long term potential for this feed stock can be debated.
One company is even reported to be using sugar to grow algae for diesel. This is quite a departure from the original allure of algal diesel. It was seen as using sunlight and waste carbon dioxide, a sustainability home run of sorts. Now we see folks going to the dark side of algae, literally. These algae are grown in the dark! The photosynthetic part is transferred to the growing of sugar. So we still have the sunlight and carbon dioxide (from the air in this case) put to use.
An interesting twist is the use of existing ethanol plants by some of these companies. This is a good trend, to deploy assets created by a flawed national policy and subsequently idled by realities.
So, what of corn and cellulose? Both are challenged by the fact that the chemical structure renders them more difficult to convert to alkanes. Of the two, corn, while simpler to react, is the worse in part because of water usage. Cellulosic materials such as grasses offer the promise of draught resistance. Price of Brazilian sugar over the long haul will be a determinant.
An interesting avenue for biomass in general is pyrolysis such as practiced by RTI International under DOE funding. This produces a liquid akin to crude oil. Close enough to merit inclusion as a portion of the feed to a refinery. This is the pre-refinery analog to a post-refinery drop-in fuel. Requiring no modification to current practices. A chemical plug and play, as it were.
Finally, the Economist story discusses the place of electric cars in this context. They opine that while alcohol will get trampled, drop-in fuels will survive. In the next thirty years, gasoline will continue to be used to a significant degree. So, ethanol will continue to be used as a means of assuring complete burn of the fuel. This use as an oxygenate came about from the outlawing of MTBE, but is only needed at the 6% or so level. Beyond that, ethanol is a liability on many grounds and will probably fade away.
But drop-ins will hang around a lot longer. The Beyond Ethanol story will feature electric cars but drop-ins will get serious second billing.