Bioethanol has already come under widespread use, especially in the United States and in Brazil. But ethanol, as many realize, is a two-carbon alcohol that is not entirely different from the corresponding one-carbon alcohol methanol, three-carbon propanol, and four-carbon butanol. Butanol, in particular, has shown promise as a potential fuel as well.

            Ethanol                          Butanol            Clostridium acetobutylicum, the "Weizmann Organism"

Biobutanol, like bioethanol, can be made from biomass, and the processing for the two different fuels diverges only at the fermentation level. Fermentation of sugars into butanol requires a specific strain of bacteria known as Clostridium acetobutylicum, sometimes called the Weizmann organism, named after chemist Chaim Weizmann. It was Weizmann who originally used this bacterium to ferment starch into acetone but, unexpectedly, Weizmann made twice the amount of an unwanted side product—butanol. Realizing that butanol could serve as a source of renewable energy, scientists have investigated its potential over the last few decades. While the fermentation process differs from that of ethanol, the distillation process—to purify the fuel—is similar, but not exactly the same due to differences in the boiling points of the two.

Why use biobutanol instead of bioethanol?

Butanol has a number of advantages over ethanol, especially in terms of transport. As a fuel mixture, butanol is more easily transported with gasoline and diesel through pipelines because of its lower tendency to separate from the fuel when contaminated with water. Because butanol has a longer hydrophobic, hydrocarbon chain than ethanol, the entire molecule is less hydrophilic, and therefore less prone to mix with water when it shouldn’t. Butanol is also less corrosive than ethanol, another reason its transport through pipes is preferable.

Resembling gasoline more closely than does ethanol, butanol has actually been demonstrated to power automobiles whose internal combustion engines are made for gasoline. More practical, however, is butanol’s potential use as a fuel mixture like ethanol. Butanol’s closer resemblance in air-fuel ratio to gasoline enables the mix to include a greater percentage of butanol than ethanol. Thus, a vehicle whose engine could tolerate 5% ethanol would be able to run on 8% butanol. This 60% increase in potential fuel replacement percentage would enable more gasoline to be saved. Additionally, butanol has a higher energy density than ethanol, which could make it more cost-effective with the proper infrastructure.

Fuel————————Energy Density (MJ/l)————————Air-Fuel Ratio

Gasoline——————————32.0———————————————-14.6

Butanol——————————-29.2———————————————-11.2

Ethanol——————————-19.6————————————————9.0

Methanol—————————-16.0————————————————6.5

Why not use biobutanol instead of bioethanol?

Butanol’s high viscosity—twice that of ethanol’s and five to ten times that of gasoline’s—could cause problems in the fuel systems either of butanol-fueled cars or those running on a butanol-gasoline mixture. This problem with viscosity is a daunting one, because viscosity at a given temperature and pressure is a characteristic property of a substance and therefore can not be changed.

In the 1970s and 1980s, when scientists began researching the possibilities of alternative fuels, bacteria were used in the fermentation process. This ABE (acetone, butanol, ethanol) fermentation process yields butanol, acetone, and ethanol in a ratio of 6:3:1, respectively. For each bushel of corn, then, the yield would be 1.3 gallons of butanol, 0.65 gallons of acetone, and 0.22 gallons of ethanol, each at a concentration of 1-2%. This ratio bodes well for butanol, but, with the use of yeast in the fermentation process rather than bacteria, ethanol has emerged as the most viable fuel. For the same bushel of corn, yeast fermentation yields 2.5 gallons of ethanol with a concentration of 10-15%. It was this great difference that likely established ethanol as the premier alcoholic fuel source.

What is in the future for biobutanol?

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Butanol has received attention lately due to its benefits described above. Especially as fuel transport plays an increasingly important role, butanol’s ability to resist fuel separation and water contamination better than ethanol could make it a more advantageous choice. Fortunately, most of the existing ethanol infrastructure (which is far more extensive than that of butanol), can be readily converted to service the butanol industry if the need arises.

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Recently, the companies DuPont and BP announced their collaboration in a “partnership to develop, produce and market a next generation of biofuels to help meet increasing global demand for renewable transport fuels.” According to the official announcement, the two companies “expected to make commercial volumes of biobutanol for market by the end of 2007.” In a second phase, they “anticipate using a higher conversion technology that will allow broader commercialization.” DuPont and BP cite butanol’s advantages mentioned above as their reason for choosing butanol over ethanol, and it is conceivable that they see long-term promise in this alternative alternative fuel source.

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Insight from the Resident Expert

Fred Dryer is a professor of Mechanical and Aerospace Engineering at Princeton University who is interested in the chemical kinetics of both pure and gasoline-blended biobutanol. Professor Dryer’s work includes designing concepts for the Homogenous Charge Compression Ignition (HCCI) engine, a new development geared toward improving valve timing and fuel addition, two processes that strongly affect engine emissions and energy conversion efficiencies. Although his current research focuses on gasoline modeling and the oxidation processes of ethanol, Professor Dryer looks to research butanol, biodiesel, and bio jet fuel systems along similar lines.

The subject of much of this page has been to compare and contrast ethanol and butanol as alternative energy sources to fossil fuels. Professor Dryer was able to comment on the relative feasibilities of the two, but noted the still very inconclusive nature of current information and the variable nature of the comparison:

“This issue depends as much on the ‘use factors’ as on the ‘production factors.’ That is, from the ‘source’ to the ‘end use’ energy efficiencies and emissions, how do the two alternatives compare? The problem is under study at this level, and while the positives and negatives exist on both sides, butanol has advantages that are not minor. Moreover, the two may be produced as co-products in some technologies.”

At a glance, the two fuels seem quite comparable, and perhaps butanol’s advantages even outweigh those of ethanol. Yet, ethanol fuel is used as a gasoline additive around the world—espeically in the U.S. and Brazil—while butanol is not. The original move to ethanol as a result of yeast fermentation was discussed above, but why hasn’t the butanol industry undergone similar expansion yet? Professor Dryer provides a reminder that scientific factors are often not the only ones at work:

“There is a lot of investment needed, and government regulations in Europe and the U.S. define ‘alternative fuels,’ by the levels with which they can be added to motor fuels. So the market place is affected both technically and politically.”

Many thanks to Professor Fred Dryer for his biobutanol wisdom.

Navigating the Bioethanol Page:


Background:……………Chemistry………………………..Sources………………..Processing


Major Issues:………….Cellulosic Ethanol……………Biobutanol……………Feasibility Disputes

Sources

BP DuPont Biofuels. www2.dupont.com/Biofuels/en_US/index.

Dolan, Kerry A. “A Competitor For Ethanol?.” Forbes Magazine, 20 June 2006.

Dryer, Frederick L. Homepage. Last updated 31 December 1997. www.princeton.edu/~fldryer.

Environmental Energy, Inc. www.butanol.com.

Green Car Congress. “Boosting Biomass-to…Butanol?” 20 July 2005. www.greencarcongress.com.

Prather, Robert. “Biobutanol — Another Biofuel Breakthrough.” Outside the Beltway. 24 June 2006.

Siemerink, Marco. “Revival of butanol production by Clostridia.” Wageningen University, Source. 9 December 2005. www.ftns.wau.nl/micr/bacgen/marco.

Thomas, Justin. “Biobutanol: A Superior, Renewable Substitute for Gasoline.” 20 June 2006. www.treehugger.com.

Wikipedia, the free encyclopedia. “Butanol Fuel.” Last modified 8 January 2007.