Ethanol as Fuel

An Environmental and Economic Analysis


by Martin Turon
U.C. Berkeley, Chemical Engineering
November 25, 1998


Gasoline is clearly the fuel of choice in the current transportation market and many motorists know of no viable alternative. Automobile manufacturers of the 1990s are actively researching the possibility of introducing hydrogen powered fuel cell and electric cars, but this represents a huge shift in infrastructure. One fuel that would work in the existing class of internal-combustion vehicles is, however, mysteriously overlooked. Ethanol is historically the first fuel used by the American cars and was staunchly advocated by Henry Ford. During the 1890s, alcohol powered engines were used in farm machinery, train locomotives, and automobiles in both Europe and the United States.[1] Use of ethanol as a fuel was significantly slowed by adverse taxation during the prohibition period and by the extreme low cost of oil. Oil prices however do not reflect the adverse environmental effects of its use nor do they explicate the military action required to maintain a stable supply from large foreign reserves. Ethanol appears to be the most viable and attractive motor fuel for use in the world market when political and environmental costs not reflected in the commercial price of oil are fully accounted for.

Ethanol is produced with a simple process that can be run either as a batch reactor in a confined space or as a continuous process. An entire plant can in fact fit in one’s own backyard making ethanol a very attractive fuel source for communities or even countries that wish to be self-sustainable and not reliant on foreign resources. Sugar, starch, or biomass is broken down and let to sit in a tank of yeast which digests the material and leaves ethanol as a waste product in a process call fermentation. The ethanol is then purified with a distillation step to result in a fuel that can be used directly in automobiles.

C6H12O6 à 2C2H5OH + 2CO2 + Heat, 234.5 x 103J [2]

The equation above describes the main chemical reaction for fermentation of ethanol from monosaccharide. The reaction is catalyzed by the Zymase enzyme which is produced by a yeast culture such as Saccharomyces cerevisiae. Control over both temperature and pH of the fermentation tank is important in order to maximize the driving force and product output. Conversion is most efficient at 36° C, and effectively halts at 43° C. The optimum acidity level is one which discourages bacterial contaminants and encourages ethanol production by the yeast and has been found to be about pH 4.5.

A variety of feedstock materials may be used to produce ethanol, the simplest being basic sugars that can be fed directly into the fermentation tank and digested by the yeast. It is also possible to use starch or cellulose by preparing it with a hydrolysis pretreatment to form glucose. Though both are sugar polymers, starch is easier to break down into sub-units than cellulose because of a fundamental difference in the structure of the bonds linking the chain (see figure 2.) The a -glucoside linkage of starch has an oxygen atom holding two hydrogen atoms next to each other such that they naturally repel. Whereas, in the b -glucoside linkage of cellulose the oxygen lies in between two hydrogen atoms thus shielding them from each other and forming a stronger bond.

The process required to prepare starch for conversion into ethanol is what enables farmers to make efficient use of their grain surplus by transforming it to a usable fuel. Unlike cellulose, the weak bonds of starch polysaccharides are ideally suited to the use of enzymes such as amylase for conversion into sugars. Such enzymes are produced naturally by grain sprouts to breakdown the seed body into sugar to feed blooming young plant. As a result of this, barley sprouts have historically been used by brewers to provide the amylase needed to react the grain into fermentable sugars. The process is called malting and can also be done with commercial amylase enzyme sources. The basic process involves heating a well-stirred mixture of one bushel ground grain with 30 gallons of water and one pound malted barley to 160° F. The mixture should be adjusted to pH 5.4 with agricultural lime or acetic acid. After an hour, bring the temperature to a boil and then let cool to 140 degrees. Add five more pounds of malted barley and enough water to bring the total to 40 gallons, and continue to agitate. [3]

Though preparation of cellulose for use as a feed for ethanol production has not had much commercial success, the process is well understood and proved invaluable during World War II when oil and rubber were scarce. Cellulose can be obtained from sources that are effectively free, including recycled paper pulp, corn or hemp stocks, or other garbage. The raw pulp must be milled and ground to maximize interface area and soaked in an acid bath or treated with bacterial enzymes to break it down into sugar. The grinding process tends be costly and since enzymatic processing requires the pulp to be more finely ground, acid treatment with H2SO4 is usually more cost effective. In fact, a patent for the manufacture of alcohol from garbage had been taken out as early as 1907. The garbage being heated with a dilute solution of an acid, by which means the carbohydrates are converted to dextrose; the acid is then neutralized and the liquid fermented and distilled. [4]

Distillation is a process whereby a mixture of two substances can be separated by exploiting a difference in vapor pressures at a given temperature. In the case of ethanol production, distillation is used to rid the output stream from the fermentation stage of water. The output of the fermentation stage is fed to a column which has a boiler on the bottom, a condenser on the top, and bubble plates in stages all the way up. Since ethanol has a lower boiling point than water, its vapor ascends up the column where it achieves continually higher concentrations with respect to water. The standard way to design and analyze distillation columns is to employ the McCabe-Thiele graphical method as shown in figure 6. A stream of up to 95% pure ethanol can be tapped from the top of a well designed tower. Blends with a purity above this amount are difficult if not impossible to achieve without a special dehydration step due to the fact that relative volatilities of the two substances are equal once the azeotrope is reached.

The only required modification to run a gasoline powered internal combustion engine on an ethanol/water mixture of 80% or more is to increase the size of the carburetor jets by about 50%. Performance is comparable to that of gas except possibly in the case of cold-starting. Adding a system to inject a little gas or warmed ethanol into the carburetor to aid in such conditions is a simple remedy. Mixing anything less than anhydrous ethanol with gasoline however is not advisable as the presence of water causes such fuels to separate into two-phase blends that undermine performance and cause stalling. Dehydration of ethanol for use in such gasohol blends can be achieved by mixing benzene, n-hexane, or some like substance to overcome the limiting azeotrope.

The macroscopic flow of energy for the production of oil and ethanol is diagrammed in figures 3 and 4. The hydrocarbons within the core of the earth which we extract to acquire petroleum are the product of hundreds of thousands of years of pressure that has been exerted on the biomass of ancient forests and plant life. Ethanol on the other hand derives its energy from the photosynthetic action of this season’s harvest. The difference in time scales involved between the two fuels must be noted here. The world’s oil supply is both a finite and highly liquid energy repository that represents thousands of generations of solar energy. It would seem most prudent for us to reserve this valuable resource for creating plastics and other synthetic compounds to which other sources are not as well suited rather than to burn it frivolously as part of our daily transportation needs. The diagram also shows that the world’s annual ethanol production directly represents how efficiently we are capturing the solar radiation that falls on the earth each year. The difference is somewhat like that of living off your income or cashing in on your hard-earned (and limited) savings.

Comparing the flow of mass, specifically of carbon atoms, between oil and ethanol production leads to some very important insights. It the case of oil, hydrocarbons are effectively being extracted from the earth and pumped into the atmosphere as they are burned in the form of carbon dioxide. These fumes contribute to the greenhouse effect, warm the globe, cause the ocean levels to rise, and may very well lead to long term catastrophic changes in the weather patterns of our planet. Burning ethanol also releases greenhouse gases, but the effects are neutralized by the biomass required to produce the ethanol because it absorbs CO2 as it is grown. This and other environmental factors are not at all reflected in the market price of the two fuels and exhibits the need to reevaluate how we come to choose between renewable and non-renewable fuels.

The international thirst for petroleum has wrought many worldwide disparities since oil became the popular fuel of choice during the 20th century. The low price of the commodity fuel does not seem to properly reflect the environmental or political impact of its wide use. Overindulgent debauchery in the leading consumer of oil (United States) can be juxtaposed with utter despair in the countries who rely on it as its major export (Iraq and Nigeria.) Our world has been faced with recurrent crises in many of its corners as multinational industrial giants exert their power to maintain economic control of the largest petroleum reserves. Political factors such as these as well as the environmental effects of global warming all point to the need for our society to make ethanol the fuel of choice for motorists during the 21st century.





Table of common sugar consuming bacterial contaminants



Acetobacter This aerobic bacteria competes with yeast to produce acetic acid from sugar in the early stages of the fermentation process leaving the tell-tale smell of vinegar.


Lactobacillus This bacterial contaminant consumes sugar in the presence of CO2 and in other slightly anaerobic conditions even when acidity levels are as low as pH 3.5.


Pediococcus Yeast stored for reuse has been known to occasionally carry this anaerobic bacterial contaminant that can vie away precious sugar from the fermentation process.


Zymomonas This robust anaerobic contaminant thrives in a wide pH range and leaves a foul odor of rotten fruit as it wastes sugar.



(Adapted from the Ethanol Fuels Reference Guide by the Technical Information Branch of the U.S. D.O.E.) [4]


Fig. 1 Ethanol production via acid hydrolysis and yeast fermentation [5]




Fig 2. Cellulose and Starch: Structure and Hydrolysis [6]




Fig 3. Energy pathway for Fig 4. Energy pathway for

ethanol production petroleum production



Fig. 6. McCabe-Thiele analysis of a typical ethanol-water distillation column. [7]






Table of Leading Export as Percentage of Total Exports for Selected Countries



Country Leading Export % of Total Exports

Iraq Crude petroleum 95.0

Nigeria Crude petroleum 94.6

Saudi Arabia Crude petroleum 90.0

Kuwait Crude petroleum 90.0

Angola Crude petroleum 88.7

Iran Crude petroleum 85.0

Congo Crude petroleum 82.6

Libya Crude petroleum 80.0

Gabon Crude petroleum 69.0

Sources: Financing Africa’s Recovery (New York, United Nations, 1988) [9]

and the CIA World Factbook, 1998.












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