by Brian Dodson

Created on: November 06, 2009   Last Updated: December 05, 2009

Ethanol as Motor Fuel - The Good, the Bad, and the Ugly

Over 140 billion gallons of gasoline per year are burned in automobiles and trucks each year in the United States, producing about 2 billion tons of greenhouse gases. In an attempt to reduce production of greenhouse gases, the US government has instituted massive subsidies (over $1/gallon) for production of ethanol as a partial substitute for gasoline. The

Energy Independence and Security Act of 2007 requires that the use of ethanol in gasoline increase to 36 billion gallons per year in the next 15 years. What environmental and economic consequences will flow from these regulatory actions?

Ethanol is generally considered a renewable fuel because the ultimate source of its energy is the light of the Sun, which is converted into usable energy as a plant grows. It is usually used blended with gasoline, and the mix is called Exx, where xx is the percentage of ethanol in the fuel by volume. That is, E15 is 15% ethanol and 85% gasoline by volume. The term 'gasohol' is usually used for compositions between E5 and E25. About 3% of the energy used in land transportation is now provided by ethanol.

Most motor fuel grade ethanol (MFGE) made in the United States for ethanol fuel is made by fermentation. Unfortunately, the techniques that are currently available to make MFGE from corn only convert the starches and sugars in the kernels - thereby wasting over 95% of the available energy in a corn plant (most of which is tied up in the cellulose of stalks and cobs). In the US, the crop of corn from an acre of land is about 20 tons. Of this, less than a ton can be converted into MFGE through fermentation.

Researchers are actively working to develop practical methods for fermenting plant cellulose. Cellulosic ethanol is produced by processes which either break cellulose down into simple sugars, followed by fermentation and distillation, or gasify the cellulose followed by chemical conversion into ethanol and distillation. Cellulosic ethanol has the potential to alleviate many of the energy and environmental problems associated with corn-based ethanol. However, the production techniques are still at the laboratory research level, and the probability of success, energy costs, and environmental impact are currently unknown. Accordingly, we will consider only current fermentation techniques here.

As gasoline and ethanol are different fuels, it is worthwhile to compare a few of their properties.

- Ethanol has a lower energy

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