by Douglas Roger Sandy Dympnus Christopher Messer

Created on: February 18, 2009   Last Updated: March 04, 2009

What are the actual numbers comparing ethanol versus conventional gasoline as fuel and as pollutants? Some time back I came upon a letter on a bulletin board asking how it was possible for a gallon of gasoline weighing around seven pounds to generate almost thirty pounds of carbon dioxide. With a degree in chemistry, I knew the answer, but refrained from supplying a response because I was more interested in what some of the other posters had to say about it. I could have gone ahead and wrote something to the puzzled poster, because no one attempted to answer his question. But it made me realize that nowhere had I seen the simple but revealing calculations that compared and contrasted the pollution potential and efficiency as fuels of conventional gasoline versus ethanol.

Gasoline is a complex blend of aliphatic (straight- and branched-chain) hydrocarbons, cycloalkanes (ring compounds in which each carbon atom has four single-bonded carbon and/or hydrogen atoms attached to each carbon atom), and aromatic (compounds based the benzene ring) hydrocarbon compounds, and may contain five to twelve carbon atoms per molecule, depending on the type of hydrocarbon. About five hundred different compounds can be used in gasoline blends, the choice of which can and does change seasonally, geographically, and in order to tailor the octane rating. To further complicate the picture, there is now a special class of components known as oxygenates that can be added to reduce the amounts of carbon monoxide produced by gasoline engines. In the United States, as a practical matter the two chemicals that have been used for this purpose are ethanol and the now-discredited MTBE; that is, methyltert-butyl ether.

With this in mind, it is evident that some simplification is needed for an explanation that is even possibly comprehensible to the average person. I propose to effect this simplification by considering the carbon dioxide generational potential of three compounds found in gasolineisooctane, benzene, and ethanoland comparing the three results. The balanced equations for complete combustion look like this:

Isooctane: 2C8H18 + 25O2 = 16CO2 + 18H2O Benzene: 2C6H6 + 15O2 = 12CO2 + 6H2O Ethanol: CH3CH2OH + 3O2 = 2CO2 + 3H2O

We can obtain a rough estimate of the amount of weight hydrocarbons, and specifically the carbon atoms they contain, gain upon being oxidized to water and carbon dioxide. What is important to notice here is that each carbon atom, which in the unburned material is (except

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