Fuels and Carbon


The relative amounts of carbon released when certain fuels are burned may be a little surprising, considering all the agitation on this subject recently, which is not characterized by quantitative evaluations, but rather by enthusiasm and emotion. Let's consider here the fuels coal (assumed pure carbon, C), methane, CH4, which is 75% carbon, ethanol (EtOH), C2H5OH, 52% carbon, and octane, C8H18, 84% carbon, which represents gasoline. Coal may be taken to give 13,000 Btu per pound, methane 912 Btu per cuft, EtOH 327.6 kcal per gram-mole, and octane 1302.7 kcal per gram-mole. These are representative tabular values, which must be converted to a common basis.

These fuels combine with atmospheric oxygen to produce carbon dioxide, CO2, and water, H2O. Actually, combustion is a little more complicated in actuality, and some carbon monoxide, CO, may be produced, as well as some nitrogen compounds from the atmospheric nitrogen. However, we only want the total carbon produced, so these details are unimportant. We may consider that the carbon released in the production and transportation of these fuels is roughly the same, except in the case of ethanol produced by fermentation. The reaction in fermentation is C6H12O6 → 2C2H5OH + 2CO2. Therefore, the total amount of carbon released when ethanol is burned is 1.5 times that appearing in the tailpipe emissions.

Let us now find the amount of carbon released to produce 1 kcal of energy (1 Btu = 0.252 kcal) for each of the fuels. The results are: coal, 0.138 g/kcal; methane, 0.0818 g/kcal; ethanol, 0.110 g/kcal; octane 0.0737 g/kcal. The surprising fact is that octane (gasoline) is the best of the four fuels, emitting roughly half the carbon than an equivalent amount of coal. Ethanol, very heavily promoted now, is not far from methane in carbon production, and produces 50% more carbon than gasoline, which it is currently proposed to replace!

The carbon associated with ethanol comes immediately from atmospheric carbon dioxide by photosynthesis, so as far as this is concerned, ethanol is carbon neutral. However, additional carbon may be released in association with the growing and processing of the corn, and the distillation and drying of the products of fermentation.

References

N. A. Lange, Handbook of Chemistry, 10th ed. (New York: McGraw-Hill, 1961). p. 817 (coal), p. 820 (methane), p. 1569 (EtOH and octane).

R. A. Young and T. J. Glover, Measure for Measure (Littleton, CO: Blue Willow, Inc., 1996). Conversion factors.


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Composed by J. B. Calvert
Created 21 April 2007
Last revised 24 April 2007