Issue Date: March 24, 2008
More on Biofuels Discussion
The Point/Counterpoint article (C&EN, Dec. 17, 2007, page 12) and related letters (Jan. 28, page 6) suffer from inaccuracies that merit correction. First, cellulosic biomass is typically composed of about 40-50% cellulose, 20–30% hemicellulose, and 12-30% lignin, with limited free sugars and starches. Acids or enzymes break down cellulose to glucose and hemicellulose to arabinose, galactose, glucose, mannose, and xylose, with their relative amounts varying with plant type. Thus David Pimentel's statement that acids or enzymes dissolve "lignin to free starches and sugars" is incorrect on the mechanism and primary source of sugars.
Although native organisms ferment arabinose and xylose poorly, bacteria and yeast have been engineered to convert all sugars to ethanol with virtually the same yield as glucose, contradicting Landis W. Doner's letter about "largely nonfermentable hemicellulose." As a result, 90–100 gal of ethanol could be made from a dry ton of corn stover or poplar wood versus the 105–114 gal per dry ton of corn, negating Pimentel's statement that "twice as much cellulosic biomass is needed to produce a given amount of ethanol" as for corn.
With 5–15 dry tons per acre per year possible for poplar, switchgrass, and miscanthus, 450–1,500 gal of ethanol could be produced annually per acre, far more than the 100 gal claimed by Pimentel. Consequently, the less than 210 billion gal of ethanol needed to replace the 140 billion annual gal of gasoline in the U.S. could be met with about 200 million acres of land, far less than the 600 million acres noted by Pimentel. Converting agricultural, forestry, and municipal residues into ethanol and using fuel-efficient vehicles would make the land need quite manageable.
Burning lignin to provide all the heat and electricity needed for conversion and exporting excess power into the grid addresses Doner's other waste concern, eliminates process fossil fuels, and counters Pimentel's statement that "about 1.5 gal of oil equivalents are required to produce 1 gal of cellulosic ethanol." To another Doner point, although carbon dioxide is released in fermentation, burning lignin, and burning ethanol, plants grow by fixing carbon dioxide, and cultivating new plants to replace those harvested results in no net buildup of this greenhouse gas.
Furthermore, fixing carbon in the soil by growing perennials and displacing coal use with excess lignin power reduces greenhouse gas emissions much more, and sequestering nearly pure CO2 from fermentation via technologies being developed for coal would result in negative release of greenhouse gases without costly CO2 purification.
Finally, although A. E. Lippman correctly notes in his letter that ethanol is not a good diesel fuel, its high octane and other properties are favorable for spark ignition engines, allowing displacement of gasoline that consumes about 44% of U.S. petroleum versus 14% by diesel.
This is serious business in that transportation uses two-thirds of U.S. oil and is responsible for about a third of U.S. CO2 emissions. Sustainable options with low greenhouse gas emissions are limited to use of batteries, hydrogen fuel cells, or biofuels, and only the latter provides liquid fuels similar to those that now power virtually all transportation. I hope these points will help clarify some key biofuels attributes.
Charles E. Wyman
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