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Miles Per Gallon of Biofuel

To the Editors:

I read with interest “Making Biofuel from Microalgae,” by Philip Pienkos and his colleagues (November–December). The authors leave out one important thing: a comparison of the energy densities of oil from petroleum and oil from microalgae—that is, the energy in a gallon of each. For instance, what kind of mileage would you get from each fuel, assuming the same car and the same driving conditions? If microalgae fuel, like ethanol, is less efficient than gasoline, it might be impractical for cars or planes, regardless of the cost.

Ted Grinthal
Berkeley Heights, NJ

Dr. Pienkos responds:

Algal biofuels are much better than ethanol, and some have greater energy density than their petroleum-based counterparts. The interest in algae is primarily due to their ability to accumulate triacylglycerols (TAGs) much like vegetable oils. Transesterification of the fatty acids in TAGs yields fatty acid methyl esters (FAMEs), whose long chain lengths make them a replacement for diesel, not gasoline. The energy density of petroleum diesel is about 130,000 British thermal units (BTU) per gallon and that of biodiesel is about 118,000 BTU per gallon. Both values are much greater than that of ethanol, which is about 76,000 BTU per gallon. Biodiesel has lower energy density than petroleum diesel because it contains oxygen, which contributes to the molecular weight but not the heating value. But TAGs can also be converted to alkanes by hydrotreating. The resulting “green diesel” actually has a heating value about 2 percent greater than that of petroleum diesel.

That doesn’t answer the question about gasoline, because algal biofuels are more closely aligned with diesel. But hydrotreating can be coupled with cracking and isomerization to produce a gasoline-like stream. I don’t have data on this material, but it would be roughly equivalent to petroleum gasoline.

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