Conventional biodiesel is produced by the processing of the fatty acids in vegetable oils which are produced from an agricultural crop.
Researchers at the University of Wisconsin, however, have been working on a method to create a diesel fuel directly from the carbohydrates that constitute about 70%–75% of a plant’s dried weight—in other words, using more of a plant’s biomass as the feedstock rather than just the processed oil from its seeds.
The processing of biomass to produce a fuel usually takes one of three primary different routes:
Fermentation of glucose to ethanol
Pyrolysis or high-pressure liquefaction to convert biomass to bio-oil, which is then further processed
Gasification of biomass to create a syngas for use in a Fischer-Tropsch process
As reported in the current issue of Science, James Dumesic and his colleagues developed a process to convert plant carbohydrates directly to the long-chain hydrocarbon alkanes that constitute diesel fuel.
The process—Aqueous Phase Dehydration/Hydrogenation (APD/H)—combines hydrogen and alkane production.
Aqueous phase reforming of the carbohydrate feedstock creates hydrogen gas
Dehydration/Hydrogenation (using the hydrogen gas) produces the alkanes.
According to the researchers, the resulting alkanes contain 95% of the original heating value of the reactant.
So far the team has managed to produce hexane (C6H14)—a lighter alkane. One of the areas of concentration of the work now is to identify catalysts and reactor designs that can produce the heavier C8–C15 alkanes from biomass reactants.
The other major challenge, from a commercialization point of view, is how to extract the carbohydrates from the plant matter, and how “dirty” the stream can be. In the research, the team used a pure carbohydrate supply.
Production of Liquid Alkanes by Aqueous-Phase Processing of Biomass-Derived Carbohydrates. Science,Vol 308, Issue 5727, 1446-1450, 3 June 2005 (subscription required)
Making Fuels from Biomass. Science, Vol 308, Issue 5727, 1421-1422, 3 June 2005 (subscription required)