ing distribution system and can substitute for gasoline directly. Its main drawback to date is the high cost of production. To reduce that cost and help initiate market entry, DuPont and BP have joined forces to retrofit an existing bioethanol plant to produce biobutanol using DuPont-modified biotechnology (Chase, 2006). Moreover, an improved next-generation bioengineered organism is projected to be available within the next few years.

Hydrocarbon Fuels from Biomass

The growing biofuel industry is based on well-established technology for producing ethanol via fermentation and distillation. This technology is energy-intensive, however, with approximately 60 percent of the product’s fuel value consumed in these two processing steps (Katzen et al., 1981; Shapouri et al., 2002). In addition, fuel ethanol is expensive to distribute, as it cannot be added to gasoline prior to pipeline transport. At an estimated 13–18¢/gal, the cost of ethanol-fuel transportation is as much as six times that of transporting traditional petroleum-based fuels (GAO, 2007). Therefore approaches to developing hydrocarbon fuels produced directly from biomass, and that are analogous to fuels produced from petroleum, are being explored (Huber et al., 2006). Other proposed approaches include a hybrid hydrogen-carbon process for producing liquid hydrocarbons (Agrawal et al., 2007) and a catalytic strategy to produce dimethyl furan from carbohydrates (Román-Leshkov et al., 2007).

Gasoline Blend Stock

One approach produces straight-chain hydrocarbons, mostly hexane, via aqueous-phase hydrogenation of biomass-derived sugars followed by dehydration. The combination of reactions is exothermic and in theory could consume no net hydrogen. Because the reactants are dissolved in water, the hydrocarbons produced form a separate phase, and distillation is not required. This process, compared with the fermentation and distillation steps used in ethanol production, has the potential for higher energy efficiency and shorter residence times, but considerable development is required to confirm that this potential can be realized in a commercially viable process (Huber et al., 2005).

The product, consisting of linear hydrocarbons, can be isomerized in a conventional refining process to form branched hydrocarbons with higher octane, which are therefore more suitable for gasoline blending. Also, conventional refinery alkylation technology can be used to process the low-boiling straight-chain



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