hydrocarbons to increase octane and boiling point to the extent needed for gasoline blending. Of course, if this production of hydrocarbons from biomass were widely commercialized, refining capability for isomerization and alkylation would likely have to be increased.

Diesel Fuel Components

Another approach to bio-hydrocarbon fuels being studied produces high-cetane diesel fuel material (Huber et al., 2005). In this process, sugars are first dehydrated and then hydrogenated to form cyclic oxygenated molecules that can undergo aldol condensation (self-addition) to form larger oxygenated molecules that remain soluble in water. The condensation products are then themselves hydrogenated and dehydrated to form mostly straight-chain hydrocarbons ranging from 7 to 15 carbon atoms per molecule. The final hydrogenation and dehydration reactions in this sequence are carried out in a four-phase reactor, with the phases being water with dissolved oxygenated hydrocarbon reactants, gaseous hydrogen, a solid catalyst, and hydrocarbons for reducing coke formation on the catalyst. The process can be modified to produce oxygenated compounds in the diesel-fuel boiling range that are soluble in the diesel fuel.

Status

Although the two processes described in this section have been shown to be feasible in the laboratory with pure feedstocks, much R&D remains before commercial applications can be undertaken. The concepts need to be tested using biomass-derived feedstocks with reactors that can be scaled for commercial operation. Based on work thus far, the keys to success in these processes appear to be the achievement of sufficient yield of the hydrocarbon product, development of highactivity catalysts with long-term stability, and minimization of coking reactions.

Bacteria- and Yeast-Based Direct Routes to Biofuels

With the rapid growth of synthetic biology and the enhanced ability to engineer organisms’ metabolic pathways so as to produce specific chemical products, new approaches to renewable fuel production are emerging (Savage, 2007). They include using well-established recombinant DNA techniques to insert existing genes into microorganisms to make specific fuel precursors or even to directly synthesize hydrocarbon fuel components. Another approach involves redesigning



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