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Sustainable Energy

Cellulosic Biofuels

Gregory Stephanopoulos explains challenges in converting biomass to biofuels.

It is now well accepted that for several reasons, corn ethanol will have a rather limited role as a renewable substitute for petroleum-derived liquid transportation fuels. The shortcomings of corn ethanol have sparked interest in the production of other types of fuels–such as higher alcohols, oils, and hydrocarbons–from renewable biomass feedstocks (see “The Price of Biofuels”). While the potential economic, environmental, and security benefits of such cellulosic biofuels are clear, many hurdles need to be cleared before they can begin to make a difference in the overall supply of liquid fuels for transportation.

There are three major challenges in the economical conversion of biomass to biofuels. The first is to optimize the yield and quality of the biomass, as well as to work out the logistics of securing, transporting, and processing the large volumes that will be required to support the operation of future biorefineries. New ways of harvesting, preprocessing, and transporting biomass will be necessary before it’s cost-effective for biorefineries to import biomass from more than 15 or 20 miles away. One scheme is to establish satellite collection and pretreatment facilities from which slurry biomass is transported by pipelines to the main biorefinery. One can envision pipelines where cellulose hydrolysis, the slow process by which cellulose is broken down into usable sugars, takes place while a slurry is transported from the satellites to the main biorefinery.

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The second challenge is to improve the way biomass is broken down, so as to yield a stream of abundant, inexpensive sugars for fermentation. This may be accomplished by modulating the plant’s content or by genetically engineering self-destruction mechanisms into it, to be initiated after harvest and at the right processing time. Another solution might be to pursue more-active and less expensive conventional cellulolytic enzymes and perhaps new physicochemical methods, such as solubilization of cellulose by ionic liquids.

The last step is to construct new pathways that convert sugars into the various target biofuels in organisms such as yeast and E. coli. Here lies the third challenge: to engineer optimal pathways. There is an important difference between stitching reactions together by importing genes from other species and constructing an optimal pathway that converts all sugars at maximum yields and efficiencies, producing biofuels at high concentrations. Making biofuels cost-competitive will require the latter, but to achieve that goal we must engineer strains of yeast, E. coli, or other organisms with high tolerance for the toxicity of both the initial biomass hydrolysate and the final biofuel product.

No single breakthrough is likely to bring us to the point of efficient biofuel production–superbugs, consolidated bioprocessing, or blooming deserts notwithstanding. Rather, it will probably take many advances on several scientific and technological fronts, underlining the importance of a systems approach. A number of promising technologies, both biological and chemical, are in development. Economics will determine the winners, no matter what kinds of plants get built in the short term. It is also important to bear in mind that specific techniques may interfere with each other to obstruct a modular approach–by, say, undermining a well-­engineered strain’s ability to work with a different feedstock hydrolysate. So far, solutions to this complex, multidimensional problem have been sought within the confines of bi­ology or chemistry, but the real answer may very well lie in a hybrid process that combines the best each field can offer.

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