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Sustainable Energy

From Waste Biomass to Jet Fuel

Fuel made from waste by-products could lower greenhouse gas emissions.

A novel chemical process developed by researchers at the University of Wisconsin-Madison converts cellulose from agricultural waste into gasoline and jet fuel. It produces fuel by modifying what until now had been considered unwanted by-products (levulinic acid and formic acid) of breaking cellulose down into sugar. The work was described in this week’s issue of the journal Science.

Biofuel tap: Liquid fuel drains out of the butene oligomerization reactor, the last part of a new chemical process for making biofuels from cellulose.

The process is one of a number of new technologies that make conventional fuels such as gasoline and diesel from biomass rather than petroleum. Unlike ethanol–today’s most common type of biofuel–these new fuels can easily be used in conventional automobiles and transported with existing infrastructure. What’s more, the jet fuel it produces stores enough energy to power commercial or military airplanes.

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Up to now, however, methods to make these advanced biofuels have often involved biological processes in which microbes break down sugars derived from biomass, including cellulose. The Wisconsin method could prove more reliable than those processes because it is a chemical process that’s easier to maintain. What’s more, carbon-dioxide created during its production can be easily captured–an advantage over conventional biofuels.

To convert cellulose, a large component of biomass, into fuel, researchers first need to break it down into simpler components, such as simple sugars. Microorganisms then process those sugars to make liquid fuels. Cellulose can be broken down by treating it with acids, but these reactions are difficult to control–the sugars are often further converted into formic and levulinic acids. “Rather than fight it, we wondered if we could start with the unwanted product to make fuel,” says James Dumesic, professor of chemical and biological engineering at the University of Wisconsin-Madison.

It is “an entirely different approach to making biofuels,” says Bob Baldwin, thermochemical process manager at the National Renewable Energy Laboratory in Golden, CO, who was not involved with the work. In the Wisconsin process, the acids are combined to form gamma-valerolactone, an industrial chemical. Catalysts made of silica and alumina then help convert this to a gas called butene, which is easily converted to liquid hydrocarbon fuels, including gasoline and jet fuel.

One advantage of the Wisconsin process compared to biological routes to biofuels is that it could decrease greenhouse gas levels, says Doug Cameron, managing director and chief science advisor at Piper Jaffray. Conventional biofuels are at best carbon neutral–growing crops for biofuels takes carbon dioxide out of the atmosphere, but this is released again when the crops are grown and processed and the biofuels are manufactured and burned. The new process produces a pure and high-pressure stream of carbon dioxide, which is easy to capture and permanently store. As a result, the net carbon emissions could be negative–part of the carbon dioxide absorbed by the plants would be prevented from returning to the atmosphere.

However, economics questions remain. Baldwin says that although the process produces high yields of the desired fuels, it requires a large number of processing steps, including separating cellulose from other components of biomass, which could make it expensive. It will also need to compete with other thermochemical processes that can be adapted to work with biomass, such as those that have been used to convert coal into liquid fuels.

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