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Better Biofuels

ORNL researchers are pooling expertise to unlock green energy

Computer simulations show how and    why lignin interacts with cellulose    and enzymes. Image: Michael    Matheson

Computer simulations show how and why lignin interacts with cellulose and enzymes. Image: Michael Matheson

Using some of the world's most powerful machines, ORNL scientists are tackling major bioenergy problems by outsmarting nature. The Energy Independence and Security Act of 2007 calls for the production of 36 billion gallons of renewable fuel by 2022. It's a lofty goal because neither the chemical nor biological process that converts biomass into biofuel is ready to be scaled to that magnitude.

"Biomass has a huge potential to be used for energy, but industrially we need better technology to mass-produce biofuels cost-competitively," says Loukas Petridis, a researcher working in the lab's Biosciences Division. "We know that we can produce ethanol from materials like grass, wood chips and even newspapers, so the question now is: How can we improve the efficiency of the biofuel production process?"

Biological blockage

One of the biggest roadblocks preventing the economically viable production of biofuels is lignin, a glue-like substance found just beneath a plant's surface that serves as a plant cell's first defense against man and beast.

Lignin intertwines with sugars called cellulose and hemicellulose in plant cell walls. Its hardiness protects a plant's inner structures from microbes and fungi, but lignin has been a major frustration for bioenergy researchers.

During biofuel production—a process that converts plant mass into alcohol— lignin blocks enzymes from breaking down cellulose into the sugars necessary for fermentation.

"Our goal is to understand, on a molecular level, exactly why biomass is so resistant, or recalcitrant, to breakdown," Petridis says. "If we can understand this, then we can suggest ways to improve biofuel production."

To solve the problem, scientists are arming themselves with neutrons and supercomputers.

Petridis is collaborating with ORNL's BioEnergy Science Center, a multidisciplinary research group with experts in math, computer science, physics, chemistry and biology who are working together to improve the biofuel conversion process.

Using simulations from ORNL's Titan supercomputer in conjunction with neutron scattering techniques at the lab's High Flux Isotope Reactor, scientists are studying how individual plant molecules change their shape and structure during biomass pretreatment. Pretreatment is an expensive process that opens cell walls and, in theory, allows enzymes to more easily break down cellulose. But these processes still aren't able to fully degrade the biomass in a timely manner.

"We wanted to understand why biomass remained recalcitrant to enzymatic degradation even after pretreatment," Petridis says. "Our group was one of the first groups to explain this observation on a molecular level."

Simulations and experiments allow researchers to resolve the structure of lignin aggregates down to 1 angstrom, which is about 1 million times smaller than what the naked eye can see. The models reveal how different temperatures can change lignin's structure, causing it to either aggregate or expand. The lignin clumps cause problems in biofuel production because they stick to the enzymes that release sugars from cellulosic biomass.

"Looking at simulations and experiments allows us to form a more holistic picture of this process," says computational biophysicist Jeremy Smith, director of ORNL's Center for Molecular Biophysics and a governor's chair at the University of Tennessee. "Researchers previously believed that lignin only clumped during the cool-down phase, but our models and experiments show us that lignin forms problematic clumps even at the relatively hot temperatures used during pretreatment."

The models also show how and why lignin interacts with cellulose and enzymes. The simulations reveal detailed multiscale structures that help people understand biomass recalcitrance and assist engineers who are trying to improve secondgeneration biofuel yield.

Other ORNL investigators involved in these computer simulations are Roland Schulz, Benjamin Lindner, Xianghong Qi and John Eblen.

Atomic details

As supercomputing power increases over the next 5 to 10 years, researchers will be able to simulate more than just the interaction between lignin and cellulose. They hope to simulate large parts of a living plant cell at atomic detail, including the enzymes and microbes that eat the biomass.

In the meantime ORNL's team has been awarded 78 million hours on Titan through the Innovative and Novel Computational Impact on Theory and Experiment, or INCITE, program. Researchers plan to use these hours to study how lignin behaves in different types of biomass, which will help them identify the plant characteristics best suited for biofuel production.

"The more we learn about biomass, the easier it will be to improve pretreatment and the biofuel production process," Petridis says. "We hope that our studies will help engineers design new pretreatment methods and engineer different types of biomass and enzymes that can harvest more energy from plant materials. In this way we can help the United States begin running on renewables." — Jennifer Brouner