From Photosynthesis to Fuel: Cellulose holds great potential as source of biofuel energy
by Elizabeth Storey
The push for alternative energy sources worldwide is leading to more advanced research in biofuels. Searching for new materials from which to produce such fuels is keeping researchers at Oak Ridge National Laboratory (ORNL) and the University of Tennessee (UT) busy.
A team led by Jeremy Smith, director of the ORNL Center for Molecular Biophysics and the UT-ORNL Governor’s Chair, will use UT’s Kraken supercomputer to run simulations that will help reveal the detailed workings of cellulose, a potential biofuel material.
Cellulose is a complex carbohydrate that forms the cell walls of plants and gives leaves, stalks, stems, and trunks their rigidity. Figuring out how to unlock its sugar subunits, which can be fermented to produce ethanol, is a major challenge of biofuel engineering. Tackling that challenge could enable full use of plants for cellulosic ethanol.
“The simulations we are performing are designed to provide a picture of biomass that will help experimentalists design plants with new, less resistant cell walls and enzymes that break the cellulose down more efficiently,” Smith said. “This is basic research designed to help underpin the current major, worldwide effort in renewable energy research.”
Biofuels past and present
As the public becomes more concerned about the environmental effects of global warming and air pollution, researchers are investigating alternative energy sources. Motor vehicles are a major source of greenhouse gas emissions, and cleaner fuels could dramatically lower the amount of carbon dioxide, methane, and pollutants released into the atmosphere.
Ethanol biofuel, one of the most viable alternatives to fossil fuels, has been blended with gasoline since 2002. Net emissions of ethanol combustion are lower than those from fossil fuels because corn planted to make ethanol converts some of the carbon dioxide back into glucose and oxygen through photosynthesis.
Corn is currently the major source of ethanol in the United States, but it is also a food staple for both people and animals. A significant chunk of American farmland is used to grow corn. If most of the corn was taken out of the fields and put into gas tanks, the price of corn in the grocery store could skyrocket. Even then, corn ethanol would not come close to meeting the nation’s energy needs. Scientists are on the hunt for nonfood materials, such as plants and even trees, from which to produce ethanol.
Plant cell walls hold great promise for biofuels. Yet, little is known about how cellulose interacts with other components of plant cells—hemicellulose, lignin, and pectin—so progress toward using plant biomass to create ethanol has been slow. Smith and his team are working at the National Institute for Computational Sciences, which manages the world’s fastest academic supercomputer. With a peak speed of more than 600 teraflops, or 600 trillion calculations per second, Kraken, a Cray XT system, can simulate the interactions between cellulose and its associated components in extremely fine detail, enabling the team to learn more about lignocellulosic biomass at its fundamental level and better understand its larger-scale characteristics
Fuel of the future
Cellulose strongly resists being broken down into glucose. Researchers call this natural resistance to decomposition “biomass recalcitrance.” Hydrolysis, a reaction that uses water to break down chemical bonds, must take place for cellulose to become glucose. In lignocellulosic biomass, noncellulosic components slow the hydrolysis of cellulose and must be removed before hydrolysis can effectively occur.
In typical ethanol production, scientists first pretreat the biomass to break it down into its individual components by mechanically chipping and grinding the plant material into dust-sized particles. They then dissolve the noncellulose components away from the cellulose by subjecting them to changes in acidity and temperature. These methods are very expensive and incomplete, and they require large amounts of energy.
With the computer time on Kraken allocated to them by the National Science Foundation, Smith and his team will use a molecular dynamics simulation method to calculate how the parts of lignocellulosic biomass are arranged and how strongly they are bonded to one another at the atomic level. With a fast, massively parallel code called LAMMPS, the researchers will use unprecedented computer power to visualize this complex biological system as the cellulose breaks down.
Also important in biomass recalcitrance are natural enzyme complexes found in cellulose that act as catalysts to break it down. Smith and his team will simulate models of the enzymes and observe how they react with each other and with cellulose. Little is known about how the enzymes interact with biomass, and this is an important aspect of understanding lignocellulosic resistance to hydrolysis.
Smith plans to create models of lignocellulosic biomass and cellulose that will show the structure, motion, and mechanics of the materials on a level never seen before. The simulations will show the team what needs to happen to get trees and weeds into the gas tank. This work will lead to a greater understanding of cellulose composition and breakdown and serve as a reference for future research in biomass fuels. And if a process can be understood, it can be engineered.
“We hope to design plants that are less resistant and microbes that can overcome recalcitrance,” Smith said. “Putting this designed plant into fields would mean cheaper ethanol production, getting more per acre. It’s a big effort, worldwide, to understand ethanol processes, and we will likely see these efforts take effect in the next five years or so.” —by Elizabeth Storey