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Feature   | Spring 2010

No Carbon Left Behind

Engineering plants for efficient biofuels production

The production method to create cellulosic ethanol has a problem that’s no secret among biofuel scientists: It’s inefficient.

Standing in the way of making the maximum amount of cellulosic ethanol from a plant is a process that is wasteful of the carbon captured by plants, and the interference of a stubborn substance called "lignin," a compound in plant cell walls that makes it difficult to break plant material down into sugars.

new lab
This new lab will soon be filled with equipment as plant cell biologists Nick Carpita and Maureen McCann and other researchers search for new methods of biofuel production using plant biomass. The research is funded by a $20 million Department of Energy grant.

Purdue University scientists are hoping to develop new methods of biofuel production from plant biomass distinctly different from that used to make ethanol. The production of advanced biofuels, particularly liquid hydrocarbons, would eliminate those inefficiencies and add to the types of fuel used to meet the world’s energy demands. This past summer, the U.S. Department of Energy awarded a group of Purdue scientists $20 million over five years as part of the economic stimulus to create the Center for Direct Catalytic Conversion of Biomass to Biofuels—aka C3Bio—to do just that.

"Right now, in  the process of converting the sugars to ethanol and the time for the organisms to grow, you lose 50 percent of the carbon as carbon dioxide," says Nick Carpita, a Purdue plant cell biologist. "The idea is to overcome the natural inefficiency of ethanol production." Another way that he often puts it is, "Every carbon is sacred," meaning the team will investigate ways to capture and utilize every carbon to get as much energy as possible from plant material.

"The goal of our center is to turn 100 percent of carbon atoms from the biomass into fuel molecules, using carbon trapped in lignin as well as in polysaccharides," says Maureen McCann, a plant cell biologist and director of the center. "Once the plant has locked up the carbon from carbon dioxide in the air in solid form, you want to make sure that none of it is lost until the fuel is burned."

McCann said the C3Bio could develop the next generation of biofuels, adding to the mix of renewable energy sources aimed at ending fossil fuel dependence. "There is no single answer right now in terms of biofuels, but this center could add to the energy options we have," McCann says.

Out with the old
Current biofuel production is a multistep process. In cellulosic ethanol, enzymes break down plant material into sugars, which are fermented using yeast. The yeast, using the sugars as food, creates ethanol.

One of the first inefficiencies is in breaking down the plant material. Lignin acts as a barrier in plant cell walls, making it difficult for enzymes to break down the biomass. According to Nate Mosier, a researcher in agricultural and biological engineering, the enzymes get to less than 20 percent of the sugars—without pre-treating the biomass, which adds expense. When yeast uses the sugars to create ethanol, the byproduct is carbon dioxide. That gas is lost, wasting valuable carbon atoms.

About 50 percent of the carbon in the sugars is lost via carbon dioxide, meaning only 30 percent or so of the carbon in a plant used to make cellulosic ethanol is actually converted into fuel. It also takes a good amount of energy to make ethanol. From the fuel needed to plow, fertilize and harvest the crops to transportation and energy used in the biological processes, Carpita estimates that only 1.4 units of energy are created for every unit of energy spent. "In essence, you’re using millions of acres of fields to create relatively little energy," he says.




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