Engineering plants for efficient biofuels production
No Carbon Left Behind
By Brian Wallheimer
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.
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.
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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.
A different approach
So one of two things would have to happen to make biomass a realistic source of primary fuels: More land would have
to be devoted to growing biomass, or the production process would have to become much more efficient.
With efficiency as a goal, the center hopes to cut out the middleman—in this case, fermentation. “Focusing on ethanol
as a fuel isn’t really an endgame here. I think it is a stepping stone until we can develop more advanced, energy-rich
biofuels from biomass,” McCann says.
Scientists at Purdue will develop a thermocatalytic process, using an implanted catalyst and heat to create fuel. A
catalyst must overcome the difficulties ethanol-production facilities face now, namely the expense of the enzymes and
their inability to break down the lignin in the cell walls, Mosier says. This new biofuel production process would rely on
plants that have a new catalyst engineered into them that is present as they grow.
Such a catalyst would be able to function at high temperatures, breaking down plant material, but this time taking lignin
and turning its previously unused carbon atoms into fuel. “There’s enormous potential for this center to advance the
basic science and to better control and manipulate matter at the atomic scale to create energy,” Mosier says.
Scientists have been adding and removing from the makeup of plants for some time. But just what sort of catalyst to add
to the plants is a difficult question to answer. One of the first steps is to better understand how lignin works so that
plants can be engineered properly.
“It’s a lot of fundamental work,” maintains Clint Chapple, a molecular biologist and head of the Department of
Biochemistry, who has worked for 20 years on understanding how lignin is formed so that scientists can work on
breaking it down. “It opens a door to whole new ways of thinking about dealing with lignin. Here’s an opportunity to alter
the chemical properties of lignin so that we can really control it.”
Chapple likened the process of restructuring lignin to rebuilding a brick wall—pieces need to be moved, but in doing so,
the wall can’t fall down. The wall would still be standing, doing its job, but the changes would make it easier to handle.
“We know that we can alter it,” he says, “and we know how. We just have to do it the right way.”
Adding a catalyst to the cell walls of plants that could break down lignin and give access to all the plant’s sugar would
significantly increase the amount of fuel that could be created per plant. “Can we redesign the cell wall so that it’s the
perfect feedstock for the conversion to fuels and other useful products? That’s the question,” Carpita admits. “You would
have a genetically advantaged plant used specifically for fuel products, just as breeders and genetic engineers have
created to get larger grains or more yield.”
Finding the proper bioenergy plant will be key in the process, Carpita says. He is trying to identify the best possible
crops to create biofuels, including a type of tropical maize that doesn’t create grains, uses less nitrogen and grows
bigger than traditional corn.
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The benefit could be enormous in terms of savings as well, Chapple adds. An inexpensive catalyst could eliminate the
need for expensive enzymes used in the current biological fermentation process.
“For any bioenergy process to be successful, it really comes down to money,” Chapple points out. “If we could eliminate
any of the processes, we’d be able to do away with steps that cost a lot of money. Tens of cents per gallon would make
a difference at the pumps.”
Reaping other benefits
Other inefficiencies of current ethanol production are time and capital. A thermocatalytic process could also shorten the
time it takes to make biofuels. “We could get the catalysts to react in minutes or seconds instead of days,” Mosier says.
A thermocatalytic process would also require less infrastructure, namely large biorefinery plants that can cost hundreds
of millions of dollars. Cutting that cost could make biofuels more attractive to manufacture and even drive down the
price of the fuel in the marketplace.
Without the need for large biorefineries, the Purdue researchers say they could envision portable mobile biofuelproduction facilities that could go to harvest sites and create fuel on the spot. Such a development would eliminate a
large piece of the transportation cost that drives up current ethanol prices.
“Once you’ve built a biorefinery, you have to get the crop there,” McCann explains. “And once your crop is beyond a 50mile radius, you may be using more energy to transport it than you can recover in fuel.”
Besides cost, a thermocatalytic process could address another difficulty biofuel producers have with ethanol—it’s tough
on vehicle engines. “We would make fuels that are very similar to gasoline, diesel and jet fuel that we are using today,”
Mosier says. “Ethanol is similar to those, but different enough that there are issues with engine design and delivery
through the marketplace.”
McCann also hopes that biomass crops tailored for their end-use with implanted catalysts for a thermocatalytic biofuel
production process would create jobs for seed producers, engineers, farmers and construction personnel.
Working together
All the science aside, the C3Bio project would never succeed without interdisciplinary collaboration. Scientists from
agriculture, science and engineering will all bring something to the table. “All of the challenging problems we have to
solve are in the overlapping areas of these disciplines,” Mosier says. “It really works to the strengths of Purdue by
integrating agriculture with engineering and science.”
Purdue’s strategic plan emphasizes the importance of interdisciplinary research as a key to solving global challenges.
The C3Bio collaboration is a prime example.
Right away, Chapple recognizes the value. His work on lignin modification goes hand in hand with the catalyst research
of Mahdi Abu-Omar, a professor of chemistry and associate director of the center. “We had never even talked before,”
Chapple says. “Now our work will come together. That’s absolutely critical.”
The group will also work with end-users, such as oil companies, ethanol producers and the auto industry, to coordinate
their needs with what may come from the research. “We absolutely need each other,” says McCann. “There are lots of
potential partners out there, and we’re interested in reaching out to them and having their expertise feed the science.”
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It’s that reaching out and learning about each others’ work across departments and schools that brought the Purdue
team together in the first place. “We all have the same goal,” McCann says. “Working together, we can find a way to
capture all of those carbon atoms into fuel molecules.”
Contact Brian Wallheimer at [email protected]
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