Researchers at the new Biomass Energy Center
are homing in on future fuels.
—By David Pacchioli
Three dollars got our attention.
We could dispute the fact that oil reserves are dwindling. We could ignore the growing evidence that we're overheating the planet. We could somehow even disconnect an ongoing war and instability in the Middle East from our unquenchable thirst for petroleum. But when gasoline prices hit three dollars a gallon during the summer of 2005, the term "energy independence" suddenly held new urgency. Finding the energy sources of the future—clean, sustainable, reliable sources—became the task of the immediate present. For many of us, that was when the idea of growing switchgrass for fuel stopped sounding like science fiction.
Energy researchers, of course, have been taking switchgrass—and hybrid poplar, and blue-green algae, and sugar cane—seriously for much longer than that. These are just a few examples of biomass, plant matter that can be transformed into fuels and other energy products. Like petroleum and coal, biomass contains carbon taken from the atmosphere via photosynthesis: turning sunlight into energy. Unlike fossil fuels, however, biomass is renewable, and can be grown domestically. In all its varieties, biomass is also plentiful, and has the potential to be much more environmentally friendly as fuel.
At Penn State, over 50 researchers are currently involved in researching biomass energy, in every discipline from fuel science to genetics. To help coordinate their efforts, the University has created the Biomass Energy Center, an interdisciplinary initiative housed in the College of Agricultural Sciences that also includes the Eberly College of Science, the College of Engineering, and the College of Earth and Mineral Sciences. According to biological engineer and Center director Tom Richard, none of these researchers thinks of biomass energy as a panacea. But all of them are convinced that it can be part of the solution.
Much more than corn
Until recently, the average American might well have been excused for equating alternative fuels exclusively with corn. After all, corn-based ethanol accounts for virtually all of the biomass-derived fuel currently produced in the U.S., a projected six billion gallons in 2007. Nationwide, there are now well over 100 ethanol fuel plants processing corn grain, with another 35 or so on the way. In Pennsylvania, large corn-to-ethanol plants are under construction in Clearfield and Westmoreland counties. Corn ethanol, mixed with gasoline, now accounts for roughly three percent of this country's automobile fuel.
This rapid growth has had significant consequences, not all of them desirable. Economists and others have worried about a dramatic rise in corn prices which, while it benefits corn growers, has also inflated food prices and raised wider fears about turning over prime farmland to the production of fuel. Too, because corn is energy-intensive to grow, there is ongoing debate over the actual net gains of using corn as fuel, both in terms of energy produced and greenhouse gases emitted.
Tom Richard Credit James Collins
All else aside, Tom Richard says, "there's just not enough of it" for corn to be the biofuels answer over the long haul. Even if all the corn U.S. farmers now grow were to be funneled into ethanol, he notes, it would replace only 15 percent of the country's gasoline demand.
Where energy is concerned, Richard and others now agree, the chaff might actually be more important than the grain. In 2004, Richard, then at Iowa State, testified before a U.S. Senate Committee about the untapped energy potential of corn stover—the stalks and other inedible parts of the corn plant which currently go to waste.
Richard estimated that a sustainable harvest of 100 million metric dry tons per year of corn stover is available for use—a bulk that could translate into over 10 billion gallons of fuel in the U.S. "Because stover is a crop residue," he added, "the incremental energy, nutrient and cost inputs for collection are relatively small, offering corn producers the potential for a valuable new co-product from existing acreage."
"Transforming corn stover, wheat straw, and other crop residues into fuel can help turn the food versus fuel debate on its head," Richard says. "Developing integrated cropping systems that provide food, fuel, and a quality environment is critical to finding win-win-win solutions that address long term societal needs."
Crop scientist Greg Roth has been working out just such integrated cropping systems for Pennsylvania agriculture, focusing particularly on alternative winter cover crops. Roth has been studying the use of canola for biodiesel and hulless barley for ethanol production, growing these energy-rich cover crops over the winter and spring when the soil was otherwise bare.
"These cover crops allow us to get three crops out of the common corn-soybean rotation, producing 115 to 200 gallons of biofuel per acre," Roth reports. "By covering the soil for most of the year, cover crops not only capture more sunlight for energy, but reduce erosion and improve water quality as well."The starch in barley could provide a near term alternative to corn ethanol, and would be available in the summer when corn prices typically reach their annual peak. And there are even larger bioenergy resources that are yet to be tapped.
Out of the woods
In a heavily forested state like Pennsylvania, one key bioenergy feedstock is the woody biomass of forest residue. "Pennsylvania's forests are rich in potential bioenergy from small-diameter trees that are overcrowded, under-utilized, and inhibit the opportunity for professional management," says Charles Ray, assistant professor of wood operations research. According to the most recent U.S. Forest Service data, Ray says, as much as 500 million tons of wood are held in these "small-diameter stems" across the state's 16 million acres of forestland. Ray has estimated that six million dry tons of this "waste wood" per year could be harvested for bioenergy—enough raw material to produce 540 million gallons of ethanol.
Corn stover and forest residue are ready sources of cellulosic biomass—fibrous plant material made up mostly of the structural compounds cellulose and lignin. Other potential options include fast-growing, high-yield grasses like switchgrass and miscanthus, which could be cultivated as dedicated energy crops. The cellulose in these sources can be converted to sugar, which is then fermented into ethanol. The process is not as straightforward as fermenting the starch from corn, experts say, but the net energy balance is far more favorable.
According to a study by the U.S. Department of Energy in 2006, the long-term potential for cellulosic ethanol is gigantic: some 120 billion gallons a year could be made in the U.S. by 2050. That won't happen, though, until researchers get more efficient at cracking the lignin barrier.
The miracle of lignin
"Nature's plastic," is what Ming Tien calls lignin. "It's a polymer of phenols that helps plant tissue stay rigid," explains Tien, professor of biochemistry. Lignin is what makes wood hard, and brown. "Aquatic plants don't have it—it was an adaptation to the terrestrial lifestyle," he says.
Ming Tien Credit James Collins
In the early 1980s, Tien, then a postdoc at Wisconsin, discovered the enzymes that fungi produce to degrade lignin. Fungi, he explains, are the predominant degraders of wood. "They have to deal with the lignin barrier to get to the cellulose, which is the carbon source that they live on." At the time, Tien's discovery was of great interest to the pulp and paper industry, still the major users of cellulose. "One idea was to use these enzymes instead of harsh chemicals to degrade lignin for making paper." He pauses. "That never really panned out. While these enzymes do degrade lignin, it's a slow process. Just think how long it takes for stumps to degrade."
Twenty years later, however, the chemical pretreatment required to remove the lignin barrier is a major expense in the making of cellulosic ethanol. As a result, Tien says, "There's renewed interest in how can we use some of these fungal enzymes." He is working with Richard on methods for adding these enzymes to biomass during ensilage, to begin breaking down lignin while biomass is still on the farm. "The trick is to enzymatically crack the lignin with a minimal loss of cellulose," Tien says.
Another idea is genetically altering trees to make them produce less lignin. "A lot of people are working on this," Tien says. "But if you lower the lignin content, you're affecting the fitness of the plant. It may not be as structurally rigid. It may blow over and break if there's a strong wind."
Instead, Tien wondered what would happen if he engineered the tree in a slightly different way, introducing proteins into the plant's cell walls that would block some of the cross linkages between lignin molecules and replace them with protein-lignin linkages. This would then make those molecules more accessible, easier to chemically "unzip." "Instead of using harsh chemicals to soften the lignin, we could use proteases," Tien says. "Protease technology is pretty well known—these are the enzymes people use to remove stains from clothing."
He took his idea to molecular geneticist John Carlson and Haiying Liang, a postdoc working in Carlson's lab. Carlson and Liang have since introduced the blocking proteins into hybrid poplar plants, and are currently testing the effect on lignin degradation.
"What's nice about this," Tien says, "is that it's a generic strategy. You could use it on many different plants. And it doesn't have to be just for fuel. Tweaking the lignin content this way could make corn stover and other waste products easier for farm animals to digest. This could have impact especially in developing countries."