Researchers are closing in on new ways to separate sugars needed for biofuels.
Oak Ridge National Laboratory's BioEnergy Science Center (BESC) was established in 2007 after a spirited competition by the Department of Energy's Office of Biology and Environmental Research to conduct fundamental research related to biofuel feedstock development, cellulosic ethanol production and investigations of the plant and microbial enzyme components of the biofuels production process. "The center's basic premise," says BESC Director Paul Gilna, "was to develop an improved body of scientific knowledge and understanding to underpin the production of biofuels from plant sources and, in so doing, to harness that knowledge to develop strategies that would significantly improve the process of generating biofuels."
Most of the plant material currently used to produce ethanol in the U.S. is the starchy part of corn. The use of corn for ethanol production, both domestically and abroad, raises serious questions about using food crops and valuable farm land for the production of biofuels. BESC's research efforts are designed to look more closely at the sugars trapped in other plant structures, as well as other plants, that could be used to produce biofuels. Plant parts such as stalks and leaves are composed of fibers made from complex carbohydrates, such as lignin, that in turn contain cellulose. Like kernels of corn, they also contain sugars that can be used to produce biofuels. Unlike corn, the structural complexity of the fibers makes the sugars more difficult to separate. If scientists can unlock the secret to extracting these sugars quickly and cheaply, they can be adapted to the same manufacturing processes currently used to generate ethanol from corn.
In one sense BESC has a single goal—to overcome "recalcitrance," a characteristic that some describe as nature's way of protecting a plant by making it difficult to access the simple sugars in the complex carbohydrate fibers in which they are bound. Once separated, these sugars are available for the production of ethanol and other biofuels. "If we can solve the puzzle of recalcitrance," Gilna says, "we can open up avenues for more efficient production of biofuels and do so in a way that does not compete with the production of food. In fact, in some cases we would be taking advantage of the waste stream of food production, like discarded cornstalks, to produce biofuel. In other cases, we could use feedstocks such as switchgrass or poplar trees that do not compete with land used for food crops." BESC scientists are looking at a variety of potential feedstocks, with an emphasis on those that do not compete for agricultural land. In particular, they are seeking crops that can be grown in more arid climates and in rotation with food crops to complement existing agricultural practices. The broader goal is to develop a diversity of biomass feedstocks for the next generation of biofuels that require less water and fertilizer and can be grown on marginal land.
As BESC researchers succeed in breaking down recalcitrance in plant material, they open up the possibility of using a number of compounds stored in plants, besides sugars, for the production of other products and chemicals. One commonly cited example is lignin, the fiber that helps plants stand up. Colleagues at ORNL are excited about the use of lignin as a raw material for producing carbon fiber, a compound which is lighter but stronger than steel or aluminum and has enormous potential as an energy-saving material for cars and planes. "Certainly among our priorities at BESC is a greater understanding of how plants produce lignin, so with that understanding in hand, we can explore how we might modify a plant to produce less lignin without affecting the plant's ability to grow. That same basic understanding of the lignin generation pathways in plants would also tell us how we might modify a plant to produce more lignin, or a different quality of lignin, that would be beneficial for carbon fiber production."
The production of biofuels from plant materials involves two main processes. The first consists of the extraction of sugars from the plant biomass. The second is the transformation of those sugars into biofuels—for example fermentation by yeasts or other microbes into ethanol. In addition to understanding the recalcitrance of feedstocks, Gilna's staff also wants to improve the fermentation process. Researchers have identified a variety of fungi an other microbes that digest the sugars bound up in the complex carbohydrate structures of plant material. Wood rot, largely the product of a fungus, is one such example. Similarly, BESC scientists have discovered thermophilic, or heat-loving, bacteria that live in the hot springs of Yellowstone National Park that also can digest cellulose.
Gilna is convinced that science has much to learn from these microbes and fungi. As a result, his team is expending considerable effort studying microbes that have developed the ability to degrade or digest cellulose and lignin for their own needs. "If we could understand better the enzyme pathways that are responsible for this degradation, we could then engineer these pathways into the strains of yeast and microbes that are used in industrial fermentation processes." Looking ahead, scientists would like to identify or engineer microbes that could digest and ferment plant material in a single step, thereby reducing the cost of the resulting biofuel. One of the major drivers behind the Department of Energy's investment in BESC is the desire to promote increased use of biofuels in the domestic market by reducing the cost of production. "If biofuels are not cost-competitive, there will never be much demand," Gilna concludes. "We have to find strategies that will lead to cost reductions."
The search for a better fundamental understanding of the science surrounding biofuels production and an opportunity to apply this understanding to America's transportation sector lie at the heart of BESC's efforts to develop affordable, sustainable biofuels. "There's a huge amount to be learned," Gilna says. "As we overcome the fundamental challenge of recalcitrance, we can move forward and continue to increase our understanding of the entire process."
One measure of this project's importance is the scale of resources dedicated to the task. BESC is one of three Department of Energy bioenergy research centers, each funded at approximately $125 million, which encompass the efforts of about 1000 scientists working on the challenge of producing sustainable biofuels. Reflecting on the magnitude of the endeavor, Gilna says. "When is the last time we rallied together 1000 scientists for a problem like this? Some would say not since the Manhattan Project."