Two Steps Forward
The path from biomass to biofuels just got shorter.
One of the primary goals of ORNL's BioEnergy Science Center (BESC) is to revolutionize the processing and conversion of biomass into biofuels, such as ethanol. A critical component of this revolution is the capacity to streamline the bioconversion process and optimize its outputs, a goal shared by Ramesh Bhave and his colleagues in ORNL's Materials Science and Technology Division as they continue on the path toward developing an unprecedented method of combining the pretreatment of biomass and the separation of its key components into a single, continuous process.
"The vision of the bioenergy center is to find innovative ways to convert biomass into fuels," says Bhave. One of the key steps in this process is to develop an effective pretreatment technique, one that breaks down the biomass effectively, making it more amenable to biochemical conversion and increasing its yield of sugars and other chemicals that might be of value. "This is where our group's expertise in the area of separations technology becomes important," Bhave says. "Developing the ability to effectively separate sugars and other biomass components could accelerate bioconversion." With that goal in mind, the efforts of Bhave's group are focused on a unique pretreatment regimen for the conversion of biomass to fuels using very selective, robust membranes that can operate under harsh pretreatment conditions, enhance the yields of the sugars and minimize byproducts that decrease the efficiency of the process.
Bhave believes that the breadth of talent represented by BESC's academic and institutional partners has been critical to the project's early success. "We are working with Dartmouth College because it is a leader in researching and understanding pretreatment," he says. "Dartmouth's engineering department has studied biomass pretreatment technologies for thirty-plus years." When the Department of Energy established BESC, Dartmouth became a very desirable research partner because its pretreatment expertise, combined with ORNL's capabilities in the area of chemical separations, enables us to make the separations process more effective and more efficient at a lower cost."
Bhave characterizes consolidated bioprocessing as a question of how biomass can be pretreated so as to become more amenable to biochemical conversion and produce more of the products, like sugars, that will increase the yield of biofuels. "If we look at the problem from that perspective," he says, "separating the products released by pretreatment becomes an important part of the process. If we could develop effective separations methods, they would assist in the bioconversion process."
Toward that end, Bhave and his colleagues are working on a novel pretreatment process for biomass conversion. The key to the process is the use of selective filter membranes that can operate under very aggressive pretreatment conditions. The ability to extract various biomass components, such as sugars, proteins and lignins, while the biomass components are still in the pretreatment environment, enables researchers to boost their yields. This increase occurs because, in a continuous process, the components spend less time in the harsh preprocessing environment and are, therefore, less likely to break down into other compounds.
The pretreatment process begins with small chips of plant material from energy crops, or "feedstocks," such as switchgrass. "The process could be applied to a range of feedstocks," Bhave explains. "What we are looking for in the various energy crops is a cell structure that is less resistant to pretreatment, so the sugars can be released from the biomass using less energy and fewer chemicals." This feedstock is pretreated to break down its cell structure, which releases liquid containing various chemical products, including the sugars needed to produce biofuels. This process typically requires the use of moderately high temperatures, along with dilute acids or bases.
At this point in the traditional pretreatment process, the liquid would be left to cool so the solubilized products could "precipitate" or separate from the liquid. In Bhave's process, a flowing water stream interacts with the heated biomass, carrying away organic components that contain sugars and other soluble products released by pretreatment. This is where the laboratory's separations expertise comes in. At this point, the dissolved material is retained by the membranes that are designed both to withstand the aggressive pretreatment environment and to separate sugars and other organic components that have been released by the biomass. One of the advantages of this integrated approach to preprocessing and separation is that it potentially minimizes the time the biomass is in the harsh pretreatment conditions. This, in turn, would minimize the production of materials, such as degraded sugars, that could stop the fermentation process.
"This all happens under pretreatment conditions," Bhave emphasizes. "We don't wait for things to cool down. In the pretreatment process, the components are basically just dissolved at high temperature. If the pretreated biomass were allowed to cool down, its dissolved components would precipitate into solids." Once this occurs, a much more difficult and energy-intensive chemical process would be required to recover the sugars from the other components, he says. "Separating sugars and other components from the rest of the biomass during pretreatment is an approach that, to our knowledge, has not been taken before," Bhave says. "It requires an understanding of both the pretreatment process and the ability to apply membrane separation technology in an entirely new way."
As they work to fine-tune the separations process, Bhave and his colleagues are addressing the challenge of creating membranes that have multiple functions. "We need membranes to do the separation, withstand the preprocessing conditions and be able to be regenerated, or cleaned, periodically," he says.
The ability of Bhave's team to consolidate separations technology using the pretreatment process suggests that BESC is making significant strides in the processing and conversion of biomass. Bhave isn't yet predicting a revolution, but he is excited by the project's potential. "The research is promising," he says, "but it is also challenging. We are hopeful that the price of the biofuel produced by this process will be lower because our pretreatment costs and energy costs are lower." Perhaps more than Bhave realizes, his hopes are shared by a nation increasingly eager for low-cost alternative fuels.