Building a sustainable biofuel production system

Chemical engineering professors Harvey Blanch and Alex Bell discuss options for producing carbon-neutral fuels.

Biofuels are making the headlines lately, mostly because of scientific and political debate about whether ethanol derived from corn is a viable fossil fuel replacement, or whether it is another subsidy to agribusiness. Meanwhile we read about biodiesel in stories that feature "greasecars" that burn used frying oil in modified diesel engines.

We hear much less about the serious efforts underway to meet the U.S. Department of Energy's target of replacing 30 percent of transportation fossil fuels with biofuels by 2025. The target also calls for replacing 25 percent of petrochemicals with biomass-based alternatives. At UC Berkeley, two professors of chemical engineering are quietly laying the groundwork to help achieve these goals.

Alex Bell, chair of the chemical engineering department and an expert on catalysis, and Harvey Blanch, a bioengineer and a veteran of biofuels research in the energy crisis of the 1970s, think that the goals are feasible, but that we have a long way to go. Ethanol, derived primarily from corn, is the major biofuel in use today. But it contributes only 2 percent to the transportation fuel mix in the United States, while biodiesel contributes only 0.01 percent.

"The 30 percent figure is reasonable, but it may take 20 years to get there," says Bell. "How do we create a steady supply of biofuels year after year? Can we do so without degrading agricultural land and using excessive amounts of water, fertilizer and other inputs? And how do we envision the whole structure of the biomass-based refinery system of the future? These are the problems we must overcome."

For Blanch the problems are political and social, not technical. "Many of the technical issues were worked out during the energy crisis in the 1970s, but the research programs were killed and the results shelved in the 1980s when gas prices fell," says Blanch. "The economics are looking better for reviving biofuels research, but solutions to these problems will take sustained political will, not just a few months of hearings in Washington, DC."

One country that did not drop its biofuels research is Brazil, which now has 30 years of R&D behind it and a thriving ethanol industry. South America's largest country is scheduled to achieve energy independence in the next few years, and it has been exporting ethanol to Europe. Brazil's fuel mix is based on roughly 25 percent ethanol and 75 percent domestic petroleum. Most autos there are designed to run on a variety of gasoline/ethanol mixtures, giving the country the flexibility to mix fuel sources and buffer itself from global supply crises.

Still, Bell and Blanch do not see Brazil as a model for the United States. "When you add up the amount of rainforest that is being destroyed in Brazil, you have to wonder how sustainable their agricultural system is," says Blanch. "Brazil's production is based on cheap agricultural labor and sugar cane, and in the U.S., cane is grown in only two states, Louisiana and Hawaii. And our agricultural system is very energy-intensive, not labor-intensive."

Along with ethanol, biodiesel has been touted as an alternative to fossil fuels. Experts from Consumers Union, publisher of Consumer Reports magazine, recently tested a diesel automobile that had been modified with a $795 kit to run on leftover restaurant frying oil, and they found that the system worked well. But Bell and Blanch are skeptical that biodiesel will ever be more than a niche fuel in an economy that is currently consuming 400 million gallons of gasoline every day.

Blanch and Bell have suggested a third path to producing biofuels, one based on converting the cellulose in biomass to liquid alkanes, which can in turn be converted to gasoline and diesel fuel. Alkanes are straight- and branched-chain hydrocarbons that, unlike sugars and alcohols, don't contain any oxygen. Gasoline is made from alkanes with anywhere from five to eleven carbons in each molecule. Diesel fuel is a little heavier, based on alkanes with nine to fifteen carbon atoms.top

In a biomass-to-fuel conversion, biomass is first treated to expose the cellulose in the plant fibers. Next, enzymes break down the cellulose into its constituent sugars. These sugars are then converted to simple hydrocarbon fuels (alkanes) and other chemicals by catalytic processes.

"The first part of what we are proposing," says Bell, "is to start with plant sugars, but instead of converting them via fermentation and distillation to ethanol, we plan to convert the sugars to alkanes via complex catalytic reactions. The big advantage to this strategy is that the overall energy efficiency of producing alkanes from sugars is about double that of producing ethanol — distilling the alcohol from water consumes about 60 percent of the energy in the ethanol that is produced."

"The second part of the proposal is cellulose conversion," says Blanch. "In the 70s, we conducted major research projects on how to convert plant cellulose to sugars that could then be converted to ethanol. Humans can't digest cellulose, so we call it fiber. Cows and termites are pretty good at it. But the champion at producing cellulases, the enzymes that break down cellulose, is the fungus Trichoderma reesei."

During WWII the army noticed that in the Pacific, tent fabric would fall apart very quickly. Elwin Reese of the U.S. Army Natick Labs discovered that a fungus was digesting the cellulose in the cotton tent fabric. Reese and co-worker Mary Mandels performed extensive research on Trichoderma reesei, which eventually was named after Reese.

"Using the techniques available in the 1970s," says Blanch, "researchers bioengineered the fungus to maximize the production of the enzymes that digest celluloses into sugars." One remaining problem is lignin, a tough fibrous plant material. Termites can partially digest lignin with the help of symbiotic bacteria in their gut, but no commercial process exists to digest it, and burning it dumps the carbon back into the atmosphere and contributes to global warming.

"One alternative," Blanch notes, "is to bioengineer a low-lignin crop that does not require fertilizer, that doesn't need much water, and that could be grown on land not suitable for food crops. The problem is that lignin is what makes the plant stalks rigid, and without it, a plant would probably be floppy and difficult to harvest. And of course," he adds, "there might be public resistance to huge plantations of a genetically-modified organism."

In essence, what Bell and Blanch are proposing is a carbon-neutral way to produce gasoline and diesel. "The fuel production system is very complex," says Bell. "There is no 'killer application' on the horizon that will revolutionize the process. What is required is a smoothly integrated set of strategies from agricultural production, through cellulosic conversion to sugars, from sugars to alkanes, and then to petrochemical plants and distribution systems."

There are not many places fostering a group of scientists with the necessary breadth of skills to achieve this — including agricultural specialists, molecular and cell biologists, chemists and chemical engineers, and energy and environmental policy experts. UC Berkeley, in conjunction with the Lawrence Berkeley National Laboratory, is one of the few places where the necessary expertise can be pooled to focus on achieving the Department of Energy's 30 percent goal.

Some advocates of hydrogen fuel might argue that producing cellulosic alkanes as a feedstock for the petroleum industry would allow our economy to maintain the status quo and forestall the transition to a hydrogen energy system. Bell strongly disagrees.

"The arguments in favor of the hydrogen economy are attractive, but the hydrogen economy lies too comfortably far in the future," Bell emphasizes. "This justifies inaction on the alternatives. It lulls us into waiting for the big technological breakthroughs that will solve our problems and enable the widespread use of hydrogen. Meanwhile, we have many technologies that we could work with today. The hydrogen transportation and storage problems may take years to solve, and even if they are solved, the infrastructure for a hydrogen economy might take 50 years to build. We don't have 50 years to start to solve the global warming problem. Let's get to work with what we have now."

Blanch remains enthusiastic about technical possibilities, but skeptical of political realities. "Global warming is just the kind of long-term problem that our political system does not respond to effectively," says Blanch. "Our system responds to crisis. If we continue to have super-hurricanes, then Washington, DC, will act. Or if the day comes when Americans drive to their local gas station and there is no gas available, then politicians will get interested. But I think as scientists and engineers we need to push ahead. Unlike the energy crisis of the 1970s, the problem of global warming is not going away. The stakes are very high."