Brewing biofuel
On Jan. 23, the U.S. Environmental Protection Agency announced a possible reduction in the renewable fuel standard for cellulosic ethanol, due to short supply.
Biofuels are made from organic materials. Because the carbon dioxide released during burning is absorbed in the next crop, biofuels have less greenhouse-gas impact than fossil fuels.
In the United States, the biofuel story is all about corn, because the kernels contain sugar that is easily fermented into alcohol. Monthly production of ethanol for gasoline reached 1.1 billion gallons in July, 2012. That year, biofuel devoured 39 percent of the giant American corn crop.
Cellulose, contained in waste from farms, forests and factories, also contains carbohydrates that can become fuel. Yet the conversion is difficult, expensive and rare, as the EPA pullback illustrates. Pending further reduction in the standard, industry must blend 6 million gallons of cellulosic ethanol into gasoline by June 30, 2014.
That fuel seems unavailable, but in Italy, Beta Renewables has started running a plant capable of converting cellulosic biomass into 60,000 tons of ethanol per year.
In the United States, several major companies are betting serious money on biofuel from cellulose. “DuPont has steel in the ground in Iowa, ADM has steel in the ground in Illinois, Iogen has it in Brazil,” says Tim Donohue, director of the Department of Energy funded Bioenergy Research Center at the University of Wisconsin-Madison.
Depending on design, cellulosic biofuel plants can accept residues from cornfields, a dedicated crop called switchgrass, or other high-cellulose materials such as tree trimmings or food factory waste.
New techniques in genetics, plant breeding and chemical engineering are integral to the long quest to convert cellulose into fuel. But there is also a growing interest in algae, simple photosynthetic organisms that can harvest sunlight, as a raw material for advanced biofuel.
Cost is always an issue, but Donohue figures the big firms have done the math. “These plants cost hundreds of millions to build, and I assume their accountants have been convinced they can make a profit on whatever technology they are using.”
Lamentable lignin
Extracting simple sugars is the first step in making biofuel from biomass. These sugars are then fed to microbes or otherwise processed into a flammable product, such as ethanol or other fuel molecules.
Cellulosic biomass is built on a durable structure called lignocellulose that contains cellulose, hemicellulose and a binder called lignin.
“Lignin is the main structural component of plants,” says Michelle O’Malley, a biofuel researcher at the University of California Santa Barbara. “Depending on the structure of the plant, it encases the cellulose in a matrix, and it also tends to inactivate enzymes that are trying to break down the cellulose. What has stumped researchers is taking this crude biomass that is complexed with lignin and other biopolymers, and gaining access to the cellulose.”
Friends of feces
Cellulosic materials are tough, but if nature can make it, nature can break it. For millennia, people have sourced useful microbes and molecules in nature. Brewers, vintners and bakers use yeasts to make beer, wine and bread; doctors learned that fungi create antibiotic molecules that kill bacteria.
Cellulosic biofuel researchers are following suit as they seek microbes and enzymes that can break down lignin and cellulose. Herbivores, which live by digesting high-cellulose food, are a natural place to look for active microbes, and so when O’Malley began working in the Boston area, she visited a horse farm to scoop poop.
From that messy raw material, O’Malley and her collaborators isolated a fungus named Piromyces and identified all the genes involved in making enzymes and other proteins. The project identified hundreds of enzymes that can break down lignin and cellulose.
The most active of those enzymes, she says, deals with “lignin in ways that other microbes have difficulty doing.” The plan is to transfer the enzyme’s genetic machinery into a common industrial yeast.
From her position in the chemical engineering department at the University of California, Santa Barbara, O’Malley has isolated other ferocious fungi from the tail ends of giraffes, elephants, sheep and goats at the Santa Barbara zoo.
We asked why fungi are such efficient decay organisms. “In the digestive tract of large animals there is a consortium of different bugs that play different roles,” she said. “Fungi are the first microbes to get to the scene. Due to their filamentous nature, they can grow into the substrate and secrete their own enzymes and also provide access to other microbes that can’t grow into the substrate by themselves, like bacteria and protozoans.”
In essence, she is using biology to break down biological structures. “The best engineer is really nature,” O’Malley says, “and so going after what nature has identified as the best degrader of lignocellulose offers our best chance of finding a breakthrough toward biofuel production.”
Fast track to sugar
Others are investigating chemical routes to that destination. Many strategies proposed to decompose lignin begin with a hot, acidic treatment that exposes the cellulose to enzymes or microbes. But that’s is expensive.
On January 17, researchers at the University of Wisconsin-Madison reported1 a technique that converted corn stover, pine and maple wood into a sugar solution in a single step.
The process relies on a chemical called gamma valerolactone (GVL) that, at moderate temperature and without acid pre-treatment, converts cellulose and lignin in shredded plant material into sugars. The output, a sugary solution, is ready for production of biofuel, and the GVL can be removed for reuse by bubbling liquid carbon dioxide through the solution. “You don’t have to boil it off or put in a lot of energy or additives,” says study co-author Jeremy Luterbacher. “Not only don’t we want to use new GVL, we don’t want GVL in the mix; it might kill your bugs if you are doing biological decomposition” of the sugars.
In a preliminary accounting, the process cut the cost of producing cellulosic biofuel by 10 percent, says Luterbacher. “People have been working on enzymes [to create cellulosic biofuels] for 30 years, we have been working on GVL for eight months, and our initial numbers show comparable prices. This is exciting, it looks good, but I think you’d get better yield as you work on the process.”
Donohue, head of the bioenergy center at Wisconsin, notes that plant biomass is only 60 percent carbohydrate. “The other carbon is tied mostly in lignin, and we are very aggressive about figuring out how to get value from the lignin. We don’t want to throw it away, or burn it to make power; we want to make something saleable from it. Lignin is too valuable to burn.”
Extracting value from lignin, and producing a variety of chemicals “can both radically change the economics of the whole cellulosic biofuel process,” Donohue says.
Similarly, Virent, a Wisconsin company that is working on catalytic conversion of cellulosic material to fuels that can be “dropped in” to existing fuel systems, is developing technologies to produce a fuel compound alongside a higher-value chemical. “We view this as a way to enable us to get the technology proven, and give us an additional margin, above the fuel price,” says Liz Woods, manager of R & D fundamentals. Crude oil prices are low, and “We want to get a bump in revenue that will make this a more economical prospect.”
Already, Virent sent synthetic gasoline for testing by Royal Dutch Shell, and received the highest (“no harms”) rating in fleet testing. Although the gasoline was made from conventional sugars, the same compound could be made from cellulosic feedstock, Woods says.
Fast and flexible
Overall, the biofuel industry sees a benefit in greater flexibility. Eventually, Donohue says, biorefineries may work like oil refineries, “which take in a barrel of crude oil and make value of every drop” in the form of auto, diesel and aviation fuels, and high-value chemicals. “The accountant sits down and says, ‘I can make so much profit per unit of production on each of these, what is the most profitable mix?'”
Flexibility seems more promising than the situation facing the first-generation corn ethanol plants, which made one product from one input, Donohue says. “Making a suite of fuels and chemicals that will increase the value proposition. Extracting value from every part of biomass is a very different equation than one product with one price.”
Beyond the economics, federal regulations are also a factor. In November, the U.S. EPA moved to reduce usage of renewable fuel (almost all ethanol) in 2014 by about 3 billion gallons below the present level of 18.15 billion gallons per year.
Exploring algae
A second area of intense biofuel interest is algae, primitive photosynthetic organisms that live in water. According to a recent government report, an all-out mobilization of U.S. water resources could produce 25 billion gallons of biofuel from algae, about one-twelfth of the 284-billion-gallon annual national demand for liquid motor fuels.
That mountain of algae would grow in shallow ponds of fresh, brackish or salty water, with phosphorus and nitrogen fertilizer. Although saltwater is limitless, it’s only available near the seacoasts, and questions about salt contamination of soil and groundwater would need to be answered in advance.
Scientists are adopting myriad techniques to derive useful chemicals from cyanobacteria, as blue-green algae are scientifically known. For example, Fuzhong Zhang, assistant professor of energy, environmental and chemical engineering at Washington University in St. Louis, is focusing on a promising cyanobacterium called Synechocystis. “My goal is to engineer microbes and turn them into microfactories that produce useful chemicals,” Zhang says. “Synechocystis is particularly interesting because it can use carbon dioxide as the only carbon source.”
In a 2013 study in the journal Marine Drugs, Zhang and colleagues concluded that production must be accelerated via new genetic tools to control the algae’s metabolism. One promising tactic would ramp up gene activity by altering stretches of “regulatory” DNA.
Another way to force cyanobacteria to work harder would be to rejigger its biological clock so the cell runs 24/7. “In cyanobacteria, the whole genome is under control of the clock,” says Carl Johnson of Vanderbilt University. “If the cell metabolizes full-time, we can have them produce more stuff than they normally do.”
Cyanobacteria get their energy from sunlight; so artificial lights would be needed to optimize the 24/7 clock; Johnson suggests these could be powered by the biofuel product. Johnson notes that he does not specialize on biofuel and has not done a cost-benefit calculation, but adds that manipulating the clock could also be handy for making drugs, hormones such as insulin, or other high value targets.
Meet “biocrude”
Even if the algae do not work full-time, it helps if the processing plant works on a flow-through basis, rather than on single batches, the usual laboratory approach. Using moderate temperature and pressure, Douglas Elliot of Pacific Northwest National Laboratory has demonstrated a way to continuously decompose a slurry of algae and water.
The raw material for the process is a soup of algae and water, so energy-intensive drying is not needed. The product is a water-based biocrude solution that can be catalytically converted into a range of hydrocarbons. Once dissolved chemicals are removed from the water, it is clean enough to grow another crop of algae.
The compounds in the biocrude “are in a whole range of molecule weights, similar to petroleum,” Elliot says. “We think this is a good set of data showing that this can serve as a basis for gasoline, diesel and jet fuels.”
Doing all of the steps in continuous flow “Is a key thing,” Elliot says. “These are not simple batch tests that are used in most of the literature. Continuous flow gives data for a scale-up that would allow this move forward commercially.”
Genifuel, of Salt Lake City, has licensed the technology and is building a larger apparatus for further tests.
Under the most optimistic assumptions, Elliot says, “our fuel might cost around $2 per gallon, but the more pessimistic calculation would reach $12 a gallon. That’s quite a range, but people are projecting biodiesel from algae at $20 a gallon, so we think we are in better shape than that.”
Price is always a factor in biofuels, and with conventional gasoline drifting down toward $3 a gallon, that superb fuel is also one of the cheapest liquids you can buy: cheaper than soda, milk, even many bottled waters.
Biofuels have environmental advantages, but they still must compete with products from natural gas and crude oil, says Tim Donohue of the Great Lakes Bioenergy Research Center. “People ask all the time, does the price of crude change the economics? We do watch the price, but we were funded by the Bush Administration in 2007, when the price of a barrel of crude was about $60. Crude is now $97 a barrel, so if it made sense at $60, it makes sense now.”
– David J. Tenenbaum
Terry Devitt, editor; S.V. Medaris, designer/illustrator; Yilang Peng, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive
Bibliography
- Nonenzymatic Sugar Production from Biomass Using Biomass-Derived ?-Valerolactone, Jeremy S. Luterbacher et al, Science, 18 Jan. 2014 ↩
- Biofuel Facts, Biofuel Information ↩
- Biofuels Basics ↩
- Bacteria and Fungi Together: A Biofuel Dream Team? ↩
- Food versus Fuel: Native Plants Make Better Ethanol ↩
- The Next Generation of Biofuels ↩