Behind the Cellulosic Scenes

Researchers in Wisconsin and the UK are developing new techniques to augment various aspects of cellulosic ethanol production.
By Anna Austin | May 21, 2010
Whether evaluating the metamorphosis of the Model-T to a Mustang or a rotary dial landline to a Blackberry Pearl, it's clear that the evolution of technology is inevitable. While hype surrounds the latest innovation, there is always someone behind the scenes, working quietly to further advance it, even when something more efficient or inventive cannot be imagined. Without fail, what is advanced today will be considered a stepping stone tomorrow.

Many companies that have successfully demonstrated cellulosic ethanol production processes at bench, pilot and semi-commercial scales are now embarked upon commercial-scale operations, some determined to accelerate progress in order to help meet the renewable fuel standard, which has a cellulosic biofuel requirement of 6.5 million gallons in 2010. Meanwhile, researchers behind the scenes continue to tweak, refine and optimize key processes, from sugar extraction to beefing up feedstocks.

For example, a research team at the University of Wisconsin-Madison recently developed a chemical method to liberate the sugar molecules trapped inside inedible plant biomass, an approach that can convert three-quarters of the sugars locked up in raw corn stover into simple, fermentable sugars. The researchers believe the process could enable crude biomass to be the sole source of carbon for a scalable biorefinery. Ron Raines, a UW-Madison professor of biochemistry, says the process is extremely efficient, and has distinct advantages over chemical and enzymatic processes currently used for producing sugars from biomass.

A New Chemical Approach
Raines, an author of more than 320 published papers and abstracts and holder of 20 U.S. patents, co-developed the process with UW-Madison graduate student Joe Binder. Their project was supported by the Great Lakes Bioenergy Research Center, a U.S. DOE bioenergy research center located at UW-Madison, as well as with a National Science Foundation Graduate Research Fellowship awarded to Binder.
The process they have developed, which Raines says is ready for the right entrepreneur, relies on a mixture of an ionic liquid (a salt in liquid state) and dilute acid, both of which can slip past lignin to dissolve the long chains of sugars in biomass into individual molecules of glucose and xylose.

Over the course of the reaction, water is added to prevent unwanted byproducts from forming. After two rounds of such treatment, a sample of corn stover released about 70 percent of its glucose and 79 percent of its xylose, producing an overall sugar yield of about 75 percent. From there, Raines and Binder used ion-exclusion chromatography (a mixture separation technique) to allow the recovery of the ionic liquid and delivery of sugar feedstocks to support the vigorous growth of ethanologenic microbes (bacteria or yeast).The sugar yields obtained using this method, Raines says, approach those typically achieved using enzymes to break down raw biomass.

Raines and Binder fermented the sugars they collected into ethanol, and, using this process, they were able to convert half the sugars in the plant biomass to liquid fuel. There are additional benefits, one being that the chemicals used in the new process are more robust and less expensive than enzymes and require no pretreatment of the biomass sample. "In the biofuels race, I feel this sort of chemical approach has a good shot at winning," Raines says. Furthermore, chloride and hydrochloride produce high sugar yields in hours at just 105 degrees Celsius (221 degrees Fahrenheit), compared to enzymatic hydrolysis that can take up to 16 days. Also, many current pretreatment methods require much higher temperatures, at 160 to 200 C. Compared to acid hydrolysis, the process avoids the use and recycling of large amounts of hazardous concentrated acid by using catalytic amounts of dilute acid. The ionic liquid used is likely to be far easier to handle as well. The target cost of the ionic liquid is 13 cents per liter of ethanol product, or 50 cents per gallon, which equals the anticipated cost of enzymes for a process based on enzymatic hydrolysis.

Though small-scale research innovations can show great promise, duplicating those results at a larger scale is always a challenge. Raines acknowledges that, and realizes certain hurdles will need to be overcome in order for the process to become economical. These include ensuring the near-perfect recovery of the costly ionic liquid, and the possibility that the larger-scale fermentation of hydrolyzate sugars might reveal inhibitors not detected in smaller-scale experiments. Despite these and other possible challenges, Raines believes the research, which was described in the March 9 issue of the Proceedings of the National Academy of Sciences, could have substantial short-term and economic political impacts.

Beefing up Biomass
Meanwhile, across the Atlantic Ocean, researchers at Manchester University are working on a different element of cellulosic production—fattening up feedstocks. Continuing a research program that began in 2003, Simon Turner, head of Manchester University's Plant Science Group and chair of the university's biological safety committee, says the original intent of the research was to evaluate how plants control the orientation and direction in which plant cells divide, rather than research aimed at increasing biomass for biofuel conversion.

Though the plant Turner and fellow researchers have worked with does not physically resemble a tree, the small, flowering Arabidopsis has a similar vascular system which circulates resources such as sugar and water throughout the plant. "We wanted to know how the cells divided to produce this [vascular] pattern, how they knew which side to divide along, and we found that it was down to the interaction of two genes," Turner explains. The resulting research paper, called "The PXY-CLE41 receptor ligand pair defines a multifunctional pathway that controls the rate and orientation of vascular cell division," was published online Feb. 10 in Development Journal.

By investigating growth in the vascular bundles, he and his team found that the genes PXY and CLE41 dictate the amount and direction of cell division. "Our work has identified these two genes that make plants grow outwards," he says. "The long, thin cells growing down the length of a plant divide outwards, giving that nice radial pattern of characteristic growth rings in trees. So you get a solid ring of wood in the center surrounded by growing cells."

During experiments, it was found that the over-expression of PXY produced more cells, but they were very disorganized and not useful because the orientation was affected, according to Turner. "It is in understanding how the both of the genes control both the amount of cell division and the orientation of division that allows us to manipulate it in a useful way," he says. When over expressing CLE41, a greater amount of growth in a well-ordered fashion was seen, thus potentially increasing biomass for biofuel or other uses. "Now we know what genes are dictating the growth process, we can develop a system of increasing growth so that it is orientated to produce more [wood]," Turner says.

On quantifying the amount of growth seen, Turner says it's difficult to convey for the Arabidopsis plant. "[because] some tissue undergoes cell divisions (and radial growth) that do not normally divide," he says. "In vascular bundles, they have somewhere between two and three times as many cells." In addition, outcomes may vary considerably when applied to the vascular systems of trees, which could largely be influenced by location. "Trees are responsive to a lot of things," Turner points out. "They stop growing in winter and start again in spring, and this changes according to the amount of light and the day length. It might take a tree 150 years to grow in Finland, but only 10 years in Portugal."

As for its applicability in other biomass crops, apart from trees, the researchers know the process works in tobacco, and although tobacco is not yet considered a biomass crop, it has made them confident it will work in any dicot crop such as soybeans or alfalfa. "What will happen in monocots such as maize or switchgrass is less clear, but we are currently testing this," Turner tells EPM. Now, he and his team are growing poplar trees in the laboratory to determine if they fit the Arabidopsis model.

Eventually, this research could aid in quickly growing biomass for biofuel, particularly in countries with growing energy needs or mandates. Turner points to estimates that, in order to reach current U.S. mandates, which call for one-third of all liquid fuel to be generated from renewable sources by 2025, about 1 billion metric tons of biomass will be needed. "This work could have substantial short-term economic and political impacts," he says. EP

Anna Austin is an associate editor for Biomass Magazine who regularly contributes to Ethanol Producer Magazine. Reach her at aaustin@bbiinternational.com or (701) 738-4968.