ISU researchers demonstrate extraction of sugars from bio-oil

By Bryan Sims | October 06, 2011

A team of researchers at Iowa State University have developed a novel process that can recover sugars from fast pyrolysis oil for conversion to advanced biofuels and biobased chemicals.

According Robert Brown, professor of mechanical engineering and director at ISU’s Bioeconomy Institute overseeing the project, the bio-oil produced consisted of two primary components: the soluble sugars and what Brown refers to as phenolic oligomers, similar to the pyrolytic lignin that’s often described in the water insoluble components found in bio-oil. Brown said that a simple water-washing technique was used to recover the sugars from the bio-oil, which leaves behind the water insoluble fraction.

Brown and his team presented a series of papers during the International Conference on Thermochemical Conversion Science in Chicago Sept. 28-30. They addressed the conference about the various steps they took from producing the sugars competitively against normal weight oxygenates and the ability to recover the sugars as a pure stream.

The exact nature of the sugars, according to Brown, consists primarily of a dehydrated monomeric form of glucose, called levoglucosan, that is suitable for upgrading in its raw form, whether it be fermented with the use of microorganisms, hydrolyzed to glucose, catalytically upgraded or be used to promote production of oligosaccharides. In addition to levoglucosan, he said maltose was also found in the sugar fractions.   

“It turned out to be a very simple thing to do,” Brown told Biorefining Magazine. “We were lucky in a sense that we had a fraction that had very little water in it to start with, and it had heavy ends consisting of a soluble fraction and an insoluble fraction so, frankly, we just easily washed it out with a very small amount of water.”

Brown said the process yielded between 20 and 40 percent sugars. “That’s far more than you would need for a fermentation process,” Brown said. “You’d actually have to dilute it before fermentation and we’re doing some studies right now fermenting the sugars.”

Astonishingly, Brown noted that the core of the research tied into the fact that there shouldn’t be any sugars in bio-oil at all.

“That was one of the things that bothered me for years,” he said. “How can you take something that has no vapor pressure and get it to pyrolyze and end up in your bio-oil? What we discovered is the sugar that’s captured in bio-oil originated as anhydrous sugars, most of it being levoglucosane. This stuff has small vapor pressure but it’s enough to vaporize material when you get up to temperatures of around 500 degrees C. That’s how those sugar compounds get out of the pyrolyzer and then they react as you’re condensing them and you end up with getting these oligosaccharides instead of some of the original monomers, but we also see a lot of the levoglucosan as well.”

One of the biggest challenges Brown said the research focused on was finding efficient methods to prevent sugar burn-off during the fast pyrolysis process. He compared this challenge to the analogy of trying to recover sugar from boiling water on a stove, adding that if left unrecovered the sugars will polymerize, similar to the texture of candy. Brown said the research has been able to transport the sugars out of the hot reaction zone as a means of circumventing this inherent challenge during the process.

“I try to instruct the students to not burn the sugar,” Brown said. “It’s very easy to burn [sugar] and that’s the same issue we’re facing. The challenge is that we have to be able to evaporate these sugars faster than they want to dehydrate. We have to figure out how to remove these sugars as fast as they’re formed. We’re getting a lot more sugar out than we do traditionally from biomass, and we’re looking at some different engineering approaches that will allow us to do that.”

In addition to corn stover and switchgrass, Brown said the research also tested loblolly pine and red oak. The reason the research used these particular biomass, according to Brown, was because he wanted to incorporate a wide range that have alkali, a form of potassium earth metal found in nature that can often interfere with the release of sugars from lignocellulosic biomass. Brown said a simple pretreatment was used on the biomass prior to entering the pyrolysis process in order to neutralize the alkali.

“There is a competition there that is due to the alkali present in biomass,” Brown said. “We made a point that the alkali content is what’s responsible, up until now, from keeping us in getting much sugar from biomass using pyrolytic methods.”

As far as utilization of the heavy-end fraction of the bio-oil, or phenolic oligomers, Brown said the research has conducted demonstrations of using the material as asphalt, “but the presence of sugars in there actually affects how you can heat it and whether it would leach out,” he said.

“With the sugar out, it remarkably transforms these heavy ends into something that flows at room temperature. By taking the sugar out we’re going to be able to use these phenolic oligomers in a number of processes that require additional heat treatment; one example being bioasphalt. Another example would be to potentially upgrade it into hydrocarbon fuels.”

Brown said he hopes to scale-up the process in the future based on engineering and performance data during lab and mini pilot trials to construct a pilot production plant in collaboration with a major partner.

In addition to Brown, key contributors to the pyrolysis research at ISU include: Brent Shanks, the Mike and Jean Steffenson Professor of Chemical and Biological Engineering and director of the National Science Foundation Engineering Research Center for Biorenewable Chemicals based at ISU; Christopher Williams, professor of civil, construction and environmental engineering; Zhiyou Wen, associate professor of food science and human nutrition; Laura Jarboe, assistant professor of chemical and biological engineering; Xianglan Bai, adjunct assistant professor of aerospace engineering; Marjorie Rover and Sunitha Sadula, research scientists at the Center for Sustainable Environmental Technologies; Dustin Dalluge, a graduate student in mechanical engineering and Najeeb Kuzhiyil, a former doctoral student who is now working for GE transportation in Erie, Pa.

The research is supported by the ConocoPhillips Biofuels Program, an eight-year $22.5 million program that was launched in 2007.