The Cornerstones of Advanced Biofuels

Novel pretreatment technologies are paving the way for the advanced biofuel industry.
By Chris Hanson | July 22, 2013

Efficiently breaking down biomass feedstocks into useable materials for biofuels and chemicals is a crucial step any developer needs to consider when selecting the best pretreatment method.  Jose Atilio de Frias, researcher at the University of Illinois’ Department of Agricultural and Biological Engineering, emphasizes that the importance of pretreatment is to remove or alter lignin, which acts as a type of glue that holds the biomass together but also inhibits the action of enzymes to release the sugars from cellulose. “So, if you introduce enzymes to the biomass without pretreatment, you will get little release of sugars,” he says. “Unless you do pretreatment, the whole biochemical scheme toward biofuels will not be accomplished.” 

Currently, one of the most common pretreatment methods is accomplished using steam explosion, but further advances and innovations in other pretreatment methods could diversify pretreatment options. In recent months, several breakthroughs and new approaches have been announced, one of which is organic solvent pretreatments with butadiene sulfone.

Salty Solutions 

Some of the newest pretreatment innovations involve the use of ionic liquids to break apart biomass into cellulose, hemicellulose and lignin. Ionic liquids, or liquid salts, are being researched at both the University of Illinois at Urbana-Champaign and the U.S. DOE’s Joint BioEnergy Institute.

Initially, the university’s research using the organic solvent butadiene sulfone to pretreat miscanthus  began for a different purpose. In the beginning, the lab was using the liquid to solubilize pure cellulose. What the researchers discovered was the biomass did not solubilize, but instead developed similar physical characteristics of pretreated material. “It actually looked very similar to what we used to do in our lab—two-stage, alkali-acid pretreatments—but in this case, one step,” de Frias explains.
“Initially, we wanted to find a solvent to actively separate lignin, hemicellulose and cellulose,” adds Hao Feng, associate professor at the University of Illinois. “However, we also found it is probably better to use this as a pretreatment because we can recover it, we can recycle it, and that way we can have that green, sustainable production.”

Using butadiene sulfone as a pretreatment offers several benefits. De Frias says the most important advantage of this method is that the solvent’s recovery process is industrially available at mild to higher temperatures. During pretreatment, he explains, the solvent decomposes into 1,3-butadiene and sulfur dioxide at 90 to 110 degrees Celsius (194 to 230 degrees Fahrenheit). In the presence of water, the sulfur dioxide changes to sulfurous acid. Together, the sulfurous acid and butadiene sulfone provide a “dual attack” to the plant cell walls, freeing over 90 percent of the hemicellulose, releasing 90 to 99 percent of the cellulose and almost 60 percent of the lignin. 

Once pretreatment is complete, the temperature is increased, and the heat breaks down the solvent, forming butadiene and sulfur dioxide. The two gases are then recombined to form the original butadiene sulfone.

The next step in developing this pretreatment method will focus on optimization experimentation. Researchers may also try different levels of solids loading, de Frias adds.

Halfway across America, the JBEI in California is also developing an ionic liquid pretreatment. Unlike the University of Illinois’ butadiene sulfone method, the institute is utilizing imidazolium chloride with mixed feedstocks. With its pretreatment technology, the institute is able to liberate 95 percent sugar yields from biomass in less than 24 hours, recovering roughly 95 percent of the ionic liquid.

Blake Simmons, vice president of the deconstruction division at JBEI, says using these “molten salts” may provide additional benefits. He explains ionic liquids can produce high sugars yields from any feedstocks. “We’ve actually asked for feedstocks from folks that they think are really recalcitrant , including pine, and we can still efficiently liberate sugars from those,” says Simmons.

Working with Idaho National Laboratory’s feedstock development unit, JBEI tested what Simmons refers to as a “witch’s brew” of feedstocks, comprised of corn stover, switchgrass, eucalyptus and pine biomass. What the researchers unexpectedly discovered was the mixtures performed better in pretreatment than single feedstocks. “Imagine if you had a biorefinery operating with ionic liquid technology that could handle any mixture that’s available regionally, be it yard trimmings, ag residues, tree residues, municipal solid waste,” says Simmons. “That’s pretty remarkable.”

Another benefit of using ionic liquids, he explains, is it allows the opportunity to “dial in the chemistry” to match biomass pretreatment characteristics by correctly choosing the appropriate anion or cation, which may come from renewable sources and have low environmental and human toxicity. “So, even in the case of a spill, they won’t pose a threat to the environment or to the humans working at a biorefinery,” says Simmons. “There are some other pretreatment chemicals that you certainly don’t want to have released into the environment or expose humans to.”

Currently, JBEI is working with the industry to commercialize the technology. Simmons hopes sugars produced from ionic liquids will be marketable within three to five years. The biggest steps that need to be taken, he says, are more process engineering and scaling to minimize risks. “We are working with user facilities within the national lab complex, post start-ups and big industry to do that,” he adds. “We are very excited about the future of the process.”

While work is being done to improve and research ionic liquid pretreatments, Leaf Energy Ltd. in western Australia is developing a glycerol-based pretreatment method.

Emerging Glycerol Pretreatment

 Formed from the merger of Aqua-Carotene Ltd. and Farmacule Bio-industries in 2010, Leaf Energy collaborated with Queensland University of Technology and Syngenta to develop a glycerol pretreatment method. Alex Baker, chief operating officer, reports the pretreatment process can liberate over 90 percent of digestible cellulose in 24 hours. Ken Richards, managing director, explains the main benefits of using glycerol over acid pretreatments is that it delivers “more sugars faster” by dissolving the lignin using a relatively inexpensive reagent at low temperatures with standard atmospheric pressure. Compared to the standard steam explosion pretreatment process, the glycerol technology produces 30 percent more in enzymatic digestibility, says Richards.  

Explaining how the pretreatment process works, Simmons says crushed bagasse from a sugar processor is churned with the glycerol in a chamber. Dissolved lignin and glycerol are then pressed out, leaving the cellulose and hemicellulose. “It’s a really simple, easy process using a very cheap substance in glycerol,” he says.

Presently, the pretreatment process has been used with sugarcane bagasse but Leaf Energy is aiming to use other regional feedstocks.  When palm oil is processed, Richards explains, there are massive amounts of biomass waste. Scientifically speaking, he adds, the process would work with other types of biomass, but challenges lie with the different proportions of lignin that can vary widely in feedstocks from sugarcane bagasse to woody biomass.  “It will work, but we’ll need to do a little work to get it to maximum efficiency.”

Other recent developments, Richards notes, include processes to purify the used glycerol for reuse and to maintain lower costs.

Moving Forward

Leaf Energy, JBEI and University of Illinois are all using different approaches to create digestible sugars, but all take aim at the same goal. "I think pretreatment is still the most expensive unit operation in biomass-to-biofuel production,” says Feng. “If you could lower the cost, including capital investment and operational costs, I think you could lower the overall cost of production. That’s why it is very important.”

Simmons believes the real challenge in biofuel production lies with inexpensive, sugar production from renewable, lignocellulosic feedstocks. He says if people are able to produce those sugars with a production cost lower or equal to corn and sugarcane-derived sugars, that “all things become possible with those sugars in terms of fuels, chemicals and others.”

Advanced biofuel, such as cellulosic ethanol, could play a big role in the pressing carbon debates, says Richards. He adds that with lower production costs, decreased enzyme costs and better technologies, cellulosic ethanol “has a very, very big task going forward to help reduce carbon.”

Author: Chris Hanson
Staff Writer, Biomass Magazine