Plant deconstruction for biofuels

By weight, cellulose in plants is the most widely available form of biomass on the planet -- and one of our best bests for developing sustainable and renewable biofuel.

In Part I of "Breaking Down the Wall," we looked at how scientists are using nature’s tools to undermine plants' natural fortresses, breaking down their lignocellulose and freeing the coveted sugars.

This article looks at some leading non-biological tools -- three chemical and engineering strategies -- which scientists are also using to break down biomass and make biofuel production more cost-effective.

Each plant is a warehouse of energy stored in the form of glucose.  

Unfortunately, getting at those stored sugars is a mighty challenge because of their difficult to access (recalcitrant) form in plants.

Plants have had more than 450 million years to evolve fortress-like walls of lignocellulose to hold themselves upright, move water, and protect themselves from disease and predators.  (These predators include humans, who may reside atop of the world’s plant and animal kingdom but lack the ability to digest cellulose.)

Fashioning fuel from plants requires deconstruction to sever the chemical bonds that make up a plant's fundamental structure and break up the cellulose and hemicellulose into free soluble sugars.

It’s the single most expensive step, accounting for more than half the costs, in making liquid fuels from lignocellulose, says Blake Simmons, Ph.D., Chief Science and Technology Officer and Vice-President of Deconstruction at the Joint BioEnergy Institute in Emeryville, Calif.

Dr. Blake Simmons and colleague of JBEI/Credit: JBEI

Today most depolymerization -- breaking plant cell wall components into their respective single units -- uses a combination of techniques. There's heat and chemistry in the form of acids or bases to breach the cell walls; enzymes to break up the more recalcitrant polysaccharides into free sugars; and microorganisms to ferment the sugars released into an alcohol-based biofuel or other products.

The latter is arguably the most straightforward part. Most of the methods in fermentation come directly from more than a millennium of experience in the brewing industry, in which the sugars in grains or grapes are fermented to create the alcohol in beer, wine and whiskeys.

But deconstruction is an expensive process:  The price of enzymes for biomass deconstruction has made it challenging to develop cost-effective cellulosic ethanol.  However, chemical and engineering techniques offer alternative -- and perhaps less expensive -- ways to break down biomass and produce fuels and other chemicals. These techniques can also be used to dramatically reduce the amount of costly enzymes needed to achieve the high yields vital to economic success.

Purely Chemical: Catalytic Pyrolysis

Heat – or burning – is possibly the oldest way to get energy from biomass, and was used to power steam locomotives, autos and ships in the not too distant past. It’s still widely used to generate heat and electricity, but it doesn’t work as a transportation fuel in today’s individualized vehicles.

However, Dr. George Huber at the University of Madison, Wisconsin and his fellow researchers have been using heat in the absence of air at temperatures up to 600 C  to decompose or pyrolysize biomass,  along with chemical catalysts to convert biomass into aromatic chemicals -- the precursors to a wide range of plastics that today are produced from non-renewable petroleum resources.

Dr. George Huber on a visit to EBI Berkeley/Credit: Saul Bromberger and Sandra Hoover

Huber, a professor of chemical and biological engineering, is one of the world’s leading experts in catalytic pyrolysis, a purely chemical process with no enzymes and no fermentation involved. It works by first feeding the plant material into a reactor, where it is heated and turned into vapor. The vapor is then blended with his patented zeolite catalyst -- which is mainly composed of cheap and readily available silica and aluminum --  that turns the gaseous material into aromatics. These are hydrocarbons typically found in gasoline such as benzene, toluene, and mixed xylenes, and are sold in a multi-billion dollar world market. (Please see Bioenergy Connection's spring 2013 profile "Dr. George Huber: On the Road to Grassoline").

In 2008 Huber received a multi-million dollar grant from the military for research into biofuels. With associates, he co-founded a startup tech company called Anellotech and has patented the catalyst in his process. They are currently planning for a pilot plant in Pearl River N.Y., to demonstrate this technology on a larger scale.

With pyrolysis alone, one can make a low-quality bio-oil, Huber said. By adding a catalyst, he said, “we produce higher value commodity chemicals and petrochemicals directly.”

Eventually, the process can also produce liquid biofuels, Huber said: “We’re targeting making products for the chemical industry that are economically competitive with aromatics made from crude oil today. “

“If the aromatics are cheap enough from chemicals, eventually they’ll be cheap enough for fuel-- but we need to show it works on a larger and larger scale," said Huber.  Within the next five to ten years, Huber and colleagues hope this technology could produce economically competitive biofuel that might replace a substantial fraction of fossil-fuel gasoline.

Better Pretreatment Technology = Cheaper Biofuel  

Since deconstruction remains the bottleneck to biofuel production, researchers are consistently looking for –and finding – better, faster and less expensive pretreatment methods. The goal:  finding a way to break up lignocellulose that requires fewer enzymes, thus helping to make biomass-based fuel commercially viable.

Recently, researchers at the University of California, Riverside (UCR) announced a new pretreatment technology that may dramatically reduce the costs of biofuels production by cutting the amount of expensive enzymes needed by 90 percent and increasing sugar yields to 95 percent of the maximum possible, making higher fuel yields attainable.  

As a plus, the novel pretreatment also dissolves and recovers about 90 percent of the stubborn lignin in biomass, presenting the opportunity to make valuable chemical products and fuels from at least some of the lignin. Currently, most plans call for burning lignin. (One old saying in bioenergy circles is that you can make anything out of lignin except money.)

The new pretreatment technology is called Co-Solvent Enhanced Lignocellulosic Fractionation, or CELF.  It works by adding THF (tetrahydrofuran) in solution with very dilute sulfuric acid (one half of one percent)  dissolved in water to lignocellulosic biomass, and holding the resulting mixture for about 25 minutes at  150 C, said Wyman. The output from CELF is a solid that contains most of the cellulose from the original biomass, along with a liquid that contains about 90% of the lignin and most of the sugars found in hemicellulose.

Dr. Charles Wyman, UC Riverside/Credit: UCR

The technology has the potential to significantly cut biofuel costs by reducing enzyme costs from about $1 per gallon of ethanol to about 10 cents or less, said Charles Wyman, Ph.D., professor of Chemical and Environmental Engineering and Ford Motor Company Chair in Environmental Engineering at UCR.

What makes this pretreatment unique is that it’s a triple whammy: it has the novel ability to remove and recover lignin along with most of the sugars in hemicellulose and requires far fewer enzymes to break down the cellulose left in CELF solids. Most other techniques only posses one or two of these attributes, he said.

The development of CELF (in conjunction with enzymatic hydrolysis) is supported through a partnership of the UCR Center for Environmental Research and Technology of the Bourns College of Engineering in the multi-institutional Bioenergy Science Center (BESC), which is funded by the DOE Office of Science.  Projecting exactly when CELF will  be in commercial use is “premature: It certainly looks promising, possibly within five years,“ said Wyman.  But, he adds, the devil is in the details.:“ A lot of work has to be done in defining an overall conversion process built around CELF and estimating capital and operating costs to develop cost-effective routes to employ it."

Mimicking Enzymes: Synthetic Catalysts

Synthetic catalysts offer another non-biological but potentially green route to depolymerization, as shown by the work of Dr. Alexander Katz and his laboratory at University of California at Berkeley.

Dr. Alexander Katz (middle) of UC Berkeley/Credit: EBI

Katz is currently an Associate Professor of Chemical and Biomolecular engineering at UC Berkeley. Contrary to the conventionally held belief that only mineral acids are strong enough chemical catalysts to depolymerize cellulose to glucose, Katz and his colleagues created a blueprint for using weak-acid sites -- prevalent on the surface of mesoporous carbon nanoparticle catalysts --  in order to accomplish this The nanoparticles can trigger the breakdown of polysaccharides, all without a single enzyme.

It is a chemical process, “but borrowing concepts from biological catalysts,” said Katz. “As an alternative, we try to, in a crude fashion, emulate the processes by which enzymes depolymerize cellulose.”

Katz was inspired by how enzymes work in nature’s biological processes, using a binding domain (the part of an enzyme that binds to a specific atom or molecule) and a catalytic domain (the part that interacts with another substance to trigger a reaction.) Recently he and his research group recently showed how the carbon surface can also act as a binding and catalytic domain – albeit in a synthetic molecular analog -- to turn "switches" on and off within the catalyst.  His group was the first to demonstrate the binding of long-chain polysaccharides that make up cellulose to the surface of mesoporous carbon nanoparticles – where the strands are thought to be in a highly strained environment that facilitates their breakdown, or depolymerization.

Nature’s catalysts -- enzymes -- switch site reactivity to fruitful conversions while excluding others entirely.  “Such switching at an active metal site permits enzymes to function in water, for example, among many other feats that include being the world’s fastest and most selective catalysts, which are used to sustain life, “ he said.  “Now, we are able to achieve some of this switching in man-made catalysts, [an accomplishment] brought about by my research group by developing a fundamental understanding of the attributes necessary to tune reactivity on surfaces.”

The discovery, researchers said, has potentially profound implications for chemical conversions involving all components of biomass. Among other things, it will help produce better catalysts that use less energy and produce fewer wasteful by-products for the greener and cheaper production of biofuels.

Katz says his research focuses on the basics of creating biofuel. “Our work is upstream—the equivalent of getting the oil out of the ground,” Katz said. “When I see a field of grass, I see glucose: How do I get the glucose out of the grass like that elephant at the zoo? Our goal is production of a concentrated glucose stream from that grass, which has fuel value.”

It takes a combo
But neither biology nor chemistry alone have the answers to a renewable and economically viable bio-fuel – at least today, said several scientists.

Dr. Patricia Bubner of UCB/Credit: EBI

Patrician Bubner, Ph.D, a researcher in the Somerville Group, is a chemist and a biochemical engineer with credentials in both camps. She thinks it will take a combination of both chemical and biological methods. At the moment, both have advantages and both have drawbacks. And both have the same basic problem: they are still too expensive. “There is still no perfect way for anything that is cost effective or has the conversion rate we are looking at,” she said.

“Enzymes have so many advantages, “ she said. Biological processes offer a greener, less toxic approach using nature’s methods, which work under lower temperatures and milder conditions. “In the spirit of green chemistry, we would like to generate biofuels without strong acids or organic solvents that have limited recyclability. That’s the appeal of enzymes.”

But enzymes are costly to produce, and recycling them is difficult. And since no one enzyme works on all plants or their chemical bonds, many different types of enzymes are necessary.

And biotechnology still requires chemistry and engineering: a pretreatment with heat, acids or other chemicals to break down the lignocellulose to make the polysaccharides more accessible for the enzymes. Chemical depolymerization might work faster, but it generally produces toxic byproducts and requires more energy.

So far, the cost of any combined process is generally too high to compete on the market with oil without subsidies.

Each of the combinations of pretreatment, hydrolysis and fermentation have drawbacks, Bubner says. "For example, enzymatic conversion are safer and more environmentally friendly, but enzymes are not as efficient in hydrolysis as acid. But acid hydrolysis produces inhibitors that might make fermentation less efficient, and the overall process cost explodes. Or, the most efficient pretreatment of the biomass might produce inhibitors for hydrolysis/ fermentation, or you might lose a lot of sugar during pretreatment. It is always a tradeoff.”

The state of science is about neck and neck between purely chemical and purely biological, she said. Today, these are two separate lines of research with little cross talk between them to date. Worldwide, chemists don’t have a good idea of what biologists are doing, and vice verse, says Bubner.

“EBI changed that to some extent, which is why EBI is so important. People are communicating and working together across disciplines to solve these big, big problems.”

Meanwhile, researchers in biofuel are tackling these challenges from many different vantage points. And that’s a good thing.
 “It’s not a matter of which biofuels technology ‘wins,'” said Charles Wyman of UCR. "The key is to get one or more biofuels that can be produced at a low enough price to reduce our dependence on fossil fuels."


Ron Kolb also reported on Dr. Alexander Katz's work for this piece.



Bioenergy Connection welcomes comments on content we publish. While we encourage differing points of view,comments should move the conversation forward constructively. We reserve the right to restrict comments that contain inappropriate content, such as offensive language, verbal abuse, etc.

Post a Comment