Breaking down barriers to fuel our world with woody biomass

Scientists have developed new methods that could overcome the obstacles to turning woody biomass into fuel efficiently. 

Historically, the complex compound found in the cell walls of woody plants, which is known as lignin, has prevented this kind of energy conversion, because it blocks access to plant carbohydrates that could be reduced into sugars, before being fermented into biofuels.

However, a research team led by Purdue University has successfully developed a way to remove lignin, creating opportunities for increased production of renewable biofuel from crop waste and biofeedstocks.

The ability to produce more of this type of biofuel would represent a significant breakthrough in more ways than one. Not least, it would lessen the global reliance on fossil fuels, with their negative environmental impact.

Nick Carpita, a Professor at Purdue University’s Department of Botany and Plant Pathology, said new research means lignin may no longer present a problem.

“We have a way of removing it and making useful products from it, as well as getting access to plant carbohydrates for production of biofuels," Professor Carpita said.

The team used a genetically modified poplar tree with an altered lignin structure in their experiments. While lignin is usually made up of three basic building blocks — guaiacyl (G), p-hydroxyl phenol (H) and syringyl (S) — the poplar tree in question contained more than 90 percent S-lignin, which is known to have weaker bonds with plant carbohydrates.

An inexpensive nickel-carbon catalyst was then proven to effectively remove the tree’s lignin without degrading the plant's carbohydrates.

However, according to the researchers, more work needed to be done to overcome additional obstacles besides the lignin, if the carbohydrates required for fuel production are to become more accessible. The compounds that hold plant cells together are particularly problematic, as are their tightly packed cell clusters.

Maureen McCann, Professor of Biological Sciences at Purdue University, said removing lignin did not eliminate all the issues of biomass recalcitrance.

"We needed to look at factors that made woody biomass difficult to degrade beyond lignin, and in its absence," Professor McCann explained.

The Purdue team therefore began exploring ways to break the tightly connected plant cells apart, in order to allow the chemical catalysts used in the biofuel refining process to be effective.

With all the lignin successfully removed from the poplar tree using the nickel-carbon catalyst, the team applied trifluoroacetic acid to loosen the tightly packed crystalline cellulose. This caused it to swell, and therefore provided easy access to the cell’s glucose molecules, which are ideal for fermentation to ethanol.

Rick Meilan, Professor of Molecular Tree Physiology at Purdue University, said we now know how to disassemble the cell walls to produce various products, including transportation fuel.

"What we're doing with poplar can help inform what's being done with other cellulosic feedstocks derived from corn stalk residues, or biomass sorghum and switchgrass," Professor Meilan said.

While the genetically engineered poplar trees cannot legally be grown commercially for the purpose of creating biofuel, the knowledge gained could be used in other crops modified through gene-editing technology.