By David Brill
There’s a century-old adage coined by the paper industry that claims “you can make anything from lignin except a profit.”
Art Ragauskas has heard this maxim countless times during his career, and it gets him a little riled up every time he hears it. As the UT-ORNL Governor’s Chair for Biorefining, Ragauskas is channeling that ire into proving that the old saying’s time has come and gone.
Lignin and its companion sibling cellulose reside side by side in the cell walls of poplar trees, switchgrass, and the residues of harvested crops—materials known as biomass.
Cellulose, the fairer of the two, is a sugar-based polymer. It can be deconstructed and fermented into bioethanol, a renewable and carbon-neutral transportation fuel. But where you find cellulose, you also find its clingy and historically less useful cellmate, lignin.
Understanding the structure of lignin and devising profitable uses for it are top priorities for Ragauskas and his multidisciplinary research team.
Tear Down the Wall
According to the US Department of Energy, the nation’s farms and forests can produce more than 1.3 billion tons of biomass annually—enough to meet future demand for bio-based fuels without relying on food grains.
Producing ethanol from corn is relatively easy. But extracting the sugars from biomass is much more difficult, partly because of the complicated relationship with lignin.
Unfortunately, the same properties that make lignin valuable to the plant—structural strength, water repellence, and resistance to decay—also hinder efforts to crack the cell walls and release sugars.
Biorefineries currently use a combination of chemicals and heat to minimize the resistance of cellulose. “But the process is far from perfect,” Ragauskas commented. “The pretreatment phase can alter lignin’s structure, and the remaining chemicals and sugar degradation products become contaminants.”
Such contaminants are of little concern for lignin’s low-value uses such as dust control on gravel roads or a resource for biopower. But for higher-value applications, these chemicals must be removed and the structure of the lignin tightly controlled.
One potential solution is to extract the lignin early in the process using organic solvents, including ethanol, and milder temperatures. This method can result in nearly pure lignin, but the cost cannot be justified until profitable uses are identified.
The Right Tools
Efforts to improve biofuel production—including finding new uses for lignin—are engaging scientists and students from multiple disciplines at UT Knoxville, Oak Ridge National Laboratory, and the UT Institute of Agriculture. They represent the vanguard of a relatively new line of research.
“Over the past century, industries that use woody plants have produced some good science,” Ragauskas said. “But in recent years, we’ve made truly significant gains in understanding and controlling the structure of lignin and other plant polymers.”
Continual advancements in technology are enabling scientists to see and model the inner workings of plant cells. ORNL’s Spallation Neutron Source can generate information on the structure of plant cells down to a nanometer. Supercomputers managed by the UT-ORNL Joint Institute for Computational Sciences can use that information to model physical-chemical processes taking place within the cell wall.
Ragauskas is putting these, and a host of other remarkable tools, to good use.
Lower-value uses for lignin have been around for decades. But high-value applications remain elusive, largely because there has been little need or urgency to develop them. That will change rapidly as full-scale biorefineries go on line and stockpiles of lignin begin to grow.
One way to avoid a lignin glut is to reduce its presence inside plants. To this end, Ragauskas and his colleagues at the ORNL-led BioEnergy Science Center have engineered switchgrass with reduced lignin content and an altered cell wall structure that shows a 34 percent increase in sugar yield.
“These improvements can aid in the release of plant sugars and boost the recovery of high-grade lignin,” Ragauskas explained.
Diverting lignin from the waste stream is important, but developing profitable co-products from it will provide biorefiners with an entirely new income stream, “just like crude-oil refining produces a range of co-products, including petrochemicals,” he said. “Many of these chemicals, like lignin, were once regarded as waste. They have since grown into a multibillion-dollar industry.”
With a few years of focused research, Ragauskas anticipates that lignin-based products will replace many of the petroleum-based items. As they do, it will help debunk the old adage and prove once and for all that you can make nearly anything out of lignin—including a handsome profit and a cleaner environment.
Efforts to develop profit-generating uses for lignin have long been frustrated by its complex structure and chemistry. Researchers at UT and ORNL are exploring ways to generate a profit from what is now regarded as waste. Here are some of the more promising ideas:
- Biodegradable grocery bags and food containers
- Feedstock for low-cost carbon fiber
- Green adhesives for wood-composite furniture
- Green industrial chemicals like solvents, lubricants, and detergents
- Natural and nonflammable furniture foams
- Replacement for graphite in lithium-ion batteries
- Resin for 3-D printers
- Sunscreens (lignin absorbs ultraviolet radiation)
- Synthetic fuels