Interview story by Jessie Waldheim
HOUSTON (ICIS)--Researchers at the US Renewable Energy Laboratory (NREL) are hoping organisms in the environment can help chart a path to finding a better use for lignin, a leftover in cellulosic ethanol production that has little use beyond being burned as a fuel.
Plant biomass consists of two kinds of polymers – lignin and polysaccharides. Industry has developed many ways to breakdown the polysaccharides and transform the resulting sugars into alcohols, fuels and chemicals.
“Lignin on the other hand, is a very different problem than sugars,” NREL senior engineer Gregg Beckham said.
Lignin, a complex and random structure of aromatic molecules, is used by plants for rigidity, water transportation and, due to its hardiness, as a barrier to pathogens.
Lignin's randomness and complexity makes it difficult to selectively be broken down by a single enzyme or chemical. It also makes lignin difficult to process commercially into useful molecules. Lignin is usually burned for process heat or energy.
But in nature, some rot fungi can break down the lignin into its individual aromatic molecules, which can be used by some bacteria as carbon or energy. The process has been studied in the environmental remediation field for many years.
This natural process could eventually point the way towards an economically viable route to using lignin as a feedstock for fuels, plastics and chemicals, an NREL study co-authored by Beckham said.
“We were inspired by this natural process and attempted to harness this approach for lignin utilisation in the biofuels industry,” Beckham said.
The researchers subjected corn stover, the plant material leftover after a corn harvest, to a low-severity alkaline pre-treatment step, which is common in the pulp and paper industry. This produced an opaque lignin-rich mixture called black liquor.
The mixture was fed to a well-known soil bacteria which has several metabolic pathways to process lignin-derived aromatic molecules. When starved of nitrogen, the organism built up a reserve of medium chain-length polyhydroxyalkanoates (PHA) which were harvested.
PHAs can be used directly as a bioplastic, can be depolymerised to hydroxyl-acids then into specialty chemicals or can be converted through a catalytic approach into hydrocarbons in the jet- or diesel-fuel range, Beckham explained.
Also importantly, the process can be tuned in several ways – by engineering biomass that contains less complex lignin, by engineering biological pathways to produce specific molecules or by altering the catalysts used to upgrade those molecules.
“The sky’s the limit for all the knobs we have to turn on using this process concept for lignin valorisation,” Beckham said.
Although the study was able to demonstrate a conceptual approach for processing lignin, the research is still in its very early stages.
“This is not a process that’s ready for industrial application tomorrow in an economically viable fashion,” Beckham said.
However, the pre-treatment is scalable and the fermentation, which the NREL team conducted on a 10-litre scale, could be readily scaled up using approaches for fermentation of other substances like sugars. But working with the biological pathways still presents some inherent challenges.
“But we’re working very hard on that,” Beckham said.