NREL Finds Easier Ways to Deconstruct Biomass
(National Renewable Energy Laboratory/Ethanol Producer Magazine) If there’s an easier, more efficient method, science will find a way. That’s certainly the case in producing cellulosic biofuels, which, at least for now, requires a two-step process to free the sugars trapped in plant matter and convert them into something else. Scientists at the U.S. Department of Energy’s National Renewable Energy Laboratory think that’s one step too many.
The process normally used to deconstruct biomass, called simultaneous saccharification and fermentation, works like this: The feedstock is pretreated with heat and chemicals to separate and remove lignin and/or hemicellulose components. The remaining biomass is deconstructed by enzymes and the resulting sugars are fed to a microorganism that converts them to ethanol (or another fuel or chemical).
“We are not doing this,” said Yannick Bomble, also a senior scientist in NREL’s Biosciences Center. “We have a microorganism that does it all.”
In a paper published last year, Bomble and Brunecky, along with NREL Senior Research Fellow Michael Himmel, detailed their work with Caldicellulosiruptor bescii, a bacteria that thrives in high temperatures. The microorganism produces an enzyme called CelA, which deconstructs biomass to sugars. The microorganism also converts these sugars into ethanol. The same can be done with another thermophilic microbe Bomble works with, Clostridium thermocellum, which can be found anywhere from soil and compost to herbivores and hot springs. It deconstructs biomass and then feeds on the resulting sugars to make advanced biofuels such as isobutanol or n-butanol.
“This is, in essence, a one-pot process,” said Bomble, who studies bacteria and how they break down biomass. The process, called consolidated bioprocessing, bypasses the need for pretreatment or added enzymes. “Pretreatment is expensive. If you can get better and better microbes and enzymes to bypass pretreatment, that’s a game changer.”
Despite its successes, CelA does run into barriers. Lignin, which acts like a glue that holds plant material together, poses a big problem.
NREL researchers are looking into other ways to overcome plant defenses, including using an enzyme called AcCel5A. This enzyme, which comes from the hot-spring bacterium Acidothermus cellulolyticus, weakens biomass by inserting a gene into the plant’s genome and then burrowing into its developing cell wall, creating nicks and voids that make biomass deconstruction easier. Scientists who have tried other enzymes found their experiments stunted the growth of plants, but AcCel5A helped NREL researchers avoid that problem while maintaining the amount of important sugars yielded by the plant.
The scientists have had particular success in using AcCel5A with Arabidopsis thaliana, a small flowering plant regarded as a weed because of how quickly it grows.
NREL researchers are now working to introduce AcCel5A into poplar.
Part of the problem, he (NREL Senior Research Fellow Michael Himmel) said, is bad timing; rising gasoline prices initially accelerated interest in bioethanol, but the cost of gasoline has since fallen. Commercial-scale biorefinery companies have also struggled to find the capital they need to upgrade from their initial process designs. “In some cases, they’re using technology that we produced here decades ago, but now it’s old technology,” said Himmel, a 39-year NREL veteran. A common pretreatment process in use, for example, requires high temperatures and the use of corrosive mineral acids, which mandates expensive pretreatment reactors.
A team led by Melvin Tucker, a biochemist and senior scientist who has worked alongside Himmel for 35 years, developed a better method. He dubbed it deacetylation and mechanical refining (DMR), and it works at a lower temperature (below boiling) and with dilute alkali instead of acid to soften the biomass. A mechanical treatment follows the chemical treatment, and after adding enzymes, yields sugars and lignin.
Learn more about NREL’s enzyme and microbial research.
News Release: Enzyme’s Worth to Biofuels Shown in Latest NREL Research (National Renewable Energy Laboratory)
The Multi Domain Caldicellulosiruptor bescii CelA Cellulase Excels at the Hydrolysis of Crystalline Cellulose (Nature Scientific Reports)