National laboratories point to sugars as key factor in ideal feedstock

Popular wisdom holds tall, fast-growing trees are best for biomass, but new research by two U.S. Department of Energy national laboratories reveals that is only part of the equation.

Of equal economic importance, according to scientists from the National Renewable Energy Laboratory and Oak Ridge National Laboratory, is the amount of sugars contained within the cellulosic biomass that can be converted into fuels.

In the production of biofuels, feedstocks represent a significant expense with costs incurred for planting, harvesting and transporting the trees. Growers typically look at how many trees they can plant per acre with little consideration given to how much fuel those trees will produce or the quality of that fuel.

The researchers analyzed 900 samples of black cottonwood trees grown in Oregon to determine how variations in their size and composition affect feedstock quality and biorefinery economics.

“Those differences do make an economic difference,” said ORNL’s Brian Davison, a biochemical engineer and a lead on the project.

The findings in a paper titled, “Economic Impact of Yield and Composition Variation in Bioenergy Crops: Populus trichocarpa,” were published in the journal Biofuels, Bioproducts & Biorefining. In addition to Davison, the other co-authors are Renee Happs, Andrew Bartling, Crissa Doeppke, Anne Harman-Ware, Mary Biddy and Mark Davis from NREL; and Erin Webb, Robin Clark, Jin-Gui Chen, Gerald Tuskan and Wellington Muchero from ORNL.

The amount of fuel produced per acre each year and the minimum fuel selling price, or MFSP, are most strongly connected to the size of a tree. Since a farmer would only plant the biggest and fastest growing trees, the researchers examined those and found that the size and sugar content in those trees were of nearly identical importance to the MFSP.

“Over the long run in the case of a biorefinery, that adds up to millions of dollars by taking the genotypes that give you the most sugar,” said Happs, an analytical chemist at NREL and lead author of the paper.

The scientists chose the black cottonwood, a type of poplar tree, to study because of its fast growth and its prevalence across North America. The tree can be ready to harvest after about seven years. In addition to the sugar content, the researchers also analyzed the amount of lignin, which forms rigid cell walls and bark and is difficult to break down. The analyses informed a techno-economic analysis using the black cottonwood as a feedstock. 

The trees with the best attributes can be cloned for rapid propagation. “We can also breed for the key genes to increase both sugar content and growth,” Davison said.

“We observed that there was no correlation between composition and size – hinting that we could selectively breed for maximizing both of these attributes simultaneously without trading one for the other,” said Bartling, who conducted the techno-economic analysis of the poplar samples.

Genetic engineering may allow for further improvement of sugar content. The researchers used computer modeling to evaluate a hypothetical scenario involving two clones in which the sugar was increased by 5% among a smaller set of the trees. The additional sugar content reduced the MFSP, highlighting how composition can begin to compensate for economic losses in smaller trees – perhaps in a region where poor conditions may not allow them to grow as large.

The research was funded by the BioEnergy Science Center and the Center for Bioenergy Innovation, which are U.S. Department of Energy Bioenergy Research Centers supported by DOE’s Office of Science.

UT-Battelle manages ORNL for DOE’s Office of Science. The single largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit https://www.energy.gov/science/.--By Wayne Hicks/NREL