A study by a team from the National Renewable Energy Laboratory (NREL) and Oak Ridge National Laboratory (ORNL) has found that the amount of sugars contained within cellulosic biomass from bioenergy crops that can be converted into fuels is of equal economic importance to the yield. The findings are published in an open-access paper in the journal Biofuels, Bioproducts & Biorefining.

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.

The amount of fuel produced per acre each year and the minimum fuel selling price (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.

—Renee 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.

—Brian Davison, an ORNL biochemical engineer and a lead on the project

Andrew Bartling from NREL, who conducted the techno-economic analysis of the poplar samples, noted that there was no correlation between composition and size—hinting that selective breeding could maximize both of these attributes simultaneously without trading one for the other.

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 US Department of Energy Bioenergy Research Centers supported by DOE’s Office of Science.

Resources

• Happs, R.M., Bartling, A.W., Doeppke, C., Harman‐Ware, A.E., Clark, R., Webb, E.G., Biddy, M.J., Chen, J.‐G., Tuskan, G.A., Davis, M.F., Muchero, W. and Davison, B.H. (2020), “Economic impact of yield and composition variation in bioenergy crops: Populus trichocarpa.” Biofuels, Bioprod. Bioref.. doi: 10.1002/bbb.2148