Bioconversion of pelletized big bluestem, switchgrass, and low-diversity grass mixtures into sugars and bioethanol

Authors: Dien, Bruce; Mitchell, Robert - Rob; Bowman, Michael; Jin, Virginia; Quarterman, Joshua; Schmer, Marty; SINGH, VIJAY - University Of Illinois; Slininger, Patricia - Pat

Submitted to: Frontiers in Energy Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/27/2018
Publication Date: 12/4/2018
Citation: Dien, B.S., Mitchell, R.B., Bowman, M.J., Jin, V.L., Quarterman, J.C., Schmer, M.R., Singh, V., Slininger, P.J. 2018. Bioconversion of pelletized big bluestem, switchgrass, and low-diversity grass mixtures into sugars and bioethanol. Frontiers in Energy Research. 6:129. https://doi.org/10.3389/fenrg.2018.00129.
DOI: https://doi.org/10.3389/fenrg.2018.00129

Interpretive Summary: Tall native prairie grasses are considered as future bioenergy crops for production of alcohols for transportation fuels. The US government estimates that up to 187 million tons of perennial grasses can be harvested annually from land too poor in quality for production agriculture. Whereas trucks filled with chopped grass or bales can be only filled 50%, pelleted materials significantly increase the amount of plant material that a truck can carry. In this study, ARS and a commercial partner investigated using grasses to make higher density pellets. Twenty tons of each big blue stem, switchgrass, and a low diversity mixture of grasses that seeks to emulate in part a natural pasture were pelletized at a commercial feed operation. We came to four conclusions. Pellets can be conveniently manufactured without the use of an external chemical binder. This is important because bioenergy and hay are different in composition because hay is harvested in spring/summer and bioenergy crops in fall/winter. The pellets bulk density was 407% greater than the beginning material and this is sufficient to allow trucks to be fully loaded. Pelletization required drying the grass, heating it, and subjecting it to high pressure. This process led to a 1- 3% loss of carbohydrates, which warrants further study. However, pellets converted more efficiently than chopped grasses into sugars and ethanol. This led to the central finding that pellets and chopped grasses had the same production yields and that pellets were easier to store and process. Overall pellets are superior to chopped grasses for production of biofuels provided the cost of pelletizing is affordable. This work is of interest to current ethanol producers considering expanding into bioenergy crops and to animal feed producers looking for new sources of biomass.

Technical Abstract: Warm-season perennials are promising candidates as bioenergy crops because of their high productivity and modest required annual inputs. However, low bulk density is problematic from the context of supply logistics and upfront processing at the biorefinery. In this study, pelletizing was evaluated as a densification step for three biomass samples: big bluestem (BBS), switchgrass (SG), and a low diversity mixture of grasses (LDM). Each biomass was successfully pelletized without adding a binder at a commercial feed operation. Pelletizing increased the bulk density by on average 407% and was equally effective on all three biomass samples (528 – 554 kg/m3). Chemical analysis of the samples indicated that glucan and xylan contents were slightly reduced during pelletizing and that theoretical ethanol yields, based upon total carbohydrate contents, were reduced an average of 6.8%. Pellets and milled straws were subsequently pretreated with either liquid hot-water or low-moisture ammonium hydroxide (LMA) and hydrolyzed with cellulases. Pelletizing did not decrease glucose or total sugar yields using either pretreatment, which suggests that pelletizing might have reduced biomass recalcitrance and this compensated for reduced carbohydrate contents. LMA pretreated samples were also hydrolyzed and fermented to ethanol using Scheffersomyces stipits yeast. Hydrolysis recovered 69.7 – 76.8% of the glucose and 66.5 – 73.3% of the xylose across all samples. Recovered sugars were converted to ethanol at 77.7 – 86.7% of theoretical yield. Pelletizing had not discernible effect on final ethanol yields. Therefore, it is concluded that pelletizing can be used to densify perennial grass feedstocks without affecting sugar and ethanol conversion yields.