In this work, we examined the behavior of feedstock blends and the effect of a specific feedstock densification strategy (pelleting) on the release and yield of structural carbohydrates in a laboratory-scale dilute acid pretreatment (PT) and enzymatic hydrolysis (EH) assay. We report overall carbohydrate release and yield from the two-stage PT-EH assay for five single feedstocks (two corn stovers, miscanthus, switchgrass, and hybrid poplar) and three feedstock blends (corn stover-switchgrass, corn stover-switchgrass-miscanthus, and corn stover-switchgrass-hybrid poplar). We first examined the experimental results over time to establish the robustness of the PT-EH assay, which limits the precision of the experimental results. The use of two different control samples in the assay enabled us to identify (and correct for) a small bias in the EH portion of the combined assay for some runs. We then examined the effect of variable pretreatment reaction conditions (residence time, acid loading, and reactor temperature) on the conversion of a single feedstock (single-pass corn stover, CS-SP) in order to establish the range of pretreatment reaction conditions likely to provide optimal conversion data. Finally, we applied the assay to the 16 materials (8 feedstocks in 2 formats, loose and pelleted) over a more limited range of pretreatment experimental conditions. The four herbaceous feedstocks behaved similarly, while the hybrid poplar feedstock required higher pretreatment temperatures for optimal results. As expected, the yield data for three blended feedstocks were the average of the yield data for the individual feedstocks. The pelleting process appears to provide a slightly positive effect on overall total sugar yield.

Environmental factors like drought impact the quality of biomass entering a bioconversion process. Drought often reduces the sugar content in lignocellulosic biomass, which could have economic impacts, particularly when compounded with losses in dry biomass yield; however, the effects on conversion efficiency are not completely understood. This study investigated how drought may impact biomass composition and sugar yields from dilute-acid pretreatment and enzymatic hydrolysis of Miscanthus, a tall fescue mixture, and switchgrass from Nebraska, Missouri, and Oklahoma, respectively, grown as part of Regional Feedstock Partnership field trials. Samples were grown and harvested in 2010 during non-drought conditions and in 2012 during extreme drought conditions. Non-structural glucose and proline were significantly greater in 2012 compared with 2010 for Miscanthus, which suggests drought stress occurred. Structural glucan and xylan were significantly decreased in 2012 for Miscanthus; however, reactivity and sugar yields from dilute-acid pretreatment and enzymatic hydrolysis were significantly greater in 2012 compared with 2010, suggesting that although structural sugars may decrease during drought conditions, sugar yields and reactivity may increase. For the tall fescue mixture, proline was greater, and structural sugars were lower in 2012, indicating drought stress, but minimal differences were observed in the conversion experiments. Few differences were observed for switchgrass composition and reactivity between years. The observed patterns are likely because of site-specific climatic conditions combined with the tolerance each species may have to drought. As drought occurrence and severity have increased, it is necessary to understand drought impacts to mitigate risks to future bioenergy industry growth.

The thermal properties of biomass over a range of pyrolysis temperatures have been measured using a Transient Plane Source (TPS) instrument and a differential thermal analyzer (DTA). In this study, thermal property measurements were made on six softwoods: subalpine fir, Douglas fir, Engelmann spruce, loblolly pine, lodgepole pine, and ponderosa pine. Results from this method show that the average thermal conductivity for these softwoods decreases by almost a factor of 3, from 0.198 to 0.091 W/(m K), as the wood goes from ambient conditions to a pyrolysis temperature of 453 °C. Over the same temperature range the average thermal diffusivity increases from 0.313 to 0.427 mm 2 /s, and the specific heat decreases from 1.58 to 0.93 kJ/(kg K). Investigation of the anisotropic nature of heat transport through lignocellulosic biomass found that heat transport is generally three to four times faster along the grain of the wood than across the wood pores, and studies on the rate at which thermal conductivity and diffusivity change with temperature revealed only a slight increase over 50−300 °C. It has also been shown that the thermal conductivity of a material correlates strongly with the density throughout the pyrolysis regime. This correlation with density has been shown before for the moisture content of green wood but not through the range of material changes associated with pyrolysis. The direct measurement of these anisotropic thermal properties has the ability to enhance modeling of lignocellulosic biomass pyrolysis and provide new insight into heat transfer through a naturally occurring lignocellulosic material.

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