Green processing of napier grass for generation of biofuel and biobased products

Abstract

Napier grass, Pennisetum purpureum, is a high yielding perennial C-4 grass that has been naturalized in Hawaiʻi and resembles the former staple crop of the state, sugarcane. Because of its high moisture content, Napier grass presents a unique and relatively unexplored opportunity for fractionation into solid and liquid components via green processing. The resulting clean, solid fibers can serve as a substrate for (advanced) biofuel production, while the nutrient-rich liquids (juice) can serve as a supplemental additive for diverse microbial co-products generation. The recalcitrant lignocellulosic fibers of Napier grass contain structural carbohydrates which require pretreatment and enzymatic saccharification to release monomeric sugars for fermentation into biofuels. In this study, the effects of dilute acid pretreatment on structural carbohydrate release of Napier grass were investigated for the first time. The optimal conditions for green processed Napier grass were determined to be 5% (w/w) sulfuric acid, 120°C, 45 minutes; producing near theoretical xylose yields and ~85% of the glucose from hemicellulose and cellulose, respectively. Preliminary trials of high value co-product generation were successful in cultivating protein-rich fungal biomass, Rhizopus oligosporus, on crude Napier grass juice. In determining the applicability of green processing for future biorefineries, an important and often overlooked consideration of the incoming feedstock is age. As Napier grass matures, significant changes may occur in its biochemical composition, subsequently affecting fractionation and biofuel and co-product generation. The composition of Napier grass was examined for the ages of 2, 4, 6, and 8 months old. Ash and lignin constituents, in particular, were found to be dependent on age and both constituents increased with feedstock age. Changes in the juice characteristics were also found to correlate with Napier grass maturation. Overall, the compositional data of Napier grass at different stages of growth represented the first of its kind for bioenergy crops grown in the (sub)tropical climate of Hawaiʻi, which has gained considerable attention for the development of biomass-to-biofuel strategies and technologies. Exploratory economic analyses of the results in this study however, suggest that future work is necessary. Ultimately, because green processing represents a biosystems engineering approach, it can be adapted and applied to a multitude of disciplines and biofuel platforms.