Benchmarking advances in pretreatment technology by performing bench-scale integrated enzymatic hydrolysis and fermentation

Recent advances at NREL in pretreating corn stover for biofuel production were compared to the 2012 benchmark using dilute-acid pretreatment. Three novel categories of pretreated corn stover were tested in a bench scale, integrated process of enzymatic hydrolysis and fermentation. The low severity pretreatment (LSP) used a mild alkaline step followed by mechanical disc refinement. The alkaline pretreatment (ALK) used a strong alkaline step and a hot water wash to remove soluble lignin. The alkali dilute-acid pretreatment (ADA) used both strong alkaline and dilute-acid steps.  Substrates were evaluated for enzymatic hydrolysis performance with a combination of commercial cellulase and hemicellulase enzymes. Fermentation was evaluated with our xylose and arabinose utilizing Zymomonas mobilis strain, 13-H-9-2. These new substrates generally were equal to or outperformed the benchmark, dilute-acid pretreatment, which had enzymatic cellulose and xylan conversions of 77% and 40%, respectively, and an ethanol process yield of 91%. The LSP substrates achieved enzymatic cellulose conversions greater than 85%, xylan conversions over 75%, and ethanol process yields of roughly 90%. The ALK substrates achieved enzymatic cellulose and xylan conversions of about 75%, but with ethanol process yields of about 93%. The ADA substrates had the best overall performance, with cellulose and xylan conversions of around 90% and ethanol process yields of about 90%. In general, recent enzyme development has been focused on cellulase production. This work demonstrates that future pretreatment strategies will likely produce substrates with high xylan content and encourages a renewed focus on advancing hemicellulase enzyme production.