Miscanthus bioprocessing using HNO3-pretreatment to improve productivity and quality of bioethanol and downstream ethylene

Miscanthus, which can grow on marginal lands and capture CO₂, has great potential as an industrial crop. A complete cycle to convert Miscanthus sacchariflorus, grown in Western Siberia (Russia), into bioethanol has successfully been implemented on a pilot scale and employed in downstream ethylene production for the first time. The Miscanthus bioprocessing technology involved mechanical comminution; HNO₃-pretreatment; hydrolysis with commercial enzyme preparations CelloLux-A and BrewZyme BGX with cellulase–glucanase–xylanase action; pre-saccharification with substrate feed; simultaneous saccharification and fermentation with delayed inoculation (dSSF) with non-GMO Saccharomyces cerevisiae Y-1693 supplemented with or without (NH₄)₂SO₄, KH₂PO₄ and yeast extract; rectification; and alumina-catalyzed dehydration to ethylene. Here we discussed pulp reactivity, yield and composition of Miscanthus products at each step (including ethylene), and compared the quality of the bioethanol obtained in the study with published data. The pulp feeding from 80 to 100 g/L resulted in a bioethanol concentration of ~40 g/L. It is the HNO₃ pretreatment that provided pure bioethanol with an impurity content as low as 6.5 g/L. Bioethanol (92%-w) contained ~0.85 g/L impurities, mainly n-propanol and isobutanol; the latter weakly influenced the catalyst activity in ethylene production. The supplement slightly increased the yields of bioethanol and fusel alcohols, thereby reduced the yield of byproducts and improved the ethylene quality. The achieved yields of bioethanol and ethylene were 260 L/ton and 115 kg/ton Miscanthus, respectively. The expected yield of absolute ethylene under pilot-scale conditions can be 122 kg/ton Miscanthus.