Bioconversion of industrial hemp to ethanol and methane: The benefits of steam pretreatment and co-production

Several scenarios for ethanol production, methane production (by anaerobic digestion) and co-production of these, using autumn harvested hemp as substrate, were investigated and compared in terms of gross energy output. Steam pretreatment improved the methane production rate compared with mechanical grinding. The methane yield of steam pretreated stems was similar both with and without pre-hydrolysis with cellulolytic enzymes. Co-production of ethanol and methane from steam pretreated stems gave a high yield of transportation fuel, 11.1–11.7 MJ/kg processed stem dry matter (DM); more than twice that of ethanol production alone from hexoses, 4.4–5.1 MJ/kg processed stem DM. Co-production from the whole hemp plant would give 2600–3000 L ethanol and 2800–2900 m3 methane, in total 171–180 GJ per 10,000 m2 of agricultural land, based on a biomass yield of 16 Mg DM. Of this, the yeast and enzymes from ethanol production were estimated to contribute 700 m3 (27 GJ) of methane.