Integrated biorefining of rapeseed straw for ethanol, biogas, and mycoprotein production

Rapeseed straw, a widely available waste stream typically disposed of by field burning, was utilized as a biorefinery feedstock for biofuel and mycoprotein production across three different scenarios. Scenario 1 focused on biomethane production and optimization, while scenarios 2 and 3 aimed at ethanol and mycoprotein production using, an edible ascomycete fungus, Neurospora intermedia in anaerobic and aerobic cultivation, respectively. Hydrolysis residues also underwent anaerobic digestion to boost up the biorefinery. Without the addition of any chemicals, the straw structure was modified by hydrothermal pretreatments at 120–180 °C for 1–3 h. The structural and morphological improvements were followed by scanning electron microscopy, X-ray diffraction, and Brunauer-Emmett-Teller methods. Specific surface area, total pore volume, and average pore width of the straw pretreated at 180 °C for 3 h were approximately 1.5, 3, and 2 times higher than that of the untreated straw, facilitating enzyme permeation. The highest ethanol yield (12.2 g/L) was achieved after pretreatment at 180 °C for 3 h. The highest methane yields were 308 and 365 N mL/g volatile solids for solid and liquid fractions, respectively, after pretreatment at 150 °C for 1 and 3 h. Hydrolysis residues from straw pretreated at 120 °C for 1 h yielded 290 N mL/g volatile solids via anaerobic digestion. In scenario 1, from each tonne of straw pretreated at 150 °C for 1 h, 241.8 m³ methane was produced (272.8 L gasoline-equivalent). In scenario 2, combined ethanol production and biogas from residues pretreated at 120 °C for 1 h enhanced energy yield to 275.2 L gasoline-equivalent. In scenario 3, 191.7 kg mycoprotein and 124.8 m³ methane were produced per tonne of straw pretreated at 150 °C for 1 h. It was concluded that biorefining of rapeseed straw holds significant promise for the simultaneous production of ethanol, biogas, and mycoprotein; however, the structure of untreated straw is highly recalcitrant, making a suitable pretreatment necessary before the bioconversions.