Efficient conversion of sweet sorghum stalks to biogas and ethanol using organosolv pretreatment

• Bioconversion of sweet sorghum stalks was greatly improved by organosolv pretreatment.
• 92% of theoretical yield in biogas production (270% improvement) was obtained.
• High sugar yield of 83% via treatment with 50% ethanol, 1% sulfuric acid, at 140 °C was achieved.
• Adding acid can improve hydrolysis and ethanol production yield but not biogas yield.
• Lignin and xylan contents and cellulose crystallinity were significantly reduced.

Improvement of bioconversion of sweet sorghum stalks to ethanol and biogas by organosolv pretreatment was investigated. The pretreatments were carried out at 100–160 °C in the presence or absence of sulfuric acid (as a catalyst) using ethanol (as an organic solvent) for 30 min. After evaporation of the solvent (ethanol), the pretreated solids (bagasse) and the liquid fraction (liquor) were subjected to anaerobic digestion for 50 days. The greatest biomethane yield, achieved from the mixture of liquor and bagasse, was 278 ml CH4/g VS, corresponding to 92% of theoretical yield and showing 270% improvement compared to the methane yield obtained from untreated stalks. The enzymatic hydrolysis of the untreated stalk and the obtained bagasse was conducted at 45 °C for 96 h using 20 FPU cellulase and 50 IU β-glucosidase per gram of the substrate. Hydrolysis of the untreated stalk resulted in a low sugar yield (34.1% of the theoretical yield). The highest sugar yield (77% of the theoretical yield) was obtained after pretreatment with 50% ethanol and 1% sulfuric acid at 140 °C. Accordingly, ethanol yield from the respective organosolv pretreated material showed a two-fold improvement compared to the untreated substrate. Ethanol production from liquor ended with a maximum yield of 65.7% after pretreatment with 50% ethanol at 100 °C in the absence of acid. The compositional and FTIR analyses showed that organosolv pretreatment at elevated temperatures significantly reduced the lignin and xylan contents and the cellulose crystallinity.

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