Biomethane production through anaerobic digestion of sunflower stalks was improved by the hydrothermal and isopropanol-based organosolv pretreatments. The pretreatments were conducted at different temperatures (140, 160, 180, and 200°C) for 30 and 60min with/without addition of 1% sulfuric acid. The pretreatment of stalks with 50% (v/v) aqueous isopropanol containing 1% w/w (based on dry stalks) sulfuric acid at 160°C resulted in the highest lignin removal. Methane production yield of the pretreated substrate was improved by 45-124% compare to 124mL CH4/g VS obtained from the digestion of untreated stalks. In the best case, hydrothermal pretreatment at 180°C for 60min and organosolv pretreatment at 160°C for 30min with 1% H2SO4 followed by 45 days anaerobic digestion resulted in 234 and 278mL CH4/g VS, respectively. Structural analyses of the stalks indicated that lignin removal, crystallinity reduction, and structural modifications by the pretreatments were the main reasons for the improved biogas production.
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