Enhanced biogas production from sunflower stalks using hydrothermal and organosolv pretreatment

•Sunflower stalks was used for biogas production using anaerobic digestion.•Isopropanol-based organosolv was studied compared to hydrothermal pretreatment.•Crystalinity and surface layer of stalks were influenced by pretreatment.•Yield of 278 mL CH4/gVS by124% enhancement over raw substrate was obtained.

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 60 min 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 124 mL CH4/g VS obtained from the digestion of untreated stalks. In the best case, hydrothermal pretreatment at 180 °C for 60 min and organosolv pretreatment at 160 °C for 30 min with 1% H2SO4 followed by 45 days anaerobic digestion resulted in 234 and 278 mL 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.