Comparison of seven types of thermo-chemical pretreatments on the structural features and anaerobic digestion of sunflower stalks

Sunflower stalks can be used for the production of methane, but their recalcitrant structure requires the use of thermo-chemical pretreatments. Two thermal (55 and 170 °C) and five thermo-chemical pretreatments (NaOH, H2O2, Ca(OH)2, HCl and FeCl3) were carried out, followed by anaerobic digestion. The highest methane production (259 ± 6 mL CH4 g−1 VS) was achieved after pretreatment at 55 °C with 4% NaOH for 24 h. Acidic pretreatments at 170 °C removed more than 90% of hemicelluloses and uronic acids whereas alkaline and oxidative pretreatments were more effective in dissolving lignin. However, no pretreatment was effective in reducing the crystallinity of cellulose. Methane production rate was positively correlated with the amount of solubilized matter whereas methane potential was negatively correlated with the amount of lignin. Considering that the major challenge is obtaining increased methane potential, alkaline pretreatments can be recommended in order to optimize the anaerobic digestion of lignocellulosic substrates.

► Comparison of seven types of thermo-chemical pretreatment on the structural features of sunflower stalks.
► Cellulose crystallinity, lignin removal and the solubilisation of cellulose, hemicelluloses, uronic acids and proteins were considered.
► Highest methane potential increase was obtained after 55 °C, 24 h, 4% NaOH.
► Methane potential and rate were linked with structural features changes.