Kinetics of SO2–ethanol–water (SEW) fractionation of hardwood and softwood biomass

• Softwood and hardwood biomass is fractionated by SO2–ethanol–water technology.
• Hemicellulose dissolution rate is similar for softwood and hardwood.
• Biomass (branches etc.) fractionation is compared with stem wood fractionation.
• Softwood biomass delignification is inferior likely due to high amount of bark.

SO2–ethanol–water (SEW) fractionation of forest residues (tree tops, stumps, branches) was investigated to demonstrate the potential of this method for forest biorefineries. The effect of fractionation time on dissolution of wood components was studied. Total mass balances of fractionation show that lignin and hemicelluloses are rapidly dissolved in the spent fractionation liquor whereas cellulose is fully preserved in the solid residue throughout the fractionation treatment. Within 20 min treatment at 150 °C (SO2:EtOH:H2O = 12:43.5:44.5, by weight, L:W ratio 6 L kg−1), 89% of hardwood lignin and 74% of hemicelluloses are dissolved. The corresponding values for softwood biomass are 64% and 74%, respectively, indicating slower delignification but equal hemicellulose removal. Additionally, sulfur content of the feedstocks, solid fractionation residues and spent liquors were analyzed to determine the degree of lignin sulfonation. The obtained results are compared with the stem wood fractionation results.