Lignin recovery from rice straw biorefinery solid waste by soda process with ethylene glycol as co-solvent

Abstract Lignin recovery and valorization is desirable to optimize the process economics of the biorefinery with lignocellulose biomass as a feedstock. The recovery of high purity lignin from the brown solid (BS) residual obtained from a mid-stream waste of the rice straw biorefinery using the soda extraction process by adding ethylene glycol (EG) as a co-solvent is investigated in this study. The best extraction result was obtained with a lignin recovery of 94% and lignin content (purity) of 92% when the reaction time was 300 min at 190 ℃. The addition of EG increased the lignin solubility in the solution so that higher lignin yield and recovery could be obtained than that from the neat Soda process. The kinetic analysis indicated that the delignification reaction was composed of a fast bulk phase and a slow residue phase with a transition time at 60 min, corresponding to different levels of fragmentation, depolymerization and solubilization of lignin. In the fast phase, about 80% lignin can be solubilized in the solution quickly due to the predominant nucleophile attacks on the ether linkages (C-O-C, α-O-4, β-O-4, etc.). In the slow phase, the carbon-carbon (C-C) bonds are broken in the remaining residue lignin.