Effect of torrefaction pretreatment on the selectivity improvement of lignin pyrolysis products

Pre-regulating the molecular composition and structural characteristics of lignin before pyrolysis is of great significance for the lignin valorization. In this work, torrefaction pretreatment of softwood kraft lignin (SKL) is carried out at different temperatures, and the characterization results show that torrefaction pretreatment is beneficial for removing oxygen element from SKL, which increases the high heating value of torrefied lignins (TLs). In addition, torrefaction pretreatment has a destructive effect on Cβ-O-C4 linkages, and reduces the molecular weight of TLs. Torrefaction pretreatment also removes the thermally unstable components from SKL and increases the release rate of pyrolysis volatiles. Furthermore, torrefaction pretreatment degrades the methoxy group (-OCH3) in SKL into phenolic hydroxyl group, which reduces the yield of guaiacyl type phenols in TLs pyrolysis products, and significantly increases the yield of catechol type phenols from 23.59% in SKL to 47.50% in TL300.