Kraft lignin has the potential to replace traditional fossil resources for the preparation of high-value chemicals because it is rich in aromatic rings and active functional groups. An effective method for the pyrolysis of kraft lignin into chemicals/fuels is microwave-assisted depolymerization. A simulation model is urgently needed to illustrate the coupling effect and mechanism of lignin conversion during the depolymerization process. In this study, COMSOL Multiphysics was used to simulate the microwave-assisted depolymerization process. The results showed that microwave power had a significant effect on the electric field and temperature distribution in the microwave cavity, while the reaction time had little effect on the electric field. The effect of the nitrogen flow rate on the electric field and temperature was negligible. The intensity of the electric field, heating rate of lignin, and final temperature of lignin depolymerization increased with increasing microwave power.

Lignin is considered an ideal natural material for the production of sustainable monophenols. In this study, a microwave-assisted depolymerization (MAD) strategy was developed. The introduction of solvent vapors in the dynamic vapor flow reaction system was performed to enhance the lignin conversion efficiency. The results showed that no liquid products were generated from the MAD of lignin without solvent vapors. With the introduction of solvents (CH₃OH, HCHO, HCOOH, and CH₂Cl₂), liquid products appeared (especially with CH₂Cl₂, which had the highest yield of 41.9 wt%). Results from gas chromatography/mass spectrometry of liquid products showed that seven kinds of compounds, including guaiacols, phenols, syringols, methoxyphenyls, heterocycles, esters, and aromatics were identified. CH₂Cl₂ can significantly enhance the production of monophenols (guaiacols, phenols, and syringols). The introduction of these solvent vapors can also facilitate the generation of porous char with high Brunauer-Emmett-Teller specific surface areas. Some carbon nanospheres deposited on the surface of the char were obtained with the assistance of CH₂Cl₂. This study provides a facile method for the utilization of lignin in the field of bio-based fine chemicals.