Supercritical water co-gasification mechanism of lignin and low density polyethylene into syngas: ReaxFF molecular dynamic simulation and density functional theory calculation study

The supercritical water gasification (SWG) of low density polyethylene (LDPE) and lignin into CO and H2 is a potential path for the conversion of biomass and waste plastics. ReaxFF molecular dynamic simulations are performed to study the supercritical water co-gasification mechanism of lignin and LDPE in the present study. The co-gasification process, the impacts of lignin content and temperature on the co-gasification of lignin and LDPE are analyzed. In addition, the nature of interaction mechanism between H2O and lignin, H2O and phenols in the co-gasification process are studied. The results showed that the primary products are CO and H2, and the proportion of latter is higher than that of former. The first step of the supercritical water co-gasification of lignin and LDPE is the cracking of lignin and LDPE, C5-C10 products and waxes are formed via the cracking of the βO4 bonds and the CC bonds, respectively. The formation of CO and H2 is facilitated by the increase of the feedstock proportion in the reaction system. Hydrogen bond formed between lignin and H2O improves the pyrolysis of lignin. This study provides a viable and green route to convert the biomass and waste plastics to syngas.