Exploration on the steam gasification mechanism of waste PE plastics based on ReaxFF-MD and DFT methods

• The steam gasification mechanism of PE plastic was explored by ReaxFF-MD and DFT. • Steam gasification is an overlapping chain depolymerization and reforming process. • H· radical from PE can highly reduce the energy barrier of water decomposition. • The effects of temperature and steam-plastic ratio are illustrated. Steam gasification of plastics is a promising way to deal with the increasing waste plastics as well as produce H 2 -rich syngas. In this paper, ReaxFF-MD simulation and DFT calculation methods were combined to study the reaction process of the PE steam gasification and the formation mechanisms of syngas especially hydrogen. According to the main reaction, the gasification process has two overlapping stages: PE thermal depolymerization followed by H 2 O reforming reaction. The depolymerization stage is mainly the terminally or intermediately breaking process of C–C bond, during which different R· radicals and ·R· double radicals fragments continuously dissociated into smaller hydrocarbons until C 1 -C 4 . The free H· radicals released through dehydrogenation and H-transfer reaction in depolymerized stage can highly reduce the energy barrier of subsequent H 2 O reforming reaction. The production of H 2 mainly comes from the reforming reaction of H 2 O with free H· radicals and the accompanying ·OH radical actively participates in the formation process of CO. Besides, the effects of temperature and water content on product distributions are analyzed. Increasing temperature and water content can promote the steam reforming consumption of hydrocarbons and thus a rise in H 2 and CO yields, but the improvement effects gradually decrease. The revelation of radical transformation and integration in this article can be a guide in later reaction adjustment as well as catalyst selection.