Unveiling the Mechanism in Joule Flash Heating Polyethylene Pyrolysis: Insights from ReaxFF MD Simulations and Experiments

In this research, the impacts of the Joule flash heating (JFH) method on the generation of monomers and three-phase products during polyethylene (PE) pyrolysis were investigated by using reactive molecular dynamics (ReaxFF MD) simulations. Temperature ranges for JFH were determined through thermal pyrolysis. Meanwhile, JFH experiments at 70, 80, and 90 V were carried out by using the JFH reactor. The trends of the calculated results are consistent with those of the experimental results. Increasing the maximum (Tmax) or minimum (Tmin) temperatures in JFH promoted secondary reactions. Then, compared to the pyrolysis characteristics of PE under the JFH and the conventional continuous heating (CCH) conditions, the results showed that the yields of char decreased in the JFH method, and the tar produced by pyrolysis exhibited a higher H/C ratio. The yield monomer (C2H4) obtained by the JFH method was nearly 12 times more than that obtained by the CCH method according to ReaxFF MD simulations. Finally, by analyzing the reaction types and reaction frequencies involved in the secondary and primary reactions of C2H4 on PE pyrolysis, the mechanisms of the effect of the JFH method on the generation of C2H4 were revealed. The enhanced C2H4 production in JFH was attributed to two key factors during the cooling stage: (1) The cooling stage inhibited hydrogen dissociation in C2H4, and decreased the reactions of C2H4 with alkyl radicals, thus reducing C2H4 consumption. (2) The decrease in temperature makes the condensation reaction of ethylene less likely to occur. These findings highlight the potential of JFH as an efficient method for optimizing PE pyrolysis, maximizing C2H4 monomer recovery and minimizing undesirable byproducts such as tar and char.