ReaxFF MD study on the early stage co-pyrolysis of mixed PE/PP/PS plastic waste

The early stage co-pyrolysis of typical plastic waste including polyethylene (PE), polypropylene (PP) and polystyrene (PS) were investigated by using the reactive force field molecular dynamics (ReaxFF MD) simulation with an automatic reaction mechanism analysis software (AutoRMA); the kinetic model, product yields and reaction process of co-pyrolysis were analyzed at atomic level. The results show that the kinetic parameters of PE/PP/PS co-pyrolysis can be obtained through the weighted sum of the parameters for the fracture of C–C and C–H bonds; the estimated activation energy is very close to the experimental one with a small error of ±3.86%, indicating that the fracture of C–C and C–H bonds can accurately characterize the co-pyrolysis process. For the co-pyrolysis of PE-PP mixture, an increase of PP content can improve the yields of oil and combustible gas, whereas for the co-pyrolysis of PP-PS mixture, the increase of PS content can improve the yields of tar and oil. In contrast, for the co-pyrolysis of PE-PP-PS mixture, a higher temperature is beneficial for the conversion of heavy oil into light oil; the light oil content increases from 44.77% at 2400 K to 56.18% at 3000 K. In addition, as a higher temperature can promote the further cracking of light hydrocarbons into gas products of smaller molecules, the yields of H₂ and CH₄ increase significantly with the increase of pyrolysis temperature, whereas the yields of C₂H₄ and C₃H₆ increase first and then decrease with the temperature. In comparison with the separated pyrolysis, the co-pyrolysis commences later, but displays shorter time to reach the first equilibrium state and generates products with smaller molecules. For the separate pyrolysis of PE and PP, their monomers emerge first, hereafter the alkanes and small molecule gases are produced; for the co-pyrolysis process, in contrast, the alkanes and small molecule gases are generated prior to the monomers. Moreover, PS tends to provide ·H radicals in the co-pyrolysis process, which can combine with the free radicals generated from PE and PP pyrolysis, forming small molecule alkanes and H₂.