Numerical Simulation of Thermal Decomposition of Polyethylene with a Single-Particle Model

Numerical simulations have been conducted to study the pyrolysis process of a low-density and a high-density polyethylene (LDPE and HDPE) particle in a hot nitrogen environment. Objective of the work is to gain an in-depth understanding of the interaction between heat transfer and pyrolysis reaction as well as its impact on the overall pyrolysis process. For that purpose, a computational tool using a spherical, thermally-thin particle has been developed, which takes the convective heat transfer and the endothermic pyrolysis reaction into account. The model has first been validated by means of thermogravimetry experiments, where the simulation results have shown a reasonably good agreement with the measured data. Thereafter, the method has been applied to simulate plastic pyrolysis at constant gas temperatures T-g, employing varying particle diameter d(p,0) and heat transfer coefficient alpha. At given T-g, the pyrolysis process is faster at decreased d(p,0) and increased alpha. This is attributed to the enhanced convective heat transfer, which leads to a higher reaction rate. While varying d(p,0) or alpha at constant T-g, the pyrolysis time tau(py) has revealed an almost inversely proportional correlation with the pyrolysis number Py, which represents the ratio of the time scales of chemical kinetics and convective heat transfer. The correlations between tau(py) and Py have been fitted by power law functions, which can be used as a first-order estimate to predict the pyrolysis time at given operating conditions.