Kinetic study of municipal plastic waste

Municipal Plastic Waste (MPW) comprises a mixture of thermoplastics (high and low density polyethylene, HDPE and LDPE; polypropylene, PP; polystyrene, PS; and polyethylene terephthalate, PET). The rate of MPW generation has increased steadily at 5% per year whilst that of MPW recycling is only at 3% per year. The remains are either incinerated or disposed in landfills. Pyrolysis, a thermochemical decomposition, provides an excellent alternative to valorise MPW into valuable products, such as hydrogen and hydrocarbons which could be further processed for fuels and chemicals like syngas (steam reforming). However, most MPW pyrolysis studies oversimplify the mechanism of reaction by assuming a first order decomposition, leading to inaccurate predictions of the process. This could cause big challenges in designing pyrolysers for MPW or scaling up pyrolysis process. This study was to develop a kinetic model to discover the true reaction mechanism of MPW pyrolysis experimentally and numerically by applying thermogravimetric analysis (TGA) and Matlab software. Several methods such as Kissinger-Akahira-Sunose, KAS; Málek and linear model fitting were applied to predict the mechanism of MPW pyrolysis and valuated by experimental data obtained from TGA. All components in MPW were decomposed in N2 atmosphere over a set of heating rates (5, 10, 20 and 40 °C/min), temperatures of 30-700 °C and sample size range of 1-4 mm. The results confirmed that a complex mechanism rather than simple 1st order occurs during the decomposition because the variation in the apparent activation energy with conversion and kinetic model with heating rate. The assumption that MPW decomposition mechanism is a mixture of series and parallel reactions agreed well with experimental data and was confirmed by the results obtained from Málek method and linear model fitting.