Conversion of real-world plastic waste into fuel grade hydrocarbon by pyrolysis: Thermal and catalytic route

Abstract

With ever-increasing plastic waste, a robust and sustainable methodology to valorize the waste and modification of the composition of the value-added product is the need of the hour. The present study describes the effect of thermal and heterogeneous catalytic pyrolysis system on the yield, composition and the nature of the pyrolytic oil produced from various real-world plastic wastes like high-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), and polystyrene (PS). These wastes were subjected to thermal as well as catalytic pyrolysis. Liquid, gas, and solid products were obtained during the pyrolysis. Pyrolysis liquid products were analyzed using gas chromatography- mass spectroscopy (GC-MS), nuclear magnetic resonance (NMR), Fouriertransform infrared spectroscopy (FT-IR), parallel plate rheometer and bomb calorimeter. In thermal pyrolysis the reaction temperature range of 450 – 480 ℃ favored the liquid oil yield. Compared with other wastes, PS waste produced the maximum liquid oil yield of 52%. Furthermore, kinetic study was carried out to understand the kinetic triplets i.e., activation energy (Eα), frequency factor (A0) and reaction mechanism (f(α)) of complex thermal pyrolysis process of plastic waste. A combined strategy of employing model-free (OFW, KAS, Starink, Tang and Boswell) isoconversional methods and model-fitting (Criado and Coats–Redfern) methods was used to study kinetics of thermal pyrolysis process. Hierarchical ZSM-5 (zeolite) catalyst was synthesized by hydrothermal method using organic template for inducing the mesoporous framework network and was used in catalytic pyrolysis in a fixed bed tubular reactor. A wide array of techniques such as XRD, BET, FE-SEM and FT-IR were used to analyze the material properties of the synthesized catalysts. The results of these analyses verified the successful formation of the mesoporous ZSM-5 framework with high surface area (approx. 295 m2/g). Usage of catalyst has reduced the temperature of the pyrolysis reaction from 450 to 420 ℃ with better product yield. In catalytic pyrolysis also, PS waste generated higher liquid yield as compared to PP, HDPE, and LDPE waste. The highest liquid yield of 63% was achieved with PS waste. However, 59%, 54%, and 45% yield of liquid oil were obtained from LDPE, HDPE, and PP wastes. The obtained liquid products consist of paraffin, naphthalene, olefin, and aromatic components. It was observed that the viscosity of the obtained liquid oil was in good relevance with that of the commercial grade diesel and kerosene oil. Also, the HHV values of the obtained liquid oils were as good as that of the petro fuels.