Catalytic Pyrolysis of Plastic Waste to Liquid Fuel Using Local Clay Catalyst

Abstract

Plastics are nonbiodegradable, and safe disposal of this waste poses an environmental challenge all over the world. Catalytic pyrolysis is superior to thermal pyrolysis as it uses lower temperatures and hence less energy. The main objective of this study was to produce liquid fuel from plastic waste using indigenous clay as a catalyst through catalytic pyrolysis. The clay from Kisumu County was characterized through an X-ray fluorescence spectrometer (XRFS) and an X-ray diffractometer (XRD). The reaction setup consisted of a round-bottom flask reactor through which plastic feed and catalyst were heated in a temperature-controlled furnace. Vapor product was condensed using a Liebig type water condenser to give pyrolysis liquid product. Solid char was recovered from the flask at the end of the reaction. Optimization studies using central composite design (CCD) and response surface methodology (RSM) were performed in design expert software to predict optimal conditions of the operating variables for maximum yield of the liquid fuel. Results show that clay has a composition of silica and alumina at 64.5 wt% and 16.3 wt%, respectively, indicating high acidity of the clay, being a requirement for a good pyrolysis catalyst. For high-density polyethylene and polypropylene, the highest liquid yield of 87.23 wt% and 60.36 wt%, respectively, was at 300°C and a catalyst concentration of 10 wt%. Indigenous clay was established to be a suitable catalyst for catalytic pyrolysis of plastic waste, with the potential to replace imported catalysts since high yields of liquid fuel were obtained at lower reaction temperatures of 300-450°C, as compared to the 600°C required for thermal pyrolysis. In conclusion, waste plastics can be used to generate alternative fuel for industrial use. The liquid fuel can be used in diesel engines as a transport fuel, in turbines for electricity generation, and as a heating source in boilers and furnaces. Further studies on the modification of the surface and structure of clay are suggested to enhance its catalytic performance in the pyrolysis process for a better fuel yield.