Catalytic pyrolysis of plastic waste to liquid fuel using local clay catalyst

Abstract

Plastics are non-biodegradable and safe disposal of this waste poses environmental challenge all over the world. One thermochemical disposal method is pyrolysis that produces clean liquid fuel. Catalytic pyrolysis is superior to thermal pyrolysis as it uses lower temperatures, hence, less energy. Imported conventional catalysts increase the production cost of the liquid fuel but local clay catalyst used in this research is a cheaper substitute. Therefore, the main objective in this study was to produce liquid fuel from plastic waste, using indigenous clay as a catalyst through catalytic pyrolysis. The liquid fuel can be used as an alternative source of energy in diesel engines as transport fuel, in turbines for electricity generation and as heating source in boilers and furnaces. The clay from Kisumu county was characterized through X-Ray Fluorescence spectrometer (XRFS) and X-Ray Diffractometer (XRD). Reaction setup consisted of a round bottom flask reactor through which plastic feed and catalyst were heated in a temperature controlled furnace. Vapour product was condensed using a liebig type water condenser to give pyrolysis liquid product. Solid char was recovered from flask at the end of reaction. Central composite design (CCD) and Response surface methodology (RSM) in Design expert software were used to study the effect of pyrolysis operating variables on the liquid fuel yield and also to identify conditions for optimal yield. Analysis of oil was done using Gas chromatography- Mass spectroscopy (GCMS). Results show that clay has composition of silica and alumina at 64.5wt% and 16.3wt% respectively indicating high acidity of the clay, being requirement for a good pyrolysis catalyst. For High density polyethylene and Polypropylene, the highest liquid yield of 87.23wt.% and 60.36wt.% respectively was at 300 °C and catalyst concentration of 10wt.%. GC- MS analysis showed that the oil from HDPE, PP and PS comprised of complex mixtures of organic compounds with aromatic, aliphatic and naphthenes as major components. The liquid fuel properties for HDPE, PP and PS were found to meet the ASTM D975 requirement for a liquid fuel. Indigenous clay was establised to be a suitable catalyst for catalytic pyrolysis of plastic waste, with a potential to replace imported catalysts, since high yields of liquid fuel were obtained at lower reaction temperatures of 300- 450 °C, as compared to 600 °C required for thermal pyrolysis. In conclusion waste plastics can be used to generate alternative fuel for industrial use. Further studies on modification of surface and structure of clay are suggested to enhance its catalytic perfomance in the pyrolysis process for a better fuel yield.