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.