Advanced pyrolysis of plastic waste for chemicals, fuel and materials

Abstract

The constant increasing generation of plastic waste, alongside the lack of plastic waste recycling capacity and the health and environmental risks from their disposal in landfills and incineration, have contributed to the search for versatile alternative management solutions e.g. pyrolysis. Thermal pyrolysis of plastic waste is a well-known and mature technology yielding very little gas and solid residue and high low quality wax (mixed C6 -C25 hydrocarbons) with limited direct application as transportation fuels. Zeolites are often introduced as catalysts for plastic waste pyrolysis to improve the quality of the wax and reduce operation temperature. However, they rapidly deactivate via coking and are expensive, opening up the search for alternative catalysts/technologies. In this thesis thermal, catalytic and cold plasma assisted pyrolysis, were evaluated in terms of the type of products obtained. Although scarcely used, sulphated zirconia, Ni/Al2O3 and char from waste biomass pyrolysis as catalysts and cold plasma proved to be potential improvements for plastic waste pyrolysis. A kinetic model of individual plastic waste, developed via iso-conversional methods and linear model fitting, showed their thermal decomposition was complex and allowed predictions for the design and operation of pyrolysis systems. The combination of waste high density polyehtylene and waste polypropylene pyrolysis with cold plasma yielded high-value chemicals e.g. ethylene (up to 55 times compared to thermal pyrolysis of waste high density polyethylene), hydrogen (40 times the H2 generation flow rate compared to waste polypropylene thermal pyrolysis) and carbon nanotubes (30-40 nm diameter from waste polypropylene), improving the overall profitability of pyrolysis. Cold plasma and catalysts (HZMS-5, sulphated zirconia and Ni/Al2O3 ) showed a synergistic effect promoting the recovery of ethylene and hydrogen from waste high density polyehtylene and waste polypropylene respectively. Sulphated zirconia catalyst (0-10 wt%) improved the recovery of up to 27-32 wt% of benzoic acid, a precursor in the food and beverage industry, alongside other high-value products recovered in the gas (CH4 and C2 -C3 hydrocarbons) from waste PET catalytic pyrolysis. Biochar, derived from waste biomass pyrolysis, as a catalysts for unsorted mixed plastic waste two-stage pyrolysis significantly enhanced the cracking process compared to non-catalytic thermal by increasing the gas (up to 85 wt%), hydrogen (up to 3.25 wt%) and methane yields (up to 55 wt%). Therefore, this thesis successfully proved the novel use of cold plasma and sulphated zirconia and biochar as catalysts to improve the value of plastic waste pyrolysis products, opening up opportunities to develop a sustainable and efficient process to re-engineer plastic wastes.