Supercritical water - actions and reactions during the thermal decomposition of plastics

Abstract

Current estimates suggest that there will be more plastic than fish in the oceans by 2050 if the mounting plastic waste problem that society is facing remains unaddressed. New technologies will be required in order to shift society towards a circular economy which will support a more sustainable future. Hydrothermal liquefaction (HTL) shows promise to be a major part of the solution for dealing with the rapidly growing quantities of plastic waste generated each year. Yet, there are still many unknowns pertaining to the reaction pathways occurring during the HTL of plastic wastes. The work within this thesis will explore the role that water plays during the decomposition of plastic waste in sub- and supercritical water. Water’s role as a solvent and a reactant will be investigated and clarified. Chapter 2 focuses on the reactivity of water with a hydrocarbon substrate. Contradictory claims in the literature about whether water or oxygen are responsible for the formation of oxygenated compounds have been clarified. Adventitious oxygen was found to be responsible for formation of oxygenated species, which is consistent with mechanisms proposed for SCWO. Chapter 3 examines the behaviour of scrap tyres in HTL. The particle size of a tyre granulate sample was varied to examine effects on the product distribution. Fundamental reaction pathways responsible for the decomposition of the polyisoprene chain are identified. Chapter 4 is a studies the HTL conversion of limonene to aromatics. The behaviour in supercritical water has been identified to diverge from previously published pyrolysis of limonene and the role of water was identified as promoting isomerisation of the double bonds within limonene. Chapter 5 focuses on the hydrothermal decomposition of polyamides. Whilst the behaviour in subcritical water is well known, the same cannot be said about the behaviour in supercritical water. A hydrolysis mechanism accounts for the observed behaviour at all conditions examined.