Advances in understanding kinetic mechanisms underlying waste ground tyre rubber pyrolysis

The depletion of natural resources and the need to reduce solid waste in urban areas have necessitated the incorporation of used materials such as waste ground tyre rubbers (WGTR), into manufacturing processes. As a result, techniques and recycling methods have been established to use tyres as feedstock for marketable products since tyres have a calorific value higher than coal and contain a significant amount of carbon black. Among several techniques, pyrolysis has emerged as the most appealing for treating WGTRs. This technique allows the recovery of valuable products like combustible gases, fuels and chemicals, and activated carbon. Studies have focused on understanding the mechanism underlying the WGTR pyrolysis through the establishment of mathematical models and reaction patterns to valorise WGTRs and efficiently produce marketable chemicals. This paper presents an overview of recent developments in understanding WGTR pyrolysis mechanisms. A general mechanism observed involves a first depolymerisation/condensation of the rubbers, then a degradation of the condensed products, and finally a devolatilisation of additives. Based on the limited information available on the chemicals' formation mechanism, it is assumed that limonene and isoprene are derived from natural rubber (NR), through a series of β-scission and depropagation reactions of polyisoprene and intramolecular cyclisation and scission of monomeric isoprene, respectively, with an equilibrium step of Diels-Alder reaction. The maximum yield of limonene and isoprene have been found to be 51% and 20.5% at temperature around 500°C respectively. The isoprene yield can be increased up to 37.57 % with the use of catalyst such as Calcium Oxide.