Kinetic mechanism study and waste tyre characterisation for valuable chemicals production.

Abstract

Researchers have extensively investigated waste tyre pyrolysis kinetics and mechanisms. However, available literature shows limitations as the studies were primarily aimed at tracking the devolatilisation of the main components of the feedstock than the formation of chemicals. In the present study, truck tyre crumbs of less than 1 mm were pyrolysed in a multi-shot pyrolyser connected to a mass spectrometer. The aim is to investigate the kinetics and reaction pathways that favour the formation of specific valuable chemicals (such as limonene and indene) in tyre-derived oil to maximise their production and content. The multi-shot pyrolyser (EGA/PY-3030D) equipment and accessories have been used to track the temperature and heating rate influence on the formation of chemicals. For temperature influence, experiments were conducted in single-shot mode and at different temperatures (400, 500, 600, and 700 °C), based on thermogravimetric analysis data. Chromatograms analysed regarding the area % of the considered chemicals proved that the selectivity to limonene decreases as temperature increases, reaching a minimum of 28.5% around 700 °C. In contrast, there was no discernible effect of temperature on indene in the considered range of temperature. The selectivity ratios of limonene to isoprene and indene to styrene presented a plateau between 400 and 600 °C. This trend evidenced that several equilibrated reactions steps could have taken place between limonene and isoprene as well as indene and styrene. Above 600 °C, the selectivity ratio of limonene to isoprene decreases, characteristic of cyclisation, hydrogenation and aromatisation reactions. The influence of heating rate on the production of the chemicals was conducted through Direct Evolved Gas Analysis (EGA) experiments at heating rates of 20, 30, 40, 50 and 100 °C /min and optimum temperature of 500 °C, obtained from temperature influence on the production of the chemical. The selectivity to limonene presented a continuous increase with the increase of heating rate from 30.8 % at 20 °C /min to 37.8 % at 50 °C /min and 42.7% at 100 °C /min. Hence, limonene formation is maximised at higher heating rates. The kinetic study conducted through combined model fitting and model-free approaches presented values in the range of 122 and 145 kJ.mol-1. The highest value has been obtained using the differential method (144.4 kJ.mol-1) with a difference of about five units with the two model-free methods of Kissinger (138.5 kJ.mol-1) and Starink's (139.7 kJ.mol-1). The Friedman's (122.0 kJ.mol-1) and Coats-Redfern (130.6 kJ.mol-1) methods present lower activation energy values with a difference of about 22 and 16 with the differential method. Therefore, a simplified reaction map has been presented and includes equilibrated reactions that contribute to the production of limonene-isoprene and styrene-indene.