Secondary reactions of volatiles upon the influences of particle temperature discrepancy and gas environment during the pyrolysis of scrap tyre chips

Pyrolysis process is one of the potential routes to convert waste scrap tyres into high-value resources such as liquid oil that is low in oxygen and high in valuable hydrocarbons. Although many studies have been conducted, the understanding of the fundamental science underpinning the pyrolysis of scrap tyre chip is still far from complete. This paper has examined the mild pyrolysis of scrap tyre chips at 600 °C, in an integrated manner by examining the co-effects of several key parameters including heating rate, volatile residence time, tyre chip size and non-inert gases (CO2 and/or H2O) on the quality of tars. In particular, we aimed to elucidate the influences of particle temperature discrepancy to the surrounding gas environment and reactive gas on the tar yield and properties, under the simulated fixed-bed/rotary kiln condition that is either heated directly by hot flue gas or indirectly by reactor wall. It has been confirmed that, in a simulated fixed-bed reactor with the absence of carrier gas, the particle temperature discrepancy can be correlated exponentially to the secondary cracking extent of volatiles in a temperature discrepancy range of 100 °C. Upon an increase on the temperature discrepancy by either increasing the heating rate or tyre chip size, the inherent long-chain aliphatics preferentially underwent scission, cyclisation and polymerisation, enhancing the yields for both heavy aromatics and methane – rich light gases. The use of carrier gas (i.e. hot flue gas) is beneficial in improving tar yield and aliphaticity. As a convective heating source, it heated particles slowly and also swept out volatile vapours immediately, thereby minimising the secondary reactions. For the two major components, CO2 and steam in hot flue gas, CO2 is rather inert at 600 °C, while steam is reactive enough to reduce the heavy hydrocarbons via steam reforming reaction, upon the catalytic effect of the nascent char derived from scrap tyre chips. The hydrogenation of unsaturated alkene and aromatics was also improved, and even the methanation reaction of CO on the nascent char surface, thus leading to the overwhelming dominance of CH4 in the pyrolysis gas.Vincent Tan, Anthony De Girolamo, Tahereh Hosseini, Jameel Aljariri Alhesan, Qiao Qiao Zhou, Lian Zhan