Kinetics underpinning the C-CO2 gasification of waste tyre char and its interaction with coal char upon co-gasification

Abstract

This study includes a thorough investigation of the char structure, the potential catalytic influence of minerals, the carbon conversion rate, the kinetic behavior and parameters of the CO2 – gasification reaction for waste scrap tyre char and its blends with coal chars. Kinetic modelling was also attempted based on the two classical methods, random pore model (RPM) and grain model (GM) using the isothermal gasification results collected from thermogravimetric analysis (TGA). As has been found, tyre char is characterised with a macroporous structure with a very low reactivity. The catalytic role of the inherent minerals within tyre char is only influential at lower temperatures below ~1373 K. As opposed to the generally assumed first order reaction, the tyre char exhibits a reaction order between 0.62 (for an intrinsic reaction at 1273 K) and 0.92 referring to a dominant diffusion control at 1573 K. The slow intrinsic reaction rate of tyre char was also found to be comparable with the gas diffusion rate, thereby exhibiting a large activation energy of 174.87 kJ/mol that is considerably higher than coal chars. The coal char – tyre char blends displayed declining reactivity with increasing tyre char content, due to the enhanced ash melting and diffusion against the inward gas diffusion towards the unreacted carbon that is encapsulated inside. This was further explained by the experiments of blending tyre char ash with coal chars, and the observation of ash residues collected from the gasification of blends. Furthermore, the grain model was found more suitable to fit the blending conversion data, agreeing with the hypothesised molten ash layer surrounding the unreacted char blend.