Waste tyre char-catalysed in-situ deoxygenation of volatile vapours and production of hydrogen-rich syngas during the pyrolysis of lignite

This paper examined the catalytic performance of tyre char, a waste-derived product on the catalytic pyrolysis of lignite at 600-900 °C, upon the use of different gas environment (i.e. argon versus steam) and different catalyst to lignite mass ratio. All the resultant products (solid, gas and liquid) and spent catalysts have been characterised intensively to elucidate the specific catalytic role of tyre char and the respective mechanisms. As has been confirmed, the tyre char rich in zinc sulphide is a strong Lewis acid that is able to catalyse the upgrading of lignite volatiles via decarbonylation and steam-reforming of the volatile vapours. The former reaction mainly refers to the conversion of aldehydes to alkanes, whilst the latter one is preferentially implemented for the long-chain heavy hydrocarbons that are prone to deposit on the tyre char surface. The presence of steam in the pyrolysis gas environment promotes the steam-reforming of volatiles and steam-gasification of char, but also enhances the acidity strength for the formation of intermediate S–O active site that is responsible for the adsorption of O-bearing volatiles and the dissociation of steam. The enhancement of Lewis acid on catalyst surface is also highly sensitive to the mass of catalyst and the extent of its exposure to the O-bearing species including steam and volatile vapours. The use of large quantity such as three times larger than the lignite are essential for the tyre char to remain highly active upon cyclic usage. An extra activation is also essential after each use. In addition, it is confirmed that the catalytic effect of tyre char is profound for the upgrading of volatile vapours released from fast heating of lignite above 700 °C. At 800 °C, the resultant synthesis gas from lignite is also highly rich in H₂ and CO than that reported from the upgraded biomass volatiles based on the use of tyre char. A larger H₂ yield was also confirmed for the lignites tested here, due to the abundance of long-chain heavy hydrocarbons derived from them.