Catalytic performance of scrap tyre char for the upgrading of eucalyptus pyrolysis derived bio-oil via cracking and deoxygenation

Catalytic pyrolysis of biomass is one of the potential routes to convert lignocellulose material to high-value bio-fuels. In this paper, we explored the use of Zn-rich scrap tyre char, a low-value byproduct from the pyrolysis of waste scrap tyre as a catalyst for in-situ upgrading of crude bio-oil derived from the pyrolysis of eucalyptus. Experiments were performed in a two-stage fixed-bed reactor at 500 °C with a catalyst loading of 0-80 wt% on the mass basis of biomass. The characteristics of catalyst and bio-oils were conducted extensively by different facilities, including Karl Fischer titration, XRF, CHNS, FTIR, solvent extraction, and synchrotron C K-edge XANES. A removal of 40.3% oxygen in the form of H2O, CO2 and CO was confirmed upon the loading of 80 wt% scrap tyre char. Simultaneously, the bio-oil yield decreased from 33.4 wt% to 21.1 wt%. The acid-soluble inorganic species in scrap tyre char preferentially cracks the heavy hydrocarbons, thereby decreasing the yields of heavy pitch and coke. The catalyst loading is critical, triggering a rapid increase in the cracking/deoxygenation extent upon increasing the catalyst loading to 40 wt%. Afterwards, the change on the extent of deoxygenation is marginal. Based on the gaseous products, the content of CO2 is the highest, confirming the preferential decarboxylation catalysed by scrap tyre char. In contrast, the amount of CO derived from decarbonylation reaction is nearly unchanged, suggesting an insignificant effect of scrap tyre char on this deoxygenation reaction. The catalytic performance of tyre char was also found comparable to and even slightly better than the zeolitic catalysts tested under the similar conditions in the literature. However, its catalysis mechanism is fully unknown. The abundant ZnS within tyre char has yet to be reported as a catalyst for bio-oil deoxygenation. This inorganic species is even deeply embedded and inaccessible to acid washing within the tyre char. Investigation on its detailed structure and transformation for the catalysis reactions with bio-oil are underway.