Thermo-catalytic pyrolysis and kinetic study of non-edible castor seeds into renewable liquid fuel and value-added chemicals

Abstract

Biomass is a promising renewable resource due to its availability, low processing cost, high conversion efficiency, and reduced life-cycle carbon emissions, making it a potential alternative to fossil fuels. However, the pyrolysis-derived pyrolytic oil typically suffers from poor quality, high viscosity, thermal instability, and corrosiveness, limiting its direct use as fuel. These drawbacks can be mitigated through catalytic upgrading. Thus, the present study investigates the kinetic behaviour and thermo-catalytic pyrolysis of castor seeds using a semi-batch reactor at 550 °C, 50 °C min⁻¹ heating rates and 100 mL min⁻¹ nitrogen flow rate. Thermogravimetric analysis was conducted at heating rates of 10, 30, and 50 °C min⁻¹, applying Ozawa-Flynn-Wall, Friedman (FM), Kissinger-Akahira-Sunose (KAS), Vyazovkin (VZ), Starink (STM), alongside the Criado model. The average apparent activation energies were determined as 174.69, 172.25, 173.53, 66.17, and 156.93 kJ mol⁻¹ for OFW, KAS, STM, VZ, and FM, respectively. The Criado model confirmed that biomass decomposition follows a complex, multi-step reaction mechanism. Further, pyrolysis using ZSM-5 and ZnO catalysts enhanced pyrolytic oil yields from 49.82 wt% (thermal) to 55.52 wt% and 54.33 wt%, respectively. Catalyst inclusion also improved the carbon content (by 14.12 % for ZSM-5 and 12.19 % for ZnO), higher heating value (by 1.72 and 1.00 MJ kg⁻¹), and pH (by 0.89 and 1.50). The resulting biochar exhibited 57.14 % carbon content, 36.23 % oxygen, 19.88 MJ kg⁻¹ Higher heating value, and a Brunauer-Emmett-Teller surface area of 84.98 m² g⁻¹. Overall, this study highlights the potential of castor seeds as a viable feedstock for sustainable fuel production through catalytic biomass valorisation.