Catalytic co-pyrolysis of waste pistachio nutshells and polystyrene: optimisation of parameters, and catalyst loading on the products yield and composition

The increasing energy demand and environmental challenges associated with fossil fuel use have spurred interest in renewable energy sources, particularly biofuels derived from waste biomass and plastics. This study investigates the catalytic co-pyrolysis of pistachio nutshells (PNS) and polystyrene (PS) to produce pyrolysis oil and char with enhanced fuel properties. The experiments were conducted in a semi-batch reactor under varying temperatures (500–650 °C), 30 °C min⁻¹ heating rate, and 100 mL min⁻¹ inert gas flow rate. Furthermore, PS loadings (20–50 wt%) and CuO loadings (5 and 10 wt%) were employed to improve the pyrolysis oil characteristics. The optimum conditions were found to be 550 °C with 30 wt% PS and 5 wt% CuO loading, yielding a maximum bio-oil output of 49 wt%. The characterisation of the pyrolysis oil revealed significant improvements in fuel quality with catalytic treatment, including an increased higher heating value (HHV) of 35.90 MJ kg⁻¹, reduced oxygen content (11.69 wt%), and lower viscosity (36.40 cP). Similarly, the resulting biochar exhibited high carbon content (76.57 wt%), low ash and moisture content (<1%), and enhanced calorific value, indicating its potential as a solid fuel or soil amendment. FTIR and NMR analyses confirmed the reduction of oxygenated functional groups and the presence of desirable hydrocarbon structures in the oil, while XRD and SEM analyses demonstrated improved structural properties of the char. The results highlight the synergistic benefits of co-processing biomass and plastic waste, as well as the catalytic role of CuO in enhancing product quality. This approach offers a sustainable waste-to-energy pathway, promoting circular economy principles and advancing the production of renewable fuels from mixed solid wastes.