Integrated pyrolysis of pine needles and cashewnut shells: optimizing process conditions for the production of renewable fuel and value-added chemicals

The sustainable conversion of lignocellulosic residues into renewable fuels and chemicals is vital to advancing a circular bioeconomy. This study presents a dual-scale thermochemical framework for the valorisation of pine needles (PN) and cashewnut shells (CNS) through integrated analytical and pilot-scale pyrolysis. Analytical pyrolysis using Py-GC/MS was employed to evaluate the temperature-dependent formation of bio-volatiles between 400 and 800 °C, revealing that 500 °C yielded the highest fraction of desirable compounds. PN generated aromatic-rich volatiles suitable for advanced fuel formulations, while cashewnut shells produced phenolic-rich vapours with applications in the renewable chemicals sector. The scale-up experiments conducted in a semi-pilot rotary kiln reactor confirmed the reproducibility of product yields and compositional trends observed at the laboratory scale. The maximum pyrolysis oil yields reached 40% for PN and 33% for CNS, while char and gas yields varied according to lignocellulosic composition. The elemental analysis indicated superior fuel quality for PN-derived bio-oil (higher heating value (HHV) = 35.08 MJ kg^-1, O/C = 0.23) compared with CNS oil (HHV = 28.14 MJ kg^-1, O/C = 0.43). Furthermore, biochars exhibited porous morphologies, indicating potential for environmental applications. This dual-scale methodology effectively bridges mechanistic understanding with process scalability, enabling tailored valorisation routes for underutilised biomass residues.