Synergistic co-pyrolysis of spent coffee grounds and polypropylene into hydrocarbon-rich bio-oil and carbon-rich biochar

Spent coffee grounds (CG) and polypropylene (PP) are widely generated food and plastic wastes that contribute to environmental pollution. This study investigates the co-pyrolysis of CG and PP across various blending ratios (100:0–0:100), focusing on product distribution, synergistic interactions, pyrolysis kinetics, and the potential of bio-oil and biochar for value-added applications. The maximum bio-oil yield (56 wt%) was obtained at a 50:50 CG:PP ratio, where synergistic effects enhanced hydrocarbon content and reduced acidic compounds in the bio-oil. The resulting oil showed improved carbon content and heating value (37.49 MJ/kg), making it a potential alternative to fossil diesel. Kinetic analysis using the Coats–Redfern method indicated that first-order kinetics and diffusion models best described the degradation behavior of CG, PP, and their blends, with activation energies ranging from 66 to 190 kJ/mol. The addition of PP also improved biochar properties by promoting higher surface area and carbon content while minimizing oxygen and sulfur levels, making it suitable for soil remediation and carbon sequestration. These results demonstrate that CG–PP co-pyrolysis is an effective waste-to-energy strategy, producing high-quality biofuels and biochar.