Thermochemical upcycling of paper-mill waste into hydrocarbon-rich fuels and functional char for rubber reinforcement

Abstract

This study investigates the thermochemical upcycling of paper-mill waste composed of mixed paper and residual plastics into high-value fuels and functional char for composite applications. Co-pyrolysis was conducted in a semi-pilot-scale rotary kiln reactor at 450–650 °C, producing maximal pyrolysis oil yields of 48 wt% and 20 wt% char at 650 °C. The pyrolysis oil was separated into heavy and light fractions, with calorific values of 39–43 MJ kg⁻¹, demonstrating strong fuel potential comparable to that of petroleum diesel. GC-MS analysis revealed the presence of hydrocarbons, alcohols, and aromatic compounds, including toluene, styrene, and xylene, indicating synergistic degradation of cellulosic fibres and polyolefin plastics. Furthermore, vacuum distillation improved oil quality by enriching the C₆–C₁₈ hydrocarbon fractions and reducing the oxygenated species. The char exhibited a high carbon content (71.20%), a moderate ash content (30.30%), and an energy value of 16.5 MJ kg⁻¹, along with a porous morphology and mineral traces, enabling its use as a composite filler and secondary fuel material. Rubber compounding trials using SBR-1502 demonstrated that replacing carbon black with pyrolytic char yielded acceptable performance: hardness (58 Shore A) and tensile strength (18 kg cm⁻²) were lower than the standard carbon-black compound (63–64 Shore A; 123 kg cm⁻²), yet significantly higher than the filler-free control. Notably, char-reinforced rubber displayed similar elongation (322%) and a reduced curing time (T_c90 = 9.1 min), highlighting faster vulcanisation due to catalytic ash components. These results confirm the potential of paper-mill waste pyrolysis for sustainable fuel production and the recovery of circular-economy materials.