From waste to fuel: controlled torrefaction (mild pyrolysis) of mixed plastics with characterization of solid fuel and by-products

The global plastic waste crisis and the depletion of fossil fuels necessitate innovative waste-to-energy solutions. Pyrolysis remains a promising pathway, however conventional high temperature processes (>500°C) are energy intensive and generate hazardous emissions, limiting their suitability for community scale adoption. This study investigates a lower temperature (<450°C) mild pyrolysis approach for mixed high density polyethylene (HDPE) and polyethylene terephthalate (PET) waste using a batch reactor with extended residence times. The objectives were to characterize thermal degradation behaviour, establish wastewater-based emission profiling (COD, BOD, pH, DO, TSS and turbidity), and evaluate pyrolytic oil quality under mild pyrolysis conditions. Pure HDPE achieved oil yields of up to 68 wt% at 320 °C, whereas an HDPE1:2PET blend produced 26 wt%. Thermogravimetric analysis (TGA) indicated maximum mass loss temperatures at 482°C for HDPE and 447°C for PET. A novel low cost emission monitoring method is introduced using water phase indicators where elevated COD (up to 8508 mg/L), reduced pH (down to 3.82) and suppressed DO (<4 mg/L) signalled the presence of oxygenated volatiles and acidic condensable originating primarily from PET decomposition and trace additive derived species. These observations were validated through FTIR and GC–MS analyses which identified C₇–C₂₅ hydrocarbons and oxygenates consistent with PET derived intermediates. Char analysis revealed fixed carbon contents of up to 21 wt% for PET rich residues. The integrated framework combining oil grading with wastewater based odour and emission profiling demonstrates a scalable and resource efficient technology particularly suitable for HDPE rich waste streams making it promising for decentralized and community level applications in resource constrained settings. Overall, the findings provide a practical pathway for the cleaner low emission conversion of mixed plastic waste into high value pyrolytic fuels using mild pyrolysis