Continuous screw-reactor co-pyrolysis of waste cooking oil sludge and polyolefin plastic waste: thermal behavior, oil quality, and potential for fuel applications

This study investigated the pyrolysis of waste cooking oil sludge (WCOS) and its co-pyrolysis with polypropylene (PP) and high-density polyethylene (HDPE) at 500 °C in a continuous double-screw reactor to evaluate product yields, oil quality, energy recovery, and preliminary economic performance. The novelty of this work lies in evaluating WCOS with individual polyolefin co-feedstocks under the same continuous operating conditions, together with an integrated assessment of product distribution, liquid-fuel characteristics, energy performance, and preliminary process economics. At a 50:50 blend ratio, co-pyrolysis with PP increased the pyrolysis oil yield from 46.0 wt% for WCOS alone to 71.0 wt%, while HDPE increased it to 64.0 wt%, with pyrochar yields reduced to approximately 2.2–2.3 wt%. GC–MS and FTIR analyses showed that plastic addition produced hydrocarbon-rich oils dominated by alkanes and alkenes, with a marked reduction in oxygenated compounds. The co-pyrolysis oils also exhibited high heating values (44–45 MJ kg −1 ), low water contents, and favorable density and viscosity, indicating improved fuel properties and potential for stationary thermal applications. The fraction of feedstock energy recovered in the pyrolysis oil increased from 73.9% for WCOS alone to 88.3% and 80.7% for WCOS–PP and WCOS–HDPE, respectively. A preliminary economic assessment further suggested shorter payback periods for the co-pyrolysis systems. Overall, WCOS–plastic co-pyrolysis shows potential for integrated waste valorization and fuel-oriented product recovery.