Staged Catalytic Co-Pyrolysis of Polypropylene and High-Density Polyethylene: Optimizing Liquid Fuel Yield and Composition

The escalating global production of plastics and the depletion of fossil fuel reserves underscore the urgency of sustainable waste-to-energy strategies. This study investigates the staged catalytic pyrolysis of polypropylene (PP), high-density polyethylene (HDPE), and their blends for the production of liquid fuels. Experiments were conducted in a semi-batch reactor at 450 °C (Stage A) and 500 °C (Stage B), with bentonite as catalyst. Product yields and compositions were quantified via mass balance and GC–FID analysis. Results revealed strong feedstock-dependent behaviors: HDPE exhibited superior liquid recovery (81.96% in Stage A, 88.24% in Stage B) with minimal char, whereas PP was prone to higher char and gas formation. Co-pyrolysis demonstrated synergistic effects, with asymmetric mixtures outperforming single-polymer systems. Notably, the 70% PP–30% HDPE blend achieved the highest liquid recovery (95.07%) and lowest gas fraction (4.92%) during secondary cracking, while the 30% PP–70% HDPE blend enhanced diesel- and kerosene-range fractions. GC–FID analysis confirmed that PP favored gasoline-range hydrocarbons, while HDPE enriched middle distillates. The tunability of hydrocarbon distribution through feed composition highlights staged pyrolysis as a robust pathway for transforming mixed plastic waste into targeted fuel-range hydrocarbons. These findings provide actionable insights into optimizing product selectivity and yield, advancing the integration of polyolefin pyrolysis into circular economy and sustainable energy frameworks.