Non-Catalytic Staged Thermal Pyrolysis of Polypropylene and High-Density Polyethylene for Fuel-Range Hydrocarbon Production

The conversion of waste plastics into liquid fuels via thermal pyrolysis presents a sustainable solution to both energy scarcity and plastic waste management. This study investigates the staged non-catalytic pyrolysis of polypropylene (PP), high-density polyethylene (HDPE), and their binary blends to evaluate liquid fuel yields, char formation, and the influence of feedstock composition on product distribution. Experiments were conducted in a 1.0 L semi-batch reactor at 450 °C (Stage A) with secondary cracking of waxes at 500 °C (Stage B). Product analysis via GC–FID quantified gasoline-, kerosene-, diesel-, and heavy-oil-like fractions. Results demonstrate that HDPE produces the highest liquid yield (39.05 g) with minimal char (0.77 g), whereas PP favors gas and char formation due to random chain scission. Binary blends exhibit synergistic behavior, where HDPE-rich mixtures (30% PP–70% HDPE) maximize liquid recovery (36.81 g) while moderating char formation, and PP-rich mixtures benefit from secondary cracking to enhance oil yield (25.1 g). Stage B pyrolysis substantially increased liquid fractions across all formulations, with negligible char generation. Fuel-range composition analysis revealed that PP favors lighter hydrocarbons (gasoline 32%), whereas HDPE yields heavier fractions (diesel 33.5%, heavy oil 26%), with intermediate profiles observed in blends. These findings highlight the tunable nature of co-pyrolysis for optimizing liquid fuel yield and quality, providing a viable route for converting heterogeneous plastic wastes into transportation and industrial fuels.