Tunable product selectivity in co-pyrolysis of mixed plastics: Enhancing olefin selectivity and optimizing oil yield via synergistic effects

The co-pyrolysis of high-density polyethylene (HDPE) and low-density polyethylene (LDPE) offers a novel strategy for the selective conversion of waste plastics into high-value olefins.This study investigates the pyrolysis characteristics, kinetic parameter calculations, synergistic interactions, and product distributions of HDPE/LDPE mixtures at ratios of 1:1 (HL11), 1:3 (HL13), and 3:1 (HL31), and reveals the underlying mechanism by which synergistic effects govern product selectivity during co-pyrolysis. Experimental results demonstrate that the HDPE/LDPE mixture with a 3:1 ratio (HL31) exhibits the most significant synergistic enhancement. The co-pyrolysis of HL31 shows the lowest activation energy (212.44 kJ mol −1 ), consistent with the random nucleation and growth model ( A 3/2 ). Product analysis further reveals that the HL31 pyrolysis achieves an oil yield as high as 85 %. The interaction between free radicals generated from HDPE and LDPE decomposition suppresses alkane formation while promoting the retention of olefins, thereby enhancing the overall yield of olefins such as 2,4-dimethyl-1-decene and 1-decene. This study elucidates the ratio-dependent synergistic mechanism in HDPE/LDPE co-pyrolysis, providing a theoretical basis for the selective conversion of mixed plastic waste. By tuning the component ratio, simultaneous optimization of pyrolysis oil yield and olefin selectivity can be achieved, advancing the industrial application of plastic recycling technologies. • HDPE promotes LDPE pyrolysis, with the strongest synergy in HL31 co-pyrolysis. • Pyrolysis oil yield reaches 85 % at the HL31 ratio, showing high efficiency. • Radical interactions during co-pyrolysis promote olefin formation. • Olefin selectivity increases to 30 %, with a 5 % and 8 % boost over pure HDPE/LDPE. • Optimizing HDPE/LDPE ratios enhances both oil yield and olefin selectivity.