Synergistic effects in co-pyrolysis of lignin and low-density polyethylene (LDPE): Impact on biochar, bio-oil, and gaseous products

The technology for co-pyrolysis of biomass and plastics represents a promising strategy for waste recycling and resource valorization. Kinetic analyses using the Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS) models revealed that the average activation energy for the blend of lignin and LDPE in 1:1 mass ratio was 243.96 kJ/mol. This was lower than those of pure lignin (300.41 kJ/mol) and pure LDPE (267.35 kJ/mol). Pyrolysis experiments conducted in a tube furnace demonstrated that co-pyrolysis promoted carbon enrichment and graphitization in biochar, while increasing the abundance of surface functional groups. This synergistic modification increased the higher heating value (HHV) to 29.15 MJ/kg for the lignin/LDPE biochar obtained at 700 °C. Co-pyrolysis suppressed the formation of small-molecular oxygenates and enhanced the selectivity toward aromatic hydrocarbons in the bio-oil, with its relative content reaching 9.7% at 700 °C. Co-pyrolysis suppressed the formation of CO and CO₂, while enhancing the yields of CH₄ and H₂ in the gaseous products. Mechanistically, hydrogen radicals donated by LDPE stabilized the reactive oxygen-containing intermediates of lignin and thereby inhibited their polymerization into biochar. They also promoted the formation of small-molecular phenols, while simultaneously suppressing the release of CO and CO₂. The hydrogen-rich environment further converted these phenols into aromatic hydrocarbons through hydrodeoxygenation (with the removal of oxygen as H₂O), while promoting the release of gaseous products, such as CH₄ and H₂. This study provides fundamental insights and a data foundation for leveraging the synergistic effects in the co-pyrolysis of biomass and waste plastics. • Kinetic analysis confirmed that the co-pyrolysis reduces the reaction E a . • TG-FTIR analysis provided real-time evidence of volatile evolution. • Co-pyrolysis enhanced the quality of products. • Optimal temperatures for targeting specific product upgrades were identified. • A mechanism underlying the co-pyrolysis of lignin and LDPE was proposed.