A Comprehensive Review on Co-Pyrolysis of Biomass, Plastics, and Waste Tires: Mechanistic Insights, Synergistic Effects, and Optimization Strategies

Fossil-fuel dependence continues to drive greenhouse gas emissions and climate change, motivating renewable and low-carbon energy pathways. Biomass pyrolysis often yields oxygen-rich, unstable bio-oil with low heating value, while co-pyrolysis with hydrogen-rich plastics or waste tires improves deoxygenation and shifts products toward higher-HHV, more hydrocarbon-like oils. While biomass-plastic co-pyrolysis is relatively well studied, biomass-tire systems remain comparatively underexplored, especially with few comprehensive comparisons between them. Therefore, this review provides a comprehensive assessment of co-pyrolysis mechanisms, synergistic effects, and optimization strategies, highlighting both common and distinct behaviors of biomass-plastic and biomass-tire systems. It shows that synergy is mainly driven by radical cross-reactions and hydrogen transfer arising from the coupling of biomass devolatilization with polymer/rubber radical chain scission, and is strongly modulated by operating severity and catalyst choice. However, reproducible scale-up is constrained by feedstock heterogeneity and operational sensitivity, while tire-containing feeds additionally require sulfur management. In addition, it identifies key research gaps, including limited understanding of cross-feedstock radical interactions, absence of standardized catalytic and process optimization frameworks, and insufficient integration of emission control with process intensification. Finally, we outline a roadmap combining blend and operating-condition optimization (often near ~450–550 °C), targeted catalysts/sorbents, and machine-learning-enabled modeling and control to enable scalable, low-emission conversion of mixed wastes into clean fuels and sustainable chemicals. Overall, this review provides comparative, mechanism-to-process insights to guide scalable, low-emission co-pyrolysis of mixed wastes. • Comparative review of biomass, plastic, and tire co-pyrolysis systems; • Synergistic mechanisms via radical transfer, hydrogen donation, and catalysis; • Feedstock properties strongly affect oil yield and product composition; • Diverse upgrading strategies evaluated for process optimization.