Biomass-plastic Co-pyrolysis for sustainable biochar: Mechanisms, reactor technologies, and circular-carbon applications

The rapid accumulation of lignocellulosic biomass residues and plastic waste poses significant environmental challenges because conventional disposal methods, including landfilling and incineration, contribute to greenhouse gas emissions and inefficient resource use. Biochar production through thermochemical conversion offers a potential pathway for valorizing waste streams within a circular carbon framework. This review critically synthesizes recent developments in biomass–plastic co-pyrolysis, focusing on the reaction mechanisms, feedstock interactions, reactor technologies, product characteristics, and life cycle assessment (LCA). It integrates physicochemical and environmental perspectives to address the gap between laboratory-scale co-pyrolysis research and system-level sustainability assessment. It examines multiple parameters, pyrolysis pathways, and feedstock compositions in relation to the biochar yield, carbon sequestration potential, and functional properties. Evidence shows that lignocellulosic biomass, composed of 40–50% cellulose and up to 30% lignin, provides stable carbon matrices, whereas plastic-derived feedstocks, dominated by polyolefins, enhance hydrogen availability and modify thermal degradation pathways. Synergistic co-pyrolysis interactions have been consistently reported to improve carbon stability and tailor porosity, although the product distribution remains highly sensitive to temperature (350–600 °C), residence time, and feedstock ratios. A national-scale assessment has estimated a mitigation potential of up to ∼0.92 Gt CO 2 year −1 for biochar deployment in China under sustainable scenarios. However, bottlenecks pertaining to system boundaries, feedstock heterogeneity, and long-term environmental impact require further assessment. Biomass–plastic co-pyrolysis is a promising circular-carbon pathway, but its scale-up requires feedstock and product standardization, pilot-scale validation, and more consistent environmental assessment.