Abstract
Renewable carbon sources, such as biomass and waste, are being explored as alternatives for sustainable plastic production. However, the significant uncertainties surrounding the environmental impact of biomass supply processes raise questions about whether these plastics positively contribute to society. Furthermore, the lack of systematic knowledge about plastics and incomplete understanding among stakeholders pose challenges to conducting comprehensive assessments and designing effective plastic life cycle systems. This study aims to clarify the carbon flow within the life cycle of biomass- and recycled-derived plastics and to design a plastic life cycle that enables the introduction of renewable carbon sources. To this end, the study analyzed the structure of plastics containing renewable carbon and conducted a flow analysis of packaging plastics in Japan. The flow analysis was conducted in the form of an optimization problem. Greenhouse gas (GHG) emissions and the proportion of renewable carbon were selected as assessment indicators and objective functions. Constraints for optimization included the availability of biomass supply, the demand for product resins, and the processing capacity of various End-of-Life (EoL) methods. As a result, it was revealed that the priority of resin types to be produced and recycled shifts in response to changes in constraints such as biomass input and recycling capacity. Furthermore, a trade-off between reducing environmental impact and increasing the renewability of carbon was identified, highlighting the necessity of designing systems that balance these factors. This research provides valuable insights for designing sustainable plastic life cycles in the future.