When More CO2 Utilization Is Not Better: Life Cycle Assessment of Trade-offs and Optimal Design in Plastic Waste-to-Hydrogen Systems

This study presents an integrated environmental assessment of plastic waste-to-hydrogen systems with varying CO2 utilization ratios, combining process-level simulation with life-cycle assessment (LCA). The environmental impacts are evaluated across key categories, including global warming potential (GWP), fine particulate matter formation (PM), fossil resource scarcity (FRC), and water consumption (WC). The results reveal a non-linear relationship between CO2 utilization and environmental impacts. As the CO2 utilization ratio increases from the N2 baseline to moderate levels (CO2-40 to CO2-50), environmental impacts decrease due to improved carbon utilization and reduced direct CO2 emissions. However, further increases in CO2 utilization lead to a reversal of this trend, with environmental burdens rising significantly due to increased energy and utility demand associated with intensified CO2 recycling. Process contribution analysis shows that the dominant impact drivers shift from direct CO2 emissions to utility-related contributions, particularly heat (steam) and electricity, at higher utilization levels. A trade-off analysis between direct CO2 emissions and utility-related impacts identifies an optimal environmental operating range around CO2-50. An integrated comparison with techno-economic performance, represented by the minimum hydrogen selling price (MHSP), reveals a divergence between environmental and economic optima. While environmental impacts are minimized at CO2-40 to CO2-50, the economic optimum occurs at higher utilization levels (CO2-60 to CO2-70). These results highlight that CO2 utilization acts as a key design variable governing the trade-off between carbon efficiency and energy demand. An optimal compromise region is identified around CO2-50 to CO2-60, providing a balanced operating window for both environmental and economic performance. This study demonstrates that maximizing CO2 utilization is not necessarily optimal from a system-level sustainability perspective and provides practical insights for the design and optimization of integrated plastic waste-to-hydrogen systems.