CO2 recycling as a tunable reaction environment in plastic waste pyrolysis: A system-level assessment

This study proposes and evaluates an integrated plastic waste-to-hydrogen process in which internally generated CO 2 is recycled and utilized as a tunable reaction environment in the pyrolysis stage. Unlike conventional plastic pyrolysis operated under an inert atmosphere, the proposed configuration introduces process-derived CO 2 as an active gas-phase component, enabling internal carbon circulation and altering carbon conversion during thermal decomposition. Steady-state simulations were performed using Aspen Plus to investigate how the CO 2 utilization ratio influences gas-phase carbon distribution, hydrogen production performance, process economics, and net process-level CO 2 emissions. The results indicate that the presence of a CO 2 -rich atmosphere promotes gasification-type interactions with nascent carbon species, increasing the fraction of reformable gaseous intermediates available for downstream upgrading. As the CO 2 utilization ratio increases, hydrogen production initially improves due to enhanced carbon conversion into CO-enriched syngas, while excessive recycling leads to performance deterioration caused by thermodynamic limitations and residence-time constraints in downstream reactors. An optimal CO 2 utilization ratio of 0.7 was identified, at which hydrogen productivity was maximized and the minimum hydrogen selling price reached 1.97 USD kg/H 2 , accompanied by a significant reduction in net process-level CO 2 emissions compared to the base case. These findings demonstrate that internally generated CO 2 can serve as a controllable reaction-environment variable influencing carbon partitioning across the integrated system and provide system-level insights for balancing carbon conversion, process operability, and emission reduction in waste-to-hydrogen platforms.