CO2-assisted control of H2/CO ratio in plastic waste pyrolysis

Abstract

Plastic waste pyrolysis offers a promising route to produce precursors for chemical synthesis. Nonetheless, the compositional heterogeneity of pyrogenic products requires extensive separation and purification of platform chemicals, which critically limits the economic feasibility of circular valorization. To address this limitation, this study investigates carbon dioxide (CO2)-mediated pyrolysis of plastic waste for producing syngas with a controllable hydrogen-to-carbon monoxide (H2/CO) molar ratio. Plastic optical cable (POC) waste was selected as a model feedstock, and characterization revealed two main polymeric constituents: polymethyl methacrylate (PMMA) and poly(ethylene-co-vinyl acetate) (EVA). Conventional POC pyrolysis yielded a broad distribution of hydrocarbons (C6-C38), yet homogeneous interactions between CO2 and the heavy molecular hydrocarbons derived from POC were limited. To promote these interactions, an isothermal catalytic bed containing a Ni-based catalyst was integrated downstream of the pyrolyzer. The catalytic bed facilitated the cracking of long-chain hydrocarbons into lighter species, which enhances their reactions with CO2 and promotes CO formation. Increasing the catalytic bed temperature from 500 to 700 °C intensified CO2 reactivity, leading to greater syngas yields and more effective modulation of its composition. The effect of CO2 concentration (0–80 vol%) was also examined, revealing that higher CO2 contents increased total syngas production while exponentially decreasing the H2/CO molar ratio. The tunable H2/CO ratio achieved via CO2-assisted catalytic pyrolysis offers a promising platform for synthesizing fuels and chemicals, thereby contributing to the circular valorization of plastic waste.