Co-production of H₂ and CNTs from coal gangue-supported co-pyrolysis of PP/PE/PVC mixture in a two-stage reactor

As a persistent environmental pollutant, the management of waste plastics is critical for mitigating soil and water contamination. In the quest for carbon neutrality, this study presents a sustainable approach to waste plastic valorization through catalytic pyrolysis, focusing on hydrogen(H₂) production and carbon nanotubes (CNTs) synthesis. A coal gangue-based catalyst was utilized in a two-stage fixed-bed reactor for the pyrolysis of polypropylene (PP), polyethylene (PE), and polyvinyl chloride (PVC) to evaluate the co-production efficiency. H₂ was yielded by PP and PE at 63.9 vol% and 66.5 vol%, respectively, with PP yielding 6.23 wt% more CNTs than PE. Co-pyrolysis of PP and PE increased H₂ concentration by 11.23 vol% and 7.71 vol% over individual pyrolysis, and yields of CNTs by 11.99 wt% and 17.15 wt%, respectively. These results highlight the superiority of mixed pyrolysis over individual processes for H₂ concentration and CNTs production. The pyrolysis of PVC, which releases a significant amount of HCl, deactivates a coal gangue-based catalyst that is rich in Fe₂O₃ and inhibits the growth of CNTs by disrupting the Fe₂O₃ structure and bonding with carbon. To harness chlorine-containing plastics for CNTs production, the pyrolytic catalytic process must be optimized to mitigate chlorine's detrimental effects. There are valuable insights provided by this research into the thermochemical treatment of waste plastics, contributing to the clean production of H₂ and CNTs, aligning with carbon neutrality objectives by reducing reliance on fossil fuels and promoting sustainable waste management strategies.