Hydrogen and aromatics recovery through plasma-catalytic pyrolysis of waste polypropylene

Plasma-catalysis pyrolysis is a promising way to solve the problem of catalyst deactivation during plastic recycling. In this study, pyrolysis of polypropylene (PP) over zeolite ZSM-5 has been carried out in a two-stage fixed bed pyrolysis system with a coaxial dielectric barrier discharge (DBD) plasma reactor. The role of plasma on the pyrolysis process, as well as the stability of the plasma-catalytic system was investigated. Compared to conventional catalytic pyrolysis, plasma-catalysis pyrolysis increased gas products from 29 wt% to 47 wt% with 4.19 mmol/g H2 formed, and improved the selectivity of BTX (benzene, toluene, xylene) whilst inhibiting the production of wax simultaneously. After 10 cycles, clear decreases in gas and oil yield (from 86 wt% to 48 wt%) and BTX selectivity (from 71 wt% to 39 wt%) were found in the conventional catalytic pyrolysis, however, nearly no variation was shown in the plasma-catalysis mode. The coupling of catalyst and plasma modified the catalysts acidic sites, while the radicals enhanced the pre-cracking of volatiles, resulting in less deposited coke. Overall, the introduction of plasma resulted in an obvious reduction in total costs and presented a feasible strategy for the recycling of waste plastic.