Pyrolysis–gasification of plastics, mixed plastics and real-world plastic waste with and without Ni–Mg–Al catalyst

Polypropylene, polystyrene, high density polyethylene and their mixtures and real-world plastic waste were investigated for the production of hydrogen in a two-stage pyrolysis–gasification reactor. The experiments were carried out at gasification temperatures of 800 or 850 °C with or without a Ni–Mg–Al catalyst. The influence of plastic type on the product distribution and hydrogen production in relation to process conditions were investigated. The reacted Ni–Mg–Al catalysts were analyzed by temperature-programmed oxidation and scanning electron microscopy. The results showed that lower gas yield (11.2 wt.% related to the mass of plastic) was obtained for the non-catalytic non-steam pyrolysis–gasification of polystyrene at the gasification temperature of 800 °C, compared with the polypropylene (59.6 wt.%) and high density polyethylene (53.5 wt.%) and waste plastic (45.5 wt.%). In addition, the largest oil product was observed for the non-catalytic pyrolysis–gasification of polystyrene. The presence of the Ni–Mg–Al catalyst greatly improved the steam pyrolysis–gasification of plastics for hydrogen production. The steam catalytic pyrolysis–gasification of polystyrene presented the lowest hydrogen production of 0.155 and 0.196 (g H2/g polystyrene) at the gasification temperatures of 800 and 850 °C, respectively. More coke was deposited on the catalyst for the pyrolysis–gasification of polypropylene and waste plastic compared with steam catalytic pyrolysis–gasification of polystyrene and high density polyethylene. Filamentous carbons were observed for the used Ni–Mg–Al catalysts from the pyrolysis–gasification of polypropylene, high density polyethylene, waste plastic and mixed plastics. However, the formation of filamentous carbons on the coked catalyst from the pyrolysis–gasification of polystyrene was low.