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.