Methane Production from the Three-Stage (i) Pyrolysis (ii) Catalytic Cracking (iii) Catalytic Hydrogenation of Polyethylene Terephthalate (PET) Plastic Waste

Polyethylene terephthalate (PET) was processed using a three-stage (i) pyrolysis, (ii) catalytic cracking and (iii) catalytic hydrogenation reactor system to produce methane. The pyrolysis of PET produced CO2 and CO and oxygenated higher molecular weight hydrocarbons. Catalytic cracking of the oxygenated hydrocarbons in the second stage increased the CO2 and CO concentrations. The third stage methanation hydrogenated the evolved CO2 and CO in a catalytic reactor with input hydrogen. Pyrolysis of PET produced a gas yield of 21 wt% (mainly CO2 and CO), but with catalytic cracking the yield of gases increased to over 59 wt% due to the cracking of the oxygenated hydrocarbons derived from PET pyrolysis. Reaction of the CO2 and CO with input H2 in the (iii) catalytic hydrogenation stage with a 10 wt% Ni-Al₂O₃ catalyst produced a maximum CH4 yield of 9.4 mmol g− 1, equivalent to 160 kg for each 1 tonne of PET processed. Higher nickel loadings on the catalyst significantly reduced the CH4 yield, due to reduced surface area and availability of active metal sites. Raising the temperature of the (iii) catalytic hydrogenation stage from 250 °C to 400 °C increased CH4 yield from PET processing, but decreased at higher temperatures due to the reaction equilibrium of the exothermic methanation reaction, resulting in a consequent increase in CO2 and CO. Evaluation of different metal-Al₂O₃ catalysts, showed that Ni-Al₂O₃, Fe-Al₂O₃, Co-Al₂O₃ and Cu-Al₂O₃ produced high CH4 yield from the three-stage processing of PET. Amongst the Ni-catalyst support materials investigated, the alumina (Ni-Al₂O₃) catalyst support produced significantly higher CH4 yield (9.4 mmol CH4 g− 1) compared to the other support materials (< 3.5 mmol CH4 g− 1). This study shows that the three-stage pyrolysis–cracking–methanation process can convert PET waste plastic to methane, emphasising the importance of upstream cracking reactions on downstream methanation effectiveness.