Hydrogen was produced from waste plastics using a novel two-stage pyrolysis-low temperature (250 °C) plasma/catalytic reforming process. The effect of operational parameters and catalysts was first investigated with high density polyethylene. Pyrolysis of the polyethylene generated pyrolysis gases which were catalytically steam reformed in the presence of low temperature non-thermal plasma (dielectric barrier discharge) to produce hydrogen gas. In the absence of catalyst, increasing the plasma power resulted in a significant increase in hydrogen yield. Different catalysts (Ni/Al2O3, Fe/Al2O3, Co/Al2O3 and Cu/Al2O3) were incorporated in the discharge region of the plasma reactor and the Ni/Al2O3 produced the highest yield of hydrogen at 1.5 mmol g-1plastic. Addition of steam to the plasma catalytic process was investigated at different steam weight hourly space velocities (WHSV) using the Ni/Al2O3 catalyst. The addition of steam to promote catalytic steam reforming reactions resulted in a marked increase in hydrogen yield, producing the highest hydrogen yield of 4.56 mmol g-1plastic at a WHSV of 4 g h-1 g-1catalyst.
Catalyst support materials such as MCM-41, Y-Zeolite, ZSM-5, Al2O3, TiO2, dolomite, BaTiO3, CaTiO3 and Mo2C were investigated to find a suitable catalyst material for the plasma environment. Some of the materials suppressed the generation of the plasma, while others enhanced it by improving the generation of micro discharges and surface discharges. Among the tested materials, MCM-41 gave the highest gas yield of 29.2 wt.% and hydrogen yield of 11 mmol g-1plastic. The coupling of the catalyst with the plasma environment resulted in synergy in terms of enhanced total gas yield and hydrogen production which was higher than in the absence of plasma (catalyst alone) or plasma alone (no catalyst). Other parameters investigated using the MCM-41 support material showed that particle size and catalyst bed depth affected the plasma discharge and the total gas yield. Impregnating nickel (10 wt.%) on the MCM-41 support further enhanced the total gas and hydrogen yield due to increased surface reactions. The 10 wt.% Ni/MCM-41 was stable when subjected to a three-hour stability test showing no significant change in the yield of the gases.
The effect of plastic structure and composition was investigated by carrying out pyrolysis-non-thermal plasma/catalytic reforming of different individual waste plastics (high density polyethylene, low density polyethylene, polypropylene, polystyrene and polyethylene terephthalate). Simulated mixed plastics and real world mixed plastics from municipal solid waste, mixed plastics from household packaging (without PET), mixed plastics from construction sites, agricultural foils and mineral water packaging were also investigated. High density polyethylene produced the highest yield of hydrogen at 18 mmol g-1plastic while the simulated mixed plastics, which is dominated by polyethylene, produced 16.9 mmol g-1plastic. Among the real world waste plastics, the agricultural foils consisting mainly of polyethylene and polypropylene produced the highest yield of hydrogen at 15.4 mmol g-1plastic.
This research shows that the two-stage pyrolysis−non-thermal plasma reactor can be used for the conversion of waste plastics to produce hydrogen gas at low temperatures and atmospheric pressures.
