Dry Reforming of Waste Plastics for Synthesis Gas Production

Thermal processing is an effective technique for recycling waste plastics in a sustainable way. The pyrolysis of waste plastics, followed by reforming reactions of the pyrolysis products generates syngas (hydrogen and carbon monoxide) that has a vast array of applications. To date, the steam reforming process has been the most researched technology for syngas production from waste plastics. However, this process produces a large amount of carbon dioxide. Due to the concern related to global warming associated with the emissions of carbon dioxide into the atmosphere, the recycling of carbon dioxide through the pyrolysis-reforming of waste plastic, (dry reforming) is environmentally attractive. The dry reforming process was the focus of this research. A preliminary thermogravimetric and kinetic analysis was conducted in order to have a general understanding on the effect of CO2 in a waste plastics pyrolysis. The results show that most plastics required lower activation energy with the presence of CO2 in the pyrolysis atmosphere (N2:CO2 ratio of 7:3). A two-stage pyrolysis-catalytic dry reforming reactor was used to investigate various process conditions and types of catalyst to maximise syngas production. The two-stage fixed bed reaction systems increased the H2 in both a N2 or CO2 atmosphere. Ni/Al2O3 based catalysts with different metal promoters (Mg, Cu and Co) were selected for the investigation of pyrolysis-dry reforming of waste plastics. Among the catalysts tested, the Ni-Co/Al2O3 catalyst presented the highest catalyst activity resulting in a syngas production of 149.42 mmolsygas g-1plastic with 58% carbon dioxide conversion, also no detectable carbon formation on the catalyst surface was observed. The dry reforming reaction was also favoured with the Ni-Co/Al2O3 catalyst with high cobalt content. Various process parameters such as catalyst preparation method, reforming temperature, CO2 feed input rate and catalyst to plastic ratio were tested. It was found that the addition of steam in the catalytic-dry reforming process manipulated the H2/CO molar ratio, based on the type of catalyst used and the CO2/steam feed ratio. Better catalyst activity in relation to H2 production was observed for the Ni-Mg/Al2O3 catalyst and Ni-Co/Al2O3 catalyst favoured CO production. Different types of plastics; individual and mixed plastics from different waste treatment plants were also processed through the catalytic-dry reforming process to determine the syngas production and catalyst activity of Ni-Co/Al2O3 catalyst. This research has suggested that the use of carbon dioxide as the reforming agent in the dry reforming process of waste plastics was comparable to the current reforming technology with an optimum syngas production of 148.6 mmol g-1SWP.