Hydrogen Rich Syngas And Carbon Nanotubes Production From Pyrolysis-catalysis Of Waste Plastics

The growing demand towards plastic materials for the development of human society contributes to a huge increase in waste plastics.Environmental crisis that comes from plastic debris in the marine environment and plastic planet has become a serious issue that urgently need to be addressed.The key to manage the waste plastic is the recycling waste in a clean and valuable way.With the support from EU Horizon 2020 program and National Natural Science Foundation of China,the study on the catalytic pyrolysis of waste plastics for the production of hydrogen rich syngas and high value carbon nanotubes was conducted in this thesis.The preparation method and modification mechanism of nickel catalyst for this technology were studied.The regulation of final gases and solid products was discussed.In addition,the synergic catalytic effect of bimetallic Ni-Fe catalyst,and the growth mechanism of carbon nanotubes was explored.The main works are shown as follows.Pyrolysis-steam reforming of waste high density polyethylene for the production of hydrogen rich syngas has been investigated using different zeolite supported nickel catalysts in a two-stage reactor system.Experiments were conducted into the influence of the type of zeolite in relation to hydrogen and syngas production.The Ni/ZSM5-30 catalyst presented the best performance,with production of 66.09 mmol H₂ g^-1_plastic and 34.63 mmol CO g^-1_plastic,and showed excellent coke resistance.While Ni/Y-zeolite-30 catalyst generated the least activity with high coke deposition?⁶ wt.%?.Results indicated that hydrogen production was less affected by the Si:Al ratio than the type of zeolite support.The excellent performance of Ni/zeolite for the pyrolysis-catalytic reforming of plastics was ascribed to the porous structure of the zeolite support,which benefits the uniform dispersion of Ni into its inner structure thus providing more chances for the interaction reactions between volatiles and active sites.Three catalyst preparation methods?co-precipitation,impregnation and sol-gel?were investigated to futher improve the catalyst activeity for the pyrolysis-steam reforming of waste plastics.Results showed that the porosity of the catalyst and Ni dispersion were significantly improved using the sol-gel method,producing a catalyst surface area of 305.21m²/g and average Ni particle size of 15.40 nm,leading to the highest hydrogen yield among the three catalysts investigated.The least effective catalytic performance was found with the co-precipitation prepared catalyst which was due to the amorphous coke deposits.In regarding to the type of plastic,polyolefins experienced more decomposition reactions at the conditions investigated,resulting in higher hydrogen and coke yield.However,the catalytic steam reforming ability was more evident with polystyrene,producing more carbon from the feedstock and converting more carbon into carbon monoxide gases.Overall the maximum syngas production was achieved from polystyrene in the presence of the sol-gel prepared Ni/Al catalyst,with production of 62.26 mmol H₂ g^-1_plastic and 36.10 mmol CO g^-1_plastic.Catalytic pyrolysis of real-world waste plastics was conducted to produce hydrogen and high value carbon nanotubes?CNT?,and the influence of catalyst composition and support materials has been investigated.The carbon yield from?-Al₂O₃ supported catalyst was lower than that from?-Al₂O₃ supported one.However,the fine metal particles on the former support contribute to the higher catalytic activity towards hydrogen and carbon nanotubes.Compared with the support materials,the growth of carbon nanotubes and hydrogen are more highly dependent on the catalyst composition.Fe/?-Al₂O₃ produced the highest hydrogen yield(22.9 mmol H₂/g_plastic)and carbon nanotubes yield(195 mg g^-1_plastic)among the monometallic catalysts,while Ni based catalyst obtained higher gas yield.Based on the catalytic effect of both Ni and Fe,the bimetallic Ni-Fe catalyst was suggested for H₂ and carbon nanotubes production from waste plastic.Different Ni-Fe/?-Al₂O₃ catalysts with Ni and Fe loading at variant molar ratios were investigated to explore the individual role of each metal compound for optimizing products.Results show that the bimetallic catalyst showed the highest catalytic activity in relation to H₂ yield because of the optimum interaction between metal and support.The hydrogen yield of 31.8 mmol H₂/g_plasticlastic with content of 62.88 vol.%,and 287 mg g^-1_plastic of uniform carbon nanotubes with diameter around 20⁴0 nm were obtained in the presence of bimetallic Ni-Fe/?-Al₂O₃.In addition,catalyst with more Fe loading produced more hydrogen and deposited carbon,due to higher cracking ability.The presence of Ni in Ni-Fe bimetallic catalyst enhanced the thermal stability and graphitization degree of produced carbons.Finally,different operational conditions were studied to optimize the product hydrogen and the yield of carbon nanotubes deposited on the catalyst,and the growth mechanism of CNTs was also discussed.Results were analysed by TEM,SAED and XRD to reveal the catalytic mechanism of bimetallic Ni-Fe catalyst for the growth of carbon nanotubes.Higher catalyst temperature favoured both the hydrogen and filamentous carbon production.However,further increase in temperature led to nonselective types of carbon with lower quality.The introduction of steam greatly promote the both yields of hydrogen and carbon monoxide but at the sacrifice of the purity of carbon nanotubes.The amorphous carbon from the catalytic decomposition of waste plastics was catalysed by Fe₃C and Ni-Fe in the catalyst to produce the graphite carbon.The process for the carbon nanotubes formation was assigned to the vapor-liquid-solid model.With the interfacial capillary interaction between carbon and metal as well as the inner tension of metal catalyst particles,the tip-growth of carbon nanotubes was dominant at higher catalyst temperature,while it was the base-growth mechanism at relatively lower temperature.