Hydrogen and carbon nano-materials from the pyrolysis-catalysis of wastes

In this work, a two-stage fixed-bed reaction system was used for the production of carbon nanotubes along with hydrogen production from waste tyres and plastics from a pyrolysis-catalysis/catalytic-reforming process. The preliminary investigations concerned different metal catalysts (Ni/Al2O3, Co/Al2O3/ Fe/Al2O3 and Cu/Al2O3), which were investigated to determine the effects on carbon nanotube and hydrogen production by pyrolysis-catalysis of waste truck tyres. The results showed catalyst addition in the pyrolysis-catalysis of waste tyre process can increase hydrogen production. The Ni/Al2O3 catalyst gave the highest hydrogen production at 18.14 mmol g-1 along with production of relatively high quality carbon nanotubes which were homogenous. The influence of catalyst support was investigated with different SiO2:Al2O3 ratios (3:5, 1:1, 3:2, 2:1) with nickel. The results showed that the Ni-based SiO2:Al2O3 supported catalyst at a 1:1 ratio at 900 oC with sample to catalyst ratios at 1:2 gave the highest hydrogen production at 27.41 mmol g-1, and the 1:1 ratio gave the highest filamentous carbon production at 201.5 mg g-1. The influence of process parameters on hydrogen and CNTs production were investigated with the Ni/Al2O3 catalyst. Hydrogen production reached the highest amount which was 27.41 mmol g-1 at 900 oC with sample to catalyst ratio was 1:2. The highest filamentous carbon production was produced with the sample to catalyst ratio at 1:1 at 900 oC catalyst temperature. The water injection rates were also investigated, the results showed that water introduction inhibited filamentous carbon production but increased the hydrogen production. An in-depth study to better understand the process involved investigation of three different tyre rubbers and five tyre pyrolysis oil model compounds to understand the mechanism of carbon nanotubes formation in waste tyres by the pyrolysis-catalysis process. The results showed that natural rubber which is the main component of tyre samples which used for this thesis, dominated hydrogen production at 25 mmol g-1 and SBR gave the highest carbon formation which was 40 wt. %. The aliphatic model compounds (hexadecane and decane) favoured gaseous hydrocarbons formation instead of solid carbon formation, but the aromatic model compounds (styrene, naphthalene and phenanthrene) favour solid carbon formation where the majority of carbon formation was filamentous carbon. The study was extended to investigate waste plastics and different types of waste plastic feedstock used in the pyrolysis catalysis/catalytic reforming process to produce hydrogen and carbon nanotubes. As carbon nanotubes separation from the catalyst is a challenge for this project, the nickel metal catalyst was loaded on stainless steel mesh and applied in the high-density polyethylene pyrolysis-catalysis process. The benefit of this catalyst has been shown in that the carbon formation could be easily separated by physical shaking from the stainless steel-nickel mesh catalyst. However, further investigation on waste plastics was concentrated on hydrogen production and where carbon nanotubes were the by-product from the process. Fe-based and Ni-based catalysts as bimetallic catalysts supported by MCM-41 with different Fe:Ni ratios were investigated using simulated mixed waste plastics. A synergistic effect of the iron and nickel was observed, particularly for the (10:10) Fe/Ni/MCM-41 catalyst where the highest gas yield (95 wt.%) and highest H2 production (46.1 mmol g-1plastic have been achieved. Along with lowest carbon deposition which was 6 wt.% with carbon nanotubes formation. Seven real world waste plastics were used to produce hydrogen and carbon nanotubes in the presence of a Fe:Ni at 10:10 ratio catalyst with an MCM-41 support. The results showed that the agricultural waste plastic gave the highest hydrogen production that was 55.99 mmol g-1 with carbon nanotubes formation. The calorific values of the produced gases from different plastic samples were in the range of 12.13 - 24.06 MJ m-3, which could provide the process fuel that shows the possibility to apply the technology for further larger scale of research.