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Abstract
Increased concerns on anthropogenic greenhouse gas emissions have renewed interest in the CO2 (dry) reforming process as an alternative to steam reforming for synthesis gas production from natural gas. However, dry reforming is highly endothermic, and suffers from carbon-induced catalyst deactivation. This thesis therefore investigates the combination of the exothermic partial oxidation reaction with dry reforming, and the effects of lanthanide oxide promoters (Ce, Pr and Sm) on alumina-supported Co-Ni catalyst.
Lanthanide promotion did not alter reaction rate significantly. However the interaction of lanthanide oxides with the surface carbonaceous species resulted in substantially reduced carbon deposition (by up to 50%) on promoted catalysts, with Ce providing the greatest coking resistance followed by Pr and Sm. A dual-site Langmuir-Hinshelwood kinetic mechanism was proposed, with associated activation energies of about 47 kJ mol-1. Two types of carbonaceous deposits were present on the used catalysts a reactive species which was present in smaller quantities on the promoted catalysts, and a less reactive pool (which may only be completely removed by O2) whose quantities remained similar in all the catalysts.
O2 co-feeding resulted in increased CH4 conversion with O2 partial pressure before levelling out at O2:CH4 = 1, while CO production decreased and H2 formation reached a maximum before a slow decline. Product H2:CO ratio increased from 0.9 for pure CH4 dry reforming and peaked at 1.7 with O2 co-feeding, showing that the oxidative dry reforming of CH4 may be used to generate near ideal syngas composition for downstream processing such as the Fischer-Tropsch synthesis. Furthermore, the overall heat demand for the reforming reaction may be reduced by managing the combination of CO2/CH4/O2. Post-reaction analysis revealed that even a low amount of O2 fed (CO2:CH4:O2 = 1:1:0.25) leads to undetectable carbon deposition.
Longevity runs showed that coke-induced deactivation was absent during O2 co-feeding or at CO2:CH4 ≥ 2. At CO2:CH4 < 2, carbon deposition via rapid dehydropolymerisation resulted in the significant loss of CO2 activity, even as H2 was produced from CH4 dehydrogenation. Conversion-time data were adequately fitted to a generalised Levenspiel reaction-deactivation model, with activation energies estimated to be 66-129 kJ mol-1.