Oxygen-carriers were prepared by supporting CuO on Al<sub>2</sub>O<sub>3</sub>, using three methods: mechanical-mixing, wet-impregnation and co-precipitation. The co-precipitated particles (containing 82.5 wt.% CuO) were best; they managed to maintain full conversion for 20 cycles of reduction (either in 2.6 vol% CO or 2.6 vol% H<sub>2</sub>, or mixtures thereof) and re-oxidation in a fluidised bed. The kinetics of the reduction (by CO and H<sub>2</sub>) and oxidation (by O<sub>2</sub>) of the co-precipitated particles were fast and controlled primarily by external mass transfer from the bulk gas to the external surface of the particles. At high temperatures (between 700 and 900°C), the reduction and oxidation took place <i>via</i> the intermediate of Cu<sub>2</sub>O: the reactions of the particles also followed the shrinking core mechanism. At low temperatures (~ 300°C), no shrinking core mechanism was detected. The particles are surprisingly reactive at temperatures as low as 250°C; the reduction proceeded directly to Cu and was controlled by the nucleation of cooper atoms to form bigger nuclei, which subsequently initiated the reaction between CuO and CO or H<sub>2</sub>. To gauge the impact of hydrocarbons present in the gasifier gases, the reaction between the co-precipitated particles and CH<sub>4</sub> and C<sub>3</sub>H<sub>8 </sub>was studied. It was confirmed, using XRD, that these gases also reduce the oxide fully to Cu. The formation of carbon was observed. It was catalysed by the reduced carrier when CH<sub>4</sub> was the fuel. With propane, the decomposition to carbon was dominated by reactions in the homogeneous gas phase and appeared to be unaffected, substantially, by the presence of reduced oxygen-carrier. When steam or CO<sub>2</sub> was added to the feed gases, the formation of carbon was totally suppressed for CH<sub>4</sub> and partially suppressed for C<sub>3</sub>H<sub>8.</sub>
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:597682 |
| Date | January 2010 |
| Creators | Chuang, S. Y. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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