This dissertation investigates a version of CLC for solid fuels, based on a single fluidised bed reactor, operated cyclically, with each cycle containing three stages. (i) Fuel is fed steadily to a bed of oxygen-carrier fluidised with steam or CO<sub>2</sub>. The fuel is gasified and the resulting syngas is oxidised by the oxygen-carrier, which is itself reduced: unreacted char accumulates. (ii) Before the oxygen-carrier is completely converted, feeding of fuel is stopped and the remaining char in the bed is gasified. (iii) The fluidising gas is changed to air and the carrier is regenerated. Experiments were undertaken on the continuous feeding of lignite char to a fluidised bed reactor containing either (i) inert silica sand, or (ii) an iron-based oxygen-carrier. The bed was fluidised with CO<sub>2</sub> in N<sub>2</sub> at 800°C during the gasification stage. The carrier was made from pure haematite (Fe<sub>2</sub>O<sub>3</sub>) powder. The gasification of the fuel was rate-limiting whether or not carrier was present, but the rate was higher, and inventory of carbon lower, with the oxygen-carrier. This was attributed to the removal, by the iron oxide, of the principal product of gasification, CO, which inhibits the rate. A mathematical model was devised to describe the fuel-feeding stage. Multiple cycles of chemical-looping combustion were performed using either (i) the granulated iron oxide oxygen-carrier, or (ii) carrier made by impregnating an alumina catalyst support with copper oxide (25 wt%CuO overall). The gasification was in 50 mol% CO<sub>2</sub>, balance N<sub>2</sub>, at 850 – 925°C. Four fuels were used. The rates of gasification of bituminous coal and activated carbon were much slower than those of the lignite and its char, resulting in an unfavourably large accumulation of carbon during stage (i). A conversion of Fe<sub>2</sub>O<sub>3</sub> to Fe<sub>3</sub>O<sub>4</sub> of close to 100 % was maintained over the course of 20 cycles for all fuels. No agglomeration was observed. At the temperatures required for gasifying the less reactive coals (> 900°C), copper oxide was problematic: the CuO formed by oxidation in stage (iii) decomposed very rapidly to Cu<sub>2</sub>O before the feed of fuel could be established. Also, the formation of unreactive copper aluminates was favoured. Both phenomena reduced the oxygen capacity of the particles. Repeated redox reactions decreased the ability of the iron oxide to withstand attrition in a fluidised bed. However, the copper carrier maintained its resistance to attrition over 10 redox cycles.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:596966 |
| Date | January 2011 |
| Creators | Brown, Tamaryn Ann |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.repository.cam.ac.uk/handle/1810/252227 |
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