Emissions of the gaseous pollutants NO and NO<sub>2</sub> (collectively known as NO<sub>x</sub>) are subject to increasingly stringent controls. The following reaction scheme has been proposed as a method of reacting the toxic gases NO and CO: 3 NO + 2 Fe ® Fe<sub>2</sub>O<sub>3</sub> + 3/2 N<sub>2 </sub>(VII) ; 3 CO + Fe<sub>2</sub>O<sub>3</sub> ® 2 Fe + 3 CO<sub>2</sub>. The net effect of these reactions is CO + NO ® CO<sub>2</sub> + ½ N<sub>2</sub>. Reaction (VII), above, has been studied in detail in a very sensitive balance, a differential reactor and a laboratory scale fluidised bed. Kinetic parameters for reaction (VII), along with associated activation energies, were measured using the balance. The reaction was found to be marginally slower in the balance than in the differential reactor. The rate of reaction was found to be substantially faster (by a factor of up to ten) in a fluidised bed. The proposed explanation for the large difference in the rate of reaction measured in the balance or differential reactor and in the fluidised bed depends on the attrition of a weakly adherent oxide from the surfaces of iron particles in the fluidised bed. The latter stages of the reaction were also studied in the balance. It was found that after a thick layer of oxide had built up on the surface of a particle of Fe, the rate became almost independent of [NO] in the surrounding gas. This behaviour was ascribed to a solid-state diffusion-mechanism controlling the rate of reaction. Solid-state diffusion coefficients were measured, along with associated activation energies. Particles were also examined in an electron microscope both before and after reaction. Those particles which had been reacted in the balance were found to be very different in appearance from particles which had been reacted in the fluidised bed. In a separate study, the rates of oxidation of coal-chars in a fluidised bed which was contained in a quartz tube were measured. Three coal-chars of differing ranks were studied. The dependence of the rate of reaction on [O<sub>2</sub>] and temperature were examined for each coal-char; rate constants were derived, along with activation energies. Chars from Cynheidre (high rank) and Reitspruit (middle rank) appeared to obey a similar rate law, the mechanistic implications of which are here studied in detail. Char from a lignite appeared to obey a very different rate law; a mechanism is also proposed for reaction of this char. A study of the method of addition of the char to the bed was also conducted. This concluded that the method for adding char to the bed was very important when considering the formation of NO<sub>x </sub>during combustion of the char; the method of addition had less effect on the rate of formation of CO and CO<sub>2</sub>. It was also found that a stainless-steel heat-shield which prevented direct radiative heating of the upper surface of the fluidised bed and the freeboard above it had a marked effect on the rate of reaction of the coal-chars.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:598972 |
| Date | January 2003 |
| Creators | Fennell, P. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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