OCR Text |
Show 13 out, the oxygen concentrations are found to be only 2-4/6 for the flame region greater than 1 meter from the burner. Thus incomplete burn-out can result in large char particles being present in the exit region of the combustor, and these are likely to be captured in the slag with the result of producing locally reducing conditions in the deposit. It is known that, under reducing conditions, initial deformation and ash fusion temperatures of iron-rich coal ash are up to 200 C lower than under oxidizing conditions. Such an explanation can account for both the molten behavior of the slag mass and the formation of iron-rich crystals on the slag surface. A proposed mechanism of deposit formation which includes both of these features is described below. Proposed Mechanism of Deposit Formation The mechanism of deposit formation can be divided into three stages: the initial deposit formation of the metal surface, followed by growth of this initial layer into a heavy deposit, and the aging of the deposited material at the temperatures and conditions inside the combustion chamber. The initial deposit is most likely formed by condensation of vaporized mineral constituents on cool heat transfer surfaces, in addition to the capture of fly ash particles covered with a sticky layer of the same condensed mineral species. Once a liquid or sticky layer is formed on the metal surface, the majority of the mass of the deposit is added by direct impaction of large ash particles, which leave the bulk flow because of inertial forces. After the sticky condensed layer becomes coated with particles, further capture of particles is quite slow unless a progressive liquid film is present to bond them to the slag surface. In the early stages of deposit growth, further condensation of vaporized mineral matter could contribute this liquid layer, but as the deposit grows and the surface temperature increases, the condensation of vaporized mineral species will cease. The iron contained in the coal ash is the most likely component to form a lower-melting compound with the silica/alumina ash particles. A well-known compound that can be formed under these conditions is fayalite, 2Fe0.Si0p, which may reduce the temperature of partial melting well below 100C C. Experimental measurements of ash fusion and initial deformation temperatures of different coals are shown in Figure 27. As iron content increases under reducing conditions, the depression of ash fusion and initial deformation temperatures becomes more severe. Vith the possibility of carbon capture discussed above and the resulting reducing conditions the iron seems to be the most likely cause of lower melting temperatures of the slag. Such a liquid phase would result in the major growth phases of the deposit by enhancing capture of airborne ash particles from the flame and also by binding the captured particles together. Reactions between the silica/alumina particles and the alkali compounds contained in the ash could also produce such a liquid layer, but this would be expected to form at lower temperatures than those under which these deposits were collected. Many changes can occur after the deposit is formed, while it is aged in the combustion chamber. The alkali compounds are reported to "distill" from the bulk of the deposit to the cooler area adjacent to the tube surface, thus contributing to metal wastage through corrosion. Any carbon or sulfur captured in the deposit is likely to have enough residence time to |