OCR Text |
Show content aluminosilicates and iron oxides whose boundaries are difficult or impossible to see in the photograph. The gray spheroidal features are lower iron content aluminosilicates which had higher viscosity and were less distorted on impact than the brightest particles. The enrichment of iron in the layer adjacent to the tube is confirmed by the analysis given in Table 4. After about three quarters of an hour, when the iron-rich deposit described above had grown to about a millimeter in thickness, the rate of deposit growth increased markedly. The structure changed to a less dense, more friable deposit superimposed on the thin, bonded layer . The outer layer had approximately the average composi tion of the coal ash, and built up and fell off repeatedly, leaving behind the iron-rich layer and a transition region of intermediate strength. Most of the deposit visible in Figure 3 is the friable material. A schematic diagram of the structure of the deposit is shown in Figure 8. When the outer, friable layer falls off, the break occurs to the left of the indicated boundary between the inner and outer layers. Similar behavior was observed by Lavasseur and coworkers (1986) during experiments with bituminous coal at Combustion Engineering. Measurements of the growth of the deposit retained on the tube are shown in Figure 9. MODEL Deposit Growth The flux of ash Fi impacting the tube (mass per unit time per unit tube area projected on a plane perpendicular to the undisturbed flow) is the product of: area, F coal' 2. 1. the coal feed rate per unit convection passage open the mass fraction of ash in the coal, Y h' as 20 3. the |