OCR Text |
Show The dry deposit may be eroded by incoming solid particles. Pieces of the deposit may also separate from the tube and drop off. The frequency of shedding of deposit may be accelerated by periodic sootblowing and load changes. As the thickness of the deposit increases, the temperature at its surface may become high enough that a few particles remain sticky after cooling to the surface temperature. At this point the deposit becomes a much more efficient collector, because the sticky portion of its surface now collects both sticky and nonsticky particles, and the sticky area is not subject to erosion. As its thickness and surface temperature increase further, the fraction of the incoming sticky particles which remain sticky on the surface also increases, accelerating deposit growth. Under favorable conditions, the mass of deposit per unit area of tube eventually reaches a steady value at which the addition of new material is balanced by the loss due to shedding. In order to account for observed influences of ash composition and temperature on deposition rate, a mechanism is needed which relates particle sticking probabilities to their composition and temperature . The mechanical properties of coal slags under steady conditions may be approximately described either as those of a Newtonian liquid or Bingham solid, depending on whether the temperature is above or below the temperature of critical viscosity (Reid and Cohen, 1944). An approach oriented toward the solid character of ash particles has been based on the work of Hertz (1881). This analysis is obtained by consideration of the elastic-plastic deformations of the impacting particles and deposit surface on collision. Extensions of the treatment to include the process of adhesion have been developed by Brenner, Wriedt, and Oriani (1981); Rogers and Reed (1984); and Ahluwalia, 1m, and Hajduk (1986a, 1986b). 4 |