OCR Text |
Show iron, silica, silicates, calcite, atomically dispersed species (sodium, calcium, potassium, magnesium, and titanium), and 'other.' The variable t is a material time scale, designating particle residence time relative to the time of injection of the particles. It typically varies between 0 and 3 seconds. The variable t designates elapsed or laboratory time, i.e., time relative to an arbitrary time of day independent of the particle residence time. It represents, for example, the time between soot blowing cycles and typically varies between 0 and 20 hours. In a steady-state or stationary system, the only relevant time scale is t and ash deposition rates, composition, and all other characteristics of the process would have the same mean values at all times at a given location. However, ash deposition clearly is a non-stationary process (li, 16.). Therefore, both the material and elapsed time scales must be addressed. Equation 1 can be thought of as an ordinary differential equation parameterized by the variable r. Practical illustrations can be used to clarify the differences in the two time scales. Changes in deposit composition from one location to another in a boiler are indicative of variation of one or more of the terms in Equation 1 with particle residence time (t). For example, commercial and pilot scale observations indicate that ash deposits formed from eastern and midwestern, pyritebearing coals are enriched in iron. This enrichment is most pronounced when the deposits are sampled near the burners, with typical enrichments of 60 %. Deposits sampled midway between the burners and the furnace exit are slightly less enriched in iron. A typical enrichment of 40 0/0 may be observed in this area. Near the furnace exit, iron enrichment in the deposit drops to 10 to 20 %. This change in deposit composition with location is a reflection of the residence time (t) dependence of the inertial impaction and particle capture efficiency terms in Equation 1. Changes in deposit composition as a function of deposit thickness are indicative of variation of one of the terms in Equation 1 with clock time ('tJ. For example, deposits formed in the convection pass of boilers typically show pronounced variation in composition between the heat exchanger surface and the outside of the deposit. These composition changes are often associated with variation in the condensation rate with r. As the deposit accumulates, its surface temperature increases and the rate of condensation decreases. Each of the major mechanisms of ash deposition indicated in Equation 1 are conceptually reviewed below. A cylinder in cross flow is used to illustrate several of the mechanisms although the same mechanistic processes describe deposition on both cylinders and water walls. Inertial Impaction I (t, r) Inertial impaction is most often the process by which the bulk of the ash deposit is transported to the heat transfer surface. Particles depositing on a surface by inertial impaction have sufficient inertia to traverse the gas stream lines and impact on the surface. The particle capture efficienc describes the propensity of these particles to stay on the surface once they impact. The rate of inertial impaction depends almost exclusively on target geometry, particle size and density, and gas flow properties. The capture efficiency depends strongly on these parameters and on particle composition and viscosity (ll). It also depends on deposit surface composition morphology, and viscosity ill). The relative magnitudes of the characteristic time and dimension of particle and fluid relaxation processes control the rate of inertial impaction. Specifically, inertial impaction occur when the distance a particle travels before it fully adjusts to changes in the fluid velocity is larger than the length scale of an object, or target, submerged in the fluid . The particle Stokes number is defined as the ratio of these length scales. 4 |