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Show 4.1 NEGLIGIBLE RADIAL GAS VELOCITY AND CHAR BURNING Figure 4 shows the results for nondimension time T (left hand ordinate) versus coal injector position R. with e (ratio of frictional to inertial force) as a parameter. Increasing values of e indicate large frictional force or small inertial force (i.e., small particle diameter) which decreases the radial velocity of particles, thus increasing the travel time to reach wall. The nondimensional time has been converted to dimensional form using the constants listed in Table 3 and the definitions listed in Section 3.2. For the burner under consideration with d = 10 ym, e ~ 100 and e °° d * When the particle size is large, the time to reach wall reaches a peak (see curve ABC for a = 20) at some radius of injection. The reason is that particles injected at Rj = 0.10 with zero radial velocity have attained a high radial velocity by the time they reach R ~ 0.4 while the particles injected at R, = 0.4 have zero radial velocity assigned. The travel time over the radial distance, 0.1 < Ry < 0.4 is less than the radial acceleration time starting at R, = 0.4 For increasing e (decreasing particle size) such a peak almost disappears due to large drag force (or small inertial force) which decreases the radial velocity. Then using Figure 3 for various values of 3 (i.e., for various values of d ), one can obtain t versus d and draw the curve CQPD in Figure 5. If the particles burn up within the free flight time prior to impact (impact time), then only ash remains and the remaining ash is believed to break up into three to five particles/ ' Thus a typical ash particle diameter reduces to one-fifth of the original diameter for a coal particle of about 15 percent ash content on the presumption that ash from each coal particle comes to about 15 pm. Most particles of this size and smaller will be either vaporized before imparting the wall or entrained in the gas flow. Hence, most of tnis ash is lost to the gas phase. Thus, from the point of view of ash rejection to wall, the particle burhup time scale is important relative to the travel time. For simplicity, it is assumed that no ash vaporization occurs during particle combustion. The total burning time scale is a sum of (i) mixing, (ii) heating, (iii) pyrolysis, and (iv) char burning time scales. The mixing time scale is estimated to be about 1 ms and heating time scale is of the order of 10 ms for -16- ^d7AVCO EVERETT |