OCR Text |
Show C0«. One way to do this is to compare measured particle temperatures with those predicted from steady-state heat balances. Complete reaction to C0~ is considerably more exothermic than reaction to CO, and thus the particle temperature will be significantly higher if C0£ is the effective product. As shown in Figure 8, such calculations support the assumption that C0« was the effective product for large particle combustion. Once the reaction stoichiometry is known, the degree of mass transfer control can be determined by comparing the observed combustion rates with those predicted by Eq. (4). Such a comparison reveals that combustion of the large coal particles (i.e., 0.6 to 0.7 cm diameter) was almost completely diffusion controlled. As particle size was reduced, there was a rapid drop-off in the importance of diffusion. This is best illustrated in Fig. 9, which is a plot of the ratio R./R^ vs particle size. Figure 9 shows that chemical control was closely approached for particles <0.1 cm. Another way to evaluate the importance of mass transfer vs chemical reaction is to compare the measured burnout times with those predicted by equations (9) and (10). As is shown in Fig. 10, there is good agreement between the data for large particles and equation (9). This strongly supports mass-transfer control. Char burnout times are also useful for illustrating the importance of swelling during devolatilization. This is clearly shown in Table 3, which summarizes the swelling ratios, initial combustion rates, and burn times for series B, I, and K. Note that although the specific combustion rates per unit area drop with the reactor temperature, the char burn times also decrease with temperature. This is because the greater swelling at lower temperature (lower heating rate) increases the effective surface area per unit mass more than enough to compensate for the reduced specific rate. In fluidized bed combustors, however, the more highly swelled particles would also tend to elutriate and abrade more easily. Phase II - Chemical reaction rate measurements The Phase II experiments were run to determine the effective activation energy, reaction order, and pre-exponential factor required 26 |