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Show also similar to those reported for other bituminous coals in the litera- 1,25,28 ture. ' ' Figures 3 and 4 illustrate the approach of the observed rates to the limiting external mass transfer. As the oxygen concentration increased from 3% to 6%, the ratio of q to q (also designated as x) increased from max about 0.3 to 0.5. Comparison of the Low and High-Temperature Results The low and high-temperature data appear to represent opposite extremes of burning. In the former case, individual char particles are burning internally, with oxygen essentially penetrating throughout (i.e., Zone 1 24 27 burning ' ). In the latter case, combustion is confined primarily to the particle surface and oxygen penetration is small (i.e., Zone 2 burning approaching Zone 3). In order to directly compare the two rate expressions, it is necessary to convert k to an "apparent" k by k = k --E- (7), s w o This has been done in Figure 5 which shows both the low and high-temperature expressions in terms of k on a single Arrhenius plot. The expression of Field 28 et al is plotted for comparison. Perhaps the most obvious difference between the two temperature regimes is the apparent activation energy; the high-temperature value is about twice the low-temperature value. One consequence of this difference is that the low-temperature expression significantly under-predicts high-temperature burning rates, and the high-temperature expression significantly under-predicts low-temperature burning rates. Interestingly, at temperatures typical of AFB coal combustion C1060-1170 K bed temperature and 1060-1370 K *It is important to keep in mind that these activation energies are for intra-particle diffusion and reaction. External diffusion has been separately accounted for. 14 |