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Show particles; this behaviour may be consistent with a shift from kinetic-dif-fusional regime for larger particles towards a more pronounced cinetic regime for smaller particles. From the experimental results obtained with different oxygen concentrations an overall order n of 0.5 with respect to oxygen pressure seems to be the most acceptable. This may be seen from figure 13 , where especially in the case of larger particles the experimental points fit acceptably well the same Arrhenius curve. This again is consistent with a kinetic-diffusional regime, at least for the larger particles, if one assumes the combustion rate to be de sorption controlled ( in which case n = 0 ). Figure 14 shows the experimental value of rate constants as a function of temperature, determined for some coals and compared to the rate constant obtained on a cenospherical soot by the same method. The apparent activation temperatures (d) range from 7000 (Beulah lignite), to 12000 K , with a surprisingly high value of 16000 to 19000 K for a given low volatile high ash coal (Pocahontas, W. Virginia). With the higher discussed assumption of a desorption controlled combustion in the kinetic -diffu si onal regime, these values suggest for the decomposition of the oxygen surface complexe an activation temperature ranging from 14000 to 24000 K, or even to 32000 to 38000 K for the particular case of Pocahontas coal. 2.4. Extinction of coal combustion. As may be seen e.g. from figure 7 in the case of coal, or from figure 10 in the case of char, the combustion in many cases ceases before total burnout of the carbon contained in the sample. The fraction of carbon burnt when the reaction ceases (F ) may be relatively small at temperatures close to the ignition temperature, increases with increasing grid temperature and approaches to unity at temperatures close to 7-25 |
