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Show 1 15.2 to 16.5 kcal/mole. Similar low-temperature results are reported by 19 20 21 22 23 Sondreal and Ellman, Schmidt, Yohe, Dryden, and Carpenter and Giddings. The B§W AFB elutriate and TVA AFB elutriate data combined are fit by k = 74.50 exp (-16,730/RT ) (3), with a correlation coefficient of 0.96. The TVA elutriate data appear to follow a slightly different trend than the B§W elutriate data, but the two data points are not sufficient to confirm this. Without including the TVA elutriate points, the pre-exponential factor and activation energy are 66.50 s atm ' and 16,764 cal/mole, respectively, yielding a correlation coefficient of 0.98. From Figure 1, it is apparent that the reactivity of the elutriate is substantially lower than that of the parent coal. The activation energies are similar, but the pre-exponential factor for the elutriate is much smaller than that for the parent coal. This implies that either the reactive surface of the elutriate is less accessible, or the number of active sites has been reduced. High-Temperature Experiments The parameters which describe particle burning rates were determined from the temperature, velocity, and size data by integrating the heat, mass, and momentum balance equations for particles burning in the entrained flow reactor. Details of these calculations are given in Mitchell. The combined intra-particle diffusion and reaction are assumed to follow an Arrhenius expression of the form (1/AJ dm/dt = q = -k P n = -A' exp (-E'/RT ) (4), a 5 b O p s r where q is the rate of carbon disappearance per unit external surface area 2 (gC/cm • s). The Arrhenius parameters, A' and Ef, are then determined by making at least-squares fit of the calculated burning-rate coefficients, k , versus the reciprocal of the observed particle temperature. External diffusion is accounted for by assuming a particle Sherwood number of 2.0." The oxygen partial pressure at the particle surface is calculated by equating the external and internal oxygen flows. 12 |
