INFLUENCE OF INITIAL PARTICLE DENSITY ON THE REACTION MODE OF POROUS CARBON PARTICLES

In combustion of porous carbon particles, the relation between particle density and diameter: sigma/sigma(0) = (d/d(0))(alpha); was obtained in an earlier analysis [1]. This present work extends that earlier analysis to show with experimental support that (1) the power index, alpha, varies with the initial char density, sigma(0), according to alpha = alpha(0)(1 - sigma(0)/sigma(s)), where sigma(s) is the He density, and (2) that the limit value of alpha at zero density, alpha(0), is inversely proportional to the square root of the velocity constant of the reaction. Available experimental data values for alpha(0) yielded an effective value of activation energy, E, of 23 kcal/mol, and 46 kcal/mol after correction for Zone II behavior in agreement with earlier analysis of the same data by a different method [4]. For values of alpha, supporting the linear decline of alpha with sigma(0), the experimental data base was a set of 37 values obtained from 15 literature sources. The set showed a surprisingly small range in alpha values, from 0 to 7, and with most in the range 1-3, in spite of the different methods of experiment, char types, origins, particle sizes, and reaction temperatures. This suggests that, while Zone II reaction may be common, the depth of penetration of the reaction in Zone II combustion may be quite shallow. It also supports the conclusion that intrinsic reactivity from different char sources may be very similar and that differences in combustion rate may be determined less by chemical reactivity differences and more by physical access to the surface and interior of the particle, with access to the interior largely determined by initial char density, sigma(0), at the start of char combustion.