Processes in highly porous chars under kinetically controlled conditions: I. Evolution of the porous structure

This paper presents a model that describes (1) structures of random pores and (2) the evolution of the pore structure of highly porous chars during oxidation under kinetically controlled conditions. Two types of pores were considered: macropores and micropores. Macropores were represented by empty cylinders with distributions of finite lengths and radii. The micropore structure is the result of a highly dense network of solid microcrystals which are represented by microrods of cylindrical shape with a distribution of lengths. Three mechanisms for the evolution of the pore structure were considered: (1) The network of the solid microrods remains unchanged during reaction except for changes in the dimensions of the microrods. The microrods are shortened due to break-restoration of the interrod joints. (2) Continuous coalescence and replacement of all microrods during reaction. (3) The subskeleton of large microrods does not change during reaction except for the change in dimensions of the microrods; however, coalescence takes place for small microrods. Using these models, expressions for the apparent density, microrod dimensions, total surface area, and the porosity were developed. Comparison with available experimental data validated mechanism 3. Shrinkage is explained by the continuous process of break and restoration of the internal joints of the subskeleton of large microrods.