On the modelling of the peripheral fragmentation during coal char gasification with CO2

Abstract In this paper, a novel mathematical model for the coal char gasification with CO2 is developed. This new approach considers the non-uniformity of pore size distribution, the longitudinal growth of pores due to the surface reaction between CO2 and char, the pore overlapping effect, and the peripheral fragmentation phenomenon. This phenomenon was simulated by incorporating a fragmentation criterion based on both a critical porosity and a critical porosity gradient. Model predictions were compared with experimental data taken by sieving, gas adsorption porosimetry and surface area analysis, and a remarkable agreement was obtained. It was found that critical porosity and critical porosity gradient do not depend either on the operating conditions or the initial particle size but on the initial microstructure of the coal char. Furthermore, these two parameters could not vary significantly between different coal char samples. Regarding the fragmentation, it is observed that as the diffusional limitation increases, this phenomenon takes place in even earlier stages of reaction due to the appearance of porosity gradients within the particle. It was also observed that a maximum on the specific surface area appears due to the pore overlapping phenomenon, and that under chemical control conditions, the average specific surface area that can be reached is larger than that obtained under conditions with diffusional limitations.