A FRACTAL APPROACH TO THE ANALYSIS OF LOW-TEMPERATURE COMBUSTION-RATE OF A COAL CHAR .2. MODEL DEVELOPMENT

There is increasingly large body of evidence for the existence of carbons characterized by a porous structure amenable to a topological representation based on the concepts of fractal geometry (Part I[1] and references therein). Moving from these findings, a model based on simple hypotheses is proposed for the prediction of the combustion rate of carbons characterized by a fractal pore structure. Its concern is the low temperature combustion behavior of carbons under conditions typical of chemical, kinetically controlled regime. The soundness of the model is checked satisfactorily by comparing its predictions against those obtained by a more detailed approach based on the recursive application of the Thiele analysis. The proposed model provides a useful framework for interpreting the experimental results presented in Part I[1], obtained in combustion of char from a bituminous coal. It is inferred that diffusion of oxygen within micropores is strongly activated, even at the moderately high temperature used in the combustion experiments. The formation of relatively stable oxygen-carbon complexes by dissociative oxygen chemisorption and the scarce mobility of oxygen in the chemisorbed state are indicated as possible reasons for the limited accessibility of the micropore surface.