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

There is increasingly large body of evidence for the existence of carb ons characterized by a porous structure amenable to a topological repr esentation based on the concepts of fractal geometry (Part I[1] and re ferences therein). Moving from these findings, a model based on simple hypotheses is proposed for the prediction of the combustion rate of c arbons characterized by a fractal pore structure. Its concern is the l ow temperature combustion behavior of carbons under conditions typical of chemical, kinetically controlled regime. The soundness of the mode l is checked satisfactorily by comparing its predictions against those obtained by a more detailed approach based on the recursive applicati on of the Thiele analysis. The proposed model provides a useful framew ork 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 dissoc iative oxygen chemisorption and the scarce mobility of oxygen in the c hemisorbed state are indicated as possible reasons for the limited acc essibility of the micropore surface.