Role of porous structure in char oxidation

The role of the porous structure and its effect on reactivity was studied from changes of macroscopic properties. We present a review of our experimental-modeling effort that concerns porosity and reactivity. Shrinkage gave an understanding into the structure, and occurs because of the reduction of microcrystal dimensions. Detailed modeling for the porous structure was developed and reproduced the shrinkage. Fragmentation increased the understanding into the behavior of the porous structure; it depends on the structure geometry and threshold porosity, and has been shown to occur when macroporosity reaches a threshold. Tiny details of the structure were gained from thermal conductivity (TC). A five-time decrease was measured from 0 to 30% burnout, then TC remained constant up to 80% burnout and increased twofold to 100% burnout. Heat transfer calculations using the same model showed an excellent fit to experimental data, indicating that the extent of connectivity between microcrystals is the single most important parameter for TC. The behavior of TC at low conversions is consistent with the reactivity data, since steep changes in TC are accompanied by generating defects which are attributed to active sites. (C) 1998 Elsevier Science Ltd. All rights reserved.