The role of porosity in char combustion

A detailed study has been conducted on the nature of porosity developed during the combustion of chars prepared from different coals, and how this relates to the apparent reactivity of the char towards oxygen. An Illinois No. 6 coal char and a Wyodak coal char have been examined in both Zone I and Zone II conditions (intrinsic rate control and diffusional mass transfer zones, respectively). These results strongly suggest that there exists some amount of free volume in a char that has not been burned off. Such an unburned char does not reveal much porosity when standard nitrogen adsorption is applied, but rapidly develops such porosity upon burnoff. The free volume opens, but is not truly accessible to reactant gases. It appears likely, based upon the present results, that the only truly available surface for reaction exists in pores larger than 10–15 Å.