Changes in reactive surface area and porosity during char oxidation

The intrinsic reactivity of coal chars has long been of fundamental scientific interest, but concern about this reactivity has grown in recent years. Both nitrogen BET and CO 2 surface areas are in common use for characterizing the surface areas available for reaction of chars under intrinsic reaction-rate control and zone II conditions. The best choice of adsorptive is not yet clear. This study compares nitrogen, argon, and carbon dioxide to explore the porosity of a subbituminous coal char burned off to differing extents in oxygen, nitric oxide, and carbon dioxide. The results confirm that reaction in zone II does not permit as effective opening up of microporosity, mesoporosity, and macroporosity as reaction under zone I conditions. It also appears possible that microporous area might be utilized to a very limited extent during reaction, as neither the size nor the population of micropores changes very much over wide ranges of burn-off. This may have important implications for the normalization of reaction rates using the traditional surface-area measurements. These traditional methods may overstate the importance of the micropores in providing reactive surface.

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