An Evaluation of the Oxygen Chemisorption Capacity of Mild Gasification Char at Various Burnoff Levels

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Abstract
Our previous results addressed the reactivity of coal chars prepared at mild and severe conditions. The active site concentration of coal chars as determined by oxygen chemisorption capacity is known to serve as a predictive parameter to reflect the reactivity of these chars. However, it is not certain whether the unoccupied active surface area at a given conversion can effectively describe the corresponding burnoff rate. In this study, changes in the oxygen chemisorption capacity of char as a function of burnolT are determined in a thermogravimetric analysis system for low-lemperaturc (500°C) chars. The oxygen chemisorption capacity undergoes changes as a function of char burnoff. The results demonstrate that the changes in oxygen chemisorption capacity does not follow the same trend as the reactivity profile. suggesting that the stable carbon-oxygen complex (present on the surface but not measured during chemisorption) may play a key role in determining the overall gasification reactivity. The changes in the oxygen chemisorption capacity as a function of burnoff can be attributed to three competing mechanisms: (a) formation of stable C-O complexes, (b) opening of previously closed micropores, and finally, (c) loss of micropores because of pore coalescence and burnoff at a high level of oxidation. This study also demonstrates that the reactivity data can be normalized by any reference time (between 10- to 70-percent burnoff). Therefore, arbitrarily defined reference times used in the literature do not necessarily signify parameters of fundamental importance.