Kinetic analysis of char thermal deactivation

• Published in: Proceedings of the Combustion Institute 2000, Vol.28 (2), p.2181-2188
• Main Authors: Zolin, Alfredo, Jensen, Anker, Dam-Johansen, Kim
• Format: Article
• Language: English
• ISSN: 1540-7489; 1873-2704
• DOI: 10.1016/S0082-0784(00)80627-X

The thermal deactivation of several fuels was investigated by measuring the reactivity of chars prepared in a thermogravimetric analyzer (TGA) apparatus at well-defined conditions in the temperature range 973–1673 K. Four coals, Blair Athol from Australia, Cerrejon from Colombia, Illinois no. 6 and demineralized Dietz from USA, and two alternative fuels, Danish leached straw and petroleum coke, were used in the experiments. The coal chars from demineralized Dietz, Illinois no. 6, and Cerrejon deactivate readily, whereas petroleum coke and Blair Athol show a relative high resistance to deactivation. Leached straw deactivates significantly, but maintains at any heat-treatment temperature a higher reactivity than the other chars. The inertinite-rich coal Blair Athol is more resistant to deactivation than two vitrinite-rich coals of the same ASTM rank, Cerrejon and Illinois no. 6. Cerrejon and Illinois no. 6 chars prepared in the TGA at 1673 K show a much lower reactivity than carbon extracts from boilers operated with these coals, possibly owing to enhanced annealing conditions in the TGA, such as low heating rates, relatively high temperatures, and long holding times. Based on the char reactivity data for all chars obtained from the TGA, kinetic parameters in an annealing model reported in the literature were determined. A shifted Γ distribution taking into account a positive activation energy at the beginning of deactivation, as well as a lower bound of char reactivity corresponding to the reactivity of a commercial graphite, provided the best fit to the experimental deactivation ratios. The annealing model predicts reasonably well the changes in reactivity of chars prepared in different reactor environments with much higher heating rates and temperatures than the TGA apparatus. This indicates that TGA experiments can be used to capture the reactivity differences of chars observed in combustion facilities.