Mathematical Modeling of Char Reactivity in Ar-O_2 and CO_2-O_2 Mixtures

摘要

The kinetics of the coal char + O_2 reaction was studied by thermogravimetry. Low sample masses were employed to ensure an approximate kinetic regime. Special emphasis was placed on clarifying how the recirculation of the flue gases (i.e. the presence of a high amount of CO_2 at low O_2 concentrations) affects the reactivity. The ambient gas concentrations varied from 100% O_2 to 5% O_2 in CO_2 or Ar. A semiempirical model is presented that can approximate the reactivity changes during the conversion and takes into account the heterogeneity of the samples. A least-squares evaluation procedure resulted in a good fit to the experimental data over a wide variety of temperature programs and ambient gas concentrations. The overall burn-off time of the samples varied from 8 min to 3 h depending on the experimental conditions. The reaction rate was found to be proportional to the O_2 concentration of the ambient gas and was not influenced by the presence of high amounts of CO_2. The reaction started with a sharp acceleration period, indicating an initial activation of the char surface.