Mathematical modeling of char reactivity in Ar-O-2 and CO2-O-2 mixtures

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 CO2 at low O-2 concentrations) affects the reactivity. The ambient gas concentrations varied from 100% O-2 to 5% O-2 in CO2 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 CO2. The reaction started with a sharp acceleration period, indicating an initial activation of the char surface.