The influence of iron oxide on the oxidation kinetics of synthetic char derived from thermogravimetric analysis and fixed-bed experiments under isothermal and temperature-programmed conditions

The catalytic effect of iron oxide on the oxidation kinetics of synthetic char was investigated in a fixed-bed reactor and in a conventional thermobalance for comparison. Synthetic char doped with iron oxide was obtained by pyrolyzing hydrochar at 800 °C, which had been synthesized by hydrothermal carbonization of cellulose in the presence of iron oxide. Isothermal char oxidation in the fixed-bed reactor resulted in the most reliable kinetic results. According to model-free kinetic analysis of these experiments at 15% conversion, iron oxide decreased the activation energy of char oxidation from 149 kJ/mol to 133 kJ/mol. Modeling of the conversion-time curves was first performed by using the uniform reaction model and then improved by using a n-th order power law. In the temperature range of 440-490 °C a very good agreement with the experimental data was achieved using n = 0.6. Activation energies amounting to 149 kJ/mol and 134 kJ/mol were derived for the undoped and iron oxide-doped char, respectively, well in line with the model-free analysis.