CO₂ gasification of chars prepared by fast and slow pyrolysis from wood and forest residue. A kinetic study

The CO₂ gasification of chars was investigated by thermogravimetric analysis (TGA). The chars were prepared from spruce and its forest residue. Prior to the gasification, the raw materials were pelletized and pulverized. Part of the samples was directly gasified in the TGA when the char was formed at low heating rates before the gasification. Another sort of char was prepared in a drop-tube reactor (DTR) at a heating rate of around 1 × 104 °C/s and a residence time of 0.2 s at 1200 °C. The kinetic evaluation was based on TGA experiments with linear, modulated, and constant-reaction rate (CRR) temperature programs. The gasification of the DTR chars took place at temperatures 80–100 °C lower than the chars formed at low heating rates. The chars formed at low heating rates exhibited a side reaction that occurred 80–100 °C below the main peak of the mass-loss rate curves during the gasification. Accordingly the gasification kinetics of these chars was described by assuming two pseudocomponents. The thermal annealing (thermal deactivation) of the chars during the gasification experiments was taken into account by the pre-exponential factors, which were allowed to have different values at different temperature programs. A strong compensation effect was observed between the activation energy (E) and the rest of the kinetic parameters. Nevertheless, the obtained E values varied in a narrow interval (from 219 until 227 kJ/mol) and were very close to the ones obtained for other chars with similar kinetic evaluation procedures (Wang et al., Energy & Fuels 2013, 27, 6098−6107 and Wang et al., Energy & Fuels 2014, 28, 7582–7590).