CO2 gasification rates of char particles from torrefied pine shell, olive stones and straw

Single char particles with a diameter of 8 mm were produced in situ from pellets of raw and torrefied pine shell, olive stones and straw. The single char particles were then gasified at 800, 900 and 1000 degrees C in two or three CO2 concentrations at each temperature. In the experiments, CO concentrations were measured from the product gases. From the measured CO concentrations, the char conversions versus time were determined. The reactivity of the char from the torrefied olive stones was lower than the reactivity of the char from the raw olive stones, while the reactivity of the char from the torrefied straw was higher than the reactivity of the char from raw straw. For the raw and torrefied pine shell the char reactivities were similar. At 900 degrees C the influence of torrefaction on char gasification rates was minor, however, since the conversion occurred under Regime II conditions, i.e., the conversion rate is partly limited by mass transfer. A detailed single particle model, taking into consideration mass transfer effects, was used to extract kinetic parameters from the experimentally determined char conversions. For the six chars, activation energies of the adsorption step were in the range 175 and 285 kJ/mol, while activation energies of the desorption step were in the range 145 and 195 kJ/mol. In the study it was also tested whether the char conversion could be computed with unified activation energies for all the chars. The results show that by using unified activation energies - 240 kJ/mol for the adsorption step and 168 kJ/mol for the desorption step - the computed char conversions were in good agreement to the experimental char conversions.