TGA and kinetic study of different torrefaction conditions of wood biomass under air and oxy-fuel combustion atmospheres

Abstract

Combustion and oxy-fuel combustion characteristics of torrefied pine wood chips were investigated by Thermogravimetric Analysis (TGA). Three torrefaction temperatures (250, 300, and 350 °C) and two residence times (15 and 30 min) were considered. Experiments were carried out at three heating rates of 10, 20, and 40 °C/min. The isoconversional kinetic methods of FWO, KAS, and Friedman were employed to estimate the activation energies. The assessment of uncertainty in obtaining the activation energy values was also considered. The obtained results indicated that due to torrefaction, the O/C and H/C atomic ratios decreased, resulting the 300ºC-30 min and 350ºC-15 min torrefied biomass to be completely embedded in lignite region in van-Krevelen's diagram. Oxy-fuel combustion affected the decomposition of cellulose and lignin components of biomass while the impact on the hemicellulose component was negligible. The kinetic analysis revealed that with the evolution of conversion degree, the activation energy values increased during hemicellulose degradation, remained approximately constant during cellulose decomposition and showed a sharp decrease for lignin decomposition. The activation energy trends were comparable in both air and oxy-fuel combustion conditions, however slight changes in activation energy values were noticed. The highest activation energy value was obtained for 250ºC-30 min torrefied biomass at 183.40 kJ/mol and the lowest value was 72.93 kJ/mol for 350ºC-15 min biomass. The uncertainty values related to FWO method were lower than KAS and Friedman methods. The uncertainty values for FWO and KAS methods were at the range of 5–15%.