**5. Reaction kinetics of biomass torrefaction**

To a large extent, torrefaction process can influence the overall kinetics and mechanism [35] as well as the reactivity, reaction behavior, and thermal conversion performance of biomass which is often estimated through thermogravimetric analysis (TGA). Through TGA, the reaction kinetics parameters including the mechanism function, pre-exponential factor, and activation energy [36, 37], as well as the thermal features including temperature at peak value, reaction period, peak value, and thermogravimetric (TG) data can be determined. In addition, biomass torrefaction is often regarded as a complex mechanism due to the several reactions involved including the decomposition of the common biomass components including lignin, cellulose, and hemicellulose as well as moisture evaporation.

The decomposition of the biomass components depends to some extent on the temperature at which the torrefaction process is carried out. For instance, at the temperature range of 160–900°C, lignin slowly decomposes, cellulose decomposes at 315–400°C, while the hemicellulose decomposes at the lowest temperature ranging from 220–315°C [38]. For instance, while the torrefaction of hemicellulose leads to more devolatilization and carbonization, the depolymerization of cellulose plays a vital role in the decomposition mechanism. The study on the kinetics of biomass torrefaction is very important because it represents the torrefaction reaction thereby predicting the optimum thermal degradation conditions which can ultimately enhance the process control for continuous torrefaction reactor [39, 40]. Meanwhile, at the temperature range of 230–300°C [41], the kinetics of torrefaction reactions can be best described by a two-step mechanism; cellulose and hemicellulose decomposition.
