Biomass torrefaction is a thermal pretreatment which takes place at a temperature between 200–300 °C in a non-oxidative environment. The process requires thermal energy for drying and torrefying the raw biomass. The amount of the required heat may vary depending on the biomass moisture content, operating temperature and residence time. The volatiles released during the torrefaction are usually burnt in a combustor to meet the heat requirement of the process. If the energy content of the volatiles is less than the thermal energy required for the process, the operation of the torrefaction unit is below the autothermal mode so an auxiliary fuel such as natural gas is burnt together with the volatiles.
This paper investigates autothermal operation of a torrefaction unit which consists of a dryer, a torrefaction reactor, a combustor, and two heat exchangers. An experimentally validated process model is employed to identify a relation between the moisture content, torrefaction temperature, and residence time at autothermal operation. The model is capable of predicting the composition of volatiles and torrefied biomass, mass and energy yields, process heat requirement, and CO2 emissions. The results are graphically presented allowing one to determine whether a torrefaction unit operates below or above the autothermal mode at given torrefaction temperature, residence time and moisture content. Furthermore, the effect of the main operating parameters on the carbon dioxide emissions of the torrefaction unit is discussed.