# Sugarcane Bagasse Torrefraction for Fluidized Bed Gasification
## 2 The Torrefaction Process  
Torrefaction is a thermochemical pretreatment process that improves the characteristics of biomass to be used as fuel in fluidized bed gasifiers. The enhancements are related to removing the volatiles and cracking part of the feedstock fibers. However, these improvements should be accomplished without losing too much energy to volatile products throughout the process.  
Solid-phase forms the main torrefaction product, comparable to the pyrolysis process, and is referred to as the charred residue or torrefied feedstock. The chemical structure of feedstock is altered in the process, forming diverse volatile products, and some liquids at room temperature (condensable). Other than torrefied feedstock, the main process product is carbon monoxide, carbon dioxide, methanol, acetic acid, and water [32]. All these non solid reaction products contain relatively more oxygen compared to untreated biomass. Hence, the C/O ratio of torrefied biomass is higher than untreated biomass, increasing the torrefied feedstock's heating value.  
High values of the C/O ratio (such as coal) are related to greater gasification efficiencies, while for low values of this ratio (such as biomass in general), the gasification efficiency drops. Therefore, fuels with high oxygen content are not ideal for gasification from an exergetic perspective [33].  
In the torrefaction process, biomass loses its tenacious nature and part of its fibrous structure. The biomass usually has a darker color through torrefaction and becomes more suitable to be crushed into smaller pieces. Furthermore, the biomass is wholly dried during the process, and after it, the uptake of moisture is minimal, varying from 1-6% depending on the torrefaction conditions and the storage of the torrefied feedstock afterward.  
The main reason for the hydrophobic behavior of torrefied biomass is the reduction of the hydrogen bonding capability by eliminating OH groups. The most reactive biomass polymer during torrefaction is hemicellulose [34; 35; 36]. After torrefaction, it has been entirely reacted to alternative char structures and volatiles. Most weight loss can be attributed to hemicellulose because torrefied biomass mainly consists of cellulose and lignin [25; 37; 38].  
The temperatures of the torrefaction process range between 200 \({}^{\circ}\)C and 300 \({}^{\circ}\)C, according to previous studies [39; 40]. Torrefaction above this temperature would result in extensive devolatilization and carbonization of the polymers, both undesirable for torrefaction products [39]. Also, the loss of lignin in biomass is very intensive above 300 \({}^{\circ}\)C. Furthermore, fast thermal cracking of cellulose causes tar formation, which starts around 300-320 \({}^{\circ}\)C [41]. These reasons fix the upper limit of torrefaction temperature as 300 \({}^{\circ}\)C for most biomasses.  
The residence time exerts a significant influence on the thermal degradation of biomass. Longer residence times usually lower the mass yield and increase the energy density. However, the residence time influences the torrefaction products; but it is not as dominant as the final torrefaction temperature. The effect of residence time on mass loss diminishes after 60 min [42].