## Materials and Methods
### Process Description
#### 2.2.3 Productive Process  
As previously mentioned, this production line, in the final stage of assembly, included pelletizing equipment. The line can be divided into several sections, as shown in Figure 5, which correspond to the different stages of the process. After the raw material shredding unit, which can be considered a pre-processing step, since it is in this step that the material is received directly from the forest in the form of logs and is transformed into an acceptable format for the production process, the second phase of the process begins. The material, now in the form of chips, is placed in the self-feeder, whose main function is to feed the hammer mill, where chips will undergo a reduction in size of approximately 50% and will be transported to the dryer. The purpose of drying is to reduce the moisture content to levels below 25% and, thus, allow a more efficient torrefaction process. Then, the material is transported again to another hammer mill, where it undergoes a further reduction in size to obtain a dimension on the largest axis of 6 mm or less. This material is sent to a sieve, where it is separated into two fractions. The fraction between 0 and 2 mm is taken to a silo, where it is stored and later used in biomass furnaces that supply thermal energy to the dryer and to the torrefaction reactor. The fraction between 2 and 6 mm is sent to another silo, from where the torrefaction reactor is fed. The torrefaction reactor is the central equipment of the entire process. As already mentioned, this is a rotary drum reactor with indirect heating. In other words, the drum where the raw material circulates is heated from the outside, with no direct contact between the hot air stream, resulting from the combustion of biomass, and the raw material. The heat transfer is by means of thermal conduction, while the material circulates inside the central tube, covering the distance between the entry point and the exit point in a determined residence time. This residence time can be reduced by simply increasing the drum's rotation speed so that the material circulates more quickly. During the torrefaction process, parts of the constituent material of biomass, namely hemicellulose, are volatilized, which, in fact, defines the process temperature ranges. The volatilized material appears in the form of gases, consisting of two groups. The first is composed by permanent gases, that is, compounds that remain in the gaseous state whatever the pressure and temperature conditions. The second group is composed by condensable gases, which, when there is a change in pressure or temperature conditions, condense and return to the liquid state, which is their natural state of stability, at least within a certain temperature range, and may subsequently exhibit high viscosity, as in the case of tars, or even solidify. Figure 6 shows schematically the different types of compounds that are formed during the torrefaction process. Contrary to what happens during the pyrolysis process, where the ratio tends to the side of the formation of gaseous and liquid compounds, in torrefaction the ratio tends to the side of the products in the solid state. This solid fraction is formed essentially by the remaining material, from lignin and cellulose, that is not volatilized during the process, since the gas fraction derives mainly from hemicellulose, which is the compound most affected by the increase in temperature. However, in addition to this solid material, there is still a percentage that corresponds to the mineral fraction, namely the ashes. This fraction of ash is directly related to the content of metallic elements present in the biomass used in the torrefaction process, and can vary, even within the same type of biomass, depending on the type of soil or contamination to which it was exposed. Figure 5: Different sections that form the production line of YGE—Yser Green Energy SA. Figure 6: Main constituent compounds of each of the fractions formed during the torrefaction process.