## Materials and Methods
### Identification of Faults, Malfunctions and Process Conditions
#### 2.3.2 Second Testing Phase (2014-2015)
The liquid formed then dripped on the solid material, creating a paste that, although the temperature being already lower than in the reactor, usually around 300 \({}^{\circ}\)C (\(\pm\)10 \({}^{\circ}\)C), suffered the effects of a long residence time and was carbonized. This carbonized material created a thick covering over the shaft and the helix which, in the course of time, became detached, leaving the fragments trapped in the space between the helix and the inner wall of the auger, creating friction and even stopping the movement of the auger, as verified several times when the continuous operation time of 4 to 5 consecutive days was exceeded. This made it necessary to develop a solution to prevent this deposition from happening, because of the more important fact shown in the laboratory that this condensed material impregnated the torrefied biomass pellets, significantly increasing the risk of self-heating and ignition, mainly after the further heating to which the material is then subjected after pelletization, since 94 \({}^{\circ}\)C is easily reached when leaving the die. The tests in the laboratory were carried out in a simple way but were capable of simulating a real situation that could easily happen. That is, the torrefied biomass pellets impregnated by the liquid resulting from the condensation of the torrefaction gases could be subjected to a significant increase in temperature, volatilizing again, and this situation could become dangerous if there was an ignition source nearby, or if the auto-ignition temperature of any of the gases was reached. Figure 14 shows the results of laboratory tests, where a set of pellets of torrefied biomass were deposited on a tray and placed inside a greenhouse previously heated to 95 \({}^{\circ}\)C, leaving the door open. The torrefied biomass pellets started igniting immediately, burning without visible flame, demonstrating that the same could easily happen, for example, in material stored in a silo on a summer day. Another experiment was carried out with a large amount of torrefied biomass pellets, which were collected immediately after leaving the pelletizer. This material was deposited outside, ensuring a safe distance, and was left exposed to the weather. After 4 days, the pile of pellets ignited and had to be extinguished with water. The process was repeated several times, including on several rainy days, when the auto-ignition again occurred and rekindled, even after apparently being extinguished with water. Figure 15 shows one of those moments, with one of the operators of the industrial unit extinguishing another ignition, which occurred on a rainy day. Figure 14: Torrefied biomass pellets submitted to a temperature of approximately 95 \({}^{\circ}\)C.  
This situation of self-heating and auto-ignition is associated with the existence of black particles, which, despite being cooled on the outer surface, continue to have a heated core, which slowly transmits the heat stored by thermal conduction to the outside, volatilizing the condensed materials; if there are many black particles, this can cause the materials to auto-ignite. This phenomenon occurs when the particles are not completely cooled. Currently, there is technology available for the detection and control of this type of particle, even in motion, preventing them from accumulating and becoming a problem. However, the best option will always be to cool the particles effectively. In addition to the danger associated with the self-heating and auto-ignition phenomena, there is also the possibility that the condensed materials will again volatilize and recondense, forming runoff in storage and use areas. Examples of this are shown in Figures 16 and 17, where, respectively, runoff of condensed material from the deposit of a boiler, and runoff on the outer surface of the storage silos of the torrefied biomass pellet production unit itself, can be observed. Figure 16: Condensate runoff in a boiler where the torrefied biomass pellets were being tested.