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Influence of Moisture Content on Wood Pelletizing

Influence of Moisture Content on Wood Pelletizing
With the expanding range of application, wood pellet machine has gained much popularity. During the biomass pellets production, moisture content of raw material is a crucial factor, which has the most considerable effect on both the output of wood pellet machine and wood pellet shaping.
Generally, the moisture content of raw material is 15-20%. When the materials are fed into the pelletizing chamber, they first contact with the roller. The materials between the roller and die are pressed into thin layer, as more and more materials are fed into the pelletizing chamber, the bottom part of the thin layer will be pressed into the die holes by roller. Only with the help of friction, proper temperature and pressure can we get the good-quality pellets finally.

wood pellets

Raw Materials with Low Moisture Content
If we feed the too dry raw materials into the pelletizing chamber, the dust in the materials will also enter into the pelletizing chamber because of the rotation of roller and die, thus affecting the purity of the pellets. The low density of raw materials leads to the materials can not be pressed into the die holes by roller smoothly. Besides, the friction between dry materials is not enough, producing not enough pressure, so the materials can pass through the mould without any extruding. Only enough pressure can produce enough heat to soften the lignin, so the materials will not stick each other to form a thin layer. During the compression process, the pellet surface appears the phenomenon of carbonization and fracturing. And it has a potential of spontaneous combustion.
In order to get the good quality wood pellets, we need to add water to the raw materials. In a certain scope, the more water added, the higher pressure produced. Some vegetable oil needs to be added to the raw materials with low lignin content, which can play a role of lubrication and auxiliary bonding. More water and binder need to be added to the materials with low density.

quality wood pellets

Raw Materials with High Moisture Content
If we feed the too wet raw materials into the pelletizing chamber, there will be very high pressure produced when the roller trying to press the wet raw materials into the die hole, which leads to the production of large amount of vapor. The continuous production of high pressure results in the engine of pellet machine overload, even stalling, which reduces the mechanical life of pellet machine. Large amount of vapor is produced again when forming the thin layer. When the materials pass through the die holes, huge pressure produced, which can decrease the passing speed. Even so, the heat and pressure are not sufficient to soften the natural lignin. The final pellets are soft and loose, presenting small lump. Because of the high water content, the final pellets dilate first to release water. Therefore, the final pellets are not the ideal ones. In order to get pellet with high quality, a pellet dryer is necessary for wet materials.