Pressing wood pellets is a process that involves compressing wood feedstock through a die, where it is extruded and cut into pellets.
How do you press wood pellets? 7 key steps to know
1. Feedstock Compression
The wood feedstock is introduced into a press channel where it encounters rollers.
Each pass through the rollers compresses the material further.
This compression forces the material into the channels of the die.
2. Extrusion and Cutting
As the material is pressed into the die channels, it forms cylinders of compressed material that are extruded from the outside of the die.
These cylinders are then cut into pellets by blades.
3. Heating and Drying
The process of extrusion generates frictional heating within the die, raising the temperature of the pellets to between 90–105°C.
This heat causes the moisture within the pellets to evaporate, effectively drying them.
The final moisture content of the wood pellets is typically between 7–10% on a wet basis, which is necessary to meet standard requirements.
4. Formation and Quality
The pellets are cylindrical with flat ends, and their thickness depends on the amount of material compressed and the force applied.
The dies used in the process are not fastened but align with the press ram, allowing for quick reloading and efficient operation.
5. Press Mechanism
The compression force is typically generated by hand pressure on a lever attached to a large steel cam.
This mechanism provides a mechanical advantage, allowing a small force on the lever to generate a much larger force on the press ram, sufficient to produce firm pellets.
6. Scale and Variations
While large-scale pellet mills are used for industrial production, smaller-scale mills, such as screw presses or hydraulic presses, are also available.
These operate on similar principles, using a die to shape the pellets and a platen to apply compression.
Some smaller presses may include heating elements to speed up the process and improve pellet quality.
7. Technology and Applications
Pelleting technology is primarily used for biomass, such as wood, to produce pellets as a solid biofuel.
The process involves extruding the material through a die under high pressure and temperature, which plasticizes the lignin in the biomass, acting as a natural binder.
The pellets harden and gain mechanical resistance after cooling.
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