Sawdust pellets, also known as biomass pellet fuel, are primarily made from agricultural waste, such as wood, grains, peanut hulls, alfalfa, corn hulls, cottonseed hulls, and straw. These materials are crushed into wood chips, which are then processed by a wood pellet machine. The material is forcibly extruded through a die hole of a fixed diameter and compressed into wood pellets, making it one of the most popular environmentally friendly energy sources today.
With the rise of wood pellet fuel, the biomass pellet production industry has flourished. I believe that with just a pellet machine, wood pellets could be continuously transported from the conveyor belt to all corners of the country, and even the world.
However, during the production of wood pellets, various issues may arise, such as the following:
- Pellets are not formed or the forming rate is low
- Loose particles and powder
- Short or cracked particles
- Freshly pressed pellets are good, but crack after some time
There are several reasons for the cracking of solid wood pellets. It is recommended to investigate gradually, starting with the die and roller ring, and then address the root cause to resolve the particle cracking issue.
Reason 1: Raw Material Moisture
The moisture content of the raw materials used for production should be around 10-15%. If the moisture is too high or too low, the wood pellets will not form properly or may form poorly. Due to the hardness of the raw materials and other factors, the specific moisture value should be based on your actual production conditions.
Reason 2: Raw Material Granularity
The particle size of the raw material is another factor affecting the formation of wood pellets. Sawdust with a particle size between 1-5mm is easier to shape, and the resulting pellets have a smoother appearance with relatively fewer cracks. If the particle size is too large and exceeds the mold's aperture, it will be pressed into the mold in one piece during production, causing poor adhesion, breakage at the contact points between particles, and resulting in short pellets.
Reason 3: Ring Die Compression Ratio
The compression ratio of the wood pellet machine's ring die is an important parameter for determining the molding time and the compression strength of the pellets. A small compression ratio will result in a shorter compression time and lower compression strength, which can lead to the pellets being misshapen or failing to form at all.
Reason 4: Gap of the Ring Molding Roller
The gap between the ring die and the pressure roller should typically be between 0.1-0.3mm, measured from the outer diameter of the pressure roller to the high point of the ring die. When the equipment is idling, it should not make any noise. After feeding, the pressure roller rotates and the roller shell reverses to feed the material. If the gap is too large, there will be insufficient pressure for extrusion; if the gap is too small, the material may be compressed more smoothly, but this increases friction between the ring die and the pressure roller, reducing the service life of the die and affecting pellet output.
It is also important to note that, due to wear between the pressure roller and the ring die, the gap between them should be adjusted regularly—typically once every 7 days, depending on actual production conditions. Failure to adjust the die roller after prolonged use can cause misalignment, resulting in an uneven gap that leads to unbalanced pressure on the material.
Reason 5: Equipment Temperature
The temperature of the equipment has a significant impact on pellet formation. If the temperature is too low, insufficient heat is provided to form the material, and it increases the power consumption of the pellet machine. If the temperature is too high, the surface of the pellets may undergo excessive thermal decomposition, causing cracks and reducing their strength, or in severe cases, making them unable to form.
Reason 6: Ring Die Wear
Over time, as the ring die reaches the end of its service life, the inner walls of the die holes wear down, causing the diameter of the holes to expand. This can lead to deformed or cracked pellets, or even powdered particles. Additionally, the increased roughness of the inner walls after wear reduces the smoothness of the particle surfaces and obstructs the material's feeding and extrusion, ultimately lowering the pellet output.
