Cause Analysis and Improvement of Cracks in Wood Pellets

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Abnormal wood pellets

In the production of wood pellets, sometimes abnormal wood pellets appear. Next, we will talk about the various situations of wood pellet cracks and their improvement methods.

Cracks in Wood Pellets

Wood pellets bend and show many cracks on one side

This phenomenon usually occurs when the wood pellets leave the ring die. When the cutter position is set far from the ring die surface and the cutter edge is blunt, pellets are easy to be broken or torn by the cutter rather than cut off. At this time, some wood pellets bend to one side and many cracks appear on the other side. In the process of cooling or transporting into the cooler, this kind of sawdust pellets often break from these cracks, resulting in too much sawdust powder or too short pellets.

Improvement methods

  1. Increasing the compression pressure of the ring die on wood pellets (increasing the compression ratio of the ring die) to increase the density and hardness of the pellets.
  2. To improve the density of wood pellets and prevent wood pellets from softening, sawdust raw materials should be crushed more finely.
  3. Adjust the distance between the cutter and the surface of the ring die, or replace the sharper blade. The distance between the knife edge and the outer surface of the ring die is usually not greater than the diameter of the wood pellets produced.

Horizontal cracks traverse the whole wood pellet

Similar to the above phenomenon, cracks occur on the cross section of wood pellets, except that the pellets are not bent. This may occur when fluffy sawdust containing more fibers is granulated. When the sawdust pellets are squeezed into the pelletizing holes of the ring die, they contain fibers with a longer diameter than the holes. When the pellets are extruded, the expansion of the fibers causes cross-sectional cracks in the cross-section of the pellets, resulting in the appearance of fir bark-like sawdust pellets.

Improvement methods

Increasing the compression pressure of the ring die on wood pellets, that is to say, increasing the compression ratio of the ring die. Controlling the fineness of fibers, the maximum length of fibers should not exceed one third of the pellet size. Reducing the output to reduce the speed of sawdust pellets passing through the die hole and increase the density.

Vertical cracks in wood pellets

Because of the type of dryer, some customers can not even dry sawdust, resulting in uneven moisture content of raw material. After the ring die compression granulation, it will pop up due to the role of moisture and the elasticity of raw material itself, resulting in vertical cracks.

Improvement methods

Improve the drying effect of the dryer and increase the effective length of the die hole.

Radiation cracking of wood pellets from a source point

The main reason for the occurrence of this kind of situation is that the sawdust contains relatively large sawdust. Materials with similar fiber content will squeeze and fuse each other during granulation. If there are large fibers, the interaction between fibers will be affected. It is not as easy to soften as other finer raw materials, but in cooling, due to different softening degree, resulting in different shrinkage and radiation cracks.

Improvement methods

Properly control the fineness and uniformity of powdery raw materials, add simple sieve powder before granulation.

The surface of particles is uneven

Raw materials contain large size raw materials which have not been crushed or semi-crushed. Because they are relatively hard and large, they can not be well combined with other raw materials when passing through the die hole of the pellet machine, making the pellets uneven.

Improvement method

Proper control of the fineness of powdery raw materials.

Individual or inter-individual pellet color discrepancy

This situation is relatively rare. The main manifestation is that the color of individual pellet extruded from the ring die is darker or lighter than that of other normal pellets, or the surface color of individual pellet is inconsistent, thus affecting the appearance quality of the whole batch of pellets.

Improvement methods

Controlling the return material. For the pellets which are prone to produce "flower material", the return material should be mixed with the raw material and then re-crushed. The ring die with guaranteed quality is used to control the smoothness of the die holes. If necessary, the die holes are sanded and then used.

In short, there are many reasons for these phenomena, and they are not completely isolated from each other. This requires us to make more analysis and grasp the key to solve these problems in our practical work.

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