Cracks in wood pellets may be caused by a variety of reasons, the following are several common situations and their solutions:
1. The wood pellets are bent and have many cracks on one side
Reason: This phenomenon usually occurs when wood pellets leave the ring mold. When the position of the cutter is far away from the surface of the ring die and the knife edge is blunt, the pellets are easy to be broken or torn by the cutter when extruded from the die hole rather than cut off. This causes some of the wood pellets to bend to one side and many cracks to appear on the other.
The solution:
Increasing the compression force of the ring die on wood pellets means increasing the compression ratio of the ring die, thereby increasing the density and hardness value of the pellets.
The wood pellet raw material wood chips are crushed finer to improve the compactness of the wood pellets and prevent the soft wood pellets.
Adjust the distance between the cutting tool and the surface of the ring die, usually the distance between the knife edge and the outer surface of the ring die is not greater than the diameter value of the wood pellets produced, or replace the use of a sharper cutting blade.
2. Horizontal cracks across the entire wood particle
Reason: Similar to the first case, the crack occurs on the cross section of the wood particle, but the particle is not bent. When the fluffy wood chips containing more fibers are pelleted, because they contain fibers longer than the pore size, when the pellets are extruded, the pellets produce transverse cracks on the cross section due to the expansion of the fibers, forming the appearance of wood pellets similar to fir bark.
The solution:
Increasing the compression force of the ring die on wood pellets means increasing the compression ratio of the ring die.
The fineness of the fiber is controlled, and its maximum length cannot exceed one-third of the particle size.
Reduce the yield to reduce the speed of wood pellets passing through the die hole and increase the density.
3. Wood pellets have vertical cracks
Reason: Some customers in the production process, due to the choice of the type of dryer, the wood chips can not be evenly dried, resulting in uneven moisture content of raw wood chips. After the ring mold compression granulation, it will spring open due to the action of water and the elasticity of the raw material itself, resulting in vertical cracks.
The solution:
Improve the drying effect of the dryer to ensure that the water content of the raw wood chips is uniform.
Increase the effective length of the die hole to improve the compression effect.
4. Wood pellets have radiating cracks from a source point
Reason: The main reason for the occurrence of this situation is that the wood chips contain relatively large wood chips. When raw materials with similar fiber degree are pelleted, they will be squeezed and fused with each other, such as the presence of larger fibers, which affects the interaction between fibers. During cooling, due to the different degree of softening, the difference in the amount of shrinkage is caused, resulting in radiation cracks.
The solution:
Properly control the thickness and uniformity of powdered feed raw materials, and add simple screening powder to screen wood chips with larger pellets before granulation.
5. Uneven surface of granular material
Reason: The powder used for granulation contains large granular raw materials that have not been crushed or semi-crushed, because the pellets are relatively hard and relatively large, they can not be well combined with other raw materials when passing through the die hole of the granulator, making the pellets appear uneven.
The solution:
Properly control the thickness of powdered raw materials.
6. Improper moisture ratio of raw materials
Reason: raw material moisture ratio of about 13-20%, if too high or too low will cause the wood pellets do not form or molding is not good.
The solution:
Adjust the moisture ratio of raw materials according to the actual production situation.
7. Improper clearance between ring die and press roll
Reason: the gap between the ring die and the press roll is normal because the distance between the outer diameter of the press roll and the high point of the ring die is 0.1-0.3mm. The gap is too large, the pressure is not enough, can not extrusion molding; The gap is too small, which increases the friction of the ring die press roll, reduces the service life of the ring die and affects the yield of the granulator.
The solution:
Adjust the gap between the ring die and the press roll regularly, and generally adjust it once every 7 days.
8. The device temperature is inappropriate
Reason: The equipment temperature is too high or too low will have an important impact on the molding density of the pellets and the effect after molding. The temperature is too low, and the input heat is reduced, which is not enough to make the material forming; If the temperature is too high, the thermal decomposition of the particle fuel surface will be serious, cracks will appear, the strength of the particle fuel will decrease, and even it can not be formed.
The solution:
Control equipment temperature within the appropriate range.
Through the above analysis and solutions, the problem of wood pellet cracks can be effectively reduced and product quality can be improved.