Besides Pelletizing Equipment, What Factors Affect Pellet Strength
Besides pelletizing equipment, there are factors influence your pellet strength and durability. In order to improve your pellet quality and the working efficiency of your pelletizing equipment, it’s better to learn about the factors, such as moisture content, particle size, preheating condition, chemical composition of raw materials, binders, and/ or lubricants, and other processing factors concerning the pelletizing equipment.
Moisture content is always one of the most influencing factors on pellets strength and durability. The right amount of moisture functions as a agent during the process of pelletization which will influence the pellet strength and durability by spreading the lignin which will form a natural “glue” that holds the pellet together as it cools down. Moisture within certain amount also functions as lubricant which will improve the flowing of the raw materials through the die holes of the pelletizing equipment but too much moisture will effect badly to the pellet combustion efficiency. So the right moisture content is very important to the pellet durability.
What’s more, moisture content has significant effect on the glass transition temperature. According to Kaliyan and Morey, the increasing moisture content of corn stover from 10% to 20% resulted in a decrease in the glass transition temperature. It seems that the primary impact of increased moisture content is to lower the glass transition temperature of the constituents, such as lignin, starch, and gluten . The effect of the moisture on the glass transition temperature of the constituents are shown in the following diagram.Effect of moisture on glass transition temperature (recreated from Chirfe and Del Pilar Buera, 1994)
Particle size is one of the most important factors affecting overall quality of pellets. Finer particle sizes generally correspond with greater pellet strength and durability as larger particles serve as fissure points(MacBain 1966). A mixture of particle sizes will produce optimal pellet quality due to the increased inter-particle bonding (mechanical interlocking) and the elimination of inter-particle spaces( attractive and adhesive and cohesive forces). What’s more, the particle size also is also an significant factor to the pelletizing equipment working life for the finer particles is, the longer the serving life of the pelletizing equipment will be.
Preheating is essential to activate inherent binders in the raw materials. The natural binders in the material itself such as lignin, proteins, and starches, etc., needs to be activated and reorganized to form a solid bridges and a primary mechanism of particle agglomeration through preheating. What’s more, the plastic deformation of particles is facilitated by the heat and moisture, which will increase the inter-particle contact area and decrease the inter-particle distance, simultaneously increasing intermolecular bonding and eventually improving the strength and durability of the pellets.
Lubricants and Binders
Lubricants and binders are used to assist the pellet formation and increase the pellet durability to avoiding decomposition during storage. Water and vegetable oil are the most commonly used lubricants to reduce friction between the die and the feedstock.
Compressibility of Raw Materials
Raw material compressibility has a large impact on compaction behavior. Compressibility is the change in bulk density of a feedstock before and after loading. At the relatively high loading conditions around 125MPa, plastic deformation of particles is expected, which is similar to the loading conditions experience in commercial pelletizing equipment. Compressibility can also be evaluated at lower pressures at around 100 KPa, by which the materials are in the elastic range. Little particle deformation takes place. Rather, the rearrangement of particles resulted in changes in bulk density, measured as volumetric strain.
The ultimately of interest is the bulk material response which is driven by individual particle interactions. The particle density, shape, strength and surface characteristics are important in determining the load-response of a system. And also the thickness swelling, internal bond strength, and linear expansion of particle board are all affected by particle shape. The measurement of the particle shape and surface roughness is difficult however the measurement of these characteristics can provide useful information regarding compaction behavior, even qualitatively.
Besides the factors mentioned above, there are other factors affect the pellet strength and durability such as the processing factors related to the pelletizing equipment. The factors that concerns to the pelletizing equipment is the pelletizing equipment itself, and the die dimensions, die speed and the gap between the roller and die. Different pelletizing equipment is used according to the raw materials to produce optimal quality pellets. (More detailed information on the process viable on the pelletizing equipment is discussed in How does a pellet mill impact the pellet quality.)
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