Every step in the production process can affect the final quality of the pellets, with key steps including raw material pretreatment, pelletizing, and cooling.
1.Raw material pretreatment:
Particle size: Raw materials should be crushed to a size of less than 3-5mm (wood materials can be slightly coarser, straw should be finer). Uneven particle size will result in large variations in pellet density and localized looseness; too fine a particle size will increase energy consumption and the amount of powder produced.
Mixing uniformity: When mixing multiple raw materials (such as sawdust and straw), uneven mixing will result in pellets with varying degrees of softness and hardness, and they will be prone to clumping or extinguishing during combustion.
2.Pelletizing parameters:
Mold compression ratio: The ratio of the mold's "aperture length/aperture diameter" directly determines the density and hardness of the pellets. A higher compression ratio (e.g., 1:8) results in longer compression time for the pellets, resulting in higher density and harder hardness (suitable for industrial pellets). A lower compression ratio (e.g., 1:4) results in loose and brittle pellets (suitable only for low-demand applications). Different raw materials require different compression ratios (woody raw materials are typically 1:6-1:8, while straw requires 1:8-1:10).
Roller pressure and speed: Insufficient pressure results in low pellet density and easy breakage; excessive pressure increases energy consumption and may even cause overheating and carbonization. The speed must be matched to the raw material characteristics (for raw materials with long fibers, slow speed is recommended to avoid excessive friction and heat generation).
Pelletizing temperature: Friction generates heat during the extrusion process (usually 80-120°C). Too low a temperature will result in insufficient viscosity (making it impossible to form); too high a temperature (>130°C) will cause volatile components to escape prematurely, reducing their calorific value and even producing a burnt odor.
3.Cooling and screening:
Insufficient cooling: Freshly produced pellets can reach temperatures as high as 60-80°C. If packaged directly, moisture inside cannot dissipate, causing the pellets to become damp, soft, and moldy. Cooling is required to reduce the temperature to room temperature ±5°C and reduce the moisture content to below 10%. Incomplete screening: Unscreened pellets may contain fines (>5%), which not only affects combustion efficiency (fines can easily cause furnace deflagration) but also reduces the commercial value of the pellets.
Even if high-quality pellets are produced, improper storage and transportation can lead to quality degradation:
Storage Environment: Humid environments cause pellets to absorb moisture, increasing their moisture content (above 15% will cause clumping and mold). High temperatures accelerate pellet oxidation, reducing their calorific value. Pellets should be stored in a ventilated, dry, and cool warehouse. Stacking heights should not be too high (to prevent crushing of the bottom granules).
Transportation Conditions: Inadequate sealing during transportation can absorb moisture. Severe vibrations can cause pellets to break and produce fines. Use sealed trucks and package in moisture-proof woven bags or bulk bags.