Key Factors Influencing Biomass Pellet Hardness

Biomass pellets are a type of clean energy that play a crucial role in energy conversion and environmental protection. The hardness of pellets is an important quality indicator that directly affects their combustion efficiency, storage stability, and transportation convenience. This article will thoroughly analyze the various factors that influence the hardness of biomass pellets produced by pellet machines and provide optimization suggestions to help biomass pellet production companies improve product quality.

1. Raw Material Particle Size

1.1 Impact of Particle Size on Pellet Hardness

The particle size of raw materials is one of the key factors affecting pellet hardness. Finer particle sizes allow the raw materials to gelatinize more easily during the pelletizing process, enhancing the internal bonding of the pellets and thereby increasing their hardness. Fine particles can combine more tightly during compression, reducing internal gaps and making the finished pellets less prone to breakage and harder.

1.2 Optimization of Particle Size Distribution

To improve pellet hardness, the particle size distribution ratio of the raw materials can be adjusted as follows:

• Coarse Particles (particle size above 900μm): No more than 15%. Excessive coarse particles can lead to a loose internal structure, reducing hardness.
• Medium Particles (particle size around 700μm): Approximately 35%. Medium-sized particles help enhance the structural stability of the pellets.
• Fine Particles (particle size below 500μm): Over 50%. Fine particles provide a good bonding foundation, increasing pellet hardness.
• Ultra-Fine Particles (particle size below 250μm): At least 25%. Ultra-fine particles further strengthen the bonding force, enhancing overall hardness.

This particle size distribution not only facilitates pellet formation and improves appearance quality but also ensures appropriate hardness while reducing pellet friability, thereby enhancing overall product quality.

2. Type of Raw Material

2.1 Impact of Different Raw Materials on Pellet Hardness

Different types of biomass raw materials exhibit varying hardness characteristics during the pelletizing process. For example:

• Woody Materials: Such as sawdust and wood chips, have high cellulose and hemicellulose content, providing good bonding properties and resulting in harder pellets.
• Herbaceous Materials: Such as straw and grass, have high fiber content but relatively lower bonding properties, leading to softer pellets.
• Other Biomass: Such as agricultural waste and energy crops, have hardness influenced by their specific composition and fiber structure.

2.2 Importance of Formula Adjustment

Proper adjustment of the proportions of different raw materials is an effective way to regulate pellet hardness. By scientifically balancing different materials, it is possible to ensure both the hardness of the pellets and the cost-effectiveness and utilization rate of the raw materials. For instance, increasing the proportion of woody materials can enhance pellet hardness, while adding herbaceous materials can reduce production costs and improve raw material utilization.

3. Moisture Content of Raw Materials

3.1 Impact of Moisture Content on Pellet Hardness

The moisture content of raw materials is a critical factor influencing pellet hardness. Generally, raw materials should have a moisture content controlled between 13% and 18%. Specific requirements may vary depending on the type and nature of the raw material. For example, woody materials can tolerate a wider range of moisture content, whereas herbaceous materials require more stringent moisture control.

3.2 Negative Effects of Excessive Moisture

Excessive moisture in raw materials can increase the air content within the pellets, affecting their compactness and reducing hardness. Additionally, high moisture levels may lead to mold growth and deterioration during storage, compromising product quality. Therefore, it is essential to adequately dry raw materials before pelletizing to ensure the moisture content meets the required standards.

3.3 Impact of Environmental Humidity

Environmental humidity also plays a significant role in affecting pellet hardness. In high-humidity environments, raw materials are more prone to moisture absorption, increasing their moisture content. Therefore, controlling the production environment is crucial to maintaining pellet hardness.

4. Compression Ratio of the Die Ring

4.1 Relationship Between Die Ring Aperture and Compression Ratio

The aperture and compression ratio of the pellet machine’s ring die are key technical parameters that determine pellet hardness. The compression ratio refers to the ratio of the inner diameter to the outer diameter of the die, directly affecting the density and hardness of the pellets.

4.2 Impact of Compression Ratio on Pellet Hardness

Under the same aperture, a higher compression ratio results in higher pellet density and increased hardness. Specifically:

• High Compression Ratio: Produces denser pellets with higher hardness but may cause pellet discoloration, increased energy consumption, and reduced production efficiency.
• Low Compression Ratio: Results in lower pellet density, insufficient hardness, higher friability, and challenges in storage and transportation.

Therefore, selecting an appropriate compression ratio die ring can ensure pellet hardness while improving production efficiency and optimizing product quality.

5. Machine Equipment Factors

5.1 Impact of Machine Speed on Pellet Hardness

The speed of the pellet machine directly affects pellet formation and hardness. If the machine speed is too slow or unstable, pellets remain in the die longer, leading to a loose internal structure and reduced hardness. Conversely, too high a speed may result in incomplete pellet formation, negatively affecting hardness and quality.

5.2 Impact of Die Wear on Pellet Hardness

The degree of die wear also influences pellet hardness. Severely worn dies can reduce the surface smoothness of pellets, affecting their compactness and hardness. Regular maintenance and replacement of dies are essential measures to ensure pellet hardness.

5.3 Impact of Overall Equipment Performance

In addition to speed and die wear, the overall performance of the pellet machine, such as compression force and heating system stability, affects pellet hardness. Efficient and stable equipment ensures uniform force distribution during pelletizing, enhancing the overall hardness and quality of the pellets.

6. Cooling Time

6.1 Importance of Cooling Treatment

Pellets exit the machine at temperatures between 75-90°C with moisture content of 14-16%, making them prone to breakage and mold growth during storage. Therefore, cooling treatment is a crucial step in ensuring pellet quality.

6.2 Optimization of Cooling Time and Airflow

Cooling effectiveness is primarily influenced by cooling time and airflow rate. Typically, cooling time ranges from 5 to 25 minutes, and airflow should be adjusted based on pellet diameter:

• Low Airflow, Long Cooling Time: Ensures thorough cooling both inside and outside the pellets, enhancing hardness and reducing cracking and friability.
• High Airflow, Short Cooling Time: While cooling quickly, it may cause surface cracking, reduce hardness and wear resistance, and increase friability.

Thus, the cooling process should be tailored to the specific characteristics of the pellets, selecting appropriate cooling parameters to ensure hardness and stability.

7. Pellet Length

7.1 Relationship Between Pellet Length and Hardness

In the absence of defects, pellets with the same diameter tend to be harder when their length is longer. This is because longer pellets can better distribute stress during compression, enhancing overall hardness.

7.2 Factors Affecting Pellet Length

The main factors influencing pellet length include:

1. Pellet Machine Speed: Higher speeds result in shorter pellet lengths.
2. Die Ring Aperture Rate: Higher aperture rates lead to shorter pellet lengths.
3. Release Hole Design: Shorter release hole lengths result in more uniform pellet lengths.
4. Raw Material Viscosity: Higher viscosity leads to more uniform pellet lengths.
5. Knife Sharpness: Sharper knives produce more uniform pellet lengths.
6. Material Maturation: Higher material maturation improves pellet length uniformity.

7.3 Methods to Adjust Pellet Length

To maintain uniform pellet hardness, the following methods can be used to adjust pellet length:

• Adjusting Knife Position: Ensures consistent pellet length by maintaining the appropriate knife position.
• Optimizing Speed and Aperture Rate: Adjust the pellet machine speed and die ring aperture rate as needed to control pellet length.
• Maintaining Equipment: Ensure knives are sharp and dies are not excessively worn to enhance pellet length uniformity.

8. Other Influencing Factors

8.1 Use of Additives

Adding binders and lubricants can improve pellet formation performance and hardness. For example:

• Binders: Such as lime or bentonite can enhance the bonding force of pellets, increasing hardness.
• Lubricants: A small amount of oil can reduce pellet sticking to the die, improving surface smoothness and hardness.

8.2 Temperature Control

Temperature control during pelletizing significantly affects pellet hardness. Appropriate temperatures promote the gelatinization and bonding of raw materials, enhancing hardness. Temperatures that are too high or too low can negatively impact pellet formation and hardness.

8.3 Molding Pressure

Molding pressure is a critical parameter influencing pellet hardness. Adequate molding pressure increases pellet compactness and hardness, but excessive pressure may cause pellet discoloration and increased energy consumption. Therefore, molding pressure should be adjusted based on specific conditions to achieve the best results.

9. Optimization Measures and Suggestions

9.1 Raw Material Pre-treatment

Proper crushing and drying of raw materials ensure that particle size and moisture content meet pelletizing requirements, enhancing pellet hardness.

9.2 Formula Optimization

Scientifically adjusting the proportions of different raw materials based on their characteristics balances their effects, improving the overall hardness and quality of the pellets.

9.3 Equipment Maintenance

Regularly inspect and maintain all parts of the pellet machine to ensure stable speed, sharp dies, and efficient operation, thereby enhancing pellet hardness.

9.4 Process Control

Optimize pelletizing process parameters such as compression ratio, cooling time, and airflow rate to ensure hardness and stability during pellet formation and cooling.

9.5 Quality Inspection

Establish a comprehensive quality inspection system to regularly check pellet hardness and other quality indicators, promptly identify and address issues, and ensure consistent and stable product quality.

10. Conclusion

The hardness of biomass pellets produced by pellet machines is influenced by a combination of factors, including raw material particle size, type, moisture content, die ring compression ratio, machine equipment factors, cooling time, and pellet length. By scientifically and reasonably adjusting and optimizing these factors, pellet hardness and quality can be significantly improved. Biomass pellet production companies should consider these factors comprehensively based on their actual production conditions and implement effective optimization measures to enhance product market competitiveness and meet the evolving market demands.