what affects the quality of wood pellet

Pellet quality is always a top concern for end users. As a pellet manufacturer, do you have a clear understanding of the factors that contribute to your product’s quality? Typically, the composition of raw materials, the type of binding agent used in the pelletizing process, the condition of the feedstocks, and the operating conditions of the pellet machine all play crucial roles in determining pellet quality. But how do we evaluate the quality of our pellets? Which factors are most influential? Let’s delve into these factors one by one.

Moisture content

Moisture content is a crucial aspect throughout the entire pellet production process. Elevated moisture levels in raw materials can lead to increased moisture in the final pellets. If the moisture content of the pellets exiting the pellet machine exceeds 15%, it can significantly compromise pellet quality. High moisture levels can lead to rotting due to battery decomposition and fungal growth. Conversely, when the final pellet moisture content falls below 5%, the pellets become more susceptible to breakage during transportation and storage. Therefore, managing moisture content effectively is essential to ensure high-quality pellets.

Density

Density, encompassing both unit density and bulk density, is a crucial aspect of pellet quality. Multiple factors determine the density of pellets, including the moisture content of raw materials, particle size, processing pressure applied by the pellet machine, and die temperature. Typically, higher moisture content and larger particles tend to decrease both unit and bulk density. Conversely, increased pressure and temperature during the pelletizing process enhance the density. The unit and bulk density of pellets are primarily influenced by factors like moisture content, particle size, die temperature, and processing pressure. Optimizing these parameters, such as maintaining a higher die temperature, lower moisture content, and smaller particle size, while applying appropriate pressure, can yield pellets with high density.

Durability

The durability of pellets, which reflects their strength and resistance to breakage during transportation and storage, can be evaluated by measuring the amount of fines or dust generated when spinning a quantity of pellets in a metal box, as studied by Kaliyan and Morey in 2009. The moisture content, along with water-soluble compounds like starch and lignin, acts as a binding agent and lubricant, enhancing pellet durability by increasing particle contact. However, excessive lignin and other extractives, exceeding 35%, can reduce durability. Fat content in the feedstocks also decreases durability, functioning as a lubricant during the pelletizing process. Additionally, particle size plays a crucial role; finer particles lead to higher pellet durability due to increased preconditioning, such as preheating or steaming, which activates natural binders. Finally, pellet machine variables, including die dimensions and speeds, affect durability. Smaller die dimensions produce more durable pellets. According to Stevens’ 1987 study, die speeds of 126–268 rpm are suitable for small pellets, while speeds of 6–7 m/s are preferred for large-sized feed pellets.

Fines percentage

Fines, the result of pellet breakage caused by frictions during transportation and storage, are an indicator of pellet quality. The fewer fines or dusts produced, the higher the pellet quality. Poor-quality pellets are more likely to generate fines, posing a risk to burning equipment by increasing the likelihood of dust explosions and other combustion issues. A study by Shanke et al. revealed that storage temperature impacts pellet quality, with higher temperatures around 30 to 50℃ causing a significant increase in fines percentage during two months of storage, exceeding 1%. Managing storage conditions is crucial to maintaining pellet quality and preventing fines formation.

Heating value

The heating value of pellets is influenced by various processing factors, including die temperature, particle size of the feedstocks, and their pretreatment. Typically, pellets with higher density exhibit higher heating values. Standard wood pellets generally have a heating value of approximately 17 to 18 MJ/kg. However, effective pretreatment can significantly enhance the heating value, reaching 20-22 MJ/kg, especially when pellets are torrefied at temperatures ranging from 200 to 300 ℃. This pretreatment not only improves the quality of the pellets but also enhances their combustion efficiency.