How to make good wood pellet

The increasing demand for wood pellets as an alternative fuel to fossil fuels has driven the growth of the wood pellet production industry. Global wood pellet production, estimated at around 12 million metric tons in 2009, is projected to surge to 100 million metric tons by 2020 (Bloomberg, 2010a). Consequently, there is a significant need for high-quality wood pellets to meet this growing demand. Let’s explore the process of producing quality wood pellets.

Woody materials

The primary raw materials for creating pellets are wood and its derivatives. Pellet plants associated with forestry operations make use of byproducts that would otherwise be considered waste. Collecting debris from timber harvesting for economic utilization and utilizing cleared brush from tree farms are common practices. However, the predominant sources for manufacturing wood pellets are the byproducts of primary wood processing at sawmills and plywood plants, which include bark, sawdust, wood chips, and shavings.

Grinding

Effective size reduction technology is a critical factor in pelletization when using wood-based materials. This is because the size reduction process significantly impacts the operating, maintenance, and lifecycle costs of other equipment in the production line, including the wood pellet mills. Inadequately sized materials reaching the mill can put excessive strain on the die, necessitating it to not only form the pellets but also size them. This dual role can significantly reduce die lifespan and lead to increased energy consumption.

Dying

The moisture content of the raw materials is another crucial factor influencing the quality of wood pellets. It is essential to ensure that all raw materials are dried to the desired moisture content, typically around 8-10%. If the moisture content is too low, the raw materials cannot be pelletized effectively. Conversely, if the moisture content is too high, it can block the die hole of the wood pellet mills and result in pellets with a low combustion rate and poor quality.

Pelletization

Once the raw materials have been appropriately sized and dried, the next step is pelletization. Wood pellet mills utilize pressure to push the raw materials through holes that shape them into pellets. This process involves the application of pressure and friction to increase the temperature of the wood, allowing the lignin to soften and the wood fibers to be reshaped into pellets. The compression exerted by the wood pellet mill directly determines the density of the pellets. Therefore, the quality of the wood pellet mill significantly impacts the quality of the wood pellets produced.

In addition, the use of a binder agent is another important consideration. While the natural lignin in the wood acts as a binder, additional binding agents may be used if necessary. Possible binding agents for wood pellets include starch, molasses, natural paraffin, plant oil, lignin sulfate, and synthetic agents. A small amount of starch, typically less than 2 percent by mass, can enhance pellet strength and mechanical durability.

Cooling

Following the pelletization process, the pellets exit the extruder and undergo air-cooling, which allows the lignin to solidify and the pellets to take their final form.

Screening

Screening the pellets serves to minimize the fines, accomplishing two key objectives: recycling the raw materials to lower production costs and reducing the sinter in the ash after burning. It is advisable to specify the amount of fines for each bulk delivery, with measurement typically conducted at the final stage of the factory production chain. Ideally, fines should account for less than 1% of the total weight.

Packaging

Wood pellets can be packaged in bulk or bags based on industrial requirements; however, bagged pellets are preferred due to their tendency to have fewer fines compared to bulk delivery.

Following the outlined procedures, high-quality wood pellets can be produced with the desired size, moisture content, mechanical strength, minimal fines, low ash content, and high bulk density. Attention to each step in the process is crucial for ensuring wood pellet quality, emphasizing the importance of addressing all aspects of wood pellet mill and production seriously. In essence, the production of quality wood pellets relies on the use of premium raw materials, high-quality wood pellet mills, and responsible wood pellet manufacturers.