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How to make quality wood pellets

By Jack Huang    Feb 13, 2014

Wood pellets as an alternative fuel of fossil fuels are needed highly in the energy market. Global wood pellet production was approximately 12 million metric tons in 2009, and is predicted to increase to 100 million metric tons by 2020 (Bloomberg 2010a). The huge demand of the pellets strivings the development of wood pellet production. Then let’s see how to make quality wood pellets?

Woody materials: Wood and its derivatives are the raw materials most widely used to create pellets. Forestry operations often establish pellet plants to make use of byproducts that might otherwise be discarded as waste. Debris from the harvesting of timber is collected for economic use. Cleared brush on tree farms is another source. However, the most common sources of material for the manufacture of wood pellets are the by-products of primary wood processing at sawmills and plywood plants: bark, sawdust, woodchips and shavings.

Usually, the raw materials could be processed in to wood pellets after going through the following procedures, such as grinding, drying, pelletizing, cooling, screening, and packaging.

Grinding: When using wood-based materials, size reduction technology is an important consideration in pelletization because size reduction determines, to a great extent, the operating, maintenance, and lifecycle costs of other equipment in the line, such as the wood pellet mills. If the materials are not sized properly when it reaches the mill, the die need not only act as a forming device, but also as a sizing device, which will greatly decrease the die life and increases energy consumption.

Dying: Well, then the moisture content is another factor decides the quality of wood pellets. All the raw materials should be dried to the desired moisture content which is around 8-10 % for if the moisture content is too low, the raw materials cannot be pelletized into pellets and if too high, the raw materials make block the die hole of the wood pellets mills, and the pellets produced will have low combustion rate and poor quality.

Pelletization: When the raw materials have been sized and dried in a desirable conditions, comes the pelletization. Wood pellet mills apply pressure to force the raw materials through holes that shape the pellets. Pressure and friction are used to increase the temperature of the wood, which in turn allows the lignum to soften and the wood fibers to be reshaped into pellets. The compression of the wood pellet mill directly decides the density of the pellets. So the higher the quality of the wood pellet mill is, the better the quality of wood pellets will be.

 

Pellets in the pelleting chamber of the pellet mill plant

Wood pellets in the pelleting chamber of Gemco Pellet mill
 

Beside, another factor needs to be considered is binder agent. In pelletization, binder is not needed for the lignin functions as binder but if necessary, the possible binding agents for wood pellets include starch, molasses, natural paraffin, plant oil, lignin sulfate and synthetic agents. Small amount of starch, less than 2 percent by mass, increase pellet strength and mechanical durability.

Cooling: After pelletizaion, the pellets leave the extruder, they are air-cooled, the lignums solidify, and the pellet is formed.

Screening: To screen the pellets means to reduce the fines, which not only recycle the raw materials lowering the production cost but also reduce the sinter in the ash after burning? The amount of fines should preferably be declared for each bulk delivery, and is measured at the final point in the factory production chain. Fines should preferably be less than 1% by weight.

Packaging: Pellets can be packaged in bulk and bags according to industrial purpose, however, the bags are preferred for pellets in bags tend to have less fines than those delivered in bulk.

After going through the above procedures, the wood pellets could be produced with high quality with desirable size, moisture content, and mechanical strength, less fines, low ash content and high bulk density. Attention to each of the procedures is the grantee of the wood pellet quality and anything concerns the wood pellet mill and wood pellet production needs to be treated seriously. How quality wood pellets are made? Simply, it is made by the good raw materials, quality wood pellet mills and responsible wood pellet manufacturers.