Wood Pellets Production

Pelletizing Mill

Pelletizing mill, always referred to as pellet presses or pellet press, are used to produce wood pellets for producing energy from prepared raw material such as wood chips, sawdust, planer shavings, etc. Pelletizing mill varies in size from 3 kilowatt to 375 kilowatt (5 horsepower to 500 horsepower) or larger. Production volume for wood pellets per pelletizing mill can range from 90 kilograms per hour to 8 tones per hour (200 pounds to 9 tons).  Loose wood products raw material will be compacted from about 100 kg/m3 to 650 kg/m3 (6.5 lb/ft3 to 40.5 lb/ft3) providing an extremely dense and efficient product.


The wood pellets are produced from a pelletizing mill through pressing the raw material through small holes in a die. The die holes are round and the pellets are pushed from the inside out by rollers.  The pellets are formed as a continuous cylinder shaped material and cut to length by a knife at the outside of the die.


Pelletizing mill contains one or more conditioning units installed above where liquids can be added to improve pellet ability. Water is sometimes added in the form of steam resulting in firmer pellets.


From the conditioner of a pelletizing mill, the wood feed stock falls into the centre of the pelletizing mill.  In the pelletizing mill, rollers and feed ploughs push the material through the holes of the die plate. The die plate is stationary and rollers rotate around the inner surface of the die, or the die rotates around the rollers. Outer knives cut the pellets to length required.


There are generally two types of dies in the pelletizing mill – flat die and ring die.  Die hole sizes usually range from approximately 2 mm (.050”) up to 10 mm (.400”).  The thickness of the die plate helps to determine the compactness and stability of the pelletizing mill. Pellet dies vary in thickness up to approximately 100 mm (4”) thick.  Pelletizing mill dies are usually manufactured from metal alloys, stainless steel or high chrome. Pressures in the die can reach up to 172,000 kPa (25,000 psi).


In a pellet plant, the wood pellets are transferred from the pelletizing mill, to a cooler/drier to remove the heat which is generated during the press process.  The moisture content of the pellets also is needed to be reduced for durability during storage and shipping. Pellets are then normally stored in silos or bins for bulk shipping or to be processed by packaging systems.


The pellets produced by pressing in the pelletizing mill are used in energy producing systems such as industrial boilers, pellet stoves and pellet barbeques.  Each wood pellet will have close to the same characteristics in size, moisture content and energy value.