Wood pellets have a dense, convenient form and are used as a wood fuel, animal feed, animal bedding etc. Most wood granules are being made in sawmills, but, nowadays it is also widespread to make wood pellets in a small pellet mill at home. As feedstock it could be used any organic material such as grass, bean stubble, leaves, straw and so on.
Pelletizing is a process of wood pellet making that involves putting raw materials in a round opening (die) under high pressure. Some types of raw materials are used to make wood pellets of high quality without any additives, while other requires some binder that keeps granules together. There are few steps and aspects that should be carried out if the final pellets are to be of high quality. Thus, raw materials must be passed through such processes as grinding, moisture control, extrusion, cooling, and packaging.
There are many wood pellet mill designs that could be used to make wood pellets. Some of them use sawdust or wood chips, others could manufacture pellets from lumber mill scrap or even full trees. The raw materials may be dry or partially dry, green or freshly cut etc. All these factors have a great influence on the final product, defining the level of moisture and ash content, heat value and burn characteristics.
Many pellet plants start the process by using a chipping machine for processing of large pieces of wood. However, it is only necessary for mills that use non-uniform raw materials. In other cases, a hammer mill is needed to grind up the feed stock at the beginning of the pellet milling. This machine makes the process of pellet drying and pressing consistent and quick, breaking the feed stock down into small sizes. When fresh cut raw material is used, pellet mills have to dry the sawdust to a consistent moisture level. Dryer drums are used for drying off the extra moisture. After this, dies are used for pressing the sawdust, what causes the sawdust to release natural lignins. Passing through all these stages determines such pellet characteristics as the diameter, the density, the durability and the length. To make wood pellets more cool and hard pellets cooling tower is used, as wood granules could have the temperature of around 250 degrees. After cooling, the wood pellets are put in a large silo to their further packaging and distribution.
First of all, it must be said that different types of raw materials have different burn characteristics. To make wood pellets it is better to prepare wood and ground it to the required size, as the standard-sized mills require wood particles of no more than 3 mm. It could be done by using a chipper and a hammer mill.
Secondly, feedstock should not have contain any heavy metals or halogens, as while burning they could be allocated into air. The bark presence gives higher ash levels, as well as it may cause wear on the die. Also it must be known, that some wood spices (for instance, iroko) have a high mineral level and so have higher ash content. The norm of moisture level for wood pellets is said to be 15 per cent. Pelletizing of the feedstock with the higher moisture level (some wood species could have over 60 per cent water) is expensive and unprofitable. And the most important, check if there is a possibility of a constant supply of a high quality materials nearby.
So, wood pellet manufacturing is a multistep process that requires high quality materials and special equipment. The basic stages of pelletizing are grinding, moisture control, extrusion, cooling, and packaging. Each step must be carried out with care, as it determines the quality of the final product (ash and moisture content, heat value etc.).
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