Wood Pellet Machine and Wood Pellet Mill

Pellet making machines are known by several names, including wood pellet machine, wood pellet mill and wood pellet press among others. The basic principle of all wood pellet machines is the compression of the wood residue between a roller and die. The pressure and heat involved melts the wood residue together and forms it into a pellet shape. Once the pellets cool they hold the pellet shape and quality pellets are durable. However the above comment is to make the process appear much more simple than it actually is, as the characteristics and qualities of the raw material directly impact on how well the material performs in the pellet mill. The key factors which effect the materials performance are particle size, material density, moisture content and lignin percentage.

The general rule for particle size before going into the pellet is to be smaller than the diameter of the pellet to be produced. For example if the pellet mill is using a 6mm die, the raw material particle size should be 6mm or below. However several studies have shown that a smaller particle size of around 3mm performs better in the wood pellet machine producing a more compressed pellet. One of the reasons for this due to a smaller particle size, the pellet mill has better heat penetration into the raw material to melt the natural lignin to help form the pellet. However, reducing the particle size below 3mm can have a negative effect on pellet compression. As a fine dust the rollers of the pellet find it hard to trap the material between the roller and die. Also as the fibre of the raw material has been broken down to such a level, compression results are severely effected. Once the correct particle size has been established the wood is processed in a hammer mill with a suitably sized screen to prepare the wood particles for the wood pellet machine. However material density also effects the performance of the pellet mill.

For example the pellet mill will have a different productivity with softwood residues compared to hardwood residues. The reasons for this is it is easier to compress softwood and heat penetration is easier, therefore a higher productivity can be achieved with softwood residues. Some wood pellet manufactures create a blend of softwood and hardwood residues to improve the performance of the wood pellet machine.

Wood Pellet Manufacturing and Moisture

One of the key reasons wood pellets burn so efficiently is due to the fact their moisture content is so low, however a certain percentage of moisture is required in the pellet production process to help to melt the natural lignin to bind the pellet together. This percentage varies between wood species and different pellet machines and processes. For example, large scale production uses steam conditioners to prepare the particle and pre-melt the lignin, here a lower moisture content is suitable. However for smaller scale production where steam conditioning is not practical or affordable, a higher moisture content is required.

Some raw materials may also require additional binder and lubricants as the percentage of natural lignin is not sufficient. If this is the case the most common additional binders are based around modified corn starch. The input is below 1%, and can improve pellet quality and also increase pellet mill productivity.

PelHeat Wood Pellet Production Guide

At PelHeat we have many years of experience in pellet production, and we provide a pellet production consultation service. The PelHeat guide download will show you how to make wood and other fuel pellets.

 The Wood Pellet Production Guide © PelHeat Ltd - Wood Pellet Machine