Wood Pellet Manufacturing Details

The process of producing wood pellets is not as simple as may seem. The core process is compression of the wood into pellet form, however the raw material requires certain characteristics and features for the wood to be able to be compressed into pellet form. The equipment its self also needs specific abilities, that not all pellet production equipment has. The reason for this is the majority of pellet manufacturing equipment was produced to process grass and other low density materials into animal feed pellets. However there is still a large quantity of animal feed equipment promoted for the use of wood pellet manufacturing. The simple fact is without changes to the raw material or equipment it is simply not possible to produce durable and quality wood pellet fuels. For starters, producing wood pellets puts much more load on pellet manufacturing equipment, therefore many pellet mills designed to produce animal feed pellets cannot cope.

Other general differences between a wood pellet mill and an animal feed pellet mill is rotation speed and compression temperatures. Animal feed pellet mills rotate at much higher speeds, and are geared as such to obtain the highest productivity figures possible. A animal feed pellet mill can rotate at higher speeds as the raw material is easier to compress, and generates less friction through the die. A wood pellet mill on the other hand has to rotate slower, for two reasons. The first reason is due to the increased density of the wood fibre, it requires more force to push the wood through the die. Placing wood into a animal feed pellet mill will not produce quality pellets, and more than likely will stop as there is not enough power due to the gearing. The second reason for the lower rotation speed is to melt the natural lignin within the wood to bind the pellet together, the wood needs to remain in the hot die for a longer period. If the wood is pushed through the die too quickly, the lignin will not melt and the wood will simply leave the pellet mill as dust not in pellet form. To help to increase the flow of the wood through the pellet mill and get the wood up to the required temperature, a steam conditioner is often used in large scale production before the pellet mill. This reduces the resistance of the wood as it passes through the pellet mill while still achieving the required pellet compression. Steam conditioning is very energy intensive, however the reduced load on the pellet justifies the energy input. In small scale production, steam processing is not practical due to the expense and the danger of high temperature steam. If pellet quality is an issue the wood fibre can be mixed with binders and lubricants such a modified corn starch.

Wood Pellet Production Conclusions

From the above information, it should be clear that there are several variables that are not commonly considered in pellet manufacturing. Another important point is the characteristics of every wood species and biomass feed stock behave slightly differently. For example a sample of sawdust with and without bark will perform differently in the pellet mill. The reason is the lignin within the bark melts at a lower temperature in the pellet mill and helps to bind the pellet together. However most wood fuel pellets do not contain bark, as the bark will increase the ash content of the pellets, and many pellet stoves and boilers are not designed to be able to burn higher ash content wood fuel pellets.

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 Manufacturing