Seven Steps to Making the Perfect Wood Pellet

Seven Steps to Making the Perfect Wood Pellet

Pellets are made by compacting sawdust and other wood waste from saw milling.  The lining in the wood binds the pellet together, causing it to hold its shape without the need for any additive.

1. Raw Material  Obtain Raw material from an approved source, such as saw-dust, off-cuts from wood manufacturing or, occasionally, round wood from coppicing.  Make sure that the material is not contaminated in any way and that it is certified as being from a sustainable source.

2. Drying  Firstly, the raw material is dried to a moisture content below 4%.  Any excess moisture will cause problems during the manufacturing process.  This low moisture content means the resulting wood pellets (in which the moisture is not greater than 8%, but is usually between 5 and 8%) are able to burn efficiently, produce very little smoke and will result in lower maintenance requirements for the boiler over the longer term.  The more consistent the material used, e.g. one hundred per cent sawdust, the more consistent the resulting pellets will be in quality.

3. Mincing  Once dried, the material is processed.  Any round wood will first need to go through a chipper and shredder.  The hammer mill then breaks the material up into consistent, fine particles.  The smaller the particles, the stronger the final pellets will be, with less chance of any breaking apart.

4. Conditioning This is the stage where the raw materials are mixed with water or steam added which can start the process of the lignin melting.   The next step of the raw material preparation is to check and adjust the amount of water in the mixture.  Getting the percentage right is crucial to the quality of the final pellets – a moisture content that is just 1% too high or low can cause the pellet mill to use a fifth more energy during the production process.  The pellets need just the right amount of moisture to hold their shape, but no extra as this will affect its heat output.

5. Pellet Mill The fine particles are then pressed into cast metal that has pre-drilled pellet sized holes.  This is known as the wood pellet die.  The material is compressed into the pellet shape.  The friction caused by this compression heats the pellet, releasing the natural lignin in the wood particles – this is used a natural binding agent so the pellets will keep their shape even after they have passed through the die, allowing them to cool, harden and remain intact.

Temperature control is a key requirement at the production stage.  It must reach a high enough temperature in the pellet mill for the lignin to melt.  On the other hand, if the temperature becomes too high this can damage mechanical parts such as the die, rollers and bearings.

Another crucial aspect at this stage is that the mill be set up correctly or the quality of the pellets will be affected: points to check are that the rotation speed of the roller and die and the distance between the die and roller so that there is simultaneously a good balance between the production and compression.

6. Cooling Lay the hot, wet pellets out to cool and dry.

7. Sieving and Packaging  Sieve the pellets to remove stray particles.  Stray particles, or fines, will lead to the boiler working less efficiently and will result in greater ash.  The resulting pellets should be firm and shiny (or blow air through them to remove dust) before bagging in sealed, plastic bags or, if to be kept loose, removal to a dry storage area.

 

If you would like more information on wood pellets and if a biomass boiler is best for you, then visit www.woodpellet-boilers.com

 

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