Pellet making machines are known with more than a few names, including biomass pellet machine, biomass pellet mill as well as biomass 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 biomass residue together and forms it into a pellet shape. Once the pellets cool they hold the pellet shape plus quality pellets are durable. However the above comment is to make the process appear much more uncomplicated than it actually is, as the characteristics as well as qualities of the raw material directly impact on how well the material performs in the pellet mill. The key factors which result the materials performance are particle size, matter density, moisture content as well as lignin percentage.
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The general rule for particle size before going into the pellet is to be smaller than the diameter of the pellet to be formed. For instance if the pellet mill is using a 6mm die, the raw material particle size should be 6mm or below. On the other hand quite a few studies have shown that a smaller particle size of around 3mm performs better in the wood pellet machine generating 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 matter to melt the unrefined lignin to help form the pellet. However, dropping the particle size below 3mm can have a negative outcome on pellet compression. As a fine dust the rollers of the pellet find it hard to trap the matter between the roller and die. Also as the fibre of the raw matter has been broken down to such a level, compression results are severely effected. Once the correct particle size has been established the biomass is processed in a hammer mill by a suitably sized screen to organize the biomass particles for the wood pellet machine. On the other hand matter density also effects the performance of the pellet mill. For case the pellet mill will have a unlike productivity by softwood residues compared to hardwood residues. The reasons for this is it is easier to compress softwood along with temperature penetration is easier, therefore a higher productivity can be achieved by way of softwood residues. Some biomass pellet manufactures create a blend of softwood along with hardwood residues to improve the performance of the wood pellet appliance.
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One of the key reasons biomass pellets burn so efficiently is through the fact their moisture content is so low, though a certain percentage of moisture is required in the pellet production process to help to melt the organic lignin to bind the pellet together. This percentage varies between biomass species in addition to unlike pellet machines along with processes. For example, large scale manufacture uses steam conditioners to prepare the particle and pre-melt the lignin, here a lower moisture content is suitable. On the other hand for smaller scale production where steam conditioning is not practical or affordable, a higher moisture content is required. Some raw materials may also necessitate additional binder as well as lubricants as the percentage of untreated lignin is not ample. If this is the case the most common additional binders are based around modified corn starch. The input is below 1%, along with can improve pellet quality and also increase pellet mill productivity.
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