Wood Pellet Production Facts and Challenges


    Making pellets is not as simple as it may seem. Every raw material has its own characteristics during pellet production, and not all pellet machines can process all materials into pellets. For example making pellets from low-density materials is much easier than producing quality wood pellets. Therefore some pellet mills cannot produce wood pellet fuels.
    Most people in the pellet industry, including PelHeat, regard pellet production as more of an art than an exact science. To produce pellets from different raw materials will require a firm understanding of the necessary changes that may need to be made to the raw material and pellet mill to consistently produce quality pellets. The necessary changes are as follows:
    Changes in Raw Material Particle Size
    Before pellet compression in the pellet mill can take place, the wood, straw, grass or any other form of biomass must be reduced in size. One fact that is not widely stated, however is one of the most important facts in pellet production is “Only a raw material of consistent quality can produce consistent quality pellets”. Part of this consistency is the size of raw material particles used in the pellet mill. Particles which are too small or too large can severally effect pellet quality.
    Changes in Raw Material Moisture Content and Pellet Quality and Production Rates
    One of the reasons pellet fuel is so popular, is pellets have a moisture content below 10%, and this enables the pellets to burn very efficiently, and produce virtually no smoke during combustion. Pellet production is a high temperature process, the right moisture content will produce the best quality pellets, reduce energy consumption and reduce pellet mill downtime.
    Changes in Raw Material Composition and The Inclusion of Binders and Lubricants
    In pellet production every raw material behaves differently, and some materials produce quality pellets easier than others. Depending on the equipment used, the composition of the raw material may need to be changed to produce a quality pellets at a reasonable productivity. Changing the composition can include adjusting particle size or moisture content, however it may also include adding binders and lubricants to help produce higher quality fuel pellets.
    Changes in Raw Material Pellet Mill Feed Rate
    Another adjustment that is not well known to impact on pellet production, is the rate of feed into the pellet mill. Adjustments on feed rate, and maintaining a consistent feed rate can make the key difference on how well the pellet mill operates, even if the raw material is perfect.
    Changes in Raw Material Conditioning and Steam

    Conditioning is the pre-treatment of the raw material before it reaches the pellet mill. Conditioning can include specific mixing techniques and the introduction of additional water or steam. Steam can be used to pre-anneal the raw material and start the lignin melting process. Though conditioning can have several benefits, in some cases the benefits are negligible and in other cases it is simply not practical to use conditioning.
    Changes in Pellet Mill Operating Temperatures
    Temperature is a key requirement in pellet production. Unless a certain temperature is reached in the pellet mill natural lignin will not melt, and it is not possible to produce some biomass pellets, for example wood pellets. However, if the temperature is too high this can damage the pellet mill, and particular consumables such as the pellet mill die, rollers, bearings and seals.
    Changes in Pellet Mill Roller and Die Clearance
    Another adjustment which can impact heavily on how successfully the pellet mill operates, is the distance between the roller and die template. The roller and die are wearing consumable parts, due to the abrasive nature and pressure of compression. The distance set between the roller and die can impact on how much energy the pellet mill uses, the quality of the pellet, pellet mill productivity and the amount of fines produced. Correctly setting up the die on a pellet mill will also increase the life of the roller and die, and reduce the cost of parts.
    Changes in Pellet Mill Die Template Rotation Speed
    The speed at which the roller and die turn effect the complex relationships during pellet compression. Some materials require a greater time under compression, and therefore require a slower rotation speed. Also, speed and torque requirement of the pellet mill change.
    Changes in Pellet Mill Die Design and Metal Used
    Many different forms of metal alloy are used to produce pellet mill die templates. Different metal alloys have specific advantages and disadvantages. Using the correct alloy is critical to reduce wear and increase production.
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