why compressed biomass pellets turn black?

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There are a number of reasons why compressed biomass pellets made by pellet mills turn black. These reasons are outlined below:

compressed pellets

  • Carbonization: The biomass pellets turn black via carbonization when they are passed through the holes of a pellet mill die. The pellet fuel that comes from these carbonized and blackened pellets is perhaps of a better quality due to the increased ability to hold heat within them. This fuel hence becomes of better value.
  • Dry Feedstock: When pellets are compressed in the machine, they could end up creating fuel of different quality, depending on the moisture content of the feedstock used. When the moisture content is between 20 and 25 percent, the compression of the pellets may produce a large amount of vapour. The surface of the pellets will have multiple fracture points with low density. This is a very low quality product. When the moisture content is between 15 and 20 percent, the pellet fuel produced will be high in thermal value among other things. This is very high quality fuel. For this reason, people prefer the percentage of moisture to be between 15 and 20. Some like the moisture to be even lower (between 10 and 15 percent). Unfortunately, this will also result in low quality fuel.
  • Altered Enzymes: The raw materials for these fuels will have undergone various changes in weather (such as rain and wind), resulting in compressed pellets turning black (this is due to the changing of the raw materials’ enzymes). This enzyme change results in the thermal qualities of the pellets to be lowered, even though the surfaces will not have many fractures. Nonetheless, the quality of the fuel will be lowered, and hence pellets that have undergone this are not well liked.

How is good quality pellets made? 

The following is one of our biomass pellet plant located in Chile. The capacity of this pellet fuel production is 4.5~6 tones per hour. The main raw material of this plant is pine wood and eucalyptus wood. Feel free to contact us for more details of our successful large scale pelletizing projects.

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Size of Feedstock

Prior to feeding the raw biomass materials into the pellet mill, they ought to be crushed using a straw crushing machine. The resultant crushed material has to adhere to the size of the die holes through which they will be passed in the pellet mill. This means that they should be smaller than the hole, though not too small. If the crushed material is not just the right size, it will show in the quality of the oil produced (it will be poor quality). Of course, the size of this feedstock will depend on the factory that is making the oil.

Adhesive Ability

The biomass feedstock, once crushed, should have some adhesive force about it. The adhesive ability is mainly based on the lignin content of biomass feedstock.

Moisture Content

There are certain standards that the crushed biomass materials must adhere to in relation to their moisture content. Basically, just as with the size of the crushed material, it should be just right. Too wet or too dry will have a negative effect on the final output. Again, as with the previous point, the standards of moisture content for these crushed pellets depends on the factory to which they are taken for processing. Generally, the finished products should have a moisture content of less than 13 percent after drying. This is the standard for most biomass pellet mills.

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