Quality pellet characteristics

Quality pellet characteristics

In the premium wood pellet market a “quality pellet” refers to a pellet with very low ash, for example, 0.3 percent. Some of the pellets produced will have higher ash content. A quality pellet is defined by its mechanical durability and moisture content.

Mechanical durability

Mechanical durability simply refers to how dense the pellet is, and how well it is formed. Pellets that are denser are of course stronger, and can better withstand the impact from transportation. They also function more efficiently in the pellet burner. When a quality pellet has exited the pellet mill, it should have a smooth surface, with little or no cracks. If the pellet is cracking and expanding it is because there is too much moisture within the pellet, or there was poor compression within the pellet mill. Once a quality pellet has cooled, it should be like a coloring crayon. The surface of the pellet should be smooth and lustrous.

Wood pellets tend to shine more than others; the most important thing is the pellets smooth compact state. Try tapping the pellet against a hard surface to see if the pellet stays intact, or if it crumbles easily. The length of the pellet is not really that important. However if pellets are too long (above 1 inch) they can cause damage to the auger in the pellet burner.

Moisture content

The less moisture within a pellet, the more energy the pellet burner can use. However, a certain percentage of moisture is required in the pelleting process, so the target is to keep moisture as low as possible while still creating quality pellets. The target should be to obtain a finished pellet moisture content below 10 percent.

Pellets with more than 10 percent moisture will still burn, but at the cost of efficiency.

Ash content

Wood is overwhelmingly preferred to other widely available options, notably agricultural wastes. This is in part due to agricultural wastes’ low density—which increases transportation and storage costs–its greater ash content, and to its high levels of undesirable elements such as silica, calcium, potassium, chlorine and sulfur. These minerals are imparted in the crops by fertilization. When burned, agricultural materials such as straw tend to leave more slag—a sort of black crust—on the interior of the boiler and heat-transmission equipment. Chlorine corrodes the metal. This is bad, but can be mitigated with specialized boilers, and by controlling the amount of fertilization and time of harvest.

Quality pellet test

As stated, quality pellets should have moisture content below 10 percent and be mechanically strong and dense. The simplest way to test pellet quality is to place a pellet in a glass of water, if the pellet sinks to the bottom the pellet has a high density, and was formed under sufficient pressure. However if the pellet floats it will be a poorer quality pellet with a lower density, lower mechanical durability and more likely to crumble and produce fines.

The second test is to take a vessel, which can hold at least 1 liter of water and weight it. Fill the container to the top with pellets and weigh again. Now fill the container with water and weigh. Deduct the weight of the container from both measurements, and then divide the weight of the pellets by the weight of the water. For quality pellets the results should be between 0.6 and 0.7 kilograms per liter—a figure that may also be referred to as the pellets’ specific gravity. Specific gravity is a crucial indicator that the pellets were produced under the correct pressure. Poor quality pellets, for example with a specific gravity under 0.6, will break/crumble easily, and produces excessive fines.

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