Fines in wood pellets

What fines are

Fines (The part of the fuel that passes through a sieve with 3.15 mm round holes — dust, crumbs and fragments of pellets.)The part of the fuel that passes through a sieve with 3.15 mm round holes — dust, crumbs and fragments of pellets. are everything that passes through a sieve with 3.15 mm round holes: wood dust, crumbs and broken pieces of pellets. The dimensions refer to sieves with round holes according to ISO 3310-2^[1]. The result is given as a percentage by mass, as received.

ENplus also distinguishes coarse pellet fines (Pellet fragments of 3.15–5.6 mm — no longer dust, but not yet full-value pellets (coarse pellet fines, CPF).)Pellet fragments of 3.15–5.6 mm — no longer dust, but not yet full-value pellets (coarse pellet fines, CPF). — pieces measuring 3.15 to 5.6 mm. There is no limit for them, but the value must be stated^[1]. This is useful information: a large share of such crumbs shows that the pellets are breaking, even if there is still little dust.

Requirements and point of measurement

With fines, what matters is not only the value but also where it was measured. Dust forms at every reloading, which is why ENplus ties its limits to a specific point in the supply chain^[1].

The separate, stricter 0.5% limit for bags is an ENplus requirement — neither the Polish regulation nor the ISO draft standard provides for it^[36],[12]Data require verification: The ISO 17225-2 values were read from the FDIS draft; they need to be confirmed in the published standard.. For class B, ENplus sets no limit for bags^[1].

Converting to mass is simple: 1% in a tonne is 10 kg of dust, and 0.5% in an example 15 kg bag is 75 g.

Method: ISO 5370 instead of ISO 18846

Fines are determined by sieving the sample and weighing what has passed through the sieve. The current standard is ISO 5370:2023 — it replaced the withdrawn ISO 18846:2016^[19]. The change is reflected in the 2nd edition of ENplus ST 1001, in which the test method for fines was changed from ISO 18846 to ISO 5370 and the former “coarse fines” were renamed coarse pellet fines (CPF). The threshold values themselves did not change^[1].

Effects in the auger and burner

• Dosing. The auger moves a defined volume of fuel. Dust fills the spaces between the pellets and changes the mass of fuel in each dose — and therefore the amount of energy fed to the burner. Settings chosen for clean pellets no longer fit.
• Hopper and auger. Dust settles at the bottom of the hopper and in the auger channel. It can form compacted layers, cause hang-ups and hinder the flow of pellets, and in extreme cases block the feed system.
• Burner. Fine particles burn differently from pellets: faster, partly in suspension. Some are carried away by the air before they burn, while others cover the combustion bed and restrict the air supply. The result can be an unstable flame, more unburnt particles in the ash and more particulate matter in the flue gas.

Why dust increases along the way

Pellets that leave the plant with little dust do not necessarily reach the boiler room in the same condition. Every operation — discharge into a tanker, vibration in transport, reloading pallets, blowing into the silo and finally the auger — wears the pellets down. How much dust forms depends on mechanical durability, which is tested according to ISO 17831-1^[18], and on how the fuel is handled.

That is why a fines measurement at the end customer usually gives a higher result than one at the factory gate. The current ENplus table has no separate column for delivery to the customer — the bulk limit applies at the factory gate or when loading for end customers^[1]. In a study of 30 samples from the Polish market, 10 failed the requirements for durability and/or fines^[47].

How to assess and reduce dust