Mechanical durability of wood pellets

What mechanical durability is

Mechanical durability (The resistance of pellets to abrasion and crumbling under mechanical stress — expressed as the percentage of mass that remains intact after a standardised test.)The resistance of pellets to abrasion and crumbling under mechanical stress — expressed as the percentage of mass that remains intact after a standardised test. describes how well pellets withstand impact, friction and tipping. The result is given as a percentage by mass: the share of the sample that remained as whole pellets after a standardised test. In ENplus the requirement refers to the as-received state, and the test method is specified in ISO 17831-1^[1],[18].

It is a parameter that buyers feel directly, even if they rarely think about it. Poor durability means dust in the bag, a clogged auger and uneven combustion — even when ash and calorific value are faultless.

Why pellets crumble

A pellet is held together mainly by lignin (A natural wood polymer that softens at pelletising temperature and binds the particles together once it cools.)A natural wood polymer that softens at pelletising temperature and binds the particles together once it cools., which becomes plastic under the high temperature and pressure in the die of the pellet press. Whether the bond will be strong depends on many factors at once:

• the raw material — wood species, share of bark, particle size and the presence of contamination;
• the moisture content of the raw material before pelletising — material that is too dry binds poorly, while material that is too wet gives pellets that crack as they dry;
• the pelletising parameters — die condition, die geometry and operating temperature;
• cooling and storage — pellets harden only once they have cooled, and if they get damp later they lose cohesion.

The producer may also use a small amount of a binding additive of agricultural or forestry origin, such as starch. ENplus limits the total share of additives to 2.0% by mass^[1]. More on pellet cohesion in the article pellet hardness.

Why it matters in transport and in the boiler room

Every handling step is a mechanical stress on the pellets: discharge into a bag or onto a truck, stacking pallets, vibration in transport, reloading and — for bulk deliveries — pneumatic blowing into the silo. At the end there is also the auger screw. Pellets with low durability wear down at each of these stages, and the broken-off material ends up in the hopper as fines.

For this reason ENplus measures durability at the point where the means of transport is loaded at the production plant^[1] — before the pellets have covered most of the way to the customer. A high result at this point leaves a margin for losses further down the supply chain.

How durability is tested (ISO 17831-1)

The method is described in ISO 17831-1, the current edition of which dates from 2025^[18]. The principle of the test is simple:

1. The fines are screened out of the sample so that only whole pellets go into the test.
2. A weighed portion of pellets is subjected to controlled mechanical stress in a rotating device in which the pellets tumble and strike each other and the walls.
3. After the test, the sample is screened again and the material retained on the sieve is weighed.
4. Durability is the mass of pellets remaining after the test, expressed as a percentage of the mass before the test.

The detailed test conditions — type of device, duration, number of revolutions and sieves — are laid down in the standard. Results are comparable between laboratories only if they were obtained with the same method; that is why the report should state the standard and its edition. A result of 98.0% means that at most 2.0% of the sample mass broke off in the test.

ENplus, ISO and Polish regulatory requirements

ENplus is stricter than the standard in two places. In class A1 it requires at least 98.0% for both diameters, whereas the ISO draft allows 97.5% for 8 mm pellets. In class B, ENplus requires 97.5% and the ISO draft 96.5%^[1],[12]Data require verification: The ISO values were read from the FDIS 17225-2 draft; they need to be confirmed in the published ISO 17225-2:2021.. A difference of 0.5–1 percentage point looks small, but it means noticeably more material may break off in the test: 2.5% instead of 2.0% for 8 mm A1 pellets and 3.5% instead of 2.5% for class B.

What a study of the Polish market showed

A peer-reviewed 2024 study analysed 30 samples of pellets sold in Poland for residential use. Ten of the 30 samples failed the requirements for mechanical durability and/or fines. The authors also found that the values declared by producers often differed from those measured^[47].

This result does not mean that one in three pellets on the market is poor — the sample was small. It does show, however, that durability is a parameter that cannot be assumed from the description on the bag. The authors also cite data according to which only about 30% of Polish wood pellet production held ENplus certification in 2022^[47].