Pellet hardness and mechanical durability

Hardness versus durability

Buyers talk about “hard” or “soft” pellets. The standard uses the term mechanical durability (The percentage of the pellet mass that remains as whole pellets after a standardised mechanical load in the test according to ISO 17831-1.)The percentage of the pellet mass that remains as whole pellets after a standardised mechanical load in the test according to ISO 17831-1.: how much material remains in pellet form after a controlled load that simulates handling and transport^[18]. This is a better measure than the hardness of a single pellet, because it describes the behaviour of the whole batch.

ENplus requirements: at least 98.0% in class A1 and 97.5% in classes A2 and B, determined at the point where the means of transport is loaded at the production plant^[1]. In practice, this means that no more than 2% of the mass of A1 pellets may be abraded in the test — every point below 100% is material that will turn into dust on its way to the boiler.

How durability is measured

The principle of the ISO 17831-1 test is simple: a pellet sample with the fines removed is subjected to a controlled mechanical load in a test device, then screened and weighed again. The result is the proportion of the mass that remained in pellet form^[18]. The details of the procedure (device, duration, screen) are set out in the standard — which is why the report should state the number of the standard according to which the test was carried out.

In addition, the report may include the particle density (The mass of individual pellets relative to their volume (excluding the spaces between pellets), determined according to ISO 18847.)The mass of individual pellets relative to their volume (excluding the spaces between pellets), determined according to ISO 18847. — ENplus requires it to be stated, without a limit value^[1],[27].

What it depends on

• The raw material and its particle size — a uniform meal with the right particle size binds more evenly.
• Moisture content before pressing — raw material that is too dry or too wet pelletises less well.
• Conditioning and the die — the temperature and the degree of compression decide how well the lignin binds the particles.
• Cooling — the pellet only hardens once it has cooled; pellets that are not cooled enough are more prone to crumbling.
• Fuel handling — drop heights, the number of transfers, the method of bulk unloading. Even good pellets lose some durability when transported roughly.

What market studies show

In a peer-reviewed study of 30 samples of pellets sold in Poland for domestic use, 10 failed to meet the requirements for mechanical durability and/or fines^[47]. This is one of the most common quality problems — and one of the easiest for buyers to spot from the condition of the bag.

How to assess it yourself

You will find the full checklist in the guide how to choose wood pellets, and details of the parameter on the mechanical durability page.