What are wood pellets?

Definition

Wood pellets are a solid fuel in the form of small cylinders, produced by compressing comminuted wood. ISO 17225-2 refers to them as graded wood pellets — pellets classified according to quality parameters^[11]. For domestic use, pellets have a diameter of 6 ± 1 mm or 8 ± 1 mm and a length of 3.15 to 40 mm^[1].

The key word is graded. Wood pellets have defined requirements for moisture content, ash, net calorific value, mechanical durability, nitrogen, sulphur and chlorine content, and the behaviour of the ash at high temperature. This is what allows a boiler to run automatically — and buyers to compare products.

What they are made from

The best wood pellets are made from clean wood: sawdust, shavings and wood chips from the wood industry, and stemwood. ISO 17225-1 classifies the origin of the raw material into groups — for example stemwood (Group 1.1.3 under ISO 17225-1: wood from the tree stem, without branches or roots.)Group 1.1.3 under ISO 17225-1: wood from the tree stem, without branches or roots. and chemically untreated wood-processing residues (Group 1.2.1 under ISO 17225-1: e.g. sawdust and shavings from sawmills that have not been chemically treated.)Group 1.2.1 under ISO 17225-1: e.g. sawdust and shavings from sawmills that have not been chemically treated.^[13]. For class A1, the ENplus raw-material table (ST 1001:2022, 2nd edition, Annex A, Table 5) lists only these two groups^[1]. The footnotes to the table allow, among other things, negligible amounts of glue or grease from the processing of virgin wood — provided that the chemical parameters of the pellets are clearly within the limits.

Pellets are bound by lignin (A natural polymer in the cell walls of wood. It softens at pressing temperature and binds the particles together once it cools.)A natural polymer in the cell walls of wood. It softens at pressing temperature and binds the particles together once it cools. — a component of wood that softens during pressing and hardens the pellet once it has cooled. The producer may add a small amount of an additive of agricultural or forest origin, such as starch; under ENplus, the total proportion of additives must not exceed 2% by mass^[1].

Why a standardised fuel

A pellet boiler doses fuel automatically: the auger moves a set volume of pellets and the fan supplies a set amount of air. If the fuel changes from bag to bag — in moisture content, density or the amount of dust — the settings no longer fit. Hence the requirements for each of these parameters.

Pellets, briquettes and firewood

Briquettes are also compressed wood, but in large blocks or cylinders — intended mainly for fireplaces and manually loaded stoves. Firewood is a non-uniform fuel: its moisture content depends on seasoning and storage. What sets wood pellets apart is that they are free-flowing and uniform, which allows automatic feeding and stable combustion.

Legal requirements in Poland

Since 24 May 2025, a regulation of the Polish Minister of Climate and Environment has been in force setting quality requirements for pellets and briquettes made from woody biomass and placed on the market for households and installations with a rated output below 1 MW (Dz.U. 2025 poz. 618 — Polish Journal of Laws 2025, item 618)^[36]. According to the ministry, the limit values are consistent with the EN ISO 17225 standard^[37]. However, the regulation does not include, among other things, requirements for ash melting behaviour or heavy metals, which are covered by ENplus.