Raw material for wood pellet production

Which wood goes into the pellet mill

The typical raw material for heating pellets is wood that would be a by-product at another plant. Sawdust from sawmill saws, shavings from planers, offcuts and chips from processing sawn timber are uniform, already comminuted and — if the wood was debarked beforehand — very clean. The second source is stemwood chipped specifically for pellet production.

Why does this matter? Because different parts of the tree differ in their mineral content. Finnish data from VTT show that the average ash content of the stemwood of birch, pine, spruce and aspen is 0.46%, while the bark of these species contains 2.97% (dry basis). Branches with bark average 1.52%, and leaves 4.97%^[44]. Debarked pine sawdust contained just 0.08% ash in these studies^[44]. The more bark, branches and needles there are in the raw material, the more ash the pellets contain.

Raw-material classification under ISO 17225-1

ISO 17225-1 classifies the origin of solid biofuels in a hierarchy of codes. All codes relevant to wood pellets begin with 1 — woody biomass — and the following digits indicate the source and the part of the tree^[13]. The groups you will come across most often are:

• 1.1 — forest, plantation and other virgin wood, including 1.1.1 whole trees without roots, 1.1.3 stemwood and 1.1.4 logging residues (branches, tops);
• 1.2.1 — chemically untreated by-products and residues from the wood-processing industry, i.e. sawdust, shavings and offcuts from sawmills and joinery plants;
• 1.3.1 — chemically untreated used wood, i.e. wood that has been used before and has not undergone chemical treatment.

The code describes the origin of the raw material, not the species. “Stemwood” may be pine, beech or birch — the standard classifies them all in the same way. We cover differences between species in the articles on softwood pellets and hardwood pellets.

Raw material and ENplus class

In Table 5, ENplus ST 1001 assigns each quality class a list of permitted raw-material groups under ISO 17225-1^[1]. The higher the class, the narrower the choice — and the less material from bark, branches or needles.

Class A1 therefore has no room for whole trees with branches or for logging residues — precisely the materials that bring in the most bark, needles and leaves. Class A2 allows them, but has a more lenient ash limit (≤ 1.20% instead of ≤ 0.70% on a dry basis). Class B additionally allows chemically untreated used wood, and its ash limit is ≤ 2.00%^[1].

What must not be used

On raw material, ENplus goes further than the ISO standard. In class B, ISO 17225-2 allows chemically treated by-products of the wood-processing industry and untreated used wood, provided that the content of heavy metals and halogenated organic compounds does not exceed the values typical of virgin wood^[11]. ENplus, by contrast, excludes demolition wood and chemically treated wood in all classes^[1].

Regardless of class, ENplus also does not allow raw material that is:

• rotten — biological decay changes the composition of the wood and impairs its properties;
• contaminated — e.g. with soil, sand, stones or plastics;
• high in bark^[1].

The finished pellets must not contain any material other than wood, nor wood fibre in any form other than pellets — such as wood chips or loose particles^[1].

Additives: what may be added to the wood

Wood pellets do not need glue — the particles 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.. The producer may, however, use additives (A substance added to the raw material to improve pelletising or durability, or to reduce dust. Under ENplus, only of agricultural or forest origin.)A substance added to the raw material to improve pelletising or durability, or to reduce dust. Under ENplus, only of agricultural or forest origin. that make pelletising easier or improve durability. ENplus allows only additives made from agricultural and forestry products — such as starch, maize flour, potato flour, vegetable oil or lignin from the kraft (sulphate) process — up to a total of ≤ 2.0% by mass (≤ 1.8% during production and ≤ 0.2% after production, e.g. coating oils)^[1].

The presence of an additive is not a defect. It is still worth knowing whether the producer uses one — for example when comparing the test results of several batches, or when a boiler reacts noticeably differently to a new fuel.

What this means for buyers

The wood species is not a mandatory marking on an ENplus bag — the mandatory markings include the quality class, the ENplus ID number, the diameter and the net weight^[2]. This is why it is worth asking about the raw material directly:

• whether the raw material is sawmill sawdust and shavings, or chipped roundwood or whole trees;
• whether the wood is debarked before processing;
• whether the raw-material mix is constant or changes between batches;
• whether the producer uses additives, and which ones.

The answers are no substitute for test results. The raw material explains why the pellets have a given ash content or ash melting temperature, but only a measurement can confirm these values. How to read a test report is covered in the laboratory testing section, and what to look for when buying in the guide how to choose wood pellets.