How wood pellets are made

From raw material to pellet

Pellet production essentially turns a loose, non-uniform material — sawdust, shavings, wood chips — into a fuel with consistent dimensions and properties. The raw material most often comes from the wood industry as a by-product, or from stemwood. ISO 17225-1 describes these groups as chemically untreated wood-processing residues (Group 1.2.1 under ISO 17225-1: sawdust, shavings and offcuts from sawmills and joinery plants that have not been chemically treated.)Group 1.2.1 under ISO 17225-1: sawdust, shavings and offcuts from sawmills and joinery plants that have not been chemically treated. and stemwood (Group 1.1.3 under ISO 17225-1: wood from the tree stem, without branches, needles or roots.)Group 1.1.3 under ISO 17225-1: wood from the tree stem, without branches, needles or roots.^[13]. These are the only two groups allowed in the highest ENplus class, A1^[1].

Each stage of the production line determines a different parameter of the finished fuel. Drying sets the moisture content; milling and pressing set the density and durability of the pellet; cooling stabilises it; and screening decides how much dust ends up in the bag. The diagram below shows the typical sequence; at individual plants the stages may differ slightly.

Cleaning, drying and milling

Cleaning

The first step is to separate out anything that is not wood or should not end up in the fuel: stones, sand, metal, excess bark. ENplus explicitly excludes raw material that is contaminated, rotten or contains “a large amount of bark”, and the finished pellets must not contain any material other than wood^[1]. Sand and soil do not burn — they stay in the ash pan, so every gram of them raises the ash content. There is more on this in the article on raw material quality.

Drying

Fresh sawdust contains too much water to be pelletised straight away. The dryer brings the raw material to the moisture content at which the press forms a compact pellet. Material that is too wet produces soft pellets with poor durability, and the moisture content of the finished pellets may exceed the permitted 10.0% as received^[1]. Material that is too dry binds less easily. There is no single “correct” value for every line — it depends on the raw material and the design of the press, so producers determine it by experience and monitor it continuously. The moisture content of the finished fuel is determined using the methods described in ISO 18134^[15].

Size reduction

A hammer mill evens out the particle size. The aim is to feed a uniform meal into the die: particles that are too large create weak spots in the pellet where cracks start. Uniform milling translates into consistent pellet density and fewer crumbs in the bag.

Conditioning and pelletising

Before the press, the meal is often conditioned (Moistening the meal with steam or water before pressing to make it more plastic and help the particles bind.)Moistening the meal with steam or water before pressing to make it more plastic and help the particles bind. with steam or water. Heat and moisture soften 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 natural component of wood that acts as the binder in the pellet. At this stage the producer may also add a binding agent. ENplus allows only additives made from agricultural and forestry products, such as starch, maize or potato flour, vegetable oil or lignin from the kraft (sulphate) process; up to a total of 2.0% by mass, including a maximum of 1.8% during production and 0.2% after production^[1].

The heart of the line is the pellet mill. Rollers force the meal into the holes of the die (A steel ring or disc with hundreds of holes the diameter of a pellet. The length and shape of the holes determine how strongly the material is compressed.)A steel ring or disc with hundreds of holes the diameter of a pellet. The length and shape of the holes determine how strongly the material is compressed.. As the material is pushed through the channels it heats up from friction, the lignin softens and binds the particles, and at the outlet knives cut the pellets to length. The hole diameter sets the pellet diameter — 6 or 8 mm, with a tolerance of ± 1 mm — and the knife setting affects the length, which must be within 3.15–40 mm^[1].

Cooling, screening and packing

Cooling

A pellet leaving the die is hot and soft. In the cooler, with air flowing through it, it cools and hardens — the lignin sets and fixes the shape. Pellets that are not cooled enough crumble more easily. ENplus sets a specific requirement here: the producer must ensure that bulk pellets leaving the plant are no warmer than 40 °C, and warmer pellets must not be delivered to the end user; the same rule applies to traders^[1].

Screening

After cooling, the pellets pass over a screen that separates out the fines — dust and crumbs. The screenings are usually returned to the process. ENplus sets two thresholds here for particles smaller than 3.15 mm, determined using the ISO 5370 method^[1],[19]:

Packing

Pellets are packed into bags (e.g. 15 kg) or big bags, or shipped in bulk. An ENplus-certified bag must carry, among other things, a serial number identifying the packing company, the plant and the packing date^[2] — which means the batch can be traced back if a complaint arises.

Quality control at the plant

Pellet production does not end with packing. The producer takes samples according to set procedures — ISO 18135, ISO 21945 and ISO 14780 describe the sampling and sample preparation of solid biofuels^[26] — and tests them in-house and, within the certification scheme, also in an accredited laboratory.

It is worth noting where the standard requires individual parameters to be measured. ENplus determines mechanical durability at the point where the means of transport is loaded at the production plant^[1], using the ISO 17831-1 method^[18]. This means the result describes the pellets as they leave the factory. Every subsequent reloading, transport and transfer abrades the pellets — which is why well-run storage and transport matter as much as the press itself.

What this means for buyers

• You cannot judge the process by the name or the colour. Light and dark pellets can be equally good; colour depends on the species, the bark and the pressing temperature.
• Ask for batch test results. Moisture content, ash, mechanical durability and fines are direct “traces” of process quality. How to read a report is explained in the laboratory testing section.
• Check the bag once opened. A lot of dust at the bottom of a fresh bag signals poor screening, pellets with low durability or poor transport.
• Verify the certificate in the register. The certification scheme covers production control, but only if the certificate is valid — check this on the certificate verification page.