Mixed wood pellets

What mixed wood pellets are

Mixed wood pellets are made from raw material that contains both softwood and hardwood. The blend may be deliberate — the producer combines sawdust from two sources to achieve the desired properties or to secure continuity of supply — or it may result from a sawmill or joinery plant processing different species.

As far as the quality standards are concerned, mixed wood pellets are nothing special. ENplus describes the raw material by its origin, not by species^[1], and there is no obligation to state the species or the proportions on the bag^[2]. This means that “mixed” can mean 90/10 just as well as 50/50 — and that the proportions can change between batches if the producer does not control them.

How the proportions affect the parameters

Many fuel properties — ash, calorific value, nitrogen content — behave in a blend roughly as a weighted average of the components. If both components are clean, the differences are small, because the wood groups themselves differ only slightly:

Some parameters, however, do not “average out” so simply. The ash deformation temperature (The temperature at which an ash sample starts to change shape (its edges become rounded). Determined according to ISO 21404.)The temperature at which an ash sample starts to change shape (its edges become rounded). Determined according to ISO 21404. depends on the chemical composition of the ash as a whole, and components from different raw materials can react with one another. This is why the result for a blend cannot be reliably predicted from the results for its components — it has to be measured using the ISO 21404 method^[24]. The same goes for mechanical durability: raw materials with different structures may require different press settings, and a blend may need its own set of parameters.

Consistency: the key to a good blend

A pellet boiler doses fuel by volume — the auger moves a set volume of pellets. If batch A has a different bulk density, moisture content or ash content from batch B, settings chosen for one will no longer suit the other. ENplus allows a bulk density in the range 600–750 kg/m³^[1] — a range wide enough that the difference between two compliant batches can be noticeable.

In single-species pellets from a single sawmill, the raw material usually changes more slowly. In mixed wood pellets there is an additional variable — the proportion of the components. A good producer controls it just as it controls the moisture content of the dried material. A weaker one mixes whatever happens to be in the yard.

Advantages and risks

Advantages

• Raw-material flexibility for the producer — being able to use several sources of sawdust supports continuity of production, which indirectly translates into fuel availability.
• The ability to meet the highest class — provided both components are clean and debarked, and the process is adapted to the blend.

Risks

• Variability between batches — fluctuating proportions can change the density, the ash content and the behaviour in the burner.
• “Hiding” a weaker component — lower-quality raw material (e.g. with bark) can be diluted in the blend. A test result will show this in the ash content.
• Declarations that do not match reality — in a study of pellets from the Polish market, the values declared by producers often differed from those measured^[47].

How to assess mixed wood pellets

Mixed wood pellets are neither better nor worse than single-species pellets. They are simply a fuel whose quality has to be assessed in the same way as any other: by the test results, the certificate and consistency. More on comparing types of wood pellets in the softwood or hardwood comparison.