Softwood or hardwood pellets?

In brief

• Per kilogram of dry matter, softwood usually has a slightly higher calorific value than hardwood — the difference is in the order of a few per cent^[44],[45].
• The ash content of clean stemwood is low in both groups. Bark matters far more: it contains many times more ash than wood^[44],[45].
• The ENplus A1 requirements are the same for every species. A very good spruce pellet can have better parameters than an average beech pellet — and vice versa^[1].
• Species genuinely matters when the boiler manufacturer recommends a particular type of fuel, or when the burner is sensitive to a certain type of ash.

Species compared: spruce, pine, beech, oak and blends

The table is deliberately, and largely, qualitative. We have verified figures for individual species for only some of the parameters — mainly the calorific value of stemwood from Finnish VTT research and typical values for species groups from the FAO handbook. Where we describe a tendency without a reliable source, we say so explicitly.

You will find details for individual species in these articles: spruce pellets, pine pellets, beech pellets, oak pellets, birch pellets and mixed-species pellets.

Where the differences between species come from

Calorific value: lignin and resins

Finnish VTT data for dry stemwood give 19.31 MJ/kg for Scots pine, 19.05 MJ/kg for Norway spruce and around 18.6–18.7 MJ/kg for downy birch. Softwoods come out slightly higher because they contain more lignin (A natural polymer in wood that binds the fibres together. It has a higher calorific value than cellulose and acts as a binder when pellets are pressed.)A natural polymer in wood that binds the fibres together. It has a higher calorific value than cellulose and acts as a binder when pellets are pressed. and resin^[44]. The typical values in the FAO handbook paint a similar picture: 19.2 MJ/kg for softwood and 19.0 MJ/kg for hardwood, expressed on a dry, ash-free basis (Dry, ash-free basis (daf): the result recalculated as if the fuel contained neither water nor ash.)Dry, ash-free basis (daf): the result recalculated as if the fuel contained neither water nor ash.^[45].

These are differences of around 1–4%. For comparison, every percentage point of moisture lowers the as-received calorific value by about 0.2 MJ/kg^[44] (more on this below).

Ash: species versus bark

According to the typical values quoted by the FAO, softwood and hardwood have similar ash contents — about 0.3% (range 0.2–0.5%). Bark is on an entirely different scale: typically about 4% (2–6%) for softwood bark and 5% (2–10%) for hardwood bark^[45]. VTT measurements for birch, pine, spruce and aspen gave an average of 0.46% ash in stemwood and 2.97% in bark, while debarked pine sawdust contained only 0.08%. VTT also notes that hardwoods contain more ash than softwoods^[44].

A simple calculation with typical values: if 5% bark with 4% ash finds its way into raw material with 0.3% ash, the ash content of the mix rises to about 0.49% (0.95 × 0.3 + 0.05 × 4). A few per cent of bark therefore changes the result more than the choice between spruce and beech. That is why ENplus prohibits raw material with a high bark content in all classes^[1].

Myth: “Hardwood pellets are always better”

The visualisation below shows two hypothetical pellets, A and B, both of which meet the A1 requirements. For A you can read a very good spruce pellet, for B an average beech pellet (or the other way round). The standard allows both, yet the difference in use is real: in the ash to be emptied, the water in every tonne, the energy and the dust.

Worked example: moisture matters more than species

Let us calculate the as-received calorific value using the formula Q_ar = Q_d × (100 − M)/100 − 0.02443 × M^[44] for three species from the VTT data at two moisture levels. We use birch as the example of a hardwood, because we have no verified data for beech and oak from the same source.

Dry birch pellets (6%) contain more energy per kilogram than damper spruce pellets (9%), even though spruce itself has the higher calorific value. Here, three percentage points of moisture outweigh the difference between species.

The conclusion goes beyond this example: the spread in quality between products made from the same species can be larger than the typical difference between species. This is because class limits are wide, and the result is decided by the purity of the raw material, the share of bark, the moisture content and the quality of the process. A study of 30 pellet samples from the Polish market found that 10 of them failed to meet the requirements for mechanical durability and/or fines, and some had elevated ash, too low an ash melting temperature or admixtures of bark^[47] — problems of production and raw material, not of species.

Why hardwood pellets have a better reputation

The belief that beech and oak are superior has several roots — and only some of them carry over to pellets.

• Habits from firewood. Firewood is often sold by volume, and the denser hardwoods deliver more energy in the same stacked cubic metre. Pellets are bought by weight, and pellets of every species are compressed — ENplus requires a bulk density of 600–750 kg/m³ for all classes^[1]. In pellets, the density advantage of the wood largely disappears.
• The impression of a “longer burn”. A boiler with an automatic auger feeds fuel according to heat demand. A bag lasting longer comes down to more energy per kilogram, and that is determined by the as-received calorific value, not by species.
• Bag weight and volume. A bag with the same net weight holds the same number of kilograms of fuel, whatever the species. The volume can differ — which matters for hopper capacity, but not for the amount of energy.
• Marketing. “Beech”, “oak”, “hardwood” all sound good on a bag. Yet the name of the species is not a quality parameter in ENplus or in ISO 17225-2^[1],[11].
• Less resin. Some users prefer resin-free fuel. That is a legitimate preference — but it is not proof of better quality.

When species really does matter

• The boiler manufacturer's recommendations. The appliance manual may specify a fuel class or type. ENplus bags must carry a note that the pellets are to be used in accordance with the appliance manufacturer's instructions and the applicable regulations^[2] — this is not a formality.
• Burner design. Burners differ in how well they tolerate clinker and in how they remove ash. If a particular pellet forms clinker in your burner, changing the product (not necessarily the species) and the settings usually achieves more than switching wood group alone.
• Regional availability of raw material. The make-up of the forests influences which raw material is readily available. In Poland, pine covers 59.1% of the forest area^[48], while in Slovakia beech has the largest share, at 35.8%^[50]. Neither the country of origin nor the species decides quality, however — more in the pellets by country section.
• Consistency of blends. If a producer changes the proportions of species from batch to batch, the ash content and burner behaviour can change too. Ask for the results of several successive batches.

How to decide

1. Start with the certificate and the number in the register — that is the minimum, whatever the species (how to check it).
2. Compare the parameters in the test report: ash on a dry basis, moisture content and calorific value as received, durability, fines and ash deformation temperature.
3. Check what your boiler manufacturer recommends.
4. Buy a small trial quantity first and assess the ash and burner performance at the same settings.
5. Treat species as additional information, not as a quality criterion.

You will find the full buying checklist in our guide how to choose good wood pellets.