Raw material quality and wood pellet quality

Why the raw material decides

Wood burns almost completely. What is left is ash (The non-combustible mineral residue left after the fuel is burnt. In testing, it is determined by ashing a sample at a specified temperature (ISO 18122).)The non-combustible mineral residue left after the fuel is burnt. In testing, it is determined by ashing a sample at a specified temperature (ISO 18122). — mineral compounds the tree took up from the soil, plus anything that stuck to the raw material along the way. Ash content is determined using the ISO 18122 method^[16], and ENplus A1 pellets may contain no more than 0.70% on a dry basis^[1].

Clean stemwood contains little ash — in data from Finland’s VTT, debarked stemwood typically contains 0.3–0.7%, and debarked pine sawdust only 0.08%^[44]. Problems start when the raw material contains things that should not be there. This is exactly why ENplus prohibits raw material that is rotten, contaminated or contains a large amount of bark^[1].

Bark

Bark is the typical “carrier” of ash in wood pellets. It contains several times — even ten or more times — more mineral matter than wood: according to typical values cited by FAO, softwood bark has about 4% ash (range 2–6%) and hardwood bark about 5% (2–10%), while the wood itself has about 0.3% (0.2–0.5%)^[45]. VTT measurements for the bark of specific species gave: pine 1.70%, birch 1.60%, spruce 2.34–2.80%; the average for the bark of the four species — including aspen, which we do not list here — is 2.97%^[44].

What does this mean in practice? The table below is a simple calculation based on the VTT averages: stemwood 0.46% ash, bark 2.97%^[44]. It shows how quickly the ash content of the mix rises as the proportion of bark increases.

The conclusion is not “bark is bad”. Bark has a respectable calorific value — FAO gives a typical figure of about 20 MJ/kg for it on a dry, ash-free basis^[45]. The problem is that every admixture of bark pushes the pellets towards higher ash, more frequent boiler cleaning and a lower class.

Sand, soil and stones

Mineral contamination gets into the raw material during skidding, when wood is stored on bare ground, during transport and when sawdust is stored on an unpaved yard. Sand and soil do not burn at all — they remain in the ash in their entirety. Even a small admixture therefore shows up in the ash determination.

The second problem is less obvious. Silicon (An element whose main carrier in sand and soil is silica. It is determined among the major elements of a biofuel (ISO 16967).)An element whose main carrier in sand and soil is silica. It is determined among the major elements of a biofuel (ISO 16967). from sand, combined with potassium and other components of wood ash, can form compounds that soften at the temperatures found in a burner — this is one of the mechanisms by which clinker (Hard, glassy lumps of melted and fused ash that build up in the burner and restrict the air supply.)Hard, glassy lumps of melted and fused ash that build up in the burner and restrict the air supply. formsData require verification: The chemical mechanism (potassium silicates) is described on the basis of general knowledge; there is no dedicated source in the site’s register.. Silicon is one of the major elements determined in biofuels according to ISO 16967^[25], and the behaviour of ash at high temperature is described by testing according to ISO 21404^[24].

A study of pellets from the Polish market found, among the problems, both elevated ash and too low an ash melting temperature, as well as undesirable admixtures: bark, inorganic matter and petroleum products^[47]. This is a good illustration of the point that raw-material defects later show up in the fuel test results.

Branches, needles and leaves

Branches and tops have a lot of bark relative to wood, and needles and leaves are the most “mineral” part of the tree. In the VTT data, branches with bark contained an average of 1.52% ash, and leaves 4.97%^[44]; FAO describes softwood logging residues with a typical value of 2% (1–4%)^[45].

This is why logging residues and whole trees without roots are allowed in ENplus only from class A2 downwards, while class A1 allows only stemwood and untreated wood-processing residues^[1]. A detailed overview is given in the article on raw material for pellet production.

Soundness and freshness of the wood

ENplus excludes rotten raw material^[1]. Biological decay changes the structure and composition of the wood, so such material should not go into production. From the buyer’s point of view, what matters is that poorly stored raw material — sawdust or wood chips kept on an open yard for a long time — is more exposed both to decay and to contamination with soil.

A separate issue is wood from sanitary felling, for example after bark beetle outbreaks. Coming from such felling does not in itself determine quality: if the wood is sound, clean and debarked, it can be a good raw material. If it has stood in the forest for a long time, has started to decay or is processed with its bark on, the risk increases. We cover the situation in the spruce forests of Central Europe in more detail in the article on spruce pellets.

Batch consistency

For a boiler user, it is just as important that good parameters are consistent. A burner set up for one batch of fuel will perform worse on a batch with a different density, moisture content or ash content. A constant raw material — such as sawdust from a single sawmill processing similar wood — gives a more stable fuel than raw material that changes from week to week.