Ash melting temperature and clinker

What clinker is

Ash in the burner does not always stay loose. If it starts to soften at combustion chamber temperature, its particles stick together and, once cooled, form hard, often glassy lumps — clinker (Fused, partly molten lumps of ash that form in the burner. Once cooled, they are hard and difficult to remove.)Fused, partly molten lumps of ash that form in the burner. Once cooled, they are hard and difficult to remove. (also called slag). Clinker covers the holes in the grate through which combustion air flows, hinders the movement of fuel and the removal of ash. The result is unstable combustion, more unburnt particles and, in extreme cases, a boiler shutdown.

How ash fusibility is tested (ISO 21404)

The behaviour of ash at high temperature is tested according to ISO 21404:2020, which replaced the earlier specification CEN/TS 15370-1:2006^[24]. First, ash is prepared from the fuel sample. In the ENplus scheme it is prepared at 815 °C^[1]. A small test piece is then formed from the ash and heated in a furnace while changes in its shape are observed. The temperatures at which successive changes occur are the characteristic temperatures (The temperatures at which an ash test piece in the ISO 21404 test reaches defined stages: shrinkage, deformation, hemisphere and flow.)The temperatures at which an ash test piece in the ISO 21404 test reaches defined stages: shrinkage, deformation, hemisphere and flow.:

• SST (shrinkage starting temperature) — the test piece begins to shrink;
• DT (deformation temperature) — the first signs of softening, rounding of the edges;
• HT (hemisphere temperature) — the test piece takes on a roughly hemispherical shape;
• FT (flow temperature) — the ash spreads out over the support.

For the boiler user, the deformation temperature is the most important: it marks the point at which the ash in the burner may start to stick. The higher the DT, the greater the safety margin.

ENplus and ISO requirements

ENplus also requires the test report to state all the characteristic temperatures listed in ISO 21404, not just DT^[1]. In ISO 17225-2, ash behaviour at high temperature became a mandatory requirement only in the 2021 edition, which added DT thresholds to the class table^[11]; the draft of this standard gives the same values as ENplus^[12].

Why similar ash behaves differently

Two pellets with almost the same ash content can behave completely differently in the burner. Fusibility is determined by the chemical composition of the ash — the proportions of the major elements, which are determined according to ISO 16967: calcium, magnesium, potassium, sodium, silicon, phosphorus and iron, among others^[25].

The general relationships are well known, although their strength depends on the specific composition:

• Calcium and magnesium usually raise the ash melting temperature — ash rich in these elements stays loose at higher temperatures.
• Potassium and sodium lower it. The combination of potassium with silicon is particularly unfavourable, as it forms low-melting silicates.
• Silicon in wood comes mainly from mineral contamination — sand and soil that get into the raw material together with bark, during skidding or when stored on bare ground.

That is why ash content alone does not determine the tendency to form clinker. A small amount of ash with an unfavourable composition can cause more trouble than a somewhat larger amount of calcium-rich ash.

What the measurement data show

The Finnish VTT report lists ash deformation temperatures (in an oxidising atmosphere) for various wood fuels. The overall range for wood ash is 1150–1490 °C^[44].

These data lead to two important observations. First, even clean pine sawdust had a DT below the class A1 threshold (1200 °C) in this compilation — so “clean wood” does not automatically guarantee a high deformation temperature. Second, bark had a markedly higher DT than sawdust here, even though it contains much more ash^[44]. Bark therefore mainly increases the amount of ash, while its effect on fusibility depends on the composition — in practice, the bigger problem is often the mineral contamination that gets into the raw material along with it.

In a study of 30 pellet samples from the Polish market, an ash melting temperature that was too low was one of the problems found, alongside elevated ash content and undesirable admixtures such as bark, inorganic matter and petroleum-derived products^[47].

What the user can do

A laboratory test measures ash under controlled conditions, whereas conditions in the burner vary: the temperature can be locally higher and the air supply uneven. The boiler and its settings therefore also affect whether clinker forms.