Ash content of wood pellets

What ash is

Ash is the non-combustible mineral residue left after complete combustion of a fuel. In wood pellets it has two sources: mineral elements that the tree took up from the soil, and contamination picked up by the raw material along the way — sand, soil or small stones. ISO 16967 describes the determination of the major ash-forming elements in solid biofuels: aluminium, calcium, iron, magnesium, phosphorus, potassium, silicon, sodium and titanium^[25].

Ash content is expressed as a percentage by mass on a dry basis (A result recalculated to fuel with the water removed. The Polish regulation defines it as the state after drying for more than 2 hours at 105–110 °C.)A result recalculated to fuel with the water removed. The Polish regulation defines it as the state after drying for more than 2 hours at 105–110 °C. and determined according to ISO 18122^[1],[16]. This way the result does not depend on how much water the sample contained at the time of testing^[36].

Ash is not distributed evenly in a tree. In Finnish measurements on birch, pine, spruce and aspen, stemwood contained on average 0.46% ash, bark 2.97%, branches with bark 1.52% and leaves 4.97%^[44]. The FAO handbook gives a typical value of 4% for softwood bark (range 2–6%) and 5% for hardwood bark (range 2–10%)^[45]. This is why ENplus allows only stemwood and chemically untreated wood-industry residues in class A1, and prohibits raw material with a large amount of bark in all classes^[1].

Why low ash matters

Ash releases no energy, but it affects boiler operation in three ways.

• Cleaning. More ash means emptying the ash pan and cleaning the burner and heat exchanger more often. In a boiler with automatic de-ashing, the ash container fills up faster.
• Burner and heat transfer. Ash builds up on the grate and in the combustion chamber. When it covers the air holes, combustion becomes less stable. Deposits on the heat exchanger walls hinder heat transfer to the boiler water.
• Emissions. Some of the mineral matter does not stay in the ash pan but leaves with the flue gas as fine particulate matter. A clean fuel with little mineral matter makes it easier to keep emissions low.

Requirements: ENplus, ISO and Polish regulation

Ash content is the parameter that most clearly separates the quality classes. The ENplus limits in the current 2nd edition of ST 1001 are the same as in the 1st edition^[1]. The 2025 Polish regulation adopts the same values for A1 and A2 pellets, but has no class B^[36].

How many kilograms per tonne

Percentages are easy to convert into mass: 1% ash means 10 kg of ash from every tonne of fuel on a dry basis. The A1 limit is therefore about 7 kg, the A2 limit 12 kg and the B limit 20 kg per tonne^[1]. The scale below shows the class limits and the conversion into kilograms.

Pellets are bought as received, i.e. together with their water. At a moisture content of 8%, a tonne of fuel contains 920 kg of dry matter, so ash at the A1 limit amounts to about 6.4 kg (0.70% × 0.92). The difference is small, but it shows why every result has to come with the basis on which it is reported. The ash pan may also hold more material than the analysis suggests — if combustion is incomplete, unburnt carbon ends up in the ash.

What counts as a very good result

The 0.70% limit is the upper boundary of class A1, not a typical value. Clean wood usually contains noticeably less ash. The FAO handbook, citing typical values from CEN/TS 14961, gives a typical 0.3% (range 0.2–0.5%) for both softwood and hardwood^[45]. The Finnish VTT report gives usually 0.3–0.7% for debarked stemwood and only 0.08% for debarked pine sawdust^[44]. The same data show that hardwood species contain more ash than softwoods^[44].

The draft of ISO 17225-2 (FDIS) included a recommendation to use pellets with an ash content below 0.5% in residential stoves^[12]Data require verification: The recommendation comes from the FDIS draft; its wording in the published ISO 17225-2:2021 has not been confirmed.. A reasonable practical interpretation:

• well below 0.5% — a very good result, typical of clean stemwood and sawdust;
• 0.5–0.70% — still class A1, but closer to the limit; more cleaning, especially in small stoves;
• above 0.70% — the pellets do not meet A1 requirements, whatever the bag says.

Amount of ash versus its behaviour

Ash content tells you how much ash there is, but not how it will behave in the burner. Two pellets with a similar ash content can look completely different: one leaves light, powdery ash, the other hard, glassy lumps — clinker (Fused, partly molten lumps of ash that form in the burner. They restrict the air supply and make ash removal difficult.)Fused, partly molten lumps of ash that form in the burner. They restrict the air supply and make ash removal difficult.. This is decided by the chemical composition of the ash, which is described by the ash melting temperature. ENplus requires an ash deformation temperature of at least 1200 °C in class A1 and 1100 °C in classes A2 and B^[1].

How to check it

• Test report. Look for a result determined according to ISO 18122, stating that it refers to the dry basis, with the date and the name of the laboratory^[16]. More on reading reports in the laboratory testing section.
• ENplus bag. If the producer states fuel properties on the bag, they must be the threshold values from the ENplus table or stricter values confirmed by laboratory results within the scheme^[2].
• Your own observation. Compare the amount of ash from successive deliveries at the same boiler settings. A sudden increase is a signal to ask the supplier for the batch test results.