Moisture content of wood pellets

What moisture content means

Moisture content is the share of water in the total mass of the fuel, expressed as a percentage. Pellets with a moisture content of 10% contain 100 kg of water and 900 kg of dry wood matter in every tonne. Moisture content is determined by the methods described in ISO 18134, which has three parts: a reference method, a simplified method and the determination of moisture in the general analysis sample^[15].

Wood pellets are an exceptionally dry fuel, because the raw material is dried before pelletising and the pellet itself is highly compacted. This distinguishes them from firewood, whose moisture content depends on seasoning. We describe the process in the article how wood pellets are made.

As received and dry basis

Fuel test reports use two ways of reporting results:

• as received (Fuel in the state in which it was delivered — including the water it contains.)Fuel in the state in which it was delivered — including the water it contains. — the result for the fuel “as bought”, including its water. Moisture content, net calorific value, durability and bulk density are reported on this basis^[1];
• dry basis (Fuel with the water removed. The Polish regulation defines it as the state after drying for more than 2 hours at 105–110 °C.)Fuel with the water removed. The Polish regulation defines it as the state after drying for more than 2 hours at 105–110 °C. — the result recalculated to water-free fuel. Ash, nitrogen, sulphur and chlorine are reported on this basis^[1],[36].

A dry-basis value from a report is converted to the as-received basis by multiplying it by (100 − M)/100, where M is the moisture content in per cent. For calorific value there is an additional correction for the energy needed to evaporate the water — we give the full formula on the calorific value page^[44].

Effect on combustion and boiler efficiency

Water in the fuel works against you in several ways at once:

• Less energy per kilogram. The more water, the less wood in the same mass of fuel.
• Energy for evaporation. Before the wood starts to burn, the water has to heat up and evaporate. The heat needed for this comes from the fuel, and the steam escapes up the chimney.
• Lower combustion chamber temperature. Evaporation cools the combustion zone. Combustion becomes less complete, which favours the formation of carbon monoxide, soot and particulate matter.
• Settings out of tune. The boiler feeds fuel in doses of a defined volume. Damper pellets deliver less energy in each dose, so settings chosen for dry fuel no longer fit.

In addition, more water vapour in the flue gas means a larger volume of hot gases leaving the boiler — and more heat lost with them. That is why boiler efficiency, which the documentation relates to a fuel with specified properties, falls in practice when the fuel is damper.

How much energy water takes away

The numbers show the scale of the effect clearly. Below we have calculated the as-received net calorific value at different moisture contents for pellets whose wood has 19.0 MJ/kg on a dry basis. We used the formula from the VTT report, which is consistent with ISO 18125^[44].

In this example, the difference between pellets at 6% and 10% moisture is about 0.86 MJ/kg — close to 5% of the energy in every kilogram. Both samples meet the requirements, but the buyer pays per tonne, not per unit of energy.

Requirements and the 20% anti-smog threshold

ENplus allows a moisture content of at most 10.0% as received in all three classes (A1, A2, B), determined according to ISO 18134^[1]. The Polish regulation Dz.U. 2025 poz. 618 (Polish Journal of Laws 2025, item 618) adopts the same value (≤ 10.00%) for pellets; for briquettes the limit is 12.00%^[36]. The draft of ISO 17225-2 also gives ≤ 10% for all classes^[12].

Regional anti-smog resolutions in Poland address moisture differently — as a ban on burning biomass that is too wet. In the Małopolska region (outside Kraków), burning wood and biomass with a moisture content above 20% is banned^[41]. In the Silesia region, burning solid biomass whose as-received moisture content exceeds 20% is prohibited^[43]. In Kraków, a ban on solid fuels, including biomass, has been in force since 1 September 2019, so pellet boilers are not permitted there^[42].

Storage and swelling

Pellets leave the plant dry, but they can take up moisture later. They absorb water from humid air, and on direct contact with water they swell, lose cohesion and fall apart back into sawdust. Such material not only has a lower calorific value but also clogs the auger. It is no accident that every ENplus bag must carry an instruction to store the pellets in dry conditions (in the ENplus document: “Store in dry conditions”)^[2].

Moisture damage is usually visible to the naked eye: the pellets turn dull, crack lengthwise and more sawdust collects at the bottom of the bag. This is a sign that the problem also affects fines and mechanical durability.