Wood pellet laboratory testing

What can be tested in wood pellets

Every parameter that appears in the ENplus classes and in ISO 17225-2^[1],[11] has its own standardised test method. These methods are developed by the technical committee ISO/TC 238 “Solid biofuels”, and their current editions are published in the committee’s catalogue^[14]. This means an ash result from a laboratory in Poland can be compared with one from Austria — provided both were determined using the same method and reported on the same basis.

In practice, pellet testing falls into three groups:

• Physical properties — dimensions, bulk density, particle density, mechanical durability, fines. They tell you how the pellets will cope with transport and how they will behave in the auger.
• Fuel properties — moisture content, ash, calorific value, ash melting behaviour. They tell you how much energy you are buying and how much work the boiler will need.
• Chemical composition — nitrogen, sulphur, chlorine, major elements and heavy metals. They tell you about emissions, corrosion and the purity of the raw material.

Parameters and test methods

The table pairs each parameter with its method standard (current edition), the sample basis on which the result is usually reported and what the result tells the buyer. You will find the class limit values in the certificates section.

A few changes to the standards are worth knowing about when reading older reports: fines are now determined according to ISO 5370:2023, which replaced ISO 18846, and sulphur and chlorine according to ISO 16994:2026 instead of the 2016 edition. Ash melting behaviour is covered by ISO 21404:2020, which replaced CEN/TS 15370-1:2006^{[19],[23],[24]}. Older standard numbers in a report do not mean it is wrong — but they do suggest that the analysis is not recent or that the laboratory has not updated its methods.

ENplus requires the ash melting report to state all the characteristic temperatures listed in ISO 21404, and the ash for this test is prepared at 815 °C^[1].

Sampling — half of a result’s credibility

A laboratory tests what it receives. A result is exactly as representative as the sample. That is why there are separate standards for sampling and sample preparation for biofuels: ISO 18135:2017 (sampling), ISO 21945:2020 (a simplified method for small-scale applications) and ISO 14780:2017 with its 2019 amendment (sample preparation)^[26].

Who took the sample?

This question matters more than the accreditation number. We distinguish between three situations:

• Sample taken by an independent party (an inspector, the laboratory, the buyer) from a defined batch, following a set procedure — the highest credibility.
• Sample bought on the market — a bag from a wholesaler or a shop. It shows what actually reaches the customer, but a single bag is not necessarily representative of the whole production.
• Sample supplied by the producer — the report may be entirely sound, but we do not know whether the sample came from a typical batch. We label such results as source: producer’s declaration until they are confirmed by analysis of an independently taken sample.

Where was the sample taken?

Some parameters change along the way. ENplus specifies the measuring points: mechanical durability is determined when the means of transport is loaded at the plant, fines in bulk pellets at the factory gate or when loading for end customers, and fines in bags when the bags are filled^[1]. A bag that has travelled several hundred kilometres and been reloaded several times may contain more dust than on the day it was packed. A result obtained from the market is therefore not directly comparable with a limit measured at the producer’s site — although for the buyer it is just as important.

What a professional test report must contain

The list below sets out our editorial criteria: without these elements, we do not treat a document as a complete source of data on a product. If a seller shows “test results” as a small table in an offer, ask for the full report.

Laboratory accreditation: ISO/IEC 17025

The competence of testing laboratories is covered by ISO/IEC 17025 (International standard setting out requirements for the competence, impartiality and consistent operation of testing and calibration laboratories.)International standard setting out requirements for the competence, impartiality and consistent operation of testing and calibration laboratories.. Accreditation to this standard is granted by a national accreditation body — in Poland, the Polish Centre for Accreditation (Polskie Centrum Akredytacji)Data require verification: No source in the site’s source register. The ENplus scheme requires testing bodies involved in certification to hold ISO/IEC 17025 accreditation under the EA/ILAC agreements^[8].

The key word is scope. Accreditation does not cover “the whole laboratory”, only the specific methods listed in its scope. A laboratory may be accredited for determining ash and moisture content yet test mechanical durability using a non-accredited method. So:

• ask for the accreditation number and check the scope in the accreditation body’s register;
• compare the standard numbers in the scope with the methods in the report;
• check whether results obtained with accredited methods are marked as such in the report.

A lack of accreditation does not mean a result is bad — scientific research and internal quality control are often carried out rigorously without it. It does mean, however, that the laboratory’s competence has not been confirmed by an independent party.

Who tests solid biofuels

Pellets and other solid biofuels are tested by several types of organisation. Each has a different role and a different value as a source of information:

Internal control is part of the requirements of some schemes — DINplus provides for mandatory internal quality control alongside the annual external inspection^[28]. We do not name laboratories here; verified organisations will be published in the list below.

How to read results against class limits

A result complies with a requirement if the value reported by the laboratory is within the limit — as stated in the note to the ENplus table of limit values^[1]. Before you compare a result with a limit, check three things:

1. The sample basis. Ash, nitrogen, sulphur and chlorine are compared on a dry basis; moisture content, calorific value, durability and fines as received.
2. The unit. Calorific value may be given in MJ/kg or kWh/kg (16.5 MJ/kg = 4.6 kWh/kg). Heavy metals are given in mg/kg, other constituents as a percentage by mass.
3. The class and diameter. The ash limit is 0.70% for A1 and 1.20% for A2. A result of 0.9% falls within A2 but not within A1.

Dry basis vs as received — an example

The net calorific value as received is calculated from the dry-basis value using a formula that accounts for the mass of water and the heat needed to evaporate it: Q_ar = Q_d × (100 − M)/100 − 0.02443 × M, where M is the moisture content in per cent^[44],[17]. Pellets with 18.9 MJ/kg on a dry basis and 8% moisture content therefore have 18.9 × 0.92 − 0.02443 × 8 ≈ 17.19 MJ/kg (4.78 kWh/kg) as received. The same mechanism works the other way round for ash: 0.70% on a dry basis corresponds, at 8% moisture content, to around 0.64% as received. A result without a stated basis can therefore look better than it really is.

You can do your own conversions with our calculators.

Compliance is not everything

Two A1-compliant pellets can differ markedly: ash contents of 0.3% and 0.7% mean around 3 kg and 7 kg of ash respectively per tonne of dry fuel. That is why, beyond compliance, we look at where the result sits within the class and at consistency across successive batches — details in our methodology.

Scope of testing, time and cost

We do not quote prices — they depend on the laboratory, the country, the number of determinations and the turnaround time. The scope of the analysis has the greatest effect on cost and waiting time. Practical options:

• Basic set — moisture content, ash, calorific value, mechanical durability, fines. Enough to compare offers and check deliveries.
• Full set for a class — every parameter in the table of limit values, including nitrogen, sulphur, chlorine, ash melting behaviour and heavy metals. Needed when you want to confirm compliance with class A1.
• Diagnostic tests — ash composition (major elements), when clinker appears in the boiler even though the basic parameters look fine.

Before commissioning an analysis, ask the laboratory how much sample it needs, how it should be packed, the turnaround time and which methods are covered by its accreditation. If the result may be used in a dispute with a supplier, agree on the sampling procedure in advance and keep a sealed arbitration sample.

List of laboratories

The list will cover laboratories and testing organisations that test wood pellets in Poland, the Czech Republic, Slovakia, Germany and other EU countries — added only once their accreditation and its scope have been checked.