Wood pellet specifications are the language every biomass trade is written in. Before a Japanese utility, a Korean IPP or a European heating distributor sends an order, it sends a spec sheet, and the whole negotiation turns on whether a mill can hit those numbers cargo after cargo. Getting wood pellet specifications right is therefore not a laboratory exercise. It decides which buyers shortlist you, what price band you sit in, and whether a shipment is accepted, discounted or rejected at the discharge port.
This guide explains the wood pellet specifications that matter in 2026, how ISO 17225-2 and the ENplus scheme define them, what each property actually measures, and how pellets made from Vietnamese acacia and rubberwood residues typically test against those limits. It is written for buyers who need to write a contract and for producers who need to defend one.
Why wood pellet specifications decide the deal
A pellet is a commodity only on paper. In practice two pellets with the same diameter can behave completely differently in a boiler, and the difference shows up entirely in the wood pellet specifications. Ash content decides how often a plant shuts down for cleaning. Moisture decides how much energy the buyer actually paid for. Durability decides how much of the cargo arrives as dust instead of fuel. Each of those numbers converts directly into operating cost, which is why buyers argue about them so hard.
There is a commercial layer too. Most supply contracts attach a price adjustment or a rejection right to specific wood pellet specifications, so the spec sheet is really a risk allocation document. A mill that quotes tight numbers it cannot hold every month will meet penalties. A mill that quotes realistic numbers with a stated tolerance, and proves them with third-party test certificates, builds the repeat business that biomass supply is actually made of.
ISO 17225-2: the standard behind every spec sheet
ISO 17225-2 is the international standard for graded wood pellets and it is the reference almost every serious buyer quotes. It splits pellets into non-industrial classes A1, A2 and B, intended for residential and small commercial appliances, and industrial classes I1, I2 and I3 intended for power and heat plants. When a buyer writes wood pellet specifications into a contract, they are usually copying a class from this standard and then tightening one or two properties.
The classes differ in two ways. First, they set different numerical limits for ash, nitrogen, sulphur and chlorine. Second, and this is often overlooked, they define which raw materials are allowed. Class A1 is restricted to stem wood and chemically untreated wood residues, while class B permits a broader input including chemically untreated used wood. Two mills can both claim to meet wood pellet specifications for ash and still be selling very different products because their feedstock rules differ.
| Property (ISO 17225-2) | A1 | A2 | B |
|---|---|---|---|
| Diameter | 6 or 8 mm | 6 or 8 mm | 6 or 8 mm |
| Length | 3.15 to 40 mm | 3.15 to 40 mm | 3.15 to 40 mm |
| Moisture, as received | 10.0% max | 10.0% max | 10.0% max |
| Ash, dry basis | 0.7% max | 1.2% max | 2.0% max |
| Mechanical durability | 97.5% min | 97.5% min | 96.5% min |
| Fines below 3.15 mm | 1.0% max | 1.0% max | 1.0% max |
| Net calorific value, as received | 16.5 MJ/kg min | 16.5 MJ/kg min | 16.5 MJ/kg min |
| Bulk density | 600 kg/m3 min | 600 kg/m3 min | 600 kg/m3 min |
| Nitrogen, dry basis | 0.3% max | 0.5% max | 1.0% max |
| Sulphur, dry basis | 0.04% max | 0.04% max | 0.05% max |
| Chlorine, dry basis | 0.02% max | 0.02% max | 0.03% max |
Treat that table as an orientation map rather than a contract. Standards are revised, and the authoritative text is the current edition published by ISO. Always confirm the exact edition and clause number before you paste wood pellet specifications into a purchase agreement, and state the edition year in the contract so both sides are testing against the same document.
ENplus A1, A2 and B: the certification layer
ENplus is a certification scheme built on top of ISO 17225-2 and administered through the European Pellet Council. It matters because it audits the whole chain, production, storage, bagging and delivery, rather than just testing a sample. For bagged residential pellets in Europe, an ENplus seal is close to a licence to trade, and many distributors will not discuss wood pellet specifications at all with an uncertified mill.
ENplus generally applies limits at least as strict as the matching ISO class and adds handling requirements the standard does not cover, such as a maximum pellet temperature at loading and tighter control of fines at the point of dispatch. If your buyer is European and sells into the residential market, plan on ENplus. Confirm current scheme rules directly with ENplus because the requirements are updated periodically.
Industrial buyers usually take a different route. Utility scale plants in Japan and Korea rarely ask for ENplus. They write their own wood pellet specifications around ISO industrial classes and then layer sustainability certification such as SBP or GGL on top. Knowing which of the two worlds your buyer lives in tells you which certificate to invest in first.
Calorific value, moisture and ash: the three numbers buyers check first
Every enquiry starts with the same three properties. Net calorific value is the usable energy per kilogram, moisture is the water the buyer is paying freight on, and ash is the residue the plant has to remove. These three dominate every discussion of wood pellet specifications because they translate most directly into money per delivered tonne.
Calorific value is quoted two ways and the difference causes constant confusion. Gross calorific value is measured in a bomb calorimeter and includes the energy recovered by condensing water vapour. Net calorific value as received subtracts that, and it is the number that matters for a boiler. Insist that wood pellet specifications state which basis is used, in MJ/kg or kcal/kg, and at what moisture level.
| Property | Typical unit | What it drives | Common contract limit |
|---|---|---|---|
| Net calorific value | MJ/kg or kcal/kg | Energy delivered per tonne | 16.5 to 18.0 MJ/kg |
| Moisture | % as received | Freight waste, storage mould | 8 to 10% max |
| Ash | % dry basis | Boiler cleaning, slagging | 0.7 to 1.5% max |
| Bulk density | kg/m3 | Container and hold utilisation | 600 to 750 |
| Durability | % | Fines and dust on arrival | 97.5% min |
Ash deserves special attention with tropical feedstock. Bark, sand and soil picked up during harvesting and yard handling all report as ash, so a mill with poor debarking or an unpaved log yard will drift above its stated wood pellet specifications even when the wood itself is clean. Ash control is a housekeeping discipline as much as a raw material choice.
Mechanical durability and fines
Mechanical durability measures how much of a pellet sample survives a standardised tumbling test without breaking into dust. It is expressed as a percentage, and a figure of 97.5% means only 2.5% of the sample degraded. Durability is arguably the most practically important of all wood pellet specifications because it predicts what the cargo will look like after transhipment, silo drops and pneumatic conveying at the receiving plant.
Fines are the direct consequence. A pellet that fails durability arrives as a mixture of pellets and powder, and powder blocks feed screws, dusts the plant and creates an explosion risk. Contracts commonly cap fines below 3.15 mm at 1.0% at the factory gate, but a buyer measuring at discharge after a five week ocean voyage will always find more. Sensible wood pellet specifications state clearly where and when fines are measured.
Durability is driven by die compression ratio, feedstock particle size, conditioning steam and cooling. Pellets that leave the cooler warm keep hardening for hours, which is why loading hot pellets straight into a container is a classic cause of both dust and self heating. A mill that understands its own process can hold durability far more consistently than one that simply reports whatever the batch happened to test.
Diameter, length and bulk density
Diameter is usually 6 mm or 8 mm and it is a fixed choice, not a preference. Residential stoves and small boilers are engineered around 6 mm, while many industrial plants prefer 8 mm because it moves better through large conveying systems. Confirm the diameter before quoting, because switching dies mid contract is expensive and any change to this part of the wood pellet specifications affects throughput and energy consumption at the mill.
Length is capped to prevent bridging in feed systems, typically at 40 mm with a small tolerance for a limited percentage of longer pellets. Bulk density matters commercially because it determines whether you fill a container by weight or by volume. At 650 kg per cubic metre a 40 foot container loads out very differently than at 600, so bulk density belongs in the wood pellet specifications even when the buyer does not ask for it.
Chlorine, sulphur, nitrogen and trace elements
The minor elements look like small print until a plant has a corrosion problem. Chlorine attacks superheater tubes and forms dioxins in poorly controlled combustion. Sulphur and nitrogen drive SOx and NOx emissions that a plant must permit for. Heavy metals matter where ash is spread on land as a soil improver. These properties appear in every serious set of wood pellet specifications precisely because they constrain what the buyer can legally do with the fuel and the residue.
Clean plantation wood normally passes these limits comfortably. Problems appear when contaminated inputs enter the mix, for example painted or treated demolition wood, or residues that carry adhesives from panel production. That is exactly why ISO ties raw material rules to each class. If your wood pellet specifications promise class A1 chemistry, your feedstock policy has to match, and your incoming material inspection has to be able to prove it.
| Element | Why the buyer cares | Typical A1 limit | Typical source of failure |
|---|---|---|---|
| Chlorine | High temperature corrosion | 0.02% max | Salt exposure, contaminated wood |
| Sulphur | SOx emissions | 0.04% max | Contaminated or treated wood |
| Nitrogen | NOx emissions | 0.3% max | Bark, leaves, adhesives |
| Heavy metals | Ash disposal and land spreading | Reported in mg/kg | Painted or treated wood |
| Ash melting point | Slagging in the furnace | Reported in deg C | Sand, soil, high alkali feedstock |
Ash melting behaviour and slagging
Ash content tells a plant how much residue it will handle. Ash melting behaviour tells it whether that residue will fuse into hard clinker on the grate. Deformation temperature is the figure usually reported, and a low value predicts slagging that forces unplanned shutdowns. Sophisticated buyers add it to their wood pellet specifications even though many suppliers still leave it off the standard test sheet.
Potassium, sodium and silica are the drivers, and all three are elevated by bark, leaves and soil contamination. Feedstock that includes agricultural residues almost always melts at a lower temperature than clean stem wood, which is why blending straw or husk into a wood pellet without declaring it is a serious breach rather than a technicality.
How Vietnamese acacia and rubberwood pellets typically test
Vietnam has become one of the largest pellet exporters in the world, supplying a very large share of Japanese and Korean demand, and the reason is the raw material base. Acacia, eucalyptus and rubberwood plantations feed a large wood processing sector, and the residues from sawmills and panel plants are exactly the input a pellet mill wants. The table below shows the wood pellet specifications a well run Vietnamese mill can typically hold on a routine basis.
| Property | Typical Vietnamese acacia pellet | Basis |
|---|---|---|
| Diameter | 6 mm or 8 mm | as produced |
| Moisture | 8 to 10% | as received |
| Ash | 0.7 to 1.5% | dry basis |
| Net calorific value | 4,100 to 4,400 kcal/kg | as received |
| Mechanical durability | 97.5 to 98.5% | as produced |
| Fines | under 1.0% | factory gate |
| Sulphur | under 0.03% | dry basis |
| Chlorine | under 0.03% | dry basis |
Note the ash band. Pure debarked sawmill residue can sit near 0.7%, but mixed plantation residue including some bark will land closer to 1.2 to 1.5%. Rather than quoting an optimistic single figure, state a band in your wood pellet specifications, back it with a run of recent test certificates and explain what drives the variation. Buyers respect that far more than a headline number that fails on the second shipment.
If you are evaluating Vietnamese supply, look at the wood pellets product page for current grades, the wider biomass energy range for chips and briquettes, and the factory page to see how the production line is set up. Comparable alternatives such as bamboo pellets and rice husk pellets carry very different ash and chlorine profiles, so never assume one spec sheet covers them all.
Sampling, testing and who you can believe
A test certificate is only as good as the sample behind it. Sampling should follow a recognised procedure, taking increments across the whole lot rather than a scoop from the top of a pile, and the composite should be split down properly. Ask any supplier how the sample supporting their wood pellet specifications was taken, because a beautiful certificate from an unrepresentative sample is worth nothing at the discharge port.
For commercial quantities, appoint an independent inspector such as SGS, Intertek or Bureau Veritas to sample and seal at loading, with a retained sample held for the contractual claim period. Split the sample three ways: one for the seller, one for the buyer, and one sealed and held by the inspector as the umpire sample. That structure resolves nearly every dispute about wood pellet specifications before it becomes a legal problem.
Writing wood pellet specifications into a contract
A workable clause does five things. It names the standard and the edition, it lists each property with a limit and a basis, it names the test methods, it states where and when sampling happens, and it defines the consequence of a deviation. Most disputes come from the last two. Wood pellet specifications that say nothing about the point of measurement invite an argument the moment the vessel berths.
Build in a tolerance band with a price adjustment rather than a simple pass or fail. A common structure sets a target, an acceptable range with a pro rata price adjustment, and a rejection threshold outside it. Energy content is often settled on an as delivered calorific basis so both sides share the moisture risk. That approach keeps a marginal cargo moving instead of stranding it, which is usually better for everyone.
- Name the standard and its edition year, not just the class letter.
- State the basis for every number: as received or dry basis.
- Fix the sampling point, the method and who appoints the inspector.
- Set a tolerance band with price adjustment before any rejection right.
- Require a full test certificate per shipment, not an annual type test.
Common mistakes buyers and mills make
The most expensive mistake is quoting a single ash figure with no tolerance. Feedstock varies seasonally, and a mill that promises 0.7% ash every month will eventually ship 1.1% and face a claim. The second mistake is confusing gross and net calorific value, which can look like a 6 to 8% difference in energy and destroys a price comparison between two otherwise identical sets of wood pellet specifications.
The third mistake is ignoring the raw material clause and assuming any pellet that hits the numbers qualifies as class A1. The fourth is testing durability only at the mill and never at discharge, so nobody learns how the cargo actually travels. The fifth is treating certification as a substitute for testing. A certificate proves a system exists, while a test certificate proves this particular cargo met the wood pellet specifications you paid for.
Frequently asked questions
What are the most important wood pellet specifications?
Net calorific value, moisture, ash and mechanical durability. Those four decide energy delivered, freight efficiency, cleaning cost and how much of the cargo arrives as usable fuel rather than dust.
Is ENplus required to export wood pellets?
Not universally. ENplus is close to essential for bagged residential pellets in Europe, but industrial buyers in Japan and Korea typically write their own wood pellet specifications around ISO industrial classes and ask for SBP or GGL sustainability certification instead.
What ash content should I expect from Vietnamese pellets?
Typically 0.7 to 1.5% on a dry basis depending on how much bark is in the feedstock and how clean the log yard is. Ask for a run of recent certificates rather than a single best result.
What is the difference between gross and net calorific value?
Gross includes the energy recovered by condensing water vapour, net does not. Net calorific value as received is the number a boiler operator uses, and it is the basis most contracts settle on.
Why do fines increase between loading and discharge?
Every transfer point breaks pellets. Ship loading, ocean voyage vibration, discharge grabs and conveying all add fines, so wood pellet specifications should state whether the limit applies at the factory gate, at loading or at discharge.
Where to read next
Specifications are only one leg of an export deal. Once the numbers are agreed, the same cargo has to clear customs, travel safely and satisfy a sustainability regime. Read wood pellet HS code 4401.31 for classification and duty, shipping wood pellets safely for the carbon monoxide and self heating rules that decide how the cargo is loaded, EUDR wood pellets compliance if the buyer is European, and biomass certification for the scheme your market expects.
Bottom line
Wood pellet specifications are the contract. Anchor them to a named edition of ISO 17225-2, state the basis for every figure, quote realistic bands rather than best case numbers, define the sampling point, and prove each cargo with an independent test certificate. Do that and the technical conversation becomes short, because both sides are reading the same document and measuring the same thing at the same place.
Woodbase Corp. is a Vietnamese manufacturer and exporter of wood pellets, wood chips, wood powder, wood briquettes, sawdust, shavings, rice husk pellets and firewood, produced from residues of sustainably managed acacia, eucalyptus and rubberwood plantations. We supply by container and bulk vessel with third-party inspection, full test certificates and traceable documentation. Contact Woodbase for a specification sheet, a sample and a quotation.