ENplus certification has a reputation for being a paperwork exercise. It is not. The document that governs it, ENplus ST 1001, sets numeric limits on the product and then requires the producer to prove, shift by shift, that the plant can hold those limits. The second edition of ST 1001 was published on 1 October 2025 and entered into force on 1 January 2026, and it is the fourth major revision of the scheme, issued by Bioenergy Europe and the German Pellet Institute — which means a lot of existing quality manuals and supplier datasheets are now describing an older version of the requirements.
For a plant being designed or commissioned in 2026, the useful question is not “can we pass the audit” but “does the equipment we are buying make the audit passable”. Six requirements decide that, and five of them are decided by equipment long before the auditor arrives.
What A1 actually requires
The quality classes are defined in Table 4 of the standard, and the values below are taken directly from the edition now in force.
| Параметр | A1 | A2 | B | Test method |
|---|---|---|---|---|
| Moisture (as received) | ≤ 10.0 % | ≤ 10.0 % | ≤ 10.0 % | ISO 18134 |
| Ash (dry basis) | ≤ 0.70 % | ≤ 1.20 % | ≤ 2.00 % | ISO 18122 |
| Mechanical durability (as received) | ≥ 98.0 % | ≥ 97.5 % | ≥ 97.5 % | ISO 17831-1 |
| Насыпная плотность | 600–750 kg/m³ | ISO 17828 | ||
| Fines below 3.15 mm, bulk | ≤ 1.0 % | ISO 5370 | ||
| Fines below 3.15 mm, bagged | ≤ 0.5 % | ISO 5370 | ||
| Net calorific value | ≥ 4.6 kWh/kg | ISO 18125 | ||
| Additives | ≤ 2.0 % | — | ||
| Nitrogen / sulphur / chlorine (dry) | ≤ 0.3 / 0.04 / 0.02 % | ≤ 0.5 / 0.04 / 0.02 % | ≤ 1.0 / 0.04 / 0.03 % | ISO 16948 / 16994 |
| Diameter and length | 6 ± 1 or 8 ± 1 mm; 3.15 ≤ L ≤ 40 mm | ISO 17829 | ||
The full table, including heavy metal limits and the footnotes on how each value is measured, is in the standard itself, which is free to download from the ENplus technical documentation page. Read the footnotes before you copy the numbers into a tender document: several parameters are defined “as received” and others “dry basis”, and mixing the two produces a specification that no laboratory can test against.
1. Durability is 98.0 %, not 97.5 %
This is the number most often wrong in Chinese supplier datasheets and in tender documents copied from them. A2 and B require 97.5 % mechanical durability; A1 requires 98.0 %. It is a one-point difference that changes the die specification, the cooling regime and the whole fines story.
On a ring die press, durability at that level comes from four things working together: a compression ratio matched to the raw material, a die with clean, correctly tapered holes, a roller gap that is actually set rather than guessed, and a cooling step that removes heat without stripping moisture. The press is rarely the limiting item; the die specification and the cooler sizing usually are. This is the same relationship we describe in the wood pellet line design guide, where the die and the dryer are sized from the same moisture balance.
2. Ash at 0.70 % is decided at the raw material gate
Ash is not something a pellet mill can fix. Bark, sand, soil and contamination in the feedstock all report as ash, and a plant that accepts mixed demolition wood or unbarked residues will not hold 0.70 % no matter how well it presses. The standard addresses this directly: the producer must accept incoming goods against documented requirements for origin, quality and contamination, and must retain the delivery documentation for the raw material and any additives (ENplus ST 1001:2022, clause 5.2.1).
Practically, this means a debarking decision and a screening decision, both made at the design stage. If your feedstock includes roundwood, a drum dryer will not remove bark; a debarker will. If it includes forest residues, a pre-cleaning screen before the chipper is cheaper than rejecting finished pellets at the lab.
The same logic applies to the trace element limits, which are easy to overlook because they are not in the headline table. A1 caps arsenic at 1 mg/kg, cadmium at 0.5 mg/kg and chromium at 10 mg/kg, measured on a dry basis and reported with the rest of the certificate. Painted wood, treated pallet material and mixed post-consumer wood are the usual sources of a failed cadmium or chromium result, which is why the raw material classes in the standard are narrow and why the incoming goods check is a documentary control as much as a visual one. A supplier who cannot tell you where a load came from cannot tell you whether it is A1 material.
Additives are capped at 2.0 % for all classes, and the standard requires them to be declared and controlled in the same way as raw material. For most producers this is a short list — a binding agent, sometimes a pressing aid — but the record has to exist, with the delivery documentation retained and the inclusion rate traceable to a batch.
3. Moisture is measured at loading, not at the press
Ten percent moisture “as received” applies to the pellet the customer receives, which means the sample is taken at loading or bagging, after cooling and storage — not at the die. Pellets re-absorb moisture in a silo, in a big bag, and in a truck. A plant that runs at 8 % at the press and loads at 10.4 % has a storage and cooling problem, not a drying problem.
Two design items control this. The cooler has to bring the pellet down to a temperature where it will not sweat in the silo, and the storage and loading system has to keep rain and humid air out. On a line we commission, the moisture check is done at three points — dryer outlet, cooler outlet, and loading — because the difference between them is the only way to see where the moisture is coming back.
4. Fines have two limits, and bagging is the stricter one
Bulk pellets may carry up to 1.0 % fines below 3.15 mm; bagged pellets may carry 0.5 %. The reason is mechanical: every handling step after the cooler generates fines, and a bagging line adds a drop, a weigher and a stitch. A plant that screens before the silo and then loads through three conveyors will fail the bagged limit while passing the bulk one on the same day’s production.
The fix is a screener at the last point before bagging, not only at the cooler outlet, and a bagging line designed to minimise drop height. It is a layout decision, and it is much cheaper to make on a drawing than after commissioning.
5. Self-monitoring is per shift, with records
The standard does not accept a monthly laboratory certificate as proof of routine control. It requires self-monitoring at defined frequencies, with records, and the frequencies are specific.
| Check | Where | Minimum frequency |
|---|---|---|
| Overlength (visual plus measurement of unusual pellets) | Production process | Once per shift, per production line |
| Влажность | Production process | Once per shift, per production line |
| Механическая прочность | Production process, or loading/bagging if not measurable there | Once per shift, per production line |
| Overlength at dispatch | Loading point or bagging station | Once per day with loading or bagging |
| Fines below 3.15 mm | Loading point or bagging station | Daily or weekly, depending on sieving equipment and loading speed |
A durability test per shift requires a tumbler and someone trained to run it. That is a laboratory bench, a procedure, and a person — small items that are often missing from a new plant’s budget, and the first thing an auditor asks to see. If the plant has one press, one tumbler and no backup, a single failed test can stop production while the instrument is repaired.
6. The mass balance account has to add up
The producer must keep a mass balance account covering production and all sales, able to identify certified pellets by quality class, diameter and bag design, separately from non-certified pellets. In a plant that runs certified and non-certified product on the same line, this is a documentation system that depends on physical segregation: separate silos, separate bag designs, or a clean-out procedure that is written down.
Designers tend to leave this until the end. It is much easier to provide two bagging lines or a dedicated certified silo at the layout stage than to explain in an audit why certified and non-certified pellets shared a bin for three weeks in March.
Next Steps
If you are building or upgrading a plant for ENplus A1 certification, work in this order, because each step constrains the next. The equipment side of the line — chipper, dryer, biomass pellet mill, cooler, screener and bagging — should be specified against these steps rather than in parallel with them.
Fix the raw material specification first: species mix, bark content, contamination limits, and the supplier documentation you will require at the gate. Everything downstream follows from this, including whether you need a debarker and a pre-cleaner.
Build the moisture balance from the feedstock, not from a catalogue. Drying capacity, dryer type and heat source have to be sized on the wettest month of the year, with the target moisture at the press set low enough to leave room for re-absorption before loading.
Specify the die and the cooler as a pair against the 98.0 % durability limit, and put the test method in the purchase order. Ask the supplier for the compression ratio they recommend for your material and the basis for that recommendation.
Place a screener at the last point before bagging and lay out the bagging line to minimise drop height, so the bagged fines limit of 0.5 % is achievable on a normal day rather than on a good one.
Budget the laboratory and the records before commissioning: a durability tumbler, a moisture analyser, an ash crucible and furnace if you intend to test in-house, plus the shift sheets that make the self-monitoring requirement a routine rather than a scramble.