A class code opens the discussion
The EN ISO 21640 class code communicates three important fuel characteristics consistently, but it cannot describe everything that affects storage, feeding, combustion, emissions control or product quality. Two fuels with the same NCV, chlorine and mercury classes can still differ materially in moisture, ash, particle shape, bulk density or other trace elements.
That is why a buyer normally places a plant-specific acceptance matrix beside the class code. The matrix should state targets, absolute rejection limits, test methods and the reporting basis. Permit conditions and operating experience may be more restrictive than the broad class boundary.
What a useful specification covers
A workable data sheet identifies the origin and permitted input categories, traded form, particle dimensions, moisture, ash, NCV on the agreed bases, chlorine, mercury and the other trace elements relevant to EN ISO specification. The parties may also request biomass content, bulk density, volatile matter, sulphur and other halogens, ash behaviour, preparation method and visible impurities.
Operational details matter as much as laboratory chemistry. A specification may control oversize pieces, fines, metal, PVC-rich fragments, bale dimensions, bale integrity, dust, self-heating or storage stability. The right list depends on how the receiving plant unloads, stores, meters and burns the fuel.
Illustrative receiving envelopes
The examples below are condensed from the European Investment Bank’s 2024 sector paper. They describe broad technical envelopes observed for different end uses; they are not legal EU grades, guaranteed acceptance values or a substitute for a plant’s current permit and tender specification.
| Application | NCV | Moisture | Ash | Chlorine |
|---|---|---|---|---|
| Cement main burner | >21 MJ/kg | <15% as received | <15% dry | Usually <0.8% dry |
| Cement calciner | >14.7 MJ/kg | <20–25% as received | <20% dry | Usually <0.8% dry |
| Lime rotary kiln | >23 MJ/kg | <10% as received | <7% dry | <1.0% dry |
| RDF grate plant | 11–18 MJ/kg | ≤35% as received | ≤30% dry | ≤2.0% dry |
| RDF fluidised bed | 11–20 MJ/kg | ≤35% as received | ≤25% dry | ≤2.0% dry |
Physical fit decides whether fuel can be used
A cement main burner generally needs a fine, light fraction that heats rapidly in a short flame residence time; the EIB examples cite particles below 30 mm and roughly 60–200 kg/m³ bulk density for fluff. A calciner can often accept larger material, while grate systems can handle much coarser RDF. These examples cannot be transferred between plants without checking the actual feeder and combustion design.
Chemistry is equally process-specific. Chlorine, sulphur, alkalis, aluminium metal and ash composition can affect corrosion, build-up, slagging or the mineral product. Trace-metal limits may be set individually or as grouped totals. The correct commercial approach is therefore EN ISO classification plus a receiving-plant matrix—not a claim that one class fits every outlet.
Build the contract around the destination
Before regular supply begins, producer and buyer should align the fuel description, intended operation, test basis, delivery form and response to variation. A technically attractive average is not enough if individual lots can exceed a permit or disrupt the feeding system.
For initial screening, the broad envelopes can help identify plausible outlets. Final acceptance should rely on representative laboratory data, a plant-approved specification and clear contractual remedies for material outside the agreed limits.