Overview
EN ISO 21404:2020 - Solid biofuels: Determination of ash melting behaviour is a harmonized CEN/ISO test method for characterizing how ash from solid biofuels changes shape and melts when heated. The standard defines an empirical procedure to prepare biofuel ash, produce test pieces, heat them under controlled conditions and record characteristic temperatures that describe ash melting behaviour (shrinkage, deformation, hemisphere and flow). Results are used to compare ash fusibility and assess risks such as deposit formation or bed agglomeration in combustion systems.
Key topics and requirements
- Scope: Specifies a method to determine characteristic temperatures for ash melting behaviour of solid biofuels.
- Ash preparation: Ash is produced at a controlled ashing temperature of (550 ± 10) °C (alternatives of 710 ± 10 °C or 815 ± 10 °C can be used for specific purposes and must be reported).
- Characteristic temperatures defined:
- SST (Shrinkage Starting Temperature) - start of area loss.
- DT (Deformation Temperature) - first visual sign of melting.
- HT (Hemisphere Temperature) - when the specimen approximates a hemisphere.
- FT (Flow Temperature) - when ash spreads to a defined thin layer.
- Test principle: A homogenized ash test piece is heated at a constant rate in a controlled atmosphere while continuously observed; temperatures at shape-change events are recorded (images and/or automated shape-factor evaluation).
- Test atmosphere and calibration: Specifies use of controlled gases (CO₂, CO, H₂ mixtures) and calibration/checks using materials with known melting points (e.g., gold, nickel, palladium).
- Apparatus and reagents: Inert ashing dishes (platinum, graphite), ethanol, metal calibration wires/plates and image-capture equipment; guidance on dish preparation and test-piece dimensions.
- Documentation: The standard includes precision data, procedural steps (ash prep, test-piece formation, temperature recording) and reporting requirements.
Applications and users
- Who uses it: Fuel testing laboratories, pellet and wood-chip manufacturers, power plant and boiler operators, combustion system designers, researchers studying biofuel ash behaviour, and conformity/quality assessment bodies.
- Practical uses:
- Evaluating ash fusibility to predict slagging, fouling and bed agglomeration risks.
- Comparing biofuel batches or feedstocks (wood, agricultural residues, pellets) for combustion suitability.
- Supporting fuel specification, boiler maintenance planning and mitigation strategies for deposit control.
- Research and development of combustion systems and ash-related materials handling.
- Value: Provides standardized, comparable data on ash melting behaviour to inform operational decisions and reduce unplanned downtime.
Related standards
- ISO 16559 - Terminology for solid biofuels (terms and definitions referenced).
- Historic/related methods: DIN 51730, ISO 540:2008, and CEN/TS 15370‑1 (EN ISO 21404:2020 supersedes CEN/TS 15370‑1:2006).
Keywords: ash melting behaviour, solid biofuels, ash fusibility, hemisphere temperature, flow temperature, ash testing, biofuel quality, bed agglomeration, combustion systems.