ISO 17225-6:2021

Overview - ISO 17225-6:2021 (Graded non-woody pellets)

ISO 17225-6:2021 specifies fuel quality classes and technical specifications for graded non-woody pellets, a part of the ISO 17225 series for solid biofuels. It covers pellets made from non-woody biomass origins defined in ISO 17225‑1: herbaceous biomass, fruit biomass, aquatic biomass, and blends/mixtures. Thermally treated pellets (e.g., torrefied) are excluded. The standard supports clear classification for trading, appliance compatibility and regulatory reporting.

Key topics and technical requirements

This part defines the measurable properties, sampling and testing references needed to declare pellet quality. Key technical topics include:

• Fuel classes and origin - pellets are classified by origin (herbaceous, fruit, aquatic, blends) per ISO 17225‑1.
• Physical dimensions - permitted pellet diameters (D06 to D25) and length ranges with dimensional tolerances (see ISO 17829).
• Moisture and calorific value - moisture limits and net calorific value (ISO 18134, ISO 18125).
• Ash and ash behaviour - ash content limits and informative ash melting behaviour (ISO 18122, ISO 21404).
• Mechanical durability and fines - minimum durability and maximum fines to protect handling and combustion (ISO 17831-1, ISO 18846).
• Bulk density - declared bulk density for logistics and storage (ISO 17828).
• Chemical limits - maximum contents for nitrogen, sulfur, chlorine and selected heavy metals (N, S, Cl by ISO 16948 / 16994; metals by ISO 16968). Example property limits for Classes A and B include:
  • Ash: ≤ 6% (A) and ≤ 10% (B)
  • Moisture: ≤ 12% (A) and ≤ 15% (B)
  • Mechanical durability: ≥ 97.5% (A) and ≥ 96.0% (B)
  • Chlorine: ≤ 0.10% (A) and ≤ 0.40% (B)
  • Metals (e.g., As ≤ 1 mg/kg for A)
• Additives and contamination - additives must be declared; chemically treated raw material or contaminated feedstocks require disclosure and possibly additional analysis.

Normative test and sampling methods are referenced throughout to ensure consistent measurement and reporting.

Practical applications and users

ISO 17225-6 is intended for:

• Pellet producers and suppliers - to classify and declare product quality for market access.
• Heating appliance and boiler manufacturers - to specify compatible pellet types and avoid corrosion, fouling or slagging.
• Test laboratories - for standardized sampling and analysis.
• Commercial and residential users, utilities and procurement teams - to select appropriate pellets for small-scale to industrial burners.
• Regulators and certifiers - for conformity assessment and reporting.

Practical advice from the standard: non-woody pellets often have higher ash, Cl, N and S - they are best used in appliances designed or adapted for such fuels, and blending with woody biomass can improve combustion behaviour.

Related standards

• ISO 17225-1:2021 - General requirements and origin classification (recommended companion document)
• Sampling and test methods: ISO 14780, ISO 18135, ISO 18134‑1/2, ISO 18122, ISO 18125, ISO 17831‑1, ISO 17828, ISO 17829, ISO 16948, ISO 16968, ISO 16994, ISO 21404

Keywords: ISO 17225-6:2021, graded non-woody pellets, solid biofuels, fuel specifications, pellet quality, herbaceous biomass, aquatic biomass, pellet testing.