ISO 17225-4:2021

Overview

ISO 17225-4:2021 - Solid biofuels: Graded wood chips defines fuel quality classes and specifications for graded wood chips used as solid biofuels. The standard applies to chips made from forest/plantation wood, by‑products and residues from the wood processing industry, and chemically untreated used wood. It covers only chips produced with sharp tools (excludes hog fuel produced with blunt tools) and focuses on clear classification to support trading, equipment compatibility and regulatory reporting.

Key topics and requirements

• Property classes - Defines A and B property classes (e.g., A1/A2 for virgin and untreated residues, B1/B2 for wider sources including certain treated residues) with guidance on declaring origin and treatment.
• Particle size classes - Specifies particle size/size distribution classes (P16, P31, P45, P63, including “s” variants for smaller applications) and fines classes (F02–F30+) using ISO 17827-1 oscillating screen method.
• Moisture and ash - Sets moisture and ash content classes (A classes represent lower moisture; A1 typically for drier fuels). The 2021 edition updated the moisture and ash class definitions.
• Chemical thresholds - For B‑class fuels, the standard sets limits for nitrogen, sulfur, chlorine and a range of minor elements (e.g., As, Cd, Cr, Cu, Pb, Hg, Ni) to manage contamination risks. A‑classes assume minimal contamination and therefore have no chemical thresholds.
• Sampling and analysis - Requires sampling, sample preparation and laboratory methods in accordance with referenced standards (ISO 18135 / ISO 21945 for sampling, ISO 14780 for sample prep, ISO 18122, ISO 18134‑2 and relevant ISO methods for chemical analysis).
• Declarations and documentation - Fuel quality shall be provided in the product declaration; producers/suppliers are responsible for accuracy whether using measured values, calculated typical values, or reference/simplified analyses.
• Bulk density guidance - Annex A gives informative guidance on bulk density by moisture range.

Applications and users

Practical uses include small and medium residential, commercial and public building heating systems, boilers, heaters and some gasifiers. Typical users:

• Fuel producers and suppliers (product specification and marketing)
• Heating system manufacturers and equipment designers (fuel compatibility)
• Testing and certification laboratories (sampling and analysis)
• Asset owners, facility managers and procurement teams (fuel selection)
• Regulators and policymakers (compliance and reporting)

Related standards

• ISO 17225-1 (General requirements for solid biofuels)
• ISO 17827-1 (Particle size distribution)
• ISO 18134-2 (Moisture determination)
• ISO 18122 (Ash determination)
• ISO 16948 / ISO 16968 / ISO 16994 (elemental and minor element analysis)

Keywords: ISO 17225-4:2021, graded wood chips, solid biofuels, fuel specifications, particle size classes, moisture classes, ash content, sampling methods.