Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
ISO/TS 28923 — Solid Biofuels — Fuel Specifications for Graded Wood Fuels for Non-Industrial Use
Standardized Quality Classes for Residential and Commercial Wood Fuel Products
ISO/TS 28923 establishes a comprehensive classification and specification framework for graded wood fuels intended for non-industrial use — primarily residential, commercial, and institutional heating applications. As the global bioenergy sector expands, driven by decarbonization policies and rising fossil fuel costs, the need for standardized fuel quality specifications has become paramount. End-users, appliance manufacturers, and fuel suppliers require clear, enforceable quality criteria to ensure consistent combustion performance, emissions compliance, and appliance reliability. This technical specification defines property classes for four major wood fuel product forms: pellets, briquettes, wood chips, and firewood, setting threshold values for key parameters including moisture content, ash content, calorific value, particle size distribution, and mechanical durability.
ISO/TS 28923 is closely coordinated with ISO/TS 28924 (which covers graded non-wood biofuels) and EN ISO 17225 (the base standard for solid biofuel specifications). Together, these standards form an integrated quality assurance framework for the entire solid biofuel supply chain, from forest management and primary processing to final end-use in heating appliances.
Fuel Classification and Quality Parameters
The specification divides graded wood fuels into quality classes (A1, A2, and B) based on their origin, processing level, and permissible impurity content. Class A1 represents the highest quality, comprising untreated stem wood and chemically untreated wood residues with the strictest limits on ash content (≤0.7% for pellets), nitrogen, sulfur, and chlorine. Class A2 permits a broader range of forest and plantation wood including whole trees and early thinnings, with relaxed ash limits (≤1.2% for pellets). Class B includes chemically treated wood such as construction and demolition waste, with correspondingly relaxed impurity thresholds but still within acceptable environmental limits for controlled combustion in approved appliances.
A critical engineering feature of ISO/TS 28923 is its treatment of fuel property interactions. The specification recognizes that certain parameter combinations — such as high ash content combined with low ash melting temperature — can lead to severe operational problems including slagging, fouling, and corrosion. To address this, ISO/TS 28923 provides guidance on ash melting behavior testing and recommends that fuel suppliers report the ash deformation temperature (DT) and ash softening temperature (ST) for wood fuels intended for use in appliances with high combustion temperatures or extended burn cycles.
| Property | Wood Pellets A1 | Wood Pellets A2 | Wood Pellets B | Wood Chips A1 | Firewood A1 |
|---|---|---|---|---|---|
| Moisture content (%, w/w) | ≤10 | ≤12 | ≤15 | ≤35 | ≤25 |
| Ash content (%, w/w, dry) | ≤0.7 | ≤1.2 | ≤2.0 | ≤1.0 | ≤1.5 |
| Net calorific value (MJ/kg, as received) | ≥16.5 | ≥16.0 | ≥15.5 | ≥10.5 | ≥12.0 |
| Nitrogen (%, w/w, dry) | ≤0.3 | ≤0.5 | ≤1.0 | ≤0.3 | ≤0.5 |
| Particle size (mm) | 6 ± 0.5 or 8 ± 0.5 | 6 ± 0.5 or 8 ± 0.5 | 6-12 | P16-P100 classes | Length ≤400 |
| Mechanical durability (%, w/w) | ≥98.0 | ≥97.5 | ≥97.0 | N/A | N/A |
| Ash melting temperature DT (°C) | ≥1200 | ≥1100 | ≥1000 | ≥1200 | ≥1100 |
A major practical benefit of ISO/TS 28923 classification is that appliance manufacturers can design combustion systems optimized for specific fuel classes, achieving higher efficiency and lower emissions than with ungraded fuels. Modern pellet boilers certified for Class A1 fuel routinely achieve seasonal efficiency above 92% with particulate matter emissions below 20 mg/MJ — a dramatic improvement over the 60-80% efficiency and >100 mg/MJ emissions typical of uncontrolled wood burning.
Engineering Design Considerations for Fuel Quality Management
From an engineering systems perspective, ISO/TS 28923 has profound implications for the design of fuel handling and combustion equipment. The moisture content specification is arguably the most critical parameter because it affects nearly every downstream process: storage stability, flowability in fuel delivery systems, ignition characteristics, combustion temperature, flue gas volume, and overall thermal efficiency. Wood pellets at 10% moisture produce roughly 15% more net energy per kilogram than pellets at 20% moisture, while also generating less creosote and fewer condensable organic compounds in the flue gas. Fuel handling system designers must account for the specified moisture range when sizing storage bins, conveyors, and metering equipment.
The particle size specification is equally important for automated combustion systems. Pellet boilers use auger-feed mechanisms with specific channel diameters (typically 45-70 mm for residential units). Oversized pellets or pellets containing excessive fines (particles smaller than 3.15 mm) can cause bridge formation in the hopper, erratic feed rates, and incomplete combustion. ISO/TS 28923 limits fines content to ≤1.0% for Class A1 pellets. The specification also defines the test method for mechanical durability using a tumbling canister — pellets must retain at least 98% of their mass after 500 revolutions, ensuring they can withstand the mechanical stresses of transportation, storage, and automated handling without generating excessive dust.
One of the most challenging operational issues addressed by ISO/TS 28923 is ash sintering and slag formation. Wood fuels with high potassium and silicon content can form low-melting-point silicate eutectics that fuse into hard glassy slag in the combustion chamber. The specification recommends blending problematic fuels with low-ash wood varieties or using fuel additives (such as kaolin or limestone) to raise the ash melting temperature. Combustion engineers should verify the DT value against the maximum combustion zone temperature of the appliance.
The calorific value specification deserves special attention from design engineers. ISO/TS 28923 expresses calorific value on both a dry basis and an as-received basis. The as-received value (which accounts for actual moisture content) is the relevant parameter for system sizing and energy yield calculations. For wood pellets, the net calorific value as received typically ranges from 16.0 to 18.5 MJ/kg, corresponding to roughly 4.5-5.1 kWh/kg. Designers of thermal storage systems and annual fuel supply logistics should use the lower bound of the specified range for conservative sizing to ensure adequate capacity during peak winter demand when fuel moisture content may be at its highest.
Sampling, Testing, and Compliance Verification
ISO/TS 28923 references the ISO 18100 series of solid biofuel sampling and testing standards for compliance verification. A statistically based sampling plan is required for each fuel batch, with the sampling frequency determined by the production volume and the historical variability of the fuel properties. For critical parameters such as ash content and calorific value, the specification requires that the reported value represent the 80% upper confidence bound of the batch mean rather than a simple average, providing a conservative quality guarantee to the end-user.
The specification also addresses the important issue of fuel declaration and labeling. Each fuel batch must be accompanied by a declaration document listing the fuel class, all measured property values, and the testing laboratory accreditation status. For retail sales to residential consumers, ISO/TS 28923 recommends a simplified labeling format showing the fuel class, nominal particle size, and moisture content on the packaging, along with a QR code linking to the full declaration document. This transparency enables consumers to make informed purchasing decisions and provides traceability in case of quality disputes.
A significant safety concern in wood fuel quality management is spontaneous combustion risk during storage. Wood pellets stored in bulk can undergo oxidation and self-heating, particularly when the moisture content is elevated or when fines accumulate in the stored mass. ISO/TS 28923 requires that suppliers provide storage safety guidelines for each fuel class, including maximum stack height, ventilation requirements, and temperature monitoring recommendations. Carbon monoxide monitoring is recommended for enclosed storage areas, as CO accumulation from slow oxidation is a recognized hazard in pellet storage facilities.
Q1: Can ISO/TS 28923 Class B wood fuels (recycled/demolition wood) be used in residential appliances?
A: Generally no. Class B fuels may contain treated wood with paint, coatings, or preservatives that can release hazardous compounds during combustion. They are intended for industrial or large commercial appliances with advanced flue gas cleaning systems. Residential appliances should use only Class A1 or A2 fuels.
A: Generally no. Class B fuels may contain treated wood with paint, coatings, or preservatives that can release hazardous compounds during combustion. They are intended for industrial or large commercial appliances with advanced flue gas cleaning systems. Residential appliances should use only Class A1 or A2 fuels.
Q2: How does the specification address the variability of wood chip properties?
A: Wood chips exhibit greater inherent variability than manufactured pellets. ISO/TS 28923 defines chip classes by particle size distribution (e.g., P16 for fine chips <16 mm, P100 for coarse chips <100 mm) and allows wider moisture and ash limits. Suppliers must declare the actual measured values rather than relying on class default limits.
A: Wood chips exhibit greater inherent variability than manufactured pellets. ISO/TS 28923 defines chip classes by particle size distribution (e.g., P16 for fine chips <16 mm, P100 for coarse chips <100 mm) and allows wider moisture and ash limits. Suppliers must declare the actual measured values rather than relying on class default limits.
Q3: Is ISO/TS 28923 applicable to torrefied or thermally treated wood fuels?
A: The current edition addresses only conventional (untreated and mechanically processed) wood fuels. Torrefied wood pellets, which have different hydrophobicity, grindability, and combustion characteristics, are being considered for inclusion in a future edition. At present, producers and users of torrefied fuels should refer to CEN/TS 17242 or negotiate bilateral specifications.
A: The current edition addresses only conventional (untreated and mechanically processed) wood fuels. Torrefied wood pellets, which have different hydrophobicity, grindability, and combustion characteristics, are being considered for inclusion in a future edition. At present, producers and users of torrefied fuels should refer to CEN/TS 17242 or negotiate bilateral specifications.
Q4: What are the key differences between ISO/TS 28923 and the corresponding EN standards (EN ISO 17225)?
A: ISO/TS 28923 is substantially harmonized with EN ISO 17225-2 (wood pellets), 17225-4 (wood chips), and 17225-5 (firewood). The primary difference is that ISO/TS 28923 includes additional guidance on fuel property interactions (ash melting behavior, corrosion risk assessment) and provides more detailed sampling protocols for non-industrial supply chains.
A: ISO/TS 28923 is substantially harmonized with EN ISO 17225-2 (wood pellets), 17225-4 (wood chips), and 17225-5 (firewood). The primary difference is that ISO/TS 28923 includes additional guidance on fuel property interactions (ash melting behavior, corrosion risk assessment) and provides more detailed sampling protocols for non-industrial supply chains.
