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ISO 18846-17:2017 – Solid Biofuels — Determination of Moisture Content — Oven Dry Method
Technical Guide to Accurate Moisture Measurement in Solid Biofuels for Quality Assurance and Trade Compliance
The quality and energy value of solid biofuels such as wood pellets, chips, briquettes, and agricultural residues are heavily influenced by their moisture content. Excessive moisture reduces heating value, increases transport costs, and can lead to biological degradation during storage. ISO 18846-17 (also adopted as CAN/CSA-ISO 18846-17) specifies the reference oven dry method for determining the total moisture content in solid biofuels. This method provides the most accurate and reproducible results, forming the basis for trade contracts, regulatory compliance, and quality control.
1. Scope of ISO 18846-17
ISO 18846-17 applies to all solid biofuels including:
- Wood pellets and briquettes
- Wood chips and hog fuel
- Forestry and agricultural residues
- Torrefied biomass and char
The standard covers the determination of total moisture content (free and bound water) by drying a test portion in a ventilated oven under controlled temperature conditions. It is applicable for materials with moisture content up to 60% (wet basis). The method is intended for laboratory use and serves as the reference procedure against which alternative methods (e.g., near-infrared, capacitive sensors) are calibrated.
Tip: For routine process control, alternative moisture meters can be used, but they must be regularly calibrated against the oven dry method as per ISO 18846-17.
2. Technical Requirements
2.1 Sample Preparation
The test sample must be representatively collected according to ISO 18135. The sample mass should be at least 300 g for particles up to 8 mm, and larger for coarser materials. The sample is placed in a pre-dried and tared container (non-corroding, with a lid).
2.2 Drying Oven
A ventilated oven capable of maintaining the required temperature within ±2 °C. The oven must have sufficient ventilation to remove evaporated water and avoid condensation.
2.3 Drying Temperature
The standard defines two temperature regimes depending on sample type:
| Sample Type | Drying Temperature | Typical Time |
|---|---|---|
| Wood pellets, chips, other standard biofuels | (105 ± 2) °C | 12–24 h |
| Biomaterials with significant volatile oils (e.g., pine, eucalyptus) | (70 ± 2) °C | 12–24 h |
| Torrefied biomass | (105 ± 2) °C | 8–12 h |
| Fine-grained residues (sawdust) | (105 ± 2) °C | 4–6 h |
The lower temperature (70 °C) is used when the sample contains volatile compounds that would be driven off at 105 °C, leading to overestimation of moisture content.
2.4 Calculation
Moisture content (wet basis) is calculated as:
M = (mw / msample) × 100%
where:
- mw = loss in mass after drying (initial – final dry mass)
- msample = initial sample mass
Results are reported to one decimal place.
2.5 Repeatability and Reproducibility
The standard provides precision data. For most biofuels, the absolute difference between two single test results obtained under repeatability conditions should not exceed 0.5% for moisture < 40%, and 1.0% for higher moisture.
3. Implementation Highlights
Successful implementation of ISO 18846-17 requires attention to several critical points:
- Use an oven with forced air circulation and stable temperature control. Calibrate thermometers regularly against a traceable standard.
- Do not overload the oven; ensure hot air can circulate freely around containers.
- Desiccator cooling before weighing to avoid moisture adsorption from ambient air.
- Weighing must be performed with a balance of 0.01 g resolution for samples under 300 g.
- If the sample may contain volatile components above 0.2% by mass (e.g., methanol, terpenes), use the 70 °C method and confirm by a preliminary test.
Warning: Drying at 105 °C for materials containing volatile organic compounds (VOCs) can lead to loss of VOCs, overestimating moisture. Always verify the appropriate temperature using the standard’s classification table.
Benefit: Adopting the standardised oven dry method eliminates disputes between buyer and seller and ensures accurate payment based on net calorific value.
4. Compliance Notes
Accredited laboratories seeking compliance with ISO 18846-17 should:
- Demonstrate competency in sample handling and oven operation.
- Maintain records of oven temperature monitoring (including continuous chart recorders or data loggers).
- Participate in interlaboratory proficiency testing for moisture content.
- Implement regulatory requirements for moisture determination in solid biofuels (e.g., EU RED, EPA regulations).
The Canadian adoption (CAN/CSA-ISO 18846-17) includes a national foreword but is technically identical to the international standard. Labs in Canada should reference this version for conformity assessment.
Critical: Failure to use the correct drying temperature for a given biofuel type can result in significant measurement errors, potentially exceeding 5% absolute moisture. This can impact commercial transactions and product classification.
Frequently Asked Questions
Q: Why is moisture content so critical in solid biofuels?
A: Moisture directly reduces energy yield per tonne, increases transportation costs, and promotes microbial degradation. An accurate moisture value is essential for pricing, energy calculation, and storage management.
A: Moisture directly reduces energy yield per tonne, increases transportation costs, and promotes microbial degradation. An accurate moisture value is essential for pricing, energy calculation, and storage management.
Q: Can I use a halogen moisture analyzer instead of the oven method?
A: Halogen analyzers are faster but considered secondary methods. They must be validated against ISO 18846-17 for each material type. The oven method remains the reference for dispute resolution.
A: Halogen analyzers are faster but considered secondary methods. They must be validated against ISO 18846-17 for each material type. The oven method remains the reference for dispute resolution.
Q: What is the typical drying time?
A: For standard wood pellets at 105 °C, a minimum of 12 hours is recommended until constant mass is achieved (mass change less than 1 mg per gram of sample after 1 hour additional drying).
A: For standard wood pellets at 105 °C, a minimum of 12 hours is recommended until constant mass is achieved (mass change less than 1 mg per gram of sample after 1 hour additional drying).
Q: Is there a difference between ISO 18846-17 and CAN/CSA-ISO 18846-17?
A: The Canadian adoption is technically identical to the ISO edition. The main difference is the national foreword and adoption status, which makes it the recognized standard in Canada for regulatory purposes.
A: The Canadian adoption is technically identical to the ISO edition. The main difference is the national foreword and adoption status, which makes it the recognized standard in Canada for regulatory purposes.