ISO 21656:2021

Overview

ISO 21656:2021 - Solid recovered fuels - Determination of ash content specifies laboratory methods for measuring ash content in all types of solid recovered fuels (SRF). The standard defines the principle, apparatus, sampling and sample-preparation requirements, two combustion procedures, calculation rules (including treatment of coarse inert material), precision statements and reporting requirements. It is intended for laboratories, producers, suppliers, purchasers, regulatory authorities and inspection organizations involved in SRF quality control.

Key topics and technical requirements

• Principle: Combustion in air with controlled time, temperature and sample mass; ash determined from mass of residue after ignition. Differences between determinations at 550 °C and 815 °C are explained (carbonate decomposition, loss of volatile inorganic compounds, further oxidation).
• Two methods:
  • Method A - 550 ± 10 °C (commonly used for biomass-rich SRF and for many elemental analyses).
  • Method B - 815 ± 10 °C (higher temperature option; consult standard for full heating routine).
• Heating routine (example elements): initial ramp to 250 ± 10 °C over 50 min with a 60 min hold to remove volatiles, then further heating to the target temperature following the standard-specified schedule.
• Apparatus requirements: inert dish (porcelain/silica/platinum) with depth 10–20 mm and sample loading ≤ 0.1 g/cm²; furnace with uniform temperature and ventilation (recommended 5–10 air changes/min); balance with 0.1 mg resolution; desiccator without desiccant; sieve ≤ 1 mm; airtight container.
• Sampling & sample prep: follow ISO 21645 and ISO 21646; grind to ≤1 mm; pre-drying per ISO 21646; remove coarse inert material (removed ash contributors - rac) where necessary to protect equipment. rac are considered dry and must be included in total ash calculations (Clause 8.2).
• Quality & validation: minimum two determinations per general analysis sample; automatic equipment allowed if validated against reference method; precision, repeatability and reproducibility limits provided (Clause 9).
• Reporting & environmental aspects: test report requirements in Clause 10; Annex B discusses environmental aspects.

Applications and who uses it

• Quality control and specification of solid recovered fuels for fuels producers, waste-to-energy plants and fuel suppliers.
• Acceptance testing by purchasers and contractual verification between suppliers and buyers.
• Regulatory and inspection laboratories performing compliance testing and environmental assessments.
• Research labs validating thermogravimetric or automated combustion equipment for ash determination.

Related standards

• ISO 21637 - SRF vocabulary
• ISO 21645 - SRF sampling methods
• ISO 21646 - SRF sample preparation
• ISO 21660-3 - Moisture determination (oven dry) for general analysis samples

Using ISO 21656:2021 ensures consistent, reproducible ash measurements for SRF, supporting fuel specification, process optimisation and regulatory compliance.