ISO 21644:2021

Overview

ISO 21644:2021 - "Solid recovered fuels - Methods for the determination of biomass content" specifies three validated approaches to quantify the biogenic (biomass) fraction in solid recovered fuels (SRF). The standard defines procedures for the 14C (radiocarbon) content method, the selective dissolution method (SDM) and the manual sorting method (M_sort), covering sampling, sample preparation, result expression and performance characteristics. ISO 21644 supports consistent determination of the renewable energy fraction used for greenhouse‑gas accounting, fuel classification and regulatory compliance.

Key technical topics and requirements

• Methods covered
  • 14C (C) content method - measures radiocarbon (14C) to estimate biogenic carbon; detailed in Annex A. Suitable for all SRF types. Note: liquid scintillation counting (LSC) has a practical lower application around 10% biogenic carbon.
  • Selective dissolution method (SDM) - based on dissolution of biomass in a sulphuric‑acid / hydrogen‑peroxide mixture; detailed in Annex B. Applicable for biomass fractions between ~10% and 90%, with specific material limitations.
  • Manual sorting method (M_sort) - visual separation of fractions by origin and nature; detailed in Annex C. Intended for coarse particles >10 mm.
• Sampling & sample preparation
  • Sampling and transport follow ISO 21645; laboratory sample preparation follows ISO 21646.
  • Minimum test sample sizes: C method: 0.4–2 g (nominal top size ≤1 mm); SDM: ≥5 g (≤1 mm); M_sort: at least the minimum sample size per ISO 21645/21646.
• Limitations
  • SDM may be unreliable when SRF contains >10% natural/synthetic rubber or when the combined content of coal, degradable fossil plastics, certain polymers (wool, viscose, nylon, polyurethane), oils/fats or silicon rubber exceeds ~5% - see Annex D.
• Expression of results
  • Biomass can be reported as percentage by mass (wB), percentage by calorific value (wB,cal) or percentage by carbon content (wB,TC).
• Performance & reporting
  • Annex E summarises round‑robin and validation data for uncertainty estimates; Clause 9 defines test report requirements.

Practical applications and users

• Intended users: laboratories, SRF producers, suppliers, purchasers, regulatory authorities and inspection bodies.
• Typical applications:
  • Determining the renewable energy fraction for greenhouse‑gas accounting and energy policy compliance.
  • Quality control and classification of SRF for waste‑to‑energy operations.
  • Contractual verification between suppliers and buyers, and for sustainability certification and reporting.
  • Supporting permit and emissions assessments where biogenic carbon content affects accounting.

Related standards

• ISO 21637 (terminology for SRF)
• ISO 21645 (sampling)
• ISO 21646 (sample preparation)
• ISO 21654 (calorific value)
• ISO 21656 (ash content)
• ISO 21663 (total C, H, N, S analysis)

Keywords: ISO 21644:2021, solid recovered fuels, biomass content determination, 14C method, selective dissolution, manual sorting, biogenic carbon, renewable energy fraction.