ISO 21654:2021

Overview

ISO 21654:2021 - "Solid recovered fuels - Determination of calorific value" specifies a standardized laboratory method to determine the gross calorific value (GCV) of solid recovered fuels using a combustion vessel calorimeter. Measurements are made at constant volume and at the reference temperature 25 °C, and the calorimeter is calibrated by combustion of certified benzoic acid. The standard yields the GCV with all combustion water considered as liquid water and provides procedures and formulae to convert results to net calorific value (NCV) (constant pressure or volume) used in practical combustion contexts.

Key topics and technical requirements

• Scope and principle: Method for determining GCV of solid recovered fuels at constant volume; GCV result basis explicitly defined.
• Calibration: Calibration of the calorimeter using certified benzoic acid; calculation and validation of the effective heat capacity (ε) and its working range.
• Calorimetric procedures: Preparation of test samples, vessel preparation, combustion reaction, temperature measurement, corrected temperature rise, thermometer corrections and reference temperature adjustments.
• Apparatus and laboratory conditions: Requirements for equipment (balances, auxiliary equipment) and controlled laboratory conditions to ensure repeatability.
• Types of calorimeters: Normative guidance for adiabatic, isoperibol, static-jacket, and automated combustion vessel calorimeters (specified in Annexes A–C).
• Analysis and calculations: Product-of-combustion analysis, calculation of GCV on different bases, conversion to NCV, and formulas for removed ash contributors (Annex D).
• Quality and precision: Procedures addressing precision, repeatability and reproducibility limits, and calibration repetition.
• Safety: Explicit warning to follow provisions to avoid vessel rupture or blow-out; vessel integrity and correct procedure are emphasized.

Practical applications and users

ISO 21654:2021 is intended for organizations and professionals who need reliable, comparable calorific value data for solid recovered fuels:

• Testing laboratories performing fuel characterization and certification.
• Waste-to-energy and cement plants evaluating fuel energy content for combustion control and fuel mix optimization.
• Fuel producers and recyclers validating product quality and energy value for commercial contracts.
• Regulators and certification bodies ensuring compliance with fuel specifications and reporting requirements.
• Calorimeter manufacturers and instrument developers implementing standardized calibration and reporting features.

Practical uses include energy content verification, fuel quality control, contractual verification of delivered fuels, process optimization in combustion systems, and conversion of laboratory results into operational NCV values for plant design and emissions calculations.

Related standards

• ISO 21637 - Solid recovered fuels - Vocabulary
• ISO 21644 - Methods for determination of biomass content
• ISO 21646 - Sample preparation
• ISO 21660-3 - Moisture determination (oven dry method)
• ISO 16993 - Conversion of analytical results between bases
• EN 15400 - Related European standard (ISO 21654 is based on EN 15400)

Keywords: ISO 21654:2021, solid recovered fuels, calorific value, gross calorific value, net calorific value, combustion vessel calorimeter, benzoic acid calibration, fuel testing, calorimetry.