Refuse-Derived Fuel (RDF/SRF) Analysis and the Use of a Calorimeter

Waste-derived fuels help many facilities — from cement plants to thermal power stations — reduce their fossil fuel consumption. However, the trade and efficient use of these fuels depend on accurate laboratory analysis, and in particular on calorific value determination performed with a calorimeter. In this guide we take a detailed look at what RDF/SRF analysis is, which standards govern it, and the role of the bomb calorimeter in this process.

Table of contents

• What are RDF and SRF, and how do they differ?
• Why is RDF/SRF analysis carried out?
• Which parameters are measured?
• The role of the calorimeter: calorific value determination
• Standards: EN ISO 21654 and EN ISO 21640
• The SRF classification system
• Sample preparation
• Calorimeter measurement, step by step
• Common mistakes
• Frequently asked questions

What are RDF and SRF, and how do they differ?

RDF (Refuse-Derived Fuel) is a fuel with a high combustible fraction, obtained by separating and shredding municipal and industrial waste. Plastic, paper, textiles, wood and mixed packaging waste are the main components of RDF.

SRF (Solid Recovered Fuel) is the standardised, quality-controlled form of RDF. SRF is produced according to an international standard (EN ISO 21640) and classified by measurable criteria such as calorific value, chlorine and mercury content.

In short: every SRF is an RDF, but not every RDF is an SRF. To qualify as SRF, a fuel must be documented and classified in accordance with the standard — and laboratory analysis lies at the heart of that certification.

Why is RDF/SRF analysis carried out?

The composition of waste-derived fuel is variable; even two batches from the same facility can have different energy contents. Analysis serves three fundamental needs:

• Trade and pricing: The fuel's energy content (calorific value) directly determines its price. Buyer and seller reach agreement based on independent laboratory results.
• Process efficiency: A cement rotary kiln or a boiler operates most efficiently within a specific calorific value range. Analysis makes it possible to optimise the fuel feed settings.
• Environmental and regulatory compliance: Chlorine leads to corrosion and dioxin formation, while mercury causes atmospheric emissions. These parameters must be kept below legal limits.

Which parameters are measured in RDF/SRF analysis?

Among these parameters, calorific value is the most critical from both an economic and a technical standpoint — and this is exactly where the calorimeter comes into play.

The role of the calorimeter: calorific value determination

Calorific value is the amount of heat released when a unit of fuel undergoes complete combustion (MJ/kg). For RDF/SRF, this value is measured using a bomb calorimeter (combustion vessel calorimeter).

Working principle

A weighed sample is placed inside a steel "bomb" (combustion vessel) filled with high-pressure oxygen. The sample is ignited electrically and complete combustion is achieved. The released heat raises the temperature of the water bath surrounding the bomb. This temperature rise is measured precisely, and the fuel's gross calorific value (GCV) is calculated using the known heat capacity (the calorimeter's calibration constant).

Gross calorific value (GCV) and net calorific value (NCV)

The calorimeter directly measures the gross calorific value — the value that includes the heat of condensation of the combustion water. In practice, facilities generally use the net calorific value (NCV), because the water in the flue gas leaves the stack as vapour and its heat of condensation cannot be recovered. NCV is calculated by subtracting a correction from GCV based on the hydrogen and moisture content. SRF classification is also carried out on the basis of NCV.

The calorimeter is calibrated by burning certified benzoic acid. This ensures that measurements are traceable and consistent with international standards. Regular calibration is a prerequisite for reliable results.

Standards: which norms govern RDF/SRF analysis?

Waste-derived fuel analysis must be carried out according to internationally recognised standards. Two key standards stand out in calorific value determination.

EN ISO 21654 — Determination of calorific value

This standard defines the laboratory method for determining the gross calorific value of solid recovered fuels (SRF), measured at constant volume and corrected to a reference temperature of 25 °C. The measurement is performed with a combustion vessel calorimeter calibrated using certified benzoic acid. EN ISO 21654 supersedes the previously used EN 15400 and covers apparatus requirements, sample preparation, the calorimetric procedure, calibration, and the calculation of both gross and net calorific values.

EN ISO 21640 — Specifications and classes

This standard defines the classification system that describes the properties of SRFs. It facilitates the trade and use of these fuels while supporting environmental protection. SRF is divided into five classes based on net calorific value, chlorine content and mercury content. EN ISO 21640 is the international equivalent of the former EN 15359.

Standard numbers and limit values may be revised over time. When selecting a laboratory or analysis service, confirm that the accredited body works according to the current version of the standard.

How does the SRF classification system work?

EN ISO 21640 builds a class code from three characteristics: an economic characteristic (net calorific value, NCV), a technical characteristic (chlorine content, Cl) and an environmental characteristic (mercury content, Hg).

Each parameter is assigned a class from 1 to 5. Class 1 represents the highest quality (highest energy, lowest pollutants), while Class 5 represents the minimum acceptable quality. For NCV and chlorine, the arithmetic mean of 10 measurements is used; for mercury, the median and 80th percentile values are considered.

The cement industry generally demands Class 1–2 fuels, while Class 3–4 fuels are more commonly used in the pulp & paper and chemical industries.

Sample preparation: the most critical step of the analysis

RDF/SRF is a heterogeneous material; a small sample taken from a large pile must represent the entire batch. Poor sample preparation renders even the most expensive calorimeter meaningless. The process generally involves the following steps:

1. Representative sampling: Multiple incremental samples are taken from different points of the pile and combined.
2. Size reduction: The sample is reduced to smaller particles using a shredder/mill.
3. Dividing (reduction): The quantity is reduced representatively by coning and quartering or a sample divider.
4. Drying: Moisture content is determined; the laboratory sample is brought to an air-dry state.
5. Fine grinding: For the calorimeter, it is typically ground to below 1 mm.
6. Pelletising: The powdered sample is usually pressed into a small pellet for complete combustion.

Moisture loss or gain during sample preparation directly distorts the results. For this reason, every result is reported stating the basis on which it is given (air-dry, dry basis, as-received).

Calorimeter measurement, step by step

1. The prepared sample (~0.5–1 g) is weighed precisely on an analytical balance.
2. The sample is placed in the combustion vessel (bomb) in contact with the ignition wire.
3. The bomb is pressurised with high-purity oxygen (~30 bar).
4. The bomb is immersed in a constant-temperature water bath and the system is brought to thermal equilibrium.
5. The sample is ignited electrically; complete combustion is achieved.
6. The rise in water temperature is recorded precisely.
7. The gross calorific value is calculated using the calibration constant.
8. Moisture and hydrogen corrections are applied to obtain the net calorific value (NCV).
9. The result is converted to the relevant basis and reported in a standard-compliant format.

Choose the right solution for your RDF/SRF analysis

Whether you need a bomb calorimeter to set up your own laboratory, or you want your samples analysed at an accredited laboratory, let us determine the right solution together. You can review our calorimeter instruments or request a quote for our analysis service.

Common mistakes in RDF/SRF analysis

• Non-representative sample: Taking a sample from a single point of a heterogeneous pile is the most common mistake.
• Neglecting moisture tracking: If the reporting basis is not stated, the data becomes impossible to compare.
• Skipping calibration: A device that is not regularly calibrated with benzoic acid does not give reliable results.
• Ignoring chlorine: A high calorific value alone is not enough; chlorine can cause corrosion and damage the furnace.
• Insufficient grinding: Coarse particles may not fully combust, resulting in a lower calorific value reading.

Frequently asked questions