🔥 Calorific value (heat of combustion)
In short
Determination of the gross calorific value of coal and coke at constant volume, at the reference temperature of 25 °C, by burning the analysis sample in oxygen under pressure in a combustion vessel calorimeter calibrated with certified benzoic acid. The test is performed according to PN-ISO 1928:2020-05 (English version; adopts ISO 1928:2009 [IDT]); Quantity measured: gross calorific value at constant volume, reference temperature 25 °C, water in the combustion products as liquid. The procedure comprises 6 steps; it is used for: Gross calorific value of coal and coke at constant volume, at the reference temperature of 25 °C, Calculation of the net calorific value at constant pressure (practical combustion conditions) or at constant volume, Quality testing of solid fuels — PN-ISO 1928:2002 or PN-ISO 1928:2020-05 specified in the regulation on test methods for the quality of solid fuels (point 5.3).
At a glance
- Standard: PN-ISO 1928:2020-05 (English version; adopts ISO 1928:2009 [IDT])
- Category: Petrochemistry
- Procedure steps: 6
- Quantity measured: gross calorific value at constant volume, reference temperature 25 °C, water in the combustion products as liquid
- Quantities calculated: net calorific value at constant pressure and at constant volume
- Oxygen: purity at least 99.5 % (V/V), free from combustible matter; pressure in the vessel (3.0 ± 0.2) MPa, the test is discarded if charged above 3.3 MPa (ISO 1928:1995)
Overview
STATUS (as of 06.11.2026). According to the PKN catalogue card (read on 06.11.2026) the current standard is PN-ISO 1928:2020-05 (English version, published 28.05.2020, 68 pages), which implements ISO 1928:2009 [IDT] and replaced PN-ISO 1928:2002 (Polish version, implementing ISO 1928:1995; withdrawn 28.05.2020); the card has no withdrawal note, and the PKN search shows no newer edition. The designation "2020" therefore does not mean the content of ISO 1928:2020 — the PN implements the 2009 edition. ISO editions: ISO 1928:2009 (third edition, "Solid mineral fuels"), ISO 1928:2020 (fourth, "Coal and coke — Determination of gross calorific value") and ISO 1928:2025 (fifth, 13.08.2025, superseded the 2020 edition). According to the foreword of the 2025 edition the changes include the title coal and coke, updated symbols in formulae, allowance for an alternative material for the calorimeter can, removal of ambiguity around crucible masses, specification of the analysis sample, more concise rules on the use of a combustion aid, warnings in the body of the text and additional ignition methods. The description is based on previews of ISO 1928:2025 (foreword, scope, references), ISO 1928:2020 (Clauses 1–6.2.1: definitions, principle, reagents, start of apparatus) and ISO 1928:1995 (Clauses 6.2–8.5: apparatus, sample preparation, procedure); we did not have the text of the 2009 edition implemented by the PN. The regulation on test methods for the quality of solid fuels (consolidated text Journal of Laws 2021 item 1366, point 5.3 of the annex as worded in Journal of Laws 2023 item 2661) specifies PN-ISO 1928:2002, PN-G-04513:1981 or PN-ISO 1928:2020-05; the same standards are described in a separate Knowledge Base entry on determining the calorific value of solid fuels according to PN-ISO 1928.
SCOPE. The standard specifies a method for determining the gross calorific value of coal and coke (2020 edition: "a solid mineral fuel") at constant volume and at the reference temperature of 25 °C, in a combustion vessel calorimeter calibrated by combustion of certified benzoic acid. The result is the gross calorific value of the analysis sample at constant volume, with all the water of the combustion products as liquid water. In practice fuel is burned at constant (atmospheric) pressure and the water leaves as vapour with the flue gases — the relevant quantity is then the net calorific value at constant pressure; the net calorific value at constant volume can also be used, and the standard gives formulae for both. Procedures for types of calorimeter (adiabatic, isoperibol and static-jacket, automated) are in Annexes A–C, checklists in Annex D, examples of calculations in E, and guidance on the safe use and testing of the combustion vessel in F. Liquid fuels are covered by another standard, e.g. ASTM D240 (heat of combustion of liquid hydrocarbon fuels).
PRINCIPLE OF THE DETERMINATION. The effective heat capacity of the calorimeter is determined by burning certified benzoic acid under conditions similar to those of the test. The corrected temperature rise is established from measurements before, during and after combustion. Water is added to the vessel to saturate the gas phase with vapour so that all the water formed from the hydrogen and moisture of the sample can be regarded as liquid. The gross calorific value is calculated from the corrected temperature rise and the effective heat capacity, with corrections for ignition energy, combustion of the fuse and side reactions (formation of nitric acid), and for the energy difference between the sulfuric acid formed in the vessel and gaseous sulfur dioxide.
WHAT WE DO NOT GIVE HERE. We did not have the clauses on calibration, calculations, precision and calculation of the net calorific value (Clauses 9–12) — so we do not give formulae or repeatability and reproducibility limits. We have removed figures and statements without a source or contradicting the sources: application to liquid and gaseous fuels, biomass, waste and building materials, "the only reference method" of authorities and exchanges, a precision of "0.1–0.3 %", a gross calorific value of benzoic acid of 26 454 J/g (the value is taken from the certificate of the standard), a net calorific value formula with the number 24.42 (from outside this standard), step times, the volume of water in the can, the drying temperature, brands and prices of apparatus.
Method principle
A weighed portion of the analysis sample of the fuel is burned in high-pressure oxygen in a combustion vessel calorimeter under specified conditions. The effective heat capacity of the calorimeter is determined beforehand by burning certified benzoic acid under similar conditions. The corrected temperature rise is established from temperature readings before, during and after combustion. Water added to the vessel saturates the gas phase with vapour, so that all the water formed from the hydrogen and moisture of the sample is liquid in the result. The gross calorific value at constant volume is calculated from the corrected temperature rise and the effective heat capacity, with corrections for ignition energy, combustion of the fuse, formation of nitric acid and the conversion of sulfur from sulfuric acid to gaseous sulfur dioxide. The net calorific value at constant volume is calculated from the gross calorific value using the moisture and hydrogen contents of the analysis sample, and at constant pressure — in principle also using the oxygen and nitrogen contents.
Applications
- Gross calorific value of coal and coke at constant volume, at the reference temperature of 25 °C
- Calculation of the net calorific value at constant pressure (practical combustion conditions) or at constant volume
- Quality testing of solid fuels — PN-ISO 1928:2002 or PN-ISO 1928:2020-05 specified in the regulation on test methods for the quality of solid fuels (point 5.3)
Key parameters
| Parameter | Value |
|---|---|
| Quantity measured | gross calorific value at constant volume, reference temperature 25 °C, water in the combustion products as liquid |
| Quantities calculated | net calorific value at constant pressure and at constant volume |
| Oxygen | purity at least 99.5 % (V/V), free from combustible matter; pressure in the vessel (3.0 ± 0.2) MPa, the test is discarded if charged above 3.3 MPa (ISO 1928:1995) |
| Combustion vessel | internal volume 250 ml to 350 ml, material resistant to the acids of combustion |
| Sample (ISO 1928:1995) | analysis sample passing a 212 µm test sieve (250 µm acceptable for low and medium rank coals), in moisture equilibrium with the laboratory atmosphere |
| Temperature measurement (ISO 1928:1995) | resolution at least 0.001 K; temperature intervals of 2–3 K determined with a resolution of 0.002 K |
Standard
- Standard number
- PN-ISO 1928:2020-05 (English version; adopts ISO 1928:2009 [IDT])
- Title (PL)
- Paliwa stałe — Oznaczanie ciepła spalania metodą spalania w bombie kalorymetrycznej i obliczanie wartości opałowej
- Title (EN)
- Solid mineral fuels — Determination of gross calorific value by the bomb calorimetric method and calculation of net calorific value
Step-by-step procedure
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Analysis sample
According to ISO 1928:1995 use the analysis sample passing a 212 µm test sieve, well mixed and in moisture equilibrium with the laboratory atmosphere; determine moisture at the same time according to the moisture standards (the 2025 edition refers to ISO 687, ISO 5068-2 and ISO 11722).
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Preparing the combustion vessel
Weigh the sample in the crucible (about 1 g) with a resolution of 2 parts in 10⁴, i.e. 0.1 mg; fasten the ignition wire between the electrodes, tie the fuse and bring it into contact with the sample. Add the prescribed amount of water to the vessel (e.g. (1.0 ± 0.1) ml per 1 g of sample), assemble the vessel and charge it slowly with oxygen to (3.0 ± 0.2) MPa without displacing the air; if charged above 3.3 MPa, discard the test and begin again.
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Assembling the calorimeter
Fill the calorimeter can with water within ± 0.3 K of the selected initial temperature, in the same quantity (to within 0.5 g) in all tests; mount the vessel, check for leaks, connect the ignition leads and the thermometer. If gas escapes, discard the test. The calorimeter normally reaches a steady state after about 5 min.
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Combustion and temperature measurements
Read the temperature every 1 min with a resolution of 0.001 K (fore period), fire the charge and continue readings every 1 min (main and after periods); during the rapid rise 0.02 K is adequate, but for no longer than 5 min from the start of the main period. The lengths of the periods are given in Annexes A and B of the standard.
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Analysis of combustion products
After the after period remove the vessel, release the pressure at a moderate rate, dismantle it and examine the interior, the crucible and any residue; discard the test if there are signs of incomplete combustion. Analyse the vessel solution when needed for the nitric acid and sulfur corrections.
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Calibration and calculations
Determine the effective heat capacity of the calorimeter by burning certified benzoic acid in the same way; calculate the gross and net calorific values according to Clauses 9–12 of the standard (formulae are not given here).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Combustion vessel (bomb) | Withstands the pressures developed during combustion, allows complete recovery of liquid products, resistant to acids; volume 250–350 ml; parts inspected regularly (especially the threads of the main closure), each vessel used as a complete unit | — |
| Calorimeter with thermostat (jacket) | Calorimeter can with water covering the vessel head, stirrer at constant speed, water jacket with an air gap of about 10 mm; the working principle (adiabatic, isoperibol, static, aneroid) determines how the corrected temperature rise is evaluated | — |
| Temperature measuring instrument | Resolution at least 0.001 K; mercury-in-glass thermometers according to ISO 651, 652, 1770 or 1771 or platinum resistance sensors, thermistors, quartz crystal resonators (ISO 1928:1995) | — |
| Ignition circuit | According to ISO 1928:1995 6–12 V alternating current from a step-down transformer or direct current from batteries; spring-loaded, normally open firing switch | — |
| Crucible | Of silica, nickel-chromium, platinum or similar unreactive material; for coke lined with aluminosilicate cement | — |
| Ancillary pressure equipment | Pressure regulator for filling with oxygen, pressure gauge (e.g. 0–5 MPa, resolution 0.05 MPa), relief valve or bursting disk at 3.5 MPa; free from oil and grease | — |
| Balances | For sample and fuse — resolution at least 0.1 mg (0.01 mg recommended for samples of about 0.5 g or less); for the calorimeter water — 0.5 g | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Oxygen | 7782-44-7 | At a pressure sufficient to fill the vessel to 3 MPa, purity at least 99.5 % (V/V), free from combustible matter (electrolytic oxygen can contain up to 4 % hydrogen) |
| Certified benzoic acid | 65-85-0 | The sole substance recommended for calibrating the calorimeter; of calorimetric-standard quality, certified by a recognized authority; burned as pellets; the gross calorific value is taken from the certificate for the conditions of use |
| Ignition wire and fuse | — | Nickel-chromium wire 0.16–0.20 mm or platinum wire 0.05–0.10 mm (or another conducting wire with well characterized combustion behaviour); fuse of white cellulose cotton |
| Crucible lining materials | — | Paste of aluminosilicate cement (passing a 63 µm sieve), fused aluminium oxide (106–180 µm fraction), silica fibre disk — for coke, anthracite and high-ash coals |
| Solutions for analysis of combustion products | 17194-00-2 | Only when analysis of the vessel solution is required: barium hydroxide 0.05 mol/l, sodium carbonate 0.05 mol/l, sodium hydroxide 0.1 mol/l, hydrochloric acid 0.1 mol/l, screened methyl orange, phenolphthalein |
| Water | 7732-18-5 | Deionized or distilled, specific conductivity at most 0.2 mS/m at 25 °C |
Health and safety (OHS)
- The standard warns that only strict adherence to its provisions ensures against explosive rupture of the combustion vessel or a blow-out — provided the vessel is of proper design and in good mechanical condition
- Vessel parts are inspected regularly for wear and corrosion, especially the threads of the main closure; manufacturers' instructions and regulations are observed; parts of vessels of the same design must not be swapped (this can lead to a serious accident)
- Do not reach over the vessel during charging with oxygen; do not extend any part of the body over the calorimeter during firing and for 20 s thereafter (ISO 1928:1995)
- Equipment for high-pressure oxygen must be kept free from oil and grease; the pressure gauge is not tested with hydrocarbon fluid; oxygen (CLP) may cause or intensify fire — oxidizer (H270)
- Benzoic acid (harmonised CLP classification): causes skin irritation (H315), causes serious eye damage (H318), causes damage to the lungs through prolonged or repeated exposure by inhalation (H372)
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-ISO 1928:2020-05 (English version; adopts ISO 1928:2009 [IDT]) — “Solid mineral fuels — Determination of gross calorific value by the bomb calorimetric method and calculation of net calorific value”.
How does this method work?
A weighed portion of the analysis sample of the fuel is burned in high-pressure oxygen in a combustion vessel calorimeter under specified conditions. The effective heat capacity of the calorimeter is determined beforehand by burning certified benzoic acid under similar conditions.
What is the measuring range and accuracy?
Quantity measured: gross calorific value at constant volume, reference temperature 25 °C, water in the combustion products as liquid; Quantities calculated: net calorific value at constant pressure and at constant volume; Oxygen: purity at least 99.5 % (V/V), free from combustible matter; pressure in the vessel (3.0 ± 0.2) MPa, the test is discarded if charged above 3.3 MPa (ISO 1928:1995); Combustion vessel: internal volume 250 ml to 350 ml, material resistant to the acids of combustion.
How long does the test take?
The procedure comprises 6 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
Combustion vessel (bomb), Calorimeter with thermostat (jacket), Temperature measuring instrument, Ignition circuit, Crucible, Ancillary pressure equipment, Balances.
Where is this test used?
Gross calorific value of coal and coke at constant volume, at the reference temperature of 25 °C; Calculation of the net calorific value at constant pressure (practical combustion conditions) or at constant volume; Quality testing of solid fuels — PN-ISO 1928:2002 or PN-ISO 1928:2020-05 specified in the regulation on test methods for the quality of solid fuels (point 5.3).
What safety precautions apply?
The standard warns that only strict adherence to its provisions ensures against explosive rupture of the combustion vessel or a blow-out — provided the vessel is of proper design and in good mechanical condition; Vessel parts are inspected regularly for wear and corrosion, especially the threads of the main closure; manufacturers' instructions and regulations are observed; parts of vessels of the same design must not be swapped (this can lead to a serious accident); Do not reach over the vessel during charging with oxygen; do not extend any part of the body over the calorimeter during firing and for 20 s thereafter (ISO 1928:1995); Equipment for high-pressure oxygen must be kept free from oil and grease; the pressure gauge is not tested with hydrocarbon fluid; oxygen (CLP) may cause or intensify fire — oxidizer (H270); Benzoic acid (harmonised CLP classification): causes skin irritation (H315), causes serious eye damage (H318), causes damage to the lungs through prolonged or repeated exposure by inhalation (H372).
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-ISO 1928.