ISO 540:2025

Hard coal - Determination of ash fusibility

This document specifies a method of determining the characteristic fusion temperatures of ash from hard coal. NOTE Descriptors: fossil fuels, ash, ashes, tests, high temperature tests, determination, and fusibility.

Houille — Détermination de la fusibilité des cendres

General Information

• Status: Published
• Publication Date: 07-Aug-2025
• ICS: 73.040 - Coals
• ICS: 75.160.10 - Solid fuels
• Technical Committee: ISO/TC 27/SC 5 - Methods of analysis
• Drafting Committee: ISO/TC 27/SC 5 - Methods of analysis
• Current Stage: 6060 - International Standard published
• Start Date: 08-Aug-2025
• Due Date: 20-May-2026
• Completion Date: 08-Aug-2025

Relations

• Revises: ISO 540:2008 - Hard coal and coke - Determination of ash fusibility
• Effective Date: 27-May-2023

Overview

ISO 540:2025 - Hard coal - Determination of ash fusibility defines a standardized laboratory method to determine the characteristic fusion temperatures of ash derived from hard coal. The fifth edition (2025) updates the method (title change, coke removed from scope) and preserves the Seger-cone–style approach used to characterize ash fusibility under controlled high temperature tests. The standard provides repeatable procedures for preparing ash test pieces, heating under defined atmospheres, observing shape changes and reporting characteristic temperatures.

Key topics and technical requirements

• Scope and purpose: determination of deformation, sphere, hemisphere and flow temperatures (DT, ST, HT, FT) that describe ash melting behaviour.
• Principle: controlled heating of an ash test piece and observation of morphological changes at specified temperature intervals.
• Test atmospheres: reducing (CO/CO2 or H2/CO2 mixtures with specified volume fractions) or oxidizing (air or CO2). Safety warnings on CO and H2 ventilation/purging are included.
• Temperature range and equipment:
  • Furnace capable of reaching up to ~1 500 °C with a uniform heating zone and controlled heating rate from 815 °C upward.
  • Pyrometer/thermocouple (platinum/platinum‑rhodium) located in the temperature zone.
  • Optical instrument (graticule/CCTV/video recommended) for continuous profile observation.
• Test piece preparation: ash mixed and formed into specified shapes - pyramid (height 12–19 mm), cube, upright cylinder or truncated cone - with sharp edges for clear observation.
• Reagents and supports: dextrin binder, non‑reactive supports (sintered alumina, mullite, or platinum interfaces) and reference wires/plates (gold, nickel, palladium) for calibration checks.
• Quality and reporting: clauses on calibration, precision (repeatability/reproducibility limits), and required test report content.

Practical applications and users

ISO 540:2025 is used where understanding ash melting behaviour is critical:

• Coal analysis laboratories conducting ash determination and characteristic fusion tests.
• Power plant fuel testing and boiler/furnace operators assessing slagging and clinker formation risk.
• Combustion engineers, fuel suppliers and consultants evaluating fuel selection and combustion performance.
• Researchers and materials scientists studying high‑temperature ash chemistry and deposit formation. The standard helps predict tendencies for ash fusion and slagging under controlled conditions - a valuable indicator for furnace design, operational troubleshooting and fuel quality control.

Related standards

• ISO 1171 - Coal and coke - Determination of ash (referenced normative standard).
• Developed by ISO/TC 27 (Coal and coke), Subcommittee SC 5 - Methods of analysis.

Keywords: fossil fuels, ash, ashes, tests, high temperature tests, determination, fusibility, coal ash, slagging, ash fusibility.

Standards Content (Sample)

International
Standard
ISO 540
Fifth edition
Hard coal — Determination of ash
2025-08
fusibility
Houille — Détermination de la fusibilité des cendres
Reference number
© ISO 2025
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
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or ISO’s member body in the country of the requester.
ISO copyright office
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Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
5 Reagents . 2
6 Apparatus . 2
7 Test conditions . 4
7.1 Test atmosphere .4
7.2 Shape of test piece .4
8 Calibration check . 6
9 Preparation of the test piece . 6
10 Procedure . 7
11 Precision of the method . 7
11.1 Repeatability limit .7
11.2 Reproducibility limit .8
12 Test report . 8
13 Precision statement . 8
Bibliography . 9

iii
Foreword
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The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
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Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 27, Coal and coke, Subcommittee SC 5, Methods
of analysis.
This fifth edition cancels and replaces the fourth edition (ISO 540:2008), which has been technically revised.
The main changes are as follows:
— change of the document title;
— removal of coke from scope;
— increase of the allowance of the pyramid to between 12 mm and 19 mm.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
The method for determination of the fusibility temperatures of coal ash described in this document provides
information about the fusion and melting behaviour of the composite inorganic constituents of the ash at
high temperatures. The standard method is based on the “Seger Cone” method, which is well known in the
ceramic industry, the use of which predates the year 1900. The conditions of the test, as well as basic studies
on the influence of ash chemistry and of gas composition on ash fusibility temperatures (which have led to
[1]
the standardization of the method), arose from the pioneering work of Fieldner, Hall and Field .
In the laboratory, the ash used for the test is a homogeneous mixture prepared from a representative sample
of the coal, and the determination is performed at a controlled rate of heating in either a reducing or an
oxidizing atmosphere. In contrast, under industrial conditions, the complex processes of combustion and
fusion involve heterogeneous mixtures of particles, heating rates (that can be several orders of magnitude
greater than those used in the standard test) and variable gas composition.
During the first quarter of the 20th century, laboratory, pilot-scale and field studies were undertaken to
establish that the ash fusibility test can provide a reasonable indication of the propensity of ash to form
[2]
fused deposits (referred to as “clinker”) in stoker and other fuel-bed type furnaces. Subsequently, the test
has been used as a general indicator of the tendency for ash to fuse on heating and of ash slagging propensity
in pulverized coal-fired furnaces.

v
International Standard ISO 540:2025(en)
Hard coal — Determination of ash fusibility
1 Scope
This document specifies a method of determining the characteristic fusion temperatures of ash from hard coal.
NOTE Descriptors: fossil fuels, ash, ashes, tests, high temperature tests, determination, and fusibility.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 1171, Coal and coke — Determination of ash
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
deformation temperature
DT
temperature at which the first signs of rounding, due to melting, of the tip or edges of the test piece occur
Note 1 to entry: Shrinkage or distortion of the test piece, or rounding of cracks and fins, are not criteria for deformation
temperature and should be ignored if the tip and edges remain sharp. However, for some hard coals, the temperature
at which the test piece shrinkage begins can be of interest and should be reported as a feature noted during the
determination.
3.2
sphere temperature
ST
temperature, in the case of pyramidal and truncated-cone test pieces, at which the height is equal to the
width of the base, and temperature, in the case of cubical or cylindrical test pieces, at which the edges of the
test pieces become completely round with the height remaining unchanged
3.3
hemisphere temperature
HT
temperature at which the test piece forms approximately a hemisphere, i.e. when the height becomes equal
to half the base diameter
3.4
flow temperature
FT
temperature at which the ash melt is spread out over the supporting tile in a layer, the height of which is
one-third of the height of the test piece at the hemisphere temperature

4 Principle
A test piece made from the ash is heated under standard conditions and continually observed at intervals
of temperature change. The temperatures at which characteristic changes of shape occur are recorded. The
characteristic temperatures are defined in Clause 3 (See also Figures 2, 3 and 4).
Although the determination is usually performed in a reducing atmosphere, additional information can
sometimes be obtained by performing a further determination in an oxidizing atmosphere. In general, the
reducing atmosphere in 7.1 gives the lowest characteristic temperatures.
5 Reagents
5.1 Dextrin, 100 g/l solution.
Dissolve 10 g of dextrin in 100 ml of water.
5.2 Petroleum jelly.
5.3 Gold wire, of diameter 0,5 mm or larger, or gold plate, of thickness 0,5 mm to 1,0 mm, with a purity of
99,99 % and a melting point of 1 064 °C.
5.4 Nickel wire, of diameter 0,5 mm or larger, or nickel plate, of thickness 0,5 mm to 1,0 mm, with a
purity of 99,9 % and a melting point of 1 455 °C.
5.5 Palladium wire, of diameter 0,5 mm or larger, or palladium plate, of thickness 0,5 mm to 1,0 mm
with a purity of 99,9 % and a melting point of 1 554 °C.
5.6 Carbon dioxide.
5.7 Hydrogen or carbon monoxide.
6 Apparatus
6.1 Furnace, electrically heated, which satisfies the following conditions.
a) It shall be capable of reaching the maximum temperature at which the properties of the ash are
determined (a temperature of 1 500 °C or more can be required).
NOTE Some furnaces can have a practic
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