🔥 Fusibility of ash of solid fuels — characteristic temperatures: deformation (DT), sphere (ST), hemisphere (HT) and flow (FT) of an ash test piece heated in a tube furnace in a reducing or oxidizing atmosphere; PN-ISO 540

Petrochemistry PN-ISO 540

Overview

WHAT THE STANDARD COVERS. ISO 540:1995 (clause 1) specifies a method for determining the characteristic ash fusibility temperatures of solid mineral fuels. Definitions (clause 3): deformation temperature (DT) — the first signs of rounding of the tip or edges of the test piece due to melting (shrinkage, distortion and rounding of cracks are not criteria of deformation if the tip and edges remain sharp; for some fuels the temperature of the onset of shrinkage may be reported as an additional observation); sphere temperature (ST) — for pyramidal test pieces and truncated cones the height equal to the width of the base, and for cubes and cylinders completely rounded edges with unchanged height; hemisphere temperature (HT) — the test piece forms approximately a hemisphere, i.e. the height equal to half the diameter of the base; flow temperature (FT) — the ash has spread over the support in a layer whose height is one third of the height of the test piece at the hemisphere temperature. The "Scope" field of the PKN card of PN-ISO 540:2001 (Polish version): "Gives a method for determining the melting temperature of ash from solid fuels in an oxidizing and reducing atmosphere in a high-temperature tube furnace" (our translation).

PRINCIPLE, REAGENTS, APPARATUS (clauses 4–7). The ash test piece is heated under standardized conditions and observed continuously, recording the temperatures of characteristic changes of shape. The determination is usually carried out in a reducing atmosphere, and additional information may be obtained from a second determination in an oxidizing atmosphere; the recommended reducing atmospheres generally give the lowest characteristic temperatures. Reagents: dextrin solution 100 g/l (10 g in 100 ml of water), petroleum jelly, wire or foil of gold (purity 99.99 %, melting point 1064 °C), nickel (99.9 %, 1455 °C) and palladium (99.9 %, 1554 °C), carbon dioxide, hydrogen or carbon monoxide sufficiently free from oxygen. Apparatus: an electrically heated furnace which reaches the temperature required for the given ash (1500 °C or more may be required), has a zone of uniform temperature, allows the test piece to be heated at a uniform rate from 815 °C upwards, maintains the test atmosphere and allows the change of shape to be observed (cobalt glass protecting the operator's eyes is recommended); a pyrometer with a platinum/platinum-rhodium thermocouple; a mould of brass or stainless steel; a support which does not deform, react with the ash or absorb it (usually sintered alumina or fine-grained mullite, in case of difficulty an interlayer of platinum foil); two flowmeters for the components of the reducing gas; an agate mortar; a sieve of 0.075 mm aperture; an optical instrument with a graticule (camera or video recording — optional, but recommended).

ATMOSPHERE AND TEST PIECE (clauses 7–8). Reducing atmosphere: 55–65 % (V/V) of carbon monoxide with 35–45 % (V/V) of carbon dioxide or 45–55 % (V/V) of hydrogen with 45–55 % (V/V) of carbon dioxide, with a linear flow velocity past the test piece of at least 400 mm/min (at ambient temperature); oxidizing atmosphere — air or carbon dioxide. The gases leaving the furnace contain carbon monoxide and must be vented outside; with hydrogen, the furnace is purged with carbon dioxide before it is introduced and after it is shut off, to prevent an explosion. The test piece has sharp edges and a mass ensuring temperature equalization: a pyramid with an equilateral triangular base, a height of not more than 19 mm and 2–3 times the side of the base; a cube with a side of 3–7 mm; a cylinder 3–9 mm high with a diameter equal to the height; a truncated cone 4 mm high with diameters of 3 mm (base) and 1.5 mm (top). The calibration is checked regularly under routine conditions: the pyrometer — with the melting point of gold and if possible of palladium, the reducing atmosphere — with the melting point of nickel. The ash is moistened with demineralized water (if necessary with dextrin solution), formed into a paste and pressed into the mould, which may be thinly coated with petroleum jelly to make it easier to remove the test piece.

EDITIONS. PN-ISO 540:2001 adopts the third edition ISO 540:1995 (which replaced ISO 540:1981, "technically revised"). In the iTeh catalogue (read on 27.10.2026) ISO 540:1995 and the fourth edition ISO 540:2008 ("Hard coal and coke — Determination of ash fusibility") have the status "Withdrawn", and the current one is the fifth edition ISO 540:2025 (August 2025) under the title "Hard coal — Determination of ash fusibility". The introduction of ISO 540:2008 describes the method as based on the Seger cone method and notes that under industrial conditions the heating rates and gas compositions differ from those of the laboratory test. We did not compare the substantive differences between the editions of 1995, 2008 and 2025 — of the newer editions we read only the forewords, contents and introduction.

STATUS (PKN card as of 27.10.2026). The card of PN-ISO 540:2001 (Polish version) has no withdrawal label: publication 05-12-2001, 8 pages, Mining Sector, KT 220 Natural Solid Fuels, "Adopts: ISO 540:1995 [IDT]", ICS 75.160.10; the card has no "Replaces" field. PKN search, search term "ISO 540" (27.10.2026): with this number there is only PN-ISO 540:2001 — the results contain no Polish standard adopting ISO 540:2008 or ISO 540:2025.

HOW MANY LABORATORIES AND IN WHAT FORM (copy of the accreditation scope database refreshed from the production server on 27.10.2026 and current PCA documents downloaded on that day). Number 540 (form "PN-ISO 540") appears in 7 records at 6 Polish laboratories: AB 005, AB 081, AB 103, AB 300, AB 550 (two entries), AB 1698. The standard is in all 6 current PCA documents. At AB 103 the entry with this standard is printed in bold italics: "Accreditation suspended at the request of the body in the part of the scope marked in bold italics from 18.02.2026 to 17.02.2027" (document issue 42 of 18.02.2026) — so active accreditation for this test is held by five laboratories. Forms: "PN-ISO 540:2001" (AB 081 with the addition "(ISO 540:1995 IDT)", AB 103, AB 300, AB 550, AB 1698), "PN-ISO 540" without the year (AB 550 — waste) and in one cell with a second standard: "PN-82/G-04535 PN-ISO 540:2001" (AB 005), and at AB 300 and AB 550 PN-82/G-04535 alongside PN-ISO 540:2001. The "Laboratories" tab ("PN-ISO 540") shows 10 laboratories: these 6 Polish ones and 4 Czech ones (tab query on labcoda.pl, 27.10.2026) — including AB 103, whose entry is suspended.

WHAT THE LABORATORIES TEST (database copy and PCA documents). Characteristic ash fusibility temperatures: hard coal, lignite and coke from hard coal (AB 005: 860–1550 °C, high-temperature microscopy), the same and processed solid fuels (AB 081: 815–1650 °C, "high-temperature method with image observation"), hard coal (AB 300: 800–1550 °C, high-temperature microscopy), hard coal and lignite, coke, processed solid fuels and waste with several dozen codes (AB 550: 800–1500 °C, high-temperature microscopy), solid biomass — solid biofuel (AB 1698: 815–1500 °C, high-temperature microscopy). AB 103 (suspended): "determination of ash fusibility at high temperature in an oxidizing atmosphere", 750–1600 °C, tube method — hard coal and lignite, coke, semi-coke, processed solid fuels.

ALONGSIDE THIS STANDARD (database copy, 27.10.2026). Ash fusibility is in the scopes of 7 Polish laboratories (15 entries). Solid biomass is tested according to PN-EN ISO 21404:2020-08 by AB 053, AB 081 and AB 550, according to NPR-CEN/TS 15370-1:2006 and an in-house procedure — by AB 300; solid recovered fuels — according to CEN/TR 15404 (AB 081, AB 550). The Polish standard PN-82/G-04535 is held alongside PN-ISO 540 by AB 005, AB 300 and AB 550.

WHERE A SEEMINGLY CORRECT RESULT IS EASY TO GET. First — atmosphere: a result without the atmosphere stated is incomplete; the standard usually carries out the determination in a reducing atmosphere, which generally gives the lowest temperatures, and in an oxidizing one the temperatures may be higher — AB 103 has precisely the oxidizing atmosphere in its scope (entry suspended until 17.02.2027). Second — object: ISO 540:1995 concerns solid mineral fuels; solid biomass (AB 1698) and waste (AB 550) are tested by this standard outside its scope, while other laboratories use PN-EN ISO 21404 for biomass and CEN/TR 15404 for recovered fuels. Third — deformation: shrinkage of the test piece and rounding of cracks and burrs are not deformation as long as the tip and edges are sharp (note 1). Fourth — calibration: the pyrometer is checked regularly with the melting of gold (1064 °C) and palladium (1554 °C), and the reducing atmosphere with the melting of nickel (1455 °C); without this, an error in temperature or atmosphere carries over to all four temperatures. Fifth — edition: the Polish standard adopts the ISO edition of 1995, two editions behind the current ISO 540:2025.

WHAT WE DO NOT GIVE. The fragment of ISO 540:1995 we read ends at the beginning of the clause on the preparation of the test piece (clauses 8–9). We do not give the heating rate, the course of the determination, the method of preparing the ash (ashing temperature), precision or the report requirements.

Method principle

An ash test piece (pyramid, cube, cylinder or truncated cone of specified dimensions) is heated in an electric furnace from 815 °C upwards at a uniform rate in a reducing (CO + CO2 or H2 + CO2) or oxidizing (air, CO2) atmosphere and observed continuously; the deformation (DT), sphere (ST), hemisphere (HT) and flow (FT) temperatures are recorded (ISO 540:1995, clauses 3–7). WE DO NOT GIVE the heating rate and the course of the determination.

Applications

• Ash from solid mineral fuels: hard coal and lignite, coke (ISO 540:1995, clause 1; accreditation scopes)
• Assessment of ash behaviour at high temperature — reducing atmosphere (usually) and oxidizing (additional information)
• In accreditation scopes: 6 Polish laboratories (1 with a suspended entry), ranges 750–1650 °C; at some of them also biomass and waste — outside the scope of the standard

Key parameters

Standard

Standard number

PN-ISO 540:2001 (wersja polska; wprowadza ISO 540:1995 [IDT])

Title (PL)

Paliwa stałe — Oznaczanie topliwości popiołu w wysokiej temperaturze metodą rurową

Title (EN)

Solid mineral fuels — Determination of fusibility of ash — High-temperature tube method

Step-by-step procedure

1. Calibration

   Regular check of the pyrometer with the melting point of gold (and palladium) and of the reducing atmosphere with the melting point of nickel, under routine conditions (clause 8).

2. Test piece

   Ash moistened with demineralized water or if necessary with dextrin solution, formed into a paste and pressed into the mould (which may be thinly coated with petroleum jelly); one of four shapes with sharp edges (point 7.2). WE DO NOT GIVE the preparation of the ash from the fuel.

3. Atmosphere

   Reducing (CO + CO2 or H2 + CO2, flow ≥ 400 mm/min) or oxidizing (air, CO2); with hydrogen, purging with carbon dioxide before and after (point 7.1).

4. Heating and observation

   Heating from 815 °C at a uniform rate, continuous observation of the profile; recording of the DT, ST, HT and FT temperatures. WE DO NOT GIVE the heating rate.

5. Report

   Four temperatures with the atmosphere stated; PN-ISO 540:2001 — current (PKN card as of 27.10.2026), although it adopts ISO 540:1995.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Gases from the furnace in a reducing atmosphere contain carbon monoxide — vent them outside through a hood or fan (warning in point 7.1)
• With hydrogen there is a risk of explosion — purge the furnace with carbon dioxide before introducing hydrogen and after shutting it off (point 7.1)
• Radiation from the furnace at high temperature — cobalt glass protecting the retina of the operator (note 3)
• TECHNICAL NOTE: a result without the atmosphere is incomplete; a reducing atmosphere generally gives the lowest temperatures

Frequently asked questions