🔥 Ash in solid fuels (coal, coke) — gravimetric method at 815 °C
In short
Combustion of about 1 g of the analysis sample in air with a prescribed rate of temperature rise up to 815 ± 10 °C and weighing of the residue; the result as the mean of two determinations to 0. The test is performed according to PN-ISO 1171:2002; Final temperature: 815 ± 10 °C, at least 60 min. The procedure comprises 7 steps (about 1 h 10 min in total); it is used for: Proximate analysis of hard coal, lignite and coke (the balance of the ballast: moisture + ash), Assessment of the commercial quality of coal and settlement of deliveries for the power industry, Conversion of the results of other determinations to the dry ash-free basis (daf).
At a glance
- Standard: PN-ISO 1171:2002
- Category: Petrochemistry
- Procedure steps: 7
- Total time of stages: 1 h 10 min
- Final temperature: 815 ± 10 °C, at least 60 min
- Heating programme: Uniformly to 500 °C in 60 min, held for 30 min (lignite 60 min), then to 815 ± 10 °C
- Sample: About 1 g, particle size < 212 µm; loading ≤ 0.15 g/cm² (coal), ≤ 0.10 g/cm² (coke)
Overview
The standard specifies a method of determining ash in all solid mineral fuels. The ash remaining after the combustion of coal or coke in air comes from the inorganic complexes present in the coal substance and from the accompanying mineral matter — for that reason the result of the determination is "ash" and not "ash content", because coal does not contain ash. The quantity of sulfur retained in the ash depends in part on the conditions of ashing, so for the results to be comparable the prescribed conditions have to be strictly observed (the description is based on the preview of ISO 1171:2010, the fourth edition, a minor revision of ISO 1171:1997, on which PN-ISO 1171:2002 is based).
The mineral matter of coal undergoes transformations during combustion: the carbonates decompose with the evolution of CO2, the silicates and aluminosilicates lose their water of crystallisation, pyrite and marcasite are oxidised to Fe2O3 and SO2, calcium oxide binds SO2 into sulfates and the alkali metal chlorides sublime. These reactions take place up to about 800 °C, and for that reason an ashing temperature of 815 ± 10 °C and a sufficiently long ignition time have been adopted, so that a stabilised mass of ash is obtained; the amount of ash is usually smaller than the content of the original mineral matter.
The combined content of moisture and ash is a measure of the ballast in a fuel. In laboratory practice a distinction is made between the method of slow ashing (all solid fuels, especially those of low rank: biomass, peat, lignite), of fast and of very fast ashing (hard coal and carbonisation products). The gravimetric method according to PN-ISO 1171:2002 is in the scope of accreditation of the Central Mining Institute (GIG), for a range of 1.05–96.30 %.
SCALE OF CITATIONS — A MEASUREMENT ON THE DATABASE OF ACCREDITATION SCOPES (read on 23-09-2026, matching on the BEGINNING of the notation "PN-ISO 1171", i.e. the way the "Laboratories" tab of this entry searches): 62 entries at 49 laboratories.
Method principle
The analysis sample (ground to pass a 212 µm sieve, well mixed and in moisture equilibrium with the atmosphere of the laboratory) is heated in air at a prescribed rate to 815 ± 10 °C and kept at that temperature to constant mass; the ash is calculated from the mass of the residue after combustion: A = (m3 − m1)/(m2 − m1) × 100, where m1 is the mass of the empty dish, m2 the mass of the dish with the sample and m3 the mass of the dish with the ash. The furnace must provide a zone of uniform temperature and a ventilation of 5–10 air changes per minute; the loading of the dish must not exceed 0.15 g/cm² for coal and 0.10 g/cm² for coke. The result is given as the mean of two determinations to 0.1 % by mass, on the analysis basis (conversions to other bases according to ISO 1170).
Applications
- Proximate analysis of hard coal, lignite and coke (the balance of the ballast: moisture + ash)
- Assessment of the commercial quality of coal and settlement of deliveries for the power industry
- Conversion of the results of other determinations to the dry ash-free basis (daf)
- Classification and quality control of solid fuels in accredited laboratories (GIG)
- Preparation of ash for further analyses (for example the sulfur in ash at 815 °C)
Key parameters
| Parameter | Value |
|---|---|
| Final temperature | 815 ± 10 °C, at least 60 min |
| Heating programme | Uniformly to 500 °C in 60 min, held for 30 min (lignite 60 min), then to 815 ± 10 °C |
| Sample | About 1 g, particle size < 212 µm; loading ≤ 0.15 g/cm² (coal), ≤ 0.10 g/cm² (coke) |
| Balance | Accuracy 0.1 mg |
| Criterion for the end of ashing | Further heating in 15 min periods until the change of mass is ≤ 1 mg |
| Precision | Ash < 10 %: r = 0.2 % abs., R = 0.3 % abs.; > 10 %: r = 2.0 %, R = 3.0 % of the mean result |
Standard
- Standard number
- PN-ISO 1171:2002
- Title (PL)
- Paliwa stałe — Oznaczanie popiołu
- Title (EN)
- Solid mineral fuels — Determination of ash
Step-by-step procedure
-
Preparation of the sample
Use the general analysis sample (particle size < 212 µm), well mixed and in moisture equilibrium with the atmosphere of the laboratory.
-
Conditioning of the dish
Before weighing, ignite the silica or porcelain dish at 815 ± 10 °C for 15 min and cool it under the conditions of the determination; weigh the clean dry dish to 0.1 mg.
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Weighing out
Spread about 1 g of the sample evenly in the dish and weigh to 0.1 mg.
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Ashing
Put the dish into the furnace at room temperature; raise the temperature uniformly to 500 °C over 60 min and hold for 30 min (lignite: 60 min). Continue the heating to 815 ± 10 °C in the same furnace or transfer the dish to a second furnace heated to 815 ± 10 °C; hold for at least 60 min. Coke: the dish on the plate may be put straight into the furnace at 815 ± 10 °C for ≥ 60 min.
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Cooling and weighing
Take the dish out, cool it for 10 min on the thick metal plate, transfer it to a desiccator without a desiccant, cool to room temperature and weigh to 0.1 mg.
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Check of the completeness of combustion
In case of doubt (visible unburnt particles of coal) heat further at 815 ± 10 °C in 15 min periods until the change of mass does not exceed 1 mg.
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Calculation and test report
A = (m3 − m1)/(m2 − m1) × 100; give the mean of two determinations to 0.1 % on the analysis basis, with a reference to the standard, the identification of the sample, the date and the basis of the calculation. The permissible difference between parallel results: 0.2 % (ash < 10 %) or 2.0 % of the mean result.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Muffle furnace | A zone of uniform temperature, ventilation of 5–10 air changes/min; alternatively two furnaces: about 500 °C and 815 ± 10 °C | — |
| Dishes | Of silica, porcelain or platinum, 8–15 mm deep | — |
| Plate under the dishes with coke | Of silica or heat-resisting steel, 6 mm thick, slid into the furnace | — |
| Analytical balance | Accuracy 0.1 mg | — |
| Desiccator or another closed vessel | Without a desiccant; it may be flushed with a dry gas and the dish covered with a lid | — |
| Thick metal plate | For cooling the dish for 10 min after it is taken out of the furnace | — |
Health and safety (OHS)
- A furnace at 815 °C — laboratory tongs and heat-resistant gloves, cool the dishes on the plate, not in the desiccator straight from the furnace
- Coal dust from grinding and weighing out — goggles and a dust mask
- Ash dusts are alkaline and irritant — do not blow them off, handle them carefully
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-ISO 1171:2002 — “Solid mineral fuels — Determination of ash”.
How does this method work?
The analysis sample (ground to pass a 212 µm sieve, well mixed and in moisture equilibrium with the atmosphere of the laboratory) is heated in air at a prescribed rate to 815 ± 10 °C and kept at that temperature to constant mass; the ash is calculated from the mass of the residue after combustion: A = (m3 − m1)/(m2 − m1) × 100, where m1 is the mass of the empty dish, m2 the mass of the dish with the sample and m3 the mass of the dish with the ash. The furnace must provide a zone of uniform temperature and a ventilation of 5–10 air changes per minute; the loading of the dish must not exceed 0.
What is the measuring range and accuracy?
Final temperature: 815 ± 10 °C, at least 60 min; Heating programme: Uniformly to 500 °C in 60 min, held for 30 min (lignite 60 min), then to 815 ± 10 °C; Sample: About 1 g, particle size < 212 µm; loading ≤ 0.15 g/cm² (coal), ≤ 0.10 g/cm² (coke); Balance: Accuracy 0.1 mg.
How long does the test take?
The times given for the individual stages add up to approximately 1 h 10 min. The procedure comprises 7 steps.
What equipment is required?
Muffle furnace, Dishes, Plate under the dishes with coke, Analytical balance, Desiccator or another closed vessel, Thick metal plate.
Where is this test used?
Proximate analysis of hard coal, lignite and coke (the balance of the ballast: moisture + ash); Assessment of the commercial quality of coal and settlement of deliveries for the power industry; Conversion of the results of other determinations to the dry ash-free basis (daf); Classification and quality control of solid fuels in accredited laboratories (GIG); Preparation of ash for further analyses (for example the sulfur in ash at 815 °C).
What safety precautions apply?
A furnace at 815 °C — laboratory tongs and heat-resistant gloves, cool the dishes on the plate, not in the desiccator straight from the furnace; Coal dust from grinding and weighing out — goggles and a dust mask; Ash dusts are alkaline and irritant — do not blow them off, handle them carefully.
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 1171.