🔥 Volatile matter in coal and coke — gravimetric method at 850 °C

Petrochemistry PN-G-04516

Overview

Volatile matter is the percentage loss of mass of the analysis sample of a solid fuel, arising during a strictly prescribed heating of the sample with no access of air (degassing) in a vessel of standardised shape, in an electric furnace at a temperature of 850 °C, reduced by the moisture content. The determination belongs to the typically conventional analytical determinations — the result is affected by the particle size and the mass of the sample, by the shape of the vessel and by the way the prescribed temperature is reached. The present description is based on the laboratory instructions of the Wrocław University of Science and Technology and of the Poznań University of Technology, carried out according to the earlier edition PN-81/G-04516; the current edition PN-G-04516:1998 was not available and before implementation it has to be checked.

Volatile matter consists of the gaseous products and tar vapours formed by the thermal decomposition of the fuel with no access of air; its quantity falls as the rank rises (lignite 45–65 %, hard coal 14–45 %, anthracite 1–10 %, semi-coke 4–15 %, coke 1–5 % Vdaf), and for that reason the volatile matter content on the dry ash-free basis is an approximate indicator of rank and one of the parameters of the Polish classification of hard coals. Fuels with a large amount of volatile matter give a long flame and require additional air for smokeless combustion; the appearance of the coke button after the determination allows a rough assessment of the suitability of the coal for coking.

The gravimetric method according to PN-G-04516:1998 is in the scope of accreditation of GIG (volatile matter content 0.40–48.10 %; the fixed carbon index is also calculated from it).

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Method principle

A test portion of 1 ± 0.1 g of the analysis sample is heated in an electric furnace in a closed porcelain crucible of standardised dimensions, with no access of air, at a temperature of 850 ± 15 °C for 7 minutes, and the volatile matter content is calculated as the difference between the total loss of mass and the loss caused by the loss of moisture: Va = Δm/m × 100 − Wa, where Δm is the loss of mass during the degassing, m the mass of the sample and Wa the analysis moisture. The furnace must have such a heat capacity that after cold crucibles are put in it returns to 850 °C within 3–4 min. Conversion to the dry ash-free basis: Vdaf = Va × 100/[100 − (Wa + Aa)].

Applications

• Proximate analysis of hard coal, lignite and coke
• Assessment of rank and classification of hard coals (types 31–43)
• Assessment of the suitability of a fuel for energy purposes (length of the flame, demand for air)
• Preliminary assessment of the suitability of a coal for coking (the appearance of the coke button)
• Calculation of the fixed carbon index (GIG)

Key parameters

Standard

Standard number

PN-G-04516:1998

Title (PL)

Paliwa stałe — Oznaczanie zawartości części lotnych metodą wagową

Title (EN)

Solid fuels — Determination of volatile matter content by gravimetric method (tłumaczenie robocze)

Step-by-step procedure

1. Crucibles

   Weigh two numbered, ignited porcelain crucibles with their lids on the analytical balance.

2. Weighing out

   Put 1 ± 0.1 g of the fuel under test into each crucible (mix the sample thoroughly before weighing), weigh them with their lids, level the surface of the coal by tapping the bottom of the crucible lightly against a hard surface and cover them with the lids.

3. Putting into the furnace

   Place the crucibles on the stand and put them quickly into the furnace heated to 850 °C, into the zone of uniform heat; close the furnace at once and start the stopwatch. The furnace is to reach 850 ± 10 °C within 3–4 min; if it does not reach it within 4 min — repeat the determination.

   🌡 Temperature: 850 ± 10 °C

4. Heating

   Heat for 7 minutes, counted from the moment the furnace is closed.

   ⏱ Time: 7 min

5. Cooling

   Take the stand with the crucibles out, leave it for 5 min in air, transfer the crucibles to a desiccator and cool them to room temperature, as far as possible not longer than 30 min from the removal from the furnace.

   ⏱ Time: 5 min + ≤ 30 min

6. Weighing and calculation

   Weigh the crucibles with their contents (0.0002 g). Va = Δm/m × 100 − Wa; determine the analysis moisture Wa in parallel on the same sample.

7. Result

   Take as the result the mean of two determinations meeting the criterion for the difference (0.3 % abs. up to 10 %; over 10 % — 3 % of the higher result), rounded to 0.1 %; if the difference is greater — carry out a third determination. In addition convert to Vdaf.

Required equipment and apparatus

Health and safety (OHS)

• A furnace at 850 °C — tongs and heat-resistant gloves; put the stand with the crucibles in and take it out quickly but carefully
• During the degassing flammable gases and tar vapours are evolved from the crucibles — provide ventilation, do not open the furnace during the 7 min
• Cool hot crucibles in air first (5 min), so as not to damage the desiccator

Frequently asked questions