Volatile matter content testing is a core analytical procedure used to quantify the gaseous decomposition products released by coal, coke, plastics, rubber, and solid biofuels under controlled heating conditions. This technical guide explains the testing principle, applicable standards such as ISO 562, ASTM D3175, and GB/T 212, sample preparation requirements, step-by-step operating procedures, and result calculation methods. It serves as a practical reference for quality engineers, laboratory technicians, and procurement professionals who need reliable volatile matter data for material evaluation, trade settlement, and regulatory compliance.
Technical Overview
Volatile matter refers to all gaseous and vaporous products, excluding water vapor, that are driven off when a test sample is heated to a specified temperature for a defined period under conditions that exclude or severely restrict the presence of oxygen. It is one of the four determinations in proximate analysis, alongside moisture, ash, and fixed carbon, with fixed carbon conventionally obtained by difference.
The volatile matter value provides direct insight into the thermal behavior of a material. High-volatile coals ignite readily and burn with long flames, making them suitable for pulverized-fuel combustion, while low-volatile coals are preferred for metallurgical coke production. In polymers, volatile content reflects residual monomers, processing solvents, moisture, plasticizers, and other low-molecular-weight species that can influence molding quality, dimensional stability, and emission performance.
Testing is governed by internationally recognized standards, and the procedures must be followed without deviation because results are highly sensitive to temperature, residence time, and atmosphere control:
- ISO 562 and ISO 17246 for the determination of volatile matter in hard coal and coke
- ASTM D3175 for volatile matter in the analysis sample of coal and coke
- GB/T 212 for the proximate analysis of coal under the Chinese national standard system
- ISO 18123 for the determination of volatile matter in solid biofuels
- ISO 248 for volatile matter in raw rubber
- ISO 11358 for thermogravimetric analysis of polymers
Testing Samples
Coal and Coke
The analysis sample is prepared by crushing and dividing a representative gross sample, then pulverizing it until at least 99 percent passes a 212 micron sieve (ISO) or 250 micron sieve (GB/T 212). The sample is brought to air-dried equilibrium in a controlled atmosphere room before testing. Metallurgical coke, semi-coke, and coal char follow the same general sample preparation route with particle size limits defined by the applicable method.
Plastics and Polymer Materials
Samples include thermoplastic pellets, polymer powders, compounded resins, and composite materials. The specimen should be representative of the lot, and predrying conditions must be recorded because absorbed moisture will otherwise be reported together with volatiles unless the method requires separate moisture determination.
Rubber and Elastomer Compounds
Raw natural rubber, synthetic rubber, and uncured compounds are tested according to ISO 248 using either a hot mill method or an oven method. Volatile matter in rubber typically consists of moisture plus residual low-boiling processing substances.
Solid Biofuels and Derived Fuels
Wood pellets, chips, briquettes, and agricultural residues are ground to a particle size of 1 millimeter or less in accordance with ISO 18123 and equilibrated before analysis. Refuse-derived fuels may be tested with reference to ASTM D5142 or equivalent proximate analysis procedures.
Testing Items
A complete volatile matter determination normally delivers the following reported items:
- Volatile matter content on an air-dried basis, expressed as a mass percentage
- Moisture content of the analysis sample, required as a correction term
- Volatile matter converted to dry basis, dry ash-free basis, or as-received basis
- Mass of the carbonaceous residue remaining after devolatilization, used to derive fixed carbon by difference
- Observations of abnormal behavior such as flame ejection, lid displacement, or sample projection
Testing Methods
Apparatus and Equipment
The gravimetric method requires the following calibrated equipment, typically maintained by accredited third-party laboratories and in-house quality control facilities:
- A muffle furnace or vertical tube furnace capable of maintaining 900 degrees Celsius plus or minus 10 degrees Celsius with a validated uniform temperature zone
- Lidded crucibles of platinum, silica, or porcelain with a closely fitting cover to exclude air, sized for approximately 1 gram of sample
- An analytical balance with a readability of 0.1 milligram
- A crucible stand or rack that allows rapid and simultaneous insertion and removal of multiple crucibles
- A desiccator with fresh desiccant and a cold metal plate for staged cooling
- A calibrated timer and a calibrated thermocouple or pyrometer for furnace verification
Sample Preparation
The analysis sample is homogenized and equilibrated to the laboratory atmosphere so that its moisture status is stable. The simultaneous determination of moisture content on a separate portion of the same sample is mandatory for coal and coke, because the mass loss measured in the volatile matter test includes water and must be corrected.
Standard Gravimetric Procedure for Coal and Coke
The following sequence reflects the requirements of ISO 562, ASTM D3175, and GB/T 212:
- Ignite the crucible and lid to constant mass, cool in a desiccator, and record the mass m0 to 0.1 milligram.
- Weigh 1 gram plus or minus 0.1 gram of sample into the crucible and spread it in an even layer without compacting; record the combined mass m1.
- Confirm the furnace is stabilized at 900 degrees Celsius plus or minus 10 degrees Celsius and place the loaded rack at the furnace entrance.
- Insert the rack into the hot zone, close the furnace, and heat for exactly 7 minutes from insertion, ensuring the furnace recovers to within the tolerance band within about 4 minutes.
- Remove the rack, cool the crucibles briefly on a metal plate, then transfer them to a desiccator to reach ambient temperature.
- Reweigh the cooled crucible and residue and record mass m2 to 0.1 milligram.
- Inspect the crucible; if the lid was displaced, the sample projected, or free carbon was lost, discard the determination and repeat.
Procedure Variants for Other Materials
For solid biofuels under ISO 18123, a covered crucible is heated at 900 degrees Celsius plus or minus 10 degrees Celsius for 7 minutes under conditions analogous to coal testing, with moisture separately determined. For polymers, volatile content is commonly evaluated by oven conditioning at a material-specific temperature, such as 105 degrees Celsius for drying-type volatiles, or by thermogravimetric analysis under nitrogen according to ISO 11358, in which the mass-loss step below the decomposition temperature is recorded. For raw rubber under ISO 248, a hot mill or oven method is applied and the mass loss is reported as volatile matter.
Calculation and Expression of Results
The volatile matter content on an air-dried basis is calculated as the percentage mass loss during heating minus the moisture content of the analysis sample. In formula form: VM (air-dried basis) equals the quantity of m1 minus m2, divided by the quantity of m1 minus m0, multiplied by 100, minus the moisture percentage Mad. Conversion to other bases is performed as follows: dry-basis VM equals air-dried VM multiplied by 100 and divided by 100 minus Mad; dry ash-free VM further divides by 100 minus the ash percentage on the same basis. All results are reported to two decimal places together with the basis designation.
Precision and Quality Control
Duplicate determinations should agree within the repeatability limit stated in the governing standard, typically on the order of 0.3 to 0.7 percentage points for coal depending on volatile matter level, with reproducibility limits between laboratories generally within 1.0 to 1.5 percentage points. Quality control measures include furnace temperature mapping at defined intervals, crucible mass verification, the use of certified reference materials, control charts, and full traceability of balance calibration. Any determination showing evidence of ignition, spattering, or lid lift must be rejected and repeated, because oxygen ingress oxidizes the char and depresses the measured value.
Applications
Volatile matter data are used across the entire value chain of solid fuels and polymeric materials, supporting commercial, technical, and regulatory decisions:
- Coal classification and grade assignment, including coal typing under national classification systems that rank coal by dry ash-free volatile matter
- Trade settlement and contract specification verification for coal, coke, and biomass imports and exports
- Boiler and gasifier design, combustion tuning, and flame stability assessment in power plants
- Coke oven charge formulation and prediction of coke strength in the iron and steel industry
- Quality control of plastic resins, detection of residual solvents or monomers, and evaluation of drying efficiency
- Compounding development and incoming material inspection for rubber processors
- Regulatory compliance, emission characterization, and certification support through accredited third-party laboratories
FAQ
Question: Why must the crucible be covered during the volatile matter test?
The cover excludes air so that the sample decomposes by pyrolysis rather than burning. If oxygen enters, the residual char oxidizes to ash, the measured mass loss becomes too small, and the volatile matter result is biased low. A determination with a visibly displaced lid must be rejected.
Question: Why is moisture subtracted from the mass loss?
During heating, both volatile decomposition products and residual moisture in the analysis sample are lost. Because moisture is defined and reported separately in proximate analysis, its percentage must be deducted from the total mass loss so that volatile matter represents only the thermal decomposition products.
Question: What temperature and heating time are specified for coal testing?
ISO 562, ASTM D3175, and GB/T 212 all specify heating at 900 degrees Celsius plus or minus 10 degrees Celsius for exactly 7 minutes in a covered crucible. The furnace must recover to the tolerance band within a few minutes after the cold crucibles are inserted, and the total heating duration is strictly timed.
Question: What does it mean if a flame is observed escaping from the crucible?
A visible flame indicates that gaseous decomposition products have ignited, usually due to air leakage or lid movement. Such a determination is invalid because part of the combustible volatile stream has been consumed, and the test must be repeated with a properly seated lid.
Question: How is fixed carbon obtained?
Fixed carbon is not measured directly. It is calculated by difference as 100 minus the sum of moisture, ash, and volatile matter, all expressed on the same reporting basis. Accurate volatile matter determination is therefore essential for a correct fixed carbon value.
Question: What are the common reporting bases and how do they differ?
Results may be reported as received, on an air-dried basis, on a dry basis, or on a dry ash-free basis. Trade contracts and classification systems specify the required basis, and laboratories must clearly state the basis with every reported figure to prevent misinterpretation.
Question: How should samples be sent for testing?
Samples should be collected representatively, sealed in airtight moisture-proof containers, accompanied by a chain-of-custody record, and shipped promptly to the laboratory. Sufficient quantity, typically at least 100 grams of the analysis sample plus reserve, should be provided along with the required standard, reporting basis, and any special observations.