Fixed carbon content is one of the most important quality indicators for coal, coke, graphite, and other carbonaceous materials, and in China it is determined under the national standard (GB) proximate analysis system. The fixed carbon value is derived by difference from moisture, ash, and volatile matter results, following standards such as GB/T 212, GB/T 30732, GB/T 2001, and GB/T 3521. This article explains the test principles, procedures, calculation formulas, precision requirements, and typical applications of these GB methods.
Technical Overview
Fixed carbon (FC) is defined as the solid, combustible residue that remains after moisture, ash, and volatile matter have been removed from a carbonaceous sample under standardized heating conditions. It is not measured directly; instead, it is calculated "by difference" from the three measured components of proximate analysis. Because the fixed carbon result depends entirely on the accuracy of the moisture, ash, and volatile matter determinations, the complete analytical chain of the relevant national standard must be strictly followed.
The principal Chinese national standards (GB standards) governing this determination include:
- GB/T 212-2008 — Proximate analysis of coal (classical gravimetric method)
- GB/T 30732-2014 — Proximate analysis of coal by automatic thermogravimetric instruments
- GB/T 474-2008 — Preparation of coal samples
- GB/T 483-2007 — General rules for analytical and testing methods of coal
- GB/T 211-2017 — Determination of total moisture in coal
- GB/T 2001-2013 — Determination of moisture, ash, and volatile matter of coke
- GB/T 3521-2008 — Methods for chemical analysis of graphite (fixed carbon by difference)
Internationally comparable methods include ISO 17246 (proximate analysis of coal), ISO 562 (volatile matter), ISO 1171 (ash), ISO 11722 (moisture), and ASTM D3172 / ASTM D5142, allowing results obtained by GB methods to be aligned with global trade specifications.
Testing Samples
Coal and Coal Products
Bituminous coal, anthracite, lignite, coking coal, thermal coal, washed and blended coals, and pulverized coal for injection in blast furnaces are the most frequently tested materials. Coal-water slurry samples can also be characterized using the corresponding national proximate analysis series after appropriate solid-matter determination.
Coke and Solid Carbonized Fuels
Metallurgical coke, foundry coke, ferroalloy coke, semi-coke, green and calcined petroleum coke, and other carbonized solid fuels are tested to verify their fixed carbon and volatile matter specifications.
Graphite and Carbon Materials
Natural flake graphite, amorphous (microcrystalline) graphite, spherical graphite for lithium-ion battery anodes, graphite electrode materials, graphitized petroleum coke recarburizers, and foundry carbon raisers are routinely tested, as fixed carbon content is the primary grade-defining parameter for these products.
Other Sample Types
Charcoal, activated carbon precursors, carbon black feedstocks, biomass-coal blends, and research samples can be evaluated using the same by-difference approach, provided the heating conditions are validated for the material class.
Testing Items
A complete fixed carbon determination under the GB system comprises the following measured and derived parameters:
- Air-dried moisture M(ad) and, where required, total moisture M(t)
- Ash content A
- Volatile matter V
- Fixed carbon FC, calculated by difference
- Reporting-basis conversions: air-dried basis (ad), dry basis (d), dry and ash-free basis (daf), and as-received basis (ar)
Testing Methods
Sample Preparation
Laboratory samples are air-dried, crushed, and divided to obtain a general analysis sample with a particle size below 0.2 mm, in accordance with GB/T 474. Test portions of approximately 1 g are weighed to 0.0002 g. Proper sample preparation and airtight storage are essential, because any moisture change directly shifts the calculated fixed carbon value.
Moisture Determination (GB/T 212)
For the reference method, a 1 g test portion is heated in a nitrogen-flushed drying oven at 105–110 °C until constant mass, typically about 1.5 hours; an air-drying variant is permitted for coals that are not susceptible to oxidation. The percentage mass loss is reported as air-dried moisture. Total moisture of the as-received sample is determined separately under GB/T 211.
Ash Determination (GB/T 212)
In the slow-ashing procedure, the sample is placed in a cold muffle furnace, heated to about 500 °C over no less than 30 minutes, held to burn off volatiles without flaming, and then raised to (815 ± 10) °C and kept until constant mass. A rapid ashing variant with controlled temperature staging is also permitted. The residue mass percentage is the ash content.
Volatile Matter Determination (GB/T 212)
A 1 g test portion is placed in a standardized lidded volatile-matter crucible and heated at (900 ± 10) °C for exactly 7 minutes in the hottest zone of the muffle furnace, with strict limits on crucible placement and recovery times. The mass loss, corrected for moisture, is the volatile matter content. The lid prevents oxidation of the residue during the test.
Calculation of Fixed Carbon
Fixed carbon is calculated by difference on a consistent basis:
FC(ad) = 100 − M(ad) − A(ad) − V(ad), with all values in percent by mass.
Conversion to other reporting bases is performed as follows:
- FC(d) = FC(ad) × 100 / (100 − M(ad))
- FC(daf) = FC(ad) × 100 / (100 − M(ad) − A(ad))
Automatic Thermogravimetric Method (GB/T 30732)
Automatic proximate analyzers use programmed temperature ramps and controlled atmospheres, nitrogen for moisture and volatile matter and air or oxygen for ash, to determine all three parameters in a single run. Instruments must be calibrated with certified reference materials and periodically verified by parallel testing against the classical GB/T 212 procedure. This method is widely used in high-throughput industrial and commercial laboratories.
Coke and Graphite Determinations (GB/T 2001 and GB/T 3521)
For coke, GB/T 2001 applies the same gravimetric logic with temperature and timing conditions adapted to the dense, low-volatile character of coke. For graphite, GB/T 3521 determines moisture, ash, and volatile matter on the graphite sample and calculates fixed carbon as 100 − M − A − V; for high-purity graphite products, fixed carbon contents above 99 percent are common and serve as the key grade criterion.
Precision and Quality Control
Repeatability limits under GB/T 212 are typically 0.2 percentage points for moisture and range from about 0.2 to 0.5 percentage points for ash and 0.3 to 0.8 percentage points for volatile matter, depending on the analyte level. Because fixed carbon is a derived result, the permitted uncertainty is the combined effect of the three component determinations, which is why calibration of balances, furnaces, and thermocouples, together with certified reference materials, is mandatory. For contractual disputes, the classical reference method is generally preferred.
Applications
Fixed carbon data determined by GB methods support a wide range of industrial and commercial activities:
- Coal trading and pricing, where fixed carbon is a contractual specification, especially for anthracite
- Power plant boiler design, combustion modeling, and burnout performance evaluation
- Cokemaking blend design and coke quality prediction in iron and steel production
- Foundry recarburizer selection, where high fixed carbon ensures efficient carburization of molten iron
- Graphite grade classification and quality control for battery anode, electrode, and sealing materials
- Aluminum smelting carbon anode production and petroleum coke calcination control
- Academic research, emission accounting, and process optimization for solid fuels
FAQ
Why is fixed carbon calculated instead of measured directly?
Fixed carbon represents the nominal carbonaceous residue after standardized removal of moisture, ash, and volatile matter. It is a defined, operational quantity rather than a discrete chemical species, so the national standards specify its calculation by difference. This approach is simple, reproducible, and harmonized with ISO and ASTM proximate analysis practice.
What is the difference between fixed carbon and total carbon?
Fixed carbon is a proximate analysis parameter, while total carbon is an ultimate analysis parameter measured by combustion-based elemental analysis. Total carbon includes the carbon present in volatile matter, so it is generally higher than fixed carbon. Both results are complementary in full fuel characterization.
Which reporting basis should be used?
GB standards report proximate results on the air-dried basis by default, with conversions to dry, dry-and-ash-free, or as-received bases depending on the contractual or engineering purpose. All components used in the by-difference calculation must be on the same basis to avoid systematic errors.
Are automatic analyzers acceptable alternatives to the classical method?
Yes, when the equipment and procedure comply with GB/T 30732, the instrument is calibrated with certified reference materials, and periodic verification against GB/T 212 is documented. For arbitration and certification purposes, the classical reference method is typically specified.
How should samples be submitted for testing?
A representative sample of at least 100 grams should be sealed in airtight moisture-proof packaging to prevent moisture change during transport, accompanied by material description and the required reporting basis. Accredited third-party laboratories operating under ISO/IEC 17025 quality systems can perform the full GB proximate analysis chain and issue recognized test reports.
What turnaround time is typical?
Classical GB/T 212 proximate analysis with fixed carbon calculation is usually completed within two to three working days per sample, while automated thermogravimetric systems can deliver same-day or next-day results for routine production control.