Operating Procedure for the Determination of Fixed Carbon Content

Fixed carbon content is one of the most important quality parameters in the proximate analysis of coal, coke, and carbonaceous materials, directly reflecting the solid combustible residue available for energy release and metallurgical reduction. This article presents a complete professional operating procedure for fixed carbon determination, covering applicable standards, sample preparation, step-by-step gravimetric methods, calculation formulas, precision requirements, and quality control measures. Engineers, laboratory technicians, and quality inspectors can follow this guide to obtain accurate and reproducible fixed carbon results in accordance with recognized international and national standards.

Technical Overview

Fixed carbon (FC) is defined as the solid, non-volatile combustible residue remaining after a sample of coal, coke, or other carbonaceous material has been heated in the absence of air and the volatile matter has been expelled. Together with moisture, ash, and volatile matter, fixed carbon constitutes one of the four components of proximate analysis, the most widely used framework for evaluating solid fuels and carbon materials. A higher fixed carbon value generally indicates a higher fuel rank, greater calorific contribution from the solid residue, and superior performance in reduction and carburizing applications such as blast furnace operation and foundry melting.

Because fixed carbon cannot be isolated and weighed directly as a distinct substance, it is determined indirectly, either by the classical calculation-by-difference method based on gravimetric proximate analysis, or by direct high-temperature combustion for specific materials such as amorphous graphite. The calculation-by-difference method is standardized in standards such as GB/T 212, ASTM D3172, and ISO 17246, and requires accurate determination of moisture (ASTM D3173, ISO 11722), ash (ASTM D3174, ISO 1171), and volatile matter (ASTM D3175, ISO 562). Any error in these three determinations is propagated into the fixed carbon result, so strict adherence to temperature, timing, weighing, and cooling procedures is essential.

This operating procedure describes the equipment, reagents, sample preparation, step-by-step operations, calculations, result expression, precision limits, and safety precautions required for reliable fixed carbon determination in coal, coke, semi-coke, charcoal, activated carbon, petroleum coke, graphite, and related carbon products.

Testing Samples

Coal and Anthracite

All ranks of coal, including lignite, sub-bituminous coal, bituminous coal, and anthracite, are routinely tested for fixed carbon. Coal samples shall be air-dried, crushed, and ground to pass a 212 micron (0.2 mm) sieve in accordance with sample preparation standards such as GB/T 474 or ISO 18283. Anthracite typically exhibits fixed carbon values above 90 percent on a dry ash-free basis, while low-rank coals may show fixed carbon values below 55 percent, making correct sample handling and moisture protection critical.

Coke, Semi-Coke, and Charcoal

Metallurgical coke, foundry coke, petroleum coke, semi-coke (lan coke), and charcoal are high fixed carbon materials widely used in steelmaking, casting, silicon and ferroalloy production, and gas generation. These samples should be prepared to the same 0.2 mm fineness and stored in airtight containers to prevent moisture pickup before analysis. Coke volatile matter determination follows the same 900 degree Celsius principle, while charcoal may require attention to its high moisture sensitivity.

Graphite and Carbon Products

Natural graphite (crystalline, flake, and amorphous), graphitized petroleum coke, carbon anodes, cathode blocks, electrode materials, and other shaped carbon products are evaluated using fixed carbon as a purity indicator. For amorphous graphite, standards such as GB/T 3521 specify that fixed carbon is obtained after separate determinations of moisture, ash, and volatile matter. For high-purity graphitized materials, fixed carbon values above 99 percent are common, so microbalances and rigorously controlled blank corrections become significant.

Activated Carbon, Carbon Black, and Briquettes

Activated carbon, carbon black, coal-based briquettes, and biomass-char composites are also tested for fixed carbon to characterize devolatilization degree, adsorption precursor quality, and fuel performance. Biomass-derived samples generally contain higher oxygen and volatile fractions, and analysts should verify that the standard selected is appropriate for the material class.

Testing Items

The fixed carbon determination comprises the following measurable items, each reported on the air-dried basis before conversion:

• Moisture content (M): mass loss on drying at 105 to 110 degrees Celsius under specified atmosphere.
• Ash content (A): inorganic residue after complete incineration at 815 degrees Celsius in air.
• Volatile matter (V): mass loss, corrected for moisture, after heating at 900 degrees Celsius plus or minus 10 degrees Celsius for 7 minutes in a covered crucible with minimal air contact.
• Fixed carbon (FC): the combustible solid residue obtained by difference, calculated from the above three determinations.
• Result conversion: fixed carbon on dry basis (d), dry ash-free basis (daf), and as-received basis (ar) as required by the customer or specification.

Testing Methods

Apparatus and Materials

• Analytical balance readable to 0.1 mg, calibrated and verified before use.
• Programmable muffle furnace with ventilation, capable of maintaining 815 plus or minus 10 degrees Celsius and 900 plus or minus 10 degrees Celsius, with temperature uniformity verified by a calibrated thermocouple.
• Forced-convection drying oven or nitrogen-purge oven capable of 105 to 110 degrees Celsius.
• Porcelain or silica crucibles with well-fitting lids for volatile matter; shallow porcelain dishes for ash; glass weighing bottles with ground lids for moisture.
• Crucible tongs, heat-resistant gloves, desiccator with fresh silica gel or equivalent desiccant, timer, and dry nitrogen supply (purity 99.9 percent or better) for moisture determination of reactive coals.

Sample Preparation

Verify that the laboratory sample has been air-dried, ground to pass a 0.2 mm sieve, and mixed thoroughly. Before weighing, allow the sample to reach ambient laboratory temperature and humidity equilibrium. Weighing shall be performed rapidly, and samples hygroscopic in nature shall be kept in sealed containers except during the actual weighing and transfer operations. Record the laboratory temperature and humidity, and ensure the analytical balance is level and zeroed.

Procedure for Moisture Determination

• Heat a clean, dry weighing bottle with lid at 105 to 110 degrees Celsius, cool in a desiccator, and weigh to the nearest 0.1 mg.
• Weigh 1.0000 g plus or minus 0.1000 g of the prepared sample into the bottle, spreading it in an even layer.
• For general analysis coal, dry in a nitrogen purge oven or ventilated oven at 105 to 110 degrees Celsius; for low-rank or highly oxidized coals, nitrogen atmosphere drying is recommended to prevent oxidation.
• After the prescribed drying period, typically 1.5 to 2 hours, transfer the covered bottle to a desiccator, cool to room temperature, and weigh.
• Repeat drying in 30-minute increments until the change in mass is 0.0010 g or less. Calculate moisture from the total mass loss.

Procedure for Ash Determination

• Heat a porcelain dish at 815 degrees Celsius, cool in a desiccator, and weigh.
• Weigh 1.0000 g plus or minus 0.1000 g of sample into the dish, spreading it thinly and evenly.
• Place the dish into a cold or 100 degree Celsius muffle furnace, then raise the temperature slowly so that the sample burns without flaming; hold at approximately 500 degrees Celsius for about 30 minutes to allow volatile matter to escape gently.
• Raise the furnace to 815 plus or minus 10 degrees Celsius and hold for one hour to complete incineration.
• Remove the dish, cool for 5 minutes in air, then in a desiccator to room temperature, and weigh. Repeat 20-minute ignition cycles until the mass change is 0.0010 g or less. The residue mass is the ash content.

Procedure for Volatile Matter Determination

• Heat the muffle furnace to exactly 900 plus or minus 10 degrees Celsius in advance; the furnace must have sufficient thermal mass to recover to within 5 degrees Celsius of the set point within 4 minutes of crucible insertion.
• Weigh a pre-ignited, lidded volatile matter crucible, add 1.0000 g plus or minus 0.1000 g of sample, tap gently to level, and cover immediately to exclude air.
• Place the covered crucible on a suspended or supported rack, insert into the 900 degree Celsius zone, close the furnace, and heat for exactly 7 minutes from insertion. Timing shall be measured with a calibrated timer; the lid must remain seated throughout.
• Remove the crucible, cool in air for no more than 5 minutes avoiding moisture pickup, then cool in a desiccator to room temperature and weigh.
• Mass loss percentage minus the moisture percentage equals volatile matter. Inspect the residue for evidence of ignition or flame sparring; discard and repeat if a lid was displaced or the sample ignited.

Calculation and Expression of Results

Fixed carbon on the air-dried basis is calculated as:

FC(ad) = 100 minus M(ad) minus A(ad) minus V(ad), where all terms are percentages by mass on the air-dried basis.

For reporting on other bases, apply the following conversions:

• Dry basis: FC(d) = FC(ad) multiplied by 100 divided by (100 minus M(ad)).
• Dry ash-free basis: FC(daf) = FC(ad) multiplied by 100 divided by (100 minus M(ad) minus A(ad)).
• As-received basis: FC(ar) = FC(ad) multiplied by (100 minus M(ar)) divided by (100 minus M(ad)), when total moisture M(ar) is separately determined.

Report results to two decimal places and state the basis of the result, the standard used, and the date of analysis.

Precision and Quality Control

Under GB/T 212, ASTM D3172 to D3175, and ISO 17246, the repeatability of each proximate component in a competent laboratory is commonly within 0.2 to 0.5 percentage points, and the reproducibility between laboratories is wider by a factor of two to three. Because fixed carbon is obtained by difference, the combined uncertainty of the moisture, ash, and volatile matter determinations governs its accuracy. The following quality control measures are mandatory:

• Verify furnace temperature profiles quarterly with a calibrated thermocouple or temperature-measuring device traceable to national standards.
• Run certified reference materials or in-house control coal samples with each batch; results must fall within the certified or control limits.
• Perform duplicate determinations at a defined frequency, for example one in ten samples, and confirm that duplicates meet repeatability limits before release.
• Use matched crucibles of similar mass and thermal properties, and handle all vessels exclusively with tongs after ignition.
• Maintain desiccant activity, balance calibration records, and a complete audit trail of weighing data.

Safety Precautions

• Wear heat-resistant gloves and face protection when operating the muffle furnace; hot crucibles can cause severe burns.
• Ensure adequate ventilation or an exhaust hood above the furnace, as sulfur oxides and other gases may be released during ashing and devolatilization.
• Handle nitrogen cylinders with regulators and check for leaks; nitrogen asphyxiation risk requires adequate room ventilation.
• Allow all heated ware to cool in designated zones; never place hot crucibles directly on the balance or bench.
• Dispose of ash residues and spent desiccant according to the laboratory waste management procedure.

Applications

Fixed carbon determination supports a wide range of industrial, commercial, and regulatory activities:

• Coal mining, washing, trading, and pricing, where fixed carbon and calorific value serve as core contract indices for steam coal and anthracite.
• Power generation, for fuel blending, boiler combustion modeling, and verification of delivered coal quality against specification.
• Iron and steel production, where coke fixed carbon content governs blast furnace permeability, reduction capacity, and coke rate.
• Foundry and casting operations, in which cupola and induction furnace charge calculations depend on the fixed carbon of coke and recarburizers.
• Ferroalloy, silicon metal, and calcium carbide manufacturing, which consume high fixed carbon reductants such as semi-coke, petroleum coke, and low-ash char.
• Graphite and carbon product manufacturing, where fixed carbon functions as a purity specification for electrodes, anodes, seals, and battery materials.
• Activated carbon and charcoal production, for process control of carbonization degree and product classification.
• Environmental and customs compliance, providing data for emission inventories, resource utilization assessments, and import-export quality inspection by accredited third-party laboratories.

FAQ

What is the difference between fixed carbon and total carbon?

Fixed carbon is a proximate analysis parameter representing the solid combustible residue after moisture, ash, and volatile matter are removed, whereas total carbon is an ultimate analysis parameter obtained by complete combustion instrumentation, measuring all carbon including the carbon contained in the volatile matter. Total carbon is always equal to or greater than fixed carbon, and the two must never be interchanged in specifications.

Why is fixed carbon calculated by difference instead of measured directly?

During the standardized volatile matter test, the residue still contains some decomposed carbon and condensed products, and there is no physical operation that can isolate pure fixed carbon without alteration. The difference method, defined in GB/T 212, ASTM D3172, and ISO 17246, is therefore the internationally accepted approach, and its accuracy depends on the quality of the three supporting determinations.

What are the critical control points of the procedure?

The most influential control points are the 900 plus or minus 10 degrees Celsius furnace temperature and the exact 7-minute heating time for volatile matter, the covered state of the crucible to exclude air, the slow ramp program for ashing to prevent mechanical loss of sample, complete cooling in a desiccator before every weighing, and protection of hygroscopic samples from atmospheric moisture during handling.

How is the volatile matter test affected by sample ignition?

If air leaks into the crucible or the lid is displaced, the sample may ignite, which oxidizes fixed carbon and produces an erroneously high volatile matter and low fixed carbon result. Any crucible showing soot ejection, a displaced lid, or visible burning must be discarded and the determination repeated.

How long does a complete fixed carbon determination take?

With parallel operation of the three gravimetric determinations, a single sample can typically be completed within 3 to 4 hours of bench time including cooling and weighing cycles, while full batch processing with duplicates and reference material checks is usually organized over one working day.

Which basis should be used when reporting fixed carbon?

The reporting basis must match the purpose: air-dried basis for laboratory comparison, dry basis for fuel quality grading, dry ash-free basis for rank classification and combustion calculations, and as-received basis for commercial settlement. The basis used must always be clearly stated alongside the numerical result.