Fixed Carbon Content Determination: Experimental Procedure and Test Steps

Fixed carbon content is a core parameter in the proximate analysis of coal, coke, petroleum coke, and other carbonaceous materials, and it directly reflects the solid combustible fraction that determines heating value and fuel performance. This article explains the complete experimental procedure for fixed carbon determination based on recognized standards such as GB/T 212, ISO 17246, and ASTM D3172, covering sample preparation, moisture, ash, and volatile matter measurement, the calculation formula, and key quality control points.

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 specified conditions. In routine industrial analysis, fixed carbon is not measured directly; instead, it is calculated by difference from the other three proximate analysis components, which makes the accuracy of every individual determination critical to the final result.

The determination follows internationally recognized procedures, including GB/T 212 (Proximate Analysis of Coal), ISO 17246 (Coal and Coke Proximate Analysis), ISO 562 (Hard Coal and Coke Determination of Volatile Matter), ISO 1171 (Solid Mineral Fuels Determination of Ash), and the ASTM D3173, D3174, D3175, and D3172 series. Fixed carbon content is a key specification in coal trading contracts, coke production control, boiler and furnace design, and the evaluation of carbon products such as recarburizers and activated carbon.

Testing Samples

Coal

Anthracite, bituminous coal, and lignite samples are the most common materials tested. Fixed carbon is used together with volatile matter to classify coal rank, estimate calorific value, and predict burnout behavior in power plant boilers.

Coke and Semi-Coke

Metallurgical coke, foundry coke, and semi-coke are tested to verify mechanical strength precursor properties and carbon purity. High fixed carbon and low volatile matter are essential quality indicators for blast furnace and casting applications.

Petroleum Coke

Green and calcined petroleum coke are analyzed to control volatile matter release during calcination and to confirm the carbon yield for electrode and anode manufacturing.

Activated Carbon and Carbon Additives

Activated carbon, graphite powders, recarburizers, and carbon raisers are tested to verify carbon purity and consistency, since ash and volatile matter directly reduce effective carbon delivery.

Biochar and Charcoal

Biomass chars are characterized by fixed carbon content to assess their stability, fuel quality, and suitability for soil amendment or metallurgical use.

Testing Items

A complete fixed carbon determination requires the following measurements, each reported on a defined analytical basis:

• Total moisture and air-dried moisture content
• Ash content
• Volatile matter content
• Fixed carbon content, calculated by difference
• Result conversion between air-dried (ad), dry (d), and dry-ash-free (daf) bases

Testing Methods

Calculation Principle

Fixed carbon is obtained by subtracting the percentages of moisture, ash, and volatile matter from 100 percent. On the air-dried basis, the fundamental equation is expressed as FC(ad) = 100 − M(ad) − A(ad) − V(ad), where M is moisture, A is ash, and V is volatile matter. Results are then converted to the dry basis using FC(d) = FC(ad) × 100 / (100 − M(ad)).

Apparatus and Materials

• Analytical balance with a readability of 0.1 mg
• Electrothermally heated drying oven capable of 105 to 110 °C with ventilation
• Programmable muffle furnace with temperature control to 815 ± 10 °C and 900 ± 10 °C
• Vented porcelain or silica crucibles with lids for volatile matter determination, or platinum crucibles
• Shallow porcelain or silica dishes for ash and moisture determination
• Desiccator with fresh silica gel desiccant
• Crucible tongs, heat-resistant gloves, and personal protective equipment
• High-purity nitrogen gas where an inert atmosphere is specified

Step 1: Sample Preparation

• Air-dry the gross sample to reach approximate equilibrium with laboratory atmosphere, then crush and divide it by coning and quartering or mechanical riffling.
• Grind the sample to pass a 0.2 mm (approximately 72 mesh) sieve and homogenize thoroughly.
• Spread the prepared sample in a shallow dish, weigh approximately 1 g portions to 0.1 mg precision, and record each mass accurately.
• Complete all determinations promptly to avoid moisture change during storage; keep the prepared sample in a sealed container.

Step 2: Moisture Determination

• Place the weighed sample in a ventilated oven at 105 to 110 °C for one hour for hard coal, or use the nitrogen atmosphere procedure for coals susceptible to oxidation, such as lignite.
• Remove the dish, cool it in a desiccator for approximately 20 minutes, and weigh it accurately.
• Repeat heating in 30-minute increments until the mass change is 0.1 percent or less, confirming constant mass.
• Calculate M(ad) as the percentage mass loss relative to the original sample mass.

Step 3: Ash Determination

• Place the weighed sample dish in a cold muffle furnace or on the hearth at room temperature.
• Raise the temperature gradually to approximately 500 °C within no less than 30 minutes to avoid flaming, and hold for 30 minutes to allow volatile matter to escape without ignition losses.
• Continue heating to 815 ± 10 °C and maintain this temperature for approximately 40 minutes until all carbonaceous matter is burned away and only incombustible residue remains.
• Cool in a desiccator and weigh; check constant mass by further 20-minute heating cycles until the mass change is within 0.1 percent.
• Calculate A(ad) as the percentage of residue mass relative to the original sample mass.

Step 4: Volatile Matter Determination

• Place approximately 1 g of sample in a lidded volatile matter crucible, ensuring the lid seats tightly to exclude air, and weigh precisely.
• Position the crucible in a muffle furnace already stabilized at 900 ± 10 °C (or 950 ± 20 °C where the ASTM procedure is applied) and heat for exactly 7 minutes.
• Ensure the temperature recovery within the first 4 minutes stays within specification, and avoid any flame contact or lid displacement.
• Remove the crucible, cool it on a clean metal plate for about 5 minutes, then transfer it to a desiccator to reach room temperature before weighing.
• Calculate V(ad) as the percentage mass loss after subtracting the moisture content from the total loss.

Step 5: Calculation and Reporting of Fixed Carbon

Apply the difference formula FC(ad) = 100 − M(ad) − A(ad) − V(ad) and report the result to two decimal places. Convert to the dry basis with FC(d) = FC(ad) × 100 / (100 − M(ad)), or to the dry-ash-free basis with FC(daf) = FC(ad) × 100 / (100 − M(ad) − A(ad)). Report the analytical basis, the standard applied, the furnace temperatures, and any deviation from the reference procedure.

Direct High-Temperature Method

For research purposes and high-purity carbon materials, fixed carbon can be approached directly by heating the sample in an inert atmosphere at approximately 900 to 950 °C to remove volatile matter, followed by combustion of the remaining carbon in air or oxygen. The mass loss during the combustion stage represents the fixed carbon fraction, and the final residue corresponds to ash. This approach is more labor-intensive but provides a useful cross-check for materials with unusual volatile matter behavior.

Instrumental Thermogravimetric Method

Automatic proximate analyzers and thermogravimetric (TGA) instruments replicate the classical temperature program in a single run under controlled atmospheres, applying integrated mass-loss algorithms to deliver moisture, volatile matter, ash, and fixed carbon results. Instrumental systems must be validated against the classical gravimetric methods and calibrated with certified reference materials before routine use.

Quality Control and Precision

• Perform all determinations in duplicate; the difference between results must meet the repeatability limits of the applied standard, typically 0.2 to 0.5 percent absolute depending on the constituent and its concentration level.
• Because fixed carbon is calculated by difference, the combined uncertainty of moisture, ash, and volatile matter directly propagates into the FC result, so strict control of temperature, timing, and cooling conditions is essential.
• Use certified reference materials to verify furnace performance and operator technique at regular intervals.
• Replace desiccant frequently and limit the time crucibles spend in the open laboratory, as ash and char residues are hygroscopic.
• Accredited third-party laboratories should participate in interlaboratory proficiency testing to demonstrate ongoing competence.

Applications

Fixed carbon data support a wide range of industrial and commercial decisions:

• Coal trading and contract settlement, where fixed carbon and volatile matter define price and quality grades
• Power plant boiler combustion optimization, burnout prediction, and unburned carbon control in fly ash
• Coke oven and foundry operation, where high fixed carbon ensures adequate carbon yield and hot metal quality
• Steelmaking recarburizer and electrode paste selection based on carbon purity requirements
• Activated carbon production control, where volatile matter governs activation behavior
• Research and development of biochar, gasification feedstocks, and alternative carbon materials

FAQ

What is the difference between fixed carbon and total carbon?

Fixed carbon comes from proximate analysis and represents the combustible solid residue after volatile matter is expelled, whereas total carbon comes from ultimate analysis and measures all carbon in the sample, including carbon in volatile hydrocarbons and even in mineral carbonates. The two values describe different properties and should not be interchanged.

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

The difference method is robust, fast, and fully standardized. Each of the three measured components (moisture, ash, and volatile matter) is determined gravimetrically with well-controlled conditions, so the calculated fixed carbon is reproducible across laboratories without requiring a separate direct combustion procedure.

What temperatures and times are specified for each step?

Moisture is determined at 105 to 110 °C, ash at 815 ± 10 °C after a controlled heating program, and volatile matter at 900 ± 10 °C for exactly 7 minutes under GB/T 212 and ISO procedures, or 950 ± 20 °C under the ASTM procedure. Deviations from these conditions directly affect the fixed carbon result.

How are results converted between analytical bases?

Multiply the air-dried fixed carbon value by 100 and divide by 100 minus the moisture content to obtain the dry basis value. For the dry-ash-free basis, divide by 100 minus the sum of moisture and ash percentages instead.

What repeatability should be expected?

Under well-controlled conditions, duplicate fixed carbon results typically agree within approximately 0.5 percent absolute, consistent with the combined repeatability of the moisture, ash, and volatile matter determinations. Larger discrepancies indicate a procedural deviation that must be investigated before reporting.

Can instrumental analyzers replace the classical procedure?

Yes, provided the automatic proximate analyzer or TGA system has been validated against the classical gravimetric reference method and calibrated with certified reference materials covering the relevant sample types and concentration ranges.