Determination of Fixed Carbon Content in Coal Briquettes (Type Coal): Standards, Methods, and Testin

Fixed carbon content is one of the most important quality indicators for coal briquettes (type coal), directly reflecting calorific value, combustion stability, and burnout performance. Determination of fixed carbon in type coal is performed through proximate analysis in accordance with standards such as ISO 17246, ASTM D3172, and GB/T 212, with fixed carbon calculated by difference from moisture, ash, and volatile matter. This guide explains the testing principles, sample requirements, procedures, calculation bases, and typical applications of fixed carbon determination for coal briquette products.

Technical Overview

Fixed carbon (FC) is the solid, combustible residue that remains in coal briquettes after moisture, ash, and volatile matter are accounted for. In the proximate analysis system, fixed carbon is not measured directly but is calculated by difference using the formula: FC(ad) = 100 − M(ad) − A(ad) − V(ad), where M is moisture, A is ash, and V is volatile matter, all expressed as mass percentages on the same analytical basis. Chemically, fixed carbon is composed primarily of elemental carbon together with small amounts of bound nitrogen, sulfur, and residual inorganic constituents in the char.

For type coal (shaped coal and coal briquettes), fixed carbon content is a decisive quality parameter because it governs heat release rate, flame length, burnout time, and resistance to thermal disintegration. Briquettes produced from anthracite typically show fixed carbon values of roughly 60 to 75 percent on an air-dried basis, while bituminous-based and binder-rich briquettes generally fall in the range of 40 to 60 percent. Because binders such as clay, starch, molasses, sodium silicate, or pitch alter both ash and volatile matter, accurate fixed carbon determination is essential for evaluating the true fuel value of the finished briquette rather than the feed coal alone.

• Fixed carbon is the key index linking briquette composition to calorific value and combustion behavior.
• It is determined by difference from the three measured proximate components.
• Results may be reported on air-dried (ad), dry (d), dry ash-free (daf), or as-received (ar) bases.
• Testing is carried out under ISO, ASTM, and GB standard systems in accredited third-party laboratories.

Testing Samples

Civil Coal Briquettes (Honeycomb and Ovoid Briquettes)

Honeycomb briquettes, ovoids, and nut-shaped briquettes used for household cooking and heating are the most common type coal samples submitted for fixed carbon determination. Representative increments are taken from a production lot, mechanically crushed, reduced by riffling or coning and quartering, and ground to an analysis fineness below 0.2 mm in accordance with sample preparation standards such as GB/T 474 and ISO 13909 series.

Industrial Shaped Coal for Boilers and Gasification

Briquettes and pellets intended for chain-grate boilers, lime kilns, and fixed-bed gasifiers are tested to verify that fixed carbon levels meet feedstock specifications. High fixed carbon is particularly critical for gasification feed, where it controls syngas yield and thermal stability of the briquette bed.

Composite and Biomass-Blended Briquettes

Briquettes formulated from coal fines blended with biomass, semi-coke, or chemical binders require fixed carbon testing to assess how the blend design and binder fraction influence final fuel performance. A minimum laboratory sample of approximately 500 g to 1 kg of crushed material is normally requested so that replicate determinations can be performed.

Testing Items

A complete fixed carbon determination for type coal comprises the following measured and calculated parameters:

• Total moisture, M(t), and air-dried (analytical) moisture, M(ad)
• Ash content, A(ad) or A(d)
• Volatile matter, V(ad) or V(d)
• Fixed carbon, FC(ad), calculated by difference
• Optional supporting items: calorific value, total sulfur, and carbon and hydrogen content for classification purposes

All four proximate components are reported on a consistent basis, and fixed carbon is additionally converted to dry, dry-ash-free, or as-received bases according to client or contract requirements.

Testing Methods

Moisture Determination

A one-gram portion of the 0.2 mm analysis sample is dried at 105 to 110 degrees Celsius in a nitrogen atmosphere to constant mass in accordance with ISO 11722, ASTM D3173, or GB/T 212. The percentage mass loss defines the air-dried moisture M(ad), which is the first term subtracted in the fixed carbon calculation. Total moisture M(t) is determined separately on the as-received sample, typically by a two-stage air-drying and oven-drying procedure following GB/T 211 or ISO 589.

Ash Determination

A weighed one-gram sample is heated in an oxidizing atmosphere in a muffle furnace, with the temperature progressively raised to 815 degrees Celsius for ISO 1171 and GB/T 212, or to approximately 750 degrees Celsius for ASTM D3174. The residue is cooled and weighed, and the ash percentage A(ad) is calculated. Because inorganic binders report entirely to ash, this step strongly influences the final fixed carbon result for type coal.

Volatile Matter Determination

A one-gram sample in a lidded platinum or silica crucible is out of contact with air and heated at 900 plus or minus 10 degrees Celsius for exactly seven minutes in accordance with ISO 562 and GB/T 212; ASTM D3175 employs 950 plus or minus 20 degrees Celsius for coal and coke. The percentage mass loss, corrected for moisture, is the volatile matter V(ad). Strict control of temperature, timing, and crucible lid fit is essential, since volatile matter is the largest subtractive term and the dominant source of dispersion in fixed carbon results.

Calculation and Basis Conversion of Fixed Carbon

Fixed carbon on the air-dried basis is calculated as FC(ad) = 100 − M(ad) − A(ad) − V(ad). Conversion to other bases is performed as follows:

• Dry basis: FC(d) = FC(ad) × 100 / (100 − M(ad))
• Dry, ash-free basis: FC(daf) = FC(ad) × 100 / (100 − M(ad) − A(ad))
• As-received basis: FC(ar) = FC(ad) × (100 − M(t)) / (100 − M(ad))

Repeatability and reproducibility limits for each proximate component are specified in the governing standards, and duplicate determinations are performed until the agreement criteria are satisfied. Accredited third-party laboratories report all measured values, the calculated fixed carbon on the requested basis, the standard methods applied, and any observations on binder-derived ash or unusual volatile matter behavior.

Applications

Fixed carbon determination for type coal supports quality control, trade settlement, and regulatory compliance across the entire briquette value chain. Typical applications include:

• Production quality control of civil honeycomb and ovoid briquettes, verifying compliance with clean-heating fuel requirements such as GB 34170 for civil briquettes.
• Acceptance testing and commercial settlement between briquette producers, traders, and end users, where fixed carbon and calorific value define fuel grade and price.
• Feedstock qualification for industrial boilers, lime kilns, and fixed-bed gasifiers that specify minimum fixed carbon thresholds.
• Research and development of binder formulations and coal-biomass blends, quantifying how each component shifts ash, volatile matter, and fixed carbon.
• Environmental and energy accounting programs, including emission inventories and clean air initiatives that limit volatile matter and sulfur in residential solid fuels.
• Classification of briquette feed coal according to international classification systems based on proximate composition.

FAQ

What is the difference between fixed carbon and total carbon?

Fixed carbon is a proximate analysis parameter representing the combustible solid residue after volatile matter expulsion, and it still contains small amounts of other elements and bound inorganic matter. Total carbon is an ultimate analysis result measured by instrumental combustion analysis and always exceeds fixed carbon in a given coal sample.

Why is fixed carbon determined by difference instead of directly?

During volatile matter testing, the char residue retains minor amounts of volatile species, oxygen, nitrogen, and sulfur that cannot be separated gravimetrically. Calculating fixed carbon by difference is therefore the internationally standardized and reproducible approach used in ISO, ASTM, and GB methods.

How do binders affect the measured fixed carbon of briquettes?

Clay and sodium silicate binders increase ash and thereby lower calculated fixed carbon, while starch and molasses raise volatile matter. Pitch and semi-coke binders tend to increase fixed carbon. Interpretation of results should always consider the binder type and dosage declared by the manufacturer.

What fixed carbon level is expected for good quality coal briquettes?

Anthracite-based honeycomb briquettes commonly show fixed carbon of 60 to 75 percent on an air-dried basis, whereas bituminous-based and biomass-blended briquettes typically range from 40 to 60 percent. Purchase specifications should state both the required value and the reporting basis.

How much sample is needed and how long does testing take?

Around 500 grams to 1 kilogram of representative briquette material is sufficient for full replicate proximate analysis. Routine determination of fixed carbon is normally completed within two to three working days once the sample is received and prepared.

Which standards govern the test?

The principal standards are ISO 17246, ISO 562, ISO 1171, ISO 11722, ASTM D3172 through D3175, and GB/T 212 together with GB/T 211 and GB/T 474 for moisture and sample preparation. Accredited third-party laboratories select the standard system according to the destination market and contractual requirements.