Dry ash-free fixed carbon analysis (FCdaf) is a fundamental coal and coke quality test that quantifies the non-volatile combustible fraction of a solid fuel after excluding moisture and mineral ash, providing a normalized indicator of coal rank, combustion behavior, and commercial value. This article explains the definition and calculation of fixed carbon on a dry ash-free basis, the applicable international and national standards such as ISO 17246, ASTM D3172 and GB/T 212, typical sample types, analytical procedures, basis conversion formulas, and common industrial applications.
Technical Overview
Fixed carbon (FC) is the solid, carbon-rich residue that remains after the volatile matter of a solid fuel has been expelled under standardized high-temperature conditions. In proximate analysis, moisture (M), ash (A) and volatile matter (V) are determined directly, and fixed carbon is obtained by difference: FC = 100 − M − A − V, calculated on the same reporting basis. Because fixed carbon is a derived value, the accuracy of each constituent measurement directly influences the reported FC result.
The dry ash-free basis (daf) removes the diluting effects of both inherent moisture and mineral matter, so that fixed carbon and volatile matter together describe only the organic portion of the fuel. On this basis, the calculation simplifies to FC(daf) = 100 − V(daf), where V(daf) is the volatile matter content reported dry ash-free. Fixed carbon on a dry ash-free basis rises systematically with coal rank, from approximately 50–60% in lignites to more than 90% in anthracites, making it a core parameter for coal classification, combustion modeling, gasification assessment and commercial specification.
- Fixed carbon is a proximate-analysis parameter calculated by difference, not measured directly.
- On a dry ash-free basis, FC(daf) = 100 − V(daf), because moisture and ash terms are excluded by definition.
- FC(daf) is a primary rank indicator and a key input for fuel ratio (FC/V) evaluation in combustion engineering.
- Governing standards include ISO 17246, ISO 562, ISO 1171, ISO 11722, ASTM D3172/D3173/D3174/D3175, GB/T 212 and JIS M 8812.
Testing Samples
Coal Samples by Rank
Accredited third-party laboratories routinely perform dry ash-free fixed carbon analysis on anthracite, bituminous coal, sub-bituminous coal and lignite. High-rank coals yield FC(daf) values above 90% for anthracite and roughly 55–90% for bituminous coal, while low-rank coals typically fall between 50% and 65%. The parameter is used extensively in coal classification systems, including GB/T 5751, which uses dry ash-free volatile matter as a principal classification input.
Coke and Semi-Coke
Metallurgical coke, foundry coke, semi-coke (char) and calcined petroleum coke can be characterized using the same proximate framework. Coke exhibits very high fixed carbon on a dry ash-free basis, generally above 96–98%, because coking drives off most volatile matter. These results support furnace charge design, quality verification and price settlement in metallurgical applications.
Coal Blends and Washed Products
Blended coals, washed coals, coal slurries and coal-water slurries are tested to verify blend consistency and specification compliance. Because washing changes the ash content, reporting on a dry ash-free basis allows the organic fuel quality of different washed products to be compared on an equivalent footing.
Other Carbonaceous Materials
The method is also applied to anthracite-based carbon feedstocks, activated carbon precursors, electrode paste materials and coal–biomass blends, where the non-volatile carbon fraction determines yield and performance in downstream carbonization or activation processes.
Sample Preparation Requirements
Analysis samples are air-dried and crushed to a nominal particle size of 212 µm (0.2 mm) following ISO 18283, ASTM D2013 or GB/T 474. A laboratory sample of approximately 50–100 g is normally sufficient, and homogeneous division is critical because test portions are typically only about 1 g. Proper preparation minimizes segregation of bright and mineral-rich particles, which is a major source of scatter in FC results.
Testing Items
A complete dry ash-free fixed carbon determination includes the supporting proximate parameters from which FC is derived:
- Fixed carbon content, dry ash-free basis — FC(daf), the primary deliverable.
- Volatile matter on air-dried, dry and dry ash-free bases — V(ad), V(d), V(daf).
- Air-dried (inherent) moisture — M(ad), with total moisture M(t) where required for as-received reporting.
- Ash yield on air-dried and dry bases — A(ad), A(d).
- Fixed carbon on air-dried, dry and as-received bases for comparison and contract reporting.
- Fuel ratio FC/V, a key combustion characteristic indicating ignition and burnout behavior.
- Basis conversions among as-received (ar), as-determined/air-dried (ad), dry (d) and dry ash-free (daf) results.
Testing Methods
Gravimetric Proximate Analysis Procedure
Moisture is determined by heating a 1 g test portion at 105–110 °C to constant mass, in nitrogen for readily oxidized coals per ISO 11722 or in a drying oven per ASTM D3173 and GB/T 212. Ash is determined by controlled incineration of the sample to 815 °C per ISO 1171 and GB/T 212, or to 750 °C per ASTM D3174, with a staged heating program that avoids flame loss and ignition of volatile products. Volatile matter is determined by heating a covered crucible at 900 °C ± 10 °C for exactly 7 minutes per ISO 562 and GB/T 212, or at 950 °C ± 20 °C per ASTM D3175, excluding air so that only thermally liberated volatiles, not combusted carbon, are accounted for. Fixed carbon is then calculated by difference on the desired reporting basis.
Automated Thermogravimetric Analysis
Modern automatic proximate analyzers based on thermogravimetry perform moisture, volatile matter and ash determinations sequentially on a single crucible with automated gas switching between nitrogen and oxygen or air, in accordance with ASTM D5142 and ISO 17246. These systems improve throughput and reproducibility for high-volume coal inspection while remaining traceable to the classical gravimetric reference methods.
Calculation and Basis Conversion
The standard calculation chain from as-determined (air-dried) results to the dry ash-free basis is as follows:
- FC(ad) = 100 − M(ad) − A(ad) − V(ad)
- FC(d) = 100 − A(d) − V(d)
- FC(daf) = 100 − V(daf)
- V(daf) = V(ad) × 100 / (100 − M(ad) − A(ad))
- FC(daf) = FC(d) × 100 / (100 − A(d))
Worked example: for a coal with M(ad) = 1.6%, A(ad) = 12.4% and V(ad) = 26.0%, FC(ad) = 60.0%. Converting, V(daf) = 26.0 × 100 / (100 − 1.6 − 12.4) = 30.2%, and therefore FC(daf) = 100 − 30.2 = 69.8%. This demonstrates why identical coals can appear very different on an as-determined basis while being directly comparable on a dry ash-free basis.
Quality Assurance and Precision
Reliable FC(daf) results depend on calibrated analytical balances readable to 0.1 mg, controlled furnace temperatures and heating rates, pre-conditioned crucibles and standardized cooling and weighing procedures. Accredited third-party laboratories run duplicate determinations and certified reference materials with every batch. Repeatability limits for the underlying determinations are typically on the order of 0.3–0.8 percentage points absolute, depending on constituent level and the specific standard applied. For coals with significant carbonate mineral content, corrections for carbon dioxide release during ashing and volatile matter determination are applied as specified in the relevant standards.
Applications
Dry ash-free fixed carbon results are used throughout the coal value chain, from exploration and mining to trade settlement and end-use engineering:
- Coal classification and rank assessment: FC(daf) and V(daf) are primary rank descriptors in classification frameworks such as GB/T 5751 and ISO 11760.
- Utility boiler design and combustion optimization: the fuel ratio FC/V correlates with ignition delay, flame stability, burnout time and unburned carbon in fly ash, guiding boiler selection and operational tuning.
- Coal trading and contract compliance: dry ash-free parameters provide an ash- and moisture-neutral basis for comparing cargoes and enforcing contractual specifications.
- Coke making and metallurgy: fixed carbon levels in coal blends and cokes support oven charge design, coke yield estimation and blast furnace injection programs.
- Gasification and chemical processing: FC(daf) informs carbon conversion efficiency, syngas yield modeling and reagent demand in gasifier design.
- Carbon materials manufacturing: the non-volatile carbon fraction governs char yield, activation performance and product quality for activated carbon, electrode carbon and graphite precursors.
FAQ
What is fixed carbon on a dry ash-free basis?
It is the percentage of the organic (moisture-free, mineral-matter-free) portion of a solid fuel that remains as non-volatile combustible residue after volatile matter is expelled. It is calculated as FC(daf) = 100 − V(daf).
How is fixed carbon different from total carbon?
Fixed carbon is a proximate-analysis parameter obtained by difference, whereas total carbon is an ultimate-analysis parameter measured directly by combustion and infrared or thermal conductivity detection per ASTM D5373, ISO 17247 or GB/T 30733. Total carbon includes the carbon released within volatile matter, so it generally exceeds fixed carbon for the same sample.
Why report on a dry ash-free basis?
Moisture and ash vary independently of the organic quality of the fuel due to weathering, washing and geological factors. Removing both terms allows fuels of different ash and moisture levels to be compared purely on their organic composition, which is essential for rank classification and fair commercial evaluation.
What are typical FC(daf) values?
Lignite and sub-bituminous coals generally show FC(daf) of about 50–65%, bituminous coals about 55–90%, and anthracite above 90%. Metallurgical coke typically exceeds 96–98% on a dry ash-free basis.
Which standards govern the analysis?
The principal standards are ISO 17246 with ISO 562, ISO 1171 and ISO 11722; ASTM D3172 together with D3173, D3174 and D3175, or D5142 for instrumental methods; GB/T 212 and GB/T 474 in the Chinese system; and JIS M 8812. Sampling standards such as ISO 18283 and ASTM D2013 govern sample preparation.
How accurate are the results?
Since fixed carbon is calculated by difference, its uncertainty combines the uncertainties of moisture, ash and volatile matter determinations. Under accredited laboratory conditions with proper quality control, repeatability is typically within ±0.5–1.0 percentage points, which is adequate for classification, combustion engineering and commercial settlement purposes.