Determination of Fixed Carbon Content in Coal Water Slurry: Methods, Standards and Applications

Fixed carbon content is one of the most important quality parameters of coal water slurry (CWS), directly reflecting the combustible solid fraction that governs heating value, combustion stability, and gasification efficiency. Professional determination of fixed carbon in coal water slurry is performed through gravimetric proximate analysis in accordance with recognized standards such as the GB/T 18856 series, GB/T 212, ASTM D3172 and ISO 17246, with the fixed carbon value calculated by difference from moisture, ash and volatile matter. This article explains the technical principles, sampling requirements, analytical procedures, result expression, and typical applications of fixed carbon testing for coal water slurry.

Technical Overview

Coal water slurry (CWS) is a pumpable, liquid-like fuel consisting of finely ground coal particles, water, and small quantities of chemical additives, typically containing 60–70% dry coal solids by mass. It is widely used as a substitute for fuel oil in industrial boilers, utility boilers, and as feedstock for entrained-flow gasifiers. Because the energy delivered by the slurry is concentrated in its solid combustible matter, the fixed carbon content is a key indicator of fuel quality and economic value.

In fuel analysis, fixed carbon is defined as the solid, carbon-rich combustible residue remaining after the volatile matter has been driven off under standardized high-temperature conditions, leaving aside the incombustible ash. Unlike moisture, ash, and volatile matter, fixed carbon is not measured directly by a single instrumental step; instead, it is obtained by difference from the complete proximate analysis. A higher fixed carbon content generally corresponds to a higher calorific value, better burnout characteristics, and more stable flame behavior, while a lower fixed carbon content may indicate dilution by ash or excessive volatile-rich, low-rank coal blending.

For coal water slurry, the determination presents specific technical considerations: the very high inherent water content of the slurry, the need to distinguish between results expressed on an as-received slurry basis and on a dry coal basis, and the influence of slurry concentration on reported values. Reliable results therefore depend on standardized sampling, careful sample preparation, and strictly controlled furnace conditions in an accredited third-party laboratory or a well-equipped in-house quality control facility.

Testing Samples

Commercial Coal Water Slurry Fuel for Boilers

Samples taken from delivered slurry tankers, storage tanks, or pipeline transfer points at power plants, district heating plants, and industrial steam users. These samples are tested to verify contract specifications and to confirm combustion performance before the fuel enters the boiler.

Gasification Feedstock Slurry

Coal water slurry prepared as feedstock for entrained-flow gasifiers, where the fixed carbon and total carbon conversion directly affect syngas yield, oxygen consumption, and cold gas efficiency. Feedstock slurries are usually produced from bituminous coal or blended coals with carefully controlled ash and fixed carbon levels.

Slurries from Different Coal Ranks

Laboratory-prepared or pilot-plant slurries based on bituminous coal, sub-bituminous coal, anthracite, or coal blends. Fixed carbon determination helps characterize how coal rank affects slurry energy density, since anthracite-based slurries typically show higher fixed carbon than slurries from lower-rank coals.

Research and Formulation Development Samples

Experimental slurries produced during additive screening, particle size distribution optimization, and rheology studies. Fixed carbon data allow researchers to compare energy content on a consistent basis while varying solids concentration and grind fineness.

Testing Items

The core deliverable of the analysis is the fixed carbon content of the coal water slurry, supported by the three directly measured components of proximate analysis:

• Fixed carbon content (FC), calculated by difference
• Total moisture / slurry water content (M), determined gravimetrically
• Ash content (A), determined by controlled incineration
• Volatile matter content (V), determined by standardized pyrolysis at 900 °C
• Slurry concentration (dry solids percentage), derived from moisture determination
• Optional supporting parameters: calorific value, total sulfur, and elemental carbon for correlation studies

Results can be reported on multiple bases depending on client requirements: as-received slurry basis, air-dried basis, dry basis (d), or dry-ash-free basis (daf). For commercial settlement, the dry-basis fixed carbon of the coal solids is most commonly specified, while combustion calculations often use the as-received slurry basis value.

Testing Methods

Method Principle

Fixed carbon is determined by the classical proximate analysis approach, in which the three other components—moisture, ash, and volatile matter—are each measured gravimetrically under precisely defined thermal conditions, and fixed carbon is then obtained by difference:

• FC (as-received) = 100 − M(total) − A − V
• FC (dry basis) = 100 − A(d) − V(d)
• FC (dry, ash-free basis) = 100 − V(daf)

This method is applicable to coal water slurry after appropriate sample preparation and is the basis of the GB/T 18856 series for CWS testing, GB/T 212 for coal proximate analysis, ASTM D3172 for the proximate analysis of coal and coke, and ISO 17246 for coal proximate analysis.

Sample Preparation

The slurry sample is homogenized by mechanical agitation. A portion is weighed immediately for total water determination; a further portion is dried at low temperature, crushed, and ground to pass a specified sieve (typically 0.2 mm) to produce the analysis sample for ash and volatile matter testing, following the sample preparation procedures of the GB/T 18856 series or ISO standards for solid mineral fuels.

Moisture Determination

Total water content of the slurry is determined by drying a weighed portion at 105–110 °C to constant mass, in air or in a nitrogen atmosphere for oxidation-sensitive coals, with the mass loss taken as water. The dry solids concentration of the slurry is the complement of the water content. Reference methods include GB/T 212, GB/T 18856 series, ASTM D3173, and ISO 11722.

Ash Determination

A weighed analysis portion is heated progressively in an electric muffle furnace and incinerated to constant mass at 815 ± 10 °C. The residue mass, expressed as a percentage of the test portion, gives the ash content. Reference methods include GB/T 212, ASTM D3174, and ISO 1171.

Volatile Matter Determination

A weighed sample, covered with a lid to exclude air, is placed in a muffle furnace at 900 ± 10 °C for exactly seven minutes without stirring or intermediate weighing. The mass loss, corrected for moisture content, is the volatile matter. Reference methods include GB/T 212, ASTM D3175, and ISO 562.

Calculation and Quality Control

After the three determinations are completed, fixed carbon is calculated on the required reporting basis and cross-checked for mass balance closure. Because fixed carbon is obtained by difference, its uncertainty combines the precision of the moisture, ash, and volatile matter results. Reputable laboratories therefore apply strict repeatability control, calibrated balances, verified furnace temperature profiles, and certified reference materials to keep repeatability of fixed carbon typically well within about one percentage point, in line with the precision statements of the referenced standards.

Typical Instrumentation

• Electric muffle furnaces with digital temperature control (815 °C and 900 °C zones)
• Analytical balances with 0.1 mg readability
• Forced-air or nitrogen-purged drying ovens (105–110 °C)
• Platinum or ceramic crucibles with matched lids for volatile matter testing
• Optional automatic proximate analyzers with thermogravimetric detection for high-throughput testing

Applications

• Commercial quality control and settlement: verification of fixed carbon specifications in CWS purchase contracts between producers, logistics providers, and end users
• Boiler operation support: input data for combustion calculations, thermal efficiency testing, fuel blending optimization, and burner adjustment
• Gasification plant feedstock qualification: assessment of slurry energy density and expected carbon conversion for entrained-flow gasifier operation
• Fuel research and development: formulation of high-concentration slurries, evaluation of new coal sources, and study of additive performance on an energy-consistent basis
• Environmental and regulatory compliance: supporting data for energy accounting, emissions inventories, and carbon utilization assessments
• Process benchmarking: comparison of slurry quality across production batches, plants, and coal suppliers

FAQ

Why is fixed carbon calculated instead of measured directly?

Fixed carbon is, by definition, the combustible solid residue remaining after moisture, ash, and volatile matter are accounted for. No single selective weighing step can isolate it, so the internationally accepted approach is to determine the other three parameters gravimetrically under standardized conditions and obtain fixed carbon by difference using the 100 percent mass balance.

What is the difference between fixed carbon on a slurry basis and on a dry coal basis?

On an as-received slurry basis, the 30–40% water in the slurry is included in the mass balance, so the fixed carbon value is correspondingly lower. On a dry coal basis, the water is excluded and only the solid coal matter is considered, giving a value that characterizes the coal itself. Both results are useful: the slurry basis reflects delivered energy per ton of fuel, while the dry basis enables fair comparison between coal sources.

Which standards govern the determination?

The determination is typically performed following the GB/T 18856 series for coal water slurry test methods, together with GB/T 212 for coal proximate analysis. International equivalents include ASTM D3172 with its supporting moisture, ash, and volatile matter practices (ASTM D3173, D3174, D3175), ISO 17246 for coal proximate analysis, ISO 11722 for moisture, ISO 1171 for ash, and ISO 562 for volatile matter.

How should slurry samples be taken and preserved?

Representative samples should be drawn from well-mixed tanks or flowing pipeline streams using standardized sampling procedures, then sealed in airtight containers to prevent water evaporation, which would bias the moisture and concentration results. Homogenization before subsampling is essential because coal particles settle rapidly.

What factors most commonly cause erroneous results?

Frequent error sources include water loss or evaporation before weighing, incomplete homogenization of settled slurry, incorrect furnace temperature or heating time in volatile matter testing, oxidation of the sample during ashing, and reporting-basis confusion. Strict adherence to standardized conditions and participation in interlaboratory comparison programs minimize these risks.

How long does a typical determination take?

A complete proximate analysis with fixed carbon calculation is usually completed within one to two working days for a prepared sample, allowing for constant-mass checks at each gravimetric step. Automatic thermogravimetric proximate analyzers can shorten the turnaround for routine quality control applications.