Understanding Coal Specifications: Ash, Sulfur, Moisture and Fixed Carbon

A coal specification is useful only when each value has a unit, analytical basis, test method and acceptance meaning. Ash, sulfur, moisture and fixed carbon describe different properties. None alone proves that a coal will meet the receiving plant's requirements.

Ash and sulfur

Ash is the residue from the specified laboratory combustion procedure. The percentage helps quantify non-combustible residue, while ash chemistry and fusion behaviour can be important to the process. A low ash percentage does not establish a particular composition or guarantee freedom from slagging.

Sulfur should have a stated basis and maximum limit. The plant must evaluate it with its operating and environmental requirements. “Low sulfur” is a marketing description until the buyer defines the limit and receives representative test results. Rank and origin do not remove the need for testing.

Total moisture and analysis moisture

Total moisture describes water in the received sample under the agreed method. Analysis moisture relates to the prepared sample used for laboratory measurements. They should not be silently substituted for one another. Moisture affects delivered energy per tonne and the relationship between as-received, air-dried and dry results.

Contracts should specify the sampling point and procedure because transport and storage can alter moisture. A source COA may not describe exactly the condition at destination. Agree in advance which result controls settlement and how discrepancies will be investigated.

Volatile matter and fixed carbon

Volatile matter is measured under a defined heating procedure. It is relevant to combustion and coal classification, but is not a direct statement of all gases released under actual plant conditions. Fixed carbon in proximate analysis is calculated by difference after moisture, ash and volatile matter on the same basis. It is not synonymous with elemental carbon from ultimate analysis.

Turn results into enforceable limits

Separate minimum, maximum, typical and rejection values. State whether price adjustments apply inside an acceptable band and whether exceeding a limit allows rejection. Include particle size and any process-specific properties omitted from the basic proximate analysis.

Require a COA that identifies the lot, sampling date, sampling method and laboratory. For a large or recurring transaction, agree independent inspection and sealed retained samples. A certificate with no traceable lot may be useful background but weak evidence for the contracted delivery.

Can fixed carbon be calculated from mixed bases?

No. Components must refer to the same basis before a difference calculation is meaningful.

Read the calorific-value guide and use the coal RFQ checklist to specify the missing fields.

Turn the guidance into a comparable offer

Ask the proposed supplier to separate confirmed information from open items. Record the product, source country, analytical basis, shipment quantity and named delivery point on the same comparison sheet. An offer that omits one of these fields should be clarified before it is ranked against complete offers. Do not treat an indicative market description as an accepted consignment specification.

For a recurring programme, agree how changes in source or quality will be approved and how each delivery will be sampled. Keep technical approval separate from payment and freight approval, then reconcile them in the purchase contract. This helps prevent a commercially attractive offer from bypassing the receiving plant's acceptance conditions.

Related coal sourcing

• Coal
• Steam Coal

Duara Group assesses sourcing requirements from Kenya for African and international buyers. Origin, availability, assay, price and delivery terms require transaction-specific confirmation. Request a bulk coal quotation with your specification, metric-ton quantity, destination and delivery window.

Sources and scope

Sources reviewed on 26 October 2026. Technical guidance is a purchasing framework; the signed specification and applicable current rules control each transaction.

• SGS: analytical bases and calculations
• USGS: coal properties and analysis