HOW TO DETERMINE COKING COAL QUALITY By Ricardus A. Entrepreneur...

HOW TO DETERMINE COKING COAL QUALITY
By Ricardus A. Entrepreneur

Coking Coal quality characteristics refer to either the composition of the coal or properties of the coal that directly influence its ability to perform the function required in the end use of the coal. Using Proximate Analysis, coal is comprised of Fixed Carbon, Volatile Matter, Ash and Inherent Moisture.
Fixed Carbon and coal Volatile Matter can be further divided into the elements Carbon, Hydrogen, Nitrogen, Sulphur and Oxygen using Ultimate Analysis.

Vitrinite Reflectance (R0max or Rvmax) is an accurate measure of coal rank and is also used to evaluate coking coal blends. Coke quality increases with coal rank. Older, higher rank coals tend to have a lower inherent moisture content due to their longer exposure to heat and geological pressure. Similarly, volatile matter tends to decrease as the rank of a coal increases. By contrast, the fixed carbon content and calorific value of coal tends to rise with the rank of the coal.

The Crucible Swelling Number (CSN) indicates the capacity of the coal to expand when subjected to a standardized heat and is used to evaluate the coking properties of the coal. Coking coal is determined primarily by its coking properties. The Crucible Swelling Number ranges from 0 to 9 and tends to be positively related to coke quality. That is, a Crucible Swelling Number of 9 implies the coal has very good coking properties.

The Fixed Carbon (FC) content of the coal influences the specific energy content or calorific value of the coal. The Calorific Value is the amount of heat released by the complete combustion of the coal under specified conditions and is usually measured in kilocalories per kilogram (kcal/kg). The Fixed Carbon content is measured as a percentage of the air dried coal sample.

Volatile Matter (VM) is the proportion of the air dried sample that is released in the form of gas or vapor during a standardized heating test. Volatile Matter is a positive influence on the ability of the coal to sustain combustion, but is inversely related to coke yield. A high Volatile Matter content — generally exceeding 30 percent (> 30%) of the air dried coal — increases the potential risk of spontaneous combustion.

The Total Moisture (TM) content refers to the water in coal, and is the sum of Inherent (or air dried) Moisture and excess moisture. Excess moisture can be removed by coal preparation. Transport costs increase directly with moisture content. Extremely low Total Moisture increases the risk of spontaneous combustion.

Ash is the residue remaining after the complete combustion of all coal organic matter and oxidation of the mineral matter present in the coal. Ash is therefore the incombustible material present in the coal and is measured as a percentage of the air dried coal sample. A higher Ash content results in both higher transport and handling costs per unit of energy contained in the coal and waste that, in general, requires disposal. For coking coals, Ash remains in the coke and becomes incorporated in blast furnace slag. Coal contracts typically contain penalties for ash outside the specified range.

Sulphur (TS) increases operating and maintenance costs by causing fouling and corrosion of metal surfaces. Coal contracts typically contain penalties for Sulphur outside the specified range.

Fluidity of Coking Coals increases up to a point of Maximum Fluidity when subjected to increasing temperature. Some coking coal End Users pay a premium for increased fluidity despite the fact that there is little evidence of a positive correlation with the quality of the coke produced.

Coke Strength After Reaction (CSR) is strongly positively related to coke quality. For the purpose of blast furnace applications, the coal used must be able to form large angular coke that retains its form despite constant abrasion and collision in the blast furnace.