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Coal classification

There are many compositional differences between the coals mined from the different coal deposits worldwide. The different types of coal are most usually classified by rank which depends upon the degree of transformation from the original source (i.e., decayed plants) and is therefore a measure of a coal's age. As the process of progressive transformation took place, the heating value and the fixed carbon content of the coal increased and the amount of volatile matter in the coal decreased. The method of ranking coals used in the United States and Canada was developed by the American Society for Testing and Materials (ASTM) and is based on a number of parameters obtained by various prescribed tests:

• Heating value: The energy released as heat when coal (or any other substance) undergoes complete combustion with oxygen.
• Volatile matter: The portion of a coal sample which, when heated in the absence of air at prescribed conditions, is released as gases. It includes carbon dioxide, volatile organics and inorganic gases containing sulfur and nitrogen.
• Moisture: The water inherently contained within the coal and existing in the coal in its natural state of deposition. It as measured as the amount of water released when a coal sample is heated at prescribed conditions. It does not include any free water on the surface of the coal. Such free water is removed by by air-drying the coal sample being tested.
• Ash: The inorganic residue remaining after a coal sample is completely burned and is largely composed of compounds of silica, aluminum, iron, calcium, magnesium and others. The ash may vary considerably from the mineral matter present in the coal (such as clay, quartz, pyrites and gypsum) before being burned.
• Fixed carbon: The remaining organic matter after the volatile matter and moisture have been released. It is typically calculated by subtracting from 100 the percentages of volatile matter, moisture and ash. It is composed primarily of carbon with lesser amounts of hydrogen, nitrogen and sulfur.

The ASTM ranking system is presented in the table below:

Classification of Coals by Rank ^(a)

Class or Rank	Group	Fixed Carbon (b) (wt % dry mmf)		Volatile Matter (b) (wt % dry mmf)		Gross Heating Value (c) (MJ/kg moist mmf)
		Equal or greater than	Less than	Greater than	Equal or less than	Equal or greater than	Less than
Anthracitic	Metaanthracite (d) Anthracite (d) Semianthracite (d)	98 92 86	98 92	2 8	2 8 14
Bituminous	Low-volatile bituminous (d) Medium-volatile bituminous (d) High-volatile A bituminous High-volatile B bituminous High-volatile C bituminous (e) High-volatile C bituminous (f)	78 69	86 78 69	14 22 31	22 31	32.55 30.23 26.74 24.41	32.55 30.23 26.74
Subbituminous	Subbituminous A Subbituminous B Subbituminous C					24.41 22.09 19.30	26.74 24.41 22.09
Lignite	Lignite A Lignite B					14.65	19.30 14.65
(a) This classification does not include a few coals (referred to as unbanded coals) having unusual physical and chemical properties falling within the fixed carbon and heating value ranges of the high-volatile bituminous and subbituminous ranks. (b) Percentage by weight on a dry and mineral matter free basis (mmf). (c) Gross Heating Value on a moist and mineral matter free basis. Moist refers to the natural inherent water contained in a coal but does not include visible water (if any) on the surface of the coal. Multiply MJ/kg by 430.11 to convert to Btu/lb. (d) Coals containing 69 wt % or more fixed carbon on a dry mmf basis are ranked according to their fixed carbon content regardless of their Gross Heating Value. (e) A high-volatile C bituminous coal that may be agglomerating or non-agglomerating. (f) A high-volatile C bituminous coal that is an agglomerating coal, which means that it tends to become sticky and to cake when heated. The agglomerating character of a coal is determined by heating a sample to 950 °C under certain conditions. If the residue is coherent and supports a weight of 500g without pulverizing, the coal is classified as being agglomerating.

The anthracitic coals, with the highest contents of fixed carbon and lowest contents of volatile material, have the highest rank. The lignite coals,with the lowest contents of fixed carbon and highest contents of volatile matter, have the lowest rank. The bituminous and subbituminous coals (in that order) are ranked between the anthracitic and lignite coal.

As a broad generality, the anthracitic coals have the highest heating value and the lignite coals have the lowest heating values.

Examples of Proximate and Ultimate Analyses

Coal Rank	Proximate Analysis (wt % ar)				Ultimate Analysis (wt % maf)					Net Heating Value (maf) (MJ/kg)
	Fixed carbon	Volatile matter	Moisture	Ash	C	H	O	N	S
Anthracite	81.8	7.7	4.5	6.0	91.8	3.6	2.5	1.4	0.7	36.2
Bituminous	54.9	35.6	5.3	4.2	82.8	5.1	10.1	1.4	0.6	36.1
Subbituminous	43.6	34.7	110.5	11.2	76.4	5.6	14.9	1.7	1.4	31.8
Lignite	27.8	24.9	36.9	10.4	71.0	4.3	23.2	1.1	0.4	26.7
Notes: • wt % = percent by weight     ar = as received     maf = moisture and ash free • C = Carbon     H = Hydrogen     O = Oxygen     N = Nitrogen     S = Sulfur • Multiply Net Heating Values in MJ/kg by 430.11 to convert to Btu/lb.