Coal Classification and Analysis: Coal Rank, Classification by Heat value

November 10, 2010 (Coal Geology): Coal Companies don’t sell a black rock. They sell heat values. So, it is extremely important to analyze multiple coal samples for any project to properly classify the coal rank. Coal rank drives the market value of the coal. Often both RAW COAL and WASHED coal are analyzed for various coal properties.

The composition of a coal is usually reported in terms of its proximate analysis and its ultimate analysis:

  • The proximate analysis consists of four items: fixed carbon, volatile matter, moisture and ash, all on a weight percent basis.
  • The ultimate analysis provides an element-by-element composition of the coal’s organic fraction, namely: carbon, hydrogen, oxygen and sulfur, all on a weight percent basis.

Both the proximate and the ultimate analysis may be reported on an as received (ar) basis, a dry (d) or moist basis, an ash-free (af) basis, a mineral matter-free (mmf) basis and various combinations of those bases. For example, an analysis may report the basis to be: as received (ar), dry and ash-free (daf), dry and mineral matter-free (dmmf) or moist mineral-matter free (moist mmf).

Ash and mineral matter are two distinctly different entities. Mineral matter consists of the various minerals contained in the coal. Ash is the inorganic solids remaining after the coal is completely combusted. The ash is usually less than the mineral matter because of the weight changes that take place during coal combustion such as the loss of gaseous carbon dioxide from mineral carbonates, loss of water from silica minerals and loss of sulfur (as gaseous sulfur dioxide) from iron pyrites.

Some examples of proximate and ultimate analyses are given in the table below:

he composition of a coal is usually reported in terms of its proximate analysis and its ultimate analysis:

  • The proximate analysis consists of four items: fixed carbon, volatile matter, moisture and ash, all on a weight percent basis.
  • The ultimate analysis provides an element-by-element composition of the coal’s organic fraction, namely: carbon, hydrogen, oxygen and sulfur, all on a weight percent basis.

Both the proximate and the ultimate analysis may be reported on an as received (ar) basis, a dry (d) or moist basis, an ash-free (af) basis, a mineral matter-free (mmf) basis and various combinations of those bases. For example, an analysis may report the basis to be: as received (ar), dry and ash-free (daf), moist and ash-free (maf), dry and mineral matter-free (dmmf) or moist mineral-matter free (moist mmf).

Ash and mineral matter are two distinctly different entities. Mineral matter consists of the various minerals contained in the coal. Ash is the inorganic solids remaining after the coal is completely combusted. The ash is usually less than the mineral matter because of the weight changes that take place during coal combustion such as the loss of gaseous carbon dioxide from mineral carbonates, loss of water from silica minerals and loss of sulfur (as gaseous sulfur dioxide) from iron pyrites.

Some examples of proximate and ultimate analyses are given in the table below:

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.

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 organic 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 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. The diagram below provides the estimated percentage of the world’s coal reserves for each coal rank. It also provides the typical uses of each coal rank.

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

Source: visit http://en.citizendium.org/wiki/Coal for more.




About Editor
Ankan Basu is a Certified Professional Geologist (CPG) with 10+ years of experience in the field of geology, hydrogeology and geochemistry.

2 Comments on Coal Classification and Analysis: Coal Rank, Classification by Heat value

  1. “maf” is not ASTM standard nomenclature.

    ASTM abandoned this term long ago because it is ambiguous.
    Why [the hell] is the coal industry so stubborn about it?
    Everyone knows what “daf” means. maf is ambiguous.
    It’s ambiguous. It’s ambiguous. It’s unambiguously ambiguous.

  2. Simon Brooksbank // July 5, 2012 at 11:37 am // Reply

    Jeff

    Does MAF = DAF?

    Also note moisture content typo in sub bit. 110.5% H20 should read 10.5%, I think.

    Regds, S

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