1. The Origin of Coal

[GLOSS]Coal[/GLOSS] is a combustible, dark and compact stratified mass of decayed plant debris interspersed with smaller amounts of inorganic matter and covered with sedimentary rock. As will be seen below, the length of deposition time and the conditions experienced during deposition all affect the quality of the coal. For these reasons coal is included within the group of primary fuels known as fossil sources of energy ([GLOSS]Fossil fuel[/GLOSS]).

2. The Quality of Coal

The chemical and physical properties, which are of significant importance when a particular coal is used in a given combustion environment, depend upon the relative proportions of the different chemical components that were present in the parent plant debris. The nature and extent of the changes that these components have undergone since their deposition, and the nature and quantity of inorganic matter and moisture present also play a significant part in the quality of the final coal. These latter parameters vary widely for the different kinds of coals.

Although land plants first developed in the Lower Palaeozoic Era, it was not until the Upper Palaeozoic Era, particularly the Carboniferous period that adequate plant coverage was established and preserved to produce the necessary accumulations of organic matter to produce significant coal reserves.

These differences give rise to the concept of coal rank, which is used to indicate the stage of alteration, or degree of coalification, obtained by a particular coal; the greater the alteration, the higher the rank of the coal. The nomenclature describing the coal rank contain primarily, [GLOSS]Peat[/GLOSS], [GLOSS]Lignite[/GLOSS] or [GLOSS]Brown coal[/GLOSS], [GLOSS]Sub-bituminous coal[/GLOSS], [GLOSS]Bituminous coal[/GLOSS], [GLOSS]Semi-anthracite[/GLOSS] and [GLOSS]Anthracite[/GLOSS].

3. The age of Coal

The oldest coals are thought to be those of the Middle and Late Devonian age in the Canadian Artic.

From this these periods onwards, throughout the geological column to the Quaternary period, it is generally accepted that coal was formed – see Table 1 below.

Two major episodes of coal formation are currently accepted. The first took place during the late Carboniferous-Early Permian Period. The second occurred during the Tertiary. These coals make up the majority of coal traded on the world market. Coals of the Triassic, Jurassic and Cretaceous periods also make up significant worldwide reserves but are not globally as widespread.

The eras and the associated coal formation are summarised in Table 1 below.

Table 1: Geological ages and main coal formation periods

The inclusion of Peat as a type of coal is almost universal and therefore a class of Fossil Fuel – see CF84.

4. The analysis and classification of Coal

The formation route of a coal, its geographical location, the variation in its composition, its microstructure and its chemical reactions, indicate that a coal rank does not give a full picture of a coal’s properties.

To this end a range of somewhat more detailed analyses were developed to give a more concise picture of a coal’s properties. The most important of these are known as the Proximate and Ultimate analyses.

The former gives a basic outline of the important coal parameters, namely Ash content (inorganic material), Volatile matter, Fixed Carbon (sic) and Moisture on a mass basis.

The latter Ultimate analysis comprises a chemical analysis of a coal’s major constituents i.e. C, H, O, N, S and ash (normally from an associated Proximate analysis, as it is rare to perform one analysis without the other). The Ultimate analysis is also reported on a mass basis.

More advanced analyses are available; examples are discussed in CF48. However, The Proximate and Ultimate analyses still form the basic parametric assays of a coal for trade and combustion assessment purposes.

Although the aforementioned Proximate and Ultimate analyses are used extensively to determine a coal’s potential performance, they often do not give enough detail. This has been borne out many times by the fact that two coals, by inspection of their Proximate and Ultimate analyses appear similar, but behave totally differently when fired.

This can be generally attributed to the fact that The Proximate and Ultimate analyses give only a basic outline of a coal’s potential behaviour, particularly in a combustion environment.

The difference in actual behaviour can be attributed to a coal’s geographic origin, the mode of changes it has undergone and its geological age. Thus, more advanced analytical techniques are used with varying degrees of success to provide a more comprehensive picture of a coal and its potential behaviour. These techniques are introduced in CF48.

Sources

[1] E. Stach et al., Stach’s Textbook of coal petrology, ISBN 3443390684, Borntraeger, Berlin (1975).
[2] D. W. Van Krevelen, Coal – Typology, Chemistry, Physics and Constitution. ISBN 044440600, Elsevier, Amsterdam, 1981.
[3] J. F. Unsworth, D. J. Barratt and P. T. Roberts. Coal Quality and Combustion Performance – An International Perspective. ISBN 0444887032, Elsiver – Amsterdam (1991).
[4] Kirk-Othmer et al., Kirk-Othmer Concise Encyclopaedia of Chemical Technology, ISBN 0471869775. Wiley-Interscience, New York, 1985