The widespread occurrence and the diversity of coal for various uses have resulted in the development of numerous classification systems. Indeed, these systems have invoked the use of practically every chemical and physical characteristic of coal. Consequently, it will be useful to review the major classification systems in current use. In addition, several of the lesser known classification systems are also included because they often contain elements of coal terminology that may still be in current use by the various scientific disciplines involved in coal technology, although they may not be recognized as part of a more formalized classification system.
2.3.1 Geological Age
Coals have at various times been classified according to the geological age in which they were believed to have originated.
For example, coal paleobotanists have noted that three major classes of plants are recognizable in coal: coniferous plants, ferns, and lycopods. Furthermore, these plant types are not usually mixed in a random manner in a particular coal, but it has been observed that one particular class of these plant types usually predominates in a coal bed or seam. Thus, because of the changes in character and predominant types of vegetation during the 200 million years or so of the coal-forming period in the history of the Earth, it has often been found convenient and, perhaps, necessary to classify coal according to the age in which the deposit was laid down (Speight, 2013).
It should be noted, however, that deposits of vegetable matter are not limited to any particular era or period, but while these deposits occur even in pre-Cambrian rocks, the plants (i.e., terrestrial plants) that were eventually to become coal were not sufficiently abundant until the Devonian period and it appears that such deposits really became significant during the Carboniferous period.
2.3.2 Banded Structure
Reference has been made elsewhere to the three general classes of coal: banded coal, non-banded coal, and impure coal (White, 1911; Thiessen, 1931) and further discussion is not warranted here. Nevertheless, the fact that many types of coal have a laminated structure consisting of layers which may vary considerably in thickness, luster, and texture has led to an attempt to classify coal by virtue of these differences (Table 2.3).
Since this banded structure persists in all types of coal from lignite to anthracite (although it is most obvious in the bituminous coals), there may, of course, be some merit in such a classification (Stach et al., 1982). However, the failure of such a classification system to take into account the elemental composition of the coal is a serious deficiency. Indeed, a similar statement may be made relative to all of the classification systems that involve the physical appearance of the coal. To all but the well initiated, there is little, if any, difference between one piece of coal and another. Therefore, classification systems which rely on a physical property are not only difficult to rationalize but are even more difficult to accept.
Furthermore, the wide variation in the elemental (ultimate) composition of coals, irrespective of the banded structure, is the major objection to classification by physical methods alone.
2.3.3 Rank
There is a need to accurately describe the various coals in order to identify the end use of the coal and also to provide data which can be used as a means of comparison of the various worldwide coals. Hence, it is not surprising that a great many methods of coal classification have arisen over the last century or so (ASTM D388; ISO 2950; Montgomery, 1978; Speight, 2013).
An early method that attempted a definitive classification of coals on the basis of their composition and heating value was based on the ratio of the fixed carbon to the volatile combustible matter [C/(V.Hc)] (Frazer, 1877, 1879) in which the ratio of the volatile to fixed combustible matter was a logical basis for the classification of coals.
Table 2.3 Classification by banded structure.
| Designation | Thickness of band (mm) | Remarks |
| Coarsely banded | >2 | |
| Finely banded or stripped | 2–0.5 | |
| Microbanded or striated | <0.5 | Bands not visible to naked eye |
| Mixed banded | Both coarse and fine bands | |
| Nonbanded (little or no lamination) | Cannel and boghead coals that break with conchoidal fracture |
Source: Davis et al. (1941).
After various attempts to make the fuel ratio of the different coals fit the descriptions of the varieties of coal, it was concluded that coal could be classified according to the fuel ratio within wide limits, and the following divisions were suggested:
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 the age of the coal. As the process of progressive transformation took place, the heating value and the fixed carbon value of the coal increased and the amount of volatile matter in the coal decreased.
Coal contains significant proportions of carbon, hydrogen, and oxygen with lesser amounts of nitrogen and sulfur. Thus, it is not surprising that several attempts have been made to classify coal on the basis of elemental composition. Indeed, one of the earlier classifications of coal, based on the elemental composition of coal (Seyler, 1899), was subsequently extended (Seyler, 1900, 1931, 1938). This system (Figure 2.2) offered a means of relating coal composition to technological properties and may be looked upon as a major effort to relate properties to utilization. Indeed, for coal below the anthracite rank, and with an oxygen content less than 15%, it was possible to derive relationships between carbon content (C% w/w), hydrogen content (H% w/w), calorific value (Q, cal gm), and volatile matter (VM, % w/w):
Since these relationships only apply to specific types of coal the application is often limited and it is unfortunate that composition and coal behavior do not exist in the form of
