Designation
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 simple relationships. In fact, classification by means of elemental composition alone is extremely difficult. Nevertheless, the attempt by Seyler to classify coal should not be ignored or discredited as it offered an initial attempt at an introspective look at coal behavior.
The American Society for Testing and Materials has evolved a method of coal classification over the years; it is based on a number of parameters obtained by various prescribed tests for the fixed carbon value as well as other physical properties which can also be related to coal use (Table 2.4, Table 2.5). In the ASTM system (ASTM, D388), coal is classified based on certain gradational properties that are associated with the amount of change that the coal has undergone while still beneath the earth. The system uses selected chemical and physical properties that assist in understanding how the coal will react during mining, preparation and eventual use.
Figure 2.2 Classification by the Seyler System.
Thus, coal can be divided into four major types; (i) anthracite coal, (ii) bituminous coal, (iii) subbituminous coal, and (iv) lignite coal which show considerable variation in properties (Table 2.5). For the purposes of this text, peat is not classified as being a member of the coal series and, therefore, in this book peat is not included in this system of coal classification (Chapters 1, 2).
Anthracite is coal of the highest metamorphic rank; it is also known as hard coal and has a brilliant luster, being hard and shiny. It can be rubbed without leaving a familiar coal dust mark on the finger and can even be polished for use as jewelry. Anthracite coal burns slowly with a pale blue flame and may be used primarily as a domestic fuel.
Bituminous coal ignites relatively easily coal burns with a smoky flame and may also contain 15-20% w/w volatile matter. If improperly burned, such as a deficiency of oxygen, bituminous coal is characterized with excess smoke and soot. It is the most abundant variety of coal, weathers only slightly, and may be kept in open piles with very little danger of spontaneous combustion, although there is evidence that spontaneous combustion is generally considered to be a factor of extrinsic conditions such as the mining and storage practices and the prevalent atmospheric conditions (Chapter 4) (Berkowitz and Schein, 1951; Berkowitz and Speight, 1973; Chakravorty, 1984; Chakravorty and Kar, 1986; Speight, 2013). Bituminous coal is used primarily as fuel in steam-electric power generation, with substantial quantities used for heat and power applications in manufacturing and also to produce coke.
Table 2.4 Coal classification according to rank (ASTM D388).
| Class and group | Fixed carbona (%) | Volatile mattera (%) | Heating valueb(btu/lb) |
| Anthracitic | |||
| 1. Meta-anthracite | >98 | <2 | – |
| 2. Anthracite | 92–98 | 2–8 | – |
| 3. Semianthracite | 86–92 | 8–14 | – |
| Bituminous | |||
| 1. Low-volatile bituminous coal | 78–86 | 14–22 | – |
| 2. Medium-volatile bituminous coal | 69–78 | 22–31 | – |
| 3. High-volatile A bituminous coal | <69 |
>31
|