Coal-Fired Power Generation Handbook. James G. Speight
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before launching into a discussion of the means by which coal is classified, it is perhaps necessary to become familiar with the nomenclature (the equivalent terms nomenclature and terminology are used synonymously here) applied to coal even though this terminology may be based on a particular classification system or may simply be described as the grade of coal.
Briefly, coal grade is a term used to indicate the value of coal material as determined by the amount and nature of ash yield and the sulfur content following the complete oxidation of the organic fraction. Calorific value is one of the principal measures of the value of coal as a fuel and is directly influenced by mineral impurities. Coal mineralogy is not only important to combustion characteristics, but also as materials that can be passed on to secondary products such as metallurgical coke. Alkalis-containing compounds derived from coal minerals can contribute to excessive gasification of coke in the blast furnace and attack of blast furnace refractories, whereas phosphorus and sulfur from coal minerals can be passed on to the hot metal, thus reducing the quality of the coal for steelmaking.
For example, within certain parts of the industry, coal is classified (or described) as two main categories: (i) high-grade coal and (ii) low-grade coal, which refers to the energy content of the coal. Among high-grade coals are anthracite and bituminous coal while low-grade coals include subbituminous coal and lignite. On the other hand, the grade of Indian coal is based on the calorific value of the coal (Table 2.2).
In the United States and many other countries the grade is determined mainly by the sulfur content and the amount and type or amount of mineral ash produced during combustion rather than on a single property. Such properties are not always of use or recommended for estimation of coal resources – definitive statements related to the sulfur content and the chemical types of the sulfur derivatives as well as the mineral matter (that produces the mineral ash that is formed during combustion) ash are preferable. Statements indicating high, medium, or low grade are inappropriate (and may even be subject to misinterpretation without supporting analytical data. Another means of classifying coal grade is using the mineral matter content or the propensity for ash production. Thus: 50% w/w yield of ash limit for coal, 80% w/w ash is the limit for middlings formed during coal washing, and 100% w/w/ yield applies to for shale.
This general system of nomenclature offers little, if anything, in the way of a finite description of the various coals. In fact, to anyone but an expert (who must be presumed to be well versed in the field of coal technology) it would be extremely difficult to distinguish one piece of coal from another. Therefore, the terminology that is applied to different coals is much more logical when it is taken in perspective with the classification system from which it has arisen and becomes much more meaningful in terms of allowing specific definitions of the various types of coal that are known to exist.
Table 2.2 Grades of coal as designated by the Indian System.
| Grade | Calorific value (kcal/kg) |
| A | >6200 |
| B | 5600-6200 |
| C | 4940-5600 |
| D | 4200-4940 |
| E | 3360-4200 |
| F | 2400-3360 |
| G | 1300-2400 |
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.