*Volatile matter - dry mineral matter free basis. In coal, those products, exclusive of moisture, given off as gas and vapor determined analytically.
A three-digit code number is then employed to express the coal in terms of this classification system. The first figure indicates the class of the coal, the second figure indicates the group into which the coal falls, and the third figure is the subgroup (Table 2.8). For example, by this system a 523 coal would be a class 5 coal with a free swelling index of 2.5 to 4 and an expansion (dilatation) falling in the range 0 to 50.
Table 2.8 Classification of coal by the International System.
2.3.6 Coal as an Organic Rock
Coal, in the simplest sense, consists of vestiges of various organic compounds that were originally derived from ancient plants and have subsequently undergone changes in the molecular and physical structure during the transition to coal (Chapter 1) (Speight, 2013 and references cited therein).
Some mention will be made of the nomenclature and terminology of the constituent parts of coal, i.e., the lithotypes, the macerals, and the microlithotypes (Stopes, 1919, 1935; Spackman, 1958; Speight, 2013). It is unnecessary to repeat this discussion except to note that this particular aspect of coal science (petrography) deals with the individual components of coal as an organic rock whereas the nomenclature, terminology, and classification systems are intended for application to the whole coal. Other general terms that are often applied to coal include (i) coal rank and (ii) coal grade, which are employed to describe the particular characteristics of coal.
The kinds of plant material from which the coal originated, the kinds of mineral inclusions, and the nature of the maturation conditions that prevailed during the metamorphosis of the plant material give rise to different coal types. The rank of a coal refers to the degree of metamorphosis; for example, coal that has undergone the most extensive change, or metamorphosis, has the highest rank (determined from the fixed carbon or heating value (Chapters 1, 5, 6). The grade of a coal refers to the amount and kind of inorganic material (mineral matter) within the coal matrix (Chapter 1). Sulfur-containing organic structures are, perhaps, the most significant of the non-hydrocarbon constituents because of the potential to generate sulfur dioxide during combustion.
2.3.7 A Hydrocarbon Resource
There have also been attempts to classify coal as one of the hydrocarbon-type resources of the Earth (Figure 2.3) but the term hydrocarbon is used too loosely and extremely generally since coal is not a true hydrocarbon and contains atoms (nitrogen, oxygen, and sulfur) other than carbon and hydrogen – a hydrocarbon (by the true chemical definition) contains carbon and hydrogen only (Fessenden and Fessenden, 1990). Even crude oil, despite the errors of the general nomenclature, is not an assemblage of true hydrocarbon derivatives (because of the occurrence of nitrogen-containing, oxygen-containing, sulfur-containing, and metal-containing species) (Speight, 2014).
Figure 2.3 Classification of the various hydrocarbon and hydrocarbon-producing resources (Speight, 2013).
2.4 Coal Petrography
Coal is an extremely complex heterogeneous material that requires several parameters for chemical and physical characterization (Chapters 5, 6) – one single parameter is completely lacking for the true characterization of coal. However, the standard method of characterizing the organic (maceral) and the inorganic (mineral) constituents of coal is known as coal petrography (Speight, 2013 and references cited therein).
Coal petrography is the description of the components of coal as studied macroscopically and in thin and polished sections under the microscope. As the use of such methods became more common, coal petrography became concerned with the composition, structure, and origin of coals. In other words, it developed into the field of coal petrology. The principal task of coal petrology is the study of the elementary components of the organic matter of coal beds. These components are the remains of plants, which, in the process of biochemical decomposition into the peat stage of coal formation, lost or preserved to some extent their form and structure. A distinction is made here between the tissue elements and the matrix, which, taken together, constitute the components of coal. Also, macro- components, such as vitrain and fusain, are distinguished from micro-components, such as spores and cuticles.
Coal petrography employs various methods of investigation. In methods of optical microscopy, for example, the specimen may be viewed in air or in immersion; both transmitted-light and reflected-light methods are used, and the light may be ordinary, polarized, or ultraviolet. Specimens may be separated into groups of components of similar density in heavy liquids (mixtures, for example, of carbon tetrachloride tribromomethane). Depending on the source material and the conditions of the transformation of this material (in the so-called peat stage of coal formation) the micro-petrographic components are classified into three basic groups (vitrinite or gelinite, fusinite, and liptinite) (Table 2.9). The composition of the source plants and the quantitative relations between micro-petrographic components determine the genetic types of coal, which are characterized by definite chemical and technological properties within each stage of coalification.
Thus, coal petrology is the science in which coal type is related to the type of plant material in the peat and the extent of the biochemical and chemical alteration. Type can be assessed in terms of variety of petrographic analysis. Coal petrology is concerned with the origin, composition and properties of the distinct organic and inorganic components of different coals. To date, the principal practical application of coal petrology has been in the specification and selection of coals for carbonization (Speight, 2013 and references cited therein).
From the data obtained from coal petrology, the rank (defined by vitrinite reflectance) and composition (maceral proportions) of coal can be determined. Coal macerals are optically homogenous, discrete microscopic constituents of the organic fraction of coal, and they constitute the building blocks
