Coal-Fired Power Generation Handbook. James G. Speight
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there is a need to define coal in terms of a distinct molecular structure (Speight, 2013, 2020). In fact, this is a challenge for the analyst insofar as it is a challenge that may never be revolved. On the positive side, indications can be given by tracing the possible chemical precursors in the original mess of pottage that can lead to a variety of hydrocarbon and heteroatom chemical functional groups in coal and which can be determined by application of appropriate standard test methods.
2.3 Classification Systems
Coal (unless otherwise specified, the term is used generically throughout the book to include all types of coal) is a black or brownish-black organic sedimentary rock of biochemical origin which is combustible and occurs in rock strata (coal beds, coal seams) and is composed primarily of carbon with variable proportions of hydrogen, nitrogen, oxygen, and sulfur. Coal occurs in seams or strata and is a fossil fuel formed in swamp ecosystems where plant remains (often referred to as plant detritus) were preserved by water and by mud from oxidation and biodegradation (Speight, 2013, 2020).
The plant material (vegetal matter) is composed mainly of carbon, hydrogen, oxygen, nitrogen, sulfur, and some inorganic mineral elements. When this material decays under water, in the absence of oxygen, the carbon content increases. The initial product of this decomposition process is known as peat (which is not classed as a type of coal) and the transformation of peat to lignite is the result of pressure exerted by sedimentary materials that accumulate over the peat deposits. Even greater pressures and heat from movements of the crust of the Earth (as occurs during mountain building), and occasionally from igneous intrusion, cause the transformation of lignite to bituminous and anthracite coal (Chapter 1) (Speight, 2013, 2020).
Coal classification, which is based on coal properties and utilization, is at least 200 years old and was initiated by the need to establish order to the confusing terminology of different coals. However, in spite of the need to bring order to the confusion, several types of classification systems arose which are essential for the buyer to know and are (i) the scientific systems, which are concerned with the origin, composition, and fundamental properties of coal, and (ii) the commercial systems, which focused on market issues such as technological properties, and (iii) the suitability of coal for certain end uses (utilization). The latter systems were designed to assist coal producers and users, with many being specific to the properties and use of coal in a particular country.
It is helpful for the scientist or engineer working with coal to understand the various systems so that, for example, the scientist or engineer working with coal to produce electricity can immediately understand reference to coal type made by their counterparts working in other countries who use different classification systems.
Coal occurs in different forms or types (Table 2.1). Variations in the nature of the source material and local or regional variations in the coalification processes cause the vegetal matter to evolve differently. Thus, various classification systems exist to define the different types of coal. In addition to being defined as a sedimentary rock, coal has also been considered to be a metamorphic rock, which is the result of heat and pressure on organic sediments such as peat. However, the discussion in favor of coal as a sedimentary rock is because most sedimentary rocks undergo some heat and pressure and the association of coal with typical sedimentary rocks and the mode of formation of coal usually keeps low-grade coal in the sedimentary classification system. Anthracite, on the other hand, undergoes more heat and pressure and is associated with low-grade metamorphic rocks such as slate and quartzite. Subducted coal may become graphite in igneous rocks or even the carbonate rich rocks (carbonatites).
The degree of metamorphosis results in differing coal types, each of which has different quality. However, peat is not actually a rock but no longer just organic matter and is a major source of energy for many non-industrialized countries. The unconsolidated plant matter is lacking the metamorphic changes found in coal. Thus, coal is classified into four main types, depending on the amount of carbon, oxygen, and hydrogen present. The higher the carbon content, the more energy the coal contains. Thus, as geological processes increase their effects over time, the coal precursors are transformed over time into coal, each coal type having different properties from the other types (Table 2.1).
Coal classification systems are based on the degree to which coals have undergone coalification. Such varying degrees of coalification are generally called coal ranks (or classes). The determination of coal rank has a number of practical applications such as the definition of the coal properties. The properties include the amount of heat produced during combustion, the amount of gaseous products released upon heating, and the suitability of the coals for producing coke.
Some of the classification systems are currently in use in several countries and include: the American Society for Testing and Materials (ASTM) system (used in North America), the National Coal Board (NCB) system (UK), the Australian system, and the German and International Systems (for both hard and soft coal) classifications (Carpenter, 1988; Speight, 2013). Each system involves use of selected coal properties (chemical, physical, mechanical and petrographic) as the determining factors leading to classification of coal but the variation of these properties can lead to a poor fit of a coal within the relevant classification system. It is not the purpose to enter into such details here but to give a general description of the ASTM system and the classification systems that relate to the United States through coal trade and other aspects of coal technology.
Generally and from the mineralogical aspects, coal can be defined (sometimes classified) as an organic sedimentary rock-like natural product. The resemblance of coal to a rock is due to the physical nature and composition of the coal as well as the inclusion of a natural product term in a general definition and is not an attempt to describe coal as a collection of specific, and separate, natural product chemicals. Such chemical species are universally recognized (for example, see Fessenden and Fessenden 1990; Ramawat and Merillon, 2013) and are more distinct chemical entities with more specific use than coal.
However, the designation of coal as a natural product is no stretch of the truth and arises because of the oft-forgotten fact that coal is the result of the decay and maturation of floral remains (which are natural product chemicals) over geologic time. Indeed, the organic origins and the organic constituents of coal are too often ignored (Speight, 2013). But there are more appropriate definitions of coal and the manner by which this complex natural product can be classified.
The need to classify coal arose in order to describe each individual sample of coal in terms that would accurately (even adequately) depict the physical and/or chemical properties (Kreulen, 1948; van Krevelen and Schuyer, 1957; Francis, 1961). Consequently, the terminology that came to be applied to coal essentially came into being as part of a classification system and it is difficult (if not impossible) to separate terminology from classification to treat each as a separate subject. This is, of course, in direct contrast to the systems used for the nomenclature and terminology of crude oil, natural gas, and related materials (Speight, 2011, 2014, 2020). Indeed, coal classification systems stand apart in the field of fossil fuel science as an achievement that is second to none insofar as the system allows classification (on the basis of standardized parameters) of all of the coals that are known.
Of particular importance here is the carbon content of the coal, which is part of the basis for the modern classification system of coal, Thus, whereas crude oil does not exhibit a wide variation in carbon content – all of the crude oil, heavy crude oil, extra heavy crude oil and tar sand bitumen (sometimes incorrectly referred to as natural asphalt) that occur throughout the world fall into the range of 82.0 to 88.0% w/w – coal, on the other hand exhibits a wide variation in carbon content – all of the coal types in the world have carbon contents varying over the range 75.0 to 95.0% w/w (Speight, 2013, 2014, 2020). While little room is left for the design of a standardized system of crude oil classification and/or nomenclature based on carbon content, the door is wide open for coal to be classified using carbon content as one of the parameters.
However,