Coal-Fired Power Generation Handbook. James G. Speight
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to use each coal type most effectively.
First and foremost, coal is a sedimentary rock of biochemical origin and is formed from the accumulations of organic matter which occurred along the edges of shallow seas and lakes or rivers. Flat swampy areas that are episodically flooded are the best candidates for coal formation. During non-flooding periods of time, thick accumulations of dead plant material pile up. As the water levels rise, the organic debris is covered by water, sand, and soil. The water (often salty), sand and soils can prevent the decay and transport of the organic debris. If left alone, the buried organic debris begins to go through the coal series as more and more sand and silt accumulates above it. The compressed and/or heated organic debris begins driving off volatiles, leaving primarily carbon behind.
In addition to recognizing coal as an organic combustible sedimentary rock, another part of the challenge is to identify the chemical pathways followed during the thermal conversion of coal to liquids or gases (Speight, 2013, 2020). This is accomplished by tracing the conversion of specific chemical functional groups in the coal and studying the effects of various inorganic compounds on the conversion process. Significant progress has been made in this area by combining test reactions with a battery of characterization techniques. The ultimate goal is to relate the structure of the native coal to the resulting conversion products.
There is also a major challenge to the coal analyst and this involves recognizing the heterogeneity of coal – even during the formation of one coal seam, conditions vary and, hence, the types of coal vary depending upon the character of the original peat swamp (Speight, 2013, 2020). Within a swamp some areas might be shallow and other areas deep. Some areas might have woody plants and other areas grassy. The environment might be changing over time, making the bottom (the older part) of the coal seam different to the top (the younger part) of the seam. Varying water level and movement changes the degree of aeration and hence the activity of aerobic bacteria in bringing about decay. The different types of chemical substance present in plants (such as cellulose, lignin, resins, waxes, and tannins) are present in different relative proportions in living woody tissue, in dead cortical tissue as well as in seed and leaf coatings, In addition, these substances show differing degrees of resistance to decay.
Thus, as conditions fluctuate during the accumulation of plant debris, the botanical nature and chemical composition of the material surviving complete breakdown will fluctuate also, not only on a regional basis but also on a local basis. This fluctuation is the origin of the familiar banded structure of coal seams, which is visible to the naked eye, and provides strong support case for the different chemical and physical behavior of coals.
Furthermore, coal seams, sandstone, shale, and limestone are often found together in sequences hundreds of feet thick. The key to large productive coal beds or seams seems to be long periods of time of organic accumulation over a large flat region, followed by a rapid inundation of sand or soil, and with this sequence repeating as often as possible. Such events happened during the Carboniferous Period – recognized in the United States as the Mississippian and Pennsylvanian time periods due to the significant sequences of these rocks found in several states; other coal-forming periods are the Cretaceous, Triassic, and Jurassic Periods (Chapter 1).
To complicate matters even further, coal is also considered (perhaps without sufficient scientific foundation) to be a metamorphic rock – the result of heat and pressure on organic sediments such as peat – but most sedimentary rocks undergo some heat and pressure and the association of coal with typical sedimentary rocks and its mode of formation usually keep low-grade coal in the sedimentary classification system. On the other hand, anthracite undergoes more heat and pressure and is associated with low grade metamorphic rocks and is justifiably considered to be an organic metamorphic rock. Thus, the degree of natural processing results in different quality of coal including such coal types as (i) lignite, which is the least mature of the true coals and the most impure; it is often relatively moist and can be crumbled to a powdery, (ii) subbituminous coal, which is poorly indurated and can be brownish in color, but is more closely related to bituminous coal than to lignite, (iii) bituminous coal, which is the most commonly used coal; it occurs as a black, soft, shiny rock, and (iv) anthracite, which is the highest rank of coal and is considered to be a metamorphic organic rock; it is much harder and blacker than other ranks of coal, has a glassy luster, and is denser with few impurities (Table 2.1) (Chapters 1, 2).
As anticipated because of local and regional variations in the distribution of floral species (i.e., site specificity) the relative amounts can vary considerably from one site to another (Chapter 1). In addition to variations in the types of flora, there is also the potential for regional variations in the physical maturation conditions – these include differences such as variations in the oxygen content of the water as well as acidity/alkalinity and the presence (or absence) of microbial life forms. Variations of the plant forms due to climatic differences between the geological eras/periods would also play a role in determining the chemical nature of the constituents of the mature coal (Chapter 1) (Bend et al., 1991; Bend, 1992; Speight, 2013a).
Table 2.1 Types of coal.
| Rank | Properties |
| Lignite | Also referred to as brown coal; the lowest rank of coal and used almost exclusively as fuel for steam-electric power generation. Jet is a compact form of lignite that is sometimes polished and has been used as an ornamental stone since the Iron Age – since (approximately) 1200 BC. |
| Subbituminous coal | The properties range from those of lignite to those of bituminous coal and are used primarily as fuel for steam-electric power generation. |
| Bituminous coal | A dense coal, usually black, sometimes dark brown, often with well-defined bands of bright and dull material, used primarily as fuel in steam-electric power generation, with substantial quantities also used for heat and power applications in manufacturing and to produce coke. |
| Anthracite | The highest rank; a harder, glossy, black coal used primarily for residential and commercial space heating. |
Thus, it is not surprising that coal differs markedly in composition from one locale to another. Indeed, pronounced differences in analytical properties of coal from one particular seam are not uncommon (Speight, 2013a), due not only to the wide variety of plant debris that could have formed the precursor but also to the many different chemical reactions that can occur during the maturation process. Indeed, the continuation and development of analytical studies related to maturation indices may enable scientists to determine the precise pathways by which maturation occurred (Speight, 2013a and references cited therein).
Since the resurgence of coal science in the 1980s and the need for new and reconstituted environmental legislation, there has been a pronounced resurgence in the attempts to determine the composition of coal through the development of up-to-date analytical methods (Speight, 2015). But it is not obvious that there has been a concomitant increase in understanding and formulating the molecular make-up and molecular structure of coal. Indeed, the concept of a coal structure (often referred to as an average structure for coal) has continued for several decades and it is very questionable, in the minds of many scientists and engineers, as to whether any progress has been made down the highways and byways of uncertainty than was the case some 40 years ago. There are those who can, and will, argue convincingly for either side of this question. Or it might be wondered if (even denied