Coal-Fired Power Generation Handbook. James G. Speight
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primarily of carbon with variable amounts of hydrogen, nitrogen, oxygen, and sulfur as well as mineral matter and gases as part of the coal matrix. The types of coal, in increasing order of alteration, are lignite (brown coal), subbituminous, bituminous, and anthracite (Chapter 2).
Coal is the most abundant fossil fuel in the United States, having been used for several centuries, and occurs in several regions (Figure 2.1) (Speight, 2013). Knowledge of the size, distribution, and quality of the coal resources is important for governmental planning; industrial planning and growth; the solution of current and future problems related to air, water, and land degradation; and for meeting the short- to long-term energy needs of the country. Knowledge of resources is also important in planning for the exportation and importation of fuel.
Coal begins as layers of plant matter that has accumulated at the bottom of a body of water after which, through anaerobic metamorphic processes, changes in the chemical and physical properties of the plant remains occurred to create a peat-like solid material. It is believed that with further passing of time, lignite is formed from the peat-like product which is metamorphosed (due to thermal and pressure effects) to lignite. With the further passing of time, lignite increases in maturity to subbituminous coal thence to bituminous coal and finally to anthracite.
There are many compositional differences between the coals mined from the different coal deposits worldwide. The different types of coal are typically 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 (Chapter 1) (ASTM D2011). As the process of progressive transformation took place, the properties of the coal changed markedly, leading to the differentiation of coal based on rank (which is often cited incorrectly as carbon content but there are other factors involved in determining coal rank). Nevertheless, changes on properties can cause changes in efficiency of power plant operations.
Figure 2.1 Coal reserves and distribution in the United States (DOE/EIA, 1995).
Coal remains in adequate supply and at current rates of recovery and consumption, the world global coal reserves have been variously estimated to have a reserves/production ratio of at least 155 years. However, as with all estimates of resource longevity, coal longevity is subject to the assumed rate of consumption remaining at the current rate of consumption and, moreover, to technological developments that dictate the rate at which the coal can be mined. Moreover, coal is a fossil fuel and an unclean energy source that will only add to global warming. In fact, the next time electricity is advertised as a clean energy source, consider the means by which the majority of electricity is produced – almost 50% of the electricity generated in the United States is from coal (Speight, 2013, 2020).
However, there are considerations which can impact significantly on efficiency including: (i) moisture content, which influences latent and sensible heat losses, (ii) ash production from the mineral matter, which impacts on heat transfer and auxiliary plant load, (iii) sulfur content, which influences design limits on boiler flue gas discharge temperature, (iv) use of flue gas cleaning technologies, such as selective catalytic reduction (SCR), fabric filtration, flue gas desulfurization (FGD) and carbon dioxide capture, which increase on-site power demand, and (v) use of low-NOx combustion systems, which require excess combustion air and increases unburned carbon.
Thus, a plant designed for high-moisture, high-ash coal, fitted with flue gas desulfurization units and bag filters, and operating with a closed-circuit cooling system, could not be expected to achieve the same efficiency as one without flue gas desulfurization units using high-rank, low-ash, and low-moisture bituminous coal at a coastal site with cold seawater cooling. In most cases, there is little that can be done to mitigate these effects; it is sufficient to recognize that their impact is not necessarily a result of ineffective design or operation, but merely a function of real plant design constraints.
The efficiency of converting coal into electricity is of prime importance since more efficient power plants use less fuel and emit less climate-damaging carbon dioxide. However, with many different methods used to express efficiency and performance, it is often difficult to compare one coal-fired plant with another, even before accounting for any fixed constraints such as coal quality and cooling-water temperature. Guidelines are required that allow the efficiency and emissions of any plant to be reported on a common basis and compared against best practice. Such comparisons start with the classification of coal and, amongst other parameters, allow less efficient plants to be identified and steps taken to improve these plants. Having such information available will allow better monitoring of plant performance and, if necessary, regulate the means by which coal is used for power generation, leading to a more sustainable use of coal.
2.2 Nomenclature of Coal
Coal is classified into three major types, namely anthracite, bituminous, and lignite. However, there is no clear demarcation between them and coal is also further classified into subcategories as semi-anthracite, semi-bituminous, and subbituminous. Anthracite is the oldest coal from geological perspective and it is a hard coal composed mainly of carbon with little volatile content and practically no moisture. Lignite is the youngest coal from geological perspective and it is a soft coal composed mainly of volatile matter and moisture content with low fixed carbon. Fixed carbon refers to carbon in its free state, not combined with other elements. Volatile matter refers to those combustible constituents of coal that vaporize when coal is heated (Speight, 2013).
The different types of coal contain both organic and inorganic phases. The latter consist either of minerals such as quartz (SiO2) and various clay minerals that may have been brought in by flowing water or by wind activity or minerals (such as pyrite, FeS2, and marcasite) that are formed in place (authigenic minerals). The minerals can have a major effect on the efficient use of coal and should be removed before use. Other properties, such as hardness, grindability, ash-fusion temperature, and free-swelling index (a visual measurement of the amount of swelling that occurs when a coal sample is heated in a covered crucible), may affect coal use (especially when coal is used for power generation). Hardness and grindability determine the kinds of equipment used for reducing the size of the coal that enters the combustor (or the gasification unit) and the ash-fusion temperature influences the design of the furnace as well as the operating parameters of the furnace. The free- swelling index provides preliminary information concerning the suitability of a coal combustion and gives an indication of the potential of the coal for coke production, which is another indication of the suitability of the coal for combustion that leads to power generation.
Because of all of these varying properties, the nomenclature of coal – as might be expected – is not straightforward and requires considerable thought to elucidate the precise meaning of some of the terminology (Chapter 1). However, since coal and coal products will play an increasingly important role in fulfilling the energy needs of society it is essential that coal types be understood before use. In fact, future applications will extend far beyond the present major uses for power generation and chemicals production (Speight, 2013, 2020). A key feature in these extensions will be the development of means to provide analytical data that will help in understanding the conversion of coal from its native form into useful gases, liquids, and solids in ways that are energy efficient, nonpolluting, and economical.
The design of a new generation of conversion processes will require the analyst to have a deeper understanding of the intrinsic properties of coal and the ways in which coal is chemically transformed to produce energy under process conditions. Coal properties – such as the chemical form of the organic material, the types and distribution of organics, the nature of the pore structure, and the mechanical properties must be determined for coals of different ranks (or degrees of coalification)