Coal-Fired Power Generation Handbook. James G. Speight

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СКАЧАТЬ of the coal-forming plants, particularly details of structure that affect chemical composition or resistance to decay, (iv) the chemical composition of the coal-forming debris and its resistance to decay, (v) the nature and intensity of the peat-decaying agencies, and (vi) the subsequent geological history of the residual products of decay of the plant debris forming the deposits. In short, coal composition is subject to site-specific effects and is difficult to generalize on a global basis (Speight, 2013).

      In summary, there are advantages and disadvantages of both theories. While the coal purist may favor one or the other, there are the pragmatists who will recognize the merits of both theories. Whichever theory is correct (if that is possible) and whatever the origin of coal, there are expected to be differences in properties and behavior.

      Finally, Hilt’s law is a geological term that states the deeper the coal seam, the deeper the rank (grade) of the coal – i.e., anthracite would be expected to lie in deeper buried seams than lignite (Figure 1.1) (Elphick and Suggate, 1964; Suggate, 1974; Ward, 2008). The law holds true if the thermal gradient is entirely vertical, but metamorphism may cause lateral changes of rank, irrespective of depth. Furthermore, increasing depth of burial results in a decrease in the oxygen content of the coal.

      Chemically, coal is a hydrogen-deficient hydrocarbon with an atomic hydrogen-to-carbon ratio near 0.8, as compared to crude oil hydrocarbon derivatives, which have an atomic hydrogen-to-carbon ratio approximately equal to 2, and methane (CH₄) that has an atomic carbon-to-hydrogen ratio equal to 4. For this reason, any process used to convert coal to alternative fuels must add hydrogen or redistribute the hydrogen in the original coal to generate hydrogen-rich products and coke (Speight, 2013).

      The chemical composition of the coal is defined in terms of its proximate and ultimate (elemental) analyses (Chapter 5) (Speight, 2013, 2020). The parameters of proximate analysis are moisture, volatile matter, ash, and fixed carbon while the ultimate analysis (also referred to as the elemental analysis) encompasses the quantitative determination of carbon, hydrogen, nitrogen, sulfur, and oxygen within the coal. Additionally, specific physical and mechanical properties of coal and particular carbonization properties are also determined.

      Figure 1.1 Schematic showing tendency of coal rank to increase with depth of burial*.

      *Numbers are approximate and used for illustration only; peat is included only for comparison and it should not be construed for this diagram that peat is a type of coal.

1.3 Occurrence

      The current estimates for the longevity of each fossil fuel are estimated from the reserves/ production ratio (BP, 2019) which gives an indication (in years) of how long each fossil fuel will last at the current rates of production. The estimates vary from at least 50 years of crude oil at current rates of consumption with natural gas varying upwards of 100 years. On the other hand, 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. But most importantly, 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 just consider the means by which the majority of electricity is produced – almost 50% of the electricity generated in the United States derives from coal (Energy Information Administration, 2007; Speight, 2013).

      Coal is the most abundant and widely distributed fossil fuel in the world and possibly the least understood in terms of its importance to the world economy. Currently, approximately five billion tons are mined in more than 40 countries. Coal provinces (clustering of deposits in one area) occur in regional sedimentary structures referred to as coal basins. More than 2,000 sedimentary, coal-bearing basins have been identified worldwide but less than a dozen contain reserves of more than 200 billion tons.

Although the majority of mined coal continues to be consumed within the country of production, the value of traded coal is increasing. The United States and Australia account for approximately 50% of world coal exports. This figure increases to 70% if exports from South Africa and Indonesia are included. Japan is the largest recipient of exported coal – approximately 25% of the world coal trade – and as such, agreements with coal suppliers and Japan can have a great influence the world coal price. Japan, Taiwan, and South Korea together import approximately 45% of all coal exports and countries of the European Union account for another 30% of total coal exports.

      Coal is burned to produce energy – in the United States, coal still accounts for over 50% of the domestic electricity generating industry requirements, all from domestic production. The European Union, on the other hand, must import approximately 50% of its energy requirements (in the form of oil, gas, uranium, and coal).

      Production of coal is both by underground and open pit mining (Speight, 2013). Surface, large-scale coal operations are a relatively recent development, commencing as late as the 1970s. Underground mining of coal seams presents many of the same problems as mining of other bedded mineral deposits, together with some problems unique to coal. Current general mining practices include coal seams that are contained in beds thicker than 27 inches and at depths less than 1,000 feet. Approximately 90% of all known coal seams fall outside of these dimensions and are, therefore, not presently economical to mine. Present coal mine technology in the United States, for instance, has only 220 billion tons (220 x 10⁹ tons) of measured proven recoverable reserves out of an estimated total resource of three to six trillion tons (3 to 6 x 10¹² tons) tons.

      Problems specific to coal mining include the fact that coal seams typically to occur within sedimentary structures of relatively moderate to low strengths. The control of these host rocks surrounding the coal seams makes excavation in underground mining a much more formidable task than that in hard, igneous rocks in many metal mines. Another problem is that coal beds can be relatively flat-lying, resulting in workings that extend a long distance from the shaft or adit portal (an almost horizontal entrance to mine). Haulage of large tonnages of coal over considerable distance, sometimes miles, is expensive.

      Coal, being largely composed of carbonaceous material can also catch fire, in some cases spontaneously (Speight, 2013). Coal, for the miner, has not been an attractive occupation. Interestingly though, the problem of methane, may in the future become a profitable by-product from closed coals mines. Many countries are reported to millions of cubic feet of coal bed methane trapped in abandoned coal mines.

      As coal contains both organic and inorganic components, run-of-mine coal contains both these components in varying amounts. In many instances coal beneficiation is required to reduce the inorganic matter (ash) so that a consistent product can be more easily marketed. Most coal beneficiation consists of crushing in order to separate out some of the higher ash content, or washing that exploits the difference in density between maceral and inorganic matter.

      Coal is far from being a worn-out faded commodity and offers much promise for future energy supply (Kavalov and Peteves, 2007; Malvić, 2011; Speight, 2011b, 2013, 2020). Much research has gone into improving the efficiency of coal use, especially the implementation of coal-fired plants based on clean coal technology (pressurized fluidized-bed combustion).

However, there is considerable uncertainty related to the actual amount of proved coal reserves in many coal-producing countries СКАЧАТЬ