Coal-Forming Processes

The coalification process (coal-forming process) is, simply defined, the progressive change in the plant debris as it becomes transformed from peat to lignite and then through the higher ranks of coal to anthracite (Francis, 1961 Sunavala, 1990). 

Whereas the degree of coaliflcation generally determines the rank of the coal, the process is not a series of regular, or straightforward, chemical changes. Indeed, complexity is the norm insofar as the metamorphosis of the plant debris relies not only on geological time but also on other physical factors, such as temperature and pressure. Thus, the occurrence of a multitude of different and complex chemical reactions (no matter how simple they appear on paper or how simply they occur in laboratory simulations) is inherent in the coaliflcation process. 

Furthermore, in the early stages of coaliflcation, microorganisms may also play a role (possibly an important one) indeed, in a somewhat paradoxical manner, they may interact with the plant debris under aerobic (oxidation) conditions as well as anaerobic (reductive) conditions. 

Laboratory investigations into the nature of coal formation and the chemical transformations involved have been carried out for many years and perhaps a brief discussion is warranted here not only to indicate the complexity of the issue of coal-forming processes but also to add a caution of the speculation involved in this aspect of coal science. But flrst, a general comment, it is generally 

of course, is on the presumption that a high temperature is equivalent to, or a substitute for, geological time. Be that as it may, and as well as it may sean, increases in the temperature at which a reaction occurs not only iuCTeases the rate of the reaction (for the nonchemists, an increase in the temperature of 10°C, IST, usually doubles the rate of a chemical reaction) but also is more than capable, even likely, of changing the chemistry. Thus, caution is advised when applying these principles (no matter how sound the logic may appear) to coal formation. 

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Wang Yun-Quan Meng Fan-Shun. 1994. The influence of sedimentary environment of the Yima formation on coal-forming processes in the Yima coal field, Hean. Sedementary Facies and Palaeogeography, 14(l) 24-33. 

Indeed, in order to emphasize the complexity of coal and the complexity of the coal-forming processes, some consideration of the potential precursors are worthy of consideration. However, there is also cautious assumption that there are little, if any, differences (natural selection notwithstanding) between the natural product chemicals known now and those in existence at the time of deposition of the coal beds perhaps a reasonable assumption, but it is fraught with uncertainties. 

The deposits in the shrinkage cracks or cleats, which may, for example, be kaolinite, calcite, gyp-snm, or pyrite, are usually thick surface deposits and may not always be present and were presnmably deposited later in the coal-forming process. On the other hand, the larger adventitious bodies of foreign matter in the coal (clay veins, sandstone intrusions, washouts, and the like) may be the result of rather unusual geological conditions dnring and after the time the coal was laid down (Chapters 3 and 4). 

The red tetrathiomolybdate ion appears to be a principal participant in the biological Cu—Mo antagonism and is reactive toward other transition-metal ions to produce a wide variety of heteronuclear transition-metal sulfide complexes and clusters (13,14). For example, tetrathiomolybdate serves as a bidentate ligand for Co, forming Co(MoSTetrathiomolybdates and their mixed metal complexes are of interest as catalyst precursors for the hydrotreating of petroleum (qv) (15) and the hydroHquefaction of coal (see Coal conversion processes) (16). The intermediate forms MoOS Mo02S 2> MoO S have also been prepared (17). 

Partial oxidation of heavy Hquid hydrocarbons requires somewhat simpler environmental controls. The principal source of particulates is carbon, or soot, formed by the high temperature of the oxidation step. The soot is scmbbed from the raw synthesis gas and either recycled back to the gasifier, or recovered as soHd peUitized fuel. Sulfur and condensate treatment is similar in principle to that required for coal gasification, although the amounts of potential poUutants generated is usually less (see Coal conversion processes, gasification). 

The formation of mineral coal is not an instantaneous process, but is an extremely lengthy one, spread over an extended period. Millions of years ago, when the temperature was moderate and rainfall was heavy, vegetation was quite thick, especially in the low-lying areas of the Earth. Coal-forming plants probably grew in swamps, and as the plants died, their debris gradually formed a thick layer of matter on the swamp floor. Over a prolonged period, this matter hardened into a substance called peat. The peat deposits became buried under sand or other mineral matter. As the mineral matter accumulated, some of it turned... 

Acidity problems tend to be localized in bodies of water near industrial operations that discharge acidic materials or near active or abandoned mines. Acids form when water flows through all kinds of mines, including coal mines and mines for the extraction of various metals. Probably the most common acid-forming process in such cases occurs when iron pyrites (FeS2), found in coal seams and in many metal mines, is oxidized by atmospheric oxygen or oxygen dissolved in water to produce iron(II) sulfate ... 

Based on the above description of the coal combustion process several conclusions become apparent. First, the type and amount of ash accumulated during coal combustion greatly depends on the mineralogy of the coal being used, the combustion process, and the presence of emission control devices. Secondly, the chemical forms in which elements are found in ash are affected by coal combustion process variables such as combustion temperature and the mode of combustion (e.g., pulverized-coal fired, fluidized bed, cyclone, stoker). Lastly, the amount of CCPs accumulated by power plants is predominantly a consequence of the presence of emission control devices. The latter is supported by the fact that the total amount of CCPs produced in the US has increased significantly since the use of electrostatic precipitators became prevalent in the early 1970s (Simsiman et al. 1987). 

In the coal liquefaction process, scales and sludges, formed on the reactor walls, cause severe problems, which limit long operation. The crystal growth of chlorides and carbonates appears to trigger their formation, trapping the other solids. Hence major problems may come from cations, which react with carbon dioxide or chloride ions to form insoluble crystalline solids. The intrinsic solids may not initiate the problem. 

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