Misz-Kennan and FabiaÅ„ska reported in 2011 that coal wastes produced during mining activities are commonly deposited in nearby dumps. These wastes mostly composed of minerals and variable amounts (usually 20–30%) of organic matter start to weather immediately after deposition. Oxidation of the organic matter can lead to self heating and self combustion as a result of organic and mineral matter transformations. The degree of alteration depends on the properties of the wastes, i.e., the maceral and microlithotype composition of the organic matter and its rank.
Alteration of wastes also depends on the heating history, i.e., the rate of heating, final heating temperature, duration of heating, and the degree of air access. Although air is probably necessary to initiate and drive the heating processes, these usually take place under relatively oxygen depleted conditions. With slow heating, color of organic matter particles changes, irregular cracks and oxidation rims develop around edges and cracks, and bitumen is expelled. As a result, massive and detritic isotropic and strongly altered organic matter forms. On the other hand, higher heating rates cause the formation of devolatilization pores, oxidation rims around these pores and along cracks, vitrinite-bands-mantling particles, and bitumen expulsions.
Organic compounds generated from the wastes include n-alkanes, iso-alkanes, alkylcyclohexanes, acyclic isoprenoids, mainly pristane, phytane and, in some cases, farnesane, sesquiterpanes, tri- and tetracyclic diterpanes, tri- and pentacyclic triterpanes, and steranes, polycyclic aromatic hydrocarbons (mostly with two- to five rings, rarely six rings), and phenols. The compounds formed change during the heating history. The fact that phenols are found in dumps where heating has not yet been completed, but are absent in those where heating ceased previously suggests the presence of water washing. The organic compounds formed may migrate within the dumps. However, when they migrate out of the dumps, they become a hazard to environment.
The authors reviewed the transformations of organic matter (both maceral composition and reflectance and chemical composition) in coal wastes deposited in coal waste dumps. Immediately after deposition the wastes are exposed to weathering conditions and sometimes undergo self heating processes. Oxidation, self heating, and self combustion are the most important processes that transform both organic and mineral matter in coal wastes deposited in dumps. The scale of these processes depends on the properties of the wastes (maceral and mineral composition, rank of organic matter) and the heating history (heating time, final heating temperature, heating rate, air access). Commonly, the thermal alterations of the wastes take place under oxygen-depleted conditions. Air is necessary to start these processes and intensify them.
Forms of organic matter present in the wastes reflect the heating conditions. Microscopically unaltered organic matter is typical of wastes that did not undergo heating. Wastes heated slowly show cracks in vitrinite, oxidation rims around the external edges of particles and along cracks, and their color is paler. In such wastes exposed to slow heating for a long time, liptinite macerals disappear and organic matter became strongly thermally altered, develops a yellowish color and displays high reflectance. High heating rates cause organic matter to plasticize, reflected in devolatilization pores, bands and plasticized edges. Organic matter is lacking in coal wastes exposed to high temperature for a long time.
A variety of organic compounds are generated during self heating from organic matter in wastes. Organic compounds may be completely absent in wastes containing strongly thermally altered organic matter. The presence of phenols is typical for wastes that are currently undergoing self heating and they are lacking in wastes in which heating ceased years ago. They may have been destroyed by heating or leached elsewhere. The investigations of self heating occurring in coal waste dumps show that all dumps differ but that all have the potential to cause serious environmental problems. Apart from heavy metals, the environmental danger of coal waste dumps lies in the fact that they are a source of VOCs, gas emissions and of mobile and easily-leached organic compounds, many of which would be considered hazardous in one way or another.
Up to now, few studies on the detailed organic geochemistry of self heating coal wastes have been undertaken and those that have mostly concern coal-waste dumps in the Upper Silesian Coal Basin in Poland. They pose a challenge. The mechanisms and processes that lead to the formation of liquid- and semi-liquid pyrolytic products and their chemical composition are very complex and are easily influenced by additional external and internal factors. The latter are dump geometry, original organic-matter rank and oxygen access, self heating temperature and the presence/absence of minerals acting as catalysts in pyrolysis to mention just a few. After the heating and combustion, weathering/oxidation and leaching can disturb the distributions of biomarkers and make definitive conclusions difficult to draw. More geochemical attention needs to be given to the active, complex physical- and chemical entities that coal-waste dumps are, and to the potential environmental- and health hazard they represent.
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