Application of organic petrology and geochemistry to coal waste studies

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Abstract

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.

This paper is a review on 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.

Highlights

► Oxidation, pyrolysis, and hydropyrolysis dominate in self heating coal waste dumps. ► Various forms of macerals alterations reflect the conditions within dumps. ► Generated chemical compounds migrate within dumps. ► Heating history and properties of organic matter play key role in its alterations.

Introduction

Coal wastes are generated during the preparation of coal seams for exploitation (mining wastes) and as slurry wastes. The origin of mining wastes is connected with preparatory mining works and coal seam exploitation. Individual fragments of these wastes are up to 500 mm. Slurry wastes are generated during coal separation. They are commonly divided into (a) coarse grained (10–250 mm) wastes produced from suspension plants, (b) fine-grained (0.5–30 mm) wastes produced from sedimentation processes, and (c) very fine-grained (< 1 mm) slurries (tailings) resulting from flotation processes (Skarżyńska, 1995).

Constituent elements of wastes vary in size and composition. Mineral matter, the main component, typically occurs as fragments of sandstone, shale, mudstone and, less often, conglomerate and carbonate (Skarżyńska, 1995). Organic matter, usually comprising 3–30% of the waste material (Skarżyńska, 1995), occurs as laminae and lenses of variable length and width, and as dispersed organic matter. When they come in contact with oxygen, these wastes start to oxidize and may generate fires.

In general, two types of fires occur in coal-waste dumps. Exogenic fires are caused by some external source of heat, such as dumped hot slag and badly protected welding works (Urbański, 1983). Endogenic fires are uncontrollable and occur as the end result of self heating processes (Itay et al., 1989, Krishnaswamy et al., 1996a, Krishnaswamy et al., 1996b, Lu et al., 2004, Sensogut and Cinar, 2000, Shi et al., 2005, Singh et al., 2007, Urbański, 1983, Walker, 1999). In this paper, endogenic fires and their origins will be discussed. Although much research (Beamish, 2005, Beamish et al., 2001, Brooks et al., 1988, Clemens and Matheson, 1996, Kaymakçi and Didari, 2002, Krishnaswamy et al., 1996a, Krishnaswamy et al., 1996b, Liu and Zhou, 2010, Walker, 1999) has been conducted, the origin of endogenic fires is not understood completely. However, it is clear that these fires occur in coal wastes that contain organic matter of various rank, such as bituminous coals (Misz et al., 2007, Misz-Kennan, 2010, Misz-Kennan and Fabiańska, 2010, Misz-Kennan et al., 2011a) and anthracites (Ribeiro et al., 2010a, Ribeiro et al., 2010b, Ribeiro et al., 2010c).

Immediately after deposition in coal-waste dumps that are usually sited close to the source mines, the coal wastes undergo a process of oxidation that, in some cases, can lead to self heating and self combustion (Fig. 1). For self combustion to take place, three key conditions are required to co-exist at the same time. These conditions are the presence of organic components and pyrite that easily react with air, easy access for air into the interior of the dump, and the conditions for heat to accumulate (Barosz, 2002, Barosz, 2003, Brooks et al., 1988, Kaymakçi and Didari, 2002, Pone et al., 2007, Szafer et al., 1994, Tabor, 1999, Tabor, 2002, Urbański, 1983).

Two critical stages are involved in self heating. During the first stage no change in temperature is observed (Sawicki, 2004, Walker, 1999). The alterations of both organic and mineral matter happen at low temperatures. Low-temperature oxidation of organic matter is believed to be the precursor of self heating (van Krevelen, 1993). The initial oxidation stage is followed by self heating of the coal wastes during which the temperature rises continuously (Sawicki, 2004, Walker, 1999). The critical temperature is 60–80 °C (Pone et al., 2007, Sawicki, 2004, Sokol, 2005); above this value, the temperature rises rapidly, also described as thermal runaway, until it reaches the self-ignition stage. The temperature of self-ignition of coal is strongly rank dependent and is ~ 150 °C for subbituminous coal, ~ 200 °C for bituminous coal, ~ 250 °C for coke and ~ 300 °C for anthracite (Sawicki, 2004, Sokol, 2005). The burning of wastes can result in a temperature of as high as 1300 °C (Heffern and Coates, 2004, Sawicki, 2004, Sokol, 2005, Querol et al., 2008, Ribeiro et al., 2010a, Ribeiro et al., 2010c).

Self heating and self combustion processes occurring in coal waste dumps are very dynamic. The temperatures fluctuate and the heating centers, which usually are located at a depth of 1.5–2.5 m beneath the dump surface, migrate (Misz-Kennan, 2010, Misz-Kennan and Fabiańska, 2010, Misz-Kennan and Tabor, 2011, Misz-Kennan et al., 2011b, Tabor, 2002–2009). In some cases, the burning waste is visible on the surface (Ciesielczuk, accepted for publication). As organic material is progressively burnt, the centers of heating migrate deeper into the waste dump (Szafer et al., 1994, Urbański, 1983).

Many factors influence self heating within coal waste dumps that may be classified as internal and external factors (Urbański, 1983). Internal factors are those intrinsic to the coal wastes, especially their mineral composition, the maceral composition of the organic matter present, and its rank, and moisture content (Kaymakçi and Didari, 2002, Rosiek and Urbański, 1990, Urbański, 1983, Walker, 1999). External factors are those that influence the filtration properties of dumps and their heat balance. These factors include the shape and height of the dump, the atmospheric conditions, and particle size and distribution (Kaymakçi and Didari, 2002, Krishnaswamy et al., 1996a, Krishnaswamy et al., 1996b, Urbański, 1983, Walker, 1999).

The organic matter in coal wastes is represented by all three groups of macerals, i.e., huminite/vitrinite, liptinite, and inertinite (International Committee for Coal and Organic Petrology, 1998, International Committee for Coal and Organic Petrology, 2001, Sýkorová et al., 2005, Taylor et al., 1998). Macerals of the vitrinite group are, of all three maceral groups, the most prone to oxidation and self heating (Machnikowska et al., 2003, Strumiński and Rosiek, 1990, Taylor et al., 1998). Liptinite macerals also show a propensity to spontaneous combustion (Mastalerz et al., 2010, Misra and Singh, 1994). However, Beamish et al. (2001) have shown that resinite is not prone to these processes. Macerals of the vitrinite and liptinite groups are the more susceptible to spontaneous combustion because they are intrinsically the most reactive macerals; in a heating situation, their temperature increases more sharply and the ignition time is shorter (Mastalerz et al., 2010). Inertinite macerals are widely considered to be the least reactive group of macerals (International Committee for Coal and Organic Petrology, 2001, Taylor et al., 1998) although some believe that fusinite can be the cause of endogenic fires because of its high specific area (Strumiński and Rosiek, 1990).

Individual macerals respond differently to heating when the particles are composed entirely of a single maceral rather than when a particle comprises an association of macerals belonging to various groups (microlithotypes) and/or when organic matter occurs together with minerals as carbominerites (Suárez-Ruiz and Crelling, 2008, Taylor et al., 1998). The fact that vitrite undergoes oxidation before durite is especially marked during the earliest stages of oxidation (Machnikowska et al., 2003, Taylor et al., 1998).

The other important internal factor influencing self heating of coal wastes is organic-matter rank. Coals and coal wastes of every rank undergo self heating — from lignites through bituminous coals (Misz et al., 2007, Misz-Kennan, 2010, Misz-Kennan et al., 2011b, Misz-Kennan and Fabiańska, 2010) to anthracites (Ribeiro et al., 2010a, Ribeiro et al., 2010b). However, in general, the tendency of coal wastes to spontaneous combustion decreases with increasing rank (Beamish et al., 2001, Mastalerz et al., 2010, Rosiek and Urbański, 1990, Walker, 1999). Of all coals, the most prone to spontaneous combustion is bituminous coal (Beamish, 2005, Beamish et al., 2001) which, as it is commonly inferred, reflects the fact that lower-rank coals contain greater amounts of reactive macerals (Mastalerz et al., 2010).

In coal wastes, water is always present in varying amounts. It is a component of some minerals (e.g., clay minerals) and also derives from atmospheric precipitation and from the decomposition of organic and mineral compounds during heating. Moisture participating in chemical reactions enhances self heating. At moisture content greater than 6%, coals are more prone to self heating (Rosiek and Urbański, 1990). However, too much water inhibits the process (Rosiek and Urbański, 1990, Sawicki, 2004, Smith and Glasser, 2005).

The shape and size of a dump are important factors that influence water filtration and heat accumulation. Self heating commonly occurs on the steep, windward slopes of cone-shaped dumps (Misz-Kennan and Tabor, 2011, Moghtaderi et al., 2000, Urbański, 1983).

Coal-waste dumps are a source of numerous gaseous and liquid pollutants that are emitted to the atmosphere and leached to surface and ground waters (Carras et al., 2009, Finkelman, 2004, Grossman et al., 1994, Hower et al., 2009, Nelson and Chen, 2007, O'Keefe et al., 2010, Pone et al., 2007, Querol et al., 2008, Skręt et al., 2010, Stracher and Taylor, 2004, Zhao et al., 2008). It has been estimated that the combustion of 1 ton of mine wastes can generate 99.7 kg CO, 0.61 kg H2S, 0.03 kg NOx, 0.84 kg SO2, and 0.45 kg smoke (Liu et al., 1998). The gaseous pollutants have a strong smell of hydrocarbons and they can be suffocating.

Much research has focused on self heating and self combustion in coal seams (Beamish et al., 2001, Brooks et al., 1988, Carras et al., 2009, Clemens and Matheson, 1996, Hower et al., 2009, Jones, 2000, Liu and Zhou, 2010, Misra and Singh, 1994, Pone et al., 2007, Stracher and Taylor, 2004, Urbański, 1983, Walker, 1999). However, it is only relatively recently that similar attention has been paid to the changes of organic matter caused by these processes in coal wastes. Coal seams and coal wastes differ mainly with regard to amount of organic matter (Liu and Zhou, 2010).

Until relatively recently, efforts concentrated on attempts to explain the mechanisms of self heating, the influencing factors, and on methods for preventing and fighting coal-waste fires (Brooks et al., 1988, Jones, 2000, Kaymakçi and Didari, 2002, Krishnaswamy et al., 1996a, Krishnaswamy et al., 1996b, Mahadevan and Ramlu, 1985, Querol et al., 2011, Singh et al., 2007). Growing knowledge of the types of gases produced during self heating and self combustion, and their potential polluting influence on air, water, and soils prompted increased attention.

Section snippets

Sampling of coal wastes

In order to examine coal wastes, samples of at least 1–2 kg should be collected from about 20 cm beneath the dump surface. Typically, sampling sites are places differing macroscopically in color, hardness, compactness or from places differing in temperature. The most abundant lithologies present in a coal wastes, i.e., coal, sandstone, clay, quartzite, cleat minerals and sulfide minerals, need to be sampled (Querol et al., 2008). Additionally, samples of soils covering the dump may also be

Methods of investigation of coal wastes

In recent years, self heating of coal wastes, its causes, and its detrimental effects on environment have been the focus of much research. The detection of the initial stage of self heating in coal-waste dumps, the emission of greenhouse gases, volatile organic compounds (VOCs) and trace elements to the atmosphere have been investigated. In addition, leaching of inorganic compounds, particularly heavy metals, and soluble organic compounds to soils and groundwater and resultant changes in water

Data obtained from organic petrology and geochemistry and their application in coal waste studies

Oxidation, self heating and self combustion cause the transformation of coal wastes to varying degrees. In terms of their organic-matter content, four types of wastes can be distinguished in all dumps: 1— wastes with organic matter in the form of microscopic constituents and organic compounds; 2 — wastes with organic matter as microscopic constituents but organic compounds are absent; 3 — wastes with organic compounds but lacking microscopic constituents; 4 — wastes lacking both microscopic

Discussion

Petrographic- and geochemical data from coal-waste dumps are mutually complementary. The experience gained investigating dumps located in the USCB suggests that all dumps are different. This is not surprising considering their history and the varied properties of the material dumped in them.

In the literature, the term “coal waste fires” is found. The studies carried out on four dumps in the USCB have shown that the term does not always reflect the nature of the processes taking place within

Conclusions

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

Acknowledgments

The authors are very grateful to two anonymous reviewers for their comments that improved the quality of the paper. We are also very grateful to Tim Horscroft and Dr. Özgen Karacan for proposing to prepare this review paper. We would like to acknowledge Dr. Pádhraig S. Kennan, University College Dublin for his help with the English script.

The research was funded by grant N307 016 32/0493 from the Polish Ministry of Science and Higher Education.

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