Development of a petrographic classification system for organic particles affected by self-heating in coal waste. (An ICCP Classification System, Self-heating Working Group – Commission III)
Introduction
The importance of coal for countries reliant on it as the main source of primary energy production and consumption, is, at present, decreasing due to the growing consciousness of environmental consequences of its utilisation and low priced power generation from natural gas. However, according to (IEA, 2017), coal will continue to play a vital role in the global energy mix. Therefore, for the foreseeable future coal will remain, the key fossil fuel source for the supply of electricity and energy in certain countries.
Large industrial-scale coal mining has a number of significant environmental and human health impacts to be dealt with during and many years after exploitation (e.g., Suárez-Ruiz and Crelling, 2008; Carras et al., 2009; Bian et al., 2010; Hendryx, 2015; Woolley et al., 2015; Dulias, 2016; Munawer, 2018; Popa and Predeanu, 2018; Anastasiu et al., 2018; Zheng et al., 2019). One of the key impacts related to coal extraction and processing in coal mining regions world-wide is the phenomenon known as self-heating. Self-heating occurs in both coal mines and coal waste dumps. The process of self-heating takes place in-situ in coal seams and coal heaps (e.g., Stracher, 2007; Stracher et al., 2011, Stracher et al., 2013, Stracher et al., 2015, Stracher et al., 2018 and references therein). It can also occur in coal waste dumps during storage at selected sites for waste disposal.
Coal occurring in coal-mining dumps with other rocks such as claystones, mudstones, and coal-shales is deemed a waste component. These waste are generally accumulated in cone-shaped dumps located adjacent to the mines. In some cases, coal waste undergoes self-heating, causing severe environmental problems related to gaseous emission and changes in the landscape (e.g., Misz-Kennan, 2010; Misz-Kennan and Fabiańska, 2010; Pone et al., 2007; Ribeiro et al., 2010c; Stracher, 2007; Stracher et al., 2011, Stracher et al., 2013, Stracher et al., 2015, Stracher et al., 2018 and references therein).
In recent years various aspects of research on self-heating of coal and coal waste have been undertaken by many scientists all over the world (e.g., Carras et al., 2009; Deng et al., 2015; Garrison et al., 2017; Hooman and Maas, 2014; Hussain and Luo, 2018; Hussain et al., 2018; Kříbek et al., 2017; Kus, 2008, Kus, 2017a, Kus, 2017b; Kus et al., 2007, Kus et al., 2009, Kus et al., 2010a, Kus et al., 2010b; Kus et al., 2017; Laufek et al., 2017; Liang et al., 2018; Liévanos et al., 2018; Lin et al., 2017; Misz et al., 2007; Misz-Kennan, 2010; Misz-Kennan and Fabiańska, 2010, Misz-Kennan and Fabiańska, 2011; Misz-Kennan et al., 2009, Misz-Kennan et al., 2010a, Misz-Kennan et al., 2015b, Misz-Kennan et al., 2015a; Misz-Kennan and Tabor, 2015; Nádudvari and Fabiańska, 2016a; Pallarés et al., 2017; Pone et al., 2007; Ribeiro et al., 2010a, Ribeiro et al., 2010b, Ribeiro et al., 2010c, Ribeiro et al., 2016, Ribeiro et al., 2017; Song et al., 2015; Sýkorová et al., 2016; Zheng et al., 2017). The scientific aspects of self-heating of coal and coal waste also received an attention from the International Committee for Coal and Organic Petrology (ICCP), leading in 2008 to the establishment of the Self-heating of Coal and Coal Waste Working Group within Commission III during the 60th ICCP meeting in Oviedo, Spain (ICCP, 2008). The aim of the Working Group was: (1) to assemble examples of microscopic forms related to self-heating transformations of organic matter in coal waste and coals of various ranks; and, (2) to establish a classification of transformed organic particles in coal waste and coal that would reflect the dynamic conditions in the coal waste dumps and coal seams. It was expected that this information would give an insight into the thermal history of the given coal waste dump or coal seam, allowing for conclusions on the dynamics of the self-heating processes. The Self-heating of Coal and Coal Waste Working Group performed Round Robin Exercises in 2008–2017 period with samples being previously collected in coal waste dumps from Poland, Portugal, China, USA, and South Africa (Fabiańska et al., 2017; Misz et al., 2007; Misz-Kennan, 2010; Misz-Kennan and Fabiańska, 2010; Misz-Kennan et al., 2015a; Ribeiro et al., 2010c). The Round Robin Exercises enabled the examination of various microscopic forms of transformed and newly formed organic matter particles and enabled their classification based on the incident light microscopy approach.
The resultant classification of microscopic forms related to self-heating transformations of organic matter in coal waste presented in this paper is based on several years of research of the Self-heating of Coal and Coal Waste Working Group.
Coal wastes in the coal mining industry are divided into two main groups according to their origin: (1) mining wastes associated with preparation procedures and exploitation of coal seams (particle size commonly >500 mm) and (2) washery waste obtained during coal beneficiation processes. Washery waste are further divided into: (a) coarse-grained washery waste with diameters of 10–250 mm, and (b) fine-grained washery waste with diameter of 0.5–30 mm (Skarżyńska, 1995).
Coal mining waste disposed at dumps are, in general, composed of minerals, mainly clay minerals, sulphides (mainly pyrite), carbonates, and quartz. From the lithological point of view coal waste are usually composed of claystone, mudstone, carbonaceous shale, sandstone, and its coarse grained counterpart, i.e., conglomerate, and rarely, carbonate (Skarżyńska, 1995). The finely dispersed or often layered, laminated organic matter is an important component of the waste rocks and its content is in the range of <40%, usually 7–15% (Skarżyńska, 1995). It often occurs as laminae, lenses, interlayers, or dispersed organic particles among minerals (Misz-Kennan, 2010; Misz-Kennan and Fabiańska, 2011).
After deposition in dumps, coal mining wastes undergo the process of oxidation, which might lead to self-heating and self-combustion. Both, self-heating and self-combustion are dynamic processes and can occur at various intensities in different parts of the waste dumps through many years (e.g., Misz-Kennan, 2010; Misz-Kennan and Fabiańska, 2010). Commonly, hot spots characterised by an elevated or anomalous temperature rise are located 1.5–2.5 m beneath the surface of the dump (Urbański, 1983; Szafer et al., 1994), being able to migrate within the entire dump.
Three factors must be fulfilled at the same time for self-heating to take place: (1) the presence of oxygen-reactive components (e.g., organic matter, pyrite) that may easily react with air; (2) access of air into the interior of a waste dump; and, (3) conditions that enable heat accumulation and retention (Urbański, 1983; Brooks et al., 1988; Kaymakçı and Didari, 2002; Pone et al., 2007). As described by Kus (2017a), self-heating is attributed to the effects of exothermic reactions taking place during low-temperature oxidation (<250 °C), giving rise to the primary source of heat released and spontaneous combustion processes (Huggins and Huffman, 1989; Schmal, 1989; van Krevelen, 1993; Lopez et al., 1998). Self-heating is defined as an onset of exothermic chemical reaction (i.e., oxidation) and a subsequent temperature rise within the combustible material without the action of an additional ignition source (DMT/BAM, 2000). The self-heating process occurs in two stages (Walker, 1999; Sawicki, 2004). During the first incubation stage, oxidation of organic matter takes place and no visible changes can be seen in the waste. This stage is followed by the second stage during which a rapid temperature rise exceeding the range of 60–80 °C may cause the coal waste to undergo self-ignition (Sawicki, 2004; Sokol, 2005). The respective ignition temperature is dependent on the rank of organic matter present in the coal waste and increases with an increase in rank, i.e., it is about 150 °C for subbituminous coal, 200 °C for bituminous coal, 250 °C for coke, and 300°C for anthracite. The temperature of burning waste usually is of several hundred °C, but it can exceed even 1300 °C (Sawicki, 2004; Sokol, 2005; Ribeiro et al., 2010c).
A number of internal and external factors can influence the oxidation, self-heating, and combustion processes (Kus et al., 2017). Internal factors reflect the reactivity of the coal waste and are represented by maceral composition of organic matter, its rank, grain size, as well as moisture and pyrite contents (Walker, 1999; Kaymakçı and Didari, 2002). Among the external factors that affect self-heating are: the storage conditions (the shape of a dump, compaction of waste), climate factors (the direction and speed of wind, air pressure and atmospheric precipitation), (Krishnaswamy et al., 1996a, Krishnaswamy et al., 1996b; Walker, 1999; Kaymakçı and Didari, 2002). In addition, the self-combustion of coal-waste deposits can be related with forest fires since these structures are often forested for remediation purposes and therefore are integrated in forest areas (Ribeiro et al., 2010c). Other factors like lightning strikes, exposure due to erosion can also induce these fires.
During both oxidation and self-heating processes, organic matter in coal waste dumps undergoes oxidative and thermal alteration. The extent of these processes depends on the properties of organic matter, such as maceral composition and rank of organic matter as well as heating history, generally heating rate and time, end heating temperature and presence/absence of air and moisture (Misz et al., 2007; Misz-Kennan, 2010; Misz-Kennan and Fabiańska, 2010, Misz-Kennan and Fabiańska, 2011; Misz-Kennan et al., 2015c). The dynamic nature of self-heating processes is reflected in a wide variety of oxidatively and thermally affected organic particles present in the coal waste. The presence of diverse forms of self-heating related organic particles can be the key to understanding the heating history of the given dump, and potentially indicate subsequent recovery and/or management.
The Self-heating of Coal and Coal Waste Working Group was established during the 60th ICCP Meeting in Oviedo (Spain) in 2008 within the Commission III (ICCP, 2008).
During 2008–2017, a total of six Round Robin Exercises based on sets of photomicrographs were organised and conducted in 2009, 2010, 2012, 2013, 2015, and 2016 (Misz-Kennan et al., 2009, Misz-Kennan et al., 2010a, Misz-Kennan et al., 2010b, Misz-Kennan et al., 2014, Misz-Kennan et al., 2015c, Misz-Kennan et al., 2016). The number of photomicrographs used in the Round Robin Exercises was 30–210. A number of discussions followed each of the performed exercises both during the successive ICCP annual meetings and electronically, leading to an updated methodology of classified oxidatively and thermally affected organic particles. In 2011 and 2014, an intensive discussion on the existing classification and the respective identification procedure of self-heating affected organic particles took place, enabling a thorough revision and evaluation of the methodology applied and the assessment of results.
The Round Robin Exercises involved up to 24 coal and organic petrographers, ICCP members and non-members. The participants belonged to 20 institutions located in 15 countries (Table 1). In consecutive years, 14–23 participants took part in the Round Robin Exercises organised within the Self-heating of Coal and Coal Waste Working Group at the ICCP.
The Round Robin Exercises performed in 2008–2017 were based on photomicrographs of dispersed organic matter enclosed in self-heated coal waste samples provided from various research projects and supplied by both the convenors and the participants of the Self-heating of Coal and Coal Waste Working Group. In each case, the coal waste samples were crushed to <1 mm, embedded in epoxy resin, and polished blocks were prepared according to procedures described in the applied standards (DIN 22020-2, 1998-08; ISO, 7404-2 2005; Taylor et al., 1998). All photomicrographs were taken under incident white light and/or blue light excitation (in fluorescent mode), at 500× and/or 1000× magnifications, using different digital fluorescence cameras and acquired using different image software. During the 2015 and 2016 Round Robin Exercises, random reflectance measurements were taken in accordance to DIN, 22020-5, 2005–02to allow for a better differentiation between unaltered and altered particles and determination of the degree of particle alteration. The photomicrographs were taken at (1) University of Silesia, Faculty of Natural Sciences; (2) Federal Institute for Geosciences and Natural Resources (BGR) in GEOZENTRUM; (3) Faculdade de Ciências da Universidade do Porto; and (4) University of Witwatersrand, School Chemical and Metallurgical Engineering. The results discussed below are related solely to the Round Robin Exercises performed in 2015 and 2016.
All of the conducted Round Robin Exercises focused solely on incident-light microscopy studies of the self-heating related alteration of organic matter, with the inorganic matter being omitted. In the period from 2008 to 2017, the following Round Robin Exercises were performed on photomicrographs taken in reflected white light and/or fluorescence (Table 2):
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2009 Round Robin Exercise was carried out on photomicrographs of self-heated organic matter enclosed in both coal and coal waste. In total, 14 participants were asked to recognize 210 images according to the proposed classification of oxidatively- and thermally-affected organic particles in the provided Excel sheet. Following discussion of the obtained results during the 61st 2009 ICCP Meeting in Gramado, Brazil, it was decided for future Round Robin Exercises to solely focus on organic matter from self-heated coal waste.
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2010 Round Robin Exercise was based on photomicrographs of self-heated organic matter collected only from coal waste. In the exercise, a total of 182 photomicrographs were assessed with 15 participants being asked to apply a modified classification of oxidatelively- and thermally- affected organic particles. The results were presented and discussed during the 62nd 2010 ICCP Meeting in Belgrade, Serbia.
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During the 63rd 2011 ICCP Meeting in Porto, Portugal, an extensive discussion on the modified classification, the nomenclature of self-heated organic matter as well as the mode of identification was conducted. As a result, the classification of oxidatively- and thermally-affected organic particles was revised. The new nomenclature was based on degree of particle alteration. Three groups of particles were established and distinguished: unaltered particles, altered particles, and newly formed particles. Further, altered particles were further categorised based on their appearance (cracked and microfractured), presence of brighter and darker rims, plasticised edges, bands, particles paler in colour), structure (massive, devolatilisation pores), and texture (isotropic, anisotropic). Within the group of newly formed particles, pyrolytic carbon and bitumens were recognized. In addition, the mean random vitrinite reflectance values of the to-be-identified self-heated organic matter in the corresponding photomicrographs were added.
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2012 Round Robin Exercise was based on photomicrographs of self-heated organic matter collected from coal waste. In the exercise, a total of 30 photomicrographs were assessed with 16 participants being asked to apply the revised classification of oxidatively- and thermally-affected organic particles. The results, which were presented and discussed during the 64th 2012 ICCP Meeting in Beijing, China, showed that there was a general need to apply an adequate mode of identification (via cross-hair, arrow, square, etc.) of the particle to be analysed, especially for the categories of structure and texture, as well as, the general appearance (pale in colour particles, cracks and microfractures) instead of solely under the spot of cross-hair.
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2013 Round Robin Exercise was likewise based on photomicrographs of coal waste associated organic matter subjected to self-heating processes. The exercise was performed on 32 photomicrographs with 15 participants applying the same revised classification of oxidatively- and thermally-affected organic particles as in the previous year. The results presented and discussed during the 65sth 2013 ICCP Meeting in Sosnowiec (Poland) were encouraging. A cross-hair applied as the new mode of identification of self-heated organic matter in coal waste was regarded as problematic as the identification of the respective categories was related only to the particle level classification.
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In 2014, taking into account the lessons learned from previous years, a new classification of organic forms in coal waste was prepared. The structure of the new classification schema followed the structure of classification of chars in fly ash (Lester et al., 2010; Suárez-Ruiz et al., 2015, Suárez-Ruiz et al., 2017) and contained six levels: level 1 – nature of particle (organic components, inorganic components), level 2 – optical character (fused, unfused), level 3 – optical structure (dense, porous and vesiculated), level 4 – optical texture (isotropic, anisotropic), level 5 – origin, and level 6 – type of particle (char classification).
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2015 Round Robin Exercise was likewise based on photomicrographs showing organic matter subjected to self-heating processes in coal waste. The exercise was performed on 30 photomicrographs with 23 participants applying the newly established classification of oxidatively and thermally affected organic particles. The new classification contains six levels and its general structure was adapted from classification of chars in fly ashes (Lester et al., 2010; Suárez-Ruiz et al., 2015, Suárez-Ruiz et al., 2017). The discussion during the 67th ICCP Meeting in Potsdam, Germany, gave rise to further improvement of the classification, including detailed descriptions for the new categories and rearrangement of the sub-categories in level 6.
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2016 Round Robin Exercise was based on the same photomicrographs as in the former 2015 Round Robin Exercise with the aim to test the newly introduced changes to the established classification. The results of 16 participants were obtained. The applied terminology was clarified and some categories in the new classification of oxidatively- and thermally-affected organic particles in coal waste were rearranged, but maintaining the same categories. During the 68th 2016 ICCP Meeting in Houston, USA, it was decided that the classification reflects the factual presence of unaltered, altered and newly formed components and that the information should be published.
Section snippets
Petrographic classification system of organic particles in coal waste subjected to self-heating and self-combustion processes
The petrographic classification of altered organic particles as a result of oxidation, self-heating and self-combustion processes, in coal waste was developed following numerous Round Robin Exercises performed during 2008–2017 period, taking into account the ICCP fly ash char classification (Lester et al., 2010; Suárez-Ruiz et al., 2015, Suárez-Ruiz et al., 2017). A wide variety of forms of transformed organic matter related to the optical properties and optical appearance is represented in the
Results and discussion
The present classification of organic particles in coal waste was a subject of debate and discussions during the ICCP Commission III Meetings from 2008 to 2017. As mentioned above, classification criteria established for transformed organic particles in coal waste were successively modified and adopted during the Round Robin Exercises carried out within the Self-heating of Coal and Coal Waste Working Group. The subsequently implemented classification criteria were numerous and included among
Summary and conclusions
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The paper presented results of multiple Round Robin Exercises performed within the Self-Heating Working Group in the Commission III of the International Committee for Coal and Organic Petrology (ICCP) between the years 2009 and 2016. The Round Robin Exercises were conducted on photomicrographed organic matter obtained from sampled coal waste dumps and involved the overall participation of 24 participants from different laboratories worldwide. The aim of the numerous Round Robin Exercises was to
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The samples used for preparation of the Round Robin Exercises within the Self-heating Working Group of the ICCP were obtained within the framework of the grants 2011/03/B/ST10/06331 and N307 016 32/0493 acquired by Magdalena Misz-Kennan from the National Center of Science, Poland. The research work conducted by Jolanta Kus within the Self-heating Working Group of the ICCP was supported and is published with the permission of the Federal Institute for Geosciences and Natural Resources, Hannover,
Authorship statement
All persons who meet authorship criteria are listed as authors, and all authors certify that they have participated sufficiently in the work to take public responsibility for the content, including participation in the concept, design, analysis, writing, or revision of the manuscript. Furthermore, each author certifies that this material or similar material has not been and will not be submitted to or published in any other publication before its appearance in International Journal of Coal
Authorship contributions
M.Misz-Kennan, J. Kus, D. Flores – preparation of the manuscript, tables, figures for publication; in 2008-2016 – organised round robin exercises that lead to elaboration of the classification of morphological forms of organic matter present in self-heated coal wastes.
C. Avila, Z. Büçkün, N. Choudhury, K. Christanis, J.P. Joubert, S. Kalaitzidis, A.I. Karayigit, M. Malecha, M. Marques, P. Martizzi, J.-M.K. O'Keefe, W. Pickel, G. Predeanu, S. Pusz, J. Ribeiro, S. Rodrigues, A.K. Singh, I.
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2021, International Journal of Coal GeologyCitation Excerpt :Organic matter present in the raw coals comprises macerals of vitrinite, liptinite, and inertinite groups following the terminology recommended by ICCP (1998, 2001) and Pickel et al. (2017). For description and identification of altered forms of organic matter in heated coals, the nomenclature employed by Kwiecińska and Petersen (2004) and Misz-Kennan et al. (2020), and discussed in Kus and Misz-Kennan (2017), was applied. Contents of the vitrinite, liptinite, and inertinite maceral groups present in the raw-coal samples are typical for USCB coals (Zdanowski and Żakowa, 1995).