1 Introduction

Spontaneous combustion causes an increase in the temperature of a thermally segregated accumulation of coal or other combustible materials due to the chemical reactions between this material and oxygen (Davidson 1990). The low-temperature oxidation exists when the heat produced is absorbed by the surrounding environment (Kim and Sohn 2012). Spontaneous combustion of coal will eventually occur if nothing is done to minimize it (Onifade and Genc 2018c; Phillips et al. 2011). The liability of coal to spontaneous combustion is a function of the coal properties, geological, environmental and mining factors, which are in turn functions of various contributory factors (Smith and Glasser 2005).

The recent challenge faced by a number of coalfields in South Africa is spontaneous combustion of coals and coal-shales (Onifade et al. 2018; Onifade and Genc 2018d, e; Fig. 1), which frequently causes loss of revenue and at several intervals it has led to loss of precious resources, increase in production cost, loss of properties, and an increase in rehabilitation cost. Sedimentary rocks such as coals and coal-shales contain different volumes of organic and inorganic matter in which pore spaces are embedded in the solid together with carbon-rich matter (Alpern and de Sousa 2002; Dullien 1979; Onifade and Genc 2018e). This renders the rock to be permeable to water and air, and with the increased surface area, the organic materials may have reactive oxidation sites (Dullien 1979). Studies reported by Mastalerz et al. (2010), Restuccia et al. (2017) and Rumball et al. (1986) indicated that for coal-shale to experience self-heating, it may contain varying proportions of sulphur (forms of sulphur), organic matter, reactive nature and rank of associated coal. Research on spontaneous combustion of coal has been examined by Beamish and Blazak (2005), Falcon (2004), Genc and Cook (2015), Gouws and Wade (1989a, b), Kaymakci and Didari (2002), Panigrahi and Sahu (2004), Panigrahi and Sexana (2001) and etc. However, a detailed investigation on the relationships between intrinsic properties of coals and coal-shales towards spontaneous combustion is limited. There is limited information to compare and contrast the intrinsic properties and spontaneous combustion liability of coal-shales in relation to coals (Onifade et al. 2018; Onifade and Genc 2018d, e). For this study, selected experimental tests on coals and coal-shales intrinsic properties (moisture, ash, volatile matter, ash, maceral compositions, total sulphur and forms of sulphur and etc.) were carried out according to the procedures of the American Society for Testing and Materials (ASTM) and International Organization for Standardization (ISO). A broad understanding of the inherent characteristic of coal-shales in relation to coal properties may be used to provide reliable information on the causes of spontaneous combustion of coals and coal-shales.

Fig. 1
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a Spontaneous combustion of highwall and b effects of shale spontaneous combustion at at Khwezela Mine (Bokgoni Pit), Witbank, South Africa

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2 Materials and methods

2.1 Sample collection and preparation

Samples of coal and coal-shale from four open cast mines in the Witbank Coalfield, South Africa using the ply sampling method were experimentally examined. A full description of sample and collections and preparation for both petrographic and chemical analyses tests is extensively described in the studies reported by Onifade and Genc (2018d, e).

2.2 Wits-Ehac tests

The Wits-Ehac Index has been developed to measure the spontaneous combustion liability of coal since the late 1980s and has been widely used in South Africa [Eroglu (1992), Genc et al. (2018), Genc and Cook (2015), Gouws and Wade (1989a, b), Onifade et al. (2018), Onifade and Genc (2018a, b, d), Uludag et al. (2001) and Wade (1989)]. Full details of the Wits-Ehac experimental procedure (Fig. 2a) are extensively explained in the studies reported by Wade et al. (1987) and Onifade and Genc (2018e). The index is calculated from the formula shown in Eq. (1) and MS Excel is used to calculate the stages and generates the thermogram (Fig. 2b).

$$ {\text{Wits{-}Ehac}}\;{\text{Index}} = \left( {{\text{Stage}}\;{\text{II}}\;{\text{slope/XPT}}} \right)*500 $$
(1)
Fig. 2
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a Schematic of the Wits-Ehac test apparatus setup (Wade et al. 1987). b Typical differential thermogram of a coal sample

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3 Intrinsic properties and spontaneous combustion liability of coal and coal-shale samples

This study used the set criterion (Table 1) documented by Onifade and Genc (2018e) to evaluate and compare the linear relationships between the intrinsic properties and spontaneous combustion liability of coals and coal-shales. Full details of the statistical analysis is reported by Onifade and Genc (2018e). The data set is divided into dependent (Wits-Ehac Index) and independent (intrinsic properties obtained from proximate, ultimate, total and forms of sulphur and petrographic analysis) variables to enable simple interpretation and analyses. The R-squared values and the correlation coefficients were used to determine the trends of relationships between the intrinsic properties and the liability index (Tables 2, 3, 4, 5, 6).

Table 1 Criterion for factors influencing spontaneous combustion liability of coals and coal-shales (Onifade and Genc 2018e)
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Table 2 Relationships between independent (proximate and ultimate analysis, wt%-ad) and dependent variables for the coal samples
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Table 3 Relationships between independent (petrographic analysis, vol%) and dependent variables for the coal samples
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Table 4 Relationships between independent (total reactive maceral, total maceral and total mineral matter-vol%) and dependent variables for the coal samples
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Table 5 Relationships between independent (proximate and ultimate analysis, wt%-ad) and dependent variables for the coal-shale samples
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Table 6 Relationships between independent (petrographic analysis-vol%) and dependent variables for the coal-shale samples
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The overall database involved the Wits-Ehac Index and intrinsic properties of 14 coals and 14 coal-shales. Data were analysed using the set criterion. Tables 2, 3, 4, 5 and 6 present the results of the linear regression analyses for both the coal and coal-shale samples.

The experimental data were analysed with the use of a linear regression analysis to establish whether any of the intrinsic properties were linearly correlated to the spontaneous combustion liability index. The relationships between independent and dependent variables using the correlation coefficients and R-squared values are seen in Tables 2, 3, 4, 5 and 6. The linear regression analysis identifies linear relationships between dependent and independent variables. The analysis of variable pairs (dependent and independent) indicated consistent trends, i.e. an increase in the liability index with increasing volatile matter and vice versa for these materials. Coals and coal-shales show inertinite as the major constituent among the maceral (Tables 3, 6). Weak linear relationships were noted between the Wits-Ehac Index and petrographic properties (total vitrinite, total inertinite, and total liptinite) for both coals and coal-shales (Table 7).

Table 7 Relationships between independent (total reactive maceral, total maceral analysis and total mineral matter-vol%) and dependent variables for the coal-shale samples
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From Tables 2, 3, 4, 5 and 6, according to the criterion set, contents of volatile matter, 0.5164; ash, − 0.6884; carbon, 0.6572; hydrogen, 0.6616; nitrogen, 0.6945; sulphate sulphur, 0.6422; and inertinite macerals-fusinite, 0.5663; with strong effects on self-heating potential are factors affecting spontaneous combustion liability of coals, while contents of moisture, 0.7715; volatile matter, 0.6389; ash, − 0.8352; carbon, 0.7962; hydrogen, 0.5795; nitrogen, 0.6446; total sulphur, 0.5791; and its forms [pyritic, 0.5704; sulphate, 0.5365; and organic sulphur, 0.5933], total inertinite, 0.7360; and its constituents [fusinite, mmf, 0.5898; total semifusinite, 0.7776; secretinite, 0.6003; and total inertodetrinite, 0.7749; total maceral, 0.7653; and mineral matter, − 0.7653] with strong effects on self-heating potential are factors affecting spontaneous combustion liability of coal-shales. Ash, some macerals and mineral matter contents for both coals and coal-shales show negative trends. Coal-shale intrinsic properties show better linear relationships to spontaneous combustion liability than the coals and hence, identifies the intrinsic properties influencing these materials toward spontaneous combustion. Despite the low contents of moisture, volatile matter, carbon, hydrogen, nitrogen and total sulphur in coal-shales compared to coals, the coal-shales shows significant correlation to the spontaneous liability index than the coals. The influence of intrinsic properties on spontaneous combustion liability of coals and coal-shales using statistical analysis has been extensively reported in a study by Onifade and Genc (2018e). The study created models which combined the effects of the main intrinsic properties affecting spontaneous combustion liability of these materials for predictive purposes (Onifade and Genc 2018e).

From Figs. 3, 4, 5, 6, 7, 8, 9 and 10, there is an increase in contents of moisture, volatile matter, ash, carbon, total sulphur, calculated oxygen, pyritic sulphur, organic sulphur, inertinite macerals and mineral matter in both coals and coal-shales. However, this appears to be more noticeable for coal-shales than for the coals, while coals seem to be more distinct in terms of hydrogen, nitrogen and sulphate sulphur content than coal-shales. It was found that spontaneous combustion liability of coals and coal-shales could be affected by varying proportions of one or more intrinsic properties.

Fig. 3
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Influence of proximate and ultimate analysis on spontaneous combustion liability of coals

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Fig. 4
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Influence of vitrinite and its group on spontaneous combustion liability of coals

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Fig. 5
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Influence of total inertinite and its group on spontaneous combustion liability of coals

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Fig. 6
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Influence of total liptinite and its group, total reactive maceral, total macerals, and mineral matter on spontaneous combustion liability of coals

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Fig. 7
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Influence of proximate and ultimate analysis on spontaneous combustion liability of coal-shales

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Fig. 8
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Influence of total vitrinite and its group on spontaneous combustion liability of coal-shales

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Fig. 9
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Influence of total inertinite and its group on spontaneous combustion liability of coal-shales

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Fig. 10
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Influence of total liptinite, sporinite, total reactive macerals, total macerals, and mineral matter on spontaneous combustion liability of coal-shales

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4 Conclusion

This study has evaluated, compared and identified the relationships between intrinsic properties and spontaneous combustion liability of coals and coal-shales. The influence of selected intrinsic properties towards spontaneous combustion liability indicated a better linear relationship for the coal-shales than the coals, thus they may have a greater effect to cause spontaneous combustion of coal-shales. The linear regression analysis shows that among the macerals, the inertinite macerals indicated a stronger linear relationship to spontaneous combustion liability. Thus, the spontaneous combustion liability index of coals and coal-shales may be influenced by the proportion of each maceral composition. A definite positive or negative correlation coefficient exists between the intrinsic factors and spontaneous combustion liability index. This paper has established a comparative analysis between the dependence of spontaneous combustion liability index on intrinsic properties of selected coal-shales and associated coals. The results obtained from the petrographic and chemical analyses may be used as a tool to predict spontaneous combustion liability and may serve as of reference when comparing characteristics of coals and coal-shales from different coalfields.