Comparative analysis of coal and coal-shale intrinsic factors affecting spontaneous combustion

Coal and coal-shales tend to undergo spontaneous combustion under favourable atmospheric conditions. Spontaneous combustion liability index and intrinsic properties of coals and coal-shales varies between (above and below) coal seams. The spontaneous combustion liability index (obtained from the Wits-Ehac Index) and intrinsic properties (obtained from proximate, ultimate, and petrographic analysis) of fourteen samples representative of in situ coal (bituminous) and fourteen coal-shales obtained in Witbank coalfield, South Africa were experimentally studied. Comparative analysis of the relationships between the spontaneous combustion liability index and intrinsic properties of coals and coal-shales were established to evaluate their effects on self-heating potential. The intrinsic properties show linear relationship with spontaneous combustion liability and therefore, identifies the factors affecting spontaneous combustion of these materials. The influence of coal-shales intrinsic properties towards spontaneous combustion liability shows higher correlation coefficients than the coals. Both coals and coal-shales show inertinite maceral as major constituents than the vitrinite and liptinite macerals, hence the reactivity of inertinite macerals may show greater influence on spontaneous combustion liability. A definite positive or negative trends exists between the intrinsic properties and spontaneous combustion liability index. This research is part of a larger project which is considering the influence of intrinsic properties of coals and coal-shales on spontaneous combustion liability.


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 coalshales 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 ( 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 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 coalshales.
2 Materials and methods

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).

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).  (Wade et al. 1987). b Typical differential thermogram of a coal sample Comparative analysis of coal and coal-shale intrinsic factors affecting spontaneous combustion 283

Wits-Ehac Index
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).
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 coalshales 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).    Total reactive is the sum of total vitrinite, total liptinite, reactive semifusinite and reactive inertodetrinite Comparative analysis of coal and coal-shale intrinsic factors affecting spontaneous combustion 285   total inertodetrinite, 0.7749; total maceral, 0.7653; and mineral matter, -0.7653] with strong effects on selfheating 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.

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 coalshales 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.  Fig. 10 Influence of total liptinite, sporinite, total reactive macerals, total macerals, and mineral matter on spontaneous combustion liability of coal-shales Comparative analysis of coal and coal-shale intrinsic factors affecting spontaneous combustion 293