Coal consists of primary macromolecules of polyaromatic-polynuclear structure with some heteroatom groups and their secondary networks, latter of which are derived from aromatic ring stacking, aliphatic side chain entanglement, and hydrogen bonds, cation bridges, charge-transfer interactions through oxygen functional groups (Solum et al. 1989; Carlson 1992; Cody et al. 1993; Nakamura et al. 1995; Larsen et al. 1996; Wu et al. 2013). The oxidation of coal is a complex and multifaceted process representing a perplexing issue for both scientists and industrialist alike. The scientist seeks to understand the physical and chemical transformations brought about by oxidation, whereas the changes in behaviour specific to a given technological processes is the dominant concern of industry. Because oxidation alters the surface characteristics of both coal and mineral particles, flotation efficiencies may be changed and coal cleaning characteristics degraded (Sun 1954; Taylor et al. 1980). Conversely, mild, low temperature oxidation affects the process of gasification, reduces the caking tendencies and increases char reactivity (Mahajan et al. 1980). Hence, changes in behaviour may relate to changes in operating cost (Lowcnhaupt and Gray 1980) and there is a need to determine the extent of oxidation.

Coal undergoes structural changes when heated to a temperature at which thermal decomposition occurs (Ozbas and Kok 2003; Acma et al. 2006; Giroux et al. 2006). Fully characterizing coal structures is still a challenging issue due to the heterogeneity, non-crystalline structure, and insolubility of coal. Inertinite is the most aromatic and most thermally stable of all the maceral groups (Pandolfo et al. 1988; White et al. 1989; Vasallo et al. 1991; Xie et al. 1991; Sun et al. 2003; Wang et al. 2010). In this group, the maceral fusinite is more aromatic than semifusinite from the same coal (Blanc et al. 1991; Diessel 1992; Morga 2010). On the other hand, the amount of mobile hydrogen is higher in semifusinite than in fusinite (Maroto-Valer et al. 1998).

In this work, the compositional and structural changes that occur as a consequence of heat treatment of a bituminous coal performed at 200–1000 °C were investigated qualitatively. The study has been undertaken to develop a better understanding of the effect that the variations of mineralogical compositions, as a result of smouldering, have on the technological properties important for coal utilisation. The effect of heat treatment on the organic matrix of the coal has been discussed in detail. The results obtained would throw light not only on the nature and on distribution of organic matter (macerals) but also on the types of mineralogical transformations, which can take place during heating. As coal is heated, the inorganic phases undergo transformations and reactions that yield a complex mixture of solid, molten, and volatile species. There are many different minerals that behave differently. The main refractory minerals are quartz, metakaolinite, mullite, and rutile, while the common fluxing minerals are anhydrite, acid plagioclases, Kfeldspars, Ca silicates, and hematite (Vassileva and Vassilev 2006; Creelman et al. 2013; Mishra et al. 2016).

Chemical structure of coal macerals can be well examined by means of FTIR spectroscopy which makes it possible to obtain good quality spectra even from relatively small grains (Painter et al. 1978, 1980, 1981a, b; Bouwman and Freriks 1980; Painter and Rhoads 1981; Solomon 1981; Kuehn et al. 1982; Meyer 1982; Solomon and Carangelo 1982, 1988; Mastalerz and Bustin 1993a, b, 1995, 1996; Guo and Bustin 1998; Bustin and Guo 1999; Walker and Mastalerz 2004; Singh et al. 2015). Therefore, Fourier transform infrared spectroscopy (FTIR) is used as a physical detection technique and is widely applied in characterizing the chemical structures of coal. Besides, to quantify the mineralogical transformation X-ray diffraction analysis (XRD) analysis has been done on the samples.

2.1 Sampling and preparation of coal

Samples were taken from Jamadoba colliery (Fig. 1) of Jharia coalfield. The stratigraphy (Table 1) of Jharia coalfield unconformably overlies the Archean basement and belongs to the Lower Gondwana of Permian age. The stratigraphic succession in the coalfield comprises Talchir, Barakar, Barren measures and Raniganj formations, from bottom to top (Fox 1934; Mehta and Murthy 1957).

Geological map of the study area (modified after Paul and Chatterjee 2011)

The collected sample was used for laboratory thermal alteration, which was performed at different temperatures. The sample was crushed, pulverized, and passed through 3 mm sieve. −3 mm size fraction weighing 1 kg was followed to heavy media separation with the relative density range from 1.3 to 1.5 gm/cm³, made by benzene (density: 0.8 gm/cm³) and bromoform (density: 2.8 gm/cm³) mixture, for heat treatment study. Thus, from a single sample, three different density samples were prepared and 5 gm of each density fractions were heated at 200, 400, 600, 800 and 1000 °C temperatures at oxidizing atmosphere for a period of 1 h to attain the temperature of 1000 °C. After that, the furnace was cooled at the rate of 6 °C/min. Beyond 1000 °C, coal is reduced into ash and the organic matter is completely burnt.

2.2 Proximate and ultimate analysis

Proximate and ultimate analysis of the representative and their fractionated coal samples were carried out in the coal characterisation laboratory using standard analytical procedures. The proximate analysis was performed taking 72 mesh size coal powder using oven and muffle furnace as per Bureau of Indian Standard (BIS 2003). The elemental analysis (C, H, N and S) was performed using Vario EL III CHNS analyser (Elementar GmbH, Germany).

2.3 Petrography, XRD and FTIR

To study the coal samples under microscope the samples were crushed and passed through −18 mesh size and pellets were prepared in cold medium of epoxy-resin and hardener. The maceral analysis was carried out on polished mounts of coal with a polarized transmitted microscope having fluorescence attachment (Leica DM4500P) using established ICCP (1963, 1998, 2001) recommendations.

Mineralogical analysis of the coal samples were performed by means of X-ray diffraction (XRD) with Ni-filtered CuKα radiation at (10°–70°)/(2θ) at a scan rate of 2°/min (D8 Discover Bruker) using High Score Plus software package to obtain quantitative mineral proportion.

IR spectra of the samples were recorded with a Perkin–Elmer (model-Paragon 500) FT-IR spectrometer in the range of 4000–400 cm⁻¹ on KBr pellet.

3.1 Proximate and ultimate

Table 2 reveals the general characteristics of the head sample and the prepared samples from heavy media separation. The yield % of the float fractions of different densities are also tabulated there. The volatile matter (VM _daf) and ash yields (A _d) are 24.04 and 19.0 wt% respectively. In the fractions, VM and moisture are decreasing with density while ash is increasing. Carbon, hydrogen, nitrogen and sulphur, all are also decreasing with density.

3.2 Organic change from petrography and FTIR

3.2.1 Petrography

All the coal samples prepared by heating in an oxidising condition at different temperatures from 200 to 1000 °C were subjected to petrography to study the heating-induced changes in micro-texture and structure of coal (Fig. 2; Table 3). The percentage of inertinite macerals increased with density in all the temperatures while the macerals which were affected by heat (heat affected maceral), seen to increase with density at only 200 °C. These two components decreased with density in the samples treated at other temperature range (400–1000 °C). Oxidised macerals were only found in 400 °C where it also decreased with density. The mean reflectance of vitrinite macerals increased with temperature (≤400 °C) and density both. Vitrinites were seen below 600 °C since it was totally transformed into heat affected macerals at higher temperature. The mineral matter relatively increased with density at all temperature ranges.

Photomicrograph of laboratory oxidative alteration of some of the macerals from Jamadoba coals (A1). Telovitrinite, semifusinite and inertodetrinite (A2). Detrovitrinite and fusinite (A3). Semifusinite is filled by clay (A4). Fusinite with macrinite and low reflectivity semifusinite sample, thermally activated cracks (B1). Thermally activated cracks with oxidised semifusinite (B2). Low reflective semifusinite surrounded by oxidised bright semifusinite (B3), Fusinite (B4). The oxidation of semifusinite is started from the grain boundary. Centre part of grain is almost not metamorphosed with fragments of inertodetrinite (B5). Thermally altered of detrovitrinite, Inertodetrinite, heat affected (C1). Heat affected inertinite filled by mineral matter (C2). Fusinite with pores increasing in size due to heat (C3). Mineral matter with trace amount of fusinite (D1). Quartz and siderite (D2). Highly metamorphosed fusinite (E1), Quartz (E2). Altered coal with showing character of coke with many macropores

Table 4, shows the IR spectra of macerals. The major FTIR spectra of vitrinite are the aromatic C=C ring stretching vibration at 1615–1585 cm⁻¹, aliphatic absorption at 1460–1450 and 3000–2800 cm⁻¹. Weak peaks occur at 1740–1700 cm⁻¹ due to C=O group absorption, and at 3050–3030 cm⁻¹, due to aromatic CHx stretching vibration (Xuguang 2005). In our coal, all the three major peaks were identified. 3039–2916 and 1603–1575 cm⁻¹ are present below 600 °C which is in a perfect match with petrography where vitrinite is not found above 400 °C. The strongest aromatic C=C ring stretching at 1600 cm⁻¹, and the weakest aliphatic CHx stretching at 3000–2800 cm⁻¹, establishes the fact that fusinite has the highest degree of aromatic substituents and condensation of aromatic rings, and aliphatic substances is minor (Xuguang 2005). Thermal stability of inertinite macerals are higher than the other macerals probably due to its aromatic nature (Sun et al. 2003; Wang et al. 2010; Roberts et al. 2015). In our coal, the presence of inertinite found throughout the whole temperature range (200–1000 °C).

At 200 °C (Fig. 2 A1–A4), the mean reflectance of vitrinite (Table 3) increased with density but did not show any change in the maceral and microtextural constituents. Telovitrinite, detrovitrinite, were observed in vitrinite group maceral. In case of inertinite group macerals, fusinite is low in concentration than semifusinite. Inertodetrinite is also seen in the float fractions of the density of 1.3 gm/cm³. Among liptinite group meceral, resinite is seen in this density but in low concentration. The clay minerals are associated with semifusinite. In the float fractions of the density of 1.4, telovitrinites were observed and the inertinite macerals were dominated by semifusinite. Fusinite and macrinite occur in low concentration. In the float fractions of density of 1.5, the dominated macerals are detrovitrinite and semifusinite. Low reflectivity of semifusinite is observed in all density fractions at this temperature.

At 400 °C (Fig. 2 B1–B5) temperature, the sub-macerals of vitrinite and inertinite are oxidised in all density fractions. In the float fractions of the density of 1.3, telovitrinite, semifusinite and inertodetrinite were observed. Thermally activated cracks were developed in semifusinite. In float fractions of the density of 1.4, fusinite and semifusinite were seen. The reflectivity of semifusinite was changed in this temperature and this oxidation was started from grain boundaries. Thus low reflective semifusinite, surrounded by bright semifusinite, found at this temperature. Thermally altered detrovitrinite, inertodetrinite, heat affected macerals were also observed. In float fractions of the density of 1.5, detrovitrinite and inertodetrinite were seen. Heat affected macerals were found in very low amount in this density. Oxidised macerals were observed at this temperature range (>200 °C, <600 °C). Beyond this range, they get altered. However, below this range, oxidation did not ensue.

At 600 °C (Fig. 2 C1–C3) temperature the sub-macerals of inertinite and heat affected macerals were very common in all density fractions. Vitrinite and liptinite were not seen in this temperature. In float fractions of the density of 1.3, the pores of fusinite became larger due to thermal alteration. In the float fractions of the densities of 1.4 and 1.5 heat affected macerals, fusinite and semifusinite were seen and the mineral matter occur either alone or associated with fusinite.

At 800 °C (Fig. 2 D1–D2) and 1000 °C (Fig. 2 E1–E2) temperatures, vitrinite and liptinite were not seen and metamorphosed inertinite occurs in all density. In the float fractions of the density of 1.5 gm/cm³, the mineral matter is highest in amount (Table 3). Organic matter gradually decreased with density fraction.

3.2.2 FTIR

Figure 3(A–C) represents the comparative FTIR spectra of the samples of different density with varying temperatures. The corresponding functional groups (Ibarra et al. 1996; Maity and Mukherjee 2006) of the bands present in each sample are tabulated in Table 4. The numbers of the peaks, coming from organic complex, decreased with increasing density. The −OH absorption in the range of 3419–3359 cm⁻¹ due to the moisture absorbed in the residues during cooling process. The strong aliphatic and aromatic absorptions with C–O–C stretching and carbonyl (C=O) groups were present in low temperatures in all density fractions while at high temperature (>400 °C) some of these peaks were lost. Most of the peaks in FTIR spectra of coal between 1100 and 400 cm⁻¹ are assigned to quartz and clay minerals such as kaolinite, illite and montmorillonite groups (Saikia et al. 2007) which increase with density.

FTIR spectra of coal samples of different density with varying temperature

The absence of vitrinite and liptinite macerals at >400 °C is also reflected by FTIR spectra where some of the peaks are lost in that temperature range (>400 °C). Therefore, it appears that with increasing temperature, the organic groups collapsed and macerals were oxidised. Oxidation at 400 °C has deleterious effects on coking properties and reduces the specific energy of steaming coal (Ignasiak et al. 1972; Fredericks et al. 1983). At >400 °C inertinite is seen in all density fractions whereas in FTIR spectra the C–O–C stretch is present with an aromatic nucleus (CH) and three to four adjacent H deformations as organic functional groups. In the higher density fraction, the mineral matter content is comparatively more (Table 4) which is also reflected in FTIR spectra.

3.3 Mineral chemistry

The XRD analysis was performed on all the residues of different density fractions obtained from burning the coal samples at different temperatures. Figure 4 represents the comparative XRD diffractogram of same density fractions at different temperatures. It indicates evidently, the presence of kaolinite at low temperatures (≤400 °C), and illite, andalusite, mullite at high temperatures (>400 °C) whereas quartz was seen as the major mineral phase in all the samples of different density fractions at different temperatures. This indicates the mineral phase transformation with increasing temperature where the clay minerals were oxidised and form different type of oxides (Fig. 4 A–C). The identification of minor minerals only by XRD in a multi component system like coal ashes is difficult due to the detection limits (normally at about 0.5%–1%) and peak overlapping (Mishra et al. 2016) and mullite peaks were only present at higher temperature (1000 °C) of float 1.5 density fraction. Hence, presence of mullite in higher density fraction indicates the higher concentration of mineral matter in that particular fraction.

XRD diffractograms of coal samples of different density and different temperature. A Float fraction of density 1.3. B Float fraction of density 1.4. C Float fraction of density 1.5. K kaolinite, Q Quartz, I Illite, A Andalusite, M Mullite

This study provides a comprehensive view of the changes in organic structure with increasing temperature and helps to predict the structure of inertinite macerals. Coal undergoes appreciable physico-chemical changes when heated in the temperature range of 200–1000 °C during which, reacted molecules break along the weakest bonds, forming free radicals which subsequently recombine with other radicals or molecules to form more highly condensed species and volatile compounds. At ≤400 °C temperature, all the macerals are seen. The dominating macerals are vitrinite (telovitrinite, detrovitrinite) and inertinite (fusinite, semifusinite). The semifusinite has a low reflectivity but at 400 °C temperature, some of the vitrinite and inertinite macerals were oxidised. At >400 °C temperature the vitrinite and liptinite were not seen while porous inertinite was observed. In FTIR spectra, the absence of vitrinite and liptinite macerals in >400 °C is indicated by absence of their peaks at that temperature range. While inertinite survived, the other two macerals could not be seen at higher temperature. Therefore, it can be concluded that inertinite is most thermally stable maceral in coal. The clay minerals were oxidised at high temperature, which was revealed by XRD analysis.