A Comparison of Mineralogical and Thermal Storage Characteristics for Two Types of Stone Coal

Abstract

Understanding the mineralogical and chemical characteristics of stone coal is imperative for exploring the thermal storage characteristics of stone coal. Two types stone coal (SC_wt and SC_hc) were sampled from the pond of a stone coal mine in Henan province (China), and the chemical composition, phase composition, and thermal behavior of the stone coal were investigated. Furthermore, the petrography of the stone coal was studied in detail. The mineral phases of the stone coal were quartz, kaolinite, roscoelite, and goethite, as distinguished by reflected light microscopy and further proven by scanning electron microscope-energy dispersive spectrometer (SEM-EDS). The thermal conductivity of SC_wt was 0.19 W m⁻¹ K⁻¹, while that of SC_hc was 0.24 W m⁻¹ K⁻¹. Stearic acid (SA) was blended with SC_wt and SC_hc to prepare SA/SC_wt and SA/SC_hc composites via an impregnation method, respectively. The thermogravimetric (TG) curves show that the loading capacity of SC_hc, at 17.40%, is higher than that of SC_wt (16.63%). The thermal energy storage capacities of SA/SC_wt and SA/SC_hc composites were 29.21 J g⁻¹ and 33.02 J g⁻¹ according to a differential scanning calorimetry (DSC) analysis. Therefore, SC_hc is a potential candidate for thermal storage applications due to more obvious thermal storage characteristics, including higher thermal conductivity and loading capacity.

1. Introduction

Nowadays, energy and environmental issues are two major challenges of society, and improving energy utilization efficiency and protecting the environment have received widespread attention [1,2,3,4,5,6]. It is obvious that inexpensive energy, which is a prerequisite for sustainable economic growth and development, can be produced from coal [7]. Some trace elements in coal also have significant potential for industrial extraction and utilization [8]. With the increasing market demand for high-performance materials and the shortage of high-grade ores, it becomes more and more necessary and urgent to exploit and utilize low-grade resources for high-performance materials [9]. Recently, utilizing low-grade resources to produce high-performance materials and high purity metals has become a research hotspot [10,11,12,13,14].

Stone coal is a combustible, low heat value, high-rank sedimentary rock mainly derived from early Paleozoic bacteria and algae after saponification and coalification in a marine-influenced environment (e.g., epicontinental sea, lagoon, or bay) [15]. It is mainly distributed in southern China, where coal resources are scarce, and the reserves are extremely rich. The total reserves of stone coal are 61.88 Gt, covering an area of about 30,000 km² [15]. Because of the simple mining conditions of stone coal and the shortage of coal resources, stone coal resources will be widely used. In recent years, the comprehensive utilization of stone coal resources has developed rapidly [16]. The utilization of stone coal is mainly in four aspects: fuel power (stone coal power generation), building materials, agricultural fertilizer, and refined rare metals [15]. While making full use of the low calorific energy value of stone coal, research on the extraction of vanadium and other associated elements can not only improve the utilization value of stone coal but also provide more urgently needed rare and precious metal materials [16]. Stone coal after the extraction of vanadium also has the potential for producing composite phase change materials. Therefore, the study of the mineralogical characteristics of stone coal is very important. So far, very little information related to the composite phase change materials that use stone coal is available, and only research in vanadium extraction from stone coal has been conducted. Stone coal is now used to extract beneficial elements, mainly small-scale extraction of vanadium pentoxide. Vanadium is an important rare metal that has been widely used in ferrous and non-ferrous alloys to improve its hardness, tensile strength, and fatigue resistance, and vanadium oxide is widely applied [17,18,19]. In China, apart from vanadium titano-magnetite, carbonaceous shale, also called stone coal, is another important vanadium-bearing resource due to the vast amount of deposits [20]. The gross reserve of vanadium in terms of V₂O₅ in stone coal accounts for more than 87% of the domestic reserve of vanadium [21]. Generally, most vanadium in stone coal exists in terms of isomorphism, where V³⁺ substitutes for Al³⁺ in the crystal lattice of aluminosilicates including muscovite, tourmaline, and garnet, because V³⁺ has a similar electronegativity and ionic radius to Al³⁺ [22,23]. In addition, small amounts of vanadium in the form of adsorbed state adsorb on the surface of clay minerals such as kaolinite [24]. After the extraction of vanadium, if research on stone coal after extracting vanadium continues, the utilization of stone coal resources will be fully, effectively, and reasonably developed. Due to the complex mineral composition of stone coal, in order to fully exploit and utilize abundant stone coal resources and guide the preparation of composite materials using stone coal as a raw material, it is necessary to thoroughly explore the mineralogical and thermal storage characteristics of stone coal.

In recent years, mineralogical characteristics have been used to study stone coal. Researchers have studied stone coal by various characterization methods so as to explore the nature of minerals, extract vanadium, and prepare materials. To explore the flotation technology of high-carbon stone coal bearing vanadium, Wang et al. [9] studied the mineralogical characteristics of stone coal by XRD, scanning electron microscope-energy dispersive spectrometer (SEM-EDS), and optical micrographs. To optimize the roasting method and conditions to improve the leaching efficiency of vanadium pentoxide (V₂O₅), Zeng et al. [25] investigated the oxidizing roasting of stone coal in a laboratory fluidized bed reactor by means of X-ray fluorescence (XRF), SEM-EDS, and XRD. Furthermore, Hu et al. [26] explored the separation of V over Fe impurities from V-bearing shale using oxalic acid as an eco-friendly leachant by XRF, XRD, and SEM-EDS. Xie et al. [16] also explored activated carbon (AC), vanadium oxide (V₂O₅), and V₂O₅/AC photocatalyst prepared by in situ utilization of stone coal ore by XRD, XRF, Fourier transformation infrared spectroscopy (FTIR), SEM, Brunauer-Emmett-Teller (BET), and thermogravimetric-differential thermal analyzer (TG-DTA). In addition, Gao et al. [27,28,29,30] explored the nature of minerals by calculating surface broken bonds combined with mineralogical features. Therefore, the mineralogical characteristics of stone coal can serve as a guide for subsequent work on stone coal. However, the use of stone coal as a raw material for the preparation of phase change composites also requires research on its thermal storage characteristics.

As a natural mineral, stone coal has the potential to be used to prepare composite thermal storage materials. However, few studied related to stone coal-based composite thermal storage materials have been reported. Some scholars have studied a composite of coal-series kaolin (coal gangue) and fatty acid as form-stable phase change material [31,32]. Stone coal with a lower carbonization degree has similar properties to coal gangue [33], so it is meaningful to study the thermal storage characteristics of stone coal. In this paper, mineralogical and thermal storage studies of stone coal were conducted with the purpose of preparing thermal storage materials by the comparison of the mineralogical and thermal storage characteristics of two types of stone coal.

2. Materials and Methods

2.1. Materials

Two kinds of stone coal (weathered stone coal and high-carbon stone coal) were sampled from the pond of a stone coal mine in Henan, China. After being transported to the laboratory, these stone coal samples were air-dried, ground to pass through a 0.074 mm metal sieve, homogenized, and stored in polypropylene plastic sample bags. The weathered stone coal and high-carbon stone coal were labeled as SC_wt and SC_hc, respectively.

2.2. Preparation of the SA/SC Composites

The SA/SC composites were synthesized via vacuum impregnation [34,35], and stearic acid (CH₃(CH₂)₁₆COOH, SA) with a phase change temperature range of 56–69.9 °C was supplied by Tianjin Hengxing Chemical Reagent Co., Ltd., Tianjin, China. The following is the fabrication process: 10 g SA (an excess) and 5.0 g SC were put into an Erlenmeyer flask, which was concatenated to a suction pump by using a unit for preventing backward suction. A vacuum was created in the Erlenmeyer flask at −0.1 MPa for 5 min, and then its temperature was maintained at 95 °C for 30 min via a constant temperature bath. The vacuum was released and the flask heated to 80 °C for 5 min in an ultrasonic bath before cooling to room temperature. The resulting mixture was filtered at 80 °C to produce the final stearic acid/stone coal (SA/SC) composites. The composites were labeled as SA/SC_wt and SA/SC_hc, respectively.

2.3. Characterization

The elemental composition of raw stone coals (SC_wt and SC_hc) was determined by X-ray fluorescence (XRF, PANalytical B.V., Almelo, The Netherlands) using radiation at an acceleration voltage of 60 kV and a current of 100 mA. X-ray diffraction (XRD, Bruker Corporation, Billerica, MA, USA) was carried out using a D8 ADVANCE analyzer (Cu-Kα) under the following conditions: voltage 40 kV, current 250 mA, scan range from 3 to 80°, and step size of 0.5°. The crystalline phases of samples were identified using the software Jade 6.5 compiled by Materials Data Inc. (MDI, California, CA, USA). The stone coal was embedded in the epoxy resin and polished for the petrography analysis, and the photomicrographs were then obtained using an Axiovert 200 mat photomicroscope; furthermore, a scanning electron microscope (SEM)-energy dispersive spectrometer (EDS, JEOL, Tokyo, Japan) was used on the polished samples under the backscattered electron (BSE, JEOL, Tokyo, Japan) mode by JEOL JSM-6490LV. The samples were pressed onto a conductive adhesive tab mounted on a copper stub and coated with gold before the test, and the corresponding microstructures were investigated using a Zeiss Sigma 500 scanning electron microscope (SEM, Carl Zeiss AG, Oberkochen, Germany) operated at an accelerating voltage. Fourier transformation infrared spectroscopy (FTIR, SHIMADZU, Japan) spectra were recorded using an IR tracer-100AH FTIR spectrometer in the range of 4000~400 cm⁻¹ at 20 kV. A thermogravimetric analysis (TGA, HENVEN HCT-3, Beijing Henven Scientific Instrument Factory, Beijing, China) was conducted at a heating rate of 15 °C·min⁻¹ up to 1100 °C in oxygen atmosphere. The thermal conductivity of two samples pressed into tablets was recorded using a DRX-I-300. Before testing the samples, the samples had to be to be prepared into wafers with a thickness of about 1 mm and a diameter of 12 mm. Differential scanning calorimetry (DSC) analysis of stearic acid and the composites was performed using a TA instruments DSC Q10 (V9.9 Build 303, New Castle, DE, USA) at a heating rate of 5 °C/min under a constant stream of argon at atmospheric pressure. Liquid nitrogen was used to cool the sample during the freezing period.

3. Results

3.1. Elemental Composition

The main components of SC_wt were Si, Al, and Fe, while the content of Ca, K, Ba, V, and P was less (

Table 1. X-ray fluorescence (XRF) results of samples (ppm).

	Na	Mg	Al	Si	P	SO3	Cl	K	Ca
SCwt	764.19	5646	41,876.47	308,186.67	4366.20	18,700	18,000	8546.81	20,642.86
SChc	487.45	2588.4	45,837.53	243,231.8	2841.96	567	0	10,652.81	2068.57
	Ti	V	Cr	Cu	Zn	As	Se	Rb	Sr
SCwt	1362	8182.42	841.58	616	1348.15	143.94	35.59	27.43	725.15
SChc	1468.8	5479.98	9713.74	393.6	95.49	237.12	0	33.83	445.08
	Y	Zr	Mo	Ba	Pb	Co	Mn	Fe	Ni
SCwt	126.02	63.63	46.67	30,623.53	176.37	15.73	123.94	38,745	330.4
SChc	57.5	46.61	42.67	20,451.50	41.77	11.8	179.72	21,886.9	815.77

). The main components of SC_hc were Si, Al, and Fe, while the content of Cr, K, Ba, V, and P was less. The content of V was 8182.42 ppm and 5479.98 ppm for SC_wt and SC_hc, respectively, indicating that the SC_wt includes more vanadium. Other valuable elements, such as Cu, Zn, Ni, and Mo, are low in the mineral composition, making little sense if recycled.

3.2. Mineralogical Composition

Figure 1 shows the XRD patterns of the SC_wt and SC_hc, indicating the respective phase composition such as quartz, kaolinite, roscoelite, and goethite [36]. It can clearly be seen from the XRD curve that the phase of the two stone coals is similar. The major phase of SC_wt and SC_hc was quartz. Combined with the results of the XRF, Fe, Ca, and Si in SC_wt were slightly more than in SC_hc. The content of V in SC_wt is slightly greater than in SC_hc, whereas the content of Al and K in SC_wt is slightly lower than in SC_hc. This may be because the goethite in SC_wt contains a small amount of vanadium [37]. Vanadium seems to be affiliated mainly with roscoelite.

3.3. FTIR Analysis

The FTIR spectra of samples are shown in Figure 2. The absorption at 3442 cm⁻¹ and 2359 cm⁻¹ in all samples was attributed to loosely bound water (ν(H–O–H)) [38] and Al–OH [39], and the band at 2924 cm⁻¹ in all samples was crystalline water (free water) [39]. The absorption bands at 1730 cm⁻¹ and 1647 cm⁻¹ were bending vibrations from adsorbed water in roscoelite [39]. The bands at 1456 cm⁻¹ in all samples were attributed to stretching vibrations from CO₃²⁻ in carbonate impurities [39,40]. The band at 1377 cm⁻¹ in all samples was attributed to symmetric stretching vibrations from adsorbed CO₂ [41]. The bands at 1094 cm⁻¹ in SC_wt and 1089 cm⁻¹ in SC_hc were attributed to antisymmetric stretching vibrations from Si–O–Si in roscoelite and quartz [42]. The bands at 692 cm⁻¹ and 798 cm⁻¹ in all samples were attributed to symmetric stretching vibrations from Si–O–Si in quartz [42]. The band at 515 cm⁻¹ in all samples was attributed to Si–O–Al^VI bending [43]. The bands at 469 cm⁻¹ in all samples were attributed to Si–O and Si–O–Si bending [39].

3.4. TG-DTA Analysis

The TG and DTA curves of SC_wt under the flow of O₂ are shown in Figure 3a. According to the TG curve, free water was released in the temperature range of 15–100 °C with a mass loss of 1.28%. Dehydration of lattice water occurred in the range of 100–689 °C with a mass loss of 4.82%. From 689 °C to 879 °C, the weight loss was relatively stable, with a mass loss of 0.23%. From 879 °C to 1140 °C, the weight loss maintained a rising trend with a mass loss of 1.66% due to the removal of the structural hydroxyl groups from aluminosilicate minerals [41]. The TG curve of SC_wt has no obvious weightlessness step, while the corresponding DTA curve is a smooth curve without an obvious endothermic peak. Figure 3b shows the TG and DTA curves of SC_hc under the flow of O₂. According to the TG curve, free water was released in the temperature range of 15–100 °C with a mass loss of 2.74%. The lattice water and structural hydroxyls (part) were released in 100–436 °C with a mass loss of 4.00%. From 436 to 600 °C, the weight loss increased seriously, with a mass loss of 22.44% due to the carbon oxidized and the dehydration of lattice water [44]. From 600 to 1130 °C, the weight loss maintained the rising trend, with a mass loss of 3.13% due to the removal of the structural hydroxyl groups from aluminosilicate minerals [41]. The TG curve of SC_hc has an obvious weightlessness step, and in the DTA curve, an endothermic peak above 500 °C suggests that the endothermic enthalpies of SC_hc increased. There were no obvious chemical reactions in SC_wt under O₂ atmosphere at room temperature to 1140 °C. This indicates that the SC_wt had good thermal stability.

3.5. Reflected Light Microscopy Images Analysis

The study of petrography is useful for the identification of rocks, minerals, and ores [38,45]. The reflected light microscopy images demonstrated that four types of mineral phases were easily distinguished in two samples (Figure 4). The optical characteristic of roscoelite is that the ⊥ Bxa interferogram can be seen on the (001) plane, and the irregular roscoelite particles were surrounded by quartz. According to the optical characteristic of roscoelite, the dark roscoelite contains Fe impurities or carbonaceous impurities (Figure 4b,c). Goethite was distributed in kaolinite in a wide range of sizes. Kaolinite is mainly distributed along the bedding planes and, to a lesser extent, occurs as disseminated fine particles (Figure 4a,d) [8]. Therefore, the mineralography compositions of these samples were quartz, kaolinite, goethite, and roscoelite. However, the amount and size of roscoelite in SC_wt was slightly larger than in SC_hc, and the brownish-yellow area was also significantly larger than in SC_hc. These results were in accord with the XRF and XRD results.

3.6. SEM-EDS Analysis

The SEM-EDS analysis of the stone coal samples is presented in Figure 5, and atomic composition contrasts are also identified. Figure 5a,c shows that the particles were well separated from each other and that the shape of these particles is irregular. The stone coal samples presented flaky and pellet structures; the size of the flaky structures was 1–2 μm, while the size of the pellet structures was 0.1–1 μm. It is revealed that flaky structures were interspersed throughout the pellet structures. According to the EDS results (Figure 5b,d), the V element emerged in both SC_wt and SC_hc. These results are in accord with the reflected light microscopy images and XRF results of the stone coal.

Secondary electron (SE) images and EDS elemental maps of SC_wt and SC_hc are shown in Figure 6 and Figure 7, respectively. These indicate that Si (Figure 6b and Figure 7b), O (Figure 6c and Figure 7c), and Al (Figure 6d and Figure 7d) emerged in the concentrated distribution, and Fe (Figure 6e and Figure 7e) and V (Figure 6f and Figure 7f) presented the uniform distribution. Si and O mainly originated from quartz, and Al was from aluminosilicate (kaolinite and roscoelite). Fe and V were geared to goethite and roscoelite, respectively. The elemental mapping diagrams of the two stone coal samples are similar, indicating that their elemental composition and phase distributions are similar. The contents of vanadium minerals are usually very low and are commonly detrital materials of terrigenous origin and, to a lesser extent, from epigenetic igneous hydrothermal solutions [15]. Vanadium and roscoelite have a relatively obvious synchronous distribution. Most of the vanadium in stone coal occurs in ground roscoelite. The presence of V leads to the possibility of having roscoelite [7]. This suggests that roscoelite was formed by a complex reaction involving a change in the oxidation–reduction potential of the groundwater and the presence in the sandstone of sensitive associations of minerals containing vanadium-rich iron and titanium oxides. The reduction spots formed by diffusion as part of the same process [46]. The occurrence of roscoelite in depositional environment indicates input from hydrothermal fluids [15]. The roscoelite is of authigenic origin and was probably derived from the interaction between V derived from penetrating V-rich solutions and kaolinite already present in the coal during an early stage of the diagenetic process [47]. The sediment-source region for the coal was mainly due to Mesozoic intermediate-felsic volcanic rocks [8]. These results were in accord with the XRF results of the stone coal.

The EDS of the SC samples under the backscattered electron (BSE) mode is presented in Figure 8, and atomic composition contrasts (Z-contrast) are also identified. The original EDS data are provided in the Supplementary Materials. Four different contrasts can be seen in Figure 8a and are labeled as A1, A2, A3, and A4, respectively. According to the EDS results (Figure 8b), the A1, A2, A3, and A3 were quartz (dark-gray large grain), kaolinite (dark-gray small flake), roscoelite (light gray), and goethite (bright gray), respectively, in the SC_wt. This may be because the primary particles of SC_hc are too small, and the color between the phases is not much different. But relying on the EDS analysis, quartz, kaolinite, and roscoelite were also discovered in the SC_hc (Figure 8c,d) and labeled as C1, C2, and C3, respectively. Moreover, quartz, kaolinite, and goethite emerged in Figure 8e, and the corresponding EDS is presented in Figure 8f (E1, E2, and E3). These results were in accord with the reflected light microscopy images and XRF results of the stone coal. In addition, relatively little roscoelite, kaolinite, and goethite were detected according to the BSE and EDS. This is also the reason why the characteristic peaks of kaolinite, goethite, and roscoelite are not obvious in the XRD results. Nevertheless, the optical properties of the minerals in the reflected light microscopy and the results of the EDS data and their elemental composition imply the possible presence of kaolinite, goethite, and roscoelite.

3.7. The Thermal Conductivity of Stone Coal

Thermal conductivity is a fundamental factor for composite thermal storage material. The thermal conductivity of SC_wt is 0.19 W m⁻¹ k⁻¹, while the thermal conductivity of SC_hc is 0.24 W m⁻¹ k⁻¹. The thermal conductivity of SC_hc is slightly greater due to the sample having a high carbon content. It indicates that the SC_hc is more suitable for use as a composite thermal storage material. According to the lattice theory, it is known that stone coal contains a large amount of silicate, and the Si–O bond is the most powerful in the composition, so the crystal vibration of the grid is the most efficient in this direction. According to the composition analysis of stone coal, the content of stone coal is complicated and the Si–O bond in the unit cell increases, which will increase the vibration coupling between the cells, and the phonon collision will increase, thus improving the thermal resistance. According to the composition analysis, the metal ion composition in stone coal is also very different. However, the metal ion generally does not participate in the phonon conduction, but the vibration of the Si–O bond skeleton also partially excites the metal ion vibration, which also increases the difficulty of phonon propagation through the Si–O skeleton, thereby reducing the thermal conductivities of stone coal [48]. The aforementioned results also provide the way for enhancing the thermal conductivity of stone coal, such as adding the carbon materials.

3.8. Thermal Stability and Loading Capacity

As shown in Figure 9a, the curves of SC_wt and SC_hc under N₂ are similar to those under O₂, but are not as weightless as under O₂. This was due to the release of free water and dehydration of lattice water, not an oxidizing reaction (Figure 9b). Only one single decomposition process existed in the tested temperature range of SA/SC_wt, which was due to the degradation of SA. The TG curve of SA/SC_hc has two obvious weightlessness steps. An endothermic peak above 200 °C suggested decomposition of pure SA, and another endothermic peak in the curve of SA/SC_hc above 500 °C was due to the dehydration of lattice water. As can be seen in Figure 9a, the maximum mass fractions of SA in SA/SC_wt and SA/SC_hc were 16.63% and 17.40%, respectively. Thus, the loading capacity of SC_hc for phase change materials is higher than that of SC_wt. The complete decomposition temperatures (T_max) for SA/SC_wt and SA/SC_hc were in the range of 320–340 °C, indicating that the stone coal-based composite phase change materials (PCMs) had good thermal stability under 300 °C. Moreover, the TG curves and DTA curves of SCwt, SC_hc, SA/SC_wt, and SA/SC_hc under the flow of N₂ are shown in Figure 9.

3.9. Thermal Storage Behavior of Composite Phase Change Materials

It can be concluded that the thermal storage behavior of the composites was very close to that of the pure stearic acid (Figure 10), indicating no chemical reaction between stearic acid and stone coal during the preparation of the composites. The phase change properties were presented in

Table 2. Thermal properties of pure SA and SA/SC composites.

Sample	Loadage (β, %)	Melting Temperature (Tm, °C)	Freezing Temperature (Tf, °C)	Latent Heat of Melting (ΔHm, J g−1)	Latent Heat of Freezing (ΔHf, J g−1)	Theoretic Values of ΔHm (ΔHth, J g−1)	Crystallinity of SA (Fc, %)	Efficient Energy per Unit Mass of SA (Eef, J g−1)
SA	100	52.91	53.10	190.2	191.5	-	100	-
SA/SCwt	16.63	52.18	50.15	29.21	30.55	31.63	92.35	175.64
SA/SChc	17.40	52.81	53.24	33.02	33.17	33.09	99.78	189.78

. The SA had a melting phase change temperature (T_m) of 52.91 °C in the heating curve and a freezing phase change temperature (T_f) of 53.10 °C in the cooling curve (Figure 10). DSC curves show that the heating curves of the two composites exhibit a peak; the phase change of SA/SC_wt was at 52.18 °C (Figure 10), while the phase change of SA/SC_hc was at 52.77 °C (Figure 10). Furthermore, two transitions (a liquid–solid phase transition of SA/SC_wt at 50.15 °C and a liquid–solid phase transition of SA/SC_hc at 53.24 °C) appeared in the cooling curve. The latent heats of melting (ΔH_m) and freezing (ΔH_f), therefore, were calculated to be 190.2 J g⁻¹ and 191.5 J g⁻¹, respectively, for stearic acid. However, the latent heats of melting and freezing were 29.21 J g⁻¹ and 30.55 J g⁻¹ for SA/SC_wt, and 33.02 J g⁻¹ and 33.17 J g⁻¹ for SA/SC_hc, respectively. These values are lower than their theoretic values (Table 2).

The decrease in the latent heats of the composites is not only attributable to the lower fraction of stearic acid within the composites but could also be related to the crystallinity of the stearic acid in the composites. Crystallization of the stearic acid in the composites may be inhibited by interactions between the stearic acid and the substrate, which would also reduce the latent heats of the composites. The crystallinity of stearic acid (F_c) and efficient energy (E_ef) in the composite were calculated by [49,50,51,52,53]:

$$F_c=\frac{{\Delta }{H}_{composite}}{{\Delta }{H}_{PCM}\beta }\times 100\%,$$

(1)

$$E_{ef}={\Delta }{H}_{PCM}\times F_c,$$

(2)

where ΔH_composite and ΔH_PCM are the latent heats of the composites and pure stearic acid, respectively, and β represents the loading of stearic acid in the composites. The crystallinity of stearic acid in the SA/SC_hc (99.78%) was clearly higher than in the SA/SC_wt (92.35%). The stearic acid in the SA/SC_hc had a higher E_ef (189.78 J g⁻¹) than the SA/SC_wt (175.64 J g⁻¹). In this experiment, the loading of stearic acid in SA/SC_wt is higher than that in SA/SC_hc, but the latent heat of phase change in SA/SC_hc is higher than SA/SC_wt, and the crystallinity of stearic acid in SA/SC_hc is also higher than in SA/SC_wt. This is because the mineral composition in stone coal is complex, and the adsorption of components in SC_wt may affect the stearic acid molecules, reducing the regularity of stearic acid molecules and resulting in a decrease in crystallinity and a decrease in enthalpy.

4. Conclusions

The mineralogical and thermal storage characteristics of two types of stone coal (SC_wt and SC_hc) were investigated by XRF, XRD, FTIR, TGA-DTA, reflected light microscopy, thermal conductivity, and DSC. The content of V₂O₅ was 1.46 wt.% and 0.9778 wt.% for SC_wt and SC_hc, respectively. The main mineral phase in the two samples was quartz, kaolinite, roscoelite, and goethite. The results of combining reflected light microscopy images and scanning electron microscopy of the stone coal show that most of the roscoelite in the stone coal is dispersed around the quartz. The thermal conductivity of SC_wt and SC_hc was 0.19 W m⁻¹ k⁻¹ and 0.24 W m⁻¹ k⁻¹, respectively. The SA/SC_wt and SA/SC_hc composites were prepared by impregnating SA into SC_wt and SC_hc, respectively. The loading capacities of SC_wt and SC_hc for SA were 16.63% and 17.40%, respectively. The latent heat value of SA/SC_hc (33.02 J g⁻¹) was higher than that of SA/SC_wt (29.21 J g⁻¹). The composite PCMs had good thermal stability under 300 °C. Therefore, the SC_hc is a potential candidate for thermal storage applications due to its higher thermal conductivity and loading capacity. The results can provide a base for preparing thermal storage materials from stone coal.