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Article

An Experimental Study on the Effect of Magmatic Thermal Evolution on the Molecular Structure of Low-Rank Coal

by
Wei Zhao
1,2,3,4,
Jingyu Jiang
1,2,3,*,
Yongzhen Huang
1,2,3,
Fang Zhang
1,2,3 and
Wanxing He
1,2,3
1
Key Laboratory of Gas and Fire Control for Coal Mines, China University of Mining and Technology, Ministry of Education, Xuzhou 221116, China
2
National Engineering Research Center for Coal and Gas Control, China University of Mining and Technology, Xuzhou 221116, China
3
School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China
4
Chensilou Coal Mine of Henan Longyu Power Co. Ltd., Yongcheng 476600, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2023, 13(21), 11907; https://doi.org/10.3390/app132111907
Submission received: 25 July 2023 / Revised: 23 October 2023 / Accepted: 23 October 2023 / Published: 31 October 2023
(This article belongs to the Section Energy Science and Technology)
Browse Figures
Versions Notes

Abstract

:
Low-rank coal accounts for over half of China’s proven coal reserves. The possibility of coal and gas outbursts in low-rank coal is higher, especially in the m·agmatic thermal evolution area. The complexity of coal’s molecular structure is one of the reasons for problems during the process of mining operations. Different analysis techniques, including XRD, FTIR and Raman spectroscopy, were used to obtain the molecular characteristics of magmatic thermal evolution coal samples and normal coal samples, so that a comparative study could be carried out to investigate the influence of the magmatic thermal evolution effect on the molecular structure of low-rank coal. The ranges of the aromatic interlayer spacing (d002), average stacking heights (Lc) and stacking layer number (Nave) of the thermally evolved coal samples are 3.41–3.51 Å, 22.76–27.02 Å, and 6.68–7.70, respectively. The ranges of the full width at half maximum ratio (FD1/FG) and the peak integral intensity ratio (ID1/IG) are 2.16–2.19 and 1.55–1.84, respectively. Compared with the normal coal samples, those affected by magmatic thermal evolution have smaller d002, ID1/IG, and FD1/FG values, but larger Lc values. The results indicate that the thermally evolved coal samples have more ordered structures and more developed microcrystalline structure sizes than normal coal samples.

1. Introduction

Magma intrusion into coal seams is a common phenomenon in major coal-producing countries worldwide [1,2]. Such intrusion results in the formation of more pronounced pore fissures within coal, leading to an increase in gas reservoir space, enhanced gas adsorption, and an elevated risk of spontaneous combustion [3], as well as coal and gas outbursts, all of which significantly impact the safety of coal mining [4,5]. Through an analysis of coal and gas outburst accidents in coal mines, it has been discovered that the proximity of magma intrusion areas is the leading cause of outburst incidents [6,7,8]. The intrusion of magma alters the molecular structure of coal, consequently influencing the physical and chemical properties of the coal [9,10,11]. Typically, coal is primarily characterized as a porous material consisting of complex organic polymers and inorganic compounds. The internal microstructure of coal, including aromatic compounds, functional groups, and side chains, among others, to a certain extent, influences both the microscopic pore characteristics of coal and the adsorption/desorption properties of gas [12,13,14,15,16]. Therefore, it is necessary to study the thermal evolutionary effects of magma on the molecular structure of coal bodies, thus providing a molecular-scale theoretical basis for studying the effects of magma intrusion on the pore characteristics and gas adsorption/desorption of coal.
The complex molecular structure of coal is a three-dimensional network of condensed aromatic rings and hydrogenated aromatic rings connected with various functional groups through various bridge bonds [12]. Analytical techniques such as X-ray diffraction (XRD), Raman spectroscopy, and Fourier transform infrared (FTIR) spectroscopy are widely used to study the molecular structural properties of coal [13,15]. XRD can be applied to determine the physical structure of crystals and is also widely used to characterize the physical structure of non-crystals. In recent years, XRD has been used for characterizing the three-dimensional carbon-infilled structure of coal [14]. Raman spectroscopy belongs to scattering spectroscopy and is widely used for characterizing carbonaceous materials’ crystal structure and the macromolecular structure of coal [17,18,19,20,21]. The Raman signal of graphite-like structures is most obvious in the first-order mode with D peaks and G peaks. The existence of D peaks is because of the existence of a large amount of heteroatoms or molecular lattice defects inside the coal, and the G peak is the only graphite-like characteristic spectral peak within the first-order Raman spectral curve, characterizing the degree of ordering of the microcrystalline structure [22,23]. Scholars usually use G-D1 and D1/G to assess information on the orderliness and crystallinity in carbon-containing materials, and use the intensity ratio of the D1 band to the G band to estimate the average lateral sizes (La) [13,20,24]. FTIR spectroscopy is widely used for studying the functional group components of coal, as well as for studying the coal rock composition and the degree of coalification [15,25,26,27]. This technique has the capability to characterize the hydrocarbon structure and heteroatomic functional groups in coal by discerning changes in the position of each distinct absorption peak in the infrared spectrogram.
At present, studies on the molecular structure of coal bodies mainly focus on magma contact metamorphic coals, with few studies on the differences in molecular structure between coal samples affected by magmatic heat and normal coal samples [9,28,29,30]. Therefore, to comprehend the impact of magmatic thermal evolution more clearly on the molecular structure of coal, magmatic thermal evolution coal samples affected by magma intrusion as well as unaffected normal coal samples from Tiefa Daxing coal mine and Shenyang Hongyang No. 2 mine were used in this paper. Three methods, XRD, Raman spectroscopy, and FTIR spectroscopy, were used to study the influence of magmatic thermal evolution on the molecular structure of coal. On the basis of studying the molecular structure of the thermal evolution coal, research on the macroscopic physical and chemical properties of thermal evolution coal may be further carried out to better predict and analyze the magmatic intrusion in the corresponding region. This study is of great significance to reveal the influence of magma intrusion thermal evolution on coal seam outbursts, coal and gas outburst prevention, and the exploitation of coal seam gas.

2. Coal Samples and Experiment Methods

2.1. Coal Samples

In this study, coal samples DX 1# and DX 2# were obtained from the outer section of the 906 transportation chute and the outer section of the 906 return air chute in the south second mining area of Tifa Daxing Coal Mine, Liaoning Province. Coal samples HY 1# and HY 2# were obtained from the −650 m bottom road in the third mining area of the southern flank of the 12-bed coal seam and the 1201 return air chute in the third mining area of the southern flank of the 12-bed coal seam in Hongyang No. 2 Mine, Liaoning Province. Among them, DX 1# and HY 1# are thermally evolved coal samples affected by the thermal evolution of magma, and DX 2# and HY 2# are unaffected, normal coal samples.

2.2. Proximate Analysis and Ultimate Analysis

Proximate analyses were performed according to ISO 17247:2020 [31] and ISO 19579:2006 [32]. Ultimate analyses were performed according to ISO 11722:2013 [33] and ISO 1171:2010 [34]. To be able to characterize the aromaticity and condensation information, calculations were performed based on data from proximate and ultimate analyses, combined with Equations (1) and (2) [35,36].
f a = 100 − V d a f × 0.9677 / C
R / C u = 1 − f a / 2 − H / C / 2
Vdaf is the volatile yield and 0.9677 is the fitting coefficient. C is the carbon yield.

2.3. X-ray Diffraction Analysis

This test was performed using a D8 ADVANCE X-ray diffractometer from Bruker, Mannheim, Germany, to scan the samples [37]. Coal samples with particle sizes of up to 0.074 mm were used for this test. The experimental conditions were an X-ray tube with a Cu target and Kα radiation (U = 40 kV, I = 30 mA, λ = 0.15405 nm). After the test, the diffraction results obtained from the experiment were processed using Origin 2022b software, and the parameters of the γ and 002 peaks were obtained by splitting the peak fit at 20° and 26°, respectively [38]. Based on this, the aromaticity (fa) of the coal sample can be calculated [39].
f a = C a r / C a r + C a l = A 002 / A 002 + A γ
A002 represents the area of the 002 peak and Aγ represents the area of the γ peak.
In addition, interlayer spacing (d002), the stacking layer number (Nave), and the stacking heights (Lc) can be calculated using Equations (4)–(6) [18,40].
d 002 = λ / 2 sin θ 002
N a v e = L c / d 002
L c = 0.89 λ / β 002 cos θ 002
In these equations, λ is the wavelength of the X-ray (γ = 0.15405 nm), θ002 is the diffraction angle of the 002 peak, and β002 is the full width at half maximum (FWHM) values of the 002 peaks.

2.4. Raman Spectroscopy Analysis

Raman spectroscopy experiments were performed using a Senterra-type laser confocal Raman spectrometer manufactured by Bruker, Mannheim, Germany. The experiments were conducted using a 532 nm laser at indoor temperature (laser power of 5 mW) and the measurement range was 1000~2000 cm−1. The Raman spectra in the range of 1000~2000 cm−1 were fitted by applying Origin 2022b. Based on the fitting results, the average lateral sizes (La) can be calculated [41].
L a = C ( λ L ) ( I D / I G ) − 1
C ( λ L ) ≈ C 0 + λ L C 1
where C(λL) is the wavelength factor, C0 is −12.6 nm, and C1 is 0.033 nm. ID and IG are the intensities of the D band and G band, respectively, and λL is the Raman laser wavelength.

2.5. FTIR Spectroscopy Analysis

In this experiment, a Fourier Transform infrared spectrometer and a micro infrared system TIR Vertex 80v infrared spectrometer were used. The test selected 1 g coal samples with particle sizes of less than 0.074 mm, and scanned 32 times in the wave number range of 4000~400 cm−1 (spectral resolution of 8 cm−1). In order to study the thermal evolution effects on the chemical structure of low-rank coals, the experimentally obtained spectra were fitted with PeakFit v4.12 software to split the peaks. Based on the fitted peak results, the apparent aromaticity (fa) and fatty structure parameters A(CH2)/A(CH3) [42] as well as the aromatic ring condensation characterization parameters (R/C)u of the coal samples could be calculated [28,43].

3. Results and Discussion

3.1. Chemical Properties of Experimental Coal Samples

The results of the proximate and ultimate analyses of the thermally evolved coal samples and normal coal samples are listed in Table 1.
As shown in Table 1, the volatile yield of the coal samples decreased under the thermal evolution of magma. Moisture also shows the same pattern with volatile yield, which is due to the fact that when the magma intrudes, the higher temperature bakes the coal body, causing a large amount of moisture to escape. Ash, on the other hand, shows the opposite pattern, which is due to the fact that a large number of mineral components gradually penetrate into the coal body through hydrothermal convection or groundwater flow during or after magma intrusion, resulting in a large increase in its ash content. The value of (R/C)u responds to the number of aromatic carbon rings in the monomer and can reflect the degree of aromatic ring condensation to some extent [43]. Comparing different types of coal samples at the same mine, the magmatic thermally evolved coal samples have higher fa and (R/C)u values than the normal coal samples. This indicates that the thermally evolved effect of magma makes the aromatic rings of coal more condensed.

3.2. Effect of Thermal Evolution on the XRD Microcrystalline Structure of Coal

Figure 1 shows that both thermally evolved coal samples and normal coal samples show a typical graphite diffraction peak (002 peak) at around 2θ = 25°. Another graphite diffraction peak (100 peak) also appears at about 2θ = 42°. It indicates that each coal sample possesses some characteristics of crystals, with both disordered and ordered structures within them [44].
Based on a previous study, the asymmetry of the 002 diffraction peak is attributed to the overlap of this peak by the amorphous carbon peak (γ peak) and the microcrystalline carbon peak (002 peak) [45,46]. To delve further into the characteristics of the XRD microcrystalline structure parameters of the magmatic thermal evolution coal samples and normal coal samples, the raw data were subjected to denoising and fitted using Origin 2022b software for peak splitting [38]. The fitting results are presented in Figure 2, with DX 1# and DX 2# serving as illustrative examples.
As can be seen in Figure 2, an obvious diffraction peak appears near the diffraction angle of 2θ = 20° as an amorphous carbon peak (γ peak), which is caused by branched microcrystals of aliphatic hydrocarbon branched chains, various functional groups, and alicyclic hydrocarbons [39,47]. A clear diffraction peak also appears near the diffraction angle of 2θ = 26° as a graphite peak (002 peak), which correlates with the stacking between aromatic ring lamellae [48]. Based on the peak position, area, and FWHM parameters of the three peaks (002 peak and γ peak) obtained from the split-peak fitting profiles, the results of the quantitative calculation of the aromatic lamellae spacing (d002), aromaticity (fa), average stacking heights (Lc), and stacking layer number (Nave) combined with Equations (3)–(6) are shown in Table 2.
As listed in Table 2, the d002 values of the thermally evolved coal samples DX 1# and HY 1# are lower at 3.41 Å and 3.51 Å, respectively. The d002 values of these two coal samples gradually converge to the aromatic layer spacing of graphite crystals (3.36–3.37 Å), which indicates that the thermally evolved effect of magma intrusion promotes the coalification or carbonization process of the coal samples. The cross-sectional comparison between DX 1# and DX 2#, and HY 1# and HY 2# reveals that the fa values of DX 1# and HY 1# are higher than those of DX 2# and HY 2#, respectively, which is an indication that the thermally evolved coal samples have a higher degree of aromatization. The combustion reactivity of thermally evolved coal samples may be weak relative to normal coal samples [29]. However, in the process of coal mine production, coal bodies affected by magmatic intrusion often have a high risk of spontaneous combustion. The factors affecting the spontaneous combustion of coal include not only the chemical and microcrystalline structure of the coal itself, but also other factors such as the pore structure, geologic endowment conditions, and moisture. Therefore, the effects of the above factors on the spontaneous combustion hazard of coal will be investigated in combination with other research methods in the following research work. In addition, DX 1# and HY 1# have higher Nave and Lc values than DX 2# and HY 2#, respectively, which indicates that the magmatic thermally evolved coal samples affected by magma intrusion have larger microcrystalline structural units. The analysis indicates that the thermal evolution of magma leads to an increase in the volume of coal microcrystalline structural units, which promotes the aromatization process and the degree of condensation of the macromolecular structural units.

3.3. Effect of Thermal Evolution on Raman Spectra of Coal

Two distinct regions of Raman frequency vibrations can be seen in Figure 3 (after baseline correction), namely the D-spectral band peak located at 1350 cm−1 nearby and the G-spectral band peak at 1585 cm−1 nearby.
In order to better compare and analyze the parameters such as the integral intensity, FWHM, and peak position difference of the D and G peaks of the magmatic thermal evolution coal samples and normal coal samples for this study, the fitted curves of the Raman spectrograms were obtained by using the Origin 2022b software and fitting the data to the peaks using the combination method IX, found by Sadezky et al. [49] (taking DX 1# and DX 2# as examples), as shown in Figure 4.
Because the FWHM of the D band has good correlation with the disordered degree of carbonaceous materials [50] and the half-peak width of the G band can reflect graphitization very sensitively [51], the FWHM ratio (FD1/FG) of the D and G bands was used as a study parameter. In an effort to better study the magmatic thermal evolutionary effects on the molecular structure of low-rank coals, the ratio of the integral intensity of the peaks in the D and G bands (ID1/IG) was also used as a study parameter to analyze the degree of structural orderliness.
Based on the results of the split-peak fitting in the 1000–2000 cm−1 range, the obtained characteristic parameters in terms of the G and D1 peak positions, the peak position difference (G-D1), and the integral intensity ratio (ID1/IG) are shown in Table 3.
As can be seen from Table 3, the Raman shift of the D1 peak of the experimental coal sample varies from 1339 to 1346 cm−1; the Raman shift of G peak varies from 1576 to 1589 cm−1; the maximum value of the peak difference (G-D1) is 249 cm−1 and the minimum value is 230 cm−1. The peaks and peak differences are relatively close, showing that the structure of order in the magmatic thermally evolved coal samples and normal coal samples are relatively similar. A cross-sectional comparison of the magmatic thermally evolved coal samples and the normal coal samples of the same coal mine reveals that the D1 and G peak positions of each experimental coal sample are shifted to some extent, and the G-D1 values of the magmatic thermally evolved coal samples, DX 1# and HY 1#, are larger than those of the normal coal samples, i.e., the thermally evolved coal samples have a lower disordered structure than the normal coal samples [13,52]. The ID1/IG ranges from 1.55 to 2.16, and the FD1/FG ranges from 2.16 to 2.36. The ID1/IG and FD1/FG values of the thermally evolved coal samples are smaller than those of the normal coal samples, indicating that the internal macromolecular defects of the DX 1# and HY 1# coal samples are of a lower degree and higher degree of graphitization, i.e., magmatic thermal evolution promoted the generation of the internal ordered structure of the low-rank coal. The trend of the value of La in the Raman spectra is consistent with that of the XRD, so it will not be repeated.

3.4. Influence of Thermal Evolution on FTIR spectroscopy Characteristics of Functional Groups in Coal

The FTIR spectra for the four coal samples are depicted in Figure 5. Various functional groups in coal exhibit distinct vibrational frequencies, corresponding to specific characteristic absorption peaks in the infrared spectrum [53] (as detailed in Table 4). The FTIR spectra of coal can be categorized into four sets of absorption peaks: hydroxyl absorption peaks within the wavenumber range of 3700–3000 cm−1, aliphatic absorption peaks spanning 3000–2700 cm−1, absorption peaks associated with oxygenated functional groups in the range of 1800–1000 cm−1, and absorption peaks reflecting the structure of aromatic compounds within the range of 900–700 cm−1 [54].
Figure 5 depicts that there are multiple absorption peaks in the infrared spectra of each experimental coal sample, indicating the existence of complex functional group structures in coal. The infrared spectra of the magmatic thermal evolution coal samples and the normal coal samples partly have the same characteristics, but there are also some differences: there are no obvious characteristic peaks in each coal sample in the 2700~1800 cm−1 band, and there are absorption peaks in other bands; the location, sharpness, and width of peaks near specific bands are different for each coal sample, indicating that each coal sample contains about the same types of functional groups, but there are differences in the contents [55].
Figure 6 and Figure 7 shows the split-peak fitting profile of the IR spectra of the test. The hydroxyl absorbance peaks of the coal samples are mainly located in the 3700–3600 cm−1 range, i.e., a large amount of free -OH is present in the coal samples, possibly with alcohols, phenols, and organic acids [28]. The distribution of the fitted peaks in the wave number range of 3000~2800 cm−1 is relatively uniform in the range of the aliphatic absorption peaks, which indicates that these four coal samples essentially contain various aliphatic functional groups such as CH3, CH2, and CH in cycloalkanes or aliphatic groups. In the range of absorption peaks of the oxygenated functional groups, DX 1# and DX 2# have similar shapes of absorption peaks, and HY 1# and HY 2# have similar shapes of absorption peaks, and both have larger and sharper absorptions within 1100–1000 cm−1. In addition, the intensities of the absorption peaks of DX 1# and HY 1# are larger than those of DX 2# and HY 2#, respectively; this indicates more S=O stretching vibrations. Meanwhile, the presence of aromatic C=C stretching vibrations and CH3 asymmetric deformation vibrations in the coal samples is indicated by the presence of distinctive characteristic peaks at 1600 and 1440 cm−1 in the four samples.
Based on the results of the peak fitting, the absorption peak areas of the various functional groups as well as the structural parameters (Table 5) could be calculated.
Since the area of each absorption peak is affected by the selection of the baseline, the intensity of the light, and other factors, it does not reflect well the effect of magmatic thermal evolution on the functional groups of coal. To better analyze the changing pattern of the functional groups in the coal samples, the area percentage of the absorption bands associated with each structure was selected as the study parameter (as shown in Figure 8). When compared to the normal coal samples, the thermally evolved coal samples exhibit a lower percentage of oxygen-containing functional groups, a higher percentage of aromatic structures, and increased aliphatic functional groups compared to DX 2# and HY 2#. This suggests that the thermal evolution induced by magma may have facilitated the breakdown of certain molecular compounds in coal and the removal of oxygen-containing functional groups, leading to an increase in aliphatic structures. Additionally, it results in the continuous condensation of smaller molecular compounds, thereby enhancing the degree of coal aromatization.
Fa and (R/C)u can reflect the information of aromatic structures in the macromolecular structure of coal to some extent [43,56]. The magmatic thermal evolution coal samples, DX 1# and HY 1#, have higher fa and (R/C)u values than the normal coal samples, DX 2# and HY 2#, respectively, indicating that the thermal evolution of magma makes the smaller molecular compounds in the coal samples condensed, which promotes the degree of coal aromatization. A(CH3)/A(CH2) can reflect the length of aliphatic chains in coal [42], and the magmatic thermally evolved coal samples, DX 1# and HY 1#, have higher A(CH2)/A(CH3) values than the normal coal samples, DX 2# and HY 2#, respectively, reflecting that magmatic thermally evolved coal samples have longer aliphatic side chains than normal coal samples in their macromolecular structures.

4. Conclusions

The molecular structure characteristics of the experimental coal samples of DX 1#, DX 2#, HY 1#, and HY 2# were characterized through proximate and ultimate analysis, XRD experiments, Raman spectroscopy experiments, and FTIR spectroscopy experiments, and the main conclusions obtained are as follows:
(1)
Based on the proximate analysis, it is evident that the thermal evolution caused by magmatic activity leads to an increase in coal ash content while reducing the moisture and volatile content. The results obtained from the ultimate analysis indicate that the magmatic thermal evolution coal sample exhibits a high aromaticity rate.
(2)
X-ray diffraction experiments revealed that magmatic thermal evolution possesses the capability to elevate the aromatization of low-rank coals and foster the advancement of their microcrystalline structure. Moreover, the magmatic thermal evolution coal samples exhibited reduced d002 values, approaching the aromatic interlayer spacing observed in graphite crystals. This implies that the magmatic thermal evolution process promotes the coalification or carbonization of low-rank coal.
(3)
The Raman spectroscopy experiments show that magmatic thermal evolution promotes the generation of ordered structures in low-rank coals. The ID1/IG and FD1/FG values of magmatic thermal evolution coal samples are smaller than those of normal coal samples, which indicates that there are less disordered structures and more ordered structures in magmatic thermal evolution coal samples affected by magma intrusion compared with normal coal samples.
(4)
FTIR spectroscopy analysis demonstrates that magmatic thermal evolution possesses the ability to deoxygenate the coal samples and augment their carbon content. Additionally, the thermally evolved coal samples display higher A(CH2)/A(CH3) values when compared to the normal coal samples, signifying a reduction in the degree of branching attributed to magmatic thermal evolution.
(5)
It is necessary to recognize the limitations of this study; the microstructures of the two groups of coal samples were analyzed and investigated through the test results, but only approximate relationships could be obtained due to the limited number of coal samples selected. The next step will be to increase the number of test coal samples for studying.

Author Contributions

Conceptualization, J.J.; methodology, W.Z.; validation, W.Z., J.J., Y.H., F.Z., and W.H.; formal analysis, W.Z.; investigation, F.Z. and W.H.; resources, J.J.; data curation, F.Z. and W.H.; writing—original draft preparation, W.Z. and Y.H.; writing—review and editing, J.J. and W.Z.; visualization, F.Z.; supervision, J.J.; project administration, J.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant number 52174217.

Institutional Review Board Statement

The study did not require ethical approval.

Informed Consent Statement

The study did not involve humans.

Data Availability Statement

Readers can email the corresponding author to request data.

Conflicts of Interest

Author Weizhao was employed by Henan Longyu Power Co. Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. The XRD patterns of magmatic thermal evolution coal samples and normal coal samples.
Figure 1. The XRD patterns of magmatic thermal evolution coal samples and normal coal samples.
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Figure 2. (a) Fitting of 002 and γ peaks in the range of 15~35° for magmatic thermal evolution coal samples (DX 1#); (b) fitting of 002 and γ peaks in the range of 15~35° for normal coal samples (DX 2#).
Figure 2. (a) Fitting of 002 and γ peaks in the range of 15~35° for magmatic thermal evolution coal samples (DX 1#); (b) fitting of 002 and γ peaks in the range of 15~35° for normal coal samples (DX 2#).
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Figure 3. The Raman spectra of magmatic thermal evolution coal samples and normal coal samples.
Figure 3. The Raman spectra of magmatic thermal evolution coal samples and normal coal samples.
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Figure 4. (a) Raman spectrum of DX 1# coal samples with the corresponding curve fitted bands; (b) Raman spectrum of DX 2# coal samples with the corresponding curve fitted bands.
Figure 4. (a) Raman spectrum of DX 1# coal samples with the corresponding curve fitted bands; (b) Raman spectrum of DX 2# coal samples with the corresponding curve fitted bands.
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Figure 5. FTIR spectra of coal samples.
Figure 5. FTIR spectra of coal samples.
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Figure 6. Fitting Gaussian peaks of Fourier transform infrared spectra for magmatic thermal evolution coal samples (DX 1#) and normal coal samples (DX 2#). ((a) 3600–3000 cm−1 FTIR spectra fitting curve of DX 1#; (b) 3000–2700 cm−1 FTIR spectra fitting curve of DX 1#; (c) 1800–1000 cm−1 FTIR spectra fitting curve of DX 1#; (d) 900–700 cm−1 FTIR spectra fitting curve of DX 1#; (e) 3600–3000 cm−1 FTIR spectra fitting curve of DX 2#; (f) 3000–2700 cm−1 FTIR spectra fitting curve of DX 2#; (g) 1800–1000 cm−1 FTIR spectra fitting curve of DX 2#; (h) 900–700 cm−1 FTIR spectra fitting curve of DX 2#)).
Figure 6. Fitting Gaussian peaks of Fourier transform infrared spectra for magmatic thermal evolution coal samples (DX 1#) and normal coal samples (DX 2#). ((a) 3600–3000 cm−1 FTIR spectra fitting curve of DX 1#; (b) 3000–2700 cm−1 FTIR spectra fitting curve of DX 1#; (c) 1800–1000 cm−1 FTIR spectra fitting curve of DX 1#; (d) 900–700 cm−1 FTIR spectra fitting curve of DX 1#; (e) 3600–3000 cm−1 FTIR spectra fitting curve of DX 2#; (f) 3000–2700 cm−1 FTIR spectra fitting curve of DX 2#; (g) 1800–1000 cm−1 FTIR spectra fitting curve of DX 2#; (h) 900–700 cm−1 FTIR spectra fitting curve of DX 2#)).
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Figure 7. Fitting Gaussian peaks of Fourier transform infrared spectra for magmatic thermal evolution coal samples (HY 1#) and normal coal samples (HY 2#). ((a) 3600–3000 cm−1 FTIR spectra fitting curve of HY 1#; (b) 3000–2700 cm−1 FTIR spectra fitting curve of HY 1#; (c) 1800–1000 cm−1 FTIR spectra fitting curve of HY 1#; (d) 900–700 cm−1 FTIR spectra fitting curve of HY 1#; (e) 3600–3000 cm−1 FTIR spectra fitting curve of HY 2#; (f) 3000–2700 cm−1 FTIR spectra fitting curve of HY 2#; (g) 1800–1000 cm−1 FTIR spectra fitting curve of HY 2#; (h) 900–700 cm−1 FTIR spectra fitting curve of HY 2#)).
Figure 7. Fitting Gaussian peaks of Fourier transform infrared spectra for magmatic thermal evolution coal samples (HY 1#) and normal coal samples (HY 2#). ((a) 3600–3000 cm−1 FTIR spectra fitting curve of HY 1#; (b) 3000–2700 cm−1 FTIR spectra fitting curve of HY 1#; (c) 1800–1000 cm−1 FTIR spectra fitting curve of HY 1#; (d) 900–700 cm−1 FTIR spectra fitting curve of HY 1#; (e) 3600–3000 cm−1 FTIR spectra fitting curve of HY 2#; (f) 3000–2700 cm−1 FTIR spectra fitting curve of HY 2#; (g) 1800–1000 cm−1 FTIR spectra fitting curve of HY 2#; (h) 900–700 cm−1 FTIR spectra fitting curve of HY 2#)).
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Figure 8. Proportion of different functional groups.
Figure 8. Proportion of different functional groups.
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Table 1. Proximate and ultimate analysis results of coal samples.
Table 1. Proximate and ultimate analysis results of coal samples.
SampleProximate Analysis (wt.%)Ultimate Analysis (wt.%)H/Cfa(R/C)u
MadAadVdafFcCHONS
DX 1#1.1423.9130.8544.1081.654.3112.321.290.430.630.820.27
DX 2#3.099.0636.2651.3980.125.3913.011.110.370.810.770.21
HY 1#1.3838.1218.9041.6081.225.0712.311.110.290.750.970.14
HY 2#5.9814.9423.1655.9280.315.6312.841.010.210.840.930.12
Mad: moisture; Aad: ash, Vdaf: volatile matter, FC: fixed carbon.
Table 2. Microcrystalline structure parameters of magmatic thermal evolution coal samples and normal coal samples.
Table 2. Microcrystalline structure parameters of magmatic thermal evolution coal samples and normal coal samples.
Sample2θ002 (°)d002 (Å)Lc (Å)Navefa
DX 1#26.113.4122.766.680.69
DX 2#25.813.4521.976.370.57
HY 1#25.383.5127.027.700.81
HY 2#25.013.5626.197.360.79
Table 3. Parameters of the fitted peaks of the Raman spectra of coal samples.
Table 3. Parameters of the fitted peaks of the Raman spectra of coal samples.
SamplePeakPosition (cm−1)Integral IntensityFWHMID1/IGG-D1FD1/FGLa (Å)
DX 1#G157687,938741.842362.1626.93
D11340161,997160
DX 2#G1576208,233762.162302.3622.94
D11346449,889179
HY 1#G1588162,252701.552492.1931.97
D11339251,734153
HY 2#G1589135,348741.872462.2226.50
D11343252,906164
Table 4. Attribution of characteristic peaks in FTIR spectra of coal [53].
Table 4. Attribution of characteristic peaks in FTIR spectra of coal [53].
NumberPeak PositionWavenumber RangeFunctional Group Assignment
136803685~3600Free -OH
235503600~3500-OH self-contained hydrogen bond
334003550~3200-OH stretching vibration
429502975~2950CH3 asymmetric stretching vibration
529202935~2915CH2 asymmetric stretching vibration
628702875~2860CH3 symmetric stretching vibration
728502860~2840CH2 symmetric stretching vibration
817501770~1720Aliphatic C=O stretching vibration
917001715~1690Aromatic C=O stretching vibration
1016751690~1660C=O stretching vibration in quinone
1116001605~1595Aromatic C=C stretching vibration
1214701480~1465CH2 asymmetric deformation vibration
1314401460~1435CH3 asymmetric deformation vibration
1413801385~1370CH3 symmetric bending vibration
1511501160~1120C-O-C stretching vibration
1611101120~1080S=O stretching vibration
1710501060~1020Si-O-Si or Si-O-C stretching vibration
18870900~850aromatic nucleus (CH), one adjacent H deformation
19820825~800aromatic nucleus (CH), three adjacent H deformation
20750770~730aromatic nucleus (CH), five adjacent H deformation
21720724~716n-Alkane side-chain skeleton (CH2)n oscillatory vibration
Table 5. Molecular structure parameters of thermally evolved coal samples and normal coal samples.
Table 5. Molecular structure parameters of thermally evolved coal samples and normal coal samples.
SampleHal/HH/CCal/Cfa(R/C)uA(CH2)/A(CH3)
DX 1#0.740.630.260.740.313.64
DX 2#0.850.810.380.620.292.99
HY 1#0.760.750.320.680.283.87
HY 2#0.690.840.320.670.243.60
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Zhao, W.; Jiang, J.; Huang, Y.; Zhang, F.; He, W. An Experimental Study on the Effect of Magmatic Thermal Evolution on the Molecular Structure of Low-Rank Coal. Appl. Sci. 2023, 13, 11907. https://doi.org/10.3390/app132111907

AMA Style

Zhao W, Jiang J, Huang Y, Zhang F, He W. An Experimental Study on the Effect of Magmatic Thermal Evolution on the Molecular Structure of Low-Rank Coal. Applied Sciences. 2023; 13(21):11907. https://doi.org/10.3390/app132111907

Chicago/Turabian Style

Zhao, Wei, Jingyu Jiang, Yongzhen Huang, Fang Zhang, and Wanxing He. 2023. "An Experimental Study on the Effect of Magmatic Thermal Evolution on the Molecular Structure of Low-Rank Coal" Applied Sciences 13, no. 21: 11907. https://doi.org/10.3390/app132111907

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