*3.5. X-ray Diffraction (XRD) Analysis of Coal Samples* 3.5.1. XRD Peak Fitting

In order to analyze the microstructure of outburst coal, the XRD patterns of coal samples were fitted by peak separation, as shown in Figure 11. The original spectrum was divided into 002 peaks, *γ* peaks and 100 peaks by Guassian [41] formula. The fitting formula is:

$$y = y\_0 + \frac{A}{w \cdot \sqrt{\frac{\pi}{2}}} \cdot e^{-2\left(\frac{x - x\_0}{w}\right)^2} \tag{10}$$

where *y*<sup>0</sup> is the baseline position, *w* is the diffraction peak width, *A* is the diffraction pattern area and *xc* is the 2*θ* angle of the diffraction center.

**Figure 11.** Peak separation fitting results of coal samples.

It can be found from Figure 11 and Table 4 that the 2*θ* angle range of 002 diffraction peak is 24.92◦–25.08◦, with high intensity and good symmetry. Moreover, the 2*θ* angle range of 100 diffraction peaks is 42.73◦–42.85◦ and the diffraction peaks are wide and low, indicating that the degree of aromatic ring condensation in coal is not high. For the *γ* band, it can be clearly seen from the graph that its area is large, reflecting the rich branched chain structure of aliphatic hydrocarbons and aliphatic hydrocarbons in coal.

**Table 4.** Diffraction peak fitting of the main 2*θ* parameters.


3.5.2. Structure Analysis of Aromatic Microcrystals

The layer spacing of *d*<sup>002</sup> and *d*100, the average stacking height of *Lc* and the ductility of *La* of the aromatic layer can be calculated from the diffraction angle and the half-peak width [49].

$$d\_{002/100} = \frac{\lambda}{2\sin\theta\_{002/100}}\tag{11}$$

$$L\_c = \frac{K\_1 \lambda}{\beta\_{002} \cos \theta\_{002}} \tag{12}$$

$$L\_{\mathfrak{a}} = \frac{K\_2 \lambda}{\beta\_{100} \cos \theta\_{100}} \tag{13}$$

$$M\_c = \frac{L\_c}{d\_{002}}\tag{14}$$

where *λ* is the wavelength of X-ray, and 0.15405 nm is used for the experiment with copper target irradiation; *θ*002/100 is the Bragg angle of 002 diffraction peak and 100 diffraction peak, respectively; *β*<sup>002</sup> and *β*<sup>100</sup> are the half-peak widths of 002 diffraction peak and 100 diffraction peak, respectively; *K*<sup>1</sup> and *K*<sup>2</sup> are Debye-Scherrer constants, with *K*<sup>1</sup> as 0.89, *K*<sup>2</sup> as 1.84 and *Mc* as the number of effective stacked aromatic slices.

In general, the interlayer spacing of the coal aromatic layer is between cellulose (*d*<sup>002</sup> = 3.975 × <sup>10</sup>−<sup>1</sup> nm) and graphite (*d*<sup>002</sup> = 3.354 × <sup>10</sup>−<sup>1</sup> nm). Therefore, the coalification degree *P* is used to determine the percentage of condensed aromatic ring, and the relative content of aromatic layer and aliphatic layer structure is obtained. The calculation formula is as follows:

$$P = \frac{3.975 - d\_{002}}{3.975 - 3.354} \times 100\% \tag{15}$$

The calculation results are shown in Table 5. It can be found from the table that *d*<sup>002</sup> of primary coal is 3.548, indicating that its coalification degree is high. The number of effective stacking aromatic slices of primary coal is about 4, and the coalification degree is 68.83%, indicating that the coal sample has a high degree of coalification, with fewer side chains and functional groups, and the internal arrangement of molecules is orderly and stable, and the condensation degree of aromatic nucleus is high. Combined with the data of *d*<sup>002</sup> and *d*100, the number of aromatic flakes in outburst coal is lower than primary coal, which suggests that the prominent heating effect increases the degree of aromatization, increases the d100 and *d*<sup>002</sup> values and, finally, changes the dense ring structure in coal. Through the coal degree index *P*, it can be seen that the primary coal is slightly higher than the outburst coal. Combined with the analysis of *d*<sup>002</sup> and *d*<sup>100</sup> as basically unchanged, this shows that the content of side chain in the primary coal is lower than that of the outburst coal, resulting in the increase of the relative number of aromatic rings, which is finally reflected in the increase of *P*.

**Table 5.** XRD microstructure parameters of coal samples.


#### **4. Conclusions**


increases the *d*<sup>100</sup> and *d*<sup>002</sup> values and, finally, changes the dense ring structure in coal. Moreover, from the coal degree index P, it can be seen that the primary coal has a smaller increase than the outburst coal.

**Author Contributions:** Conceptualization: A.J., S.T.; Methodology: H.L.; Formal analysis and investigation: A.J., H.L.; Writing-original draft preparation: A.J.; Writing-review and editing: S.T.; Funding acquisition: S.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Natural Science Foundation of China under Grant number 52104079 and the Science & Technology Foundation of Guizhou Province under Grant number [2020]4Y050.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** All data, models, and code generated or used during the study appear in the submitted article.

**Acknowledgments:** The authors appreciate the support from above funders.

**Conflicts of Interest:** The authors declare that they have no competing financial interests.

#### **References**

