*3.2. 13C-NMR Test Results and Analysis*

The 13C-NMR spectrums were fitted to obtain carbon atoms information by using Peak Fit software. In Figure 2, the main carbon signal peaks of coal were located with the chemical shift between 0 ppm–50 × <sup>10</sup>−<sup>6</sup> ppm and 100 × <sup>10</sup>−<sup>6</sup> ppm–170 × <sup>10</sup>−<sup>6</sup> ppm, which belonged to aliphatic carbon and aromatic carbon, respectively [34]. Each peak was classified based on peak position attribution results in the literature [35], and the classified results are shown in Table 3. According to the calculation method of the 12 structural parameters in the literature [36] (the ascribing positions of different peaks were determined by the results of 13C-NMR peak fitting, and the peak values were substituted into the formula to calculate the structural parameters of coal molecules) and the chemical shift attribution of carbon in Table 3, the 12 structural parameters of coal were calculated, and the results are shown in Table 4. In Table 4, the proportion of aromatic carbon (*f* <sup>a</sup> ) in gas coal was 87.70%, which was the main body of a constituting macromolecule. The proportion of aliphatic carbon (*fa*1) was 12%, and carbonyl and carboxyl carbon (*fa <sup>C</sup>*) were 0.3%, unprotonated aromatic carbons bridged with aromatic carbon (*fa <sup>B</sup>*) was 25.3%. *XBP* = *fa <sup>B</sup>*/(*fa <sup>H</sup> + fa <sup>P</sup> + fa <sup>S</sup>*) = 0.407, which was the ratio of aromatic bridge carbon to circumferential carbon, and the XBP value reflected the size of aromatic clusters in coal [37]. The types and numbers of aromatic rings in the macromolecule structure were inferred by the size of aromatic clusters in combination with the ultimate analysis.

**Figure 2.** 13C-NMR split-peak-fitting spectrum of gas coal.


**Table 3.** Carbon structure attribution and proportion.

Note: A1—lipid methyl carbon; A2—aromatic cyclic methyl carbon; A3—methylene carbon and methine carbon; A4—methine carbon and quaternary carbon; A5—oxygen-conjugated methylene and oxygen-conjugated methylene carbon; A6—protonated aromatic carbon; A7—bridgehead aromatic carbon; A8—side branch aromatic carbon; A9—oxygen replaces aromatic carbon; A10—carboxy carbon.

**Table 4.** Structure parameters of gas coal.

