3.5.1. Aromatic Structures

According to Section 2.2, the XBP of gas coal in this study was 0.407, which was close to the XBP values of phenanthrene and anthracene in the literature [21]. According to the conclusion of the molecular structure in the literature [11], the macromolecular structure was dominated by anthracene and phenanthrene, followed by the pyrene and naphthalene rings. The bridging carbon ratio was made close to the calculated value by adjusting the numbers of various aromatic rings. The types and numbers of aromatic structures in the molecular structure are shown in Table 5.

#### 3.5.2. Aliphatic Carbon Structures

The aliphatic carbon in coal mainly exists in the form of branched chain, linked aromatic rings, aliphatic rings, side chains, and bridging carbons [29]. Based on the aromatic carbon ratio, the number of aromatic carbon atoms, and Table 5, the total and aliphatic carbon atoms were calculated to be 181 and 21, respectively. From the abovementioned data, the number of hydrogen atoms in gas coal was calculated to be 150. The proportions of fa1H, fa1\*, and fa10 in gas coal were 6.5%, 4.6%, and 0.9%, respectively. Therefore, the aliphatic carbon atoms in gas coal mostly existed in the forms of methyl, methylene, hypo methyl, and quaternary carbon, and the oxygen-linked lipid carbon was the least abundant.


**Table 5.** The types and numbers of aromatic structures in gas coal.

#### 3.5.3. Heteroatom Structures

The numbers of oxygen and nitrogen atoms in gas coal were calculated to be 9 and 3, respectively, by the total number of carbon atoms and the atomic ratio. According to XPS analysis, oxygen atoms in gas coal existed in the forms of five ether oxygen bonds, two carbonyl groups, and one carboxyl group. The nitrogen atoms existed in the form of two pyridine nitrogen and one pyrrole nitrogen.
