## 3 Results
### GC/MS Analysis
The relative content of several compounds was re-calculated using Equation (1), and the value was found to be very close to the species of compounds previously screened in the NIST 14 database. As shown in Table 3, the results indicated that this method of comparison through the NIST database has a certain degree of credibility. Among them, nitrogen-containing compounds accounted for 66.28% of all the products. The probable names, retention times, and relative abundances of 26 nitrogen-containing compounds in the NIST 14 database are shown in Table 4. The classification of these 26 nitrogen-containing compounds was analyzed by GC/MS, as shown in Figure 5. The products had the highest content of five-membered nitrogen heterocycles, which accounted for 34.88% of the total. This was consistent with the XPS analysis of coal tar, but the result was only 1.20% for aliphatic amines. Amino radicals are more likely to combine with more reactive aromatic radicals than aliphatic alkane breakage radicals. The high content of five- and six-membered nitrogen heterocycles is partly due to the combination of fragmented chain radicals generated by the breakage of short-chain alkanes with one or more amino radicals, resulting in the cyclization of five- and six-membered nitrogen heterocycles. Another reason is that the oxygen atoms in the original five- and six-membered heterocycle structure of coal were replaced by nitrogen atoms to form five- and six-membered nitrogen heterocycles. Aromatic amines are mainly derived from aromatic compounds in coal with ether bonds and other easy-to-break carbon-oxygen bonds. When the carbon-oxygen bond is broken, the amino radicals in the fracture can be fully reacted, thus generating a number of aromatic amines. Although there are few phenolic compounds in coal (e.g., phenol, \(\alpha\)-naphthol, \(\beta\)-naphthol, etc. ), the p-electrons of the phenolic oxygen atom and benzene ring form a conjugated state that increases the barrier to nitrogen exchange, which is the reason that residual phenolics were detected among the products. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Most Likely Nitrogenous Compounds** & **Retention Time** & **Relative Abundance \({}^{\text{a}}\)** & **Relative Content \({}^{\text{b}}\)** \\ \hline
3-(tert-butyl)-1-methyl-1H-pyrrole & 4.901 & 23.14 & 2.57 \\
2,4,6-trimethylpyridine & 5.343 & 4.41 & 0.59 \\
2,2,5,5-Tetramethyl-4-ethyl-3-imidazole-1-oxyl & 5.564 & 93.90 & 11.70 \\
3-hydroxy-2,5-dimethylbisicontifaldehyde & 6.810 & 8.36 & 0.97 \\
4-methylbenzamideonitrile & 6.870 & 1.28 & 0.81 \\
2-methyl-5-(propyl-1-en-2-yl)pyridine & 7.091 & 2.72 & 0.33 \\
2,2,6,6-tetramethylpiperidin-4-one & 7.211 & 100.00 & 10.97 \\
4-hydroxy-1,6-dimethylpyridin-2(1H)-one & 7.563 & 28.64 & 3.00 \\
5-\(\beta\),8-\(\beta\)-Epoxy-3,5,8,8a-tetrahydro-1H-2-benzopyran & 7.964 & 1.78 & 0.13 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Retention time, relative abundance, and relative content of 26 possible nitrogen-containing compounds. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Rt (\%)** & & \\ \hline
**Aliphatic** & **Aromatic** & **Phenol** & **Nitrogen** & **Other** \\
**Alkane** & **Hydrocarbon** & **Compound** & **Compounds** \\ \hline By NIST & 4.41 & 12.48 & 7.76 & 65.19 & 10.16 \\ By isotopic & 4.52 & 12.48 & 7.76 & 66.28 & 8.96 \\ labeling & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of the relative contents of various compounds in NH4 Cl and tar from the co-pyrolysis of tar-rich coal (by direct comparison with the NIST 14 database and by isotopic labeling).