## 3 Results
### GC/MS Analysis  
Firstly, the tar-rich coal was analyzed by Py-GC/MS, as shown in Figure 3a. The relative quantification of the peaks of the target compounds in the resulting TIC plots was calculated using Equation (1). \[R_{t}(\%)=\frac{S_{t}}{S} \tag{1}\]  
where \(R_{t}\) (%) indicates the relative content of the target compound, \(S_{t}\) indicates the peak area of the target compound in the TIC diagram, and \(S\) indicates the sum of all effective target compounds in the TIC diagram, after the peak areas of impurities, interfering peaks, and residual peaks of the chromatographic column removed. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & \multicolumn{2}{c}{**N Analysis (wt \%)**} \\  & **Char** & **Tar** \\ \hline Rich-oil coal & 0.77 & - \\ Rich-oil coal pyrolysis & 0.74 & 1.83 \\ Rich-oil coal and NH4 Cl & 2.38 & 6.49 \\ co-pyrolysis & 2.19 & 5.16 \\ Rich-oil coal and chitin & 2.46 & 4.06 \\ co-pyrolysis & 5.74 & 6.03 \\ Rich-oil coal and polyamides & 4.65 & 5.77 \\ co-pyrolysis & 11.61 & 7.04 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Elemental analysis of tar and coke from co-pyrolysis of six nitrogen-containing compounds and tar-rich coal. The relative contents of various compounds in coal tar were calculated according to Equation (1), as shown in Table 2. The content of aliphatic hydrocarbons is about 11%; the content of aromatic hydrocarbons is about 60%; the content of phenols is about 11%; and the nitrogen-containing compounds are about 6%. It is well known that the tar from tar-rich coal has a low content of nitrogen-containing compounds, so we prepared nitrogen-rich tar by co-pyrolysis of tar-rich coal and nitrogen-containing compounds. Since Py-GC/MS mainly analyzes solids and liquids, NH\({}_{4}\)Cl, which is widely available, inexpensive, green, and clean, was used as the nitrogen source. At the same time, \({}^{15}\)NH\({}_{4}\)Cl was used to label the nitrogen atoms, and the nitrogen-containing compounds in the tar would have characteristic signals in the mass spectra, further identifying the nitrogen-containing compounds therein as shown in Figure 4a,b. After that, co-pyrolysis was carried out using the same temperature first, and then the collected tar was subjected to GC/MS, with the results shown in Figure 4c,d. From Figure 3, it can be seen that the TIC plots after isotope labeling have the same trend as the unlabeled TIC plots, indicating that the isotope labeling is more complete. This also proves that it is completely feasible to utilize the products obtained by the isotope labeling method during the co-pyrolysis of coal and NH\({}_{4}\)Cl. Comparison of \(m/z\) shows that the nitrogen-containing compounds obtained using the pyrolyzer are not only more diverse than those obtained by Py-GC/MS, but also have significantly higher contents. We analyzed the labeled TIC plots and found that all of the effective compounds in them were concentrated within 4-20 min, and the corresponding changes in \(m/z\) values of 26 compounds were analyzed in this range. Based on this, it can be preliminarily judged that 26 nitrogen-containing compounds were \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Type of** & **Aliphatic** & **Aromatic** & **Aromatic** & **Nitrogen** & **Others** \\
**Compound** & **Method** & **Alkane** & **Alkane** & **Phenol** & **Compound** & **Others** \\ \hline \multirow{2}{*}{R\({}_{\text{t}}\) (\%)} & Py-GC/MS & 11.47 & 60.21 & 10.96 & 5.85 & 11.51 \\  & GC/MS & 17.97 & 50.18 & 12.84 & 6.98 & 12.03 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Relative contents of various compounds in coal tar pyrolyzed from tar-rich coal. Figure 3: TIC plots of coal tar (**(a)**: TIC plot obtained using Py-GC/MS. (**b**): Tar obtained by pyrolysis furnace and TIC plot obtained by GC/MS). produced in the tar from the co-pyrolysis of tar-rich coal with NH4 Cl. By comparing the retention times of all product peaks in Figure 4c,d, it was found that the \({}^{15}\)N isotope had almost no effect on the retention times of the labeled products.