## 3 Results and Discussion  
### Coal Extraction Rate  
Table 1 shows the graded extraction rates of Shenfu coal. The extraction rates of the coal at all levels were affected by the solvent polarity size and solubility parameters, and the extraction rates of polar solvents were higher than those of nonpolar solvents, mainly because the more similar the polarity of the solute and solvent, the stronger the intermolecular gravitational force was relatively and the more favorable the dispersion of the solute in the solvent and thus of the separation. For Shenfu coal, CS\({}_{2}\), acetone, and THF extracted better. Among them, CS\({}_{2}\) extracted about 70% of the aliphatic hydrocarbons, 91% of the aromatic hydrocarbons, and 40% of the heteroatomic compounds in the coal, which is because CS\({}_{2}\) is the first stage of extraction and it could not effectively separate the different components in the coal. Acetone extracted 42% of the heteroatomic compounds. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Extractant** & **\(\boldsymbol{\xi}\)/(cal-cm\({}^{-3}\))1/2** & **Polarity** & **Extraction** & **Aliphatic** & **Aromatic** & **Heterocyclic** \\  & **Parameters** & **Rate/\%** & **Hydrocarbon** & **Content/10\({}^{-4}\) g** & **Substance** \\ \hline CS\({}_{2}\) & 10 & 0.15 & 0.7160 & 32.12 & 105.51 & 13.63 \\ \hline n-hexane & 7.3 & 0.06 & 0.2830 & 2.4141 & 0.1318 & 0.076 \\ \hline Benzene & 9.2 & 3 & 0.3067 & 1.795 & 0.91 & 0.889 \\ \hline Methanol & 14.5 & 6.6 & 0.6247 & 0.259 & 0.054 & 4.838 \\ \hline Acetone & 9.9 & 5.4 & 2.7197 & 7.275 & 8.166 & 14.176 \\ \hline THF & 9.5 & 4.2 & 7.3968 & 1.485 & 0.583 & 0.22 \\ \hline THF and methanol & 12 & 5.4 & 0.5142 & 0.7409 & 0.5815 & 0.1452 \\ \hline \end{tabular}
\end{table} 
Table 1: Extraction rate and extractant composition of Shenfu coal in graded extraction. The solubility parameter \(\delta\) can be an important parameter to measure the mutual solubility between two substances and is widely used in solvent extraction [19]. The closer the \(\delta\) of the selected solvent and solute, the higher the mutual solubility of the two. The extractants with different \(\delta\) were selected for the extraction of Shenfu coal, and the trend of the extraction rate with \(\delta\) is shown in Figure 2. From Figure 2 the extraction rate of Shenfu coal in the range of the solubility parameter of 9-10 could be maintained above 7%, and the maximum extraction rate was reached at \(\delta\) = 9.5, while the extraction rate of the solubility parameter in the ranges of 7-9 and 10-15 was only about 0.4%. This indicates that not the higher solubility parameter is better, beyond the optimal solubility parameter, and the extraction rate will drop rapidly. Table 2 shows the graded extraction rates of Huolinguole coal. The comparison shows that the total extraction rate of Shenfu coal was greater than that of Huolinguole coal, which was 12.5611%. CS\({}_{2}\), methanol, and THF/methanol mixture had higher extraction rates for Huolinguole coal. The better extraction of both coals by CS\({}_{2}\) may be due to the fact that both coals contained more small molecules [20], and CS\({}_{2}\) was the first stage of extraction. By observing the composition of the extracts, it was found that CS\({}_{2}\) extracted about 99% of the aliphatic hydrocarbons and 99.8% of the aromatic hydrocarbons from the coals. Acetone extracted 72% of the heteroatomic compounds, and THF extracted 16.3% of the heteroatomic compounds. At the third stage of extraction, the aromatic hydrocarbons were already extracted completely, while the aliphatic hydrocarbons were extracted completely at the fourth stage. Meanwhile, when the aromatic and aliphatic hydrocarbons were extracted completely, the content of the heteroatomic compounds increased rapidly. Figure 3 shows the variation trend of the extraction rate with \(\delta\). From 0, the extraction rate of Huolinguole coal could be maintained above 2% in the range of a solubility parameter of 14-15, and the maximum extraction rate was reached at \(\delta\) = 14.5.