## 3 Results and Discussion
### Clustering Analysis of GC/MS Results of Coal Extracts  
Systematic cluster analysis (HCA), also known as hierarchical cluster analysis, is one of the most commonly used methods of cluster analysis. HCA is calculated by starting with one data point as a cluster, and then for each cluster, merging the clusters based on the same criteria. This calculation is performed until finally only one cluster is left, and the hierarchy of clusters is found. Because the principal component analysis only explains 84.5% of the overall variable information and does not fully reflect its classification, it is necessary to perform a systematic cluster analysis. Using the class averaging method in systematic cluster analysis, the GC/MS results of the two coal extracts were analyzed by clustering tree diagram using the Euclidean distance to obtain the clustering of the family components based on their contents. As shown in Figure 6, the 190 compounds detected were classified into 14 groups, from left to right, aliphatic hydrocarbons, aromatic hydrocarbons, phenols, aldehydes, alcohols, esters, ketones, carboxylic acids, ethers, furans, pyrans, sulfur-containing compounds, other compounds, and nitrogen-containing compounds.  
Figure 4: Content distribution of 14 groups of extractant family composition based on GC/MS data.  
of each family component, and the color from light to dark indicates that the content of the family component gradually increases. It can be seen from the Figure that the CS\({}_{2}\) extracts of the two coal samples contained the most aromatic hydrocarbons, mainly polycyclic aromatic hydrocarbons, such as naphthalene, anthracene, pyrene, phenanthrene, and their derivatives and a small number of thick-ringed aromatic hydrocarbons, among which SF-CS\({}_{2}\) contained more bicyclic, tricyclic, and tetracyclic aromatic hydrocarbons, while HLGL-CS\({}_{2}\) contained more bicyclic and tricyclic aromatic hydrocarbons. The tree diagram above the thermogram shows that the 14 groups of coal extracts were clustered into six groups by the similarity in the types and contents of the soluble components of organic matter contained, and the five groups SF-HEX, HLGL-PE, SF-BEN, SF-THF, and SF-THF/ME were clustered into one group because they contained a large number of aliphatic hydrocarbons in addition to small numbers of aromatic hydrocarbons. The three groups SF-ME, HLGL-THF, and SF-ACE were grouped together because they contained many ketones. HLGL-ME and HLGL-THF/ME were grouped together because they contained many esters. HLGL-ACE was grouped separately because it contained many alcohols. The above results reflect that polar solvents were more effective in the extraction of oxygenated compounds [22], which was because solvents, such as methanol and acetone, could break stronger hydrogen bonds along with van der Waals forces and weaker hydrogen bonds, so the content of oxygenated compounds in their extracts will be relatively high. HLDL-BEN was clustered into a separate group because of the large number of nitrogenous compounds contained in it.  
Figure 7: Heat map of clustering of 14 groups of extract family composition based on GC/MS data. SF-CS\({}_{2}\): Shenfu coal CS\({}_{2}\) extract; HLGL-CS\({}_{2}\): Huolinguole coal CS\({}_{2}\) extract; SF-HEX: Shenfu coal n-hexane extract; HLGL-PE: Huolinguole coal petroleum extract; SF-BEN: Shenfu coal benzene extract; SF-THF: Shenfu coal THF extract; SF-THF/ME: Shenfu coal THF and methanol mixture extract; SF-ME: Shenfu coal methanol extract; HLGL-THF: Huolinguole coal THF extract; SF-ACE: Shenfu coal acetone extract; HLGL-BEN: Huolinguole coal benzene extract; HLGL-ME: Huolinguole coal methanol extract; HLGL-THF/ME: Huolinguole coal THF and methanol mixture extract; and HLGL-ACE: Huolinguole coal acetone extract.