Dissolution of low molecular weight compounds and change mechanism of macromolecular structure during coal evolution

Coal is a mixture of organic matter and a small amount of minerals, among which the composition and structure of organic matter directly affect the efficient and clean utilization of coal. It is important to study the organic composition and structural characteristics of coal for the high-value utilization of coal. In order to reveal the difference of the composition and macromolecule structure of low molecular compounds in different grade coals, this study takes long flame coal, 1/3 coking coal, coking coal, meager-lean coal, lean coal, and anthracite as the research objects. THF soxhlet extraction is used to test the coal with GC / MS and 13C NMR. The differences of THF soluble organic compounds in coals with different metamorphism degrees were compared, and the mechanism of chemical composition and structural parameters in coal gasification were discussed, so as to provide a reference for the study of coal composition and structure. The results show that ① THF soluble low molecular compounds in all kinds of coal are mainly oxygen-containing heteroatom compounds, in which long-chain aliphatic hydrocarbons and oxygen-containing compounds containing carboxyl group are mainly in low and medium metamorphic bituminous coal, and cyclic hydrocarbons and oxygen-containing compounds containing aldehyde group are mainly in high metamorphic coal. The content of aromatic hydrocarbon is less than that of aliphatic compounds, and the types of soluble compounds increase first and then decrease with the degree of metamorphism. ② THF extraction has little effect on the carbon number of aromatic structure, which shows that the calculated aromatic carbon ratio (fa′) after the dissolution of low molecular weight compounds has the same trend as that of raw coal, and decreases first and then increases with the increase of coal evolution degree. ③ The changes of composition and macromolecular structure of soluble low molecular weight compounds in bituminous coal and anthracite are staged. During the second jump of coalification (0.5%-1.3%，Ro,max), the low molecular weight carboxyl compounds gradually decrease and even disappear, while the structure of aromatic ring is changed from the benzene and naphthalene of 1-2 ring to the benzene, naphthalene and anthracene of 1-3 ring. In the third jump (Ro,max is about 2.0%), the aromatic condensation reaction intensifies, the basic aromatic structure unit increases, and the condensed ring of pyrene and other four ring structure appear. The strong pyrolysis makes the low molecular weight compounds separate from the coal, and the soluble components decrease significantly. It is considered that the variation of THF solvent extraction rate and composition of different grade coals is the result of the different content and type of soluble low molecular compounds generated in the process of coalification, the mode of occurrence, and the force between soluble low molecular compounds and solvent and macromolecular structure.