Investigation on solubility of multicomponents from semi-anthracite coal and its effect on coal structure by Fourier transform infrared spectroscopy and X-ray diffraction

As a sedimentary rock composed of organic carbonaceous matters with different molecular sizes and inorganic minerals, coal can be extracted by organic solvents and dissolved by acids to make chemical compositions and structures changed, thus influencing on the occurrence and migration of coalbed methane (CBM). For the purpose of investigating the impacts of different solvents on coal structure, semi-anthracite coal samples from Huoerxinhe coalmine in the central part of Qinshui Basin were treated by four single solvents (tetrahydrofuran (THF), carbon disulfide (CS2), hydrochloric acid (HCl) and chlorine dioxide (ClO2)), and by combined solvents (an organic solvent and an inorganic solvent with different treatment orders). Then the solubility behaviors of the organic compounds or inorganic matters from coal were investigated and the changes of the functional groups and the microcrystalline structure in coal were addressed using Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The results show the extraction yields of organic solvents are significantly greater than the dissolution yields of inorganic solvents, and the solubility of HCl is relatively higher than that of ClO2. Specifically, main functional groups in coal, including aliphatic, aromatic and oxygen-containing functional groups, have a decline tendency to different degrees under the treatment of organic solvents. Further, main microcrystalline structure parameters increase in vertical direction but reduce in parallel direction of the aromatic carbon network layer with the swelling effect, regardless of an organic solvent or inorganic solvent treatment. Only by the organic solvent treatment, do the d002 values of the basic structure unit of coal appear dramatically enlarged characteristics. Whereas, main functional groups and microcrystalline structure in coal show relatively complex features within the combined solvents treatment because of the phenomenon of pore-blockade, pore-expansion or slight solvent retention. Based on the change characteristics and mechanisms, the chemical structure model of semi-anthracite coal is put forward for providing a reference for the further study of coal structure.