Effects of CO2 adsorption on molecular structure characteristics of coal: Implications for CO2 geological sequestration

With a series of global problems such as greenhouse effect caused by the gradual increase of atmospheric CO2 concentration, CO2 geological storage has become one of the effective measures. To study the influence of the interaction between CO2 and coal on the molecular structure characteristics of coal, three coals of different ranks were selected for CO2 adsorption and were subjected to X-ray diffraction, Raman spectroscopy and Fourier transform infrared spectroscopy experiments. The results indicate that with increasing of CO2 pressure: 1) Due to the solubility differences of different minerals, the contents of quartz in the coal samples gradually increases, while the contents of calcite, pyrite and kaolinite decreases; 2) The average number of the aromatic layers gradually decreased indicates that some aromatic layers are destroyed and order degree is lowered in the coal microcrystalline structure, resulting in a gradual increase of the inter-layer spacing between aromatic layers, the stacking of aromatic layers; 3) The concentration of aromatic rings gradually decreases leading to the intermolecular structural defects are obvious and the gradual evolution of the microcrystalline structure into the more disordered crystal structure; 4) The hydroxyl groups are predominated by oxidation reactions in long-flame coal and lean coal, while they are mainly governed by swelling in anthracite; 5) The ratio between aliphatic and total hydrogen atoms (F-al/H), aliphatic structure (I-1) and 'A' factor values of the anthracite reveal a slight increase, while the aromaticity 2 (AR2) and degree of condensation 2 (DOC2) values display a decreasing trend. These values of long-flame coal and lean coal only slightly change. The influence of CO2 intrusion on coal properties due to the changes of molecular structure and CO2 trapping mechanisms are discussed. Our research contributes to understand deeply the geological sequestration of CO2 in coal seams and higher CO2 injection pressures should be adopted.