Micro Raman characterization of the response of medium rank coals structure to methane adsorption

The volumetric expansion of coal with methane,carbon dioxide and other fluids and its mechanism is always one of the important topics in the research of coalbed methane geology and coal and gas outburst prevention. Previous studies focused on macroscopic adsorption expansion and deformation and its explaination from the perspective of mechanics. In fact,as a molecular system composed of macromolecules,the interaction between coal and methane and carbon dioxide is a molecular phenomenon,and the nature of expansion should owing to the change of molecular structure. Therefore,revealing the response characteristics of coal macromolecular structure to adsorption and expansion is the basis for understanding its mechanism. The chemical structure characteristics of 8 mediumrank coal samples (the maximum vitrinite reflectance R o=1.08%-1.80%) before and after methane adsorption were studied by MicroRaman spectroscopy,and structure parameters were calculated by using Origin 8.5 software before and after methane adsorption. The results show that the band position difference of D and G peak (d(G-D)) of both raw and adsorbed coal increase and the full widths at half maximum of G band (FWHM-G) decrease with the increase of coal rank,indicating that the order degree and microcrystalline size of coal structure increases with the increase of coal rank with R o ranging from 1.08% to 1.80%. The relative amount of more ordered rings (A D/A G) of coal samples increases first and then decreases,which is the result of the transition from the formation of more ordered aromatic rings (≥6) to the formation of graphite structure. With the increase of coal rank,the “impurity” among the large aromatic rings (A S/A D) and the content of C—C structures on hydroaromatic rings (A S/A total) of raw coal have a decreasing trend,indicating that the sp 2-sp 3 carbonaceous structures or C—H on aromatic rings decrease,and the order degree increases. When R o is greater than 1.3%,compared with raw coals,d(G-D) and A D/A G of the adsorbed sample decrease,while the ratio of small aromatic rings to large aromatic rings