Role of chemical structures in coalbed methane adsorption for anthracites and bituminous coals

Abstract

An in-depth investigation was conducted on four bituminous coals and three anthracites regarding the chemical structural characteristics using Fourier transform infrared spectroscopy (FTIR). The FTIR spectra of coals can be well approximated by the combination of Gaussian/Lorentzian peaks. FTIR parameters, including aromatic structures, aliphatic structures and oxygen-containing groups (C=O), were determined from curve-fitting analysis. Methane adsorption measurements were carried out through an equilibrium volumetric method, and the Langmuir equation was applied to fit the experimental data. Correlations were established for these FTIR structural parameters and methane adsorption parameters (V L and P L). From the results obtained, it can be seen that rapid changes occur for bituminous coals with the vitrinite reflectance, R o = 1.1–2.1%. The sharp variations of the apparent aromaticity, f a, and A(CH2)/A(CH3) indicate the significant intensive aromatization and the decrease of aliphatic methylene groups, respectively. As a consequence, the size of aromatic clusters is greatly enhanced with increasing coalification. However, the oxygen-containing functional groups, ´C´, varying from 0.43 for sample WJD to 0.10 for sample ZZ, presents a negative linear relationship with R o. Once R o > 2.1% (anthracite), both f a and A(CH2)/A(CH3) tend to be steady. The defects and degree of disorder in coal structure have been reduced to a large extent, and the graphite microcrystalline structure in coal is gradually perfected for anthracites. Coals with higher f a provide more adsorption sites for methane occupation on coal surface, thus demonstrating stronger adsorption capacities. However, the aliphatic methylene groups and oxygen-containing functional groups can decrease the CH4 adsorption amount at low pressure stage (<5 MPa), further reducing the adsorption properties. These findings may have great significance for accurate estimation reserves of coalbed methane and the prevention of gas disasters underground coal mine.