Coal macromolecular structural characteristic and its influence on coalbed methane adsorption

Abstract

In order to investigate the effect of coalification on coal macromolecular structure, the combination of Raman spectroscopy and low-pressure N2 gas adsorption (LP-N2GA) was adopted to explore the chemical and physical characteristics of selected samples ranging from bituminous C to anthracite. Raman structural parameters, including band position, band area ratio (AD1/AG) and band position difference (G-D1) of G band and D1 band were derived from curve-fitting analysis. Methane adsorption properties of these samples were measured, and correlations between these Raman parameters and coalbed methane adsorption capacity (VL) were also established. The results indicate that D1 band generally shifts to the lower wavenumbers decreasing from 1365 cm−1 to 1339 cm−1, while G band shifts to higher wavenumbers ranging from 1578 cm−1 to 1609 cm−1 with increasing coal rank. The values of G-D1 increase, but the band area ratio (AD1/AG) declines with the increase of Ro. Pore parameters, including the BET SSA and pore volume, show a polynomial relationship with G-D1, but a reduction for AD1/AG. The increase of graphitization and the order degree of aromatic structures in coal can enhance the porosity in coal. The evolution of coal macromolecular has significant impact on methane adsorption, which displays a U-shape correlation with Raman parameters. Coalbed methane adsorption is not only related to the physical structures, but also to the chemical characteristics, which should be taken into account in practice.