Effect of microstructure and chemical composition of coal on methane adsorption

Abstract

Coalbed methane (CBM) is mainly adsorbed in coal reservoir. To explore the methane (CH4) adsorption mechanism in coal, this paper investigated the relationships among CH4 adsorption characteristics and microstructure and chemical composition of the coal samples through different solvent treatments. The results indicate that after THF and CH3CH2OH treatments, aromatic and aliphatic functional groups decline significantly as hydrocarbon low molecular weight compounds are dissolved, while oxygen-containing functional groups show a remarkably increase after acid solvent treatment. However, it can be found that the inter-layer spacing (d002) and stacking height (Lc) values of the basic structure unit of coal show enlarged characteristics, except for the residue treated by CH3COOH. The macropore content of coal samples under THF treatment increases greatly and the micropores all but disappear, while other samples show an increase. Actually, chemical structure also contributes to nanopore structure. Both the specific pore volume and specific surface area show an opposite trend slightly with microcrystalline structure parameters, while the d002 value gives a ‘U’ shape distribution for pore structure parameters. Further, combined with adsorption curves, adsorption potential theory and Clausius-Clapeyron equation, the adsorption capacity and adsorption heat indicate a physical adsorption process. However, there is no perfect correlation among microcrystalline structure parameters, pore structure distribution and CH4 adsorption. On the other hand, low molecular weight compounds such as aromatic hydrocarbons and aliphatic hydrocarbons combined to macromolecular structures by non-covalent bonds, are easy to form competitive adsorption with CH4 molecule by occupying CH4 adsorption sites. Meanwhile, these aromatic and aliphatic structures as the non-polar functional groups on the surface of coals can also promote CH4 adsorption, but the polar oxygen-containing functional groups (especially –OH and C–O) weaken CH4 adsorption capacity greatly. Totally, the more complex the chemical composition of coal surface is, the more important it is on CH4 adsorption.