Microwave irradiation-induced alterations in physicochemical properties and methane adsorption capability of coals: an experimental study using carbon molecular sieve

In order to comprehend the applicability of microwave irradiation for recovering coalbed methane, it is necessary to evaluate the microwave irradiation-induced alterations in coals with varying levels of metamorphism. In this work, the carbon molecular sieve combined with KMnO4 oxidation was selected to fabricate carbon molecular sieve with diverse oxidation degrees, which can serve as model substances toward coals. Afterwards, the microwave irradiation dependences of pores, functional groups, and high-pressure methane adsorption characteristics of model substances were studied. The results indicated that microwave irradiation causes rearrangement of oxygen-containing functional groups, which could block the micropores with a size of 0.40–0.60 nm in carbon molecular sieve; meanwhile, naphthalene and phenanthrene generated by macro-molecular structure pyrolysis due to microwave irradiation could block the micropores with a size of 0.70–0.90 nm. These alterations in micropore structure weaken the saturated methane adsorption capacity of oxidized carbon molecular sieve by 2.91%–23.28%, suggesting that microwave irradiation could promote methane desorption. Moreover, the increased mesopores found for oxidized carbon molecular sieve after microwave irradiation could benefit CH4 diffusion. In summary, the oxidized carbon molecular sieve can act as model substances toward coals with different ranks. Additionally, microwave irradiation is a promising technology to enhance coalbed methane recovery.