Response of Molecular Structures and Methane Adsorption Behaviors in Coals Subjected to Cyclical Microwave Exposure

Abstract

To better understand the methane adsorption behavior after microwave exposure, the importance of quantitatively characterizing the effect of cyclical microwave exposure on the molecular structures of coals cannot be overemphasized, with implications for enhancing coalbed methane (CBM) extraction. Thus, cyclical microwave exposure experiments of three different metamorphic coals were conducted, and the methane adsorption capacity before and after each microwave exposure (10 in total) for 120 s was evaluated. Fourier transform infrared spectroscopy analysis and peak fitting technology were applied to quantitatively characterize the changes in the structural parameters of coal molecules. The results showed that after modification, the structural parameters like aromatic carbon fraction (f a–F), aromaticity (I 1 and I 2), degree of condensation (DOC 1 and DOC 2), and the maturity of organic matter (“C”) gradually increased with increasing exposure times, while the length of the aliphatic chain or its branching degree (CH 2/CH 3) and the hydrocarbon generating capacity (“A”) showed a decreasing trend. The Langmuir volume (V L) of three different rank coal samples decreased from 29.2, 32.8, and 40.4 mL/g to 25.7, 29.3, and 35.7 mL/g, respectively; the Langmuir pressure (P L) increased from 0.588, 0.844, and 0.942 MPa to 0.626, 1.007, and 1.139 MPa, respectively. The modification mechanism was investigated by analyzing the relationship between the methane adsorption behaviors and molecular structures in coals. The release of alkane side chains and the oxidation of oxygen-containing functional groups caused by microwave exposure decreased the number of methane adsorption sites. As a result, the methane adsorption capability decreased. In addition, the decomposition of minerals affects methane adsorption behaviors in coals. This work provides a basis for microwave modification of coal as well as in situ enhancement of CBM extraction using microwave exposure.

Synergistic Mechanism of Ultrasonic-Chemical Effects on the CH4 Adsorption–Desorption and Physicochemical Properties of Jincheng Anthracite

“…Some oxygen-containing functional groups with low bond energy, most aliphatic molecules, and some aromatic rings in anthracite macromolecules are broken or evaporated by the mechanical vibrations and thermal effects generated by ultrasonic. Figure shows the schematic diagram of the changes in the molecular structure of coal by ultrasonic …”

“…Figure 4 shows the schematic diagram of the changes in the molecular structure of coal by ultrasonic. 32 (3) The degree of change in the area of the characteristic peaks in the T4 infrared spectrum is significantly higher than that in the other samples. Ultrasonic loading directly affects the surface group distribution pattern of coal and makes the SDS micelles more easily adsorbed on the anthracite surface.…”

Controversial Issues of Hydrocarbon Field Formation and the Role of Geomagnetic Fields

“…In terms of molecular changes in the composition of oil when exposed to microwave radiation, an increase in the degree of polycondensation of aromatic structures, reducing the length of alkane side chains, and increasing the degree of maturity of organic matter [33]. Figure 3 displays a scheme of catalytic reactions when the organic matter is exposed to microwave radiation.…”

“…Polycondensation of aromatic rings, the release of alkane side chains, and oxidation of oxygen-containing functional groups caused by microwave exposure are the causes of changes in the molecular structure parameters of organic matter [33]. Another work on the microwave exposure of bituminous coals notes that the amount of aliphatic hydrocarbons is decreased by the decomposition of methylene bridging bonds, and hydroxyl self-associated hydrogen bonds are broken by microwave radiation (phenol and carboxylic acid are converted into either groups) [34].…”

“…Figure3: Scheme of complex catalytic reactions when organic matter is exposed to microwave radiation[33].…”