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.