The anthracite was electrochemically treated in Fe 2 (SO 4 ) 3 and CuSO 4 electrolytes, and the adsorption capacity and desorption ratio of CH 4 on the anthracite were investigated using adsorption and desorption tests. The experimental results demonstrated that the saturated adsorption capacity of CH 4 on anthracite decreased from 41.49 to 38.31 mL g −1 after being modified by the Fe 3+ ion electrolyte and further decreased to 34.84 mL g −1 after being modified by the Cu 2+ ion electrolyte. The CH 4 desorption ratio increased from 63.43 to 73.66% and 84.87% after modification in Fe 3+ and Cu 2+ ion electrolytes, respectively, and this improvement was caused by the enlargement of the diffusion coefficient D. The effect of the Cu 2+ electrolyte on CH 4 adsorption/desorption was better than that of the Fe 3+ electrolyte, and the mechanism was analyzed by the changes in the surface energy, functional groups, and pore characteristics before and after modification. The results obtained from this work provide a basis for the optimization of ion electrolytes when accelerating methane extraction via an electrochemical method.

The strong adsorption capacity of methane in anthracite can seriously affect the methane extraction. Electrochemical treatment is an effective way to weaken the capacity of methane adsorption in coal. Iron, copper, aluminum and graphite as four kinds of electrode materials were selected to modify anthracite by electrochemical treatment. The adsorption of methane in anthracite, before and after modification, was tested under different adsorption pressure. Based on the changes of pore characteristics and chemical groups of anthracite, the modification process of different electrode materials was analyzed. The results showed that after electrochemical modification, the adsorption of methane decreased, when the graphite electrode was used, the methane adsorption decreases the most, followed by copper and iron electrodes, and the aluminum electrode decreased the least. After electrochemical modification using aluminum, iron, copper and graphite electrodes, the Langmuir constant a reduced by 5.22%, 8.48%, 9.24% and 11.33%, respectively, and the degree of reduction is graphite > copper > iron > aluminum. After electrochemical modification using the graphite electrode, the Langmuir constant b was reduced by 23.52%. On the contrary, after electrochemical modification using the mental electrodes, the Langmuir constant b was increased by about 5%. The surface free energy of anthracite decreased with the adsorption of CH4, the higher the pressure, the more the free energy decreased, and the reduction of surface energy decreased after electrochemical modification. The difference of the electrode reactions was the main reason for the electrochemical results, the Mn+ ions generated in the anode changed the properties of the clay mineral in the coal, and the H+ ions corroded the calcite minerals in the coal. The results obtained from this work indicate that the selection of electrode materials is crucial for the electrochemical modification, and graphite electrode is optimum for anthracite when accelerating methane extraction by electrochemical method.

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The application of electrochemical modification for accelerating methane extraction in lean coal seams is limited due to the lack of experimental and theoretical research studies. Therefore, electrochemical modification with different electric potential gradient values was selected to modify lean coals in this study; meanwhile, the amount of methane adsorption and the methane desorption ratio were tested and analyzed. The results showed that the maximum amount of methane adsorption in coal samples decreased after electrochemical modification and the decrease in methane adsorption increased with an increase in electric potential gradient. The methane desorption ratio increased from 83.20% up to 87.84 and 86.90% at the anode and cathode zone, respectively, after electrochemical modification using a 4 V/cm electric potential gradient. A higher electric potential gradient performs better in the electrochemical modification. The mechanism of electrochemical modification using different electric potential gradients was revealed based on the measurements of Fourier transform infrared spectroscopy and liquid nitrogen adsorption. It is due to an increase in acid groups in coal molecular structure and the change of the specific surface area of coal after modification. The results obtained from this work contribute to the methane extraction via the electrochemical method in lean coal seams.

The application of cyclical microwave modification for accelerating the extraction of coalbed methane (CBM) from anthracite is limited. In this study, the apparent permeability of anthracite samples before and after each microwave treatment (three in total) for 120 s was measured by a self-built permeability-testing platform. Microcomputed tomography (micro-CT) technology and image-processing technology were employed to analyze the 3D micron-scale pore structures, especially the quantitative characterization of connected pores and throats. After modification, the average apparent permeability increased from 0.6 to 5.8 × 10 –3 μm 2 . The generation, expansion, and connection of micron-scale pores and fractures became more obvious with each treatment. The total porosity increased from 3.5 to 6.2%, the connected porosity increased from 0.9 to 4.8%, and the porosity of isolated pores decreased from 2.5 to 1.4% after three cycles. The number, volume, and surface area of the connected pores as well as the number, radius, and surface area of the throats were significantly increased. In addition, the release of alkyl side chains from the anthracite surface reduced the capacity of the anthracite to adsorb CH 4 and the decomposition of minerals promoted the development and connectivity of pores. As a result, the gas seepage channels have been greatly improved. This work provides a basis for micron-scale pore characterization after cyclical microwave modification and contributes to CBM extraction.