Research On The Influence Of Electrode And Electrolyte Characteristics On Methane Adsorption In Anthracite Modified By Electrochemical Treatment

The methane in coal reservoirs is a high-quality clean energy.However,methane was strongly absorbed on the anthracite,and the permeability was poor,which seriously affects the development and utilization of methane.Electrochemical modification can change the pore and fracture structure and surface characteristics of anthracite effectively,thus reducing the methane adsorption and enhancing the methane desorption and seepage.However,the existing researches mainly focus on the influence of electric potential gradient and electrolyte concentration on pore and fracture structure and the surface groups of coal samples.The optimization of electrode materials and electrolyte types and the molecular dynamics and energy mechanisms of electrochemical modification on anthracite is limited.Given this,the No.15 anthracite sample from Sihe mine,Jincheng,Shanxi Province,was selected for modification with different electrode materials and electrolyte types in the paper.Through the comparative analysis of the adsorption and desorption characteristics of coal samples before and after modification,combined with molecular dynamics simulation to explain the microscopic mechanism of the modification effect.The results obtained from this work are expected to provide the basis for the optimization of electrode materials and electrolyte for the realization of enhancing methane extraction from anthracite via an electrochemical method.The main work and conclusions are as follows:（1）The modification effect and mechanism of different electrode materials.In the self-developed electrochemical modification experimental device,four kinds of electrodes,iron,copper,aluminum and graphite,were used to carry out electrochemical modification experiments on anthracite samples.The effects of four electrode materials on methane adsorption capacity and desorption rate of electrochemical modified anthracite were compared.The modification effects of the four electrode materials were analyzed through the changes of pore structure and surface characteristics of coal samples after modification.The results showed that the methane adsorption capacity of anthracite samples modified by the four electrode materials decreased and the desorption rates increased,the effect of graphite electrode modification is the most obvious.The difference in the modification effect of different electrode materials on anthracite lies in the difference in electrode reactions.On the one hand,the graphite electrode produces more H⁺ions to acidify the coal surface,which reduces the adsorption sites of methane on the coal surface,as a result,the amount of methane adsorption.On the other hand,the generated H⁺ions dissolve the carbonate and FeS₂ minerals in the coal,and the connectivity of pores in coal is increased,which increased the methane desorption rate.Metal electrodes such as aluminum,iron,and copper occur oxidation reactions at the anode to generate metal ions and H⁺ions.The reduction reaction of cathode and lattice substitution of minerals occupies the adsorption sites of methane,which leads to the decrease of methane adsorption capacity.The basic reason for the decrease of methane adsorption capacity in anthracite after electrochemical modification of different electrode materials is the increase of acid groups on coal surface caused by graphite electrode modification and lattice substitution caused by metal electrode modification,which reduces the surface energy of coal.（2）The modification effect and mechanism of different electrolytes.In the experimental device of electrochemical modification,anthracite was electrochemically modified with Na₂SO₄,Al₂（SO₄）₃,Fe₂（SO₄）₃,and CuSO₄electrolyte solutions,respectively.The effects of metal cation types in the electrolyte on methane adsorption capacity and final desorption rate of electrochemically modified anthracite were compared.The modification effects of the four electrolytes were analyzed through the changes of pore structure and surface characteristics of coal samples after modification.The results showed that the methane adsorption capacity of anthracite was decreased and the desorption rares was increased,Al₂（SO₄）₃ electrolyte is the most effective.The difference of modification effect of different electrolytes lies in the difference of migration ability of metal cations in electrolyte and the difference of reaction ability between metal cations and coal surface groups.In the process of electrochemical modification,the migration of sodium ions is strong,but the reaction ability with coal surface groups is poor,resulting in poor modification effect of Na₂SO₄ electrolyte.The migration ability of iron and copper ions in electrochemical modification process is poor,but the reaction ability with coal surface groups is strong,which leads to the better modification effect of Fe₂（SO₄）₃ and CuSO₄ electrolyte solution.In the process of electrochemical modification,Al₂（SO₄）₃ electrolyte solution has the best modification effect because of its strong migration ability and strong reaction ability with coal surface groups.The main reason for the decrease of methane adsorption capacity of anthracite after electrochemical modification of different electrolytes is that the surface energy of coal is reduced by the migration of metal cations and the reaction with coal surface.（3）The changes of molecular structure of anthracite after electrochemical modification.The molecular structures of anthracite used in this experiment and its change before and after electrochemical modification were studied by elemental analysis,Fourier transforms infrared spectroscopy（FTIR）,solid-state¹³C nuclear magnetic resonance(¹³C NMR),X-ray photoelectron spectrometry（XPS）,and High-resolution transmission electron microscopy（TEM）.The results show that the aromatic structure of the No.15 anthracite sample obtained from Sihe mine,Jincheng is mainly composed of naphthalene ring,anthracene ring and phenanthrene ring;there are some oxygen bridged carbon in the coal structure,which is mainly in the form of single bond,while carbonyl carbon and carboxyl carbon are less;the fatty carbon exists in the form of methylene and methylene,while the oxygen-containing functional groups are mainly carbonyl oxygen and carboxyl oxygen.Electrochemical modification mainly reduced the number of alkyl groups on the surface,increased the content of carboxyl group,and the change of the aromatic structure in anthracite is limited.Based on the test results,the macromolecular average structure model of anthracite samples was constructed.The accuracy of the molecular model was verified by carbon content,coal density and methane adsorption,which provided a model basis for molecular simulation of methane adsorption.（4）Molecular dynamics simulation of the influence of electrochemical modification on methane adsorption of anthracite molecular model.Based on the molecular structure model of anthracite in No.15 anthracite sample obtained from Sihe mine,Jincheng,the effects of electrochemical modification on methane adsorption on anthracite surface were simulated and analyzed.The results show that after electrochemical modification,the adsorption capacity of anthracite to methane molecules decreases,while the self diffusion coefficient of methane molecules increases,which is more conducive to methane desorption.Through the analysis of the interaction energy between anthracite and methane molecules before and after electrochemical modification,it can be seen that the decrease of the interaction energy between anthracite and methane molecules after electrochemical modification is the fundamental reason for the decrease of methane adsorption capacity.