| The prevention and control of mine gas disaster and precise extraction of coalbed methane(CBM)are closely related to microstructure characteristics of coal and the law of CH4 adsorption.Studying the influence of pore and functional group characteristics of different rank coals on CH4 adsorption characteristics is beneficial to accurately predict the content of CH4.In this paper,8 groups of coal samples from typical mining areas were selected,and analytical methods such as exp erimental testing,theoretical analysis,numerical analysis were used,the pore size distribution(PSD),fractal dimension(D),functional group characteristics of samples,and influencing factors on the adsorption capacity of CH4 were analyzed respectively.Experiments including the carbon dioxide adsorption(LPCDA),low temperature liquid nitrogen adsorption(LTN2A),and mercury injection porosimetry(MIP)were carried.After considering the valid pore range o f those methods,pore size of 2 nm and 100 nm were taken as the boundary points of the porosity,and the full-PSD of samples were obtained.In addition,the spearman analysis method was used to analyze the main influencing factors of pore development-The results show that pore volume(PV)exhibits the bimodal peak with the increase of pore size and the average contribution of ultra-micropares to PV is 51.44%.According to the cumulative PV of the stage,they are sorted from large to small,the order of pore size is the ultra-microporcs,mesopores,small pores,micropores,and macropores,and that of coal rank is high-rank,low-rank coal,and medium-rank.Besides,specific surface area(SSA)of pores exhibits a single peak with the increase of pore size and the average contribution of ultra-micropores to SSA is 96.30%.Similarly,the cumulative SSA of the stage also are sorted from large to small,the order of pore size is ultra-microporcs,micropores,small pores,mesopores,and macropores,and that of rank is high-rank,low-rank,and medm-rank.Although the pore development is significantly a ffected by coalization with turning points of vitrinite reflectance(R0)at 1.1%and 3.1%,the influenccs of coal quality characteristics and micro-components are more complex and less influential.The PSD characteristics are analyzed and the D or multi-fractal parameters are obtained through the fractal theory.In addition,the gray relation analysis(GRA)method was used to tnalyze the main influencing factors of fractal parameters.The results show that the D of adsorption pores is larger than that of absorption pores or seepage pores.And the heterogeneity of PSD for the adsorption pores is stronger than the absorption pores.Besides,the PSD characteristics have a greater influence on the fractal parameters than the coal quality characteristics and micro-components.The types,contents,and structural characteristic parameters of functional groups were analyzed by FTIR based on the Gaussian curve for peak fitting.It is concluded that types of functional groups include mainly oxygen-containing functional and hydroxyl groups,while aliphatic hydrocarbons and aromatic hydrocarbons are less,There is a positive correlation between the aromatic degree or condensation degree of aromatic rings and R0,while there is a negative correlation between the relative length of fatty side chain or the relative content of different oxygen-containing functional groups and R0.Based on the experiment of high-pressure capacity adsorption(HCA),the modified adsorption isotherm curves were obtained considering the influence of the adsorption phase volume,and the maximum adsorption capacity(MAC)of CH4 in the equilibrium state at 30℃was obtained by further fitting the adsorption model.In order to explore the relationship between the MAC of CH4 and 1 9 microstructure characteristic parameters,and even the main occurrence form of CH4,the principal component analysis(PCA)and multi-linear regression(MLR)methods were adopted,It was found that pores size which below 10 nm provide the main space of existing for CH4.In addition,the main influencing factors of CH4 include the PV of ultra-micropores,the SSA of ultra-micropores,the relative length of fatty side chain,the condensation degree of aromatic rings,the relative content of different oxygen-containing functional groups,the aromatic degree of aromatic rings,the D of absorption pores,the D of adsorption pores,the SSA of micropores,and the PV of micropores,and the degree of influence diminishes accordingly.Then,the theoretical MAC of CH4 was calculated based on the monolayer adsorption theory and the micropore filling theory.Through the relative error analysis,the theoretical and experimental results were verified each other,and then the mian occurrence forms of CH4 were discussed.The research results will provide the certain data and theoretical basis for improving the basic theoretical system of gas adsorption and desorption with different rank coals. |
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