Mechanism Of Small Molecule Organics Influencing On Coal Microstructure And Adsorption Characteristics

The amount of coal mining in China is increasing year by year,and the gas content is also increasing year by year.The small molecule organics in coal is one of the key internal factors that affect the microstructure and gas adsorption.Based on the analysis of the differences of pore structure,fractal characteristics,surface functional groups and methane adsorption characteristics between the raw coal and the residual coal after extraction with tetrahydrofuran,this paper deeply analyzes the effect of small molecular organic compounds on the microstructure of coal,and then discusses its effect on methane adsorption.In this paper,four kinds of coal samples with different degrees of metamorphism,including long-flame coal,fat coal,coking coal and anthracite,were selected to extract residual coal with tetrahydrofuran as extraction solvent.Lubricating oil was selected as the model of small molecule organics,and"increase"（porous medium containing lubricating oil）and"decrease"（dissolved small molecule organics in coal）were used to change the content of small molecule organics in coal.The pore structure parameters of raw coal and residual coal of four kinds of coal samples were tested by nitrogen adsorption experiment at low temperature,and the change characteristics of pore fractal dimension before and after extraction were analyzed.It is found that the pore size range of coal sample is mainly 1¹0nm.The dissolution of small molecule organics has two effects on pore structure,i.e."increasing pore"and"expanding pore".On the one hand,the pores occupied by them were enlarged into larger pores,and the fractal dimensions D₁ and D₂ are reduced,resulting in the reduction of specific surface area,the enhancement of pore structure connectivity and the reduction of complexity.On the other hand,the micropores sealed by small molecular materials were reopened,the fractal dimension D₁ and D₂became larger,a large number of micropores appeared on the surface of coal dust,the surface became rougher,and the internal pore structure became more complex.The content change of micro surface functional groups of raw coal and residual coal was measured by infrared spectrum experiment and peak fitting method.The results show that the small and medium molecular organic compounds in coal provide abundant functional groups for the surface of coal,and the extraction of tetrahydrofuran changes the chemical composition of the micro surface of coal,destroys the hydrogen bond between the small molecular organic compounds and the large molecular structure,which results in the dissolution of oxygen-containing functional groups such as ether oxygen,phenol hydroxyl,carbonyl,etc,At the same time,the aliphatic hydrocarbons with longer fat chain and aromatic substances with more fat side chain are also destroyed and dissolved.The isotherm adsorption experiments of high pressure methane were carried out at 30°C before and after extraction in porous media with and without lubricating oil.Based on Langmuir adsorption model,the effect of small molecular organic compounds on methane adsorption by coal was analyzed.Small molecule organics in coal play an important role in methane adsorption characteristics by changing pore structure and surface functional groups.However,for different coal samples,the effect of pore structure and surface functional groups is different,which will have different effects on the adsorption characteristics.