Study On The Microstructure Of Water-bearing Tectonic Coal And Its Adsorption Characteristics Of CH₄ And CO₂

Tectonic coal is formed by the effect of tectonic stress on the primary structure of coal,and the primary structure of coal is destroyed and deformed.The influence of its molecular structure on gas adsorption and desorption is an important reference for coalbed methane disaster prevention and mining.As a natural adsorbent with excellent performance,coal is an effective carrier for CO₂ storage.CO₂-Ehanced Coal Bed Methane,referred to as CO₂-ECBM,is not only conducive to the large-scale development and utilization of coalbed methane,but also can realize the coalbed storage of CO₂ and achieve the purpose of reducing greenhouse gas emissions.In this paper,in order to study the adsorption law of CH₄ and CO₂ in primary structure coal and structured coal,select primary structure coals and structural coals of different mines and different degrees of metamorphism,Through the use of industrial analysis,scanning electron microscopy and X-ray diffraction experiments to test the physical parameters of coal,observe its surface microstructure characteristics,determine and calculate crystal structure parameters;Combined with graphite unit cell parameters,using molecular simulation software to construct 1nm,3nm,5nm primary structure coal and structural coal structure models,using the Grand Canonical Ensemble Monte Carlo method(referred to as GCMC),simulated the adsorption isotherm of CH₄ and CO₂ under different environmental conditions,compared the adsorption characteristics of primary structure coal and structural coal with different metamorphic degrees,analyzed the adsorption law of CH₄,CO₂ single component and binary mixed component in primary structure coal and structural coal under different temperature,pressure,pore structure,and water content,and make a microscopic explanation of the competitive adsorption of CH₄ and CO₂ in primary structure coal and structural coal from the three perspectives of coal rock pore structure difference,gas adsorption mechanism and adsorption potential.The result shows:The adsorption law of CH₄ and CO₂ single component and binary mixed component in primary structure coal and structured coal accords with the characteristics of type I isotherm adsorption curve.With the increase in the degree of metamorphism,the adsorption capacity and adsorption rate of CH₄ and CO₂ for primary structure coal and structural coal gradually increase.Under the same conditions,the adsorption capacity and adsorption speed of coal with the same metamorphic degree for CO₂and CH₄ are higher than those of primary structure coal;Among the primary structure coal and structured coal models with different pore structures,the 1nm pore size primary structure coal and structured coal model have the largest adsorption capacity and adsorption rate for CH₄ and CO₂,followed by 3nm,and 5nm the smallest;With the increase of pressure,the adsorption amount and adsorption speed of CH₄ and CO₂ gradually increase;With the increase of temperature,the adsorption capacity and adsorption speed of CH₄ and CO₂gradually decrease.High temperature promotes the desorption of CH₄ and CO₂,and low temperature helps the adsorption of CH₄ and CO₂;In the measured temperature and pressure range,the adsorption capacity of CO₂ is greater than that of CH₄,and the adsorption speed is faster,indicating that CO₂ has preferential adsorption in primary structure coal and structural coal,and has stronger adsorption competitiveness than CH₄,in addition,the adsorption capacity and adsorption speed of structural coal for CH₄ and CO₂ are greater than that of primary structural coal;However,with the increase of temperature and pressure,the competitive adsorption capacity of CO₂ relative to CH₄ in primary structure coal and structural coal weakens,resulting in a decrease in the efficiency of CO₂-ECBM;The presence of H₂O molecules does not change the adsorption characteristics of CH₄ and CO₂ in primary structure coal and structural coal.The adsorption isotherms of CH₄ and CO₂ single component and binary mixed components still conform to the type I isotherm adsorption curve.The existence of H₂O molecule also did not change the preferential adsorption of CO₂in primary structure coal and structured coal,and the competitive adsorption capacity of CO₂in primary structure coal and structured coal is still stronger than CH₄.but it is always lower than the competitive adsorption capacity without H₂O molecules.Therefore,under the same conditions,the adsorption capacity and adsorption rate of CO₂ in primary structure coal and structured coal are both greater than CH₄,that is,the adsorption capacity of primary structure coal and structured coal for CO₂ is stronger.It shows that CO₂ can play a good ability to displace CH₄ in coal reservoirs,and further provides an important reference for the implementation of the CO₂-ECBM project.