基于吸附层厚度理论的软硬煤吸附机理解析

针对同一变质程度软/硬煤的比表面积和总孔容积相差数倍，但其对甲烷吸附量却相当这一现象,根据热力学原理及煤对甲烷吸附机理，建立了煤的孔径对甲烷吸附层厚度的方程，数值分析了吸附压力和孔径对吸附层厚度（吸附层数）的影响，同时采用软/硬煤的孔径分布拟合函数，数值计算了软/硬煤的瓦斯等温吸附曲线，并与实测结果进行了对比分析。研究结果表明：基于吸附层厚度理论，在同一吸附平衡压力下，甲烷吸附层厚度随着孔径增大呈负指数变化，即煤体对甲烷的吸附是不同分子层的集合。采用煤体中孔径与其孔体积的分段函数和煤对甲烷的吸附层厚度理论，计算得到的瓦斯吸附等温线无论是变化趋势还是定量上均与实测结果一致，误差小于6.5%。因而，吸附层厚度理论很好地揭示了软/硬煤对甲烷吸附特征。由此，只要测得煤的孔径分布特征，即可采用吸附层厚度理论对其吸附量进行计算，为预测煤层瓦斯含量提供新方法。

Mechanism analysis on adsorption properties of soft and hard coal based on adsorption layer thickness theory

Through the test of specific surface area,total pore volume and adsorption characteristic of soft and hard coal,it is found that for the same metamorphic degree,the specific surface area and total pore volume of soft coal is several times to that of hard coal,but the adsorption capacity of soft and hard coal is equivalent.In order to solve this problem,according to the thermodynamic principle and methane adsorption mechanism in coal,adsorption layer thickness theory in coal body pore was established to numerically calculate and analyzed adsorption layer thickness (adsorption layer number),which is effected by pore and adsorption equilibrium pressure.At the same time,pore size and pore volume distribution were measured by mercury injection test,and adsorption isotherms of methane in coal was calculated.The results show that based on the adsorption layer thickness theory,the methane adsorption layer thickness decreases with negative index relation to pore diameter at the same adsorption equilibrium pressure,which indicates that methane adsorption in coal has different molecular layer.The relation of pore size and its pore volume can be fitted by piecewise functions.Gas adsorption isotherms in different coal samples are calculated based on the layer thickness,and both the change trend and quantify are consistent with the measured results,the error is less than 6.5%.Therefore,the adsorption layer thickness theory reveals the methane adsorption characteristics of soft and hard coal.Thus,as long as the pore size distribution characteristics of coal is measured,adsorption quantify can be calculated by adsorption layer thickness theory,which provides a new method for the gas content prediction in coal seam.