寺河3号煤甲烷吸附解吸热力学特征

为研究煤层气排采过程CH4解吸内在热力学特征及水蒸汽在排水降压产气过程中的作用机理,在20,25,30,35,40℃五个温度点对寺河3号无烟煤(WY)进行等温吸附解吸实验,利用Clausius-Clapeyron方程计算等量吸附热和极限吸附热。结果表明,升压(吸附)过程和降压(解吸)过程极限吸附热分别为23.31 kJ/mol和24.02 kJ/mol,属于物理吸附,但后者大于前者。从热力学角度看,吸附解吸平衡体系中,降压不足以导致煤层甲烷解吸,但降压促使液态水在煤孔隙中形成局部低压,水分子汽化,水蒸汽分子在煤孔隙表面吸附产生的吸附热约为40 kJ/mol,远大于甲烷吸附热,水蒸汽吸附置换煤孔隙表面吸附的甲烷,最终导致甲烷解吸。

Thermodynamic characteristics of adsorption-desorption of methane in coal seam 3 at Sihe Coal Mine

To study the internal thermodynamic characteristics of CH4 desorption in the production process and the mechanism of water vapor in the process of drainage decompression and gas recovery，a series of methane adsorption-desorption isotherm experiments on anthracite in coal seam 3 at Sihe Coal Mine were conducted at 20，25，30，35 and 40 ℃ respectively.Based on Clausius-Clapeyron equation，the isosteric heat of adsorption and maximum heat of adsorption were calculated.These calculations indicate that the maximum heat of adsorption in process of elevated pressure(adsorption)and lowered stress(desorption)is 23.31 kJ/mol and 24.02 kJ/mol.Therefore，it belongs to physical adsorption.However，the latter is greater than the former.From the point of view of thermodynamics，in the adsorption-desorption equilibrium system，dropping pressure alone does not lead to desorption while it causes liquid water to form local low pressure in the coal pores and water molecules vaporizes.The adsorption heat produced by water vapor molecules adsorbing on the coal pore surface is approximately 40 kJ/mol，which is far greater than the adsorption heat of methane，thus the methane on the surface of coal pores will be easily replaced by water vapor，and the desorption of methane eventually occurs.