Study On Adsorption Characteristics Of Granular Coal On Methane In Supercritical State

Methane exists mainly in coal seam as adsorption state.How to accurately describe the adsorption capacity of methane in coal is one of the key problems in the estimation,development and utilization of coalbed methane reserves.With the increase of coal mining depth,part of the methane in coal has reached supercritical state,the adsorption isotherm of methane in coal would appear supercritical adsorption phenomenon: first rising after falling,if we apply directly,the adsorption amount of methane would be small,it is necessary to correct the experimental data measured before using,and the correction methods have not coming to an agreement at home and abroad.Aiming at this problem,this paper manufactures the granules coal sample from Hebi sixth coal mine and Longshan coal mine of Anyang-Hebi coal field,using magnetic suspension balance to measure the isothermal adsorption curve of12 points at the pressure is 1 ～ 24 MPa,the temperature is 308 K,313K and 318 K.Author used the adsorption model to calculate the adsorption phase density and absolute adsorption capacity,discussed the factors of the adsorption phase density and absolute adsorption capacity of supercritical methane,and the isothermal adsorption line was predicted by using the adsorption heat theory.The results are as follows:(1)The adsorbed phase density of supercritical methane increases with the increasing of pressure and tends to be stable gradually,and it is affected by temperature,pressure and coal metamorphism degree.The lower the temperature,the higher the pressure and the higher the coal metamorphism degree,the higher the adsorbed phase density of supercritical methane.(2)The adsorption phase density is calculated by intercept method,Langmuir ternary model method,L-F quaternion model method and liquid density method,substituting it into Gibbs excess adsorption model,and the excess adsorption amount is corrected to the absolute adsorption amount.The fitting degree of the model,the rationality of the correction results and the law of change with the adsorption heat are used as the judgment basis.It is found that Langmuir ternary model is the most appropriate method to describe the adsorption of critical methane in coal.(3)The corrected results of absolute adsorption quantity show that the methane in particles coal sample under the condition of supercritical can still be characterized by Langmuir adsorption model,the lower the temperature,the higher the pressure,the higher the metamorphic grade of coal,the stronger adsorption ability of coal to methane in measuring range,under the same temperature and the metamorphic grade,with the increasing of the pressure,the methane adsorption quantity tends to saturation;(4)The calculation of coal surface free energy reveals the reason why the coal with higher metamorphic degree absorbs more methane.The prediction of adsorption capacity under different temperature is carried out by using the theory of equal adsorption heat,and the error is less than 5%.This method has great practical application value.