The Characteristic And Influencing Factors Of High-pressure Methane Adsorption On Dry Coals

Isothermal adsorption experiments of coal samples of different coal metamorphic degrees carried out to study the adsorption characteristics of methane in coal under high pressure,which is of great significance for the exploration and development of deep coalbed methane(CBM)resources.Based on low temperature carbon dioxide adsorption experiment,low temperature liquid nitrogen adsorption experiment and mercury injection experiment,the pore structure of coal was characterized with full pore size.Based on the fourier transform infrared spectrum experiment(FTIR),the surface chemical structure of coal was studied.The effects of pore characteristics and surface chemical structure on methane adsorption on coal were analyzed by numerical simulation,and the dynamic adsorption process of methane in coal with the change of adsorption pressure was defined.The following results achieved:(1)The pore characteristics of coal were characterized.The pore volume of coal is mainly composed of ultramicropore,mesopore and macropore.The specific surface area is mainly provided by the ultramicropore.The fractal dimension increases with the increase of the aperture of the pore.The fractal dimension of ultramicropore,which is mainly controlled by pore geometry,is small and stable.The fractal dimension of micropore and pore is controlled by pore geometry and surface chemical structure.The fractal dimension of mesopore and macropore is the largest.(2)Semi-quantitative analysis of coal surface chemical properties with coal rank was carried.With the increase of coal rank,the coal maturity,aromaticity,degree of polymerization of aromatic rings,relative content of oxygen-containing functional groups and relative content of hydroxyl(hydrogen bond)gradually increased;the relative length of aliphatic side chain decreased and the ratio of C-O/C=O decreased.(3)The adsorption amount of methane in coal reaches a maximum at 11-15 MPa.When adsorption pressure is distributed at 11-15 MPa,the increasing density of free methane causes the contraction of coal matrix,the number of effective adsorption potential place of coal sample decreases.Some methane desorbs from the coal surface,the adsorption amount of methane on coal begins to decrease.(4)The factors affecting the adsorption characteristics of methane in coal were studied.The volume and specific surface area of ultramicropores and micropores mainly control the adsorption capacity of coal to methane by controlling the number of adsorption place provided by coal.The fractal dimension,pore type,condensation degree of aromatic ring,length of aliphatic side chain and relative content of different oxygen-containing functional groups affect the adsorption capacity of methane in coal by controlling the adsorption potential of coal surface.(5)Based on the different adsorption positions of methane in coal under different adsorption pressures,the adsorption process is divided into four stages.Stage 1: single layer adsorption on the surface of the ultramicropore and "henhouse" filling adsorption in the micropore.Stage 2: filling adsorption in ultramicropore;Stage 3: lamellar filling adsorption in micropores;Stage 4: the adsorption amount reaches the maximum and the adsorption stops.