Effect of microstructures on methane adsorption characteristics of soft and hard coal

LIU Xianfeng; SONG Dazhao; HE Xueqiu; NIE Baisheng

Abstract：The microstructural differences in coal pore characteristics and physical-chemical structure have great influence on coalbed methane adsorption.Focusing on 3 pairs of soft and hard coal samples with different ranks,the methane adsorption properties were analyzed through isothermal adsorption experiments.The pore structural characteristics were also studied using low pressure liquid nitrogen adsorption,and the control mechanism of porous structure to methane adsorption was revealed from microcosmic level.FTIR experiments were conducted to investigate the chemical microstructure features on coal surface and their microcosmic influencing mechanism on methane adsorption behavior.The results show that methane adsorption properties vary a lot among different soft and hard coal specimen.VL increases from 17.35 cm3/g to 37.89 cm3/g,and pL changes at the range of 0.88-1.75 MPa with coal rank increasing.For fat coal and coking coal,the methane adsorption capacity of soft coal is significantly higher than that of the corresponding hard coal,but the condition is opposite for anthracite.The total specific surface area of soft coal is always much greater than that of hard coal.The pore size distribution reveals that all the peak values fall in the range of 3-10 nm (micropores).The specific surface area ratio of micropores is more than 50％,indicating that the pore specific surface area is dominated by micropores.Compared with hard coal of the same rank,soft coal of fat coal and coking coal have greater specific surface area and less oxygen-containing groups,and display advancing evolution characteristics,thus exhibiting higher methane adsorption capacity.While for anthracite,the coalification degree of soft coal is lower than that of hard coal,and soft coal contains more oxygen-containing groups,resulting in lower methane adsorption capacity of soft coal.The behavior of coalbed methane adsorption is not only affected by pore structure,but also closely related to the chemical structure,which should be simultaneously considered in practical application.