Archive | 2019
Effects of the Physical and Chemical Structure of Bituminous and Anthracite Coals on CH 4 Adsorption
Abstract
CH4 adsorption in coal is the basis of coalbed methane (CBM) exploration and exploitation. In this study, five coal samples collected from the Qinshui Basin with different ranks were used for an isothermal adsorption test. Mercury intrusion porosimetry, the nitrogen adsorption method, and a CO2 adsorption experiment were used to identify the pore characteristics of the coal samples. Fourier transform infrared spectroscopy was used to calculate the contents of the main organic groups in the coal samples. Then, the effects of coal’s physical and chemical structures on the CH4 adsorption were studied. The results show that pores in coal determine the adsorption behavior of CH4. When the equilibrium pressure ≤3 MPa, the filling adsorption of CH4 in micropores determines the theoretical maximum adsorption quantity and adsorption rate of CH4, causing rapid rises in the isothermal adsorption curves. When the equilibrium pressure >3 MPa, CH4 adsorbs in macropores in the form of surface coverage, and the adsorption quantity is controlled by the pore specific surface area and adsorption layer number. The development of macropores is beneficial to the migration of CH4 to micropores and can improve the adsorption rate of CH4. The organic groups of coal mainly influence the adsorption density of CH4 and the chemical bond between CH4 and organic groups. Hydroxyl groups and nitrogen-containing groups can increase the adsorption density of CH4, thus increasing the theoretical maximum adsorption quantity of CH4. Oxygen-containing groups can form strong polar hydrogen bonds with CH4, thus increasing the adsorption stability and adsorption rate of CH4.