Adsorption Mechanisms of High-Pressure Methane and Carbon Dioxide on Coals

Abstract

Carbon dioxide (CO 2 ) geological sequestration and coal-bed methane (CBM) recovery in deep coal seams are usually operated with a pressure higher than 10 MPa. The adsorption mechanisms of methane (CH 4 ) and CO 2 on coals in such a situation, however, are not yet revealed. With the help of a high-pressure gas adsorption system, CH 4 and CO 2 adsorption isotherms were first conducted on two coal samples. Simplified local density (SLD) theory was then tailored and applied to describe the adsorption characteristics of specific CH 4 and CO 2 on coals. Next, the adsorption mechanisms of high-pressure CH 4 and CO 2 on coal samples were revealed on the basis of adsorbed and bulk density distributions within the matrix pores. The results show that the high-pressure gas adsorption on coals is different from that at low pressure. The excess adsorption capacity first increases and then decreases with pressure, and the maximum value occurs at a specific pressure referring to reverse pressure. The maximum excess adsorption capacity of CH 4 is less than that of CO 2 , while its reverse pressure is greater than that of CO 2 . The reversal of excess adsorption with pressure depends on the relative increase in adsorption and bulk phases. However, the mechanisms of adsorption reversal vary with gas types. The reversal of CH 4 excess adsorption is due to the saturation near the pore wall, while the CO 2 excess adsorption reversal is due to the dramatic bulk density changes near the critical point. Advances in the mechanism of high-pressure gas adsorption on coals suggests that coal seams deeper than 1000 m have more recovery potential, and CO 2 -enhanced coal-bed methane recovery success depends on the optimal management of its injection pressure and the associated coal swelling.

H2, N2, CO2, and CH4 Unary Adsorption Isotherm Measurements at Low and High Pressures on Zeolitic Imidazolate Framework ZIF-8

“…However, such practice is outside the scope of this study, and we report here only the measured quantities of excess and net adsorption. Furthermore, we observe crossing of the CO 2 isotherms after reaching the excess maximum when plotted as a function of pressure, consistent with previous observations on other porous media. , We attribute such observation to the rapidly increasing CO 2 density of the bulk phase at lower temperatures. , Therefore, we do not see the same effect in the isotherms of CH 4 , the bulk density of which does not show a sharp increase at the same temperatures, and the crossing of CO 2 isotherms also disappears when plotted as a function of density.…”

“…46,47 We attribute such observation to the rapidly increasing CO 2 density of the bulk phase at lower temperatures. 23,48 Therefore, we do not see the same effect in the isotherms of CH 4 , the bulk density of which does not show a sharp increase at the same temperatures, and the crossing of CO 2 isotherms also disappears when plotted as a function of density.…”

Adsorption Characteristics and Thermodynamics of CH4, CO2, and N2 on Shale at Different Temperatures

“…Valve A2 was closed, and injecting of gas into the reference cylinder was continued. The adsorption test was repeated by changing the gas pressure, and the adsorption amount (N ads ) can be expressed as 41 (3)…”

“…The constant temperature system was set at a temperature of 110 °C, and the time was set at 12 h. The valves A1, A2, and A3 were opened, and the vacuum pump was used for high-temperature vacuum degassing to minimize the effect of water content on the results. Air tightness check: The instrument was adjusted to the experimental temperature, the valve A0 was opened, and high-pressure helium was injected at a higher pressure than the maximum test pressure into the sample cylinder, which was observed continuously for 12 h. If the pressure change of the system is less than 6 × 10 –3 MPa, no bubbles came out of the interfaces and from the bolts connecting the sample cylinder, and the pressure in the system was basically stable, it was considered that the air tightness of the system was good, and the next step could be carried out. Measurement of the reference cylinder volume: The adsorption system was vacuumed for 4 h, then valves A0, A2, and A3 were closed, and valves D0 and D1 were opened, the known state of He was injected into the reference cylinder using the AJP-100 calibrator, and the pressure was recorded after equilibration. The volume of the reference cylinder can be calculated by eq : P He cal V cal Z He cal R T = P He ref V ref Z He ref R T where P He cal and P He ref are the pressures of the calibrator and the reference cylinder, respectively, MPa; V cal and V ref are the volumes of the calibrator and the reference cylinder respectively, cm 3 ; Z He cal and Z He ref are the compressibility factors of the gas in the calibrator and the reference cylinder, respectively; R is the ideal gas constant, at approximately 8.314 J/(mol·K); T is the experimental temperature, K. Measurement of the effective adsorbed volume: Valve A2 was opened, the sample cylinder was connected to the reference cylinder, and the pressure was recorded after equilibration. The volume of the effective adsorbed volume within the adsorption cylinder can be calculated by eq : P He ref V ref Z He ref R T …”

“…The volume of the reference cylinder can be calculated by eq : P He cal V cal Z He cal R T = P He ref V ref Z He ref R T where P He cal and P He ref are the pressures of the calibrator and the reference cylinder, respectively, MPa; V cal and V ref are the volumes of the calibrator and the reference cylinder respectively, cm 3 ; Z He cal and Z He ref are the compressibility factors of the gas in the calibrator and the reference cylinder, respectively; R is the ideal gas constant, at approximately 8.314 J/(mol·K); T is the experimental temperature, K. Measurement of the effective adsorbed volume: Valve A2 was opened, the sample cylinder was connected to the reference cylinder, and the pressure was recorded after equilibration. The volume of the effective adsorbed volume within the adsorption cylinder can be calculated by eq : P He ref V ref Z He ref R T = P He eq ( V ref + V ads ) Z He eq R T where P He eq is the pressure of the adsorption system after equilibration, MPa; V ads is the volume of the adsorption system after equilibration, cm 3 , the value being the average of 3–4 repeated tests, and the difference of each time is required to be less than 0.1 cm 3 ; Z He eq is the compressibility factor of the gas in the adsorption system afte...…”