Abstract
Adsorption thermodynamic characteristics are an important part of the methane adsorption mechanism, and are useful for understanding the energy transmission mechanism of coalbed methane (CBM) migration in coal reservoirs. To study the effect of coal pore characteristics on methane adsorption heat, five different types of rank coals were used for low-pressure nitrogen, low-pressure carbon dioxide, and methane adsorption experiments. Pore structure and adsorption parameters, including maximum adsorption capacity and adsorption heat, were obtained for five coal samples, and their relationships were investigated. The results show that the low-pressure nitrogen adsorption method can measure pores within 1.7–300 nm, while the low-pressure carbon dioxide adsorption method can measure micropores within 0.38–1.14 nm. For the five coal samples, comprehensive pore structure parameters were obtained by combining the results of the low-pressure nitrogen and carbon dioxide adsorption experiments. The comprehensive results show that micropores contribute the most to the specific surface area of anthracite, lean coal, fat coal, and lignite, while mesopores contribute the most to the specific surface area of coking coal. Mesopores contribute the most to the pore volume of the five coal samples. The maximum adsorption capacity has a significant positive correlation with the specific surface area and pore volume of micropores less than 2 nm, indicating that methane is mainly adsorbed on the surface of micropores, and can also fill the micropores. The adsorption heat has a significant positive correlation with the specific surface area and pore volume of micropores within 0.38–0.76 nm, indicating that micropores in this range play a major role in determining the methane adsorption heat.
Highlights
As the main source of coal mine accidents, coalbed methane (CBM) is a type of unconventional natural gas resource [1,2]
The N2 adsorption-desorption isotherms of the five coal samples at 77 K are shown in Figure 1 and can be classified as a type II isotherm based on the IUPAC [23]
According to IUPAC, the type of loops for the CZ, JJ, SS, and DL coal samples can be classified as type
Summary
As the main source of coal mine accidents, coalbed methane (CBM) is a type of unconventional natural gas resource [1,2]. The study of coal adsorption characteristics is of great significance in CBM exploration and mine gas disaster prevention [5,6,7]. Coal is a type of natural porous material with a large internal surface area due to the abundance of pores [11,12,13], and coal pores are the main storage space for CBM. The LP-N2 GA and LP-CO2 GA methods have been widely used to quantitatively analyse the pore structure of coal [20,21,22,23,24].
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