Critical pore size for micropore filling in coal samples with different rank coals

Abstract

The objectives of this study were to explore the occurrence and migration of coalbed methane in coals of different ranks and reveal the microscopic reservoir space and the mechanism of coalbed methane. To meet these objectives, this study selected six coal samples of different coal ranks for low-pressure N2 adsorption experiments, explored the critical pore filling characteristics of packed N2 molecules in the coals, and analyzed the low-pressure N2 adsorption/desorption experimental isotherms using the DFT method and DA equation based on the micropore filling theory. Finally, the critical filling pressure and pore size range for micropore filling were determined, and the analysis results were verified by combining the Langmuir, DA, and BET equations. The results showed that, from low to high coal rank, the N2 adsorption/desorption isotherms of the coal samples transition from type Ⅱ to type Ⅰ. The proportion of N2 molecules in low-rank coals in the form of micropore filling and monolayer adsorption was higher than that in high-rank coals. The critical pressure and critical pore size for micropore filling exhibited U-shaped correlations with the coal rank. Low-rank coals (lignite and long flame coal) were gradually filled in the relative pressure range P/P0 ≈ 1E-4–0.03, and medium- and high-rank coals (gas coal, 1/3 coking coal, lean coal, and anthracite) were filled in the relative pressure range P/P0 ≈ 1E-4–0.01; the corresponding critical pore size ranges were 1.7–2.19 and 1.61–2.00 nm, respectively.

Thermodynamic Characteristics of Methane Adsorption and Desorption on Varied Rank Coals: A Systematic Study

“…According to the classification method of the pore morphology of coal by Khalili et al, the samples include all morphological types. 42 Taking part of samples for example, the LNA absorption and desorption curves of M1 and M2 almost coincide, and there is no hysteresis ring, indicating that the pore shape is dominated by a closed one end, belonging to class I pores, which are conducive to gas desorption but not to storage; the curves of L1 and L2 have small hysteresis loops, meaning that pore shapes are dominated by open cylindrical or parallel plates, belonging to class II pores; the curves of H4 and H5 show significant hysteresis loops and inflection points, corresponding to the narrow-neck bottle pores, that is, class III pores, having good gas storage capacity but poor desorption capacity 43 (Figure 5).…”

Effect of micro-fractures on gas flow behavior in coal for enhanced coal bed methane recovery and CO2 storage

“…Additionally, during procedures like coal gasification or coalbed methane depletion, microfractures in coal may have an impact on permeability. These microfractures can spread or shut as the coal is subjected to heat or stress changes, changing the permeability of the coal seam [ 24 , 25 ]. Foreseeing variations in coal permeability and optimizing the extraction or utilization processes requires an understanding of how microfractures behave under various conditions.…”

Modeling Methane Adsorption Distance Using Carbon Nanotubes and Bituminous Coal Pore Models

“…In coal seams, 80–90% of the gas is adsorbed on the inner surface of the coal pore structure. − Coal is a complex porous material, in which the specific surface area and pore volume of the nanopores account for 97 and 68% of the total amounts, respectively. − Therefore, coal nanopores are the primary pores that adsorb gas. , However, owing to the limited measurement accuracy of research tools, research progress on coal nanopores is very slow. In recent years, benefiting from the progress in science and technology, molecular simulation methods such as molecular mechanics, molecular dynamics (MD), and density functional theory (DFT) have been widely used in microscopic model construction and dynamics calculations. − Accordingly, the nanopore structure of coal has received considerable attention. ,− Many researchers have studied the adsorption of methane and other gases onto coal nanopores using molecular simulation methods.…”