Filling–adsorption mechanism and diffusive transport characteristics of N2/CO2 in coal: Experiment and molecular simulation.

Enhanced coalbed methane (ECBM) is an effective development technology to improve energy utilisation and reduce greenhouse gas emissions by injecting N 2 /CO 2 into coal seams. The process involves the adsorption, desorption, and diffusion of N 2 /CO 2 in multi-scale coal pores. This results in gas storage and transport characteristics limited by the coal's structural distribution of nanoscale pores (≤100 nm). Thus, studying N 2 /CO 2 storage and diffusive transport characteristics in pores at different scales is vital for efficiently implementing N 2 /CO 2 -ECBM. This paper performed low-pressure adsorption with N 2 /CO 2 (LPA-N 2 /CO 2) to evaluate coal samples' isothermal adsorption characteristics and pore structure, revealing the dual filling-adsorption mechanism of N 2 /CO 2 in coal. A new coal pore classification method was proposed. Pores (≤50 nm) in coal-N 2 or coal-CO 2 adsorption systems were classified as inaccessible pores (<0.364 nm or < 0.33 nm), filling pores (0.364–1.65 nm or 0.33–1.36 nm), transition pores (1.65–2 nm or 1.36–1.60 nm), and adsorption pores (>2 nm or > 1.60 nm). Combining the experimental results to construct micropore and mesopore models, molecular dynamics were applied to simulate the loading and migration processes of N 2 /CO 2 in the pore models. Filling, transition, and adsorption pores completed loading gas within 0.1–1 MPa, 1–5 MPa and 5–10 MPa in that order, visualising the gas filling-adsorption process in coal. Then, the adsorption capacity of N 2 was controlled by the pore size and adsorption pressure, while CO 2 was mainly influenced by adsorption pressure. [Display omitted] • The filling-adsorption mechanism of N 2 /CO 2 in different coal pores is revealed. • A new pore classification method for coal-N 2 and coal-CO 2 systems is proposed. • The pressure stage for loading gas in various types of pores is defined. • Visualisation of gas filling-adsorption process in coal by simulation is realised. [ABSTRACT FROM AUTHOR]