Hydrogen Storage Law and Nanoscale Occurrence Mechanism of Anthracite Containing Depleted CH₄: Insights from Experiment and Molecular Simulation

Hydrogen energy is pivotal for driving sustainable development and achieving deep decarbonization; yet, its storage remains a significant challenge. Notably, depleted methane reservoirs can serve as a promising large-scale solution for underground hydrogen storage (UHS). Based on adsorption experiments, Monte Carlo and molecular dynamics methods, the adsorption behavior of H₂ and CH₄ in anthracite and the applicability of five models were discussed. The occurrence space and competitive adsorption law of H₂ and residual CH₄ in anthracite were analyzed, and the H₂ storage law and nanoscale occurrence mechanism of anthracite were revealed to evaluate the feasibility of high-pressure H₂ storage in depleted CH₄ reservoirs. The results show that the H₂ isotherm exhibited a linear trend. The Langmuir model can accurately describe H₂ adsorption, and the temperature inhibits H₂ uptake. Both D-A and Freundlich models are also capable of precisely fitting CH₄ and H₂ adsorption, while the BET model shows the poorest performance. The adsorption difference between CH₄ and H₂ narrows under a high pressure. The isosteric heat of adsorption for H₂ is lower than that of CH₄, and the depleted reservoir's low methane pressure exhibits minimal impact on high-pressure H₂ adsorption. When the H₂ molar fraction reaches 0.9, its adsorption capacity exceeds that of CH₄, with this difference amplifying under increasing partial pressure, which highlight the potential of high-pressure H₂ storage in CH₄-depleted reservoirs. Radial distribution function and coordination number analyses reveal that CH₄ exhibits a denser distribution around coal molecules compared to H₂. However, H₂ demonstrates a diffusion coefficient 1 order of magnitude higher than that of CH₄, facilitating rapid hydrogen storage. Furthermore, H₂ occupies approximately 70% and 30% of the adsorbed and free spaces in coal, respectively, compared to 90% and 10% for CH₄. This study is expected to provide scientific guidance for H₂ energy storage in depleted coal seams.