Hydrogen Storage Law and Nanoscale Occurrence Mechanism of Anthracite Containing Depleted CH

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.