Hydrogen Storage Law and Nanoscale Occurrence Mechanism of Anthracite Containing Depleted CH: 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 H and CH in anthracite and the applicability of five models were discussed. The occurrence space and competitive adsorption law of H and residual CH in anthracite were analyzed, and the H storage law and nanoscale occurrence mechanism of anthracite were revealed to evaluate the feasibility of high-pressure H storage in depleted CH reservoirs. The results show that the H isotherm exhibited a linear trend. The Langmuir model can accurately describe H adsorption, and the temperature inhibits H uptake. Both D-A and Freundlich models are also capable of precisely fitting CH and H adsorption, while the BET model shows the poorest performance. The adsorption difference between CH and H narrows under a high pressure. The isosteric heat of adsorption for H is lower than that of CH, and the depleted reservoir's low methane pressure exhibits minimal impact on high-pressure H adsorption. When the H molar fraction reaches 0.9, its adsorption capacity exceeds that of CH, with this difference amplifying under increasing partial pressure, which highlight the potential of high-pressure H storage in CH-depleted reservoirs. Radial distribution function and coordination number analyses reveal that CH exhibits a denser distribution around coal molecules compared to H. However, H demonstrates a diffusion coefficient 1 order of magnitude higher than that of CH, facilitating rapid hydrogen storage. Furthermore, H occupies approximately 70% and 30% of the adsorbed and free spaces in coal, respectively, compared to 90% and 10% for CH. This study is expected to provide scientific guidance for H energy storage in depleted coal seams.