Mechanistic Studies of H2 Adsorption and Diffusion in Low-Rank Coals: A Discussion on Geologic Hydrogen Storage

An in-depth investigation of the adsorption and diffusion mechanism of hydrogen in low-rank coals is of great significance for optimizing the technical path of geological hydrogen storage and improving the efficiency of hydrogen storage. Two kinds of coal samples with a low metamorphic degree from Foran Coal Mine and Sihe Coal Mine were used as adsorbents, and the metamorphic degree and molecular structure of the coal samples were determined experimentally, and the adsorption and diffusion mechanism of H2 molecules in the structure of low metamorphic coal was analyzed from the atomic level based on numerical simulation. It was found that the aliphatic carbon in the low-rank coal mainly links the aromatic ring in the form of a branched chain and exists as an aliphatic ring, side chain, or bridging carbon, and the lower the deterioration degree of the coal, the longer the length of the alkane side chain in the molecular structure. The branched structure present in the aliphatic carbon and the polybenzene ring structure present in the aromatic carbon can provide more effective adsorption sites and enhance the adsorption of H2 by the low-rank coal structure. High specific surface area and porosity will enhance the adsorption of H2 from coal samples, while the presence of oxygen-containing functional groups in low-rank coals will strengthen the interaction between the microporous structure and H2. These findings provide theoretical support for the application of low-rank coals in geological hydrogen storage.