Optimization of CH4 storage strategies in ZIF-8@AC system based on characteristics of adsorption-hydration hybrid CH4 storage kinetic and thermodynamic behavior

Adsorption-hydration hybrid is regarded as an innovative approach for achieving low-carbon and efficient natural gas storage and transportation. However, enhancing the adsorption-hydration hybrid methane (CH4) storage density and storage kinetics remains challenging. Methods such as bubbling, mechanical agitation and pre-freezing, while promoting efficient hydrate conversion, introduce additional energy consumption and costs that undermine the economic viability of this approach. Based on the kinetic and thermodynamic analysis, this study proposes a two-step gas injection pattern as an enhanced CH4 storage strategy suitable for adsorption-hydration hybrid. Results demonstrate that this strategy significantly enhances CH4 storage rate while simultaneously increasing storage capacity. Under defined initial pressure and ratio of water content (Rw) conditions, Zeolite imidazoline framework-8@activated carbon (ZIF-8@AC) maintained at 7 MPa and Rw = 2.0 achieved an average CH4 consumption rate of 0.0647 mol/mol·h during the hybrid storage stage with a 0.75 h storage interval, representing a 64 % improvement over one-step gas injection. The period of high heat flux for CH4 storage via adsorption-hydration hybrid was shortened to 1.62 h. This improvement stems from the injection interval that providing sufficient time for CH4 diffusion within the pore networks, thereby enhancing the adsorption-hydration hybrid rapid storage process. Moreover, ZIF-8@AC achieved a CH4 storage capacity of 279.59 V/V under this storage strategy, attaining the targets proposed by US Department of Energy (DOE). To ensure the industrial applicability of this strategy, ZIF-8@AC adopting this approach underwent five cyclic tests, demonstrating that the system retained over 90 % of its storage performance after cycling.