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[Display omitted] • CH 4 /CO 2 replacement behaviors in natural marine sediments are investigated. • CO 2 diffusion is weakened by clayey sediment during replacement process. • NGH reservoir with moderate water content has excellent potential for replacement. • Optimized P-T conditions are crucial to improve CH 4 recovery from hydrates. The use of CH 4 /CO 2 replacement from hydrate bearing sediments for CH 4 recovery and CO 2 storage is an alternative option to mitigate energy shortage and global warming. Fine marine sediments are highly attractive for abundant gas hydrate reserves and tremendous CO 2 sequestration potential. However, the CH 4 /CO 2 replacement regularity previously obtained from coarse sands may not be suitable for fine marine sediments because of their distinct differences in physical properties. In this study, fine natural marine sediments obtained from the South China Sea were used as porous media to investigate the CH 4 /CO 2 replacement characteristics. The results indicated that the pressure and the temperature were the main controlling factors affecting the replacement efficiency. The content of initial methane hydrate and water in reservoir had a more significant effect on CO 2 storage than CH 4 recovery. The gas exchange kinetics in hydrates presented in fine marine sediments were significantly different from those in coarse grained sediments. CH 4 /CO 2 replacement in fine marine sediment seemed to be inhibited by weak CO 2 diffusion as peculiarities of the sediment including fine grain size, clay swelling and high proportion of bound water. According to the observed experimental results, the pressure and temperature conditions should be comprehensively optimized to enhance the mass transfer effect and improve economic benefits. This work provided greater insights into future marine NGH exploitation and contributed to carbon sequestration with applying CH 4 /CO 2 replacement method. [ABSTRACT FROM AUTHOR]

Kinetic enhancement for capturing and storing harmful gases into hydrates simultaneously to achieve low energy penalties and environmental mitigations. • Kinetic surfactants accelerate gas captures in pore spaces. • Oscillated pressure controls are first proposed to enhance CO 2 gas captures. • Enhanced gas capture processes change microstructural features. • The hydrate technology is a safely long-term storages of harmful gases. The use of hydrate-based technology for gas capture and storage is highly attractive for environmental mitigation, as it entails low energy penalties and provides gas storage density maximization and long-term storage stability. Although this method has been investigated in extensive researches, its development is restricted by the obscure underlying gas capture micro-mechanisms, elusive micro-structures of stored forms, and insufficient hydrate film growth rates. In this study, the Magnetic Resonance Imaging technique was employed to analyze the hydrate growth micro-processes for greenhouse gas (imitated by CO 2 , CH 4 , and various fractions of CO 2 -CH 4 mixed gases) and volatile organic compound (simulated by C 2 H 4 and C 2 H 2 gases) capture and storage. The hydrate film growth was enhanced with the addition of 288 ppm sodium dodecyl sulfate (SDS), which significantly improved the hydrate growth in the cases of hydrocarbon gases, but not CO 2 gas due to the competing adsorption of bicarbonate and dodecyl sulfate ions. With SDS, hydrocarbon gas hydrates grew via the patchy model at 65–105 mm/s, and 65–95% liquid water was converted into hydrates for gas capture and storage. However, only about 1.4% water was converted into CO 2 hydrates with SDS, at 10.4 mm/s. Thus, a multi-pressure control mechanism for secondary hydrate growth was developed to promote CO 2 capture and storage, based on a large amount of dissolved CO 2 gas compared to the other investigated gases. The enhanced CO 2 capture has important implications for the optimized harmful gas sequestration, due to preferentially patchy hydrate morphologies and associated impacts on permeability. [ABSTRACT FROM AUTHOR]

• CBCMs significantly improved the hydrate formation kinetics and storage capacity. • Carbonyl groups was identified as the active sites enhancing CH 4 hydrate formation. • Functional groups tuned hydrogen-bonding network results in the enhanced kinetics. The sluggish formation kinetics is a formidable challenge for the practical application of gas hydrate-based technologies for natural gas storage and transportation. Methane hydrates in both nature and labs exclusively form with the assistant of foreign additives via a heterogeneous process. Although the process is common and widely used, it remains unclear what makes a material a good methane hydrate promoter. Additionally, the molecular mechanisms underlying the promoted hydrate formation remain largely unknown. Herein, carbon monoliths (labeled as CBCM) with finely controlled surface functional groups were produced using cellulose as precursors. The oxygen-containing groups have been identified as the active sites enhancing the nucleating ability of methane hydrates. Carbonyl oxygen is pinned down as the most effective functional group in reducing the induction time and enhancing the formation kinetics of methane hydrate. The turned hydrogen bonds between water molecules, which are close to the surface of CBCM, contribute to the enhanced formation kinetics as confirmed by the Raman spectroscopy. The CBCM with optimized carbonyl oxygen significantly improve the methane hydrate formation kinetics and storage capacity with outstanding cycle stability, which are superior than most of the previously reported promoters, particularly in shorter induction time. The finding in this work paves the way for effectively designing promoters and unraveling the underlying mechanism for enhancing gas hydrate formation. [ABSTRACT FROM AUTHOR]

• A new hydrate formation promoter was selected for methane storage. • The R141b was confirmed to improve the equilibrium condition of methane hydrate. • Thermodynamic characteristics of mixture hydrate was analysed. • Mechanism of mixture hydrate nucleation was proposed. • Gas storage procedure was optimized. Solidified natural gas is a promising alternative for improving natural gas storage owing to its safe and environmentally friendly properties. However, harsh conditions limit its application. In this study, the R141b was used as a thermodynamic promoter to enhance methane hydrate formation under mild conditions. Addition of R141b to the water system significantly shifted the phase equilibrium boundary to lower pressures and higher temperatures, in which the R141b-CH 4 hydrate can be formed at 0.1 MPa and 287.2 K. Notably, the final pressure of the cell after hydrate formation was near the atmospheric pressure even at an initial pressure of 5.384 MPa, suggesting that the R141b-CH 4 mixed hydrate had milder storage conditions and lower costs than liquefied natural gas (approximately 110 K). The higher initial pressure significantly reduces the induction time and increases gas storage. The mechanism of unusual kinetic characteristics of the R141b-CH 4 hydrates that were observed can be attributed to the different nucleation behaviour of CH 4 , R141b, and R141b-CH 4 hydrates. This study provides evidence of a new promoter using the solid hydrate method under mild conditions that can be alternatively used for the storage and transport of natural gas, thereby increasing the supply of clean energy without the high economic costs and safety issues. [ABSTRACT FROM AUTHOR]