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• 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]

• Metabolic molecules significantly promote hydrate formation. • Protein macromolecules could act as kinetic inhibitor of hydrate nucleation. • Water in the macropores is preferentially consumed during hydrate formation. Numerous efforts have been made to mimic the natural marine environments to understand the behavior of hydrate formation and accumulation in the sediments under seafloor. Yet it is found that the role of organic matters in the kinetics of hydrate formation is largely veiled, which should be much enriched as a result of the activities of the marine ecosystem. Therefore in this study, the kinetic effects of protein macromolecules and metabolic small molecules on CH 4 hydrate formation were examined; the organic compounds were extracted from the samples gathered from the hydrate reservoir located in the South China Sea. A total of 2058 species classified into 10 types of protein macromolecules and metabolic molecules were identified. Low-field nuclear magnetic resonance technique was used to in-situ monitor the hydrate formation process. The results showed that the metabolic molecules could remarkably promote hydrate formation, with more water converted into hydrate within a shorter time. Proteins have been widely studied to inhibit hydrate formation; here it is for the first time confirmed that the proteins extracted from the natural samples drilled in the hydrate reservoir could act as kinetic inhibitor, lowering the possibility of hydrate nucleation. It was further indicated that hydrate formation majorly consumed the water in the macro pores; this was followed by a diffusion-limitation process with the hydrate shell acting as mass transfer barrier slowing down the water conversion. The water in the micropores was yet found to be very difficult to participate in the reaction, potentially resulting from the unavailable gas-water contact. The findings could provide insights into the kinetics of hydrate formation in the presence of organic matters abundant in natural sediments and expand the current understandings on the occurrence of natural gas hydrate under marine environments. [ABSTRACT FROM AUTHOR]

[Display omitted] • An approach of warm brine injection to promote CH 4 /CO 2 replacement is proposed. • Warm brine injection provides a 2.0 times enhancement of the cumulative gas yield. • A potential risk of hydrate reformation is alleviated by increasing salinity. • Gas production is insensitive to injected heat as the significant heat loss. • Brine injection scheme should be optimized based on energy harvest and CO 2 storage. CH 4 /CO 2 replacement for natural gas hydrates (NGHs) exploitation is a promising method for CO 2 geological sequestration and energy recovery simultaneously. However, the puzzles of low replacement efficiency and slow reaction rate caused by the mass transfer obstacle of gas exchange are fatal bottlenecks for field application of gas replacement method. Therefore, we propose a method that uses warm brine injection during CH 4 /CO 2 replacement process to break the barrier of CO 2 diffusion and enhance CH 4 recovery as well as CO 2 storage. Benefiting from the synergistic influence of salt effect and thermal stimulation as well as water flow erosion, warm brine injection provides three dimensionally connected channels in hydrate for subsequent mass transfer, improving CH 4 /CO 2 replacement in the deep layer of hydrate. CO 2 sequestration efficiency of the newly proposed method reaches 76.46%, and the maximum amounts of CH 4 recovery is nearly treble than that achieved by single CO 2 replacement method. Notably, a potential risk of secondary hydrate formation could occur upon the pressure surge and fluid migration attached to warm brine injection, which could be effectively alleviated by increasing salinity to destabilize hydrate lattices by reducing water activity. The gas production is insensitive to the injected heat as the significant heat loss during the transportation of injected brine in pipelines and thermal diffusion through boundaries. The introduction of free water increases the complexity of the reactions in hydrate reservoirs, and the formulation of brine injection regimes should be synergistically optimized based on energy harvest, energy efficiency and CO 2 sequestration. This work extends the previous knowledge on CH 4 /CO 2 replacement and shows important practical significance for future field studies of NGHs exploitation and CO 2 geological sequestration. [ABSTRACT FROM AUTHOR]

Schematic diagram for N 2 /CO 2 gas enrichment process during hydrate growth process. Blue sticks and spheres: N 2 in the gas phase; yellow spheres: CO 2 in the gas phase or hydrate phase; red sticks: hydrate seeds. [Display omitted] • Proper subcooling can improve the hydrate growth rate and final occupancy of CO 2 molecules. • Mass transfer process could impact the hydrate growth rate, with a higher pressure improving the concentrations of N 2 and CO 2 in the free water. • Higher pressure decreased the selectivity of CO 2 for 51262, and lower subcooling assisted CO 2 in entering 51262 cages easily. The climate system is changing and becoming warmer due to the consumption of fossil energy, which results in a large quantity of CO 2 emitted into the atmosphere. The hydrate formation process can encapsulate guest molecules in hydrate cages, enabling a promising storage of CO 2 in the form of hydrate under the seafloor. Here, we performed molecular dynamics (MD) simulations to validate the hydrate-based gas storage method for low-concentration flue gas. The direct phase method was applied at four target pressures to obtain the melting temperature of mixed gas hydrate in the presence of SI hydrate seeds that are fully occupied by CO 2 molecules. Hydrate growth simulations showed that a proper subcooling can improve the hydrate growth rate and final occupancy of CO 2 molecules in newly formed hydrate cages. The mass transfer process could impact the hydrate growth rate, with a higher pressure improving the concentrations of N 2 and CO 2 in the free water. This enhanced the hydrate growth at both 260 K and low subcooling temperatures. The influence of pressure and subcooling on cage selectivity presented a different tendency for CO 2 and N 2. A higher pressure decreased the selectivity of CO 2 for 51262, and lower subcooling assisted CO 2 in entering 51262 cages easily. Interestingly, the CO 2 occupancy at 260 K is larger than that of the simulations at the melting temperature. The results could be of help in revealing the behavior of case filling during hydrate formation from a gas mixture. [ABSTRACT FROM AUTHOR]

[Display omitted] • A magnetically recoverable nanoparticle inhibitor was synthesized. • The induction time with inhibitors was 3 times longer than pure water case. • The inhibitors exhibit good cycling performance and magnetic separation efficiency. • The inhibition mechanism based on in-situ Raman was proposed. Current schemes dealing with hydrate blockage problems in gas pipelines suffer a high dosage of inhibitors and thus are associated with a significant risk of environmental damage. Overcoming these issues requires the development of novel recoverable inhibitors with persistent cycle performance. In this work, a new approach involving coating poly(vinylcaprolactam) (PVCap) and poly(vinylpyrrolidone) (PVP) onto the surface of γ-methacryloxypropyltrimethoxy silane (MPS)-modified Fe 3 O 4 nanoparticles was proposed; this procedure enables the magnetic recovery of the inhibitory particles. It was found that the onset time of hydrate formation was extended 3 times compared to pure water with the help of the synthesized inhibitors showing better performance than commercial inhibitors. Excellent magnetic separation from sandy matrices in solution was achieved with a recovery efficiency of 86%. This was followed by a good cycle performance: the induction time remained still over 2 times longer than the pure water case after 5 cycles of recovery. The in-situ Raman spectra revealed that the recyclable inhibitors functioned through disturbing the construction of the large cages; a resulting mechanism was proposed with nanoparticles isolating the early hydrate patches preventing their further explosive bulk growth. In addition, the simple synthesis method makes it possible to coat various polymer inhibitors onto recovery particles. The results indicate that magnetically recovering the inhibitory particles and reusing them to hinder hydrate generation could be an alternative method to tackle the environmental and economic problems associated with current flow assurance procedures. [ABSTRACT FROM AUTHOR]

• 3D morphologies and time-dependent parameters of the hydrate shell are obtained. • Water is more mobile than gas in hydrates in the diffusion-controlled growth stage. • Protrusions growth on the outer surface of a hydrate shell is due to water permeation. • Effective diffusion coefficients of gas and water in hydrate are determined. Understanding the transport properties of water and guest molecules in dense hydrate structures is pivotal in controlling their formation and stability; this is nevertheless hindered by challenges in characterizing the time-varying diffusion and permeation process. This paper reports experimental results on the mass-transfer characteristics of water and guest molecules across hydrate shells during their formation from water droplets. The time-dependent 3D morphologies and parameters of hydrate shells were in-situ obtained via X-ray computed tomography to reveal the transport mechanism in hydrates. It was found that in the mass-transfer-limited stage, hydrate growth occurred primarily at the hydrate-gas interface, indicating a more mobile characteristic of water molecules in those hydrate shells than gas. The resulting outward permeation of water led to the growth of hydrate protrusions on the outer surface of the hydrate shell, suggesting a transition of the water transport schema through the hydrate shell from diffusion to permeation. Consequently, a diffusion-based shell growth model was developed to quantify the diffusivity of gas and water molecules in hydrates. Combining the measurements in the temperature range of 275.15–283.15 K, the effective diffusion coefficient of gas through the hydrates was estimated to be in the range of 1.34 × 10−14 − 1.90 × 10−13 m2/s, with that of interstitial water in the range of 2.87 × 10−12 − 3.83 × 10−11 m2/s. These results are of fundamental value in developing an improved understanding of the kinetics of hydrate formation from gas-water interfaces, which is essential in the optimization of hydrate-based techniques. [ABSTRACT FROM AUTHOR]