Hydration is an alternative promising method for CO2 capture and separation from either post-combustion flue gas or pre-combustion fuel gas. The present paper gathers the researches on CO2 hydrate and the hydrates of gas mixtures of CO2+N2/H2/CH4, including studies of fundamental thermo-physical properties, molecular structures and hydrate formation equilibrium conditions. Some promoters, i.e. quaternary ammonium salt etc. are usually used in CO2 hydration process to reduce the hydrate equilibrium pressure and to enhance the hydrate kinetic and stability, hence their promotion effect on CO2 hydrate and on the hydrates of gas mixture of CO2+N2/H2/CH4 are reviewed. The paper also summarizes the applications of hydrate technology in CO2 capture and separation, and the corresponding performance is summarized and the bottlenecks are discussed. It necessitates more works to promote this technology towards industrial application.

Review of fundamental properties of CO 2 hydrates and CO 2 capture and separation using hydration method

Gas hydrates have been endowed with great potential for natural gas storage and transportation; achieving the rapid hydrate formation and high storage capacity are critical to utilize this technology. Surfactants have been confirmed as the most efficient promoters for gas hydrate formation; however, the promotion mechanisms are un-unified, and foam generation during hydrate dissociation can seriously impact their applications. However, given that non-surfactant promoters, such as porous materials and metal nanoparticles, cannot produce obvious superiority over surfactants with regard to promotion efficiency, surfactants are still considered to be the most potential promoters for the industrial applications of hydrate technology. On this account, a review focused on the surfactant-promoted gas (particularly methane) hydrate formation during the past 2–3 decades has been presented in this study, with the aim of achieving a comprehensive evaluation on the current research status and effective guidance on research prospects. First, different promotion mechanisms of surfactants in gas hydrate formation were generalized and evaluated; thereafter, the effects of the molecular structures of surfactants on the promotion efficiency were analyzed; furthermore, surfactant-supported copromoters applied in gas hydrate formation were listed; finally, novel nanopromoters developed based on the promotion of surfactants during the recent years were summarized.

Surfactant-based promotion to gas hydrate formation for energy storage

The continuously increasing trend of natural gas (NG) consumption due to its clean nature and abundant availability indicates an inevitable transition to an NG-dominated economy. Solidified natural gas (SNG) storage via combustible ice or clathrate hydrates presents an economically sound prospect, promising high volume density and long-term storage. Herein, we establish 1,3-dioxolane (DIOX) as a highly efficient dual-action (thermodynamic and kinetic promoter) additive for the formation of clathrate (methane sII) hydrate. By synergistically combining a small concentration (300 ppm) of the kinetic promoter L-tryptophan with DIOX, we further demonstrated the ultra-rapid formation of hydrates with a methane uptake of 83.81 (±0.77) volume of gas/volume of hydrate (v/v) within 15 min. To the best of our knowledge, this is the fastest reaction time reported to date for sII hydrates related to SNG technology and represents a 147% increase in the hydrate formation rate compared to the standard water–DIOX system. Mixed methane–DIOX hydrates in pelletized form also exhibited incredible stability when stored at atmospheric pressure and moderate temperature of 268.15 K, thereby showcasing the potential to be industrially applicable for the development of a large-scale NG storage system.

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Ultra-rapid uptake and the highly stable storage of methane as combustible ice

Hydrate cold storage technology has been intensively researched in recent years and plays an important role in the macro-control of energy. This paper reviews the diversity and variability of hydrate cold storage media and the new hydrate cold storage system. The diversity is embodied by the types of hydrate cold storage media, which include alkane hydrates, freon hydrates, CO2 hydrates, water-soluble organic hydrates, and mixed hydrates. In mixed hydrates, the different components in the mix media play different roles in the process of hydrate formation. The variability is reflected in the differences in the external environments (environmental compatibility: ozone depletion potential and global warming potential) and the multiple reinforcement methods of hydrate nucleation. The reinforcement methods including mechanical methods, external field functions, and additives (mainly surfactants, nanoparticles, porous materials, and thermodynamic accelerators) are compared and discussed for optimisation of the hydrate formation conditions. The operational characteristics and application advantages of the new cold storage systems with different hydrate media are summarised. The environmental impact, energy efficiency, life cycle assessment, and commercial and industrial possibilities of the hydrate cold storage system are discussed. Finally, this review provides a comprehensive outlook of hydrate cold storage from an application perspective.

Review and prospects of hydrate cold storage technology

The new scenario involving the rapid energy supply transition from oil-based to natural gas-based undoubtedly affects the future carbon capture and storage (CCS) and offers an opportunity for the use of natural gas hydrates (NGHs). NGHs account for one-third of the mobile organic carbon on Earth, acting as a carbon storage reservoir in the carbon cycle. Hydrate-based technologies including CO2 capture, CO2 separation, and natural gas storage and transportation can also be used to reduce greenhouse gas (CO2, CH4) emissions and have excellent application potential. However, the implications of the energy supply transition, NGH extraction, and hydrate-based technologies for future climate change mitigation have not been deeply recognized. This paper comprehensively discusses the global energy supply and environmental challenges and transitions, and NGHs and their role in the energy supply, carbon cycle, and historical and future climate change, and summarizes the state-of-the-art developments in hydrate-based technologies and NGH exploitation methods and their potential environmental impacts, thereby providing a perspective on the roles of NGHs and their related technologies in the future energy supply and climate change mitigation. In all of these areas, we focus on identifying future CCS challenges and the technological development risk imposed by a natural gas-based global economy and NGH utilization, which should be highlighted in the next several decades.

Natural gas hydrate resources and hydrate technologies: a review and analysis of the associated energy and global warming challenges

The hydrate phase equilibrium behaviors of tetrahydrofuran (THF) + CH4, THF + CO2, CH4 + CO2, and THF + CO2 + CH4 were investigated over wide ranges of temperature, pressure, and concentration. The dissociation conditions of THF + CH4 and THF + CO2 hydrates were shifted to lower pressures and higher temperatures from the dissociation boundaries of pure CH4 and pure CO2 hydrates. X-ray diffraction results revealed that the CH4 + CO2 and THF + CO2 + CH4 hydrates prepared from a CH4/CO2 (50:50) gas mixture formed structure I and II clathrate hydrates, respectively. Raman measurements provided detailed information regarding the cage occupancy of CH4 and CO2 molecules encaged in the hydrate frameworks. For the CH4 + CO2 hydrates, the concentrations of CO2 in the hydrate phase were higher than those in the vapor phase. In contrast, for the THF + CO2 + CH4 hydrates, the concentrations of CO2 in the hydrate phase were lower than those in the vapor phase.

Phase Equilibrium Studies of Tetrahydrofuran (THF) + CH4, THF + CO2, CH4 + CO2, and THF + CO2 + CH4 Hydrates

Easy come, easy go: Hydroquinone forms a channel structure of cages with hydrogen-bonded hexagons. These may provide an ideal route for the fast inclusion and facile release of hydrogen molecules (see figure), which can lead to reversible hydrogen storage under mild conditions.

Hydrogen Molecules Trapped in Interstitial Host Channels of α‐Hydroquinone

Using terahertz time-domain spectroscopy (THz-TDS), we have measured the absorption spectra of α-hydroquinone and methane- and methanol-loaded β-hydroquinone clathrates up to 3 THz frequency range. The α-hydroquinone exhibits 11 resonances whereas methane- and methanol-loaded β-hydroquinone clathrates show only 1 resonance. The observed THz resonances represent the intermolecular and lattice vibrations of the hydroquinone frameworks, but seem to be insensitive to the encaged guest species. The indices of refraction are found to be ca. 1.6 for α-hydroquinone and methane-loaded β-hydroquinone clathrate and 1.7 for methanol-loaded β-hydroquinone clathrate, and the anomalous dispersions are well defined at the resonance frequencies.

Characterization of hydroquinone clathrates by THz time-domain spectroscopy

The hydrate phase behavior of CO2/3-methyl-1-butanol (3M1B)/water, CO2/tetrahydrofuran (THF)/water and CO2/1,4-dioxane (DXN)/water was investigated using both a high-pressure equilibrium viewing cell and a kinetic pressure–temperature measurement system with a constant volume. The dissociation pressures of CO2/3M1B/water were identical to those of pure CO2 hydrate, indicating that CO2 is not acting as a help gas for structure H hydrate formation with 3M1B, thus the formed hydrate is pure CO2 structure I hydrate. The CO2 molecules could be encaged in small cages of the structure II hydrate framework formed with both of THF and DXN. For a stoichiometric ratio of 5.56mol% THF, we found a large shift of dissociation boundary to lower pressures and higher temperatures from the dissociation conditions of pure CO2 hydrate. From the measurements using the kinetic pressure–temperature system, it was found that the solid binary hydrate samples formed from off-stoichiometric THF and DXN aqueous solutions are composed of pure CO2 hydrate with a hydrate number n = 7.0 and THF/CO2 and DXN/CO2 binary hydrates with a molar ratio of xCO2 · THF · 17H2 Oa ndxCO2 · DXN · 17H2O, respectively. The X-ray diffraction was used to identify the binary hydrate structure and Raman spectroscopy was measured to support the phase equilibrium results and to investigate the occupation of CO2 molecules in the cages of the hydrate framework.

Yun-Je Lee

Papers

Phase Equilibrium Studies of Tetrahydrofuran (THF) + CH4, THF + CO2, CH4 + CO2, and THF + CO2 + CH4 Hydrates

Hydrogen Molecules Trapped in Interstitial Host Channels of α‐Hydroquinone

Characterization of hydroquinone clathrates by THz time-domain spectroscopy

Thermodynamicstability,spectroscopicidentificationandcageoccupationofbinary CO2 clathratehydrates