CO2 Hydrate: Synthesis, Composition, Structure, Dissociation Behavior, and a Comparison to Structure I CH4 Hydrate

Hydrate Technologies for CO2 Capture and Sequestration: Status and Perspectives

CO2 capture and sequestration based on hydrate technology are considered supplementary approaches for reducing carbon emissions and mitigating the greenhouse effect. Direct CO2 hydrate formation and CH4 gas substitution in natural gas hydrates are two of the main methods used for the sequestration of CO2 in hydrates. In this Review, we introduce the crystal structures of CO2 hydrates and CO2-mixed gas hydrates and summarize the interactions between the CO2 molecules and clathrate hydrate/H2O frames. In particular, we focus on the role of diffraction techniques in analyzing hydrate structures. The kinetic and thermodynamic properties then are introduced from micro/macro perspectives. Furthermore, the replacement of natural gas with CO2/CO2-mixed gas is discussed comprehensively in terms of intermolecular interactions, influencing factors, and displacement efficiency. Based on the analysis of related costs, risks, and policies, the economics of CO2 capture and sequestration based on hydrate technology are explained. Moreover, the difficulties and challenges at this stage and the directions for future research are described. Finally, we investigate the status of recent research related to CO2 capture and sequestration based on hydrate technology, revealing its importance in carbon emission reduction.

Interfacial tension and wettability alteration during hydrogen and carbon dioxide storage in depleted gas reservoirs

The storage of CO2 and hydrogen within depleted gas and oil reservoirs holds immense potential for mitigating greenhouse gas emissions and advancing renewable energy initiatives. However, achieving effective storage necessitates a thorough comprehension of the dynamic interplay between interfacial tension and wettability alteration under varying conditions. This comprehensive review investigates the multifaceted influence of several critical parameters on the alterations of IFT and wettability during the injection and storage of CO2 and hydrogen. Through a meticulous analysis of pressure, temperature, treatment duration, pH levels, the presence of nanoparticles, organic acids, anionic surfactants, and rock characteristics, this review elucidates the intricate mechanisms governing the changes in IFT and wettability within reservoir environments. By synthesizing recent experimental and theoretical advancements, this review aims to provide a holistic understanding of the processes underlying IFT and wettability alteration, thereby facilitating the optimization of storage efficiency and the long-term viability of depleted reservoirs as carbon capture and storage or hydrogen storage solutions. The insights gleaned from this analysis offer invaluable guidance for researchers, engineers, and policymakers engaged in harnessing the potential of depleted reservoirs for sustainable energy solutions and environmental conservation. This synthesis of knowledge serves as a foundational resource for future research endeavors aimed at enhancing the efficacy and reliability of CO2 and hydrogen storage in depleted reservoirs.

CO2 competes with radioactive chemicals for freshwater recovery: Hydrate-based desalination

Here, we introduce CO2 hydrate-based desalination (CHBD) technology for freshwater recovery from radioactive wastewater, for water particularly containing Cs and Sr. The hydrate equilibrium curves of CO2 hydrates shift towards lower temperature and higher pressure regions as the concentrations of CsCl and SrCl2 increase. X-ray diffraction and Raman spectroscopy measurements found that neither CsCl nor SrCl2 can affect the structure of CO2 hydrates. The high-pressure micro-differential scanning calorimetric results demonstrate that CO2 hydrates in the presence of CsCl and SrCl2 started to dissociate at lower temperatures due to the enrichment of CsCl and SrCl2 in the remaining solutions. The formation kinetics results indicate that increases in the concentrations of the radioactive chemicals lead to a decrease in the initial reaction rate and sub-cooling temperature. Solid-state nuclear magnetic resonance spectroscopy was utilized to confirm the exclusion of radioactive isotopes from solid gas hydrates. Importantly, the CHBD technology proposed in this study is applicable to radioactive wastewater containing Cs+ and Sr2+ across broad concentration ranges, spanning from a percent to hundreds of parts per million (ppm), and even sub-ppm levels, with comparable recovery efficiency. This study presents new insights into the potential of environmentally sustainable technologies to overcome the challenges posed by radioactive wastewater.

Carbon dioxide sequestration in natural gas hydrates - effect of flue and noble gases

Clean energy is one of the immediate requirements all over the world to tackle the global energy demands. Natural gas hydrates (NGHs) are one of the proposed alternatives that could be used to extract methane as clean energy and simultaneously sequestrate carbon dioxide. However, the formation of CH4-CO2 mixed hydrates and the first hydrate layer besides the interface reduces the rate of CO2 sequestration and methane extraction in NGHs, and thus, multistep extraction of methane is one of the proposed solutions. We report the atomic level factors that could enhance CO2 sequestration in the newly formed first hydrate layer besides the interface in the presence of flue and noble gases using DFT calculations and molecular dynamics simulations at 250 K and 0.15 kbar. The simulations show the formation of stable dual cages (large-large or small-large) that lead to the formation of a four-caged, Y-shaped cluster (growth synthon) which leads to the formation of a hydrate unit cell in heterogeneous medium. Among the flue and noble gases, only argon forms energetically favorable dual cages with itself and CO2 due to which enhanced CO2 sequestration is observed at different concentrations of Ar and CO2 where the CO2 : Ar (2.5 : 1.5) system shows the best CO2 sequestration in the first layer besides the interface. The results also provide understanding into the previously reported concentration dependent CO2 selectivity in sI hydrates in the presence of third gases (N2 and H2S).

Non-destructive analysis of a mixed H2O-CO2 fluid in experimental noble-metal capsule by means of freezing and high-energy synchrotron X-ray diffraction

High-pressure high-temperature syntheses that involve volatile-bearing aqueous fluids are typically accomplished by enclosing the samples in gas-tight welded shut noble-metal capsules, from which the bulk volatile content must be extracted to be analyzed with mass spectroscopy, hence making the analysis non-replicable. Here we describe a novel non-destructive method that ensures the identification and the quantitative estimate of the volatiles directly in the sealed capsule, focusing on fluid H₂O-CO₂ mixtures equilibrated with graphite at conditions of geological interest (1 GPa, 800 °C). We used a high-energy (77 keV) synchrotron X-ray radiation combined with a cryostat to produce X-ray diffraction patterns and X-ray diffraction microtomographic cross-sections of the volatile-bearing samples down to -180 °C, thus encompassing the conditions at which crystalline phases-solid CO₂ and clathrate (CO₂ hydrate)-form. The uncertainty of the method is < 15 mol%, which reflects the difference between the volatile proportion estimated by both Rietveld refinement of the diffraction data and by image analysis of the microtomograms, and the reference value measured by quadrupole mass spectrometry. Therefore, our method can be reliably applied to the analysis of frozen H₂O-CO₂ mixtures and, moreover, has the potential to be extended to experimental fluids of geological interest containing other volatiles, such as CH₄, SO₂ and H₂S.

Solidified-Air Energy Storage: Conceptualization and Thermodynamic Analysis

Grid-scale electrical energy storage (EES) is a key component in cost-effective transition scenarios to renewable energy sources. The requirement of scalability favors EES approaches such as pumped-storage hydroelectricity (PSH) or compressed-air energy storage (CAES), which utilize the cheap and abundant storage materials water and air, respectively. To overcome the site restriction and low volumetric energy densities attributed to PSH and CAES, liquid-air energy storage (LAES) has been devised; however, it suffers from a rather small round-trip efficiency (RTE) and challenging storage conditions. Aiming to overcome these drawbacks, a novel system for EES is developed using solidified air (i.e., clathrate hydrate of air) as the storable phase of air. A reference plant for solidified-air energy storage (SAES) is conceptualized and modeled thermodynamically using the software CoolProp for water and air as well as empirical data and first-order approximations for the solidified air (SA). The reference plant exhibits a RTE of 52% and a volumetric storage density of 47 kWh per m3 of SA. While this energy density relates to only one half of that in LAES plants, the modeled RTE of SAES is comparable already. Since improved thermal management and the use of thermodynamic promoters can further increase the RTEs in SAES, the technical potential of SAES is in place already. Yet, for a successful implementation of the concept—in addition to economic aspects—questions regarding the stability of SA must be first clarified and challenges related to the processing of SA resolved.

Local structure and distortions of mixed methane-carbon dioxide hydrates

A vast source of methane is found in gas hydrate deposits, which form naturally dispersed throughout ocean sediments and arctic permafrost. Methane may be obtained from hydrates by exchange with hydrocarbon byproduct carbon dioxide. It is imperative for the development of safe methane extraction and carbon dioxide sequestration to understand how methane and carbon dioxide co-occupy the same hydrate structure. Pair distribution functions (PDFs) provide atomic-scale structural insight into intermolecular interactions in methane and carbon dioxide hydrates. We present experimental neutron PDFs of methane, carbon dioxide and mixed methane-carbon dioxide hydrates at 10 K analyzed with complementing classical molecular dynamics simulations and Reverse Monte Carlo fitting. Mixed hydrate, which forms during the exchange process, is more locally disordered than methane or carbon dioxide hydrates. The behavior of mixed gas species cannot be interpolated from properties of pure compounds, and PDF measurements provide important understanding of how the guest composition impacts overall order in the hydrate structure.

Assessing the Alkyl Chain Effect of Ammonium Hydroxides Ionic Liquids on the Kinetics of Pure Methane and Carbon Dioxide Hydrates

In this study, four ammonium hydroxide ionic liquids (AHILs) with varying alkyl chains were evaluated for their kinetic hydrate inhibition (KHI) impact on pure carbon dioxide (CO2) and methane (CH4) gas hydrate systems. The constant cooling technique was used to determine the induction time, the initial rate of hydrate formation, and the amount of gas uptake for CH4-AHILs and CO2-AHILs systems at 8.0 and 3.50 MPa, respectively, at 1 wt.% aqueous AHILs solutions. In addition, the effect of hydrate formation sub-cooling temperature on the performance of the AHILs was conducted at experimental temperatures 274.0 and 277.0 K. The tested AHILs kinetically inhibited both CH4 and CO2 hydrates at the studied sub-cooling temperatures by delaying the hydrate induction time and reducing the initial rate of hydrate formation and gas uptake. The hydrate inhibition performance of AHILs increases with increasing alkyl chain length, due to the better surface adsorption on the hydrate crystal surface with alkyl chain length enhancement. TPrAOH efficiently inhibited the induction time of both CH4 and CO2 hydrate with an average inhibition percentage of 50% and 84%, respectively. Tetramethylammonium Hydroxide (TMAOH) and Tetrabutylammonium Hydroxide (TBAOH) best reduced CH4 and CO2 total uptake on average, with TMAOH and Tetraethylammonium Hydroxide (TEAOH) suitably reducing the average initial rate of CH4 and CO2 hydrate formation, respectively. The findings in this study could provide a roadmap for the potential use of AHILs as KHI inhibitors, especially in offshore environs.

Co-deposition of gas hydrates by pressurized thermal evaporation

Gas hydrates are usually synthesized by bringing together a pressurized gas and liquid or solid water. In both cases, the transport of gas or water to the hydrate growth site is hindered once an initial film of hydrate has grown at the water-gas interface. A seemingly forgotten gas-phase technique overcomes this problem by slowly depositing water vapor on a cold surface in the presence of the pressurized guest gas. Despite being used for the synthesis of low-formation-pressure hydrates, it has not yet been tested for hydrates of CO₂ and CH₄. Moreover, the potential of the technique for the study of hydrate decomposition has not been recognized yet. We employ two advanced implementations of the condensation technique to form hydrates of CO₂ and CH₄ and demonstrate the applicability of the process for the study of hydrate decomposition and the phenomenon of self-preservation. Our results show that CO₂ and CH₄ hydrate samples deposited on graphite at 261-265 K are almost pure hydrates with an ice fraction of less than 8%. Rapid depressurization experiments with thin deposits (approx. 330 μm thickness) of CO₂ hydrate on an aluminum surface at 265 K yield identical dissociation curves when the deposition is done at identical pressure. However, hydrates deposited at 1 MPa almost completely withstand decomposition after rapid depressurization to 0.1 MPa, while samples deposited at 2 MPa decompose 7 times faster. Therefore, this synthesis technique is not only applicable for the study of hydrate decomposition but can also be used for the controlled deposition of a super-preserved hydrate.

Toward a Robust, Universal Predictor of Gas Hydrate Equilibria by Means of a Deep Learning Regression

Due to offshore reservoirs being developed in ever deeper and colder waters, gas hydrates are increasingly becoming a significant factor when considering the profitability of a reservoir due to flow disruptions, equipment, and safety hazards arising from the hydrate plug formation. Due to low-dosage hydrate inhibitors such as kinetic inhibitors competing with traditional thermodynamic inhibitors such as methanol, accurate information regarding the hydrate equilibrium conditions is required to determine the optimal hydrate control strategy. Existing thermodynamic models can prove inflexible regarding parameter adjustment and the incorporation of new data. Developing a multivariate regression model capable of generalizing hydrate equilibria over a wide range of conditions, with results competing with thermodynamic models is worthwhile. A multilayer perceptron neural network of three hidden layers has undergone supervised training means of a backpropagation to accurately predict uninhibited hydrate equilibrium pressure for a range of gas mixtures with nine input features, excluding hydrogen sulfide and electrolytes, from a dataset of 1209 equilibrium points, 670 of which are multicomponent gases, sampled in a rigorous data sampling campaign from existing experimental studies. Statistical significance of results has been emphasized, with models validated using 10-fold cross-validation and holdout validation, facilitating hyperparameter optimization without overfitting, while stratified holdout ensures testing a wide range of conditions. The developed model has proven to outperform two popular thermodynamic models. Various scoring metrics are used, with an average cross-validated R ² of 0.987 ± (0.003). An R ² of 0.993 and mean absolute percentage error of 5.56% are yielded for holdout validation. Auxiliary models are included to determine the multicomponent prediction capability and dependency on individual data sources. Multicomponent data prediction has proven successful; results prove that the model accurately generalizes hydrate equilibria and is well suited to predicting unseen data. Positive results are largely insensitive to exact model parameters, thus indicating a robust, replicable methodology.

Pore Characteristics for Efficient CO2 Storage in Hydrated Carbons

Development of new approaches for carbon dioxide (CO₂) capture is important in both scientific and technological aspects. One of the emerging methods in CO₂ capture research is based on the use of gas-hydrate crystallization in confined porous media. Pore dimensions and surface functionality of the pores play important roles in the efficiency of CO₂ capture. In this report, we summarize work on several porous carbons (PCs) that differ in pore dimensions that range from supermicropores to mesopores, as well as surfaces ranging from hydrophilic to hydrophobic. Water was imbibed into the PCs, and the CO₂ uptake performance, in dry and hydrated forms, was determined at pressures of up to 54 bar to reveal the influence of pore characteristics on the efficiency of CO₂ capture and storage. The final hydrated carbon materials had H₂O-to-carbon weight ratios of 1.5:1. Upon CO₂ capture, the H₂O/CO₂ molar ratio was found to be as low as 1.8, which indicates a far greater CO₂ capture capacity in hydrated PCs than ordinarily seen in CO₂-hydrate formations, wherein the H₂O/CO₂ ratio is 5.72. Our mechanistic proposal for attainment of such a low H₂O/CO₂ ratio within the PCs is based on the finding that most of the CO₂ is captured in gaseous form within micropores of diameter <2 nm, wherein it is blocked by external CO₂-hydrate formations generated in the larger mesopores. Therefore, to have efficient high-pressure CO₂ capture by this mechanism, it is necessary to have PCs with a wide pore size distribution consisting of both micropores and mesopores. Furthermore, we found that hydrated microporous or supermicroporous PCs do not show any hysteretic CO₂ uptake behavior, which indicates that CO₂ hydrates cannot be formed within micropores of diameter 1-2 nm. Alternatively, mesoporous and macroporous carbons can accommodate higher yields of CO₂ hydrates, which potentially limits the CO₂ uptake capacity in those larger pores to a H₂O/CO₂ ratio of 5.72. We found that high nitrogen content prevents the formation of CO₂ hydrates presumably due to their destabilization and associated increase in system entropy via stronger noncovalent interactions between the nitrogen functional groups and H₂O or CO₂.

Macroscopic defects upon decomposition of CO2 clathrate hydrate crystals

Cracks and decomposition barriers observed in time-lapse micro-computed tomography measurements challenge existing models of gas hydrate decomposition.

Elasticity and Stability of Clathrate Hydrate: Role of Guest Molecule Motions

Molecular dynamic simulations were performed to determine the elastic constants of carbon dioxide (CO₂) and methane (CH₄) hydrates at one hundred pressure–temperature data points, respectively. The conditions represent marine sediments and permafrost zones where gas hydrates occur. The shear modulus and Young’s modulus of the CO₂ hydrate increase anomalously with increasing temperature, whereas those of the CH₄ hydrate decrease regularly with increase in temperature. We ascribe this anomaly to the kinetic behavior of the linear CO₂ molecule, especially those in the small cages. The cavity space of the cage limits free rotational motion of the CO₂ molecule at low temperature. With increase in temperature, the CO₂ molecule can rotate easily, and enhance the stability and rigidity of the CO₂ hydrate. Our work provides a key database for the elastic properties of gas hydrates, and molecular insights into stability changes of CO₂ hydrate from high temperature of ~5 °C to low decomposition temperature of ~−150 °C.

Characterization of Free Radicals in Clathrate Hydrates of Furan, 2,3-Dihydrofuran, and 2,5-Dihydrofuran by Muon Spin Spectroscopy

In addition to their importance as abundant hydrocarbon deposits in nature, clathrate hydrates are being studied as potential media for hydrogen and carbon dioxide storage and as "nano-reactors" for small molecules. However, little is known about the behavior of reactive species in such materials. We have employed muon spin spectroscopy to characterize various organic free radicals that reside as isolated guests in structure II clathrates. The radicals are formed by reaction of atomic muonium (Mu) with the guest molecules furan and two isomeric dihydrofurans. Muonium is essentially a light isotope of hydrogen and adds to unsaturated molecules in the same manner as H. We have determined muon and proton hyperfine coupling constants for the muoniated radicals formed in the clathrates and also in neat liquids at the same temperature. DFT calculations were used to guide the spectral assignments and distinguish between competing radical products for Mu addition to furan and 2,3-dihydrofuran. Relative signal amplitudes provide yields and thus the relative reactivities of the C4 and C5 addition sites in these molecules. Spectral features, hyperfine constants, and reactivities all indicate that the radicals do not tumble freely in the clathrate cages in the same way that they do in liquids.

Dissociation behavior of CO2 hydrate in sediments during isochoric heating

As CO2 is sequestered into sediments in the oceanic environment, CO2 hydrate can form as a byproduct. This study explored the dissociation behavior of CO2 hydrate in sediments in relation to pore fluid pressure evolution and sediment particle size. We synthesized CO2 hydrate in three types of particulate sediments: glass beads, fine sand, and crushed silt. We then dissociated them through isochoric heating. We observed the excess pore fluid pressure build-up and self-preservation behavior, in which the pressure-temperature state evolves along the hydrate phase boundary until either it reaches the second quadruple point or all hydrates dissociate. The pore fluid pressure evolution is limited, however, by the CO2 vapor-liquid phase equilibrium boundary due to the liquefaction of CO2. The presence of CO2 liquid in sediments forces the pressure-temperature evolution to follow the CO2 vapor-liquid phase equilibrium boundary, regardless of hydrate formation and dissociation processes. CO2 hydrate in fine-grained sediments experiences capillary pressure-induced melting point depression, but this effect vanishes when the pores exceed approximately 1 microm, such as in coarse-grained sediments. In particular, any fracture generation in sediments which involves the local release of confinement eliminates the melting point depression induced by the capillary effect.