A Review of CO₂ Clathrate Hydrate Technology: From Lab-Scale Preparation to Cold Thermal Energy Storage Solutions

Abstract

Carbon dioxide (CO₂) clathrate hydrate is gaining attention as a promising material for cold thermal energy storage (CTES) due to its high energy storage capacity and low environmental footprint. It shows strong potential in building applications, where space cooling accounts for nearly 40% of total energy use and over 85% of electricity demand in developed countries. CO₂ hydrates are also being explored for use in refrigeration, cold chain logistics, supercomputing, biomedical cooling, and defense systems. With the growing number of applications in mind, this review focuses on the thermal behavior of CO₂ hydrates and their environmental impact. It highlights recent efforts to reduce formation pressure and temperature using chemical promoters and surfactants. This paper also reviews key experimental techniques used to study hydrate properties, including Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), high-pressure differential scanning calorimetry (HP-DSC), and the T-history method. In lifecycle comparisons, CO₂ hydrate systems show better energy efficiency and lower carbon emissions than traditional ice or other phase-change materials (PCMs). This review also discusses current commercialization challenges such as high energy input during formation and promoter toxicity. Finally, practical strategies to move CO₂ hydrate-based CTES from lab-scale studies to real-world cooling and temperature control applications are discussed.

1. Introduction

The growth in global energy demand and the need for environmental protection have heightened the urgency of reducing carbon emissions (both CO₂ and CH₄). The United Nations has prioritized the reduction of carbon emissions as a key strategy to combat climate change. Buildings account for approximately 40% of the world’s energy consumption, with heating, ventilation, and air conditioning (HVAC) systems being responsible for approximately half of that usage. Energy usage is rapidly growing as the number of residential, commercial, and industrial buildings continues to rise, making the building sector the largest consumer of energy globally [1].

HVAC systems are critical for maintaining indoor comfort, but their energy demands fluctuate throughout the day and across seasons, creating peak loads that can strain the power grid and increase energy costs. During extreme temperature conditions, cooling or heating demands spike, requiring additional capacity to meet these temporary surges in space conditioning demand [2]. Thermal energy storage (TES) technologies offer a promising solution by storing excess thermal energy during off-peak periods and releasing it when demand rises.

Beyond HVAC systems, refrigeration represents another significant portion of energy consumption. Commercial refrigeration in industries such as medicine, food storage, supermarkets, plant nurseries, transportation, as well as data centers, supercomputing, and microchip manufacturing processes can be very energy intensive. For example, HVAC and refrigeration systems account for 50 to 60 percent of the total energy consumption in supermarkets [3], including both space conditioning for the comfort of the occupants and the safety of the food products stored [4]. The integration of TES technologies with HVAC systems can enhance efficiency, reduce operational costs, and mitigate peak energy demand. TES systems can operate on either sensible heat storage (SHS) or latent heat storage (LHS). SHS relies on the temperature change of the storage material, such as water, stone, or concrete, to store and release energy [5,6]. It is simple and cost-effective but requires large volumes of material due to the relatively low volumetric energy storage density. LHS, on the other hand, utilizes phase-change materials (PCMs) to store energy during phase transitions (e.g., melting and solidification), offering higher volumetric energy storage density and maintaining nearly constant operating temperatures. Individual PCMs are limited in their temperature range, but the wide variety of PCMs can be used to address the desired operating conditions.

Figure 1 (adapted from Awan et al. [7]) illustrates the thermal behavior of PCM during heating and cooling cycles. During heating, the solid phase initially undergoes sensible heating, where its temperature rises proportionally to the energy absorbed. Upon reaching the melting temperature, the material transitions from a solid to a liquid. During this process, energy is absorbed as latent heat while maintaining a constant temperature, as shown by the horizontal section of the graph. Once the phase-change process is complete, the liquid PCM undergoes further sensible heating, with the temperature increasing proportionally to the energy input. The additional latent heat storage mechanism provides significant energy density benefits but limits the operating temperature range of an individual PCM. Thus, PCMs must be carefully selected to match the expected operating temperatures. In addition, some PCMs may degrade over time, and heat transfer rates can be lower during phase transitions. Furthermore, some PCMs undergo changes in volume during phase-change processes, adding complexity to the PCM system design.

There are several different types of PCMs used today, including organic, inorganic, and eutectic substances. Organic PCMs boast a wide range of phase-change temperatures, a high latent heat of melting, and compatibility with construction materials. They also exhibit long-term stability and eliminate supercooling issues. However, their thermal conductivity is relatively low, they have high thermal expansion coefficients, and they pose flammability risks. Inorganic PCMs offer high heat storage capacity, excellent thermal conductivity, and low volumetric expansion [8]. However, some of them are prone to supercooling, they may cause corrosion, and present incongruent melting behavior. Eutectic PCMs provide a compromise in performance between organic and inorganic substances, but they often suffer from lower phase-change enthalpy and tend to be more costly [9].

Pure water ice is one of the oldest and most widely used PCMs. However, producing ice requires specialized low-temperature refrigeration equipment and consumes significant amounts of energy, which in turn directly impacts the costs. High temperature differences between the processing temperature and the ice also negatively impact the system’s efficiency, increasing the overall costs [10].

Gas hydrates, particularly CO₂ clathrate hydrates, have emerged as promising TES materials due to their higher phase transition temperature compared to that of water (improving efficiency in space conditioning applications) and superior dissociation energy. CO₂ hydrates and their slurries exhibit phase-change temperatures between 278 K and 285 K and latent enthalpies between 350 and 520 kJ/kg [11], which are ideal for air-conditioning needs. Unlike conventional PCMs, CO₂ hydrates offer environmental benefits, as they are biodegradable and non-corrosive. Beyond thermal energy storage, CO₂ hydrates also show potential for a usage in wastewater treatment and desalination, showcasing their versatility in energy and environmental applications [12].

A gas clathrate hydrate generally represents a structure where one component, known as the host (water), forms cages that enclose the molecules of another component, referred to as the guest (gas). The crystalline lattice of the host is a thermodynamically metastable phase, and the presence of guest molecules in its cavities stabilizes it. Under specific temperature and pressure conditions, the clathrate hydrate achieves thermodynamic stability when a certain proportion of its cavities is occupied by guest molecules [13].

The structure of the gas hydrate is determined by the type of guest gas molecules it contains, and it is capable of storing large amounts of gas. For example, one volume of methane hydrate can theoretically accommodate up to 172 volumes of methane gas [14]. Gas hydrates are typically categorized into three distinct structural forms: sI, sII, and sH. The sI and sII structures are both cubic crystal arrangements with lattice parameters of approximately 11.6 Å and 12.0 Å, respectively [15]. These structures are formed by varying arrangements of water molecules that create different types of cages, which encapsulate gas molecules. The size and shape of these cages significantly influence the properties of the hydrate, including its stability and gas storage capacity. Small guest molecules consisting of molecular diameters ranging between 0.4 and 0.55 nm form an sI structure, and larger guest molecules consisting of 0.6 to 0.7 nm molecular diameter help in the formation of the sII structure. The sH structure needs both a small guest molecule and a large guest molecule ranging between 0.75 and 0.9 nm [16]. As depicted in Figure 2, the clathrate hydrate structures are shaped by five distinct types of polyhedral: 5¹², 5¹²6², 5¹²6⁴, 4³5⁶6³, and 5¹²6⁸, where, for example, the designation 5¹² indicates the presence of 12 pentagonal faces, while 5¹²6² represents a structure with 12 pentagonal faces and 2 hexagonal faces. The unit cell of the sI structure has the form 2(5¹²)·6(5¹²6²):46H₂O, the sII structure has the form 16(5¹²)·8(5¹²6⁴):136H₂O, and the sH has the form 3(5¹²)·2(4³5⁶6³)·1(5¹²6⁸):34H₂O [17,18].

In crystal structures of CO₂ hydrates, water and CO₂ molecules are bonded by weak van der Waals forces. The structure becomes stable only when a guest substance CO₂ is present, and only under specific pressures and temperatures. Otherwise, it turns into a normal ice structure. The water molecule content in each structure is different, with 46, 136, and 34 water molecules for sI, sII, and sH structure hydrates, respectively [19].

Following the current developments in the fields of building energy efficiency, refrigeration, and thermal energy storage, this review provides a unique discussion of CO₂ hydrate technologies, as future cold energy storage materials and thermofluids. While earlier reviews focus on limited aspects, such as formation kinetics or a narrow class of promoters, this paper connects multiple areas of CO₂ hydrate research into a coherent narrative. It begins with a discussion of phase-change material applications, gas hydrate formation, phase behavior, and structural thermodynamics, then involves formation enhancers, including some of the most recent developments, such as metal organic frameworks (MOFs) and aziridine. The authors also address safety and environmental regulations surrounding additive use. This paper looks beyond the lab-scale experiments by considering scalability challenges, energy performance, and the full system lifecycle. By pulling together these many strands, this review supports diverse aspects to move from bench-scale hydrate studies to field-ready cold thermal energy storage that aligns with current energy performance and decarbonization goals.

2. CO₂ Hydrate Formation

CO₂ clathrate hydrate, which resembles shaved ice or Granita Siciliana desserts, is a crystalline substance composed of water ice and carbon dioxide. The first evidence for the existence of CO₂ hydrates dates back to 1882. It was associated with carbonic acid research performed by Polish physicist and chemist Zygmunt Florenty Wróblewski. Wroblewski was also the first to provide the approximate chemical formula for CO₂ hydrates-CO₂·8H₂O [20].

2.1. When and Where Does Gas Hydrate Formation Occur?

The gas hydrate formation process begins with nucleation and is followed by hydrate growth. Nucleation is the fundamental step in the formation of gas hydrates and is formed at the gas–water interface. This process begins with tiny clusters of water molecules forming around gas molecules, creating incomplete crystal structures. These early formations continuously appear and disappear due to fluctuations in temperature, pressure, and mass. Only the most stable clusters, those that reach a critical size, can grow into a full hydrate structure. Hydrates can form in two primary ways: spontaneously, without impurities (homogeneous nucleation), or with the help of impurities such as dust or pipe/reactor walls (heterogeneous nucleation). The formation of hydrates is influenced by several factors, including temperature, pressure, and the presence of impurities. The rate at which hydrates form is affected by these conditions, with lower temperatures and higher pressures usually promoting faster formation [21]. In the real world, hydrate formation occurs through heterogeneous nucleation, where the presence of interfaces or foreign surfaces significantly lowers the energy barrier for nucleation, compared to homogeneous nucleation. Surfaces with moderate hydrophobicity, such as stainless steel, silica, and carbon-based substrates, have been shown to promote hydrate nucleation by providing sites that support the alignment of water and gas molecules into pre-nucleation clusters [22]. Experimental visualization techniques such as high-pressure microscopy and cryogenic scanning electron microscopy (Cryo-SEM) have shown that hydrate formation often initiates at the solid–liquid–gas interface, especially at the microscopic surface defects such as scratches and crevices, which act as energy wells for nucleation. Nanoscale textures increase the available surface area and localize gas molecules by enhancing the probability of nucleation events. Surface-treated substrates, such as hydrophobically modified silica or polydopamine-coated carbon fibers, have been studied to reduce induction time and increase nucleation density, which indicates that surface chemistry and morphology directly influence hydrate nucleation kinetics [23].

In addition to solid surfaces, as discussed further in the upcoming sections, the presence of particulate additives such as graphene oxide, activated carbon, and metal–organic frameworks (MOFs) has been used to enhance nucleation by providing additional surface area and gas-trapping sites, which improve local gas concentration and water structuring. These materials not only serve as nucleation sites but also influence the growth patterns of hydrates, helping to make formation processes more efficient.

2.2. Memory Effect in Gas Hydrate Formation

It is a challenging task to investigate the nucleation process, both through theoretical/numerical analysis, and in situ laboratory experiments. One of the most theoretically demanding, and still not fully explained, aspects associated with hydrate formation is called a memory effect. Gas–water systems that have previously undergone hydrate formation tend to form hydrates more rapidly upon re-exposure to similar conditions. Residual water after initial hydrate formation and dissociation is called memory water.

There are several physical phenomena which are considered responsible for this increase in nucleation rate. The short list of the best-known theoretical explanations of the memory effect origins includes the following nano- and micro-scale mechanisms:

-

The residual structure mechanism;

-

The gas supersaturation mechanism;

-

The impurity imprinting theorem (for situations when hydrate formation takes place in porous media, i.e., fixed-bed continuous-flow reactors).

In thermal energy-oriented CO₂ hydrate applications such as thermofluids and cold storage, the first two mechanisms are the most important.

The residual structure mechanism theory assumes that the hydrate-like molecular structure remains in the decomposed solution and can provide nucleation sites for hydrate reformation [24].

The gas supersaturation mechanism theory considers that, after the initial hydrate formation and dissociation, the retained gas nanobubbles can affect the thermodynamic state of the water–gas system and the following nucleation process of the hydrate [25].

In general, the above behavior causing the memory effect is attributed to the existence of residual clathrate-like structures and/or trapped gas nanobubbles following the initial hydrate formation and dissociation process. They serve as nucleation sites for subsequent hydrate formation. Studies utilizing the classical nucleation theory have modeled this phenomenon, providing a framework for predicting and controlling hydrate formation in various systems [3]. It is important to highlight that in addition to the reduction in the hydrate formation time, the memory effect also influences hydrate morphology and transport properties. However, with prolonged thermal exposure, these residual structures or nanobubbles can dissolve effectively, erasing the memory effect and causing the system to behave similarly to fresh water. X-ray CT imaging revealed that hydrate reformation from memory water provides a more homogeneous distribution of hydrates, in contrast to the localized or clumped distributions typically observed during initial hydrate formation from fresh water [26].

2.3. Conditions Needed for CO₂ Hydrate Formation

The properties and formation of CO₂ clathrate hydrates have been explained by Sloan Jr. et al. [16] from the perspective of the physical reaction between CO₂ gas and water. In this process, gas molecules are trapped in the cavities of the network of water molecules. This typically takes place in the body-centered or diamond cubic lattice of structure II, which consists of 16 small cages with 5¹² cages per unit cell and 8 large cages with 5¹²6⁴ cages per unit cell. Each hydrate structure is occupied by at most one guest molecule. Both methane and CO₂ are gases with low molecular weight and form structure I, which consists of large cages (5¹²6² cages per unit cell) and smaller cages (5¹²6³ cages per unit cell) in hydrate equilibria [16]. The cubic structure sI clathrate is the only stable structure that has been formed at 0.7 GPa, as reported by Bollengier et al. (2013) [27]. It has been observed that the transition phase of CO₂ sI hydrates to a high-pressure hydrate starts below 280 K and at pressures up to 1 GPa.

Structure _SII is formed at higher pressures and has both a small cage (5¹²6²) and a large cage (5¹²6³). Due to the larger cage, sII can potentially store more guest molecules than sI. sII can be typically formed at a pressure of 300 MPa and a temperature of 249 K [28,29].

As mentioned before, CO₂ clathrate hydrates form under specific temperature and pressure conditions. This process is often influenced by the local CO₂ concentration and by the presence of a variety of additives, such as nucleation and crystal formation stimulants. The CO₂ hydrate formation process involves CO₂ interacting with water molecules to create a cage-like structure, especially near the melting point of ice, where a quasi-liquid layer (QLL) plays a crucial role. Raman spectroscopy-based observations by Zhou et al. [30] indicate that hydrate formation is spatially heterogeneous within ice particles, with the quasi-liquid layer near the melting point of ice facilitating localized conversion. The observed variability in gas concentration is attributed to incomplete conversion and mass transfer limitations between the ice core and the outer layers.

Experimental studies have shown that electrolytes such as NaCl, KCl, CaCl₂, and their binary mixtures, as well as synthetic seawater solution, impact the equilibrium conditions of CO₂ hydrate formation. The equilibrium data collected by Dholobhai et al. 1993, across a temperature range of 259 K to 281 K and a pressure range of 0.9 to 4.1 MPa, revealed that the composition of electrolytes significantly inhibits hydrate formation, altering the equilibrium temperature at a given pressure [31].

The use of methane and CO₂ mixtures in hydrate formation experiments demonstrated the importance of specific pressure and temperature ranges for equilibrium points. Methanol solutions are also used as promoters in some studies, where methanol influences hydrate formation temperatures. The results align with predictions based on the formula developed by Hammerschmidt [32].

$$∆T=K{\displaystyle \frac{1}{100-I}}$$

(1)

where K is a specific parameter dependent on the nature of the additive, and I is the amount of antifreeze in percent with respect to water. The agreement between the experimental data and the Hammerschmidt equation was reasonably good in the aqueous liquid—hydrate—gas region [33].

In the CO₂–H₂O system, the coexistence of gas, ice, and hydrates has been investigated by multiple researchers, such as Miller and Smythe [34], Adamson and Jones [35], Wendland [36], Yasuda and Ohmura [37]. Below the water melting point temperature, ice is more thermodynamically stable than liquid water, making the main type of equilibrium under such conditions a monovariant V-I-H (vapor-ice-hydrate) equilibrium. Studies by Kimura et al. 2021 confirmed that the difference in equilibrium pressures for hydrates with isotopic compositions of 12 CO₂ and 13 CO₂ is minimal, with the 12 CO₂ hydrate having slightly lower equilibrium pressure [38].

Experimental data indicate that even at temperatures below the freezing point of water, a metastable equilibrium involving vapor, supercooled liquid water, and hydrate (V–Lw*–H) can occur. This represents an extension of the typical vapor–liquid water–hydrate (V–Lw–H) equilibrium into colder temperatures. When hydrates decompose below 273 K, an ice layer forms on the surface of the hydrate particles, which slows down further decomposition. This phenomenon is known as the self-preservation of gas hydrates. Metastable supercooled water and gas hydrates can coexist, as seen in the dissociation of hydrates at sub-freezing temperatures for gases such as CO₂, propane, methane, and ethane. The presence of supercooled water during hydrate dissociation has been verified using Raman spectroscopy [39] and nuclear magnetic resonance relaxation spectroscopy [40].

Several studies have provided data on the vapor–liquid equilibrium of pure CO₂ and the three-phase equilibrium involving vapor, liquid CO₂, and hydrate (V–L CO₂–H), as well as vapor, liquid CO₂, and liquid water (V–L CO₂–Lw) in the CO₂–H₂O system. The ice melting curve in a CO₂ atmosphere (V-Lw-I equilibrium) shows a much steeper slope compared to the equilibrium line of pure water due to the high solubility of CO₂ in water, which lowers the equilibrium temperature of ice melting. A metastable three-phase equilibrium V–Lw*–I (vapor–supercooled liquid water–ice) can also occur at pressures above 1.04 MPa. The unique aspect of CO₂ as a hydrate-forming gas is that it can exist in both gas and liquid forms at typical hydrate formation conditions, unlike non-condensing gases. This complicates the phase behavior of the CO₂–H₂O system [41,42].

By plotting the phase diagram for CO₂ hydrate, we can clearly see the conditions under which CO₂ exists in different phases, such as ice, water, and hydrate. A comparative analysis shows that the experimental data for the two-phase equilibrium of pure CO₂ match well with the data for the three-phase equilibrium in the CO₂–H₂O system. Above these equilibrium lines, the stability of the CO₂ hydrate is governed by the three-phase equilibrium among liquid CO₂, liquid water, and hydrate (LCO₂–Lw–H). Reference data on the vapor–liquid equilibrium of pure CO₂ and experimental data on V–L CO₂–H (vapor–liquid CO₂–hydrate) and V–L CO₂–Lw (vapor–liquid CO₂–liquid water) three–phase equilibria for the CO₂–H₂O system have been provided by multiple studies [43,44]. Plotting the phase diagram of CO₂ hydrate provides a clear idea on the conditions under which CO₂ exits in different phases, such as ice, water, and hydrate. A comparative analysis shows that the experimental points for the two-phase equilibrium of pure CO₂ align with data for the three-phase equilibrium in the CO₂–H₂O system.

Following the combination of the experimental data analyzed by Semenov et al. [45], Figure 3 plots the phase diagram for the CO₂–H₂O system, showing various equilibrium lines and regions for the existence of liquid and gaseous CO₂. This phase diagram is essential for understanding the phase behavior of CO₂ hydrates and developing accurate thermodynamic models. The lower quadruple point Q1, where the lines of monovariant equilibria V–Lw–H, V–I–H, and V–Lw–I intersect, defines a non-variant equilibrium because four phases (vapor, liquid water, ice, and hydrate) coexist at fixed pressure and temperature values. Determining Q1 coordinates involves experimentally measuring equilibrium types and approximating the experimental points to find their intersection.

In Figure 3, the solid line V–Lw–H indicates a three-phase equilibrium involving gas, aqueous solution, and gas hydrate. The dashed line V–Lw*–H represents a metastable three-phase equilibrium featuring gas, supercooled aqueous solution, and gas hydrate. Another solid line, V–I–H, corresponds to a three-phase equilibrium comprising gas, ice, and gas hydrate. Line V–Lw–I denotes a three-phase equilibrium involving gas, liquid aqueous solution, and ice. A dotted line, V–Lw*–I, represents a metastable three-phase equilibrium involving gas, supercooled aqueous solution, and ice. The solid line V–L CO₂ signifies a two-phase equilibrium of gaseous and liquid carbon dioxide. Overlapping with it is the solid line V–LCO₂–(H or Lw), indicating a three-phase equilibrium of gaseous and liquid carbon dioxide and either gas hydrate or aqueous solution. The solid line L CO₂–Lw–H signifies a three-phase equilibrium involving a liquid carbon dioxide-rich phase, aqueous solution, and gas hydrate. Another red star, the Q2 point (283 K, 4.494 MPa), denotes the intersection showing the coexistence of gas, liquid carbon dioxide-rich phase, aqueous solution, and gas hydrate [45].

The formation of the CO₂ clathrate hydrate mainly depends on temperature [46,47] and pressure [48] parameters, as pure CO₂ clathrate hydrates are formed under high pressures and low temperatures. The dissociation pressure of CO₂ clathrate hydrates can be lowered by using mixtures of gases or different additives. For example, adding methane (CH₄) or hydrogen (H₂) to CO₂ can reduce the pressure required for hydrate formation. The presence of certain additives/promoters, such as salts or surfactants, can also influence hydrate stability, reduce induction time, and lower dissociation pressure [49].

These promoters act as catalysts in accelerating the formation process by lowering the activation energy required for hydrate growth and enabling the hydrate to form at mild pressures and viable temperature conditions. Additional discussion of promoters is provided in Section 3.

2.4. Hydrate Induction Time

Hydrate induction is an important kinetic characteristic parameter in hydrate formation, and it can also be reduced by promoters, as they accelerate the formation process and then promote the growth rate. Induction time refers to the time that is needed for the formation of the first hydrate nuclei that reach a critical size, beyond which they can grow into macroscopic hydrate structures. Depending on the process duration and formation steps, induction time is categorized into two types, including microscopic and macroscopic times, which are typically used in hydrate formation analysis. The microscopic induction time refers to the duration required for the nucleation of hydrates, that is, the formation of hydrate critical nuclei before the system reaches a phase equilibrium state. The macroscopic induction time is more commonly used in experimental studies and is defined as the time required for the system to reach a phase equilibrium state, followed by a sudden change in the state parameters, such as temperature and pressure. This is observed as the interval between the moment the system achieves phase equilibrium temperature and the appearance of the first inflection point on the temperature–time curve. Figure 4 illustrates both induction time definitions, together with the changes in the crystal structures [50]. The formation process of gas hydrates begins with the interaction between water molecules and guest molecules. Initially, water molecules arrange themselves around the guest molecules, leading to the formation of labile clusters, which are unstable and dynamic. These clusters then undergo agglomeration, where they merge and stabilize, eventually forming nucleation sites. As nucleation progresses, the hydrate structure begins to grow, ultimately leading to the formation of solid hydrates. The last part of the figure shows a reactor developed by the SBU collaboration for controlled hydrate formation [51].

3. Hydrate Formation Promoters

A variety of different types of promoters have already been evaluated for processing control in gas hydrate formation experiments, and almost every promoter behaves differently dueing gas hydrate formation. Some promoters can help with faster hydrate formation, while others might influence size and morphology and improve long-term stability during repeated cycles of formation and dissociation. In general, two basic groups of hydrate promotors are used to support CO₂ hydrate formation: (i) thermodynamic promotors and (ii) kinetic hydrate promoters.

In this chapter, the use of promoters and surfactants to enhance CO₂ hydrate formation is discussed. The following sections categorize promoter mechanisms and introduce emerging sustainable materials. They integrate thermodynamic and kinetic aspects of hydrate nucleation with quantitative performance metrics (e.g., induction time, formation rates) and environmental impact assessments.

3.1. Thermodynamic Promoters

Thermodynamic promoters enhance CO₂ hydrate formation through several mechanisms. They interact with water molecules at the water–CO₂ interface, modify the interfacial tension, and reduce the energy barrier for hydrate nucleation. These promoters selectively occupy the guest cages within the hydrate structure and create more favorable conditions for CO₂ molecules to fill the remaining cages. Promoters also influence the water molecule structure, making it more conducive to hydrate formation. They alter the thermodynamic properties of the system, such as the chemical potential of water or CO₂, helping make the formation of hydrates more energetically favorable [52]. Promoters also help lower the pressure and/or increase the temperature at which CO₂ hydrates are stable by effectively shifting the equilibrium curve to more favorable conditions for hydrate formation. For instance, when cyclopentane is added to a CO₂ hydrate system, there is a shift in the equilibrium curve, which indicates that hydrates can form at higher temperatures or lower pressures than would be possible without the promoter. This shift makes the hydrate formation process more energy-efficient and feasible under less extreme conditions, enabling practical applications. The most commonly used thermodynamic promoters are tetrabutylammonium bromide (TBAB), tetrahydrofuran (THF), cyclopentane (CP), cyclohexane, butane, propane, and argon. Figure 5 shows the structures of selected promoters.

Table 1. Thermodynamic promoters and their influence on CO₂ hydrate formation conditions.

Thermodynamic Promoter	Temperature (K)	Pressure (MPa)	Concentration (mol%)	References
Pure CO2 hydrate	273.2 to 281	5.0 to 25	No promoters	[54]
Tetrabutylammonium bromide (TBAB)	279 to 291	1.4 to 4.5	0.1 to 4.0	[55,56]
Cyclopentane (CP)	287 to 293	0.5 to 2.6	Water: CP–19:1	[57,58]
Tetrahydrofuran (THF)	279 to 291	0.18 to 3.17	4 to 10	[57,59,60]
Propane	274 to 282	0.5 to 3.7	3 to 60	[61]

presents various thermodynamic promoters that are used by different researchers and compares hydrate formation temperatures, pressure, solid fractions, and concentration ranges. Tetrabutylammonium bromide (TBAB) is effective in hydrate formation across temperatures from 279 to 291 K and pressures between 1.4 and 4.5 MPa, with concentrations varying from 0.1 to 4.0 mol%. This promoter is known for its ability to stabilize hydrates under moderate pressure conditions. Cyclopentane (CP) operates within a temperature range of 287 to 293 K and a pressure range of 0.5 to 2.6 MPa, with a water-to-CP ratio of 19:1. This shows that it can form hydrates at ambient temperatures. Cyclohexane functions at lower temperatures, from 275 to 278 K, and pressures between 0.95 and 1.81 MPa, typically at a concentration of 0.5 mol%. This promoter works under conditions similar to those of CP but at slightly lower temperatures. Tetrahydrofuran (THF) is effective over a wide temperature range of 279 to 291 K and pressures from 0.18 to 3.17 MPa, with concentrations between 4 and 10 mol%. THF is a commonly used promoter due to its efficiency in reducing the pressure required for hydrate formation. Propane operates within a temperature range of 274 to 282 K and pressures between 0.5 and 3.7 MPa, with concentrations ranging from 3 to 60 mol%. It is widely studied for its ability to facilitate hydrate formation under moderate conditions. Each promoter plays a crucial role in modifying the equilibrium conditions for CO₂ hydrate formation, enhancing the process’s feasibility under specific environmental conditions.

3.2. Kinetic Hydrate Promoters

Kinetic hydrate promoters are additives that accelerate the rate of hydrate formation without altering the equilibrium conditions. Unlike thermodynamic promoters, which interact with the crystal structure and shift the equilibrium curve, kinetic promoters primarily affect the nucleation and growth kinetics of hydrates. There are three common types of kinetic hydrate promoters: (i) surfactants, (ii) amino acids, and (iii) solid particle additives, each having its own specific mechanism and effects.

3.2.1. Surfactants

Surfactants such as fatty acids or alkyl sulfates play a crucial role in carbon dioxide (CO₂) hydrate systems. By lowering the surface tension between CO₂ gas and water, surfactants stabilize the interface, which is essential for the formation of hydrate crystals. This reduction in surface tension effectively decreases the energy barrier necessary for the nucleation and growth of hydrate crystals. They also influence the morphology and size distribution of these crystals. They achieve this by affecting the arrangement of water molecules at the gas–water interface. Through this molecular reorganization, surfactants facilitate the initial formation of hydrate crystals and impact their subsequent development and structural characteristics.

Typically, surfactants are classified according to the configuration of their polar head groups. Non-ionic surfactants have no charged groups in their head regions. Anionic surfactants are a type of surface-active agent that possess negatively charged ions, also known as anions, in their molecular structure. The head of an ionic surfactant carries either a net positive or net negative charge.

As shown in Figure 6, surfactants have hydrophobic and hydrophilic parts. The hydrophobic tail of the surfactant has a nonpolar long hydrocarbon chain and interacts with the gas phase or the nonpolar regions of the hydrate structure. In contrast, the hydrophilic head is polar and engages with the water phase through functional groups, such as -OH (hydroxyl), -COOH (carboxyl), or -SO₃H (sulfonic acid). These surfactants are further categorized into three classes, each with their own surfactant chemical, as shown in

Table 2. Examples of some widely used surfactants.

Category	Surfactants	References
Anionic Surfactants	Sodium Dodecyl Sulfate (SDS), Sodium Dodecyl Benzene Sulfonate (SDBS)	[62]
Non-Ionic Surfactants	Tween 80, Oxyethylene nonanoic acid, Span 80	[63]
Cationic Surfactant	Dodecyl trimethyl ammonium bromide (DTAB), Hexadecyl trimethyl ammonium bromide (CTAB)	[64]

.

Table 3. Surfactants and corresponding experimental hydrate formation conditions.

Surfactant	Temperature (K)	Pressure (MPa)	References
Sodium Dodecyl Sulfate (SDS)	270–277	3.0–15.0	[65,66]
Tween 80	274	3.55	[67]
Silica Gel	273	3.00	[65]
Silica Sand and zeolite 13X	274	3.00	[68]
Quiescent water	273	3.90	[69]

shows the different surfactants used, and the formation conditions studied by different researchers.

Sodium dodecyl sulfate (SDS) is the most widely used surfactant in the formation of CO₂ hydrates. It does not affect the equilibrium conditions but reduces the induction time. N. Molokitina et al. conducted experiments by varying the SDS concentration from 100 ppm to 5000 ppm and observed a decrease in the induction time [66,70].

3.2.2. Amino Acids

Amino acids serve in a similar way to kinetic promoters for CO₂ hydrate formation due to their unique ability to interact with both water and CO₂ molecules. Unlike traditional surfactants, amino acids are biodegradable and non-polluting biomass-based promoters. The effectiveness of amino acids in promoting hydrate formation is attributed to their molecular structure, which includes both hydrophobic and hydrophilic regions. As illustrated in Figure 7, the hydrophobic side chains preferentially associate with CO₂ molecules, while the polar functional groups interact with surrounding water molecules, contributing to the formation of pre-nucleation structures. This dual affinity promotes both nucleation and early-stage hydrate growth. Figure 8 presents common amino acids explored in recent studies, including L-leucine, L-methionine, and L-tryptophan, each exhibiting favorable kinetics under moderate pressure and temperature conditions. The corresponding hydrate formation conditions for these amino acids are summarized in

Table 4. Amino acid-based promoters and hydrate formation conditions.

Amino Acid	Pressure (MPa)	Temperature (K)	References
L-leucine	3.3	275	[71]
L-methionine	3.3	275	[72]
L-norleucine	3.3	273	[72]
L-norvaline	3.3	273	[72]
L-tryptophan	3.0–3.5	273–278	[73]

.

3.2.3. Nanoparticles/Solid Particles

Nanoparticles have a size between 1 and 100 nm and have been proven to be highly effective kinetic promoters in CO₂ hydrate formation due to their unique properties, such as high surface area, relatively high thermal conductivity, and their ability to interact with hydrates, forming components at the molecular level. They accelerate the hydrate formation process by providing numerous nucleation sites, enhancing overall thermal conductivity, and preventing hydrate particle agglomeration [74,75].

Silica nanoparticles significantly reduce induction time by increasing nucleation sites, and carbon nanotubes (CNTs) promote rapid hydrate formation and improve stability through their high surface area and excellent thermal properties [76]. Metal oxide nanoparticles such as TiO₂ and Al₂O₃ are commonly used to enhance the thermal conductivity of hydrates, with efficient heat dissipation and sustained hydrate growth.

Table 5. Nanoparticles and their impact on CO₂ hydrate formation temperature and pressure.

Nanomaterial	Pressure (MPa)	Temperature (K)	Reference
Nano graphite	2.5 to 6.5	272 to 277	[74,77]
Copper (Cu)	4.0	274	[78]
Zinc oxide (ZnO)	1.0	293 to 303	[79]
Silver (Ag)	2.08 to 3.24	273 to 275	[80]
Silica nanoparticles	0.6	260–276	[81]

shows various nanomaterials used as hydrate-formation promoters.

While Table 1, Table 2, Table 3, Table 4 and Table 5 summarize various hydrate additives and their effects, this section provides a critical comparison of promoter performance metrics to support application-specific material selection.

Table 6. Performance characteristics and environmental impact of hydrate-promoting additives.

Type	Induction Time	Formation Rate	Concentration Used	Effect	References
TBAB	Shortens induction time significantly (from 6 to 15 min to <5 min)	Moderate; accelerates hydrate nucleation and CO2 uptake	0.29 mol% (approx. 3–5)	Environmentally friendly	[82,83]
THF	Strongly reduces induction time (less than 1 min at 5 mol%)	High; especially with co-additives such as SDS	1–5 mol%	Acutely toxic, volatile and corrosive to equipment	[84,85]
SDS	Increased with higher SDS concentration (as low as <1 wt%)	High; rapid hydrate growth, especially in the presence of stirring	0.01–1 wt%	Toxic, highly foamy, and difficult to recycle	[79,86]
Amino Acids	Moderate (some amino acids promote hydrate nucleation, others are neutral or inhibitory)	Moderate; up to 356 mg·g⁻1 uptake in 15 min with L-methionine	0.2 wt% (typical for amino acid studies)	Non-toxic and biocompatible nature	[72,87]

shows the performance and environmental characteristics of performance-enhancing additives for CO₂ hydrates.

In addition to traditional nanoparticles such as silica and metal oxides, researchers have recently turned to metal–organic frameworks (MOFs) as promising solid-phase promoters for CO₂ hydrate formation. MOFs are crystalline porous materials made from metal ions coordinated with organic ligands, offering high surface areas, tunable pore structures, and exceptional gas adsorption capabilities. Their unique properties allow them to not only serve as nucleation platforms but also actively participate in the CO₂ capture and conversion processes at the molecular level [88]. Cao et al. [89] provided an overview of enzyme immobilization within MOFs for carbon dioxide (CO₂) conversion. MOFs such as the Zeolitic Imidazolate Framework (ZIF) series and the Materials of Institute Lavoisier (MIL) series are discussed as porous carriers that provide a controlled microenvironment for enzyme stabilization and CO₂ diffusion. The functional groups on MOF surfaces, along with pore size and geometry, influence coenzyme regeneration (e.g., nicotinamide adenine dinucleotide, NADH/NAD⁺) and substrate transport, which are important for catalytic efficiency in enzyme-driven CO₂ conversion pathways [89].

From a hydrate inhibition standpoint, Saikia et al. [90] examined the influence of MOFs on CO₂ hydrate formation kinetics in gas-dominated pipelines. They synthesized University of Oslo-66 (UiO-66) and its amine-functionalized counterpart, UiO-66-NH₂, via hydrothermal methods. The materials were characterized using Powder X-ray Diffraction (PXRD), Brunauer–Emmett–Teller (BET) surface area analysis, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). The experiments, conducted in a sapphire rocking cell at subzero temperatures and pressures exceeding 30 bar, used MOF concentrations of 0.25 and 0.5% weight/volume (w/v). UiO-66-NH₂ showed higher inhibition efficiency than UiO-66, likely due to the presence of –NH₂ groups, which can engage in hydrogen bonding with water and enhance CO₂ adsorption near the hydrate-forming interface. A BET analysis showed surface areas around 1000–1500 m²/g, indicating a high density of potential adsorption sites that could alter local phase behavior and nucleation timing. The observed hydrate inhibition was evaluated from a kinetic perspective, including shifts in induction time and suppression of crystal growth [90]. All these studies show that MOFs can serve dual roles: (1) enhancing the thermodynamic and kinetic aspects of CO₂ hydrate formation and (2) acting as smart delivery systems for other chemical promoters or catalytic agents. Compared to conventional solid nanoparticles, MOFs offer modularity in design, enabling the specific tailoring of surface chemistry, pore size, and thermal conductivity for targeted hydrate performance.

Kazemi et al. [91] synthesized a green and scalable method for fabricating Mixed-Metal MOF-74 using cobalt (Co²⁺) and nickel (Ni²⁺) by using environmentally friendly solvents (water and ethanol) and replacing hazardous organic solvents such as DMF (N,N-Dimethylformamide). DMF is a widely used polar aprotic solvent in conventional MOF synthesis because of its ability to dissolve both metal salts and organic linkers, supporting controlled crystal growth. However, it poses health and environmental risks due to its toxicity when inhaled or upon skin contact and is classified as a reproductive toxin. Its disposal requires careful handling, which makes it unsuitable for large-scale or environmentally conscious applications. To overcome these limitations, the researchers used ethanol–water mixtures along with metal acetates, eliminating the need for harmful solvents and nitrate salts. This process not only reduced the environmental burden but also enabled rapid synthesis at 90 °C for 1 h, forming MOF particles with well-defined, rod-like morphology and diameters below 100 nm. This structural feature enhances gas diffusion, facilitates faster hydrate nucleation, and increases accessibility to active sites critical for hydrate formation [91]. From an economic standpoint, the green synthesis route significantly reduces material and processing costs. Ethanol and water are inexpensive and readily available, unlike DMF and other toxic solvents that require specialized handling. The short synthesis time and low reaction temperature also improve scalability. The use of base metals such as cobalt and nickel, rather than rare or precious metals, improve economic feasibility. These advances collectively make MOF synthesis production more practical for integration into hydrate-based gas separation and storage technologies.

Stability under hydrate-forming conditions in water-rich, high-pressure environments is another important factor. Traditional MOFs suffer degradation due to hydrolysis of the metal–ligand coordination bonds in the presence of water. Water sensitivity remains a challenge for many MOFs, which can compromise their structural integrity and performance. However, the CoNi-MOF-74 synthesized by Kazemi et al. [91] exhibited excellent hydrothermal stability and sustained CO₂ adsorption capacity over ten adsorption–desorption cycles, including in flue gas conditions. The isosteric heat of adsorption was measured at 40.7 kJ/mol, indicating a strong yet reversible physisorption, which is ideal for hydrate promotion [92].

3.3. Promoters Efficiency and Process Scalability Challenges

Recent experimental work by Wang et al. [93] shows that, while kinetic promoters such as sodium p-styrene sulfonate accelerate CO₂ hydrate formation by attaining a two-stage CO₂ removal rate of 53.65%, the overall capture performance improved to 64.66% only when thermodynamic promoters (TBAB and cyclopentane) were present. This enhancement requires an additional formation stage, involving a trade-off between reducing pressure requirements (from 8.5 MPa to 1.5 MPa) and increasing process complexity. Their 4E (Energy, Exergy, Economy, Environment) analysis shows that the low-pressure formation with atmospheric dissociation (L-A) route offers the best overall performance, achieving the highest exergy efficiency (0.725) while minimizing indirect CO₂ emissions. These results suggest that although promoters can lower formation barriers and improve removal rates, scalable CO₂ hydrate technologies must carefully balance process complexity and overall cost with capture efficiency, number of stages, energy consumption, and environmental impacts through integrated system design [93]. Molecular simulations by Phan and Striolo et al. [94] show the promoter efficiency for CO₂ hydrate formation by using aziridine, a promoter that yields significant efficiency compared to traditional promoters such as THF and pyrrolidine in accelerating hydrate growth rates. At 269.1 K, the presence of aziridine more than doubled the CO₂ hydrate cage growth rate compared to a promoter-free system. This shows its strong kinetic enhancement effect. Promoter performance is governed by the sum of two key thermodynamic parameters: the free energy barrier for CO₂ desorption (ξ) and the binding free energy at the hydrate–aqueous interface (ΔG_bind). Aziridine had the most favorable combined values, correlating directly with faster and more stable hydrate formation, while THF reduced the hydrate formation pressure, and it was less effective kinetically. Ideal promoters for large-scale CO₂ hydrate applications must simultaneously promote nucleation kinetics and maintain thermodynamic stability, rather than focusing on just pressure reduction alone [94]. Experiments conducted by Pang et al. [95] show the advanced understanding of hydrate-based CO₂ sequestration scalability by constructing a 1695 L three-dimensional sediment model to mimic in situ reservoir conditions. Their study shows that while continuous hydrate formation can be achieved without mechanical wellbore blockage, large-scale operations pose challenges such as strong spatial heterogeneity in hydrate distribution, localized pore clogging, and progressive water depletion within the sediment matrix. These effects cumulatively lower the effective storage capacity and reduce CO₂ injectivity over time. Successful field-scale deployment will require dynamic control over injection rates, pressure profiles, and water management strategies to avoid early saturation and maximize reservoir utilization. While technical feasibility has been already experimentally validated at large volumes, the precise process engineering remains critical for converting laboratory-scale success into reliable field-scale CO₂ storage operations [95].

3.4. Regulatory Standards Related to CO₂ Hydrate Promoters and Surfactants

In hydrate-based carbon capture and storage (CCS) systems, the selection of suitable promoters is not only important for enhancing hydrate formation kinetics, but also for ensuring environmental safety and compliance with health regulations. While promoters such as tetrahydrofuran (THF), tetra-n-butylammonium bromide (TBAB), and cyclopentane (CP) are effective in facilitating hydrate formation, their environmental and health implications require careful consideration.

Tetrahydrofuran (THF) is a widely used thermodynamic promoter known for its ability to form structure II hydrates at relatively mild conditions. However, THF poses several health and environmental risks, as it is classified as a Group 2B carcinogen by the International Agency for Research on Cancer (IARC), showing possible carcinogenicity to humans [96]. THF is also highly flammable and volatile, capable of forming explosive peroxides upon prolonged exposure to air. Its high volatility and water solubility increase the risk of environmental contamination, particularly in aquatic systems. Regulatory agencies such as the U.S. Occupational Safety and Health Administration (OSHA) have established strict guidelines for THF handling and exposure limits to mitigate these risks [97]. Cyclopentane (CP) is another thermodynamic promoter that forms structure II hydrates and is valued for its ability to promote hydrate formation under mild conditions. However, CP is a volatile organic compound (VOC) with a relatively high vapor pressure, leading to rapid evaporation and potential atmospheric release during handling and hydrate dissociation processes. CP is also flammable and poses inhalation risks. Environmental concerns include its contribution to ground-level ozone formation and potential toxicity to aquatic life if released into water bodies. Mitigation strategies involve implementing closed-loop systems to capture and recycle CP, thereby minimizing environmental release and occupational exposure [98].

In contrast to these conventional promoters, amino acids have emerged as environmentally friendly alternatives for promoting gas hydrate formation. Amino acids such as L-methionine, L-threonine, and L-isoleucine have demonstrated efficacy as kinetic promoters, enhancing hydrate formation rates and gas storage capacities. L-methionine has been shown to reduce induction times and increase CO₂ uptake in hydrate formation processes, outperforming traditional promoters such as SDS in certain conditions [99]. Amino acids are biodegradable, non-toxic, and naturally occurring, which makes them attractive candidates for sustainable hydrate-based CCS applications.

The environmental and health concerns associated with conventional promoters can be mitigated by some strategies, including the development of closed-loop systems to prevent the release of volatile promoters, the implementation of recovery and recycling processes to minimize waste, and the adoption of alternative promoters with favorable environmental profiles. Regulatory agencies have to establish comprehensive guidelines for the use and disposal of hydrate promoters with regard to their potential environmental persistence and toxicity.

4. Experimental Setups Used in CO₂ Hydrate Formation Experiments

This section presents state-of-the-art developments in different types of experimental setups and methodologies performed by different researchers. A variety of potential gas hydrate applications, combined with different combinations of precursors and processing conditions, indicates a need for various experimental setups/configurations used in the preparation of CO₂ hydrates. These setups might vary based on reactor type, such as high-pressure stirred reactors or batch reactors, or they might vary based on driving forces such as pressure change, temperature change, and hydrate promoter addition. The following sub-sections discuss different types of reactor configurations, and some of the testing methodologies used by different researchers for each reactor type.

4.1. Stirred Reactors

Stirred reactors are the most widely used today to prepare gas hydrates at the lab scale and probably represent the easiest method to form the hydrates. However, mixing water and CO₂ while they turn into an icy solid structure can be challenging because, at some point, this mixture turns into a viscous slurry. At this stage, it is usually difficult to mix the slurry further, since it requires significantly more power than is applied at the beginning of the process. That is the reason why advancements in reactor designs are still needed before this technology can be scaled up for commercialization, which requires the continuous synthesis of hydrates. The same reactor setup usually works with different impellers and is designed to allow for variation in the volume of water added, stirring rate, impeller type, and the temperature–pressure conditions under which hydrates form. For example, Pivezhani et al. performed a numerical analysis and more than 50 experiments by changing the physical and experimental conditions, such as equipping four different types of impellers (three blades, six blades, and anchor types), adjusting stirring rates from 100 to 500 rpm, and changing the volume of water between 350 and 500 mL. The temperature and pressure varied between 274 K and 277 K and from 20.6 bar to 23 bar [100].

These types of reactors increase the interfacial area between the gas and liquid phases by promoting faster mass transfer in the formation of CO₂ hydrates. As described in numerous publications, they are best suited for lab-scale CO₂ hydrate production [101,102,103]. Because of the unique design of the impellers, stirred reactors provide better contact between CO₂ gas and water, which helps accelerate the formation of hydrate nuclei. The agitation process in this type of reactor ensures a uniform temperature profile within the reactor. By adjusting the agitation speed, control over the reaction kinetics can be attained by helping to set up a consistent and repeatable production [104]. Once the hydrates are formed inside the reactor, the agitator requires more power to overcome the viscosity of the resulting slurry. The basic components of a stirred reactor are a gas and water supply, agitator, cooling system, data logger, and an optional camera to enable visualization inside the reactor, as shown in Figure 9 [105].

Figure 10 shows a 2000 mL reactor at University of Massachusetts Lowell (UML), which was introduced to address some of the challenges associated with traditional stirred reactors. Compared to commonly used benchtop reactors with a volume below 0.5 L, this reactor is designed to handle larger volumes of water and CO₂. The increased capacity allows for more efficient mixing. Aerogel blanket insulation helps reduce temperature stratification, allowing for better control of reaction conditions, which is essential for the formation of hydrates. The reactor has an impeller system to handle the viscous slurry formed during the hydrate formation process. This system provides good mixing efficiency and maintains performance even as the mixture becomes more viscous, reducing the risk of the impeller getting stuck.

Figure 11 shows a type of reactor that has a mixing blade (impeller) that is designed to draw gas into the liquid. This helps to mix the gas and liquid together better and prevents the hydrate crystals from sticking together. The gas is pushed around inside the liquid by the impeller. It flows down towards the center and then is captured by the blades. When the impeller spins, the gas is forced out into the liquid as tiny bubbles. While this method improves the reactor’s performance, there is still a risk that the impeller could become stuck if the liquid becomes too thick with hydrate crystals [106].

While stirred reactors are effective for CO₂ hydrate formation at the lab scale, their potential for scalability is challenging due to increasing slurry viscosity during hydrate growth. This increases the torque demand on the impeller, which leads to jamming or inefficient mixing. To overcome these issues, design innovations such as variable-speed motors with torque feedback, gas-inducing impeller geometries, and non-contact magnetic stirrers can be incorporated into the reactor. Pipe reactors with the screw mixing mechanism/conveyors can be used for continuous hybrid production. Self-lubricating coatings or Teflon-lined shafts may reduce frictional resistance during high-viscosity operations, providing better control under semi-solid flow conditions.

4.2. Bubble-Forming, Ejector, and Continuous-Flow Reactors

Bubble-forming reactors, also known as a bubble-tower reactors, are a type of reactor designed for gas–liquid reactions. They use advanced microbubble technology, proposed by Takahashi et al. [107] based on an ejector-type loop reactor (ELR). In this reactor, high-velocity liquid flow creates microbubbles within the fluid, leading to thorough mixing of gas and liquid, which is a fundamental criterion impacting the hydrate formation rate [107]. Figure 12 shows the ELR with the ejector schematics. One of the greatest advantages of this kind of reactor is its ability to speed up the hydrate formation process. It can handle the heat generated during this process more effectively than other reactor types. This means that less energy is needed to compress the gas, and this makes the operation more straightforward and more efficient [108]. However, removing the formed hydrates from the bubble-forming reactors can be difficult, and managing the nucleation heat effectively is still an ongoing challenge [109].

Traditional hydrate reactors face two significant challenges: inefficient heat transfer and the buildup of hydrates on reactor walls. To address these issues, a novel reactor design incorporating a spiral groove has been developed [110]. This innovative design causes a swirling motion within the reactor, enhancing heat transfer and preventing hydrate accumulation. The utilization of nanobubbles significantly increases the surface area available for gas–liquid interaction, thereby accelerating hydrate formation. In essence, this advanced reactor design overcomes the limitations of conventional methods by optimizing heat transfer, minimizing hydrate buildup, and promoting efficient gas–liquid contact, eventually leading to enhanced hydrate production [111].

Ejectors and bubble-based reactors exhibit high gas-liquid contact efficiency, but their continuous operation is often disturbed by hydrate crystal accumulation in ejector throats, mixing chambers, and outlet lines. To address this, design improvements such as hydrophobic-coated internal surfaces, spiral groove nozzles, or vibrating ejector assemblies can be incorporated to minimize fouling. Using nanobubble injectors with size-controlled dispersion increases mixing uniformity by preventing blockages.

4.3. Continuous Process Reactor

In an effort to advance CO₂ hydrate technologies toward industrial application, it is important to transition from batch systems to continuous processes, which would closely mimic large-scale operations. Continuous-flow reactors have better control over reaction parameters such as flow rates, temperature gradients, and residence times, all of which influence hydrate formation kinetics and heat transfer behavior. Unlike batch reactors, continuous systems can maintain steady-state operations, improve energy efficiency, and provide hydrate production under flow-dynamic conditions that are relevant to pipeline and energy storage technologies.

Yang et al. [112] investigated conditions needed for continuous synthesis of CO₂ hydrates. They investigated CO₂ clathrate hydrate formation at increased linear fluid velocities and high gas volume fractions. For this purpose, they constructed a continuous-flow reactor (CFR) containing eleven major components which include a saturated CO₂ gas delivery system and water supply, a continuous-flow reactor, chillers, a gas–slurry separator, an accumulator, a flash reactor for hydrate slurry decomposition, a sample collection source, ThermoFisher, Waltham, MA, USA analyzer/sample collection stations, and a data acquisition system. Before starting to form the CO₂ clathrate hydrate, the heat exchange between the CFR and the ambient surroundings was determined by calorimetric experiments to assess the heat loss to the surroundings. The researchers first needed to account for heat loss from the apparatus itself. To ensure accurate heat transfer measurements within the reactor, researchers performed a preliminary calibration to understand the heat transfer characteristics under non-reactive conditions. First, this calibration was conducted by running the system with nitrogen instead of CO₂ and analyzing how variations in coolant flow rate and temperature affected heat transfer.

By simulating non-reactive conditions, they were able to evaluate the heat transfer between the coolant and the reactor environment and establish baseline parameters for subsequent experiments. These calibrations helped identify both the heat transfer efficiency and key operational factors, such as the impact of coolant flow rates and temperature regulation on reactor performance.

Figure 13 shows the reactor design, which allowed for the simulation of hydrate production conditions by providing a controlled evaluation of heat transfer efficiency during hydrate formation. By maintaining a specific range of coolant flow rates and temperatures below freezing, the reactor ensured effective heat management. This ability to mimic real-world hydrate production scenarios under controlled conditions demonstrated the reactor’s suitability for studying hydrate formation, while providing flexibility in adjusting process variables [112].

Continuous reactors help hydrate production under flow conditions mimicking real-world systems, but they pose challenges related to slurry handling, sedimentation, and heat balance. These can be prevented by integrating slurry-compatible positive displacement pumps, inline slurry separation chambers, and smart valve systems that respond to flow resistance. By adding modular heat exchanger segments or localized recirculation zones, one can improve thermal control and avoid cold zones by ensuring consistent hydrate growth while avoiding clogging during long operation.

4.4. Fixed-Bed Reactors

A fixed-bed reactor is a widely used type of reactor in chemical processes, where a solid catalyst or reactant bed remains stationary while a fluid (either gas or liquid) flows through it. Fixed-bed reactors are utilized to enhance the formation of CO₂ hydrates by allowing CO₂ gas to flow through a bed of water-saturated porous material under controlled temperature and pressure conditions [113].

A typical fixed-bed reactor setup for CO₂ hydrate formation, as shown in Figure 14, it consists of a cylindrical vessel packed with porous materials, such as silica gel, activated carbon or sand that is saturated with water. CO₂ gas is introduced at one end of the reactor and allowed to flow through the fixed bed. As the CO₂ gas comes into contact with the water held within the pores of the solid material, it dissolves in the water and forms CO₂ hydrates under the appropriate thermodynamic formation conditions (low temperature and high pressure). The porous material provides a large surface area for the gas–liquid interaction, which is essential for efficient hydrate formation. The fixed-bed configuration also allows for continuous gas flow, which can be useful for studying the kinetics of hydrate formation and for scaling up the process for industrial applications [114]. Table 6 summarizes various reactor types and volumes used by different researchers for CO₂ hydrate formation. It contains consolidated details on the type and size of reactors (e.g., stirred, fixed-bed, bubble reactor), the gases utilized (CO₂, N₂, H₂, CH₄), and the additives (silica gel, THF, TBAB) employed to enhance hydrate formation.

Fixed-bed reactors facilitate hydrate formation within water-saturated porous media, but scaling these systems is restricted by pore fouling, an increase in pressure drop, and thermal bottlenecks. Over time, hydrate crystal growth blocks the interstitial channels, cutting off gas flow and reducing formation rates. This can be addressed by including the use of periodically backflushed dual-flow systems, impregnated metallic foams with high thermal conductivity, and rotational bed designs that allow gentle disturbance to break early-stage blockage.

4.5. Comparisons of Different Types of CO₂ Hydrate Reactors

This section offers a short summary and comparisons of different designs of CO₂ hydrate reactors. It includes an overview of various reactor configurations and experimental conditions reported in CO₂ hydrate research.

Table 7. Summary of reactor configurations and additives used in CO₂ hydrate research.

Authors, and Year	Reactor Type and Volume	Gases Utilized	Additive	References
Seo et al., 2001	Stirrer, 50 cm3	CO2, CH4	N/A	[115]
Linga et al., 2007	Stirrer, 323 cm3	CO2, N2 and CO2, H2	THF	[116]
Seo and Kang, 2010	Stirrer, 500 cm3	CO2, H2	Silica gel	[117]
Li et al., 2010	Stirrer, 56.4 cm3	CO2, N2	TBAB	[118]
Babu et al., 2013	Fixed bed, 1240 cm3	CO2, H2	Silica gel	[119]
Sun et al., 2014	Stirrer, 1500 cm3	CO2, N2 and CH4, N2 and CH4, H2	THF	[120]

shows the reactor types, volumes, gases used, and specific additives used to promote hydrate formation across different studies.

Table 8. Comparative analysis of reactor types.

Reactor type	Advantages	Disadvantages	Induction Time	Scalability	References
Stirred reactor	High gas-liquid contact area Uniform temperature distribution Lab-scale reproducibility	High energy input for mixing Impeller fouling at high hydrate fractions Limited scalability due to viscosity issues	24–261 min (depends on RPM)	Batch-limited	[121]
Bubble-Forming (Ejector) Reactors	Rapid hydrate formation via microbubbles Lower energy than stirred reactors Continuous operation feasible	Nozzle clogging Poor heat dissipation at large scales Limited data on long-term performance	180–360 min	Scale specific optimization	[108]
Continuous-Flow Reactors	Steady-state operation Suitable for industrial-scale production Integrated heat exchangers reduce energy loss	Complex pressure/temperature control Risk of hydrate plugging in pipes High capital cost	~0 min (instantaneous)	Designed particularly for industrial continuous operation	[122]
Fixed-Bed Reactors	Low energy input High surface area (porous media) Passive operation	Slow kinetics Pore blockage Difficult to regenerate porous media	50–250 min	Limited by mass and pore clogging	[114]

has the consolidated comparative analysis of the major reactor types used for CO₂ hydrate formation and their respective advantages, limitations, induction times, and scalability characteristics.

4.6. Challenges in Scaling CO₂ Hydrate Systems

While laboratory-scale studies have significantly advanced our understanding of CO₂ hydrate formation mechanisms and promoter performance, the conversion of these findings to industrial applications is not feasible yet, due to several engineering challenges. One of the primary issues is hydrate slurry viscosity management. As hydrate crystals form and grow within a reactor, the resulting slurry can have high viscosity and non-Newtonian behavior, particularly at solid fractions above 20–30%, resulting in increased resistance to flow, greater pumping power requirements, and potential jamming in the pipelines and heat exchangers [123]. Stirred tank reactors that are widely used at lab scale face scalability limitations due to torque overload and inefficient mixing when hydrate slurries thicken during prolonged operation [124]. Addressing this requires the design of robust mechanical systems, such as screw-driven or pipe-loop reactors, which can maintain solid suspension while minimizing energy losses and impeller fouling. Another critical hurdle is heat transfer optimization during both hydrate formation and dissociation. The exothermic nature of hydrate crystallization necessitates rapid heat removal to prevent local overheating and phase instability, while the endothermic dissociation process demands uniform heat input across the reactor. Lab-scale setups often use jacketed vessels or internal coils, but industrial-scale systems require compact, high-efficiency heat exchangers such as microchannel, plate-fin, or helically coiled systems to maintain thermal control without increasing system volume or cost [125,126]. The local heat transfer coefficient drops significantly as the solid fraction increases, emphasizing the need for advanced designs that prevent wall-layer accumulation and thermal insulation by hydrate particles. Gas–liquid–solid mass transfer limitations emerge at larger scales, making it complex to maintain the uniform dispersion of CO₂ and water. Industrial systems must incorporate dynamic flow control, optimized gas injection strategies, and multiphase CFD numerical analysis to ensure reactor homogeneity and predictable hydrate growth. The integration of real-time sensors for slurry monitoring, torque, pressure drop, and thermal gradients also becomes essential to maintain process stability. Ultimately, scaling up hydrate-based CO₂ capture and thermal energy storage systems requires a synergistic approach that combines thermodynamics in mechanical design, heat exchanger integration, and flow assurance engineering.

5. Typical Characterization Techniques Used in CO₂ Hydrates Analysis

Characterization techniques are crucial for understanding CO₂ hydrates, as they provide details of their physical properties, stability, and formation mechanisms. By analyzing the crystal structure, lattice parameters, and molecular arrangements, we can better understand how hydrates form and decompose, ensuring efficient production and storage. These techniques help assess the impact of various factors, such as salts and surfactants, on hydrate stability, leading to new applications in carbon capture, gas separation, and energy storage. This chapter provides an overview of key characterization methods used to analyze CO₂ hydrates properties such as structure, thermal, and stability properties. It discusses Fourier-transform infrared (FTIR) spectroscopy for studying crystallization mechanisms, X-ray diffraction (XRD) for crystallographic analysis and CO₂ storage capacity [127], and high-pressure differential scanning calorimetry (HP DSC) for evaluating enthalpy changes and thermodynamic behavior. Finally, the T-history method is introduced as a simplified yet effective approach for characterizing phase-change behavior. Together, these techniques enable the analysis of fundamental properties of CO₂ hydrates that make their practical applications possible in various industries.

5.1. Fourier-Transform Infrared (FTIR) Spectroscopy

The Fourier-transform infrared (FTIR) spectroscopy technique has been successfully used to find/analyze crystallization mechanics in many substances where crystal formation exists. One of the common studies includes the analysis of inorganic salt-based hydrate PCMs to efficiently apply them in various thermal storage applications, such as refrigeration systems. Handling and testing the formed CO₂ clathrate hydrates is difficult due to their highly unstable behavior outside their original formation conditions. In this light, it is important to develop lab instruments [128] or convert their shapes, such as making them into thin films to keep them stable until the end of testing [129,130].

In the FTIR analysis of CO₂ clathrate hydrates, specific IR bands correspond to the vibrational modes of CO₂ within the hydrate lattice, which differ from those of free CO₂. FTIR can track these changes, providing details of formation and dissociation processes. By observing the evolution of FTIR spectra, it can be easy to monitor the kinetics of CO₂ hydrate formation by knowing the conditions under which these hydrates form. FTIR also provides the observation of hydrate dissociation by tracking the disappearance or shift of IR bands associated with the CO₂–water interactions [131].

For example, Boufares et al. measured the CO₂ consumed during the hydrate formation process by fixing the nucleation temperature ranging from 276 K to 283 K, pressure ranging from 1 to 2.5 MPa, and the concentration of CO₂ dissolved during the crystallization. As shown in Figure 15, an in situ ATR (Attenuated Total Reflection) probe is used to determine the dissolved CO₂ gas. This ATR probe is inserted through a valve from the top lid of the autoclave reactor. Then, the measurements are continuously recorded by the IR bands as CO₂ gas is injected into the reactor. This process is repeated for various temperatures, ranging from 272 K to 283 K, to measure the CO₂ absorbance by the mix.

5.2. X-Ray Diffraction (XRD)

In the analysis of gas hydrates, including CO₂ clathrate hydrates, understanding their crystallographic structure is very important. The X-ray diffraction (XRD) technique is commonly used to define/analyze the crystallographic structure analysis and the amount of CO₂ stored in the case of the CO₂ clathrate hydrate. It has an important role, as hydrates can exist in various forms, depending on factors such as water pH, pressure, and temperature. By knowing their stability, the XRD can be applied in the analysis of storage and transportation technologies for CO₂ [133].

Powder X-ray diffraction (PXRD) is a powerful technique for analyzing the crystal structure of CO₂ clathrate hydrates. PXRD works by directing X-rays at a powdered sample and analyzing the resulting diffraction pattern to determine the arrangement of atoms within the crystal lattice. This method provides some insight into the stability, formation, and decomposition of CO₂ clathrate hydrates, enabling researchers to track phase transitions, formation and decomposition kinetics, and evaluate the impact of additives on hydrate properties.

Due to the instability of CO₂ clathrate hydrates under ambient conditions, PXRD tests are conducted at high pressures (2 MPa to 10 MPa) and low temperatures (243 K to 283 K), where the hydrates remain stable. Advanced setups such as high-pressure cryogenic vessels support these experiments by providing controlled environments until the end of the test. Figure 16 shows an example of a PXRD system that allows real-time X-ray analysis during hydrate formation and decomposition.

Takeya et al. [134] showed the application of PXRD by performing temperature-dependent measurements to study CO₂ hydrates. Their setup promotes precise control of temperature and pressure, ensuring stable conditions for hydrate formation. PXRD measurements covered a 2θ range of 22.1° to 29.7°, which corresponds to the characteristic diffraction peaks of CO₂ hydrates. The fine step width of 0.02° provided high-resolution data on the crystal structure, while measurements at regular temperature intervals (10.15 K) enabled detailed tracking of structural changes under isothermal conditions. Although PXRD presents challenges such as sample preparation under high-pressure and low-temperature conditions and complex data analysis, it remains an invaluable tool for advancing our understanding of CO₂ clathrate hydrates. By revealing detailed structural information, PXRD helps researchers evaluate hydrate stability, assess the influence of additives, and optimize conditions for practical applications in gas storage, sequestration, and energy transport. This makes PXRD a critical technique for both fundamental research and industrial implementation of CO₂ hydrate technologies [134].

5.3. High Pressure Differential Scanning Calorimetry (HP-DSC)

High-pressure differential scanning calorimetry (HP DSC) is a powerful technique for characterizing the thermal behavior of CO₂ hydrates. It measures the heat flow between a sample and a reference, as the temperature changes under controlled pressure. The analysis of heat peaks observed during hydrate formation and decomposition, facilitated by High-pressure differential scanning calorimetry (HP DSC) helps determine the enthalpy change (ΔH) associated with important thermodynamic behaviors into CO₂ hydrate stability. This allows for the investigation of guest molecule exchange within hydrate cages, providing researchers with valuable data on the kinetics and thermodynamics of this phenomenon. The pressure dependence of hydrate formation and decomposition temperatures obtained through HP DSC can be utilized to evaluate CO₂ hydrate stability under varying pressure conditions [135].

The setup used for HP-DSC thermal analysis is illustrated in Figure 17. The enthalpy of dissociation of CO₂ hydrates was measured using a high-pressure micro-differential scanning calorimeter (HP-μDSC VIIa, Setaram Inc., Caluire, France). Initially, hydrates were synthesized ex situ and transferred into pre-cooled calorimetric cells under cryogenic conditions (liquid nitrogen) to prevent premature decomposition [136]. The temperature was ramped at a constant rate of 1.0 K/min, from 250 K to 305 K, under pressures up to 20 MPa. The hydrate sample mass ranged between 40 and 140 mg, and the DSC cells (volume: 0.5 cm³) were made of Hastelloy C276. Although this conventional single-scan method enables baseline enthalpy measurement, partial water conversion and overlapping ice–hydrate transitions limit its ability to resolve metastable behaviors accurately. Researchers have been using a PVT cell equipped with a visor/window to study the phase behavior of hydrates by observing the reaction going on inside. Temperature and pressure sensors are equipped for the thermodynamic properties and pressure variations, which are all time consuming and require manual work [137].

To address these limitations, the experimental approach is compared to the multicycle calorimetric technique introduced by Robustillo et al. [138]. In the multicycle method, repeated subcooling and heating cycles (typically 20–25) are conducted under controlled pressure and temperature conditions. The heating and cooling rates vary from 0.2 to 1.0 K/min to promote a gradual increase in hydrate yield while minimizing kinetic distortions. This technique achieves >99% water-to-hydrate conversion by suppressing secondary exothermic events linked to metastable ice formation. Enthalpy values derived from this method show improved precision and do not rely on thermodynamic extrapolation through the Clausius–Clapeyron equation [138].

5.4. T-History Method

In contrast to DSC and differential thermal analysis (DTA), the T-history method provides a simpler approach for characterizing PCMs. This method was conceptualized by Yinping Zhang in 1999 [139] and refined by researchers over subsequent decades [140,141]. It provides an effective means of determining enthalpy and heat capacity. It is especially valuable for analyzing non-homogeneous samples, including those prone to supercooling. It consistently delivers superior accuracy in PCM characterization. The T-history setup for studying the phase behavior of CO₂ clathrate at high pressure is similar in concept to the setup used for other PCMs. Figure 18 shows the T-history setup constructed for testing CO₂ hydrate dissociation temperature.

In this method, both the PCM and a reference material (water) are subjected to the same controlled cooling environment, and their temperature–time responses are recorded. The thermal lag during phase change is used to calculate latent heat based on a lumped thermal model. In the experimental setup, a glass tube (10 mm inner diameter, 160 mm length, 12.6 cm³ volume) was used to hold the hydrate sample.

For sample preparations, a defined concentration of TBAB (tetra-n-butylammonium bromide) was first dissolved in deionized water to create a homogenous aqueous solution. This solution was transferred into a sealed glass tube and exposed to a regulated CO₂ gas stream under pressure. The system gradually cooled using a thermostatic bath to facilitate hydrate formation. Crystallization was confirmed through the glass on the side of the reaction tube. Once hydrate formation was complete, the sample underwent controlled thermal cycling to analyze its dissociation behavior using the T-history method. A type-K thermocouple was embedded in the sample to measure temperature changes with ±0.1 °C accuracy, while the tube was submerged in a heat transfer fluid bath. To ensure one-dimensional heat transfer and accurate modeling, the Biot number was kept below 0.1. The phase-change enthalpy was calculated by comparing the temperature shift between sample and reference curves, accounting for specific heat, mass, and temperature differential. This method is advantageous for detecting metastable behavior such as supercooling and delayed dissociation because of its larger sample size and visual access to hydrate crystallization [127].

6. Dissociation of CO₂ Clathrate Hydrates and Advancements in Analysis of Phase-Change Enthalpy

Dissociation enthalpy is a crucial property of CO₂ clathrate hydrates, which represents the energy required to break the bonds between CO₂ molecules and the water lattice in the hydrate structure. This property is crucial for understanding the stability, formation, and potential applications in both designing thermal storage systems and analyzing the performance of CO₂ hydrate slurries used as thermo-fluids. The exact enthalpy change for CO₂ hydrate formation/dissociation depends on various factors, such as temperature, pressure, and the specific hydrate structure (I or II). A higher dissociation enthalpy indicates a more stable hydrate under given conditions [142,143].

The heat which is released or absorbed when CO₂ hydrates are formed from water and CO₂ under specific conditions of pressure and temperature is called formation enthalpy. In the reverse process, the heat required to break down CO₂ hydrates back into water and CO₂ is called dissociation enthalpy.

For hydrate formation: CO₂ + n H₂O → CO₂ n H₂O + Heat.

For hydrate dissociation: CO₂ n H₂O + Heat → CO₂ + n H₂O.

This enthalpy can be either positive or negative, depending on the process. Several experimental studies and computational simulations were used to determine the enthalpy of CO₂ hydrate formation under various conditions. Hydrates provide a compact and stable medium for storing CO₂ in solid form, and the enthalpy helps predict how much energy will be required for hydrate dissociation when the CO₂ needs to be released for sequestration or industrial use [144].

However, typical values for CO₂ hydrate dissociation range from 350 kJ/kg to 520 kJ/kg of water. This means that during dissociation, for every mole of water molecules involved in the hydrate structure, about 50–70 kJ of heat is released to the surroundings. Conversely, for hydrate formation (endothermic process), this range of values translates to the amount of heat absorbed from the surroundings per mole of water in the hydrate [145].

Once the hydrates are formed, they have to follow the phase equilibrium curve pressure and temperature conditions to stay as stable hydrates. Similarly, during the dissociation process, the hydrate conditions are below the equilibrium curve and follow the endothermic process [146]. This process basically follows three independent processes: thermal stimulation, depressurization, and chemical injection [147].

Thermal stimulation is a common approach where heat is applied to the hydrate reservoir to increase its temperature and to dissociate. As shown in Figure 19, the dissociation process happens following three stages, typically the fluid flow driving stage, the heat transfer driving stage, and the kinetic driving stage. From steps A to B, the rise in temperature is caused by heat transfer. The increase in this temperature is the main reason for the hydrate dissociation process. As the hydrates begin dissociating, this affects the pressure and temperature conditions, which can be seen as the kinetic driving force from steps B to C. This step sees the release of gas and the consumption of heat. Further heating can cause additional dissociation, but this may also lead to challenges such as increased pressure and decreased temperature due to the delayed flow of fluids (steps C–D). After this stage, if there is still a delayed transfer of dissociation between gas and water, steps D to E may appear in the process. In practice, the efficiency of this method can be limited by heat transfer issues. The water released from dissociated hydrates often forms a thin liquid film that obstructs heat transfer, and if dissociation occurs too rapidly, or if gas transfer is insufficient, it can lead to conditions that favor the reformation of hydrates [148].

Another method of inducing hydrate dissociation is the depressurization process, as shown in Figure 20. This occurs due to low pressure relative to the equilibrium pressure. Depressurization is another method for inducing hydrate dissociation, which involves lowering the pressure in the reservoir below the equilibrium pressure. This process starts with the release of free gas due to the reduction in pressure, which significantly lowers both the pressure and temperature in the reservoir (Step A–B). The released gas and heat then facilitate the dissociation of hydrates (Step B–C). As dissociation continues, the P-T conditions follow the equilibrium curve, and the pressure difference drives the flow of dissociated gas and water (Step C–D). As the dissociation progresses (Step D–E), the water and gas released must flow towards the outlet due to this pressure differential. If the heat supplied is insufficient to support the endothermic dissociation, the process will encounter issues, leading to step E–F [150,151,152].

Chemical injection is the process of adding chemicals to the reservoir that change the equilibrium conditions. This method can be effective but requires careful management to ensure that the concentration of inhibitors is optimal to destabilize the hydrates without introducing new stability issues [149,153].

Researchers have been using a PVT cell having a visor/window on the instrument to study the phase behavior of hydrates by observing the reaction going on inside. Temperature and pressure sensors are installed to record thermodynamic properties and pressure variations; however, these measurements are time-consuming and require manual work [154].

Later on, researchers started using high-pressure differential scanning calorimetry (HP-DSC), as this instrument maintains the formation and hydrate-stabilizing conditions until the test is complete. Therefore, it is considered one of the precise methods to extract equilibrium data [155,156,157,158,159].

For determining the heat of dissociation, and heat capacity, researchers produced hydrates within the reactor and then transferred these hydrates to a calorimetric vessel. However, the produced hydrates may become contaminated during the transfer [160,161].

Following the experimental difficulties and the possibility of sample contamination during the transfer between test instruments, as described above, there have been different approaches developed for measuring the dissociation enthalpies of CO₂ hydrates.

Xuewen et al. conducted experiments using a high-pressure tank reactor. After injecting the gas and water, the reactor was then cooled to 280 K, and CO₂ was injected to raise pressure to 3.0 MPa, with data acquisition at 20 s intervals. Hydrates formed as CO₂ was continuously injected, until the conversion was complete. Dissociation experiments were conducted by reducing pressure in steps to 2.5 MPa, 2.0 MPa, 1.5 MPa, and 1.3 MPa and by adjusting the temperature at 1.3 MPa to observe dissociation. The dissociation rate, calculated from gas flowmeter data, showed a decrease in reactor temperature due to the endothermic nature of dissociation, with stable dissociation observed at lower pressures and a significant increase in rate when the pressure was reduced further. The effect of pressure revealed two stages: stable dissociation rates at higher pressures and decreased rates at lower pressures, with significant improvement at 1.5 MPa and 1.3 MPa. Temperature effects were studied at 280 K, 282 K, and 284 K, showing increased dissociation rates of 26% and 41% at the higher temperatures, respectively, indicating that while temperature increase enabled dissociation, and pressure changes had a more substantial impact [142].

A high-pressure micro differential scanning calorimeter (HP μ-DSC) was used by Lee et al. to measure both the heat of dissociation and the dissociation temperature of the gas hydrates. This research provides an analysis of the dynamics of hydrate formation and replacement. A pressure transducer monitored the pressure within the sample cell and the formation of hydrates was achieved through a series of repeated cooling–heating cycles. For the CH₄-CO₂ replacement experiments, CH₄ was evacuated from the system, and CO₂ was subsequently injected into the sample cell. The experiments were conducted at various temperatures, generally below 273 K, to prevent the dissociation of CH₄ hydrate. Pressure was controlled and monitored throughout the experiments. CO₂ was introduced into the sample cell at pressures that were higher than those required for CH₄ hydrate dissociation but lower than those for CO₂ liquefaction. The results showed that complete conversion of water to hydrate was achieved through repeated cooling-heating cycles. The DSC method provided accurate determinations of the hydrate–liquid water–vapor equilibrium points. The CH₄-CO₂ replacement process occurred without significant dissociation or formation of new hydrates. This study also showed that the dissociation enthalpies of CH₄-CO₂ hydrates increased with the CO₂ composition, and approximately 60% of CH₄ was replaced by CO₂ in the hydrate phase [135].

Sloan and Koh et al. [16] conducted a comprehensive review of gas hydrate thermodynamics, and their work provided a detailed analysis of dissociation enthalpy using both theoretical and experimental approaches. Using the Clausius–Clapeyron equation, a fundamental thermodynamic relationship that connects the pressure–temperature (P-T) conditions of hydrate dissociation to the enthalpy change (ΔH_d), they calculated the dissociation enthalpy to be approximately 57–59 kJ/mol. This method assumes ideal behavior of the hydrate phase and a constant enthalpy change over the measured temperature range, which simplifies the analysis but requires highly accurate P-T data. This research emphasized the challenges associated with experimental measurements, such as ensuring the formation of pure CO₂ hydrates without impurities, which could skew the results. Therefore, in addition to their theoretical approach, they utilized differential scanning calorimetry (DSC) to experimentally measure the dissociation enthalpies of CO₂ hydrates under controlled pressure and temperature conditions. Their DSC experiments focused on understanding the influence of hydrate structure (structure I (sI) or structure II (sII)) and the presence of guest molecules on the enthalpy values. The results revealed that the dissociation enthalpy of CO₂ hydrates typically ranges from 50 to 60 kJ/mol, depending on the specific experimental conditions, such as pressure, temperature, and hydrate structure [162]. They demonstrated that the hydrate structure and the nature of guest molecules significantly influence the enthalpy values, with sI hydrates generally exhibiting higher dissociation enthalpies compared to sII hydrates. This dual approach—combining the Clausius–Clapeyron equation with direct DSC measurements—provided a robust and widely applicable framework for understanding CO₂ hydrate thermodynamics.

Aromada et al. (2019) [144] conducted a comprehensive study on enthalpy changes of hydrate phase transitions using a consistent thermodynamic scheme based on residual thermodynamics. This approach involved evaluating the enthalpy changes of hydrate dissociation and formation as a function of temperature, pressure, and hydration number for both CH₄ and CO₂ hydrates. It also identified the limitations of traditional methods, such as the Clausius–Clapeyron equation, which often result in inconsistent and unreliable data due to assumptions of ideal behavior and constant enthalpy over temperature ranges. By using their proposed method, as a function of pressure, temperature, and variations in hydration number, they were able to obtain more accurate and consistent enthalpy values. Their findings showed that the enthalpy change for CO₂ hydrate dissociation is approximately 10–11 kJ/mol higher than that of CH₄ hydrate within the temperature range of 273–280 K. This difference is attributed to the stronger interactions between CO₂ molecules and water in the hydrate lattice compared to CH₄, as well as the structural differences between sI (CH₄) and sI/sII (CO₂) hydrates [163].

Ma et al. (2024) [164] focused on predicting the phase equilibrium conditions and thermodynamic stability of CO₂-CH₄ mixed gas hydrates. They used theoretical models to calculate the temperature and pressure ranges for hydrate formation, which were then used to develop a multi-parameter empirical model. This model allowed for accurate predictions of hydrate formation conditions without the need for complex thermodynamic models. Their study also examined the dissociation enthalpy of CO₂-CH₄ hydrates, finding that an increase in CO₂ mole fraction led to higher dissociation enthalpy and enhanced thermodynamic stability. This is attributed to the stronger intermolecular interactions between CO₂ and water molecules compared to CH₄, as well as the structural differences between sI (CH₄-dominated) and sII (CO₂-dominated) hydrates [165]. The ability to predict hydrate formation conditions and dissociation enthalpy with high accuracy is critical for designing efficient gas separation and storage systems, especially in industries where CO₂ and CH₄ coexist, such as natural gas production and carbon capture and storage (CCS).

The enthalpy data collected from various studies shows the energy changes associated with CO₂ hydrate formation and dissociation. These measurements confirm that CO₂ hydrate formation is an exothermic process, with enthalpy values that vary depending on the condition. Understanding these energy dynamics is crucial for evaluating the feasibility of using CO₂ hydrates in different applications.

Table 9. Enthalpy measurements for CO₂ hydrate formation using different additives.

Temperature (K)	Pressure (MPa)	Additive	Enthalpy (kJ/Kg)	References
281 to 292	0.5	Cyclopentane (C5H10)	500	[166]
277 to 282	1.9 to 3.4	CO2 + Water + Ethanol	565 to 580	[11]
285 to 288	0.60 to 0.96	cyclopentane (C5H10)	1833	[167]
276.46 to 278.39	0.66 to 0.91	Cyclopentanone (C5H8O)	677	[167]
287 to 290	0.46 to 0.81	Fluorocyclopentane (C5H9F)	1334	[167]

shows the different temperature and pressure conditions along with the enthalpies resulting from the addition of different single and mixed additives.

Beyond the quantification of enthalpy changes, a comprehensive understanding of the mechanisms governing the dissociation of carbon dioxide (CO₂) hydrates is crucial for the optimization of their practical applications. These applications include carbon capture and storage (CCS), cold thermal energy storage (CTES), and gas transportation systems. The breakdown of hydrates generally proceeds through three principal methods, as explained in the earlier section of this paper: thermal stimulation, depressurization, and chemical injection. Each of these pathways exhibits unique thermodynamic and kinetic properties. For comparative analysis,

Table 10. Analysis of CO₂ hydrate dissociation mechanisms under varying thermal and pressure conditions.

Dissociation Mechanism	Pressure and Temperature Condition	Energy Source	Dissociation Rate	Thermal Effect	Structural Behavior	References
Depressurization	Depressurized from 3.7 MPa to 3.1 MPa at 275.6 K	105 to 107 J/m3 (mainly the advection heat 103 J)	Slower gas release kinetics. Gradual dissociation over 400 min	Delayed thermal recovery	No expansion or deformation modeled	[168]
Thermal Stimulation	The bath is constant at 288.15 K (15 °C)	-	Fast release at higher temperatures	Temperature dropped from 272.15 K (−1 °C) to 269.15 K (−4 °C)	No drastic structural disruption	[169]
Combined thermal stimulation and depressurization	Depressurized from 4.5 MPa to 0.1 MPa at 288.15 K (15 °C)	-	Early dissociation onset; memory effect under disappearing water layer conditions	Max cooling 266.15 K (−7 °C) at 0.1 MPa	Sediment expanded up to 176% at lowest pressure	[169]

displays a quantitative assessment of these dissociation mechanisms.

7. Future Heat Storage, Refrigeration, and Space Conditioning Applications

Maintaining the desired temperature within a system is crucial in many thermal management and energy-related applications. The process, which involves heat removal, is usually achieved through a refrigeration cycle. Heat transfer occurs from a higher temperature zone (heat source) to a lower temperature zone (heat sink) using a working fluid (refrigerant). The working fluid undergoes a cycle of compression, condensation, expansion, and evaporation, absorbing heat from the source during evaporation and releasing it to the sink during condensation. The heat removal capacity of a conventional refrigerant or CO₂ hydrate slurry depends on its ability to absorb the heat during a phase-change process (i.e., evaporation or thawing). This value is a crucial parameter for determining the effectiveness of a temperature control system. Latent heat is also one of the important parameters for the material we use in refrigeration and is defined as the amount of heat absorbed per unit mass of refrigerant during the liquid-to-gas phase change (evaporation), or a unit mass of CO₂ hydrate slurry during the solid-to-liquid (thawing) process. A higher latent heat value shows a greater capacity for heat removal per cycle [170,171].

While commercial air conditioning systems utilizing ice thermal storage (ITS) are well-established and widely adopted for their efficiency and environmental benefits, CO₂ clathrate hydrates present an innovative alternative with significant potential advantages. Unlike ice, CO₂ hydrates have a higher volumetric energy density, allowing for more compact and efficient thermal storage solutions. This characteristic parameter enables greater cooling capacity in a smaller footprint, which is particularly beneficial for commercial and industrial HVAC applications. CO₂ clathrate hydrates offer flexible phase-change temperatures, which can be optimized for specific cooling applications, unlike ice, which melts at a fixed temperature (0 °C). This flexibility enhances heat transfer efficiency and provides better integration with different refrigeration and air conditioning systems. Another key advantage of CO₂ hydrate-based thermal storage is its potential for simultaneous carbon capture and cooling, utilizing CO₂ as a working fluid while contributing to sustainability efforts. By replacing or supplementing ice-based thermal storage systems, CO₂ hydrates could reduce system energy consumption, enhance peak load shifting, and lower operational costs in cooling applications. A representative ice thermal energy storage configuration used in air conditioning systems is illustrated in Figure 21. Thus, integrating CO₂ clathrate hydrate thermal storage into commercial air conditioning systems could provide a more energy-efficient and environmentally friendly alternative to traditional ice-based cooling technologies [172].

Below example applications of CO₂ hydrate utilization in refrigeration systems are discussed, as adopted by different researchers.

7.1. CO₂ Hydrate Used in Air-Conditioning (AC) System

CO₂ hydrates and their slurries are considered as potential high-performance coolants and/or secondary refrigerants in next-generation energy-saving AC systems. In CO₂ hydrate applications, the latent heat ranges between 350 kJ/kg and 520 kJ/kg, with a phase-change temperature range between 278 and 285 K. For this temperature range, which is slightly above the freezing point of water, the key feature of clathrate hydrate slurry (CHS), as shown in Figure 22, is its high thermal density, which is almost 1.5 times greater than that of chilled water [170]. CHS has a thermal density ranging from 10 to 17 Mcal/m³, providing enhanced energy storage capacity compared to chilled water. This high thermal density enables CHS to achieve the same cooling effect, with roughly half the flow rate required by chilled water, thereby reducing the required pump power and leading to significant energy savings. CHS forms at temperatures similar to chilled water (278–285 K), allowing refrigeration equipment to operate at temperatures above freezing, and requires simpler refrigeration equipment compared to systems using ice production. Its non-cohesive nature prevents freezing and blockages in pipes and indoor units, promoting direct transportation within buildings, similar to chilled water systems. Figure 22 shows the basic schematics of how hydrates can be possibly used in air conditioning systems [173].

Despite its advantages, CO₂ hydrates require high pressures (above 3 MPa) to remain stable, which complicates their use. To address this issue, thermodynamic and kinetic promoters can be added to stabilize hydrates at lower pressures. TBAB (Tetra-n-butylammonium bromide) is one of the best-known thermodynamic promoters to form CO₂ hydrates at atmospheric pressures of around 0.1 MPa and temperatures between 275–285 K [145].

A laboratory-scale hydrate-based refrigeration system was constructed to see the technical feasibility of such systems [174]. An example of this type of setup is a 5 L stainless-steel hydrate-forming reactor with a stirrer and sight glasses for visual observation and cooled using a coil-type heat exchanger connected to a chiller with a 960 W cooling capacity. The system that generates CO₂ hydrate slurry has been also validated by the food industry. As shown in Figure 23, CO₂ hydrate technology validated its effectiveness in rapid food cooling processes, such as for cooling meat from 25 °C to 8 °C without dehydration by spraying CO₂ hydrate slurry [175].

For cold storage applications, the rheological properties of the CO₂ hydrate slurry are important. Studies by Jerbi et al. and others have confirmed that CO₂ hydrate slurry can flow efficiently through secondary refrigerant loops [176].

Jerbi et al. designed a refrigeration system in which CO₂ hydrate slurry was used as a two-phase secondary fluid [177]. As shown in Figure 24, there were two main parts in this system, one for the formation and storage of CO₂ hydrate slurry in a 25 L tank reactor that could resist pressures up to 4.5 MPa, and the other for gas–liquid separator (GHS) circulation and dissociation in a loop. The reactor provided a heat flux of 1–5 kW for a hydrate fraction of 10–30%. The heat exchanger in the loop allowed for the continuous decomposition of the CO₂ hydrate slurry, enabling efficient heat exchange and refrigeration performance [177].

The integration of CO₂ hydrate with thermodynamic promoters such as TBAB and the use of efficient system construction methods indicate the increasing feasibility of CO₂ hydrate-based refrigeration systems. These systems not only offer higher cooling capacities but also present a viable alternative to traditional refrigerants and secondary coolants, making them attractive for sustainable and energy-efficient thermal energy storage solutions.

7.2. CO₂ Hydrate Slurry as a Secondary Refrigerant System

CO₂ hydrates offer strong potential as an alternative to conventional secondary refrigerants [178]. In this system, the separated hydrate slurry is recirculated through the secondary loop to deliver cooling to end users before returning to a storage tank. At this stage, the previously separated CO₂ gas is reintroduced into the tank, which significantly reduces the overall storage volume. Studies have shown savings of up to 75%. The process also achieves a marginal coefficient of performance (COP) of up to 6, enhancing its high energy efficiency. An experimental system for CO₂ hydrate slurry-based refrigeration is illustrated in Figure 25.

The secondary refrigerant loop constructed by Oignet et al. [179], as shown in Figure 26, consists of two primary components: a reactor for forming the CO₂ hydrate slurry and a circulation loop equipped with a heating tube for heat transfer measurements. The formation reactor, a stainless-steel vessel, is designed to withstand pressures up to 3.5 MPa and has an internal volume of 26.4 L. CO₂ is injected into the reactor, where it dissolves in the liquid phase and forms hydrates when the reactor is cooled by a refrigerator fluid circulating through its jacket. A magnetic mixer is used to enhance the formation rate of CO₂ hydrate slurry by improving the mixing within the reactor. Once the hydrate slurry is formed, it is circulated through the secondary refrigerant loop, composed of stainless-steel pipes with an internal diameter of 7.74 mm. The flow rate and pressure within the loop are monitored by sensors, such as Coriolis flow and density meters, differential pressure gauges, and thermal sensors. The loop consists of a heating tube designed to measure the local convective heat transfer coefficients (CHTCs) of the hydrate slurry. By applying an electric current to the surface of the heating tube, heat is transferred from the tube’s wall to the circulating slurry, and the temperature difference caused is used to calculate the local heat transfer properties. The secondary refrigerant loop in this setup helps study the behavior of CO₂ hydrate slurries under controlled conditions. The system provides valuable data on flow rates, pressure drops, and convective heat transfer, which are important properties for optimizing the performance of CO₂ hydrate slurries as secondary refrigerants. One of the key findings is that CO₂ hydrate slurries with solid fractions between 0 and 14 vol.% have convective heat transfer coefficients that are up to 2.5 times greater than those of liquid water, making them highly efficient for cold energy transport [179].

While the thermal behavior was well quantified, the study did not address how the slurry behaved under flow, especially in terms of resistance and stability at higher hydrate concentrations. Fu et al. [180] measured how viscosity changed with increasing hydrate content using capillary rheometry under pressure. Their results showed a sharp rise in viscosity from around 12 mPa·s at 1.4% hydrate content to over 500 mPa·s at 17.2%. The slurry quickly moved from liquid-like to paste-like behavior, raising concerns about how such systems would perform under continuous circulation [180]. To improve flow and smooth transport, Sahu et al. [181] introduced sodium dodecyl sulfate (SDS) during hydrate formation. The presence of SDS reduced viscosity from 329 cP to 147 cP, which improved mobility while still allowing hydrate formation to proceed. This made the slurry easier to handle, without compromising its cooling capacity [181]. The particle size also influences the formation process. Hydrates formed with grains around 63 μm had the highest gas uptake and moved through the system without significant restriction. Finer particles below 10 μm, limited gas diffusion and blocked pore spaces by allowing the slurry slower to form and harder to circulate [182]. Patel et al. [183] introduced tetrahydrofuran (THF) as a promoter and observed how shear rate affected particle behavior. Below 154 s⁻¹ hydrate particles started to clump, interrupting uniform flow. At 1–2 wt.% THF, the slurry remained dispersed and stable, even during high solid loading, helping to maintain consistent movement through the loop [183].

7.3. Cold Thermal Energy Storage (CTES)

In cold thermal storage, chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are being phased out as refrigerants due to their severe effects on the ozone layer. Therefore, natural refrigerants such as carbon dioxide (CO₂) and ammonia have become common alternatives. Commercial refrigeration systems that use ice storage typically incur additional costs due to the energy-intensive process of freezing and maintaining ice. Systems that rely on traditional chilled water or other cooling media avoid these costs but may require larger storage capacities due to their lower energy density compared to ice [184,185].

Dealing with the problem of uneven power usage for air conditioning, implementing cold storage systems can significantly enhance energy efficiency. These systems can also ensure a consistent supply of cooling for different cooling applications. By storing cold energy during off-peak hours (such as nighttime, when electricity demand is lower), these systems can handle part of the cooling load during peak periods, reducing operational costs. In addition, refrigeration units can be designed with a smaller capacity, reducing capital costs.

A comparison of the thermophysical properties of various cold storage media, as reported by different researchers, including water, ice, a common eutectic salt (Na₂SO₄, H₂O, NaCl, NH₄Cl), TBAB hydrate, and carbon dioxide hydrate, shows the potential candidate for use in CTES [186]. Water is a common heat transfer medium but lacks the phase-change properties of ice, eutectic salts, and hydrates in terms of its energy storage capacity. Ice is highly efficient for cold storage due to its lower density of around 917 Kg/m³ and requires a temperature of 0 °C for phase change. Eutectic salts offer moderate energy storage capacity due to its higher density of around 1490 kg/m³ but has a lower dissociation enthalpy of 121 kJ/kg compared to ice. TBAB hydrate stores more energy than eutectic salts 193 kJ/kg but less than ice. It has a lower thermal conductivity of 0.42 W/m·K and variable viscosity of 4.8–9.6 mPa·s. CO₂ hydrate exceeds all the above in energy storage due to its high dissociation enthalpy of around 500 kJ/kg [187] and has a wide range of phase-change temperatures, ranging between 0.7 and 9.5 °C.

Table 11. Comparison of dissociation enthalpies of common cold thermal energy storage components.

Medium	Dissociation Enthalpy (kJ/kg)	References
Ice	333	[189]
Eutectic Salts	121	[186]
TBAB Hydrate	193	[190]
CO2 Hydrate	459 to 507	[187]
CO2 Hydrate Slurry *	370	[191]

* Changes with volume fraction.

shows various dissociation enthalpies achieved by different media used by different researchers [188].

This makes CO₂ hydrate a promising phase-change material (PCM) for air conditioning systems. The high phase equilibrium pressure of CO₂ hydrate exceeds atmospheric pressure and increases energy consumption. Numerous studies have explored ways to use CO₂ hydrate at lower pressures in air conditioning systems. The phase equilibrium pressures and dissociation enthalpies of CO₂ hydrates in the presence of various thermodynamic promoters is already discussed in Section 2 of this paper. These promoters significantly reduce the phase equilibrium pressure and increase the dissociation enthalpy, demonstrating the adaptability of CO₂ hydrate for cold storage systems.

Figure 27 illustrates a carbon dioxide hydrate-based cold thermal energy storage system that integrates with an air conditioning (AC) system. In this system, the chiller cools the water or refrigerant used to promote the formation of CO₂ hydrate within the cold storage unit during off-peak hours, when energy demand is lower. The chilled water is circulated throughout the system using a water pump, allowing the formation of CO₂ hydrate in the gas hydrate cold storage tank. This storage process is essential for capturing cold energy that can later be used during peak energy demand periods. The CO₂ hydrate serves as a phase-change material, storing a significant amount of cold energy due to its high dissociation enthalpy. When the cooling demand rises during peak hours, the stored CO₂ hydrate begins to dissociate, releasing cold energy, which is used to cool the chilled water. This chilled water is then circulated back through the AC system and to the user side, where a fan helps distribute the cooled air. In addition, the system includes a CO₂ gas storage tank, where the released carbon dioxide gas is collected. The gas compressor plays a crucial role in maintaining the necessary pressure by recompressing the CO₂ gas, ensuring efficient system operation. The system also features an expansion tank that helps regulate the thermal expansion of the water or refrigerant as the temperature changes. Various pipes and valves control the flow of gases and liquids, ensuring proper circulation and preventing backflow within the system. This design allows for more efficient energy usage by taking advantage of lower nighttime electricity rates and reducing the need for high-capacity refrigeration units. By storing cold energy and using it strategically during peak demand times, the system helps lower operational costs and improve energy efficiency [192].

Studies on methane–CO₂ replacement reactions in porous media or saline solutions could provide insights into optimizing the carbon capture process while minimizing energy consumption. Still, energy consumption remains a critical challenge for practical deployment, as the high pressures required for CO₂ hydrate formation significantly increases operational costs. Optimizing hydrate reactor designs, improving mechanical stirring systems, and enhancing the thermophysical properties of hydrates are essential for making CO₂ hydrate-based systems more competitive.

CO₂ hydrate-based systems face several key challenges that must be addressed for practical application. One major issue is the kinetics of hydrate formation, as the rate is often too slow for practical use. Additives such as surfactants and nanoparticles can enhance nucleation, but identifying cost-efficient and effective additives remains an ongoing challenge. Researchers are actively seeking catalysts to accelerate the process. Another challenge is thermal management. CO₂ hydrate formation releases a considerable amount of heat, which, if not properly managed, can lead to localized hydrate decomposition, reducing system efficiency [193]. Scaling and contamination also present significant difficulties. Industrial CO₂ streams often contain impurities such as sulfur compounds, which can inhibit hydrate formation and cause blockages in reactors. Proper pretreatment of the CO₂ stream is required to maintain efficiency and avoid fouling. In addition, ensuring the long-term stability and storage of CO₂ hydrates is difficult. Even slight variations in pressure or temperature can lead to hydrate dissociation, releasing CO₂ and negating storage benefits [194]. The material compatibility of the system components is another area of concern. The high-pressure, low-temperature conditions needed for CO₂ hydrate formation can degrade materials, increasing maintenance costs and posing safety risks. There are environmental and safety concerns related to the release of CO₂ during hydrate storage, transportation, or dissociation, which necessitate robust leak detection and safe decomposition protocols [195]. Lastly, scale-up challenges complicate the deployment of CO₂ hydrate systems. While lab-scale experiments show promise, scaling these systems to industrial levels requires precise control over pressure, temperature, and hydrate formation across large volumes. Addressing these challenges is essential for the success of CO₂ hydrate-based technologies for carbon capture and storage [196].

8. Potential Consequences of Using CO₂ Hydrates

One of the major concerns in any long-term CO₂ hydrate application, including cold thermal energy storage (CTES), is the stability of hydrates under variable conditions. Studies on geological sequestration show hydrate behavior. Zhao et al. [197] used magnetic resonance imaging (MRI) to visualize hydrate formation during simulated CO₂ leakage and found that CO₂ hydrate caps significantly reduced permeability, preventing gas migration. The hydrate layer maintained mechanical integrity under pressures up to 10 MPa and reformed quickly after temporary disturbances due to the continued availability of water and CO₂, suggesting a natural, self-repair mechanism. This behavior points to the potential for hydrate reformation to contribute to long-term stability and containment in engineered hydrate-based storage systems [197]. Gauteplass et al. [198] demonstrated that hydrate seals can form efficiently in homogeneous media and continue to function even when localized dissociation occurs. These findings highlight that under the right thermodynamic and structural conditions, hydrate systems can exhibit both sealing and self-recovery behaviors, which may benefit CTES systems exposed to cyclic operations [198]. Understanding the sealing and self-reforming behavior of CO₂ hydrates in subsurface conditions provides confidence in their durability under repeated thermal cycling in CTES systems. Such resilience is critical to maintaining system performance and preventing hydrate degradation over time.

A comparative lifecycle cost (LCC) and energy analysis between a CO₂ hydrate-based cold thermal energy storage (CTES) system and a conventional district cooling system (DCS) reveals the long-term advantages of hydrate technology despite its high-pressure formation requirements. The comparison in

Table 12. Lifecycle analysis comparing CO₂ hydrate CTES to conventional ice/PCM systems in terms of net carbon savings.

Parameter	CO2 Hydrate CTES	Ice-Based Systems	References/Calculations
Latent Heat	507 kJ/Kg	333 kJ/Kg	[192]
Formation Pressure	Below 0.5 to up to 5 MPa	0.1 MPa	Pressure required to form
Annual Cooling Required	44,917,178 kWh	44,917,178 kWh	[199,200]
Annual Energy Consumption	6,605,795 kWh	17,967,762 kWh	[174]
Average Electricity Rate	USD 0.1276/kWh	USD 0.1276/kWh	[201]
Annual Operating Cost	USD 842,899	USD 2,292,686	Energy cost = usage × rate
Operating Cost for 20 Years	USD 16,857,989	USD 45,853,729	Annual operating cost × 20
Sequestration	Yes, as this uses CO2 gas. Subject of the nucleation conversion rate	No	Because CO2 hydrate is formed from the mixture of CO2 gas and water
Environmental Effects	Some promoters are toxic, but alternatives do exist	None	Depends on the promoters used
CO2 Emissions	0 to 0.37 Kg CO2/kWh, subject to the reactor designs, and the CO2 recovery rate	0.37 kg CO2/kWh	[202]
CO2 Emissions for 20 Years	48,750,766 kg, see above calculations performed using CO2 emission of 0.13 CO2/kWh	132,602,085 kg	Annual energy consumption × CO2 emissions × 20

shows the long-term performance of CO₂ hydrate CTES and conventional ice-based cooling systems for large-scale applications. Using standard design assumptions and updated national data, key metrics such as energy use, operating costs, and emissions were calculated and compared over a 20-year period. The analysis reflects how system efficiency, electricity rates, and carbon intensity directly influence overall performance.

Table 13. Sensitivity Analysis Comparison between Hydrate and Ice based CTES System.

Parameter	Base Case	Low-End Scenario	High-End Scenario	Impact on Hydrate CTES LCC (%)	Impact on Ice CTES LCC (%)	References
Electricity Rate (USD/kWh)	0.1276	0.08	0.18	±41%	±58%	[201]
CO2 Emission Factor (kg/kWh)	0.349 (Hydrate)	0.198	0.37 (Grid)	±43%	±43%	[204]
System COP (Hydrate CTES/Ice CTES)	7.03/3.5	5.0/3.0	8.5/4.0	+58%/+50%	+50%	[205]
Hydrate Conversion Efficiency	42.7%	30%	50%	+48%/N/A	-	[206]
Carbon Pricing (USD/ton CO2)	50	30	150	±100	±200%	[207]

shows the sensitivity analysis to assess the influence of key techno-economic and environmental parameters on the lifecycle cost (LCC) of CO₂ hydrate CTES and conventional ice-based CTES systems. Sensitivity analysis is a systematic approach used to evaluate how variations in critical input parameters affect the output of a model, thereby identifying which factors most significantly impact system performance or economic viability. In this study, sensitivity analysis was conducted to understand the robustness of lifecycle cost estimates under uncertainties in energy prices, system efficiencies, emission factors, and carbon pricing scenarios. The selected parameters show the major cost drivers and risk factors over the system lifetime. The base case values as well as the low-end and high-end scenarios for each parameter were chosen based on recent literature, regulatory data, and experimental measurements, and a Tornado figure has been plotted based on the data acquired [203].

A tornado diagram is a graphical representation commonly used in sensitivity analysis to illustrate the relative influence of individual input variables on a model’s output. It visually ranks parameters by the magnitude of their impact, along with the most influential factors shown at the top, resembling a tornado. Each horizontal bar represents the range of variation in the output metric (in this case, lifecycle cost) resulting from the low-end and high-end changes of a single parameter while holding all other variables constant [208]. Figure 28 shows the sensitivity of lifecycle cost (LCC) for CO₂ hydrate CTES (blue) and conventional ice-based CTES (red) systems to variations in electricity rates, CO₂ emission factors, system COP, hydrate conversion efficiency, and carbon pricing.

9. Research Gaps and Future Research Directions

CO₂ hydrates have emerged as promising materials for refrigeration applications and cold thermal energy storage (CTES) systems, offering significant advantages, such as high latent heat (500 kJ/kg) and a phase-change temperature range of 0–15 °C. Despite their potential, numerous challenges remain before CO₂ hydrate-based systems can be fully integrated into practical applications. One of the primary challenges is the need for efficient CO₂ gas storage with large absorption capacity and suitable absorption and regeneration conditions for cyclic operations. The absence of optimized CO₂ storage solutions has been a major barrier to the real-world implementation of CO₂ hydrate-based CTES systems.

Another issue is the low mass transfer during gas hydrate formation, which is caused by limited gas–water interface, hydrate blockages, and low CO₂ solubility. To address this, future designs should focus on increasing the gas–liquid interface and enhancing mass transfer, using methods such as porous media, spraying, circulation, or bubbling. Additionally, while the dissociation temperature of CO₂ hydrates is a reliable indicator of phase equilibrium, the formation temperature—affected by stochastic nucleation—is an essential property that directly impacts the efficiency of the chiller in a CTES system. This stochastic nature of hydrate formation complicates kinetic studies and poses challenges for system optimization.

Thermal hysteresis is another obstacle, where a lag between heating and cooling during phase transitions affects heat transfer between gas hydrates and the heat transfer fluid (HTF), leading to reduced system efficiency. The mechanisms behind thermal hysteresis in CO₂ hydrates and other phase-change materials (PCMs) are still unclear, and further research is needed to address this issue. Additionally, methods for determining the onset points of hydrate formation and dissociation are currently based on the visual observation of hydrate crystals, which can be inaccurate for quantitative studies. There is no unified method for such measurements, making it difficult to compare data across different studies.

CO₂ hydrate slurry has proven to be a suitable cooling carrier for air conditioning (AC) systems. However, challenges such as maintaining the mobility of the slurry, predicting its three-phase flow behavior in piping systems, and preventing hydrate dissociation due to pressure loss remain unresolved. Future research should focus on improving the rheological properties of the slurry and finding more accurate methods for predicting and managing three-phase flow in various piping configurations.

Thermodynamic promoters such as TBAB (Tetra-n-butylammonium bromide) have shown reliability in stabilizing CO₂ hydrate formation at lower pressures. However, it is hypothesized that when promoters are used, both CO₂–promoter double hydrates and promoter hydrates form. The formation of CO₂ double hydrates is often limited by mass transfer and the driving force, leading to the dominance of promoter hydrates. To enhance gas hydrate formation, it is necessary to develop better materials and systems that improve mass transfer in the presence of promoters. The long-term effects of additives such as TBAB in CTES systems are not yet fully understood. These additives may have side effects, including the potential to reduce energy storage density, impair system safety, or negatively impact other desired properties. Careful selection and further study of additives are crucial to the advancement of CO₂ hydrate-based CTES systems.

Research comparing CO₂ hydrates with conventional PCMs and water highlights key tradeoffs. CO₂ hydrates offer a higher dissociation enthalpy compared to water, making them more effective for energy storage. However, CO₂ hydrates require high pressures for formation, increasing operational costs and energy consumption. To address this, the use of thermodynamic and kinetic promoters can help lower the required formation pressure and accelerate hydrate growth. Advancements in experimental methods, such as incorporating ultrasonic waves or magnetic fields, could enhance hydrate formation efficiency without compromising stability.

Despite these challenges, CO₂ hydrates hold significant potential for revolutionizing cold thermal energy storage and carbon capture technologies. Future research efforts should focus on equipment optimization, additive development, and process integration to improve overall system efficiency. Structural improvements to heat exchangers, especially those designed to prevent hydrate layer formation on cold surfaces, will also be crucial in maintaining high heat transfer rates and preventing clogging. With further advancements, commercially viable hydrate slurries with improved flow and heat transfer characteristics can be developed and deployed. Such developments could significantly enhance energy efficiency in large-scale cooling applications, such as air conditioning and refrigeration systems, while reducing carbon emissions.