The dissociation of natural gas hydrates is an endothermic process. This dissociation process requires the continuous absorption of heat energy from the sediment and pore fluid. This heat transfer ...governs the dissociation rate and affects gas production. In this study, a two-dimensional axisymmetric simulator is developed to model the effects of heat transfer on the process of hydrate dissociation in porous media by depressurization. A series of simulations are performed to study sensible heat effects on the sediment, heat flow transfer in the cap- and base-sediment, and the effects of conductive and convective heat transfer on gas production from methane hydrate depressurization. The results show that the porous media material and the water content are two significant factors that affect the sensible heat in gas hydrate dissociation: the porous media material can increase methane hydrate dissociation, but water inhibits the dissociation process by affecting the pressure on the inner sediment. A high thermal conductivity of the sediment can initially positively affect hydrate dissociation but may later partially inhibit the process. Convective heat transfer in the gas flow increases hydrate dissociation markedly compared to that in water flow.
Biomass-derived photothermal conversion materials are considered to be promising evaporator choices for cost-effective, sustainable, and environmentally friendly solar vapor generation. Herein we ...demonstrate a double-layer flamed straw, which is a typical solar-driven interfacial evaporator that can directly convert solar energy into heat and then localize heat at interface for vapor generation. Benefiting from the unique structure of a natural corn straw, the flamed straw exhibits a high solar absorbance of 91%, ultralow thermal conductivity (0.042 W m–1 K–1), and a sufficient water supply. Notably, the flamed-straw evaporator achieves a fast evaporation rate of 1.497 kg m–2 h–1 and a high photothermal efficiency of 86% under 1 sun illumination, showing comparable efficiency with the reported studies. Our work highlights the promise of using the low-cost biomass-derived materials as highly effective solar vapor generators in the realm of seawater desalination and wastewater treatment.
•Hydrate reformation during exploitation is caused mainly by insufficient heat supply.•Hydrate reformation is likely to occur in the depressurization process.•The depressurization process should be ...combined with thermal stimulation.•A new system is designed to investigate hydrate reformation under various conditions.
Natural gas hydrates are considered as some of the most promising energy sources. However, economical, safe, and commercial exploitation of marine hydrates has not yet been achieved. One of the main obstacles is hydrate reformation in the process of hydrate exploitation, which needs to be properly addressed. The processes of hydrate dissociation and reformation under depressurization and thermal stimulation are discussed in this review. It is necessary to understand the manner in which various factors affect hydrate reformation during dissociation. Therefore, the effects of critical factors on hydrate reformation during thermal stimulation and depressurization dissociation are analysed to provide a comprehensive explanation of the hydrate reformation mechanism. In addition, several measures used to prevent hydrate reformation during the exploitation process are discussed. Finally, an experimental system was designed to investigate the exploitation methods and optimization of various hydrate-bearing sediments. Future efforts should focus on finding effective methods and evaluating the combined sequences, which can prevent hydrate reformation with acceptable exploitation efficiency.
Understanding the mechanical behaviors of carbon dioxide/methane hydrate-bearing sediments is essential for assessing the feasibility of CO2 displacement recovery methods to produce methane from ...hydrate reservoirs. In this study, a series of drained triaxial compression tests were conducted on synthetic carbon dioxide hydrate-bearing sediments under various conditions. A comparative analysis was also made between carbon dioxide and methane hydrate-bearing sediments. The stress-strain curves, shear strength, and the effects of hydrate saturation, effective confining stress, and temperature on the mechanical behaviors were investigated. Our experimental results indicate that the newly formed carbon dioxide hydrate would keep the reservoir mechanically stable when CH4-CO2 gas exchange took place in a relatively short period of time and spatially well distributed in the pore space. Experiments of CO2 injection in methane hydrate-bearing sediments are necessary to confirm this hypothesis.
The schematic diagram of hydrate decomposition process and the gas production by using different methods. Display omitted
•Three gas production methods were evaluated with different hydrate ...saturations.•The roles of temperature, pressure, sensible heat and heat transfer were analyzed.•The driving force of hydrate dissociation at different stages was analyzed.•The combined method effectively improved the gas production and energy efficiency.
To investigate the gas production from methane hydrate-bearing sediments, the gas production processes from methane hydrate in porous media using depressurization, two-cycle warm-water injection and a combination of the two methods were characterized in this study. The methane hydrates were formed in porous media with various initial hydrate saturation (Shi) in a pressure vessel. The percentage of gas production, rate of gas production, and energy efficiency were obtained and compared using the three methods. The driving force of the hydrate dissociation at different stages of depressurization was analyzed and ice formation during the gas production was observed. For the two-cycle warm-water-injection method, the percentage of gas production and the energy efficiency increased with increasing of Shi. However, due to the large amount of warm water needed to heat the porous media at the dissociation site, the percentage of gas production was lower than the other two methods under the same experimental conditions. The experimental results proved that the combined method had obvious advantages for hydrate exploitation over the depressurization and warm-water-injection method in terms of the energy efficiency, percentage of gas production and average rate of gas production, and with increasing of Shi, the advantages are enhanced. For the Shi of 51.61%, the percentage of gas production reaches 74.87%, which had increments of 18.63% and 31.19% compared with the depressurization and warm-water-injection methods. The energy efficiency for the combined method were 31.47, 49.93 and 68.13 for Shi of 31.90%, 41.31% and 51.61%, respectively.
Kinetic enhancement for capturing and storing harmful gases into hydrates simultaneously to achieve low energy penalties and environmental mitigations.
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•Kinetic surfactants accelerate ...gas captures in pore spaces.•Oscillated pressure controls are first proposed to enhance CO2 gas captures.•Enhanced gas capture processes change microstructural features.•The hydrate technology is a safely long-term storages of harmful gases.
The use of hydrate-based technology for gas capture and storage is highly attractive for environmental mitigation, as it entails low energy penalties and provides gas storage density maximization and long-term storage stability. Although this method has been investigated in extensive researches, its development is restricted by the obscure underlying gas capture micro-mechanisms, elusive micro-structures of stored forms, and insufficient hydrate film growth rates. In this study, the Magnetic Resonance Imaging technique was employed to analyze the hydrate growth micro-processes for greenhouse gas (imitated by CO2, CH4, and various fractions of CO2-CH4 mixed gases) and volatile organic compound (simulated by C2H4 and C2H2 gases) capture and storage. The hydrate film growth was enhanced with the addition of 288 ppm sodium dodecyl sulfate (SDS), which significantly improved the hydrate growth in the cases of hydrocarbon gases, but not CO2 gas due to the competing adsorption of bicarbonate and dodecyl sulfate ions. With SDS, hydrocarbon gas hydrates grew via the patchy model at 65–105 mm/s, and 65–95% liquid water was converted into hydrates for gas capture and storage. However, only about 1.4% water was converted into CO2 hydrates with SDS, at 10.4 mm/s. Thus, a multi-pressure control mechanism for secondary hydrate growth was developed to promote CO2 capture and storage, based on a large amount of dissolved CO2 gas compared to the other investigated gases. The enhanced CO2 capture has important implications for the optimized harmful gas sequestration, due to preferentially patchy hydrate morphologies and associated impacts on permeability.
Schematic diagram illustrating the process of gas production in hydrate-bearing sediment induced by depressurization. When depressurization occurs, the reservoir pressure and temperature change along ...the trajectory of A–B–C–D. Character of gas production process is outlined. Display omitted
•Hydrate dissociation behavior was analyzed in porous media by depressurization.•The gas production process can be divided into three main stages.•Methane hydrate first dissociates simultaneously throughout the hydrate zone, and then from the outside.•The sensible heat of the reservoir and ambient heat transfer play a dominant role in hydrate dissociation.
Natural gas hydrate is a vast energy resource with global distribution in permafrost regions and in the oceans; its sheer volume demands that it be evaluated as a potential energy source. Understanding the mechanisms of natural gas extraction from hydrate-bearing sediments is critical for the utilization of hydrate accumulations. In this work, methane hydrate dissociation was performed in three kinds of porous media at production pressures of 2.2MPa, 2.6MPa, and 3.0MPa. Results show that the methane gas production process can be divided into three main stages: free gas liberation, hydrate dissociation sustained by the sensible heat of the reservoir, and hydrate dissociation driven by ambient heat transfer. In the process of gas production, hydrate dissociation occurs simultaneously throughout the hydrate zone along the phase equilibrium curve, and then spreads radially from the outside as a result of ambient heat transfer. Hydrate reformation and ice generation always occur in the reservoir interior due to insufficient heat transfer. The use of porous media with increased thermal conductivity accelerates the gas production rate; however, it has little influence on the final percentage of gas production. Furthermore, the Stefan (Ste) number and dissociation rate constant were employed to evaluate the impact of the sensible heat of the reservoir and ambient heat transfer. Results indicate that the sensible heat of the reservoir and ambient heat transfer play a dominant role in hydrate dissociation, and that both are dependent on production pressures.
The objective of this review is to analyze potential technologies and their baseline performance of producing hydrogen from catalytic steam reforming of biodiesel byproduct glycerol. High oxygen ...content and high impurity level of biodiesel byproduct glycerol, as well as the complex intermediates and high coking potential in its thermal degradation, make the modeling, design, and operation of glycerol steam reforming a challenge. Thermal decomposition characterization of biodiesel byproduct glycerol was covered, and the recent developments and methods for high-purity hydrogen production from glycerol steam reforming were illustrated. The thermodynamics constraint of water gas shift reaction can be overcome by the sorption-enhanced steam reforming process, which integrated catalytic steam reforming, water gas shift reaction and in-situ CO2 removal at high temperatures in a single stage reactor. The effectiveness of both the enhanced H2 production and the use of CO2 sorbents have been demonstrated and discussed. The technical challenges to achieve a stable high-purity hydrogen production by the sorption-enhanced steam reforming process included extending operation time, selecting suitable sorbents, finding a way for continuous reaction-regeneration of catalyst and sorbent mixture and improving process efficiencies. The continuous sorption-enhanced steam reforming of glycerol was designed by a simultaneous flow concept of catalyst and sorbent for continuous reaction-regeneration using two slow moving-bed reactors for high-purity hydrogen production and CO2 capture, and in this process, catalyst and sorbent were run in nearly fresh state for H2 production. The sorption-enhanced chemical-looping reforming was also demonstrated. The paper discusses some issues and challenges, along with the possible solutions in order to help in efficient production of hydrogen from catalytic steam reforming of biodiesel byproduct glycerol.
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Permeability in hydrate-bearing sediment critically governs fluid flow and determines hydrate nucleation, growth, and distribution, making it important to characterize the evolution of permeability ...with respect to water and gas during hydrate formation. This study uses CT scanning of krypton hydrate formation in silica sand using the excess gas method, together with a pore network model, to investigate variations in hydrate morphology and associated permeation. The results show that during hydrate formation, the growth habit mainly varies from grain-coating to patchy with an increase in hydrate saturation; however, at relatively low saturation, hydrate preferentially grows as a grain-cementing habit. Theoretical models of a capillary tube and of a Kozeny grain both predict that permeability in grain-coating hydrates will be higher than in patchy hydrates. We thus recognise a correlation of permeability and hydrate saturation for multiple growth habits that is of interest for gas production from hydrate reservoirs. Under lower water saturation, there is a decrease in relative permeability to water but an increase in relative permeability to gas, due to the reduction in pore shape factors. Conversely, an upward shift in relative permeability to water and a downward shift in relative permeability to gas is found with hydrate formation under higher water saturation, owing to the Jamin effect. Steeper curves of relative permeability to gas and water with increasing hydrate saturation resulted in a reduction of the co-cementation zone for water and gas, even if or though no gas migrates. The results suggest that, due to the low relative permeability of gas, higher water saturation sediments result in an excess water yield accompanied by low gas production that is not desirable for natural gas hydrate production. Therefore, improving the permeability and weakening the Jamin effect are critical for gas production in marine hydrate reservoirs, especially in low permeability sediments.
•CT scanning of Kr hydrate growth is used as input to modeling of multi-phase flow properties in hydrate bearing sediments•Hydrate pore habit shifts from coating to cementing as hydrate saturation increases.•Under lower Sw, hydrate formation inhibits water flow due to reduced shape factor.•Under higher Sw, hydrate growth impedes gas migration in fine pores due to the Jamin effect.
•Natural gas is recovered from hydrate deposits using combined depressurization-thermal stimulation method.•Five main stages of gas production upon occurrence of ice generation phenomena.•Two types ...of thermal stimulation methods are used.•The positive effects of thermal stimulation are confirmed.
Natural gas hydrates have gained worldwide attention asan important potential non-conventional fossil fuel resource. Understanding the gas production behavior from hydrate deposits is critical to the utilization of the gas hydrate resource. In this study, the hydrate dissociation reaction was induced by depressurization in conjunction with thermal stimulation. Profiles of temperature, pressure, gas production rate, and cumulative gas production during the gas production processes were analyzed. The results show that the gas production process upon ice generation can be divided into five main stages: (1) a free gas release, (2) hydrate dissociation along the equilibrium curve driven by the reservoir sensible heat, (3) hydrate dissociation driven by the exothermic ice generation reaction, (4) ice melting and hydrate dissociation under ambient heat transfer, and (5) hydrate dissociation under ambient heat transfer. During the gas production process, two thermal stimulation methods—ambient heat transfer and warm water injection—were employed to supply heat for hydrate dissociation. The larger the heat flux supplied by ambient heat transfer, the greater the gas production. During the warm water injection process, the gas production time decreased as the temperature of the injected water increased. These two methods can effectively promote gas production from gas hydrate deposits. The findings of this study can provide some insight for designing and implementing optimal production techniques for use of hydrate resources.
