Review of natural gas hydrates as an energy resource: Prospects and challenges ☆

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Gas clathrate hydrates were first identified in 1810 by Sir Humphrey Davy. However, it is believed that other scientists, including Priestley, may have observed their existence before this date. They are solid crystalline inclusion compounds consisting of polyhedral water cavities which enclathrate small gas molecules. Natural gas hydrates are important industrially because the occurrence of these solids in subsea gas pipelines presents high economic loss and ecological risks, as well as potential safety hazards to exploration and transmission personnel. On the other hand, they also have technological importance in separation processes, fuel transportation and storage. They are also a potential fuel resource because natural deposits of predominantly methane hydrate are found in permafrost and continental margins. To progress with understanding and tackling some of the technological challenges relating to natural gas hydrate formation, inhibition and decomposition one needs to
develop a fundamental understanding of the molecular mechanisms involved in these processes. This fundamental understanding is also important to the broader field of inclusion chemistry. The present article focuses on the application of a range of physico-chemical techniques and approaches for gaining a fundamental understanding of natural gas hydrate formation, decomposition and inhibition. This article is complementary to other reviews in this field, which have focused more on the applied, engineering and technological aspects of clathrate hydrates.

Towards a fundamental understanding of natural gas hydrates

Investigation into gas production from natural gas hydrate: A review

Application of gas hydrate formation in separation processes: A review of experimental studies

Natural-gas hydrate fields having a large amount of methane deposits have become the object of public attention as a potential natural-gas resource. An idea of methane exploitation in linkage with CO2 isolation has been presented elsewhere. In the present study, the isothermal phase equilibrium relations of pressure and compositions in the gas, liquid, and hydrate phases for the CO2-CH4 mixed hydrate system at 280 K are obtained in company with the apparent Henry constants for the methane-water system and the three-phase coexisting lines for the methane hydrate system. The averaged distribution coefficient of methane between gas phase and hydrate phase is about 2.5, that is, methane in the hydrate phase is replaced selectively by CO2. This is the first experimental evidence for the possibility of methane exploitation combined with CO2 isolation.

Methane exploitation by carbon dioxide from gas hydrates - Phase equilibria for CO2-CH4 mixed hydrate system

The three-phase coexistence curve of methane hydrate + saturated water + saturated fluid CH4 was investigated in the temperature range from 305 to 321 K and pressure range from 98 to 500 MPa. The equilibrium curve increases monotonically on a T−p diagram at these experimental conditions. The Raman spectra of the C−H symmetric vibration mode in the methane hydrate split into two peaks, while a single peak is detected in the fluid CH4 and water phases. The split of the Raman peak indicates that two kinds of hydrate cages are occupied by the CH4 molecules. The peak intensity ratio of two types of CH4 molecules is almost independent of pressure in the range up to 500 MPa; that is, the cage occupancy ratio is constant. The Raman spectrum for the intermolecular vibration mode (O−O stretching) of the water molecules changes linearly with pressure from 207 to 228 cm-1, and the Raman shifts of the C−H vibration mode in the S-cage and in the water phase vary linearly with pressure from 2915 to 2919 cm-1 and from 29...

High-Pressure Phase Equilibrium and Raman Microprobe Spectroscopic Studies on the Methane Hydrate System

Structure and stability of ethane hydrate crystal

To evaluate the possibility of CH4 exploitation in linkage with CO2 isolation, the decomposition rates of CO2, CH4 and CO2-CH4 mixed gas hydrates were obtained under isothermal isobaric conditions. The decomposition rates of CO2 and CH4 pure gas hydrates are proportional to the total amount of gas hydrate and the fugacity difference from the equilibrium state under the conditions of three-phase coexistence. The rate constant of CO2 hydrate decomposition is larger by one order than that of CH4 hydrate. In the CO2-CH4 mixed hydrate decomposition, CH4 composition in the generated gas is substantially larger than the value calculated from the pure hydrate decomposition rate while CO2 remains selectively in the mixed gas hydrate.

Decomposition of CO2, CH4 and CO2-CH4 Mixed Gas Hydrates

The composition and cage-occupancy of guest species in a CO 2 -methane mixed hydrate system are investigated by use of Raman spectroscopy. A single crystal of mixed gas hydrate shows the homogeneous mole fraction in equilibrium. The CO 2 molecule is entrapped into the M-cage prior to the S-cage comparing to methane. The relative cage-occupancy of CO 2 and methane in the structure-I hydrate lattice is scarcely affected by pressure condition.

Shinya Nakano

Papers

Methane exploitation by carbon dioxide from gas hydrates - Phase equilibria for CO2-CH4 mixed hydrate system

High-Pressure Phase Equilibrium and Raman Microprobe Spectroscopic Studies on the Methane Hydrate System

Structure and stability of ethane hydrate crystal

Decomposition of CO2, CH4 and CO2-CH4 Mixed Gas Hydrates

Relative cage-occupancy of CO2-methane mixed hydrate