Formation of Methane Hydrates from Super-compressed Water and Methane Mixtures
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1262-W04-03
Formation of Methane Hydrates from Super-compressed Water and Methane Mixtures Jing-Yin Chen1 and Choong-Shik Yoo1 1 Institute for Shock Physics and Department of Chemistry, Washington State University, Pullman, WA 99163, U.S.A.
ABSTRACT Understanding the high-pressure kinetics associated with the formation of methane hydrates is critical to the practical use of the most abundant energy resource on earth. In this study, we have studied, for the first time, the compression rate dependence on the formation of methane hydrates under pressures, using dynamic-Diamond Anvil Cell (d-DAC) coupled with a high-speed microphotography and a confocal micro-Raman spectroscopy. The time-resolved optical images and Raman spectra indicate that the pressure-induced formation of methane hydrate depends on the compression rate and the peak pressure. At the compression rate of around 5 to 10 GPa/s, methane hydrate phase II (MH-II) forms from super-compressed water within the stability field of ice VI between 0.9 GPa and 2.0 GPa. This is due to a relatively slow rate of the hydrate formation below 0.9 GPa and a relatively fast rate of the water solidification above 2.0 GPa. The fact that methane hydrate forms from super-compressed water underscores a diffusion-controlled growth, which accelerates with pressure because of the enhanced miscibility between methane and super-compressed water. INTRODUCTION Natural gas hydrates are a new class of energy source, made of hydrogen-bonded cages of water molecules containing a wide range of guest molecules (CH4, H2, CO2, etc.). Methane hydrates are the most abundant in nature and the largest energy resource of all fossil fuels. On the other hand, because methane is a greenhouse gas that is approximately 10 times more powerful than carbon dioxide, its release could potentially result in abrupt climate change and thereby living conditions on the Earth. Because of these reasons, there have been extensive studies on methane hydrates [1-8]. Methane hydrates are stable in ocean floor sediments at water depths greater than 300 meters (~ 3 MPa) and are found most at the water depth between 2000 and 3000 m (20-30 MPa). Because it is stable only within a small pressure-temperature domain, recovering and transporting the hydrate or methane in a controlled manner is a significant technical challenge. An uncontrolled sudden release of methane gas could cause geological and industrial hazards, as well as significant environmental consequences. Addressing this technical challenge requires fundamental understandings of hydrogen bonding and disorder, and, more importantly, chemical mechanisms and kinetics governing the formation and decomposition of methane hydrates under pressures. Recent development of dynamic-diamond anvil cells (d-DAC)[9-11] enables us to study high-pressure kinetics associated with the crystal growth, phase transitions, and chemical reactions, over a wide range of compression rates and pressures. Therefore, using d-DAC coupled with the Raman spectroscopy and high-speed photograph
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