Tanaka, H., Matsumoto, M. & Yagasaki, T. Efficiency and energy balance for substitution of CH4 in clathrate hydrates with CO2 under multiple-phase coexisting conditions. J. Chem. Phys. 159, 194504 (2023) doi:10.1063/5.0179655
燃える氷として知られる「メタンハイドレート」は、水分子が作ったかご状の結晶構造の中にメタン分子が閉じ込められた物質で、次世代のクリーンエネルギー資源として期待されています。近年、このメタンハイドレートからメタンを取り出すと同時に、地球温暖化の原因となる二酸化炭素（CO2）を代わりに閉じ込める「置換法」という技術が注目されています。しかし、実際に置換が起こる現場（地中など）では、気体・液体・水など様々な相（状態）が混ざり合っており、どのような温度や圧力条件で効率よく置換が進むのかという熱力学的な理解は十分に整理されていませんでした (TMY2022, TMY2023)。
本研究では、統計力学（分子のミクロな動きから巨視的な熱力学性質を導く理論）を用いて、多様な相が共存する複雑な環境下でのメタン-CO2置換反応の効率やエネルギー収支を網羅的に解析しました。その結果、3つの相（気体・液体・固体など）が共存する状態でも置換自体は可能であるものの、それが安定して起こる温度や圧力の範囲（作業ウィンドウ）は、2つの相が共存する状態に比べて非常に狭いことが明らかになりました。
さらに、この置換反応は通常あまり熱を出入れしませんが、中程度の圧力範囲では、放出されたメタンと液体のCO2が混ざり合うことで液体CO2が一気に蒸発し、周囲から大きな熱を奪う「吸熱現象」が起きることを発見しました。これは現場の温度低下を引き起こし、反応の進行に大きな影響を与える可能性があります。本研究の理論的枠組みは、CO2置換法によるメタン採掘と地中貯留を安全かつ効率的に行うための定量的な計画立案に役立つ重要な成果です (TMY2024A, MNT2024)。
Abstract
Many experimental and theoretical studies on CH4-CO2 hydrates have been performed aiming at the extraction of CH4 as a relatively clean energy resource and concurrent sequestration of CO2. However, vague or insufficient characterization of the environmental conditions prevents us from a comprehensive understanding of even equilibrium properties of CH4-CO2 hydrates for this substitution. We propose possible reaction schemes for the substitution, paying special attention to the coexisting phases, the aqueous and/or the fluid, where CO2 is supplied from and CH4 is transferred to. We address the two schemes for the substitution operating in three-phase and two-phase coexistence. Advantages and efficiencies of extracting CH4 in the individual scheme are estimated from the chemical potentials of all the components in all the phases involved in the substitution on the basis of a statistical mechanical theory developed recently. It is found that although substitution is feasible in the three-phase coexistence, its working window in temperature-pressure space is much narrower compared to the two-phase coexistence condition. Despite that the substitution normally generates only a small amount of heat, a large endothermic substitution is suggested in the medium pressure range, caused by the vaporization of liquid CO2 due to mixing with a small amount of the released CH4. This study provides the first theoretical framework toward the practical use of hydrates replacing CH4 with CO2 and serves as a basis for quantitative planning. (Geminiによる概要の機械翻訳)
Many experimental and theoretical studies on CH4-CO2 hydrates have been performed aiming at the extraction of CH4 as a relatively clean energy resource and concurrent sequestration of CO2. However, vague or insufficient characterization of the environmental conditions prevents us from a comprehensive understanding of even equilibrium properties of CH4-CO2 hydrates for this substitution. We propose possible reaction schemes for the substitution, paying special attention to the coexisting phases, the aqueous and/or the fluid, where CO2 is supplied from and CH4 is transferred to. We address the two schemes for the substitution operating in three-phase and two-phase coexistence. Advantages and efficiencies of extracting CH4 in the individual scheme are estimated from the chemical potentials of all the components in all the phases involved in the substitution on the basis of a statistical mechanical theory developed recently. It is found that although substitution is feasible in the three-phase coexistence, its working window in temperature-pressure space is much narrower compared to the two-phase coexistence condition. Despite that the substitution normally generates only a small amount of heat, a large endothermic substitution is suggested in the medium pressure range, caused by the vaporization of liquid CO2 due to mixing with a small amount of the released CH4. This study provides the first theoretical framework toward the practical use of hydrates replacing CH4 with CO2 and serves as a basis for quantitative planning.
research papers paper2023 clathratehydrate methane CO2 thermodynamics