New Study Unveils the Optimal Concentration for Effective CO₂ Capture in Gas Hydrates

Abstract
To mitigate global warming, the paramount imperative lies in curbing the emission of CO2. The guest replacement method is a prominent carbon-neutral technological advancement that involves injecting CO2 into natural gas hydrate layers to accomplish the dual objectives of energy production and carbon storage. In this study, the guest dynamics in the CH4 – flue gas replacement process were examined, and the impacts of the N2 concentration of the injected gas were systematically analyzed. A powder X-ray diffraction analysis of the cage-specific guest distributions after CH4 − CO2 (20 %) + N2 (80 %) replacement revealed that CH4 production increased in both the large and small cages compared to the CH4 – CO2 replacement. This enhancement was attributed to the N2 molecules participating in both cages. However, this simultaneously led to a decrease in CO2 storage potential, indicating a ‘complementary’ relationship for CH4 production and a ‘competitive’ one for CO2 storage with respect to CO2 and N2. In situ Raman spectroscopy revealed that the introduction of N2 resulted in a deceleration of CO2 storage kinetics. Guest composition measurements after replacement showed an upward trend in CH4 production and a simultaneous decline in CO2 storage as the N2 composition increased. Notably, an intriguing correlation was established between the CO2/N2 ratios for the injected gas and the replaced hydrates, exhibiting a strong alignment with a simple first-order equation. The findings not only contribute to a deeper understanding of the CH4 − CO2 + N2 replacement technique but provide practical insights for its application in real-world scenarios.

A research team led by Professor Yongwon Seo from the Graduate School of Carbon Neutrality at UNIST has successfully elucidated the replacement behavior and mechanisms associated with varying concentrations of carbon dioxide (CO2) in guest replacement technology for natural gas hydrates (NGHs). Their research also proposes the optimal concentration of injected gas mixtures to maximize natural gas production while enhancing CO2 capture. This guest replacement process facilitates the extraction of natural gas from NGHs and its replacement with CO2, a known greenhouse gas.

The team conducted experiments using flue gas—also known as exhaust gas or stack gas—a mixture of nitrogen (N2) and CO2 emitted from industrial combustion processes, such as those in power plants and boilers. Utilizing this flue gas for injection allows for direct introduction of CO₂ into NGH layers without the need for prior CO₂ separation. However, because N2 occupies space within NGHs instead of CO₂, further investigation is warranted to optimize this process.

Figure 1. Schematic image, showing the guest replacement process simulation, utilizing flue gas.

In this study, the dynamics of gas guest distributions in the CH₄–flue gas replacement process were explored. Systematic analysis revealed a trade-off where increased concentrations of N₂ not only delayed the rates of CO₂ storage but also CH₄ production. The research team measured replacement behavior, natural gas production, and CO₂ storage using various concentrations of flue gas, ultimately discovering a correlation equation linking the concentration ratio of CO₂ stored in gas hydrates to the concentration of the injected gas. Their findings indicate that natural gas production and CO₂ storage efficiency are optimized at a CO₂ concentration of 64%.

“The results of this study are expected to provide guidelines for utilizing natural gas hydrates, which are distributed around the world, as reservoirs for greenhouse gases and as sources of clean energy,” remarked first author Dr. Junghoon Mok from UNIST who currently is Assistant Professor at Kyonggi University.

Researchers Jonghyuk Lee and Wonjung Choi also contributed to this study. The findings were published online on October 10 in Renewable & Sustainable Energy Reviews, a leading journal in green energy technology, and are anticipated for official publication.

Journal Reference
Junghoon Mok, Jonghyuk Lee, Wonjung Choi, and Yongwon Seo, “Complementary and competitive dynamics of CO2 and N2 in CH4 – Flue gas replacement within natural gas hydrates,” Renew. Sustain. Energy Rev., (2024).