Microscopic insights on the effects of flue gas components on CH4-CO2 replacement in natural gas hydrate

YL Zhang and M Cui and GM Xin and DX Li, GAS SCIENCE AND ENGINEERING, 112, 204947 (2023).

DOI: 10.1016/j.jgsce.2023.204947

Extracting natural gas hydrates by CO2 replacement method exhibits great significances in energy supply and carbon reduction. Particularly, using real flue gas directly rather than pure CO2 could be more economical and effective. However, flue gas components other than N2 were rarely concerned and the micro mechanism of CH4flue gas replacement is still unclear. Therefore, in this work, the molecular dynamic (MD) method is utilized to investigate the effects of various flue gas components (N2, SO2, H2S, NO) on CH4-CO2 replacement. Moreover, the density functional theory (DFT) calculations are carried out to provide more thorough explanations and insights. The MD results indicate that the addition of N2 and NO can promote CH4 production but damage more hydrate structures, which is opposite to the effects of SO2 and H2S. It can be attributed to the different stability of hydrates formed by these flue gas components after replacement (SO2>H2S > CO2>NO > N2). With equal gas injection, the presence of any flue gas component will reduce CO2 storage. Interestingly, different decreasing trends are observed due to the different competitiveness in replacing CH4 of various flue gas components (SO2>H2S > CO2>NO > N2). The small addition of SO2 and H2S can cause a significant decrease in CO2 storage. The results of DFT calculations and MD simulations acquire satisfying agreements. In addition, the DFT calculations indicate that the ability of various flue gas components to replace CH4 is mainly determined by their ability to bind with and occupy large hydrate cages rather than small cages. SO2, H2S, CO2, and NO prefer to occupy the large cages, and N2 prefers the small cages. This work provides microscopic insights and a theoretical basis for using flue gas to replace CH4 in hydrate and can be helpful in natural gas hydrate extraction control, gas transportation, gas storage, gas separation, and gas pollutants control.

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