Advanced pre-combustion CO2 capture by clathrate hydrate formation with water-to-gas molar ratio optimization

Abstract

Capturing CO2 from exhaust gases by formation of clathrate hydrates, which consist mainly of water (∼85 %), is highly promising owing to the mild operating conditions, economic feasibility, and environment-friendliness. Although tremendous efforts have been made, hydrate-based CO2 capture has not yet been realized primarily due to insufficient CO2 uptake kinetics and separation efficiency. To overcome these, the effect of the water-to-gas molar ratio (W/G ratio), which was not previously considered important despite its significance, on clathrate hydrate-based pre-combustion capture was investigated. Changes in phase equilibrium temperatures for structure Ⅰ CO2/H2 and structure Ⅱ CO2/H2/tetrahydrofuran (THF) hydrates with the W/G ratio were characterized to establish suitable operation temperature and pressure conditions. In addition, a novel strategy utilizing the residual liquid water to capture extra CO2 in structure Ⅰ hydrates was demonstrated. Contrary to the general premise, limiting the THF concentration was found to be effective in preventing unfavorable H2 capture, thus dramatically improving the separation efficiency. Through a comprehensive evaluation in terms of hydrate phase equilibria, formation kinetics, and separation efficiency, 3 mol% THF + high W/G ratio was derived as the optimal condition. The present findings demonstrate that the effect of the W/G ratio and its combined effects with other parameters on phase equilibria, formation kinetics, and separation efficiency should be emphasized to achieve an advanced pre-combustion capture using clathrate hydrates toward carbon neutrality.