Microscopic Insights into the Effect of the Initial Gas-Liquid Interface on Hydrate Formation by In-Situ Raman in the System of Coalbed Methane and Tetrahydrofuran

The serious issues of energy shortage and greenhouse gas emission have led to the development of coalbed methane (CBM) with new commercial ramifications. A hydrate-based gas separation technology is introduced to recover methane from CBM. However, the mechanism of hydrate nucleation needs to be clear for enhancing the hydrate formation rate and gas recovery efficiency. In this work, we studied, by means of in-situ Raman spectroscopy, the microscopic characterizations of hydrates forming in/around the initial gas-liquid interface in the case of CBM and tetrahydrofuran (THF). It is found that the hydrates accumulate as a film with horizontal crevices in the initial gas-liquid interface. These crevices prevent the hydrate film from hindering gas-liquid contact and limiting hydrate formation. Raman spectroscopy results illustrate that the initial gas-liquid interface shows a positive impact on water aggregation, and that the holding gas molecules stay stably with the water molecules. Nitrogen molecules encage into the cavities of THF hydrates along with methane molecules. For the interface and hydrate layer, water aggregation is evaluated by the Raman intensity ratio of hydrogen-bonded water (BW) and free water (FW) without any hydrogen bonds, abbreviated as I-BW/I-FW. A value of I-BW/I-FW higher than 0.85 can symbolize the occurrence of hydrate nucleation in the interface and help assess the hydrate formation