Re-evaluation of using carbon nanotubes as the kinetic promoters for methane hydrate formation

Abstract

The use of carbon nanotubes (CNTs) to enhance methane hydrate formation for energy storage has garnered attention. However, their promotion efficiency remains inferior to that of traditional promoters (e.g., surfactants), highlighting the necessity of re-evaluation on the feasibility of CNTs as effective promoters for methane hydrate formation. Here, the promotion of CNTs to methane hydrate formation was comprehensively evaluated via both experimental and simulation methods, with the scales from macro to micro and status from suspension, to serum and water-saturated powder. It was revealed for the first time that there was a contradiction between the CNTs status and the promotion efficiency in different periods (nucleation and growth). With the increase in CNTs concentration, transitioning from suspension to serum and water-saturated powder, the enhancement of hydrate nucleation was attributed to the increase in nucleation sites within the liquid phase. The initial stage of hydrate formation was significantly shortened, dropping from 148 min to 14 min, and nearly to 0 min. However, the promotion of hydrate growth was reduced, as the final methane storage capacity decreased from 141 to 132 and then to 90 v/v. This reduction was due to the high dosage of CNTs, which adsorbed water and hindered its migration during the hydrate growth period. Regarding the inner cavity of CNTs, molecular simulations confirmed that methane hydrates could not form in this special space, indicating that pristine CNTs have limited potential as standalone methane hydrate promoters. Therefore, promising research directions may include the combination of CNTs with other promoters, the functionalization of CNT surfaces with active groups, the synergistic enhancement of heat and mass transfer in CNT-based fluids, and the application of CNTs in the industrialization of hydrate-based technologies.