Investigation on methane recovery from low-concentration coal mine gas by tetra-n-butyl ammonium chloride semiclathrate hydrate formation

Abstract This work presents an investigation of using tetra- n -butyl ammonium chloride (TBAC) semiclathrate hydrate to separate CH 4 from a low-concentration coal mine gas containing 30 mol% CH 4 , 60 mol% N 2 , and 10 mol% O 2 . Phase equilibrium data of TBAC semiclathrate hydrate formed in the presence of low-concentration coal mine gas with TBAC concentration ranging from 0.49 mol% to 3.3 mol% were measured and reported. A high pressure micro-differential scanning calorimeter (HP µ-DSC) was employed to characterize the phase behaviors of TBAC semiclathrate hydrate formed in the presence of coal mine gas with TBAC concentration fixed at 3.3 mol% (the stoichiometric concentration of TBAC semiclathrate). It was found that pure TBAC semiclathrate hydrate coexists with the TBAC-coal mine gas mixed hydrate in the pressure range of (1.4–6.2 MPa). In addition, kinetic experiments were performed to elucidate the effects of TBAC concentration, subcooling, and sodium dodecyl sulfate (SDS) concentration on CH 4 separation efficiency and an optimum experimental condition for semiclathrate-based CH 4 recovery was screened. The highest CH 4 recovery and separation factor were obtained at the conditions of 0.49 mol% TBAC, △ T  = 8 K, and 900 ppm SDS. It was found that CH 4 recovery obtained at 0.49 mol% TBAC is comparable with that obtained at 0.29 mol% TBAB, whereas CH 4 concentration of the gas mixture released from the TBAC hydrate (49.6 mol%) is increased substantially comparing to that obtained at 0.29 mol% TBAB (42.5 mol%).