Separation of Coal Mine Methane Gas mixture via sII and sH hydrate formation

Coal mine methane (CMM) also known as coal bed methane (CBM) is an unconventional natural gas resource and a significant source of clean energy, which is produced and emitted continuously from coal mine activities either from the coal seam itself or from other gassy formations available underground, mainly consisting of methane (CH4) and air [1], [2]. The exploitation of CMM is done mostly during the extraction of coal through underground mining. The release of CMM gas into the atmosphere is a wastage of natural gas resources, while it also contributes to global warming, as the greenhouse index of CH4 is 21 times higher than CO2. The separation of methane from CMM gas is thus desirable from an economic as well as an environmental point of view. A mature process such as methane capture and utilization from CMM gas would serve as an additional source of energy and provides a safer coal mine operation. The separation of methane before releasing it into the atmosphere would mitigate the global warming problem by reducing greenhouse gas emissions to the atmosphere.

The operating cost of conventional gas separation methods is not in tune with the market requirements of being economical and safe processes. Thus, low cost, safe, efficient, and sustainable separation method for methane from its associated gases in CMM is still desirable. The hydrate-based gas separation process is a promising solution for CMM gas separation as it requires benign chemicals in low concentration, leading to low inventory and recycle cost, whereas the conventional separation methods require chemicals which may not be necessarily benign. Further, if the process is optimized properly, it may produce enriched methane as the main product and contaminant free water as a by-product from the same stream and requires much lesser energy during regeneration of the working liquid

In their work, authors have considered CMM as a mixture of methane and nitrogen (CH4-N2, 30:70 mol%), and this gas mixture has been separated by the hydrate-based gas separation (HBGS) process. Formation of sI hydrate with 70% N2 in the mixture requires significantly higher pressure and thus not suitable for scale-up. Therefore, in this work with a suitable thermodynamic promoter, sII and sH hydrates were formed to study the methane separation from CMM gas mixture at moderate temperature and pressure conditions. sII hydrate formation was carried out using polar THF (Tetrahydrofuran) and non-polar cyclopentane (CP) at different molar concentrations and at 274.2 K temperature. sH hydrate formation was facilitated using polar tert-butyl-methyl-ether (TBME) and non-polar neo-hexane (NH) at different molar concentrations and at 274.2 K temperature. Further, water-soluble hydrate promoter sodium dodecyl sulfate (SDS) was used at 500 ppm to enhance the rate of hydrate formation and thus to achieve better separation efficiency in a given time. For the CMM gas mixture, sII hydrate formation showed better methane recovery compared to sH hydrate formation, whereas sH hydrate formation showed a better separation factor compared to sII hydrate formation. Hydrate dissociation was also carried out to recover the hydrated gas via depressurization and thermal stimulation to compare the effect of polar and non-polar hydrate formers.

The authors have explored the gas hydrate-based method and based upon the experimental findings, an economical process may be developed for the separation of the gases from the mixture of nitrogen and methane. The work also provides insights about the economically feasible way for the storage of the methane under controlled conditions, requiring near to water freezing temperatures.