Enhanced methane recovery from low-concentration coalbed methane by gas hydrate formation in graphite nanofluids

Abstract

Coalbed methane (CBM) is a primary type of unconventional natural gas adsorbed in coal seams. Its amount around the world is estimated to be 260 × 1012 standard cubic meters (SCM). The low-concentration coalbed methane (LCCBM) with CH4 content less than 30 mol% is difficult to utilize because of its low combustion efficiency. This work reports an investigation of employing a graphite nanofluid to enhance CH4 recovery from LCCBM via gas hydrate formation. The CH4 separation efficiency was evaluated by comparing CH4 recovery ratio, the separation factor, and gas consumption with those reported in the literature, and the behavior of gas hydrate growth in graphite nanofluid was observed. The results indicated that hydrate nucleation was promoted when adding a mass fraction of 0.5% graphite nanoparticles (GNP) into the tetrahydrofuran and sodium dodecyl sulfate solution (THF/SDS solution) and gas consumption increased significantly in comparison with that obtained without GNP. The graphite nanofluid containing 0.5% GNP is preferred for CH4 separation among the three GNP mass concentrations (0.1%, 0.5%, and 3.0%) tested in this work. CH4 recovery ratio and the separation factor obtained in graphite nanofluid were higher than those obtained in other systems such as water-in-oil emulsions. CH4 concentration in LCCBM was increased from 30 mol% to 57.1 mol% via a one-stage hydrate based gas separation process. As a consequence, it has the potential to be an efficient approach to recover CH4 from LCCBM by forming gas hydrates in graphite nanofluid.