Kinetic-separation vacuum swing adsorption for safe and efficient enrichment of low concentration coal mine gas

Abstract

CH4 enrichment by pressure swing adsorption (PSA) is promising to promote low-concentration coal mine gas (LC-CMG) utilization and reduce greenhouse gas emission. Here, kinetic-separation vacuum swing adsorption (VSA) preferentially adsorbing O2/N2 rather than CH4 and using low adsorption pressure was studied to overcome low separation efficiency and high explosion risk of conventional equilibrium-separation PSA for LC-CMG enrichment. The kinetic/equilibrium adsorption and breakthrough tests demonstrated that the carbon molecular sieve (CMS) adsorbent has the good O2/CH4 and N2/CH4 kinetic selectivity, which was yet restrained by the large equilibrium adsorption capacity of CH4. The LC-CMG with 4.3% CH4 and 19% O2 was enriched into the product gas containing 24.7% CH4 and 1.6% O2 with CH4 recovery of 86%, showing the excellent CH4 enrichment and deoxygenation performances of the kinetic-separation VSA. The effects of feed CH4 concentration, feed flow rate, bed length, desorption and adsorption pressures on enrichment performances such as output CH4 and O2 concentrations, CH4 recovery, productivity and specific energy consumption were analyzed, providing the optimization strategies for mitigating the negative impacts of adsorption equilibrium on kinetic separation. Finally, a numerical model was established to reveal the change of CH4 and O2 concentrations distribution with time and recognize the explosion zone in adsorption bed at adsorption and desorption stages, demonstrating the higher safety of kinetic-separation VSA than equilibrium-separation PSA. This work can guide the design of kinetic-separation VSA to achieve the safe and efficient LC-CMG enrichment.