Upgrading low-concentration oxygen-bearing coal bed methane by dual-reflux vacuum swing adsorption

Abstract

Fugitive methane (CH4) is a typical by-product of mining processes, which is commonly known as coal bed methane (CBM) or coal mine gas (CMG). The capture of these CH4 gases can simultaneously avoid greenhouse gas emissions and provide extra energy benefits. However, the explosion risk of low-concentration CBM (CH4 molar fraction ≤ 30%) requires strictly safe operating protocols to conduct the capture process. Dual reflux vacuum swing adsorption (DR-VSA) is a promising candidate with a vacuum operating condition which can lower the explosion risk and simultaneously reach CH4 enrichment and O2 removal targets in product and effluent streams. Herein, a low-concentration oxygen-bearing CBM (20% CH4, 16% O2 and 64% N2) can be upgraded to 69.7 mol% in the product gas while ensuring an effluent concentration of 2.5 mol% by the DR-VSA cycle using ionic liquidic zeolites (ILZ) as the adsorbents. A rigorous safety analysis has been conducted to investigate the explosion risk in the adsorption column and product tank, suggesting that the DR-VSA process is a safe technology for upgrading low-concentration oxygen-bearing methane.