Practical separation performance evaluation of coal mine methane upgrading with carbon molecular sieves

Abstract

Upgrading coal mine methane (CMM) with adsorption method is an effective way to enhance the energy utilizing efficiency and reduce greenhouse gas emissions. In this work, adsorption equilibrium and kinetics of CH4, N2 and O2, as typical components in CMM, on three carbon molecular sieves (CMSs) and the corresponding separation performances were studied. The equilibrium and kinetic selectivity were obtained from adsorption isotherms fitted with the Langmuir model and uptake curves fitted with the micropore-diffusion model, respectively. O2 with the highest diffusion time constant among three gases suggests the advantage of CMS-based kinetic separation in deoxygenation of CMM and the critical role that N2/CH4 kinetic separation plays in CH4 enrichment. CMS-1 with the highest microporosity has the largest adsorption capacities, and CMS-3 with a proper average pore size has the greatest N2/CH4 kinetic selectivity. The actual separation performances shown by CH4/N2/O2 breakthrough curves according to the principle for practical CMM upgrading processes exhibit the order of CMS-1 > CMS-3 > CMS-2 which is inconsistent with that of the conventional N2/CH4 selectivity highly due to the underestimation of effects of N2 equilibrium adsorption relative to N2 kinetic selection without covering specific adsorption range corresponding to practical working capacity region. A modified selectivity with consideration of actual N2 amount adsorbed was therefore developed, showing the correct performance order and the potential to be used as a practical adsorbent selection parameter. An advanced CMS materials design with proper existence of microporosity would benefit practical CMM upgrading in terms of reaching an optimal balance between adsorption equilibrium and kinetics.