Adsorbents selection for the enrichment of low-grade methane coal mine emissions by temperature and pressure swing adsorption technologies

Abstract

This work evaluates the feasibility of common adsorbents (carbonaceous materials, zeolites and metal-organic frameworks) for the adsorption and further methane upgrading of the ventilation air methane (VAM) emissions from underground coal mining (0.57% CH4). Concentration was achieved by adsorption through two different operational procedures based on fixed bed configurations: temperature swing adsorption (TSA) and pressure swing adsorption (PSA). All the combinations have been simulated using a rigorous mathematical model implemented in a commercial simulation package. The main purpose is to evaluate the performance of the different combinations of adsorption technique and adsorbent material, as well as establishing a valid mathematical model able to test a wide range of materials. The comparison has been fulfilled with an economic evaluation of the different combinations. Results show that carbonaceous materials provide the highest concentration factors (C/C0 = 5), with low total methane recoveries (30%) and the lowest cost per kmol of methane recovered (1.5 €/kmol). MOFs can retain substantial amounts of methane, but with lower CH4/N2 selectivities than carbonaceous materials and lower methane concentration factors (C/C0 = 2.6). For this type of materials, a high recovery of methane is achieved, but at expense of the highest costs (80 €/kmol). Finally, zeolitic materials present the lowest methane concentration factor (C/C0 = 2), with intermediate both methane recoveries (58%) and costs (25 €/kmol). Concerning the adsorption technique, TSA has shown higher final methane concentrations for carbonaceous materials and some MOFs, whereas PSA overperforms for zeolitic materials. In addition, TSA is cheaper in all cases than PSA processes. On the other hand, PSA allows higher total methane recoveries and adsorption capacities for all materials, highlighting the high dependence on adsorption pressure, especially in carbonaceous materials (PSA/TSA = 18.3, in the case of Maxsorb).