Methane enrichment from dilute sources: Performance limits and implications for methane removal and abatement

Methane (CH4), the second most potent greenhouse gas after carbon dioxide (CO2), profoundly impacts global warming, and its mitigation is crucial to meeting climate neutrality. However, most CH4 emissions originate from "non-point" distributed sources with dilute CH4 concentrations. This study investigates the feasibility of preconcentrating CH4 from these dilute sources using a three-step vacuum temperature swing adsorption (VTSA) cycle before converting CH4 to CO2 for permanent storage. The cycle performance limits are identified through integrated adsorbent-process optimizations at CH4 feed compositions of 2 ppm, 20 ppm, and 200 ppm. The optimizations focus on minimizing exergy consumption and maximizing productivity by varying adsorbent and process variables. The results demonstrated that CH4 enrichment to at least 1% is feasible using this cycle, especially from 20 ppm air feeds, when CO2 co-removal is also considered. More advanced VTSA cycles may be needed to effectively enrich CH4 from 2 ppm. The minimum exergy of the VTSA cycle ranges from 5.1 to 1.5 MJ/kg eq. CO2 for CH4 feed compositions of 2 ppm to 200 ppm, while maximum productivity varies from 0.5 to 8.3 kg eq. CO2/m3 h. Ideal adsorbent properties for CH4 exhibit moderate loadings of 0.1-0.3 mmol/g and high heats of adsorption of 31-32 kJ/mol, indicating a potential direction for future adsorbent development.