Biogas upgrading by CO2 removal with a highly selective natural amino acid salt in gas–liquid membrane contactor

For biogas upgrading, a natural amino acid salt, potassium l-argininate (PA) is studied in a membrane contactor to capture CO2 from biogas. CO2 removal performance in terms of the overall volumetric gas phase mass transfer coefficient, membrane selectivity towards CO2 and the economic cost factor is systematically investigated. It is shown that PA is a highly CO2 selective absorbent and has a better affinity towards CO2 than monoethanolamine (MEA). The highest CH4 content in the upgraded biogas can reach about 99.15 vol% by using PA, fully meeting the requirement of biogas upgrading. Furthermore, lower solvent concentration, lower liquid velocity and higher reaction temperature may be adopted when using PA in comparison to MEA. PA also has a better flexibility to the change of CO2 partial pressure and biogas flow rate than MEA. Regarding the economic cost factor of membrane process, CO2 loading of the lean PA solution can be optimized to 0.69–0.78 mol/mol as the suitable range. Moreover, CO2 removal performance of l-arginine (ARG) is also explored. Due to the large amounts of bicarbonate other than carbamate formed in CO2-rich ARG solution, ARG has a lower biogas upgrading capability than diethanolamine (DEA) but higher than triethanolamine (TEA).