The biomethanation concept has hitherto been restricted to CO2 sources without the presence of oxygen including mainly biogas and syngas . The introduction of CO2 streams, which have been diluted with a large fraction of atmospheric air, increases the complexity of the bioreactor system due to the inherent requirements of maintaining an anaerobic environment for the hydrogenotrophic methanogens. Most gas streams, such as flue gas, have therefore not been suitable for biomethanation. In this study, the utilization of flue gas from a biogas engine was achieved for biomethanation, which validated the observed effects in the experiments with synthetic flue gas (Figure ). A CH4 productivity rate of 53.1 NmL L -1 h -1 was achieved with flue gas for MDEA concentrations at 30 mM and a CH4 productivity rate of 30.3 NmL L -1 h -1 with 60 mM of MDEA. Compared to the productivity with lab-graded gas of N2:CO2 in a ratio of 80:20, the MDEA solutions scrubbed with flue gas had a consistently lower CH4 productivity. The CH4 productivity rate from CO2 from captured flue gas accounted for 83.5% of the synthetic gas stream at 30 mM MDEA and 72.6% at 60 mM MDEA. Based on the difference in the gas compositions, it must be assumed that the reduced activity would mainly be due the presence of oxygen (Table ), which is toxic to the methanogens. . The level of oxygen was therefore four times higher in the MDEA from flue gas. However, the inhibitory effect of oxygen is only thought to be transient. Measurements of dissolved oxygen in the microbial cultures showed that the dissolved oxygen level declined to an average of 0.22 mg L -1 . The decline of oxygen was most probably facilitated by oxygen consumption of the mixed microbial community in the microbial culture, which also harbors facultative anaerobic microorganisms capable of utilizing oxygen. The oxygen consumption from these facultative microbes in the mixed sludge culture was sufficient for creating an anaerobic environment promoting the methanogenic conversion of H2 and CO2 to CH4. Since the microbial culture here used was sampled from a biogas plant, the conversion of oxygen could be based on conversion of reduced organic compounds present in the sample. However, it could also be through the microbial oxidation of hydrogen with oxygen (i.e. "Knallgas Reaction"). Although the reaction with hydrogen alleviates the inhibitory effect of oxygen, it also infers a loss of hydrogen. According to mass balances where hydrogen oxidation is assumed to be the main process of oxygen reduction, the loss would account for the negligible amount of 0.051% ± 0.006% of the total consumed H2 at an MDEA concentration of 60 mM. This loss would be even lower if higher MDEA concentrations, and consequently higher CO2 absorption capacity, were used, since a higher CO2 absorption would lower the fraction between the O2 and CO2 molecules transported to the biomethanation reactor. Hence, at amine concentrations of 60 mM of MDEA, the molar ratio between the solubilized CO2 and O2 was 637:1 while the ratio at 500 mM was 5315:1. The solubility of O2 would also depend on operational temperatures as it is inversely related to the temperature of the aqueous amine medium. The temperature in a commercial CO2 absorption unit usually reaches 40°C to 65°C due to the exothermic nature of CO2 absorption (-∆Habs ≈ 60 kJ mol -1 CO2 for tertiary amines ), which were mimicked in these experiments. The utilization of thermophilic bioreactors operating at 54°C as utilized in this study would therefore minimize problems with oxygen inhibition compared to that of mesophilic reactors operating at 37-42°C.