The absorption experiments (Figure , 3) were carried out using CO2 diluted in N2 gas, which are the major components of flue gas. However, flue gas also contains oxygen, which is toxic to strictly anaerobic microorganisms like hydrogenotrophic methanogens. Testing the compatibility of the microorganisms to the presence of oxygen was therefore an essential step in the concept development. For this, flue gas from a gas engine was utilized. Most of the heat in the flue gas produced by the biogas engine was recovered in a hightemperature heat exchanger followed by a low-temperature heat exchanger. The flue gas was then collected at the entrance to the chimney, just before it was exhausted. The temperature of the flue gas was 44.5 ± 1.6°C, and the major constituents in the dry flue gas composition are shown in table . Additionally, the NOx and CO emissions in the flue gas constituted a concentration of 141 ppm of NOx and 491 ppm of CO. The water vapor present in the flue gas will condense in the reactors and is not included in the compositional analysis.