Optimization of the Ex Situ Biomethanation of Hydrogen and Carbon Dioxide in a Novel Meandering Plug Flow Reactor: Start-Up Phase and Flexible Operation

With the increasing use of renewable energy resources for the power grid, the need for long-term storage technologies, such as power-to-gas systems, is growing. Biomethanation provides the opportunity to store energy in the form of the natural gas-equivalent biomethane. This study investigates a novel plug flow reactor that employs a helical static mixer for the biological methanation of hydrogen and carbon dioxide. In tests, the reactor achieved an average methane production rate of 2.5 <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mfrac><mrow><msub><mrow><mi>L</mi></mrow><mrow><msub><mrow><mi>C</mi><mi>H</mi></mrow><mn>4</mn></msub></mrow></msub></mrow><mrow><msub><mrow><mi>L</mi></mrow><mrow><mi>R</mi></mrow></msub><mo>∗</mo><mi mathvariant="normal">d</mi></mrow></mfrac></mrow></semantics></math></inline-formula> (methane production [L_CH4] per liter of reactor volume [L_R] per day [d]) with a maximum methane content of 94%. It demonstrated good flexibilization properties, as repeated 12 h downtimes did not negatively impact the process. The genera <i>Methanothermobacter</i> and <i>Methanobacterium</i> were predominant during the initial phase, along with volatile organic acid-producing, hydrogenotrophic, and proteolytic bacteria. The average ratio of volatile organic acid to total inorganic carbon increased to 0.52 ± 0.04, while the pH remained stable at an average of pH 8.1 ± 0.25 from day 32 to 98, spanning stable and flexible operation modes. This study contributes to the development of efficient flexible biological methanation systems for sustainable energy storage and management.