Long term and demand-oriented biocatalytic synthesis of highly concentrated methane in a trickle bed reactor

The recent increasing amount of energy from different renewable sources is connected to several challenges like flexible availability, secured energy supply, energy storage and its distribution. A suitable method is the sector coupling via conversion of excess renewable energy into natural gas by electrolysis followed by biological methanation of H-2 and CO2. Gas quality, stability, controllability as well as low internal energy consumption and low technical effort of the process are of higher significance. Therefore, the biological methanation and methane enrichment by conversion of carbon dioxide and hydrogen were investigated in an anaerobic trickle bed process. In continuous long-term operation of 1200 days, a stable and significant high methane concentration of c(CH4,empty set) = 96.6% at methane formation rate of MFRmax = 3.1 Nm(CH4)(3)/(m(3).d) could be achieved. Process disturbance like interrupted availability of the input gases was examined and discussed. Methanation could be terminated and restarted almost immediately. Even after an interruption for a short (hours) or long (weeks) period, a performance at the same level was achievable. Therefore, process control is feasible. In a second and uncoupled investigation, the inner pressure was increased up to p(abs) = 25 bar. A pressure of p(abs) = 5 bar led to a maximum of MFR = 5.75 Nm(CH4)(3)/(m(3).d) and C-CH4,C-empty set = 97.7%. The higher pressure was harmful. Apart from pure carbon dioxide, biogas could also serve as a source of carbon, which enables an ex-situ methane enrichment of C-CH4,C-empty set =50% to C-CH4 > 94% and MFRmax = 2.1 Nm(CH4)(3)/(m(3).d) using the same process design without increased inner pressure. In particular, the high gas quality and process controllability cannot be found in comparable processes for biological methanation. Biogas for methane enrichment and CO2 for methanation were accepted as carbon source. The high gas quality in the product reduces further effort of gas conditioning before injection into the natural gas grid.