Ex-situ biogas enrichment by hydrogenotrophic methanogens at low H2/CO2 ratios: Effect of empty bed residence time

Abstract

Biohydrogen is a good candidate for biogas upgrading. However, the system will operate at a lower H2/CO2 ratio than the stoichiometric one due to the CO2 present in the bioH2, challenging the process. Some operational parameters can improve the performance. Therefore, this study evaluated the effect of reducing the empty bed residence time (EBRT) on ex-situ biogas upgrading using low H2/CO2 ratios. Hydrogen was added to synthetic biogas (65 % CH4, 35 % CO2) to obtain the H2/CO2 ratios of 3:1 and 4:3. The EBRTs tested were 12, 6, and 3 h. The operation at 3:1 H2/CO2 ratio and the subsequent EBRT reduction negatively affected methane production. In contrast, the highest methane concentration (80 ± 1 %, EBRT: 12 h) and CH4 productivity (1.7 L/L d, EBRT: 3 h) were obtained with the 4:3 H2/CO2 ratio. The hydrogenotrophic archaea Methanobacterium and Methanolinea, as well as the acetoclastic archaeon Methanosaeta, dominated the archaeal community. Reducing the H2/CO2 ratio and the EBRT positively affected methane concentration and productivity; their optimization could enhance the system’s performance. Biohydrogens’ use in biogas upgrading diversifies its use without the need for purification to eliminate the CO2 in the biohydrogen.