Performance Evaluation and Optimization of Ex Situ Hydrogen Biomethanation in a Mesophilic Fed-Batch Reactor

Ex situ hydrogen biomethanation represents a promising approach to converting surplus renewable electricity into CH4-rich gas through the biological reduction in CO2 with H2, mediated by hydrogenotrophic methanogens. In this study, an ex situ H2 biomethanation process was investigated in a lab-scale mesophilic anaerobic reactor operated in fed-batch mode. The system followed a cyclic operational strategy comprising sequential gas feeding, reaction, and discharge phases. Hydrogen was supplied through a pressure-controlled feeding strategy, whereas CO2 injection maintained dissolved CO2 concentrations at 25, 17, and 2 mg L−1 during the initial, intermediate, and final stages, respectively. During early operation, volatile fatty acids (VFAs) temporarily accumulated to 3 g L−1, accompanied by a decrease in pH. Progressively lowering the dissolved CO2 target restored process stability, reduced the VFA concentration to 618 mg L−1, and increased the pH to 7.6. Under stable final-stage conditions, the reactor achieved an average CH4 concentration of 93.3%, a hydrogen utilization efficiency of 99%, and a methane evolution rate (MER) of 3.95 NL CH4 LVR−1 d−1. These results show that combining pressure-controlled hydrogen injection with dissolved CO2 regulation enhances methane production and maintains stable ex situ biomethanation.