Hydrogen Mass Transfer in Anaerobic Biomethanation: A Comparative Study of Injection Strategies and Practical Limitations of Silicone Diffusion

Biological methanation of carbon dioxide (CO2) using hydrogen (H2) offers a promising route for upgrading biogas and storing renewable energy within existing gas infrastructures. However, the low solubility of H2 in aqueous systems remains a major bottleneck, limiting its bioavailability and overall conversion efficiency. In this study, three H2 delivery strategies—direct bubbling, sparging, and diffusion through submerged silicone tubing—were systematically compared in a lab-scale continuous stirred-tank reactor (CSTR) to evaluate their relative mass-transfer performance. H2 addition via silicone tubing yielded substantially higher initial H2 mass-transfer rates than sparging and bubbling, with increases of approximately 3.8-fold and 5.3-fold, respectively, under the tested conditions. Based on these findings, silicone-based H2 delivery was applied in both in situ and ex situ configurations for biomethanation. In both setups, the methane (CH4) fraction in the biogas increased from approximately 48% to over 85% upon H2 addition, while no H2 accumulation in the reactor headspace was observed. Under the applied low organic loading conditions, biogas production remained stable, indicating that H2 addition did not adversely affect process performance within the investigated operational window. Overall, this study provides a comparative assessment of H2 delivery strategies and highlights the trade-off between mass-transfer performance and engineering feasibility. While silicone diffusion enhances H2 availability in anaerobic systems at the laboratory scale, extrapolating from the experimentally determined H2 flux suggests that an impractically large silicone surface area would be required for scale-up. This reflects a fundamental limitation of diffusion-based H2 delivery in stirred-tank reactors and underscores the need for alternative reactor concepts, such as dedicated ex situ systems or high-surface-area gas–liquid contactors. Despite extensive research on hydrogenotrophic methanation, quantitative links between H2 mass transfer, reactor design, and scalability remain insufficiently resolved.