Enhancing system stability in power-to-gas applications: integrating biological hydrogen methanation and microbial electrolysis cells under hydrogen overloading in various injection modes

Volatile fatty acid (VFA) accumulation is a common issue that compromises the performance of biological hydrogen methanation systems (BHMs). This accumulation is often triggered by fluctuations in hydrogen supply, which can disrupt microbial activity and lead to system instability. To address this challenge, this study investigated the impact of employing a microbial electrolysis cell (MEC) in BHMs to mitigate system instability and acid build-up. As such, a conventional anaerobic digester (AD) and a microbial electrolysis cell, both supplemented with exogenous hydrogen, were evaluated for their performance in hydrogen methanation. The effect of exogenous hydrogen at high addition rates (> 4:1 CO₂:H₂ molar ratio) under instantaneous and gradual injection modes was investigated. The results showed that the instantaneous addition of hydrogen resulted in the total failure of the anaerobic digestion system. Propionate accumulated in the system (> 2 g/L) and resulted in low pH (pH = 5.3). Methane production stopped, and the reactor never recovered from hydrogen shock. However, the microbial electrolysis system was able to withstand the instantaneous hydrogen addition and maintain normal operation under toxic hydrogen addition levels (> 4:1 CO₂:H₂ molar ratio). Under the gradual injection mode, both MEC and AD reactors remained reasonably unaffected; even though the hydrogen injection exceeded the stoichiometric molar ratio. This study provides a new perspective on the application of MECs for reliable operation and storage of surplus renewable energy via biological hydrogen methanation.