Improved biological methanation using tubular foam-bed reactor

Background

Power-to-gas is the pivotal link between electricity and gas infrastructure, enabling the broader integration of renewable energy. Yet, enhancements are necessary for its full potential. In the biomethanation process, transferring H₂ into the liquid phase is a rate-limiting step. To address this, we developed a novel tubular foam-bed reactor (TFBR) and investigated its performance at laboratory scale.

Results

A non-ionic polymeric surfactant (Pluronic^® F-68) at 1.5% w/v was added to the TFBR’s culture medium to generate a stabilized liquid foam structure. This increased both the gas–liquid surface area and the bubble retention time. Within the tubing, cells predominantly traveled evenly suspended in the liquid phase or were entrapped in the thin liquid film of bubbles flowing inside the tube. Phase (I) of the experiment focused primarily on mesophilic (40 °C) operation of the tubular reactor, followed by phase (II), when Pluronic^® F-68 was added. In phase (II), the TFBR exhibited 6.5-fold increase in biomethane production rate (MPR) to 15.1 \(({\text{L}}_{{\text{CH}}_{4}}\text{/}{\text{L}}_{\text{R}}\text{/d)}\), with a CH₄ concentration exceeding 90% (grid quality), suggesting improved H₂ transfer. Transitioning to phase (III) with continuous operation at 55 °C, the MPR reached 29.7 \({\text{L}}_{{\text{CH}}_{4}}\text{/}{\text{L}}_{\text{R}}\text{/d}\) while maintaining the grid quality CH₄. Despite, reduced gas–liquid solubility and gas–liquid mass transfer at higher temperatures, the twofold increase in MPR compared to phase (II) might be attributed to other factors, i.e., higher metabolic activity of the methanogenic archaea.

To assess process robustness for phase (II) conditions, a partial H₂ feeding regime (12 h 100% and 12 h 10% of the nominal feeding rate) was implemented. Results demonstrated a resilient MPR of approximately 14.8 \({\text{L}}_{{\text{CH}}_{4}}\text{/}{\text{L}}_{\text{R}}\text{/d}\) even with intermittent, low H₂ concentration.

Conclusions

Overall, the TFBR’s performance plant sets the course for an accelerated introduction of biomethanation technology for the storage of volatile renewable energy. Robust process performance, even under H₂ starvation, underscores its reliability. Further steps towards an optimum operation regime and scale-up should be initiated. Additionally, the use of TFBR systems should be considered for biotechnological processes in which gas–liquid mass transfer is a limiting factor for achieving higher reaction rates.