A Critical Assessment of Microbiological Biogas to Biomethane Upgrading Systems

Microbiological biogas upgrading could become a promising technology for production of methane (CH4). This is, storage of irregular generated electricity results in a need to store electricity generated at peak times for use at non-peak times, which could be achieved in an intermediate step by electrolysis of water to molecular hydrogen (H-2). Microbiological biogas upgrading can be performed by contacting carbon dioxide (CO2), H-2 and hydrogenotrophic methanogenic Archaea either in situ in an anaerobic digester, or ex situ in a separate bioreactor. In situ microbiological biogas upgrading is indicated to require thorough bioprocess development, because only low volumetric CH4 production rates and low CH4 fermentation offgas content have been achieved. Higher volumetric production rates are shown for the ex situ microbiological biogas upgrading compared to in situ microbiological biogas upgrading. However, the ex situ microbiological biogas upgrading currently suffers from H-2 gas liquid mass transfer limitation, which results in low volumetric CH4 productivity compared to pure H-2/CO2 conversion to CH4. If waste gas utilization from biological and industrial sources can be shown without reduction in volumetric CH4 productivity, as well as if the aim of a single stage conversion to a CH4 fermentation offgas content exceeding 95 vol% can be demonstrated, ex situ microbiological biogas upgrading with pure or enrichment cultures of methanogens could become a promising future technology for almost CO2-neutral biomethane production.