Impacts of microbial competition on underground bio-methanation of hydrogen and carbon dioxide: Insights from biogeochemical simulations

The emerging technology of underground bio-methanation (UBM) from hydrogen (H₂) and carbon dioxide (CO₂) enables renewable natural gas production, large-scale renewable energy storage, and carbon utilization and sequestration. However, during the UBM process, acetogens and sulfate-reducing bacteria (SRB) may compete with methanogens for H₂ and CO₂, affecting methane production. This competition has been relatively less explored. To address this, a microbial kinetic model incorporating environmental effects and spatial constraints was developed to investigate microbial competition in UBM. The study revealed that SRB's H₂ consumption is primarily limited by sulfate availability in formation water, whereas acetogen metabolism can significantly affect conversion efficiency. In the absence of carbonate minerals, pH reduction from acetogen metabolism may even halt the conversion process. Furthermore, increased salinity, particularly above 90 g/L, along with a higher CO₂/H₂ ratio, can inhibit methanogen activity, potentially leading to more substrate being converted into acetate. The injection ratio for fully consuming both CO₂ and H₂, accounting for C/H needs in biomass synthesis, is 1:3.78. Additionally, a larger maximum biomass capacity enables methanogens to quickly consume all CO₂ and H₂, reducing competition from other microbes. The study's findings provide valuable insights for site selection and optimal design in the implementation of UBM.