Carbon Sequestration Through Syngas Biomethanation Coupled with H2 Supply for a Clean Production of Natural Gas Grade Biomethane

Biomass gasification generates a gas mixture (syngas) that constitutes a rich source of carbon and energy for the production of second-generation renewable fuels such as biomethane. However, the produced syngas composition (H₂/CO/CO₂) cannot be converted to natural gas grade biomethane due to stoichiometric limitations, and a waste stream of CO₂ is released unexploited in the atmosphere. The present study introduces the concept of biomass gasification coupled to syngas biomethanation with in-situ exogenous H₂ supply for the complete sequestration of the syngas carbon, the valorization of renewable excess electricity from wind and solar power and the production of biomethane satisfying the criteria for injection in the natural gas grid. Syngas biomethanation was executed by mixed microbial consortia in a trickle bed reactor at 37 °C and 60 °C. The assessment of the effects of the net inlet gas composition was performed according to a hereby proposed syngas quality index ([Formula: see text]), which is based on the syngas content in compounds able to act as carbon and electron donors and the stoichiometry of methane production. The SQI of the stoichiometrically ideal syngas composition is 4. Values below 4 correspond to a stoichiometric carbon-moles excess while values above 4 correspond to a stoichiometric electron-moles excess. It was demonstrated that switching the SQI of the supplied syngas from 1.44 to 3.67 increased the CH₄ content in the outlet of the reactor from 30 to 72%, accompanied by an at least 1.2-fold increase of the CH₄ productivity. A SQI of 4.78 (> 4) resulted in a significant deterioration of the quality of the produced biomethane due to a high content (52–54%) of unconverted H₂ and because of thermodynamic limitations on carboxydotrophic hydrogenogenesis in thermophilic conditions. Maximal carbon sequestration and production of natural gas grade biomethane was shown to be feasible at a SQI = 3.98 in thermophilic conditions.