In-situ biomethanation of high CO content syngas in agricultural biogas digesters

Abstract

To meet global energy demands, syngas with high CO content (>50%) generated from hydrothermal gasification of agricultural residues or wood, could be utilized as a co-substrate in anaerobic digestion (AD) processes for conversion into CH4. This study investigated in-situ biomethanation using synthetic gas (55% CO, 35% H2, and 15% CO2), mimicking wood gasification syngas, during AD of cow manure, the most abundant agricultural waste. A continuously stirred tank reactor (CSTR) served as a control and was fed only cow manure, while the other two reactors were fed syngas with or without gas recirculation at different gas loading rates (GLRs).In-situ syngas injection in AD of cow manure at GLRs ≤ 0.3 L LRV-1 d-1 improved overall CH4 productivity compared to the control (0.35±0.1 vs. 0.29±0.1 L LRV-1 d-1), though the CH4 content (45±5%) was lower than the control reactor (62±3%), indicating a need for biogas upgrading. Gas recirculation enabled higher CO and H2 conversion efficiencies of 99-100% but considerably decreased CH4 productivity from cow manure. In-situ biomethanation enhanced the relative abundance of hydrogenotrophic methanogens, highlighting their role in syngas conversion to CH4.