Unraveling the effects of temperature on mass transfer and microbiology in thermophilic and extreme thermophilic trickle bed biomethanation reactors

Numerous initiatives are currently being initiated to substitute fossil fuels with renewable alternatives. Biomethanation is one of these emerging initiatives that presents a novel platform for valorizing carbon dioxide (CO2) to produce methane (CH4) by utilizing renewable hydrogen (H-2). Process temperature is a critical factor affecting CH4 productivity and selectivity, which previously has been ascribed solely to either biological or physicochemical changes. For the first time, this study demonstrated the temperatures effect on the intertwined biological, physicochemical, and process-engineering factors in novel trickle bed reactors (TBR). It was demonstrated that CH4 selectivity was enhanced by gradually ramping temperature from 55 degrees C to 70 degrees C resulting in 62 % reduction in acetate levels. However, further temperature increases > 70 degrees C deteriorated biocatalytic activity, for which the activity completely stopped at 85 degrees C. A comparative analysis of a thermophilic TBR (50 degrees C) and extreme-thermophilic TBR (70 degrees C) demonstrated 24.6 % improvement in CH4 productivity at 70 degrees C. Hereto, the effect of temperature on the H-2 gas-liquid mass transfer rate was modeled, which indicated an increasing trend in mass transfer up to 65.4 degrees C, whereafter the driving force became too impaired by reduced H-2 solubilities and elevated moisture content in the gas phase. A contribution of only 6.4 % enhancement in the CH4 productivity from 50 degrees C to 70 degrees C could be attributed to the increased H-2 mass transfer rate, which made the temperature effect on the biocatalyst the most pronounced factor for the enhanced process performance and selectivity. Hereto, Methanothermobacter was identified as the dominant CO2-fixing biocatalyst, and Acetomicrobiaceae as the major bacterial family correlating with acetate accumulation.