Peng Hu, Dila Banjade, Spencer Bowen, and Randy S Lewis. Chemical Engineering, Brigham Young University, 331 CB, Provo, UT 84602
Currently, a major focus on ethanol production is via the fermentation of sugars following the saccharification of cellulosic biomass. However, recent work in ethanol production is also exploring the fermentation of syngas (primarily CO, CO2, and H2) following gasification of cellulosic biomass. The syngas fermentation by clostridium microbes utilizes the Wood-Ljungdahl metabolic pathway. Along this pathway, the intermediate Acetyl-CoA typically diverges to produce ethanol, acetic acid, and/or cell mass. Because main syngas constituents are used as carbon sources (CO, CO2) and electron donors (CO, H2) during fermentation, the effects of gas partial pressure, gas mass transfer rates, and redox potential of the media provide additional design constraints as compared to traditional sugar-based fermentation. A series of experiments with varying gas partial pressures and redox potentials were performed. The experimental results showed that increasing the CO2 partial pressure increased the rate of cell growth. Also, as the redox potential decreased, the rate of cell growth and acetic acid production increased. More interestingly, as the redox potential decreased to a certain level, conversion from acetic acid to ethanol was observed. To help address some of the findings, a thermodynamic analysis of the metabolic pathway was performed. The analysis showed that the conversion from CO2 to formate is very unfavorable, whereas other steps along the methyl branch are favorable. Moreover, the ratio of NADH/NAD+, which is related to the redox potential, greatly affects the Gibbs free energies of some reducing reactions along the metabolic pathway. Similarly, insights into the conversion of acetic acid to ethanol are obtained through thermodynamic analysis. Conclusively, this work provides significant insights on how varying syngas partial pressures from gasifiers (as a result of gasifier operation or varying biomass) and redox potential can affect the syngas fermentation process.
