Recently, work in ethanol production is 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. To develop strategies for process optimization, a thermodynamic analysis was conducted that provided a detailed understanding of the favorability of the reactions along the metabolic pathway. Thermodynamic analysis provided identification of potentially limiting steps. Once these limiting reactions were identified, further thermodynamic analysis provided additional insights into the ways in which reaction conditions could be adjusted to improve product yield as well as minimize the effect of such bottlenecks. In this way, strategies to enhance product formation were effectively formed. A thermodynamic analysis regarding electron utilization suggested that it would be unlikely that H2 is utilized in favor of CO for electron production when both species are present. Therefore, CO conversion efficiency to products will be sacrificed during syngas fermentation since some of the CO will make electrons at the expense of product and cell mass formation. Furthermore, the analysis showed the thermodynamic difference of ethanol production, acetate production, and acetate to ethanol conversion, at varying reaction conditions, such as at different pH and redox potential levels. These differences were then incorporated into a strategy to optimize production of desired product, improve bioreactor design, and decrease the amount of by-product formed. Based on the thermodynamics analysis, experiments with varying experimental conditions were performed. The results showed that sulfide concentration in the media changed. In order to assess the effects of experimental conditions on syngas fermentation and decrease the experimental variability, experiments with controlled sulfide, redox potential, and pH were designed and the results indicated that these factors play key roles on cell growth, product formation and product distribution. Furthermore, experimental conditions had different effects on fermentation during different phases. For example, cell growth is much better at pH=5.8 than pH=4.5. However, the ethanol production rate at pH=4.5 is better than pH=5.8. A strategy involving controlling the pH and redox potential at different phases was effectively applied to improve ethanol production. This work provided significant insights on how varying experimental conditions can affect the syngas fermentation process.



College and Department

Ira A. Fulton College of Engineering and Technology; Chemical Engineering



Date Submitted


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ethanol, syngas, fermentation, Wood-Ljungdahl metabolic pathway, thermodynamics, redox potential, pH