Comparison of Syngas-Fermenting Clostridia in Stirred-Tank Bioreactors and the Effects of Varying Syngas Impurities

In recent years, syngas fermentation has emerged as a promising means for the production of fuels and platform chemicals, with a variety of acetogens efficiently converting CO-rich gases to ethanol. However, the feasibility of syngas fermentation processes is related to the occurrence of syngas impurities such as NH₃, H₂S, and NO_X. Therefore, the effects of defined additions of NH₄⁺, H₂S, and NO₃⁻ were studied in autotrophic batch processes with C. autoethanogenum, C. ljungdahlii, and C. ragsdalei while applying continuously gassed stirred-tank bioreactors. Any initial addition of ammonium and nitrate curbed the cell growth of the Clostridia being studied and reduced the final alcohol concentrations. C. ljungdahlii showed the highest tolerance to ammonium and nitrate, whereas C. ragsdalei was even positively influenced by the presence of 0.1 g L⁻¹ H₂S. Quantitative goals for the purification of syngas were identified for each of the acetogens studied in the used experimental setup. Syngas purification should in particular focus on the NO_X impurities that caused the highest inhibiting effect and maintain the concentrations of NH₃ and H₂S within an acceptable range (e.g., NH₃ < 4560 ppm and H₂S < 108 ppm) in order to avoid inhibition through the accumulation of these impurities in the bioreactor.