Studies on Syngas Fermentation With Clostridium carboxidivorans in Stirred-Tank Reactors With Defined Gas Impurities

Abstract

Syngas fermentation processes with acetogenic bacteria like Clostridium carboxidivorans have been proven to be a promising approach for the conversion of CO-rich waste gases into short- and medium-chain alcohols. The challenge of synthesis gas impurities, on the other hand, has always been a major concern for establishing an industrial-scale process, since some of the trace components in waste gases, such as NH3, H2S, and NOx, can have inhibiting or even toxic effects on microbial growth and product formation. Thus, this study aims to identify the effects of the main trace impurities in syngas from gasification of biogenic residues by the supply of defined concentrations of trace impurities to the cultivation medium. Autotrophic gas fermentation studies were performed with C. carboxidivorans in batch-operated fully-controlled stirred-tank bioreactors with continuous gas supply (80% CO and 20% CO2). The syngas components NH3 and H2S had a positive effect on both growth and alcohol formation (ethanol, 1-butanol, and 1-hexanol). The maximum biomass concentration was increased by more than 50%, and the maximum ethanol concentration was more than doubled with 5.0 g L−1 NH4Cl or 1.0 g L−1 H2S provided by the addition of 2.2 g L−1 thioacetamide. The addition of the nitrogen oxide species nitrate and nitrite, on the other hand, reduced biomass growth as well as alcohol concentrations. Already, the supply of 0.1 g L−1 NaNO3 resulted in reduced growth and 25% reduction of the maximum ethanol concentration. The production of the longer chain alcohols 1-butanol and 1-hexanol was reduced as well. All NaNO2 concentrations tested showed a strong toxic effect on the metabolism of C. carboxidivorans, and neither CO consumption nor product formation was observed after addition. As a consequence, NOx components in syngas from the gasification of biogenic residues should be reduced by the gasification process and/or selectively removed from the syngas after gasification.

Production of Hexanol as the Main Product Through Syngas Fermentation by Clostridium carboxidivorans P7

“…Using a continuous gas-fed bioreactor, Fernández-Naveira et al (2016) obtained 5.55 g/L of ethanol and 2.66 g/L of butanol; however, hexanol was not produced. Rückel et al (2021) reported the production of hexanol using a fermenter but the concentration was only 0.16 g/L. Shen et al (2017) optimized the composition of trace metals in the medium to enhance alcohol production and applied a timed control incubation temperature (37°C for 0–24 h, 25°C for 24–144 h), yielding a total alcohol titer of 6.97 with 1.67 g/L butanol and 1.33 g/L hexanol.…”

COxFixation to Elementary Building Blocks: Anaerobic Syngas Fermentation vs. Chemical Catalysis

“…The formation of the by-product trimethylamine is triggered when ammonia is present in syngas. For syngas fermentation, one study concludes that ammonia reduces cell growth and inhibits hydrogenase [169], while increased cell growth and ethanol formation are observed in another study [170].…”

“…Substrate inhibition can occur if too much gas is transferred into the liquid phase [137,162,[164][165][166][167][168]. Syngas impurities also influence the fermentation process [163,[169][170][171] (see Sect. 4.1).…”

Microbial electrosynthesis: carbonaceous electrode materials for CO2 conversion

“…The schematic of the production of various small-chain and long-chain chemicals from the CO2 conversion using MES and carbon biocathodes is shown in Figure 5. 111 , where a gas-liquid mass transfer can be boosted through increasing impeller speed, which separates the gas stream into smaller bubbles with a higher specific surface area 112 . Acetate is among the most widely prepared small chain value-added products from CO2 conversion in MES.…”