Fermentation of H2 and CO2 with Clostridium ljungdahlii at Elevated Process Pressure – First Experimental Results

Fermentation of synthesis gas mixtures (H2, CO and CO2) with anaerobic bacteria acting as a biocatalyst is a promising process for the production of fuels and chemicals with first large-scale applications. A known bottleneck in gas fermentation is gas-liquid mass transfer of low soluble gas components like H2 and CO. One possibility to overcome these limitations could be the elevation of process pressure. Clostridium ljungdahlii is a known acetogenic bacterium for the production of acetic acid and ethanol, well investigated for the fermentation of synthesis gas at ambient pressure. Aim of the current experiments was to study the fermentation of a H2/CO2-mixture (without CO) at elevated process pressure. The 90 h-experiments were performed in a high-pressure experimental set-up, using a 4 L-semi-batch stirred tank reactor. Fermentation temperature was 37 °C and pH was constantly kept at 5.9. Total process pressure was set up to 7 bar absolute. As pressure increased, reactor cell concentrations at the end of the experiments were lower than at atmospheric conditions. Product distribution changed with pressure elevation. The concentration of the expected fermentation products, ethanol and especially acetate, decreased. Instead, the production of formatewith a mean concentration up to 98 mmol L-1 was observed. At a pressure of 7 bar, formate became the mainproduct of H2/CO2-fermentation.