(534a) Enhanced Ethanol Production Via Activated Carbon Addition during Syngas Fermentation

Syngas made by gasification of biomass, coal, pet-coke and waste materials, or present in industrial gas waste streams can be used for production of fuels and chemicals. The biological conversion of syngas components, CO, CO₂ and H₂, to fuels and chemicals at commercial feasibility requires high conversion efficiency, product specificity and operation stability. Acetogenic bacteria convert CO, CO₂ and H₂ via the Wood-Ljungdahl pathway to acetic acid and ethanol as main products. The production of these products relies on efficient transfer of CO, CO₂ and H₂ to the cells at rates that match the bacterium’s kinetic capability to process the gas. Cells are inhibited when too much CO accumulates in the fermentation broth, which also decreases H₂ utilization. Batch syngas fermentations using Clostridium ragsdalei in a 3-L CSTR with and without activated carbon were compared. Fine powdered activated carbon particles were added to the fermentation broth to alter the gas mass transfer to the bacterium and provide additional surface for growth. Modeled syngas containing 38.0:28.5:28.5:5.0 of CO:H₂:CO₂:N₂ was used. Results showed that the addition of activated carbon sustained activity of C. ragsdalei in batch fermentations increasing total CO and H₂ uptake six fold compared to no carbon. The increased gas uptake with activated carbon produced 19 g/L ethanol with less than 1 g/L total acetic acid. However, only about 1 g/L ethanol and 5 g/L acetic acid were produced without activated carbon and cell concentrations sharply decreased after 158 h of fermentation with slight syngas consumption measured after 200 h. Alternatively, syngas consumption and cell activity were sustained during the fermentation with activated carbon for over 450 h. The maximum CO and H₂ conversion efficiencies in the fermenter with activated carbon were 88% and 86%, respectively. The operation of syngas fermentation with activated carbon exhibits higher stability, selectivity and energy conservation than any previously reported results, indicating potential for commercial biofuels production and other biological gas conversion processes.