(235f) Drop-in Biofuels Production Using Mixed Culture Syngas Fermentation

Production of transportation fuels from readily available renewable resources without major changes in existing infrastructure is essential to minimizing the costs of shifting from petroleum to sustainable fuels. The development of the corn-based ethanol industry has brought ethanol into the mainstream of transportation fuels. However, ethanol only contains two-thirds of the energy of gasoline and is considered incompatible with the existing network of gasoline pipelines due to its affinity for water. Production of "drop-in" biofuels, such as n-butanol and n-hexanol, is more suitable than ethanol. These higher alcohols have greater energy density than ethanol, are more compatible with current fuel infrastructure, and can be converted to jet fuels and chemicals.  The focus of this study is on the production of n-butanol and n-hexanol through hybrid gasification-syngas fermentation technology. This hybrid conversion process begins with the gasification of biomass to syngas (CO, CO₂ and H₂) followed by the conversion of syngas to liquid fuels and chemicals using acetogenic microorganisms. Syngas fermentations with a monoculture of Alkalibaculum bacchi CP15 and a mixed culture were examined for their ability to produce higher alcohols from syngas and to convert organic acids into alcohols. Syngas (40% CO, 30% CO₂ and 30% H₂) was fed to the microorganisms every 24 h at 240 kPa. Without the initial addition of organic acids to the fermentation medium, the monoculture of CP15 only produced ethanol. However, the mixed culture produced ethanol, propanol and butanol from syngas. When propionic acid, butyric acid, and hexanoic acid were initially added in the fermentation medium, the mixed culture was 50% more efficient in converting the acids to their respective alcohols than a monoculture of CP15. There was a synergy in the mixed culture that resulted in over 60% more alcohol production than the monoculture of CP15. These results show the advantages of using mixed culture over monoculture for production of drop-in biofuels from syngas, which could have potential in commercial applications.