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Cellulosic ethanol can be made through fermentation of syngas (CO, H2 and CO2) produced from gasifying biomass. The use of low cost medium is critical to improve the feasibility of syngas fermentation. Alkalibaculum bacchi CP15 was found to be the most promising among three strains discovered recently in our laboratories for ethanol production. To reduce the cost of production medium, yeast extract (YE), minerals and vitamins were replaced with corn steep liquor (CSL). In addition, N-[Tris(hydroxymethyl)methyl]-3-aminopropanesulfonic acid (TAPS) buffer previously identified as being important for a better pH control during growth phase, was removed. Syngas fermentation was conducted with CP15 in 250-mL fermentation bottles with 20% CO, 15% CO2, 5% H2 and 60% N2 gas mixture. Treatments containing YE and various concentrations of CSL were investigated. The results showed that CP15 produced two times more ethanol using CSL than in the YE medium. In addition, over 94% of the medium cost was eliminated by replacing YE with CSL and elimination of TAPS. These results showed that using CSL enables the removal of TAPS, YE, minerals and vitamins.

Hydrogen and carbon monoxide, sparingly soluble gases are transferred to reactive sites inside cells during fermentation of synthesis gas to fuel and chemicals. The rate of conversion is set by either the transfer of gas into the cell or the kinetics of reaction inside the cell. Gas consumption, cell growth and production of ethanol and acetic acid from synthesis gas, primarily CO, H2 and CO2, by “Clostridium ragsdalei,” also called Clostridium strain P11 was followed to observe the interplay of kinetic and mass transfer limitations in the course of fermentation. Fermentations were studied under various conditions of pressure, cell concentration, agitation, gas composition, and mass transfer techniques. Mass transfer limitation was differentiated from kinetic limitation in batch bottle fermentation. Substrate uptake rate limit transitions from kinetic limitation with inhibition to mass transfer limitation in batch bottle fermentors. Fermentation during stationary growth phase was kinetic limited. Control strategy balancing mass transfer limitation and kinetic limitation of fermentation will be adapted to a stirred tank fermentor.

Fermentation medium was designed for production of ethanol and acetic acid from synthesis gas by “Clostridium ragsdalei”, also called Clostridium strain P11, which reduced the production cost by 95% compared to the standard medium. The medium was developed by serial deletion of components from the standard medium used for isolation and growth of the bacterium. Cost and purpose of individual components in the designed medium were considered to guide the revision of the medium recipe. This process resulted in the elimination of Morpholinoethanesulfonic acid (MES), a buffer used to maintain the pH near 6.0. Instead, a buffer was formed from the acetic acid produced during the fermentation and addition of bicarbonate, keeping the pH around 4.75 to enhance ethanol production. The performance of fermentation without MES, with pH control using acetate buffer was similar to that from the original rich medium. Additionally, yeast extract, an undefined growth promoter, was eliminated and trace metals for medium preparation were prepared in dilute solution without chelating agents. Fermentation without yeast extract resulted in lower growth, but comparable initial substrate uptake and production rates. Further, omission of cysteine from the medium and dependence on sulfide as nutrient sulfur source enhanced ethanol production, but did not sustain growth of strain P11. The control of pH in the designed medium and selection of appropriate sources of elemental nutrients is expected to enhance fermentation performance and further decrease cost.