Bio-syngas as ethanol fermentation substrate: Process evaluation

Syngas fermentation is being reported as alternative to carbohydrate fermentation, to obtain products such as ethanol [Daniell et al. (2012) Energies 5, 5372]. The syngas may be obtained by biomass gasification. Certain species, a.o. Clostridia, have been reported to be capable to convert syngas into ethanol [Abubackar et al. (2011) Biofuels. Bioprod. Bioref. 5, 93]. Potential benefits of this process as compared to lignocellulosic ethanol fermentation are: high yield on feedstock since lignin is also used; wider feedstock flexibility; no cellulosic enzymes needed; and no furanics and phenolics that inhibit fermentation.

To assess the technical feasibility and economic potential of such a process, it is conceptually designed on a 100,000 t/a scale. Due to the poor solubility of CO and H₂ in water, it is assumed that the mass transfer in the fermentor will be the major process rate limiting factor [Munasinghe and Khanal (2010) Bioresour. Technol. 101, 5013]. Estimates show that mega-fermentor sizes are required when using stirred tanks. Alternative bioreactors are designed. One extra degree of freedom is obtained when combining fermentation with the water-gas shift reaction before fermentation step, in order to manipulate the CO/H₂ ratio in the fermentation inlet gas. Another option considered is to use thermophilic organisms at high temperature process to transfer ethanol to the gas phase during the fermentation. This facilitates cell retention and reduces downstream processing costs.

 Initial estimates show that the lowest capital investments are obtained when using H₂-rich syngas.