H-B-E (hexanol-butanol-ethanol) fermentation for the production of higher alcohols from syngas/waste gas

CO, H-2, and CO2 are major components of syngas and some industrial CO-rich waste gases (e.g. waste gases from steel industries), besides some additional minor compounds. It was recently shown that those gases can be bioconverted, by acetogenic/solventogenic bacteria, into ethanol and higher alcohols such as butanol, but also hexanol, through the so-called HBE fermentation. That process presents some advantages over existing chemical conversion processes. This paper reviews HBE fermentation from Cl-gases after briefly describing the more conventional ABE(acetone-butanol-ethanol) fermentation from carbohydrates by Clostridium acetobutylicum, in order to allow for comparison of both processes. Although acetone may appear in carbohydrate fermentation, alcohols are the only major end-metabolites in the HBE process with Clostridium carboxidivorans. The few acetogenic bacteria known to metabolize Cl-gases and produce butanol or higher alcohols are described. Clostridium carboxidivorans has been used in most cases. Bioconversion of the gaseous substrates takes place in two stages, namely acidogenesis (production of acids) followed by solventogenesis (production of alcohols), characterized by different optimal fermentation conditions. Major parameters affecting each bioconversion stage as well as the overall fermentation process are analyzed. Although it has been claimed that acidification is required in ABE fermentation to initiate the solventogenic stage, strong acidification seems to some extent not to be a prerequisite for solventogenesis in the HBE process. Bioreactors potentially suitable for this type of bioconversion process are described as well. (C) 2017 Society of Chemical Industry