Difference between revisions of "Gas fermentation"

<onlyinclude>A '''gas fermentation''' is an [[industrial fermentation]] process that uses a gaseous feedstock like methane, CO or CO₂, and together with hydrogen, are converted by a living organism to produce a specific product like ethanol or butanol. Gas fermentation requires organisms that are able to use these kind of feedstocks as main or single carbon source for their metabolism.</onlyinclude>

For a gas fermentation, gaseous carbon sources are used as a feedstock. They can be sourced from the [[gasification]] of various organic materials (e.g., woody biomass, municipal solid waste, MSW), or directly be taken from a gas source like reformed [[Anaerobic digestion|biogas]], an [[industrial fermentation|fermentation]], an industrial point source or directly from the atmosphere via a carbon capture technology.

The input gas stream, containing the main constituents CO, H₂, CO₂, can also contain impurities such as particulates, tar, BTEX (aromatics grouped as benzene, toluene, ethylene, xylenes), sulfur compounds (e.g., H₂S and COS), halogens, and other inhibiting gases. These are generated e.g., during [[gasification]] or [[pyrolysis]] and can be present in fluctuating quantities. Gas-fermenting microorganisms are able to grow in the presence of low levels of impurities, however, some impurities necessitate near complete removal. Particulates can be removed by cyclone separators and filters. Tars can be condensed and removed by quenching hot syngas, or can be reformed by heating at 800-900°C in accompaniment with [[heterogeneous catalysis]] using nickel or dolomite , generating additional syngas.  

A gas fermentation process depends on microorganisms that are able to digest gaseous carbon sources. Best known for this ability are acetogenic bacteria using the Wood-Ljungdahl pathway or acetyl-CoA pathway to fix and convert CO/CO₂ and H₂ to biomass and products. They are able to synthesize useful products such as ethanol, butanol and 2,3-butanediol within an anaerobic, oxygen-free atmosphere, fermentation setting. For commercial applications, mainly strains from ''Clostridium ljungdahlii'' and ''C. autoethanogenum'' are used.<ref name=":0" /><ref>{{Cite journal|title=Biotechnology for Chemical Production: Challenges and Opportunities|year=2016-03|author=Mark J. Burk, Stephen Van Dien|journal=Trends in Biotechnology|volume=34|issue=3|page=187–190|doi=10.1016/j.tibtech.2015.10.007}}</ref> Other acetogenic bacteria are in development as production organisms and there is a lot of activity in synthetic biology and genetic/metabolism engineering to modify these organisms. Additionally there are developments to integrate the metabolic pathways into well-known non-acetogenic organisms like ''Escherichia coli'' or yeasts to expand the options for fermentation processes.<ref name=":0" /> Besides, also aerobic bacteria can be used for gas fermentation. Aerobic hydrogen oxidizing bacteria, or Knallgas bacteria, are able to fix CO₂ using H₂ as the electron donor and O₂ as the terminal electron acceptor using the Calvin-Benson-Bassham (CBB) cycle. Interestingly, this metabolism allows high biomass production and the synthesis of more complex  products, including polyhydroxyalkanoates (PHA) bioplastics. As such, the Knallgas model organism ''Cupriavidus necator'', formerly known as ''Alcaligenes eutrophus'', can be used for the production of single-cell-protein (SCP) and natively accumulates polyhydroxybutyrate (PHB).  

Products from a gas fermentation depend on the organism used and its specific metabolism. Examples can be different kinds of alcohols like ethanol, butanol or isobutanol, but also organic acids, proteins, hydrogen or bio-based polymers like polyhydroxyalkanoates (PHAs).