Difference between revisions of "Gas fermentation"

| Product = Hydrocarbons, alcohols

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> Others 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).  

 

In the gas fermentation step by itself the pre-treated and often cooled syngas is compressed and sparged into a bioreactor with the gas-fermenting microorganisms in an aqueous medium. Depending on the specific technologies a multitude of variables are to account for during gas fermentation. The yield and purity of the desired product depend e.g. on the bioreactor design, agitation, gas composition and supply rate, pH, temperature, headspace pressure, oxidation-reduction potential (ORP), nutrients, and amount of foaming. As gas-fermenting microorganisms consume the gas, substrate availability can become rate-limiting and the bioreactor need to have a design that allows a high solubility of the gaseous substrates. In laboratory or small scale fermentation continuous stirred tank reactors (CSTR) offer excellent mixing and homogenous distribution of gas substrates to the microorganisms and are most commonly used. In industrial scale other types like bubble column, loop, and immobilized cell columns are preferred due to high energy demand of the stirring.<ref name=":0" />

After fermentation , product separation is required as post-treatment to separate the desired metabolic product from the fermentation broth. For this [[distillation]] systems are common to separate products such as ethanol and acetone. Other technologies to separate fermentation products from broth include liquid-liquid extraction, gas stripping, adsorption, perstraction, pervaporation, and vacuum distillation, and each of these separation technologies has their own benefits and drawbacks.<ref name=":0" />

[http://www.bbeu.org/pilotplant/ Bio Base Europe Pilot Plant (BBEPP)] is a flexible and diversified pilot plant for the development and scale up of new, bio-based and sustainable processes. It is capable of development of new bioprocesses, optimization of existing processes and scale up of a broad variety of bio-based processes up to an industrial level (from 5L to 50m3 scale, depending on the process). It can perform the entire value chain, from the green resources up to the final product.

BBEPP has built up a significant expertise on gas fermentation and cultivation of acetogenic and Knallgas bacteria through several private collaborations with Arcelor Mittal, e.g. Valorco project and in an ISPT project with Syngip, Arcelor Mittal and Dow. Although the content of these private projects is confidential, the general experience gained will be helpful in the work aimed for in the proposed work. In addition, BBEPP is also involved in several European funded consortium based gas fermentation projects. In the [https://biocon-co2.eu/ BIOCONCO2] project, BBEPP is responsible for the construction mobile gas fermentation unit to be put on site at waste gas emitters and to convert these CO₂-rich gases into chemical building blocks. In the [https://biosfera-project.eu/ BIOSFERA] project, biogenic residues and wastes will be gasified and the syngas will be fermented using acetogenic bacteria to produce acetate which will be converted in a second fermentation process to bio-based triacylglycerides (TAGs). In the CO2SMOS project, biogenic CO₂ emissions and renewable H₂ are converted by innovative biotechnologica land intensified chemical conversion process to develop the production of several bio-based fine and commodity chemicals (2,3-butanediol, long chain dicarboxylic acids, benzene, cyclic carbonates and polyhydroxyalkanoates).