Gas fermentation

Technology
Technology details
Name:	Gas fermentation
Category:
Feedstock:	Gaseous carbon source (CO, CO2, methane)
Product:	several

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

Feedstock

For a gas fermentation gaseous carbon sources are used as a feedstock. They can be delivered from a gasification of biomass or other organic materials (e.g. municipal solid waste, MSW) or directly be taken from a gas source like a biogas production, an fermentation, an industrial point source or directly from the atmosphere via a carbon capture technology.

Process and technologies

Production organisms

The reductive acetyl–CoA pathway

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 hydrogen to biomass and products. They are able to synthesize useful products such as ethanol, butanol and 2,3-butanediol and they are anaerobes so need to be used in an anaerobic, oxygen-free atmosphere, fermentation setting. For commercial applications, mainly strains from Clostridium ljungdahlii and C. autoethanogenum are used.

Fermentation technology

The overall gas fermentation process can be divided into four steps:

1. accumulation or generation of syngas
2. gas pretreatment
3. gas fermentation in a bioreactor
4. product separation.

Depending on the gasified feedstock the syngas can have several impurities that lowers the productivity of the fermentation process or are toxic to the organisms. Even with gas-fermenting microorganisms' abilities to grow in the presence of low levels of impurities, some impurities necessitate near complete removal in a gas treatment by cyclone separators and filters from an operational, biological and/or product specificity perspective.

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.

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.

Product

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 polyhydroxyalcanoates (PHAs).

Patents

Currently no patents have been identified yet.