Gas fermentation

Technology
Technology details
Name:	Gas fermentation
Category:	Conversion
Feedstock:	Gaseous carbon source (CO, CO2, methane)
Product:	Hydrocarbons, alcohols, bio-based polymers

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.

Feedstock

Origin and composition

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 biogas, an fermentation, an industrial point source or directly from the atmosphere via a carbon capture technology.

Pre-treatment

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.

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 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. 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. 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).

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.

In the gas fermentation step, 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 must be 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.

The design of a gas fermentation unit requires special attention as it involves the use of potentially explosive gas mixtures and toxic gases. With respect to the former, it should be in compliance with ATEX (ATmosphères EXplosibles) safety regulations. This is especially important in case of aerobic gas fermetentations, when a gas mixutre containing H2 (highly flammable) and O2 is sparged into the bioreactor.

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 polyhydroxyalkanoates (PHAs).

Post-treatment

Depending on the desired compound, specific down-stream processing technologies are required (see Industrial Fermentation). Moreover, more complex products can be produced in a coupled process, for example:

• A coupled fermentation process f.e. acetate fermentation coupled to lipids (TAGs) fermentation
• A coupled catalytic process f.e. ethanol fermentation coupled to Alcohol-to-Jet (AtJ) catalytic process

Technology providers

Technology comparison

Company name	Country	Technology category	Technology name	TRL	Capacity [kg/h]	Temperature [°C]	Aerobic fermentation	Anaerobic fermentation	Feedstock: Food waste	Feedstock: Garden & park waste	Feedstock: CO	Feedstock: CO2
Company 1	[Country HQ location]	[Technology category (if different sub-categories are defined this has to be specified here, the available categories can be found on each technology page under the chapter Process and technologies)]	[Technology name (the "branded name" or the usual naming from company side)]	[4-9]	[numeric value]		●	●	●	●	●	●
Company 2	[Country HQ location]	[(if different sub-categories are defined this has to be specified here, the available categories can be found on each technology page under the chapter Process and technologies)]	[Technology name (the "branded name" or the usual naming from company side)]	[4-9]	[numeric value]		●	●	●	●	●	●

Bio Base Europe Pilot Plant (BBEPP)

Gas fermentation provider

General information
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Technology and process details
Technology name:		Technology category:	Conversion (Biochemical processes and technologies)
TRL:		Capacity:	kg·h-1
Atmosphere:		Nutrients:
pH:		Pressure:	bar
Temperature:	°C	Other:
Feedstock and product details
Feedstock:		Product:

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 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 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 biotechnological and 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).

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