Gas fermentation is a biological process where microorganisms convert gaseous feedstocks into valuable products. This technology contributes to sustainable industrial practices. It offers a pathway to produce fuels and chemicals from non-traditional carbon sources, reducing reliance on fossil fuels.
What is Gas Fermentation
Gas fermentation differs from traditional liquid fermentation by using gases as its primary carbon and energy sources. Conventional fermentation typically uses sugars or other liquid biomass as feedstock. It converts gaseous mixtures containing carbon monoxide (CO), carbon dioxide (CO2), and hydrogen (H2) into various compounds through microbial conversion.
These gases are abundant, often found as industrial waste streams or from the gasification of organic materials like municipal solid waste or agricultural residues. Utilizing these gaseous feedstocks allows gas fermentation to bypass resource-intensive pretreatment steps associated with solid biomass. Hydrogen can be added as an additional energy source, especially when CO2 is the sole carbon source.
How Gas Fermentation Works
Gas fermentation relies on specific anaerobic microorganisms, primarily acetogenic bacteria. These microbes, such as Clostridium ljungdahlii and Clostridium autoethanogenum, thrive in oxygen-free environments and directly convert gaseous substrates. They use a unique biochemical pathway known as the Wood-Ljungdahl pathway.
In the Wood-Ljungdahl pathway, carbon dioxide is reduced to carbon monoxide or a formyl group. This formyl group is then reduced to a methyl group, which combines with carbon monoxide and coenzyme A to produce acetyl-CoA. Enzymes like CO dehydrogenase catalyze CO2 reduction, while acetyl-CoA synthase combines CO with a methyl group to yield acetyl-CoA. This pathway allows microorganisms to fix carbon and generate energy for growth, producing various organic compounds. The process occurs in bioreactors where treated syngas is sparged into an aqueous medium containing the microorganisms.
Key Products and Applications
Gas fermentation yields biofuels and various industrial chemicals. Common outputs are ethanol, butanol, acetic acid, and butyric acid. These products have applications across different sectors. Ethanol, for instance, is widely used as a biofuel, while acetic acid serves as a chemical precursor.
The technology also produces other compounds like 2,3-butanediol, hexanoic acid, hexanol, and bio-based polymers such as poly-hydroxyalkanoates (PHAs). It can also produce single-cell proteins for animal feed and alternative protein sources. Commercial-scale ethanol production from gas has been demonstrated, showing the technology’s readiness for industrial adoption.
Environmental Benefits
Gas fermentation offers environmental advantages, contributing to carbon capture and utilization (CCU) and promoting a circular economy. It converts industrial waste gases from steel mills, petroleum refining, or chemical plants, which contain carbon monoxide and carbon dioxide, into valuable products. This process reduces greenhouse gas emissions that would otherwise be released.
By utilizing waste carbon streams, gas fermentation helps mitigate climate change and lessens industry’s reliance on new fossil resources for chemical production. The technology also supports sustainable manufacturing by transforming pollutants into useful materials, creating a more closed-loop system for industrial processes. It has a minimal land footprint and does not compete with agricultural land for feedstock.