Syngas fermentation

Syngas fermentation, also known as synthesis gas fermentation, is a microbial process. In this process, a mixture of hydrogen, carbon monoxide, and carbon dioxide, known as syngas, is used as carbon and energy sources, and then converted into fuel and chemicals by microorganisms.[1]

The main products of syngas fermentation include ethanol, butanol, acetic acid, butyric acid, and methane.[2] Certain industrial processes, such as petroleum refining, steel milling, and methods for producing carbon black, coke, ammonia, and methanol, discharge enormous amounts of waste gases containing mainly CO and H
₂ into the atmosphere either directly or through combustion. Biocatalysts can be exploited to convert these waste gases to chemicals and fuels as, for example, ethanol.[3] In addition, incorporating nanoparticles has been demonstrated to improve gas-liquid fluid transfer during syngas fermentation. [4]

There are several microorganisms which can produce fuels and chemicals by syngas utilization. These microorganisms are mostly known as acetogens including Clostridium ljungdahlii,[5] Clostridium autoethanogenum,[6] Eubacterium limosum,[7] Clostridium carboxidivorans P7,[8] Peptostreptococcus productus,[9] and Butyribacterium methylotrophicum.[10] Most use the Wood–Ljungdahl pathway.

Syngas fermentation process has advantages over a chemical process since it takes places at lower temperature and pressure, has higher reaction specificity, tolerates higher amounts of sulfur compounds, and does not require a specific ratio of CO to H
₂.[2] On the other hand, syngas fermentation has limitations such as:

• Gas-liquid mass transfer limitation[10]
• Low volumetric productivity
• Inhibition of organisms.[1][2]