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.

The main products of syngas fermentation include ethanol, butanol, acetic acid, butyric acid, and methane.Worden, R.M., Bredwell, M.D., and Grethlein, A.J. (1997). Engineering issues in synthesis gas fermentations, Fuels and Chemicals from Biomass. Washington, DC: American Chemical Society, 321-335

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 {{chem|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.{{Cite journal | doi = 10.1002/bbb.256 | last1 = Abubackar | first1 = H.N. | last2 = Veiga | first2 = M. C. | last3 = Kennes | first3 = C. | year = 2011 | title = Biological conversion of carbon monoxide: rich syngas or waste gases to bioethanol | url = http://ruc.udc.es/dspace/bitstream/2183/13730/2/Veiga_MariaCarmen_2011_CO_bioethanol_bioproducts.pdf| journal = Biofuels, Bioproducts and Biorefining | volume = 5 | issue = 1| pages = 93–114 | hdl = 2183/13730 | s2cid = 84912109 | hdl-access = free }} In addition, incorporating nanoparticles has been demonstrated to improve gas-liquid fluid transfer during syngas fermentation. {{cite journal |last1=Sajeev |first1=Evelyn |last2=Shekher |first2=Sheshank |last3=Ogbaga |first3=Chukwuma C. |last4=Desongu |first4=Kwaghtaver S. |last5=Gunes |first5=Burcu |last6=Okolie |first6=Jude A. |title=Application of Nanoparticles in Bioreactors to Enhance Mass Transfer during Syngas Fermentation |journal=Encyclopedia |date=June 2023 |volume=3 |issue=2 |pages=387–395 |doi=10.3390/encyclopedia3020025 |doi-access=free }}

There are several microorganisms which can produce fuels and chemicals by syngas utilization. These microorganisms are mostly known as acetogens including Clostridium ljungdahlii,{{Cite journal | doi = 10.1016/0141-0229(92)90033-K | last1 = Klasson | first1 = K.T. | last2 = Ackerson | first2 = M. D. | last3 = Clausen | first3 = E. C. | last4 = Gaddy | first4 = J.L. | year = 1992 | title = Bioconversion of synthesis gas into liquid or gaseous fuels | journal = Enzyme and Microbial Technology | volume = 14 | issue = 8| pages = 602–608 }} Clostridium autoethanogenum,{{Cite journal | doi = 10.1007/BF00303591 | last1 = Abrini | first1 = J. | last2 = Naveau | first2 = H. | last3 = Nyns | first3 = E.J. | year = 1994 | title = Clostridium autoethanogenum, sp. nov., an anaerobic bacterium that produces ethanol from carbon monoxide | journal = Archives of Microbiology | volume = 161 | issue = 4| pages = 345–351 | s2cid = 206774310 }} Eubacterium limosum,{{Cite journal | doi = 10.1016/S0032-9592(01)00227-8 | last1 = Chang | first1 = I. S. | last2 = Kim | first2 = B. H. | last3 = Lovitt | first3 = R. W. | last4 = Bang | first4 = J. S. | year = 2001 | title = Effect of CO partial pressure on cell-recycled continuous CO fermentation by Eubacterium limosum KIST612 | journal = Process Biochemistry | volume = 37 | issue = 4| pages = 411–421 }} Clostridium carboxidivorans P7,{{Cite journal | doi = 10.1002/bit.21305 | last1 = Ahmed | first1 = A | last2 = Lewis | first2 = R.S. | year = 2007 | title = Fermentation of biomass generated syngas:Effect of nitric oxide | journal = Biotechnology and Bioengineering | volume = 97 | issue = 5| pages = 1080–1086 | pmid = 17171719 | s2cid = 21650852 }} Peptostreptococcus productus,{{Cite journal | last1 = Misoph | first1 = M. | last2 = Drake | first2 = H.L. | year = 1996 | title = Effect of CO2 on the fermentation capacities of the acetogen Peptostreptococcus productus U-1 | journal = Journal of Bacteriology | volume = 178 | issue = 11| pages = 3140–3145 | doi = 10.1128/jb.178.11.3140-3145.1996 | pmid = 8655492 | pmc = 178064 }} and Butyribacterium methylotrophicum.{{Cite journal | doi = 10.1016/j.copbio.2007.03.008 | last1 = Henstra | first1 = A.M. | last2 = Sipma | first2 = J. | last3 = Reinzma | first3 = A. | last4 = Stams | first4 = A.J.M. | year = 2007 | title = Microbiology of synthesis gas fermentation for biofuel production | journal = Current Opinion in Biotechnology | volume = 18 | issue = 3| pages = 200–206 | pmid = 17399976 }} 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 {{chem|H|2}}. On the other hand, syngas fermentation has limitations such as:

• Gas-liquid mass transfer limitation
• Low volumetric productivity
• Inhibition of organisms.