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Syngas (from synthesis gas) is the name given to a gas mixture that contains varying amounts of carbon monoxide and hydrogen generated by the gasification of a carbon-containing fuel to a gaseous product with a heating value. Examples include steam reforming of natural gas or liquid hydrocarbons to produce hydrogen, the gasification of coal^[1] and in some types of waste-to-energy gasification facilities. The name comes from their use as intermediates in creating synthetic natural gas (SNG)^[2] and for producing ammonia or methanol. Syngas is also used as an intermediate in producing synthetic petroleum for use as a fuel or lubricant via Fischer-Tropsch synthesis and previously the Mobil methanol to gasoline process.

Syngas consists primarily of carbon monoxide, carbon dioxide and hydrogen, and has less than half the energy density of natural gas. Syngas is combustible and often used as a fuel source or as an intermediate for the production of other chemicals.

Production

Syngas for use as a fuel is most often produced by gasification of coal or municipal waste mainly by the following paths:

C + H₂O → CO + H₂

C + O₂ → CO₂

CO₂ + C → 2CO

When used as an intermediate in the large-scale, industrial synthesis of hydrogen and ammonia, it is also produced from natural gas (via the steam reforming reaction) as follows:

CH₄ + H₂O → CO + 3 H₂

The syngas produced in large waste-to-energy gasification facilities is used as fuel to generate electricity.^[3]

Coal gasification processes are reasonably efficient and were used for many years to manufacture illuminating gas (coal gas) for gas lighting, before electric lighting became widely available.

Purification

When syngas contains a significant amount of nitrogen, the nitrogen must be removed. Cryogenic processing has great difficulty in recovering pure carbon monoxide when relatively large volumes of nitrogen are present due to carbon monoxide and nitrogen having very similar boiling points which are -191.5 °C and -195.79 °C respectively. Certain process technology selectively removes carbon monoxide by complexation/decomplexation of carbon monoxide with cuprous aluminum chloride (CuAlCl₄), dissolved in an organic liquid such as toluene. The purified carbon monoxide can have a purity greater than 99%, which makes it a good feedstock for the chemical industry. The reject gas from the system can contain carbon dioxide, nitrogen, methane, ethane and hydrogen. The reject gas can be further processed on a pressure swing absorption system to remove hydrogen and the hydrogen and carbon dioxide can be recombined in the proper ratio for methanol production, Fischer-Tropsch diesel etc. However, the total energy efficiency is not very high, if the gas is used to make fuel, meaning that the purification processes are very energy intensive.

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].

There are several microorganisms which can produce fuels and chemicals by syngas utilization. These microorganisms are mostly known as acetogens including Clostridium ljungdahlii^[3], Clostridium autoethanogenum^[4], Eurobacterium limosum^[5], Clostridium carboxidivorans P7^[6], Peptostreptococcus products^[7], and Butyribacterium methylotrophicum^[8].

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 CO:H₂^[2]. On the other hand, syngas fermentation has limitations such as:

• Gas-liquid mass transfer limitation^[8]
• Low volumetric productivity, and
• Inhibition of organisms ^[1][2].

Footnotes

1. ^ ^{a b} (Brown, 2003)
2. ^ ^{a b c} 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
3. ^ Klasson, K.T., Ackerson, M. D., Clausen, E. C., and Gaddy, J.L. (1992). Bioconversion of synthesis gas into liquid or gaseous fuels. Enzyme and Microbial Technology, 14(8), 602-608.
4. ^ Abrini, J., Naveau, H., and Nyns, E.J. (1994). Clostridium autoethanogenum, Sp-Nov, an Anaerobic bacterium that produces ethanol from carbon monoxide. Archives of Microbiology, 161(4), 345-351.">
5. ^ Chang, I. S., Kim, B. H., Lovitt, R. W., and Bang, J. S. (2001). Effect of CO partial pressure on cell-recycled continuous CO fermentation by Eurobacterium limosum KIST612. Process Biochemistry, 37(4), 411-421..
6. ^ Ahmed, A, and Lewis, R.S. (2007). Fermentation of biomass generated syngas:Effect of nitric oxide. Biotechnology and Bioengineering, 97(5), 1080-1086.
7. ^ Misoph, M., and Drake, H.L. (1996). Effect of CO₂ on the fermentation capacities of the acetogen Peptostreptococus products U-1. Journal of Bacteriology, 178(11), 3140-3145.
8. ^ ^{a b} Henstra, A.M., Sipma, J., Reinzma, A., and Stams, A.J.M. (2007). Microbiology of synthesis gas fermentation for biofuel production. Current Opinion in Biotechnology, 18(3), 200-206

References

• Brown, Robert F. (2003). Biorenewable resources: engineering new products from agriculture. Ames, Iowa: Iowa State Press. ISBN 0-8138-2263-7. 

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External links

• Fischer Tropsch archive