Syngas

Syngas, or synthesis gas, is a mixture of hydrogen and carbon monoxide, in various ratios. The gas often contains some carbon dioxide and methane. It is principally used for producing ammonia or methanol. Syngas is combustible and can be used as a fuel.[1][2][3] Historically, it has been used as a replacement for gasoline, when gasoline supply has been limited; for example, wood gas was used to power cars in Europe during WWII (in Germany alone half a million cars were built or rebuilt to run on wood gas).[4]

Not to be confused with synthetic gasoline.

Production

Syngas is produced by steam reforming or partial oxidation of natural gas or liquid hydrocarbons, or coal gasification.[5] Steam reforming of methane is an endothermic reaction requiring 206 kJ/mol of methane:

CH4 + H2O → CO + 3 H2

In principle, but rarely in practice, biomass and related hydrocarbon feedstocks could be used to generate biogas and biochar in waste-to-energy gasification facilities.[6] The gas generated (mostly methane and carbon dioxide) is sometimes described as syngas but its composition differs from syngas. Generation of conventional syngas (mostly H2 and CO) from waste biomass has been explored.[7]

Composition, pathway for formation, and thermochemistry

The chemical composition of syngas varies based on the raw materials and the processes. Syngas produced by coal gasification generally is a mixture of 30 to 60% carbon monoxide, 25 to 30% hydrogen, 5 to 15% carbon dioxide, and 0 to 5% methane. It also contains lesser amount of other gases.[8] Syngas has less than half the energy density of natural gas.[9]

The first reaction, between incandescent coke and steam, is strongly endothermic, producing carbon monoxide (CO), and hydrogen H
2 (water gas in older terminology). When the coke bed has cooled to a temperature at which the endothermic reaction can no longer proceed, the steam is then replaced by a blast of air.

The second and third reactions then take place, producing an exothermic reaction—forming initially carbon dioxide and raising the temperature of the coke bed—followed by the second endothermic reaction, in which the latter is converted to carbon monoxide. The overall reaction is exothermic, forming "producer gas" (older terminology). Steam can then be re-injected, then air etc., to give an endless series of cycles until the coke is finally consumed. Producer gas has a much lower energy value, relative to water gas, due primarily to dilution with atmospheric nitrogen. Pure oxygen can be substituted for air to avoid the dilution effect, producing gas of much higher calorific value.

In order to produce more hydrogen from this mixture, more steam is added and the water gas shift reaction is carried out:

CO + H2O → CO2 + H2

The hydrogen can be separated from the CO2 by pressure swing adsorption (PSA), amine scrubbing, and membrane reactors. A variety of alternative technologies have been investigated, but none are of commercial value.[10] Some variations focus on new stoichiometries such as carbon dioxide plus methane[11][12] or partial hydrogenation of carbon dioxide. Other research focuses on novel energy sources to drive the processes including electrolysis, solar energy, microwaves, and electric arcs.[13][14][15][16][17][18]

Electricity generated from renewable sources is also used to process carbon dioxide and water into syngas through high-temperature electrolysis. This is an attempt to maintain carbon neutrality in the generation process. Audi, in partnership with company named Sunfire, opened a pilot plant in November 2014 to generate e-diesel using this process.[19]

Syngas that is not methanized typically has a lower heating value of 120 BTU/scf .[20] Untreated syngas can be run in hybrid turbines that allow for greater efficiency because of their lower operating temperatures, and extended part lifetime.[20]

Uses

Syngas is used as a source of hydrogen as well as a fuel.[10] It is also used to directly reduce iron ore to sponge iron.[21] Chemical uses include the production of methanol which is a precursor to acetic acid and many acetates; liquid fuels and lubricants via the Fischer–Tropsch process and previously the Mobil methanol to gasoline process; ammonia via the Haber process, which converts atmospheric nitrogen (N2) into ammonia which is used as a fertilizer; and oxo alcohols via an intermediate aldehyde.

See also

References
  1. "Syngas Cogeneration / Combined Heat & Power". Clarke Energy. Archived from the original on 27 August 2012. Retrieved 22 February 2016.
  2. Mick, Jason (3 March 2010). "Why Let it go to Waste? Enerkem Leaps Ahead With Trash-to-Gas Plans". DailyTech. Archived from the original on 4 March 2016. Retrieved 22 February 2016.
  3. Boehman, André L.; Le Corre, Olivier (15 May 2008). "Combustion of Syngas in Internal Combustion Engines". Combustion Science and Technology. 180 (6): 1193–1206. doi:10.1080/00102200801963417. S2CID 94791479.
  4. "Wood gas vehicles: firewood in the fuel tank". LOW-TECH MAGAZINE. Archived from the original on 2010-01-21. Retrieved 2019-06-13.
  5. Beychok, Milton R. (1974). "Coal gasification and the Phenosolvan process" (PDF). Am. Chem. Soc., Div. Fuel Chem., Prepr.; (United States). 19 (5). OSTI 7362109. S2CID 93526789. Archived from the original (PDF) on 3 March 2016.
  6. "Sewage treatment plant smells success in synthetic gas trial - ARENAWIRE". Australian Renewable Energy Agency. 11 September 2019. Archived from the original on 2021-03-07. Retrieved 2021-01-25.
  7. Zhang, Lu; et al. (2018). "Clean synthesis gas production from municipal solid waste via catalytic gasification and reforming technology". Catalysis Today. 318: 39–45. doi:10.1016/j.cattod.2018.02.050. ISSN 0920-5861. S2CID 102872424.
  8. "Syngas composition". National Energy Technology Laboratory, U.S. Department of Energy. Archived from the original on 27 March 2020. Retrieved 7 May 2015.
  9. Beychok, M R (1975). Process and environmental technology for producing SNG and liquid fuels. Environmental Protection Agency. OCLC 4435004117. OSTI 5364207.
  10. Hiller, Heinz; Reimert, Rainer; Stönner, Hans-Martin (2011). "Gas Production, 1. Introduction". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a12_169.pub3. ISBN 978-3527306732.
  11. "dieBrennstoffzelle.de - Kvaerner-Verfahren". www.diebrennstoffzelle.de. Archived from the original on 2019-12-07. Retrieved 2019-12-17.
  12. EU patent 3160899B1, Kühl, Olaf, "Method and apparatus for producing h2-rich synthesis gas", issued 12 December 2018
  13. "Sunshine to Petrol" (PDF). Sandia National Laboratories. Archived from the original (PDF) on February 19, 2013. Retrieved April 11, 2013.
  14. "Integrated Solar Thermochemical Reaction System". U.S. Department of Energy. Archived from the original on August 19, 2013. Retrieved April 11, 2013.
  15. Matthew L. Wald (April 10, 2013). "New Solar Process Gets More Out of Natural Gas". The New York Times. Archived from the original on November 30, 2020. Retrieved April 11, 2013.
  16. Frances White. "A solar booster shot for natural gas power plants". Pacific Northwest National Laboratory. Archived from the original on April 14, 2013. Retrieved April 12, 2013.
  17. Foit, Severin R.; Vinke, Izaak C.; de Haart, Lambertus G. J.; Eichel, Rüdiger-A. (8 May 2017). "Power-to-Syngas: An Enabling Technology for the Transition of the Energy System?". Angewandte Chemie International Edition. 56 (20): 5402–5411. doi:10.1002/anie.201607552. PMID 27714905.
  18. US patent 5159900A, Dammann, Wilbur A., "Method and means of generating gas from water for use as a fuel", issued 3 November 1992
  19. "Audi in new e-fuels project: synthetic diesel from water, air-captured CO2 and green electricity; "Blue Crude"". Green Car Congress. 14 November 2014. Archived from the original on 27 March 2020. Retrieved 29 April 2015.
  20. Oluyede, Emmanuel O.; Phillips, Jeffrey N. (May 2007). "Fundamental Impact of Firing Syngas in Gas Turbines". Volume 3: Turbo Expo 2007. Proceedings of the ASME Turbo Expo 2007: Power for Land, Sea, and Air. Volume 3: Turbo Expo 2007. Montreal, Canada: ASME. pp. 175–182. CiteSeerX 10.1.1.205.6065. doi:10.1115/GT2007-27385. ISBN 978-0-7918-4792-3.
  21. Chatterjee, Amit (2012). Sponge iron production by direct reduction of iron oxide. PHI Learning. ISBN 978-81-203-4659-8. OCLC 1075942093.
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