Syngas, orsynthesis gas, is a mixture ofhydrogen andcarbon monoxide,[1] in various ratios. The gas often contains somecarbon dioxide andmethane. It is principally used for producingammonia ormethanol. Syngas is combustible and can be used as a fuel.[2][3][4] Historically, it has been used as a replacement forgasoline, when gasoline supply has been limited; for example,wood gas was used to power cars in Europe duringWWII (in Germany alone, half a million cars were built or rebuilt to run on wood gas).[5]
Steam reforming of methane is anendothermic reaction requiring 206 kJ/mol of energy:
CH4 + H2O → CO + 3 H2
In principle, but rarely in practice,biomass and related hydrocarbon feedstocks could be used to generatebiogas andbiochar inwaste-to-energy gasification facilities.[7] The gas generated (mostly methane and carbon dioxide) is sometimes described assyngas but its composition differs from syngas. Generation of conventional syngas (mostly H2 and CO) from waste biomass has been explored.[8][9]
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.[10] Syngas has less than half theenergy density ofnatural gas.[11]
The first reaction, between incandescentcoke and steam, is strongly endothermic, producing carbon monoxide (CO) and hydrogenH 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 anexothermic 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 highercalorific value.
In order to produce more hydrogen from this mixture, more steam is added and thewater gas shift reaction is carried out:
CO + H2O → CO2 + H2
The hydrogen can be separated from the CO2 bypressure swing adsorption (PSA),amine scrubbing, andmembrane reactors. A variety of alternative technologies have been investigated, but none are of commercial value.[12] Some variations focus on new stoichiometries such as carbon dioxide plus methane[13][14] or partialhydrogenation of carbon dioxide. Other research focuses on novel energy sources to drive the processes including electrolysis, solar energy, microwaves, and electric arcs.[15][16][17][18][19][20]
Electricity generated fromrenewable sources is also used to process carbon dioxide and water into syngas throughhigh-temperature electrolysis. This is an attempt to maintaincarbon neutrality in the generation process.Audi, in partnership with company named Sunfire, opened a pilot plant in November 2014 to generatee-diesel using this process.[21]
Syngas that is not methanized typically has a lower heating value of 120 BTU/scf.[22] Untreated syngas can be run in hybrid turbines that allow for greater efficiency because of their lower operating temperatures, and extended part lifetime.[22]
^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.S2CID94791479.
^"Syngas composition". National Energy Technology Laboratory, U.S. Department of Energy.Archived from the original on 27 March 2020. Retrieved7 May 2015.
^Beychok, M R (1975).Process and environmental technology for producing SNG and liquid fuels. Environmental Protection Agency.OCLC4435004117.OSTI5364207.[page needed]
^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.PMID27714905.
^US patent 5159900A, Dammann, Wilbur A., "Method and means of generating gas from water for use as a fuel", issued 3 November 1992
^abOluyede, 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.CiteSeerX10.1.1.205.6065.doi:10.1115/GT2007-27385.ISBN978-0-7918-4792-3.
