The foregoing figure illustrated the concept of coal gasification, and noted resulting composition of syngas. This can vary significantly depending on the feedstock and the gasification process involved; however typically syngas is 30 to 60% CO, 25 to 30% H2, 0 to 5% methane (CH4), 5 to 15% carbon dioxide (CO2), plus a lesser or greater amount of water vapor, smaller amounts of the sulfur compounds hydrogen sulfide (H2S), carbonyl sulfide (COS), and finally some ammonia and other trace contaminants.
Wabash River Syngas Composition
The following table shows the operational syngas composition for the Wabash River Coal Gasification Repowering Project over the four year demonstration period from 1996 through 1999.
|Carbon Dioxide, %
|Carbin Monoxide, %
|Hydrogen Sulfide, ppmv
|Carbonyl Sulfide, ppmv
|Heat of Combustion, Btu/sef (HHV)
The Wabash facility, using E-Gas™ gasification technology, operated on a variety of fuels over this period including two different types of coal and petroleum coke. Syngas composition remained relatively constant despite changes in coal composition. Although the gasifier is capable of handling a wide range of feedstocks, variations from the coal used as the design basis for the system can reduce syngas and steam production (differences in production will depend on the coal feed and how it differs from the design feed). Sudden changes in feedstock can also cause disruptions in other plant processes. The syngas H2:CO ratio is relatively high (>0.7), typical of a slurry-feed gasification system. The high levels of COS during the earlier years of operation were a result of problems with the COS-hydrolysis unit (specifically, catalyst poisoning by trace metals and chlorides and surface area degradation by overheating). These problems were remedied over the course of the project1.
Syngas Composition Variability
The linked table summarizes data from several demonstrations, analyses, and reports on syngas composition across a variety of gasifiers, and types of coal feedstocks, showing the wide variations that occur. Caution must be used in examining this table, as many factors may be missing that have significant impact on the syngas produced. For example, to accurately detail effects on syngas composition a detailed account of the coal used (ultimate and proximate analysis would be best) would be required. Gasifier operating mode, operating conditions, etc., would also need to be specified (for example, the GE gasifier can be operated in three different modes: total quenched; radiant & convective cooling; radiant followed by partial quenched—the syngas could be very different for each). This table, therefore, serves as a general overview and is not detailed enough for design work. See the linked sources for information on the development of this table.
The data shows that a wide range of syngas compositions can be obtained by varying the gasifier type, feedstock, and operation parameters. The typical design process of a gasification facility entails selecting a readily available feedstock for the potential facility site and a suitable gasification technology, and then using variations on downstream processes to optimize the syngas composition for creating the desired end product.
Another linked table is taken from a NETL study2 and gives an idea of the range of syngas compositions produced from biomass processed in various gasifier types (BFB – Bubbling Fluidized Bed, FB – Fixed Bed, CFB – Circulating Fluidized Bed, MSW – Municipal Solid Waste, RDF – Refuse Derived Fuel).
For information on optimal syngas compositions for a given end product, please see the discussion about Syngas Optimization.