Gasification in Detail
Syngas in Detail
Synthesis gas, or syngas, produced from gasification, is a mixture of gases, predominantly carbon monoxide (CO) and hydrogen (H2), along with small amounts of carbon dioxide (CO2), steam (H2O), sulfur compounds of hydrogen sulfide (H2S), carbonyl sulfide (COS), ammonia and other trace contaminates. The following figure shows the types of chemical reactions which occur in a gasifier along with the typical range of syngas composition produced by coal gasification.
As shown, the composition of the syngas can vary quite significantly, depending on the coal feedstock used, and the type of gasification reactor system and its operating conditions.
Wabash River Example
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)
Ranges of Syngas Composition
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. The gasifier is capable of handling a wide range of feedstocks; however 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.
The linked table summarizes data from several demonstrations, analyses, and reports on syngas composition across a variety of gasifiers, and types of coal feedstocks. 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. Please 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.
The 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 visit the page on Syngas Optimization.