Gasification Introduction

Syngas Optimized for Intended Products

The discussion of syngas composition is of considerable importance considering the varying requirements on composition and impurities demanded according to final uses of the syngas. The following table shows the widely varying characteristics desirable for the principal uses of syngas, including use as fuel gas to fire boilers or turbines in power cycles, use of syngas as feedstock for production of synthetic fuels such as gasoline, use as feedstock for methanol synthesis, and use as feedstock for production of hydrogen.

Desirable Syngas Characteristics for Different Applications1

Product Synthetic Fuels Methanol Hydrogen Fuel Gas
  FT Gasoline     Boiler Turbine
Hydrogen to carbon monoxide ratio (H2/CO) 0.6a ~2.0 High Unimportant Unimportant
Carbon Dioxide
Low Lowc Not Importantb Not Critical Not Critical
Hydrocarbons Lowd Lowd Lowd High High
Nitrogen Low Low Low Notee Notee
Water Vapor Low Low Highf Low Noteg
Contaminants <1 ppm Sulfur
Low Particulates
<1 ppm Sulfur
Low Particulates
<1 ppm Sulfur
Low Particulates
Notek Low Part.
Low Metals
Heating Value Unimportanth Unimportanth Unimportanth Highi Highi
Pressure, bar ~20-30 ~50 (liquid phase)
~140 (vapor phase)
~28 Low ~400
Temperature, °C 200-300j
100-200 100-200 250 500-600
(a) Depends on catalyst type. For iron catalyst, value shown is satisfactory; for cobalt catalyst, near 2.0 should be used.
(b) Water gas shift will have to be used to convert CO to H2; carbon dioxide (CO2) in syngas can be removed at same time as CO2 generated by the water gas shift reaction.
(c) Some CO2 can be tolerated if the H2/CO ratio is above 2.0 (as can occur with steam reforming of natural gas); if excess H2is available, the CO2 will be converted to methanol.
(d) Methane and heavier hydrocarbons need to be recycled for conversion to syngas and represent system inefficiency.
(e) Nitrogen (N2) lowers the heating value, but level is unimportant as long as turbine or boiler system efficiencies are satisfactory. Presence of excess N2 may be unacceptable in carbon capture scenarios, however.
(f) Water is required for the water gas shift reaction.
(g) Can tolerate relatively high water levels; steam sometimes added to moderate combustion temperature to control nitrogen oxides (NOX) formation.
(h) As long as H2/CO and impurities levels are met, heating value is not critical.
(i) Efficiency improves as heating value increases.
(j) Depends on catalyst type; iron catalysts typically operate at higher temperatures than cobalt catalysts.
(k) Small amounts of contaminants can be tolerated.

Choice of coal and the gasification technology utilized will of course have significant impact on syngas composition. Additionally, downstream processes of various types for conditioning the syngas to meet its final application characteristics are available:

1. Benchmarking Biomass Gasification Technologies for Fuels, Chemicals and Hydrogen Production, Jared Ciferno and John Marano, National Energy Technology Laboratory, June 2002.



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