5.1.6. 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)	0.6^a	~2.0	High	Unimportant	Unimportant
Carbon Dioxide	Low	Low^c	Not Important^b	Not Critical	Not Critical
Hydrocarbons	Low^d	Low^d	Low^d	High	High
Nitrogen	Low	Low	Low	Note^e	Note^e
Water Vapor	Low	Low	High^f	Low	Note^g
Contaminants	<1 ppm Sulfur Low Particulates	<1 ppm Sulfur Low Particulates	<1 ppm Sulfur Low Particulates	Note^k	Low Part. Low Metals
Heating Value	Unimportant^h	Unimportant^h	Unimportant^h	Highⁱ	Highⁱ
Pressure, bar	~20-30	~50 (liquid phase) ~140 (vapor phase)	~28	Low	~400
Temperature, °C	200-300^j 300-400	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 (NO_X) 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:

Water-Gas Shift – Water-gas shift is used to adjust the ratio of H₂ to CO.
Acid Gas Removal – Used to remove sulfur (typically in the form of hydrogen sulfide) and CO₂.
Hydrogen Separation – Separates high purity H₂ from syngas streams.
Methanation – Converts carbon oxides and hydrogen in the syngas into methane and water.
Gas Cooling
Syngas Cleanup and Trace Component Removal

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

Gasifier

Gasification Introduction
- Fundamentals
- Reactions & Transformations
- Detailed Gasification Chemistry
- Thermodynamics and Kinetics
- Syngas Composition
- Syngas Optimized for Intended Products
Commercial Gasifiers
R&D for Gasifier Optimization/Plant Supporting Systems
DOE Gasification Systems Program and Benefits