Gasifipedia
Gasification in Detail – Syngas in Detail
Syngas Optimization
Raw syngas produced from gasification must be cleaned and conditioned, the requirements of which are highly dependent on the syngas’ final application. This page serves as an overview of these desired syngas characteristics and how they affect the suitability for further processing. The individual application pages, available by visiting Applications of Gasification Technology, provide more detailed coverage of each application.
Desirable Syngas Characteristics for Different Applications[1]
| Product |
Synthetic Fuels |
Methanol |
Hydrogen |
Fuel Gas |
| |
FT Gasoline |
|
|
Boiler |
Turbine |
| H2 / CO |
0.6 a |
~2.0 |
High |
Unimportant |
Unimportant |
| CO2 |
Low |
Low c |
Not Important b |
Not Critical |
Not Critical |
| Hydrocarbons |
Low d |
Low d |
Low d |
High |
High |
| N2 |
Low |
Low |
Low |
Note e |
Note e |
| H20 |
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 |
High i |
High i |
| 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 |
Some of the downstream processes that are required for conditioning the syngas to meet its final application characteristics include:
Power (IGCC)
Syngas for use in integrated gasification combined cycle (IGCC) applications must be free of contaminants such as particulates and trace metals which could cause damage to the gas turbine. The ratio of hydrogen to carbon monoxide (CO) is not as important as in other applications which use syngas derived from gasification, if the turbine has been designed to handle increased hydrogen content, as discussed below. Also, a high hydrocarbon content, makes the syngas as similar as possible to natural gas, which is ideal.
Gas turbines developed for use in IGCC applications with syngas are based on systems designed for natural gas. Despite the similarities between syngas and natural gas, there are differences which impact the design of the gas turbines used for converting them to electrical power.
Gasification derived syngas differs from natural gas in terms of calorific value, composition, flammability characteristics, and contaminants. Oxygen-blown, entrained flow IGCC plants typically produce syngas with a heating value range of 250 to 400 Btu/ft3 (HHV basis), which is much lower than the 1,000 Btu/ft3 commonly associated with natural gas. The combustor requires a specified heat input to maintain performance, so a significantly higher flow rate is required for syngas than natural gas for a similar gas turbine. Also, natural gas consists mainly of methane (CH4), whereas syngas consists mainly of CO and hydrogen (H2). The H2 composition of the syngas results in a higher flame speed and broader flammability limits, meaning the syngas produces a stable flame at leaner conditions than natural gas and the combustion speed is much quicker than natural gas. This more rapid combustion speed limits the use of conventional natural gas combustor nitrogen oxide (NOX) control. Another complication is the relatively high concentrations of hydrogen sulfide (H2S) in syngas compared to natural gas.
To combat these issues, diluents such as nitrogen or steam are used to lower the flame temperature. The lower temperature limits the formation of NOX as the syngas is combusted. Nitrogen is an ideal solution in oxygen blown applications as it should be readily available as a by-product from the air separation unit.
Data on the composition of clean syngas being used to fire gas turbines at a range of IGCC facilities is presented in the table below.[2]
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