TRACE COMPONENTS OF RAW SYNTHESIS GAS
In Chapter 2 we discussed the principal components of the raw synthesis gas from the gasification reactor, namely carbon, CO, C02, H2, H20, and CH4. A complete review of the raw gas composition would, however, be incomplete without a discussion of trace components, which can influence the selection of the downstream gas treating process, corrosion behavior, or the potential for fouling. In this respect it is, for instance, important to have an understanding of the fate of the sulfur and nitrogen in the feedstock. Similarly, one needs to be aware of those reactions that take place downstream of the reactor in the gas cooling and solids removal sections of the process, such as metal carbonyl or organic acid formation.
The existence of sulfur compounds in raw synthesis gas represents a poison for the catalysts of most chemical applications, including ammonia, methanol, Fischer — Tropsch, low temperature shift, and others. In power applications, if untreated they would be emitted with the flue gas as S02 and S03, major components of “acid rain.”
In high-temperature processes all sulfur components in the feed are converted to H2S or COS. Other compounds such as SOx or CS2 are essentially absent. This is not the case in low-temperature processes, where tars and other species have not been completely cracked. A detailed breakdown of the sulfur compounds in raw gas from a Lurgi dry-bottom gasifier is given in Table 6-2 as an example.
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Table 6-2 Sulfur Compounds in Raw Gas from a Lurgi Dry-Bottom Gasifier |
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|
Component |
|
|
COS, ppmv |
180 |
|
H2S, ppmv |
15,300 |
|
Mercaptan S, ppmv |
600 |
|
Thiophenes, ppmv |
5 |
|
CS2, ppmv |
100 |
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Source: Supp 1990 |
The relationship between the H2S and COS contents of a raw gas is determined by two reactions, the hydrogenation reaction
H2+COS ^ H2S + CO +7 MJ/kmol (6-1)
and the hydrolysis reaction
C0S + H20^H2S + C02 -34 MJ/kmol. (6-2)
Equilibrium constants for these reactions can be found in the literature (Reimert and Schaub 1989). Under typical gasification conditions, H2S is the dominant species, and approximately 93-96% of the sulfur is in this form, the rest being COS.
It is important to be aware of the COS content in the raw gas, since not all gas treatment systems will remove COS. In order to overcome this it may be necessary to perform a selective catalytic hydrolysis of COS to H2S (reaction 6-2) prior to the acid gas removal. This is discussed in more detail in Chapter 8.