Effect of Oxidant on the Gasification Process
The most significant effects of varying oxidant composition can be seen in Figure 6-13. These results have been determined using a constant gasifier temperature of 1500°C (as determined by the ash characteristics of the coal) and constant preheat temperatures for the reactants.
The loss of cold gas efficiency with increasing nitrogen content of the oxidant is immediately noticeable. It falls off from 82% at 100% 02 to 61% with air. The essential reason for this, and for the other effects visible in the Figure 6-13, is the amount of heat required to raise the nitrogen from its preheat temperature of 300°C up to the reactor outlet temperature of 1500°C. This can be partially compensated for by reducing the moderating steam, but this is only possible to the extent of reducing it to zero. For the chosen coal and conditions, this happens at about 26mol% 02 in the
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Figure 6-13. Gasification Parameters as a Function of Oxygen in Oxidant |
oxidant. At this point more carbon is combusted to C02 to maintain the heat balance, which further reduces the efficiency. Should a particular process require a minimum amount of steam, for burner protection, for example, the point at which the C02 make begins to increase will be at a correspondingly higher oxygen content in the oxidant.
An alternative approach to compensating for the heat absorption by the nitrogen might appear to be an increase in preheat temperatures. Klosek, Sorensen, and Wong (1993) discuss this aspect in their paper but point out its limitations. The required preheat temperatures of about 1200°C become “excessively high and unacceptable.”
Similarly, the drop in useful syngas (H2+CO) is visible as the oxygen content in the blast decreases.
Gasification Temperature
At lower gasification temperatures these effects are less but are nevertheless still present.
Use of Hot Syngas
Reviewing the implications of oxidant quality only on a cold-gas efficiency basis ignores the fate of the sensible heat contained in the syngas leaving the reactor. In an IGCC environment, the most efficient use of this heat would be to feed it uncooled to the gas turbine, since this has a fundamentally higher efficiency than the steam cycle (see Section 7.3). This theoretical possibility is, however, only realizable if the raw syngas quality is acceptable to the gas turbine, which in general is not the case. In particular, particulates and alkali metals need to be removed prior to the gas entering the gas turbine. These contaminants can be removed at 500°C using candle filters (see Section 6.7).
There is still an interest in “hot gas desulfurization,” especially in connection with air-blown gasifiers. This is not because the gas turbine cannot cope with the sulfur once the alkali salts have been removed, but because of the environmental requirements. Improvements in flue gas desulphurization technology provide a continuously moving target. With double-scrubbing technology, sulfur removal from the flue gas can also be as high as 99%. However, it should be noted that any improvement obtained with such technology developments will benefit oxygen — blown systems as much as they will air-blown systems. So far all attempts at hot sulfur removal have failed, and there is no indication that a good solution will be found in the near future. Further details of gas cleaning technologies are described in Chapter 8.
An effective use of the hot gas, particularly where it is generated in a small-scale biomass gasifier, is to utilize it as a gaseous fuel in a large utility boiler. This is particularly effective, because the low efficiency inevitable with small-scale plants can be avoided by tying into the larger unit. A particularly interesting example is an air-blown CFB gasifier processing waste wood and firing the gas into a 600 MWe PC power boiler at Geetruidenberg in the Netherlands.
One other albeit rare case where the full sensible heat of the syngas from an air- blown gasifier is used in the downstream process is in the production of reducing gas, for example, for nickel reduction, where the gas is fed from an oil gasifier directly to the reduction furnaces without any cooling.