PARTICULATE REMOVAL
Only one step of the syngas treating can be carried out at an elevated temperature and that is filtering. The introduction of candle filters that can remove all solids from the gas at temperatures of up to 500°C was one of the most significant developments in gasification during the last quarter of the twentieth century. In Figure 6-9 a sketch is given of such a filter installation. The solids are deposited on the outside of the candles. Intermittently from the clean gas side the filters are blown back by a pulse of nitrogen or another gas that causes the solids, which have collected at the outside of the filters, to drop down to the bottom of the vessel, whence they can be removed via a lock hopper. The importance of this development for gasification-based power
SOLIDS-FREE GAS OUTLET
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PURGE GAS Figure 6-9. Candle Filter Vessel |
stations is discussed in Section 7.3. Filtering has to be carried out at temperatures between 300 and 500°C. At about 250-300°C the filters may be blinded by deposits of NH4C1. Above 500°C the vapor pressure of alkali compounds may still be high, which means that significant amounts may pass the filters. Below about 500°C the amount of alkali compounds is negligible, provided they are properly filtered out. This is accomplished by removing them, together with the fly slag that acts as a substrate on which the alkali compounds are deposited. Operating in the higher temperature range near 500°C is also beneficial for avoiding problems with carbonyls. Carbonyls will hardly form under these thermodynamically unfavorable conditions. Any nickel or iron present in the gas will also be deposited on the fly ash at these high temperatures.
The candle filters are mostly made of ceramic material where a fine-grain ceramic layer is deposited on a wider pore support that gives strength to the filters. Special attention has to be given to the seal between the filters and the steel support plate. The filters may either rest on a steel plate or hang down from a steel plate, as shown in Figure 6-9.
Candle filter materials have also been developed on a metal basis. Although ceramic materials can be operated at higher temperatures than metals without risk of sintering, the latter are more robust and can resist localized damage without rupture. The selection depends on the location of the filter within any particular process and the situations to which it can be exposed.