Quenching
As discussed above, the most demanding syngas cooling equipment is required for single-stage entrained-flow slagging gasifiers. The key problem is the transition stage between slagging and nonslagging conditions. This transition temperature range has to be crossed directly after leaving the slagging reactor, and ideally in such a way that the gas does not contact a wall before it is sufficiently cooled. One solution is to “sleeve” the inside of the quench section with clean gas (Staudinger and van der Burgt 1977).
For entrained-flow slagging gasifiers quenching can be accomplished in four different ways.
Although attempts have been made at quenching by allowing the hot gas leaving the reactor flow into a radiant boiler, it appears to be difficult to ensure that liquid or sticky slag particles do not hit the wall and cause fouling. Moreover, radiant boilers have the disadvantage that they scale awkwardly. For a scale-up factor of say 2, keeping the gas velocities constant, the vessel diameter scales by V2, whereas the requirement of surface area increases by a factor of 2. The height therefore also has to be increased by V2. This is in contrast to other heat exchangers, for which the volume increases proportionally with the throughput of the gasifier without an increase in height. The reason is that only the surface of the vessel can be used for heat exchange. A solution has been sought in extending the wall surface of the radiant boiler by installing ribs perpendicular to the surface, but this further complicates the construction. Radiant coolers can be prone to fouling, and they are difficult to clean by rapping. Furthermore, the heat dissipated by the wall can only be used for generating saturated steam.
All this makes radiant syngas coolers an expensive piece of equipment in practice. For the 250 MWe Polk Power Station IGCC in Florida, the radiant syngas cooler (RSC) is “about 16 feet in diameter and 100 feet long, and weighs about 900 tons” (U. S. Department of Energy 2000). On the other hand the reported reliability of this cooler is satisfactory (McDaniel and Hornik 2000).
Hot gas can be quenched by evaporation of water into the gas. It is necessary to distinguish between a partial quench, in which only just enough water is evaporated to reduce the gas temperature to 900°C, and a total quench, in which sufficient water in evaporated to saturate the gas with water vapor.
A partial quench is a well-proven quenching system that was already applied in the atmospheric pressure Koppers-Totzek gasifiers between the burners and the radiant syngas cooler above them. While effective in this configuration, replacement of nozzles as a result of wear does represent a maintenance cost, though not a limitation on reactor run-time length. Noell also offers a partial quench system. The advantage of a partial quench is that it allows the sensible heat in the syngas to be exploited for high-pressure steam raising from 900°C in a downstream syngas cooler.
A total quench has been a feature of Texaco’s oil gasification process since its inception and has also been adopted in most of the Texaco coal gasifiers. It is a low-cost and effective solution but has as a disadvantage that exergetically it is not very elegant. High-level heat that potentially can be put to better use is degraded to water vapor in the still dirty gas. Upon condensation, which is in any case required for fuel gas treating as practiced in most present day plants, this water will become available as a contaminated condensate stream that requires extensive cleaning. For slurry-feed processes, this problem of a large waste water plant is diminished by using this waste water for making the coal-water slurry feed for the gasifier.
If the final product is ammonia or hydrogen, the water vapor in the gas from a total quench may prove to be advantageous, as no additional steam has to be generated for the subsequent CO shift process. The only clean-up of the gas that is then required between the gasifier and the CO shift is a thorough solids removal. Either a hot water wash or candle filters will do this job.
One point to notice about water quenches, whether partial or total, is that the introduction of the water drives the shift reaction (2-7) to the right, and thus the C02 content and the H2/CO ratio of the gas are increased to some extent.
The gas quench is used in the SCGP and the Prenflo processes. The raw synthesis gas, which has been cooled in the syngas cooler and freed of solids in a candle filter, is split into two approximately equal portions. One is recycled with a compressor and used to quench the gas leaving the gasifier from about 1500 to about 900°C, and the remaining net gas production is routed to further downstream processing.
With gas cooling it is also possible to cool the gas further, to below 900°C, but in this case the amount of recycle gas required for the cooling will increase substantially. Even for the cooling of the gas from 1500 to 900°C, the molar gas flow (although not the volumetric flow) doubles, and as the heat has to be removed eventually by indirect means to make the quench effective, this leads to voluminous heat exchangers. Therefore, in practice, quenching with gas is limited to temperatures of 900°C. This is about the same temperature obtained after a chemical quench or after passing a radiant boiler.
In a chemical quench, ideally the sensible heat in the gas leaving the first slagging stage of an entrained-flow gasifier is used in the endothermic water gas reaction to gasify a second-stage feed. The second stage may be a dry feed as in the Japanese EAGLE and CCP processes, or a slurry feed as in the E-Gas process. Where the quench medium is a coal-water slurry a significant percentage of the heat is used to heat up this medium, to evaporate the water and for pyrolysis reactions, so that at least part of the cooling is actually attributable to a partial water quench. Either way, the heat absorbed is sufficient to cool the gas such that the ash from the second stage is dry.
Injecting coal as such or as water slurry into the hot gas leaving the first slagging stage has the disadvantage that some tars may be formed. In practice, with the E-Gas process this does not happen in normal operation, although it has been known to occur during upsets or low load operation (U. S. Department of Energy August 2000).
By introducing the chemical quench or a second non-slagging stage to a dry-coal feed-entrained slagging gasifier, a gasifier is obtained that has an outlet temperature some 400-500°C lower, and thus has a lower oxygen consumption as well as a higher CGE. As a result, the duty of the costly syngas cooler is substantially reduced. This has a cost advantage, which is attributable not only to the heat transfer surface area requirement, which is reduced by some 30%, but also to the possibility of using lower cost concepts such as a fire tube boiler.
The efficiency gain for a dry-feed gasifier is limited (see Section 5.3, page 114) but offers the advantage over a single-stage gasifier with the same outlet temperature of 1000-1100°C is that the bulk of the ash in the feed be comes available as an inert slag. The second non-slagging stage can be a simple brick-lined pressure vessel.