ENTRAINED-FLOW GASIFIERS
As discussed at the beginning of this chapter, the principal advantages of using entrained-flow are the ability to handle practically any coal as feedstock and to produce a clean, tar-free gas. Additionally, the ash is produced in the form of an inert slag or frit. This is achieved with the penalty of a high oxygen consumption, especially in the case of coal-water slurries or coals with a high moisture or ash content, as well as additional effort in coal preparation.
Nonetheless, even if entrained-flow has been selected as the means of contacting the fuel and gasification agent, this still leaves a considerable variety of alternatives open in the design approach, as can be judged from Table 5-7, which outlines characteristics of some important entrained-flow processes.
The majority of the most successful coal gasification processes that have been developed after 1950 are entrained-flow slagging gasifiers operating at pressures of
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Table 5-7 Characteristics of Important Entrained-Flow Processes
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20-70 bar and at high temperatures of at least 1400°C. Entrained-flow gasifiers have become the preferred gasifier for hard coals and have been selected for the majority of commercial-sized IGCC applications.
In entrained-flow gasifiers the fine coal particles react with the concurrently flowing steam and oxygen. All entrained-flow gasifiers are of the slagging type, which implies that the operating temperature is above the ash melting point. This ensures the destruction of tars and oils and, if appropriately designed and operated, a high carbon conversion of over 99% although some water-slurry feed plants do not achieve this. Moreover, entrained-flow gasifiers produce the highest quality synthesis gas because of the low methane content. Entrained-flow gasifiers have relatively high oxygen requirements, and the raw gas has a high sensible heat content. The various designs of entrained-flow gasifiers differ in their feed systems (dry-coal feed in a high-density fluidized state or coal-water slurries), vessel containment for the hot conditions (refractory or membrane wall), configurations for introducing the reactants, and the ways in which sensible heat is recovered from the raw gas. The two best-known types of entrained-flow gasifiers are the top-fired coal-water-slurry feed gasifier, as used in the Texaco process and the dry coal feed side-fired gasifier as developed by Shell and Krupp-Koppers (Prenflo). Furthermore, there is the dry coal feed top-fired Noell gasifier. Simple sketches of these three reactor types are
given together with temperature profiles for both the coal/char and the gas in Figures 5-13, 5-14, and 5-15. Some gasifiers (e. g., E-gas) use two stages to improve thermal efficiency and to reduce the sensible heat in the raw gas and to lower the oxygen requirements. In the present coal-water slurry-feed gasifiers, a substantial part of the reactor space is used to evaporate the water of the slurry. This is reflected in the temperature profile of this gasifier (see Figure 5-13).
Contrary to the moving-bed and fluid-bed processes, virtually all types of coals can be used in these processes, provided they are ground to the correct small size. The coals may be heavily caking and may range from sub-bituminous coals to anthracite. Browncoal and lignite can in principle be gasified, but for economic reasons this is not very attractive because of the ballast of inherent moisture that has to be evaporated and heated to the high temperatures required. High-ash coals are also not selected by preference, because all the ash has to be melted and that also constitutes thermal ballast to the gasifier. Coals with very high ash melting points are generally fluxed with limestone in order to lower the ash melting point and hence the operating temperature. This improves the process efficiency, reduces the oxygen consumption, and enables the use of a refractory-lined reactor.
Currently, most entrained-flow gasifiers are single-stage gasifiers. The fuel is introduced together with the blast via one or more burners. The blast is always pure oxygen or a mixture of oxygen and steam.
The ash produced in entrained-flow gasifiers is similar to that of the slagging moving-bed gasifiers and consists of the same fine, black, inert, gritty material.
The upflow reactors, as employed in the Shell Coal Gasification Process (SCGP) and Prenflo reactors, can be essentially considered as continuously stirred tank reactors
Figure 5-13. Top-Fired Coal-Water Slurry Feed Slagging Entrained-Flow Gasifier
Figure 5-14. Top-Fired Dry-Coal Feed Slagging Entrained-Flow Gasifier
(CSTRs). The reason is the very large recirculation inside the reactor caused by small temperature differences and the way the reactants are introduced. Contrary to the fluid-bed reactors, the carbon conversion is almost 100% because most of the ash leaves the reactor as a slag that is very low in carbon. Moreover, measures can be taken to ensure that large carbon particles tend to remain longer in the reactor. This can be accomplished, as in the Noell gasifier, by introducing some swirl in the top burner or by tangential firing as, for example, in the EAGLE gasifier. The largest coal particles are thus preferentially deposited on the liquid slag flowing
vertically downwards along the reactor wall. The coal particles having a lower density than the slag will float like “icebergs” on the slag. The velocity of this slag layer is much lower than of the gas in the reactor, and thus the ideal situation is obtained where the larger coal particles get the longest residence time. Careful design is important, as too much swirl causes reverse flow in the center of the reactor and can lead to unwanted situations.
Modeling of the second nonslagging stage of the E-Gas reactor is also not simple because of the evaporation of water and the pyrolysis reactions. The first slagging stage with the side introduction of coal-water slurry and recycled char in the E-Gas process can again be described as a CSTR.
CFD modeling of entrained-flow reactors has been initiated, and the first published results are encouraging (Bockelie etal. 2002).