As discussed in Chapter 2, dry-coal feed gasifiers have the advantage over coal - water slurry feed gasifiers in that they can operate with almost the minimum amount of blast. This implies in practice that they have a 20-25% lower oxygen consumption than coal-water feed gasifiers. Also, as shown in Section 2.4, dry-coal feed entrained-flow gasifiers have in principal an additional degree of freedom that makes it possible to better optimize the synthesis gas production. Moreover, it is possible to adjust the H2/CO ratio slightly. In practice, operation at a C02 content of the gas of
0. 5-4 mol % and a temperature of 1500°C is generally adhered to.
Single-Stage Gasifiers. In particular, the single-stage entrained-flow gasifiers yield a high gas purity with only traces of hydrocarbons and with a CH4 content of well below 0.1 mol%. Together with the low C02 and high carbon conversion, this ensures that almost all carbon in the feed is converted into CO, and hence a non - selective acid gas removal can be employed, as the H2S/C02 is such that the combined acid gas may be routed directly to the Claus plant sulfur recovery. Details about the gas treating will be discussed in Chapter 8.
Examples of single-stage dry-coal feed gasifiers are the SCGP process, the Prenflo process, and the Noell process. The dry-coal-feed process SCGP is used in a 250 MW IGCC plant in Buggenum, The Netherlands, and the Prenflo process in a 300 MW IGCC plant in Puertollano, Spain. Noell has a 600 t/d plant operating on a variety of solid and liquid feedstocks in Schwarze Pumpe, Germany.
Process Performance. One of the most striking features of single stage dry-coal entrained-flow slagging gasifiers is that the gas composition is very insensitive to the coal quality. In the case of low-rank coals and high-ash coals, however, the gas yields suffer because of the ballast of water and ash, respectively. The performance of a variety of coals is given in Table 5-8. The coal analyses are those of Table 4-4.
Two-Stage Gasifiers. An improvement in the process efficiency can be obtained by adding a second nonslagging stage to the first slagging stage of an entrained-flow gasifier.
Based on the standardized, idealized conditions of Appendix E, present state of the art single-stage pressurized entrained-flow gasifiers have an efficiency of 50% LHV for IGCC applications (see data in Table 5-9). The process efficiency of these gasifiers can be increased by the introduction of a second nonslagging stage as applied in the CCP or EAGLE processes. The second stage results in a higher cold gas efficiency and lower oxygen consumption. Efficiencies are 50 and 50.9% for the one - and two - stage process-based IGCC, respectively, when operating with a dry-coal feed system (see Table 5-9). (The calculation basis for this data is given in Appendix E.)
This is achieved by operating the first stage under high temperature slagging conditions with only part of the reactants, and then adding the remainder in a second stage where the hot gas drives the endothermic reactions in the second, nonslagging stage. In practice, current processes all operate the first stage with a deficit of coal with a second stage coal feed. In principle, however, alternative staging concepts could be used, e. g. with steam addition as the second stage. Each staging concept has its advantages and disadvantages. With a coal feed second stage there is the fact that a certain amount of tars will inevitably leave the reactor with the syngas. The extent to which this constitutes a real problem depends on the feedstock quality and the actual outlet temperature of the reactor. With steam addition in the second stage, the fuel is present as unconverted char from the first stage so that the syngas would be tar free. On the other hand, in the context of a dry-feed system, this requires considerably more steam than for a single stage reactor. This increases the amount of process condensate to be handled and degrades some of the sensible heat in the gasifier exit gas to condensing temperature levels.
Irrespective of the choice staging arrangement, there are a number of interesting aspects to be considered. The lower temperature of the second stage will require more residence time than for a single stage gasifier and in practice one will have to reckon with some carbon carry over in the syngas. On the other hand precisely these lower temperatures would allow the use of a refractory wall in the second stage, which represents a significant cost saving compared with the membrane wall used in some single stage designs without the exposure to very high temperatures in other singlestage designs.
The second stage is nonslagging. The particulate matter in the syngas contains unreacted carbon and dry ash. This is removed from the gas downstream of the syngas cooler and recycled to the first stage. In this way almost all the ash is removed from the system as slag.
The overall effect is that a slagging gasifier is obtained in which the oxygen consumption is almost as low as for a gasifier operating at the temperature at which the gases leave the second nonslagging stage. This has the following process advantages:
IGCC Efficiencies for Various Entrained-Flow Gasifiers
Note: Efficiencies are based on the standardized, idealized conditions of Appendix E.
1. No gas quench and a lower syngas cooler duty.
2. A higher cold gas efficiency and a 20% lower oxygen consumption.
The methane content of the gas will slightly increase compared with the single stage gasifier and so will the C02 content.
A drawback of incorporating a second-stage to a dry-feed gasifier is the added complexity and the higher steam consumption. The extra efficiency is about one percentage point (see Table 5-8). But there are also other advantages, such as much less quenching and a lower cost syngas cooler.
Another process advantage that can be incorporated in all slagging processes is a boiling water slag bath where the steam generated can be used for example as process steam or as a quench medium. Further, the slag bath could be used as a sour water stripper.