Both the Lurgi gasifier and the BGL gasifier described in Section 5.1.2 have moving parts that are absent in other gasifiers. For caking coals and sometimes for distribution reasons, a stirrer is installed in the top of the reactor. The distribution function is required to ensure an even depth of the coal bed over the whole cross-section of the reactor. With caking coals the distributor avoids pasting-up of the coal bed due to agglomeration of the coal particles after their temperature becomes higher than the softening point. The dry-bottom gasifier also has a rotating grate at the bottom for the ash outlet.
Moving-bed gasifiers require a relatively high amount of maintenance. However, in virtually all cases many gasifiers operate in parallel, and by proper scheduling the total unit will have a high availability.
An important feature of the Lurgi dry-bottom gasifier is its low oxygen consumption and high steam demand. The exact data depend on the feedstock. Table 5-4 provides data on the basis of three different coals that cover a broad range of coalification or rank.
Typical Performance Data of Lurgi Dry Ash Gasifier
Source: Supp 1990
Moving-bed gasifiers need graded coal, whereas the total mine output will always contain a large percentage of fines (the more modern the mine, the more fines). Although it is in principle possible to make briquettes with these fines, by binding them with heavy tar that is co-produced in the gasifier, in practice part of the fines cannot be processed.
Heavily caking coals cannot be processed in moving-bed gasifiers. Mildly caking coals require the assistance of the stirrer in order to avoid pasting-up of the bed. Tars and other oxygenated compounds are also produced as by-products. These products form about 25% of the total hydrocarbon feed input in terms of energy. Correcting the oxygen consumption and the CGE (cold gas efficiency) for this fact increases the former and reduces the latter by about one-third. When you add to this that the sensible heat in the gases cannot be used very effectively due to the presence of the tars, the overall efficiencies of the moving-bed processes are not much higher than that of fluid-bed or entrained-flow gasifiers.
The most notable commercial installation forms part of the Sasol synthetic fuels operation at two sites in South Africa, where in all thirteen MK III, eighty-three MK IV, and one MK V reactors produce a total of 55 million Nm3/day syngas, the largest gasification complex in the world. The synthesis gas generated produces 170,000 bbl/day of Fischer-Tropsch liquid fuels as well as forming the basis for a substantial chemical industry (Erasmus and van Nierop 2002). The reason that Lurgi dry-ash gasifiers are used in the SASOL complex in South Africa is that at the time the complex was built it was the only pressurized gasifier available and was very suitable for the high ash-melting point coals to be processed.
The Lurgi type moving-bed dry ash gasifier is in widespread use around the world in, apart from South Africa, the United States, Germany, the Czech Republic, and China. The plants in Germany and the Czech Republic use some of the gas to fuel gas turbine combined cycle power plants. In the United States Lurgi dry ash gasifiers are used for the production of substitute natural gas (SNG). In China, the Lurgi process is used for the production of town gas, ammonia, and hydrogen. In the SNG, IGCC, and town gas applications, the high methane content (10-15%) of the product gas is an advantage, since methane is the desired product for SNG, and for IGCC and in town gas applications methane increases the heating value of the gas.