Purpose Developed Processes Pyrolysis Processes
One feature of many processes specific to waste gasification, is the use of a separate pyrolysis stage prior to partial oxidation. (In discussing waste gasification, it is important to keep a clear distinction, since the word gasification is often used indiscriminately for both.) Pyrolysis is sometimes used as a preliminary to partial oxidation of the tars and char in a separate reactor, as in, for example, the Thermoselect, Compact Power, Brightstar, PKA, and Alcyon processes. Others do not include a partial oxidation stage but have a more or less close-coupled combustion of the pyrolysis products, such as von Roll and Takuma. Also, where a partial oxidation follows the pyrolysis, there are different approaches. Thermoselect, for example, claims to operate the pyrolysis at 300°C, then gasify with oxygen and quench the syngas prior to cleaning, thus having the option to use the syngas for power or chemicals production (Calaminus and Stahlberg 1998). Compact Power, by contrast, operates with pyrolysis at 800°C, gasifies with air and burns the syngas directly in a close-coupled combustor (Cooper 2002).
Finally, there are some processes that only include a pyrolysis such as that of Thide, where the gas from the pyrolysis stage is used in a separate, not necessarily close-coupled thermal value-recovery stage.
The issue of close-coupling a combustion stage can be an important one, even if not only in the technical sense. Where a distinction is made in regulations between gasification (as a process that makes a synthesis gas) and incineration, the close — coupled combustor can be considered integral to the gasification stage and the whole unit is then classified as an incinerator. This can lead in some jurisdictions to unfortunate results, such as totally inappropriate personnel training requirements (Lockwood and Royer 2001).
There are a number of processes that use fluid-bed gasification without a separate pyrolysis stage. The coal-derived HTW and CFB processes mentioned above are examples. Others have been developed primarily for waste feeds, such as automotive shredder residues. Such a process is that of Ebara, which in one variant close — couples an air-blown fluid-bed gasifier with a cyclonic combustion chamber. The latter operates at about 1400°C and produces a molten slag. The Ebara process, which in fact originated as an air-blown incineration process, has been developed via atmospheric gasification to include pressurized gasification with a chemicals — quality synthesis gas. The TwinRec variant of the Ebara process consists of a first stage fluid-bed air-blown gasifier operating under pyrolysis conditions at about 580°C, followed by a close-coupled downflow cyclonic combustion unit (Fujimura, Oshita, and Naruse 2001). The latter operates at 1350-1450°C and the slag is tapped at the bottom of this section. Six units are operational at the time of writing. A further fourteen are in various stages of design and construction.
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LOW TEMPERATURE HIGH TEMPERATURE GASIFICATION GASIFICATION (600 — 800°C) (1300 — 1500°C) Figure 5-40. Ebara-Ube Process (Source: Steiner etal. 2002) |
A pressurized version of the fluid-bed pyrolysis unit, known as the Ebara-Ube process, has been developed in a 30t/d pilot plant operating at about 10 bar and 600-800°C (see Figure 5-40). This is close-coupled to a high temperature gasifier operating at 1300-1500°C. The latter incorporates a water quench. An additional plant for 65 t/d is under construction at the same location, and the syngas produced will, after water scrubbing, be processed in a CO shift and PSA unit to provide hydrogen for an existing ammonia synthesis plant (Steiner etal. 2002).
Another process using fluid-bed gasification is that of Enerkem. The product syngas can be made available for separate use for powering a gas engine, for example.
Finally, there are a number of approaches that cannot be included in this summary classification. One such process is the Sauerstoff-Schmelz-Vergasung (2sv), which is one of the few current examples of a co-current moving-bed gasifier. The advantage of this concept lies in the much lower tar content in the gas compared with a counter-current moving-bed (Scheidig 2002).