REACTOR DESIGN
6.4.1 Reactor Embodiment
Gasification reactors vary from process to process. At first glance, moving-bed processes may look like a simple stove, but they are in fact mechanically often the most complicated. This complexity is either due to the presence of stirrers in the top of the reactor or by the presence of a rotating ash grid in the bottom of the reactor as, for example, in the Lurgi dry-ash gasifier (see Figure 5-1). In the BGL — Lurgi slagger the rotating ash grid is absent and the ash is present as a molten slag.
With fluid-beds the complexity lies in the fluidization. For the remainder, the reactors are relatively simple. The main advantage is that temperatures are below the ash melting point, and pressures are also lower than in most other processes. Insulating refractory is used to protect the reactor pressure shell from high temperatures.
Most complex are the reactors of entrained-flow slagging gasifiers. The containment of both the high temperatures of 1500-1700°C in combination with pressures of 30-70 bar is not easy. The wall construction will be discussed in Section 6.4.2
The simplest and lowest cost construction is a refractory brick-lined reactor. In the case of top-fired single-stage gasifiers, it consists of a simple cylindrical vessel with, in principle, a hole in the top for the burner to supply the feedstocks, and a central outlet in the bottom for both the product gas and the slag. The reactor has only one burner, which has the advantage of a simple construction and a simple control system. Also, having one outlet for both the product gas and the slag is an advantage, as the chance of plugging the slag tap or the gas outlet is virtually absent even in small-capacity gasifiers. Last but not least, the construction is completely rotational symmetrical, which makes it elegant and relatively low cost.
An additional advantage of an insulating brick-wall reactor, which has been used for decades in all oil gasifiers, is that in contrast to a membrane wall the brick lining has a large heat capacity. Therefore, no extra wall penetrations are required for an ignition burner. The heat-up facility can be integrated into the main burner. The reactor and the hot face brickwork is heated first with a start-up burner, after which the start-up burner is replaced with the coal or oil burner and the reactor is (partly) pressurized. Ignition of the flame follows immediately after introduction of the reactants at the hot brick wall, after which the final reactor conditions are obtained.
Up-flow slagging entrained-flow processes provide separate outlets for the gas at the top of the reactor and for slag at the bottom. The burners are located in the cylindrical wall of the reactor near the bottom (see Figure 5-15). This construction is more complex than the simple cylindrical reactors. Wall penetrations are numerous as the gasifiers feature (at least) four burners, and a complex system is required for the ignition burner.