Gasification

Process Description

The heart of the Lurgi process is in the reactor, in which the blast and syngas flow upwards in counter-current to the coal feedstock (Figure 5-1).

Coal is loaded from an overhead bunker into a lock hopper that is isolated from the reactor during loading, then closed, pressurized with syngas, and opened to the reactor. The reactor is thus fed on a cyclic basis.

The reactor vessel itself is a double-walled pressure vessel in which the annular space between the two walls is filled with boiling water. This provides intensive cooling of the wall of the reaction space while simultaneously generating steam from the heat lost through the reactor wall. The steam is generated at a pressure similar to the gasification pressure, thus allowing a thin inner wall that enhances the cooling effect.

The coal from the lock hopper is distributed over the area of the reactor by a mechanical distribution device, and then moves slowly down through the bed under­going the processes of drying, devolatilization, gasification, and combustion. The ash from the combustion of ungasified char is removed from the reactor chamber via a rotating grate and is discharged into an ash lock hopper. In the grate zone the ash is precooled by the incoming blast (oxygen and steam) to about 300-400°C.

The blast enters the reactor at the bottom and is distributed across the bed by the grate. Flowing upward it is preheated by the ash before reaching the combustion zone in which oxygen reacts with the char to C02. At this point in the reactor the temperatures reach their highest level (Figure 5-2). The C02 and steam then react with the coal in the gasification zone to form carbon monoxide, hydrogen, and methane. The gas composi­tion at the outlet of the gasification zone is governed by the three heterogeneous gasifi­cation reactions: water gas, Boudouard, and methanation (for details see Section 2.1).

While describing the process in the form of these four zones, it should be stressed that the transition from one zone to the next is gradual. This is especially noticeable in the transition from the combustion zone to the gasification zone. The endothermic gasification reactions already begin before all the oxygen has been consumed in combustion. Thus the actual peak temperature is lower than that calculated by assuming a pure zonal model (see Figure 5-2) (Gumz 1950; Rudolf 1984).

This gas leaving the gasification zone then enters the upper zones of the reactor where the heat of the gas is used to devolatize, preheat, and dry the incoming coal. In this process the gas is cooled from about 800°C at the outlet of the gasification zone to about 550°C at the reactor outlet.

A result of the counter-current flow is the relatively high methane content of the outlet gas. On the other hand, part of the products of devolatilization are contained unreacted in the synthesis gas, particularly tars, phenols and ammonia, but also a wide spread of other hydrocarbon species. Bulk removal of this material takes place immediately at the outlet of the reactor by means of a quench cooler in which most of the high-boiling hydrocarbons and dust carried over from the reactor are con­densed and/or washed out with gas liquor from the downstream condensation stage.

Figure 5-3. Process Flowsheet of Lurgi Dry Ash Gasification (Source: Supp 1990)