*2.5. Conditions*

Using the melter–gasifier as the prototype, an unsteady simulation is performed from the computational domain consisting of the upper coke bed region of the slag layer on the packed bed region. The calculation domain size is shown in Figure 1. The bottom edge is 4690 mm, the height is 4230 mm, and the furnace wall inclination angle is 30◦, which is close to the actual size, and the tuyere spray gun is 1200 mm. Compared with the whole model, the insertion depth of the tuyere spray gun is negligible. For the sake of the simplicity of the model calculation, the inlet of the tuyere spray gun is located at the wall. The average mesh size is 40 mm in the present simulation. They are mostly structured meshes. We performed a sensitivity study of the mesh size with an average size of 100 mm, 80 mm, 60 mm, 40 mm, 30 mm, and 20 mm. The difference in the depth of the raceway (gas velocity is 200 m/s) between 60 mm and 40 mm is 2.9%, while that between 40 mm and 30 mm is within 0.5%. This suggests that a mesh size of 40 mm is reasonable and confirms the mesh independence.

In the simulation, a no-slip condition is applied to walls. The pressure–velocity decoupling is done with the PISO algorithm. The tuyere zone is the velocity inlet. An explicit scheme is used to describe the shape of the interface. The time step is 0.001 s. The numerical solution is considered to be convergent when the residual errors of the variables are less than 10<sup>−</sup>5. The main parameters used in the model are shown in Table 1.


**Table 1.** Parameters used in this model.

The unsteady simulation initialization conditions are shown in Figure 2. The lower part is the coke layer, and the porosity is 0.4; the upper part is gas.

**Figure 1.** Schematic diagram of the computational domain for the tuyere zone.

**Figure 2.** Initialization condition of the coke volume fraction in the model.

#### **3. Results**

#### *3.1. Unsteady Simulation of the Raceway Formation Process*

Taking the typical COREX melter–gasifier nozzle injection operation parameters as an example, the gas velocity is set to 200 m/s; the spray inclination angle is 4◦, and the parameters of the shape of the raceway zone and the volume fraction of the gas–solid particles at different times are tracked. The gas volume fraction at different time periods is shown in Figure 3.

It can be seen from Figure 3 that the volume fraction of the gas phase changes significantly over time. At the beginning of the injection, the gas develops radially toward the center. Then, the gas moves straight to the center, and the upper and lower sides develop slowly. Over time, the expansion of the radial direction begins to become slow and finally stagnate, at which point the gas begins to extend upwards in the axial direction, slowly forming an arc-shaped cavity region. The upper portion of the cavity region exhibits a semicircular or semi-elliptical shape, which lasts for a long period of time. Finally, the gas forming bubble-like sphere begins to drift upward when the gas pressure in the cavity accumulates to a certain extent.

Combining the above theory, it can be determined from the change of the gas phase volume fraction, and from the start of the injection to the formation of the raceway zone, that the gas first develops in the radial direction and reaches a certain degree before starting to develop upward. The reason for this development is that the porosity of the coke bed is continuously reduced under the compression of the gas; the gap between the coke particles becomes increasingly smaller, and the resistance of the gas to the depth development is increased. When compressed to a certain limit (simulated by setting the coke porosity limit to 0.37), the resistance of the gas in the radial direction is so large that the gas cannot expand forward, and the gas grows upwards.

**Figure 3.** Variation of gas phase volume fraction with time.

Figure 4 shows the depth variation of the radial development of the gas over time. It can be seen from the figure that, as the blowing time changes, the depth of the raceway zone first increases rapidly, and after 0.4 s, it begins to stabilize and finally reaches a maximum of about 800 mm.

**Figure 4.** Variation of raceway depth with blowing time.

Figure 5 shows the raceway behaviour in our previous experimental study [26]. The result indicates that particles rotate in front of the tuyere, and a stable cavity can be observed. Comparing the present numerical simulation and the previous physical simulation, the calculation result is consistent with the results of the physical experiment. The shape of the raceway shows similar features. These agreements verify the applicability of the present model for investigating the characteristics of the raceway in a melter gasifier under different conditions.

**Figure 5.** Raceway behaviour in the experiment.
