To pursue the "dual carbon" goal, China has been vigorously supporting the development of new energy industries, which has led to rapid growth in the market demand for lithium and its related materials. As one of the main thermal equipment for lithium carbonate production, the tunnel kiln consumes energy and generates a large amount of pollutants during the production process of lithium batteries. With increasing emphasis worldwide on environmental protection and sustainable development, both upstream and downstream enterprises in the lithium industry are seeking more energy-efficient, high-performance and environmentally friendly tunnel kiln technologies. This is not only a challenge for tunnel kiln technologies but also an opportunity to promote their innovative development. By adopting advanced energy-saving technologies and environmentally friendly materials, the energy consumption and emissions of tunnel kilns can be reduced, so that their competitiveness in the lithium industry can be enhanced. How to optimize the structure of tunnel kilns, reduce energy consumption and cut down pollutant emissions has become the primary issue in the development of the lithium extraction industry.

In this paper, the pre-tropical band of the tunnel kiln was simplified and simulated. The meshing of five kiln stacking structures was carried out by using the ICEM software. All the mesh qualities were above 0.9, meeting the calculation requirements of Fluent. Moreover, the mesh was verified for independence. In order not to affect the accuracy of the simulation calculation, an appropriate mesh quantity model was selected. Through theoretical calculations, the parameters of each boundary condition were set. The realizable k-ε turbulence model and the DO radiation model were selected. The Standard Wall Function was used in the turbulence model, while the Coupled algorithm was used in the algorithm. In addition, UDF software was used to write the functions of the physical parameters of flue gas varying with temperature, whereas the inlet temperature was changed in the direction of the kiln height. Through numerical calculation and analysis, it was determined that the uniformity of the flow field and the temperature field in the kiln was optimal when the kiln stacking structure of Structure Ⅳ was used.

According to the numerical simulation results, in terms of the effects the five kiln stacking structures of lepidolite roasting tunnel kilns on the flow field and temperature field inside the kilns, Structure Ⅳ is optimal for kiln stacking. For Structure Ⅳ, the average velocity non-uniformity coefficients and the average velocity means on the odd-numbered layers and the even-numbered layers are 0.38 m·s⁻¹, 0.74 m·s⁻¹, 2.1 m·s⁻¹ and 1.2 m·s⁻¹, respectively. A relatively larger average velocity is conducive to the preheating and temperature rise of the brick stacks to a certain extent. The maximum fluctuation range of the temperature standard deviation and the dimensionless subcooling temperature are 69.6% and 0.122 ℃, respectively. The smaller the dimensionless subcooling temperature, the more uniform the surface temperature of the brick stacks will be. Compared with the other four structures, Structure Ⅳ has a relatively smaller flow resistance in the transverse direction and a larger gap in the transverse stacking channels. Under the five brick stack kiln stacking structures, the velocity non-uniformity coefficients and the temperature standard deviations of the longitudinally stacked odd-numbered layers are both smaller than those of the adjacent transversely stacked even-numbered layers, while the differences between the averages of the two are 50.9% and 29.3%, respectively.

The flue gas will form a small jet structure in the top channels and longitudinal stacking channels of the five structures, generating vortices in the surrounding transverse stacking channels and increasing the intensity of convective heat transfer between the flue gas and the brick stacks in the transverse stacking channels. Comparatively, Structure Ⅳ has a relatively smaller flow resistance in the transverse direction and a larger gap in the transverse stacking channels. Therefore, small-scale vortices can grow in the transverse stacking channels and form convective vortices, thus enabling promising mixed flow near the brick stacks and hence resulting in increases in the disturbance intensity, improvement in the convective heat transfer efficiency within the brick stacks and enhancement in the velocity and temperature uniformity inside the kiln. Therefore, Structure Ⅳ is the optimal kiln stacking structure.