280590 Modelling Convective Drying of Iron Ore Pellets
Modelling convective drying of iron ore pellets
A.-L. Ljunga, T. Staffan Lundström and Vilnis Frishfelds
Department of Engineering Sciences and Mathematics
Lulea University of Technology
Lulea 971 87, Sweden;
Iron ore pellets are a highly refined product supplied to the steel making industry for use in blast furnaces or direct reduction processes. The use of pellets offers many advantages such as customer adopted products, transportability and mechanical strength yet the production is time and energy consuming. Being such, there is a natural driving force to enhance the pelletization in order to optimize production and improve quality. The aim with this work is to develop numerical models with which the drying zone of an induration furnace can be examined and optimized. Drying of a bed of iron ore pellets is therefore examined numerically on several scales.
A diffusive model taking into account capillary flow of liquid moisture and internal evaporation is developed to examine drying of a single pellet and simulations of a scanned pellet are validated with good agreement. The result clearly shows four stages of drying; i) evaporation of liquid moisture at the pellet surface, ii) surface evaporation coexisting with internal drying as the surface is locally dry, iii) internal evaporation with completely dry surface and iv) internal evaporation at boiling temperatures. A moisture front moving towards the core of the pellet will start to develop at the second drying stage and the results show that the front will have a non-symmetrical form arising from the surrounding fluid flow. With aid of the results from the single pellet model, a discrete two dimensional bed model is developed. The system of pellets is divided by modified Voronoi diagrams and the convective heat transfer of hot fluid flow through the system including dispersion due to random configuration of the pellets is modeled. The results show that the temperature front advances much faster in the gaps between pellets than in the interior of the pellets even if all the heat energy of the air goes in heating of the pellets initially. An uneven distribution in temperature and moisture content between pellets is furthermore displayed in the discrete system. This phenomenon is related to the natural dispersion occurring in random system of pellets. In an established stage of the drying, the moisture volume-fraction increases continuously from the inlet to the outlet as it is collecting water evaporation from the pellets. The latter results in a lower evaporation in the wet area close to the outlet.