Thermal and Physical Patterns of the Reversible Drying of Pellets in the Straight-grate Indurating Machines
Eventually, almost all straight grate traveling indurating machines for pellet production operating with a reverse supply of the heat carrier into the pellet bed. First of all, updraft drying begins and then downdraft proceeds (downward). However, selection of the parameters of this option is still empirical. In particular, a length of the drying first section with the heat carrier updraft (D1) fluctuates from 9 to 18 m. The studies have shown that the drying process in this section considers an over-watering (excessively high pellet moisture). There are two types of over-watering: the first type relates to condensation of water steam within the pellet bed and filling of the free pores of pellets and the second one is “super over-watering” of the pellets on the pellet bed surface as the free water drops. As a result of the experimental data comparison obtained on the indurating machines with an area 520 m2 and the results of estimated experiments on the TOREX math model have shown that an increase of initial heat carrier temperature in the D1 section within 250-400°С reduces a total duration of the pellet drying. Albeit it causes an increase of the condensate discharge onto pellet bed surface. Afterwards, it leads to decomposition of the pellets and reduction of the pellet bed gas permeability. Only by increasing the heat medium rate through the pellet bed (filtration rate) one can decrease the pellet drying time significantly and thereby decrease the amount of the condensate both in the pellet bed, and on the surface of the latter. The special calculations have been performed to analyze the regularities of the reversible drying process. Numerical simulation showed that there is an optimum value for the length of the first drying section. This value ensures the minimum total duration of pellet drying in a straight-grate (conveyor-type) indurating machine. Drying sections with updraft and downdraft are interrelated, and therefore the optimal sizing of the first section depends highly on the numerous factors.