Heat transfer mechanisms in an indirectly heated rotary kiln with lifters and its role in scaling

Abstract

This present research aims to obtain a fundamental understanding on solid transport, solid mixing and the complex heat transfer mechanisms related to the important installed segmented lifter in an indirectly heated rotary kiln. To accomplish these objectives in a systematic manner, the experimental and modelling studies on solid transport and mixing were first carried out in pilot-scale cold kilns at Curtin University of Technology, Perth, Western Australia then followed by heat transfer study in a pilot-scale hot kiln system available at ANSAC Pty Ltd, Bunbury, Western Australia.For design and scaling purposes, dimensional analysis was carried out. A series of experiments in cold kilns were also carried out, considering lifter design, lifter configurations, helix, a wide-range of solid and various kiln designs under different kiln operating conditions. The results showed that a flat bed depth profile can be achieved using purposely-designed segmented lifters at favourable practical low ReѠ values (less energy input to drive the kilns and more throughputs), with a low total kiln filling fraction and a high degree of axial mixing (Pe < 50 or Dz = 10-5 - 10-3 m2/s). This is essential to good heat transfer performance. The effects of helix, L/D, Fr and d/D have relatively insignificant differences on solid transport and mixing under the current experimental conditions. These findings demonstrate the unique advantages of the purposely-designed segmented lifters compared to other conventional lifters (e.g. single throughout lifters), providing important information for scaling criteria and modelling work.A preliminary DEM simulation, as an emerging simulation tool at a particle level, confirmed that axial displacement is mainly due to the function of the folded lifter sections of the segmented lifters. The folded lifter sections push the solids towards the kiln discharge end along the bed arc length and such effect increases as Reincreases. This finding leads to the development of a transport model, limited to underloading regimes, to predict the average bed depth in this type of kilns. The model predictions are in good agreement with experimental data. A set of global power-law dimensionless empirical correlations on solid transport and mixing, applicable to all three (over-, design-, underloading) regimes, were also developed based on the data obtained from our systematic experiments. The validity was tested with data presented in selected previous studies. It is found that the developed correlations are largely specific to the present lifter design and configurations.A steady-state axial heat transfer model has also been developed for a kiln with segmented lifters. The model predicts the temperature profiles at inner heat tube, in the freeboard gas, in the bed, on the tube wall and the outer heat tube in the flue gas. The model incorporates developed solid transport and mixing correlations, as well as suitable heat transfer modes and reaction model. It takes the form of ordinary differential equations which were solved numerically. The input data necessary for the model were obtained by our own experiments and/or extracted from the literature. The model was validated by the temperature profiles obtained from the hot kiln. Among the heat transfer modes considered, it is found that the limiting step of heat transfer in the kiln is the heat transfer from covered inner kiln wall to covered bed, which is highly influenced by solid transport and mixing. Under the current experimental conditions, the typical overall heat transfer coefficient was found to be 31 - 35 W/m2.K.The present research advances the fundamental understanding on solid transport, solid mixing and the complex heat transfer mechanisms in an indirectly heated rotary kiln with segmented lifters. The obtained data and knowledge are important to improve the kiln energy efficiency, reduce kiln manufacture and operation costs and widen the kiln applications at different scales.