3D-DEM-CFD simulation of heat and mass transfer, gas combustion and calcination in an intermittent operating lime shaft kiln

Abstract This work presents DEM-CFD simulations of the transient processes occurring in an industrial scale PFR-kiln. DEM allows the numerical simulation of the moving and reacting limestone bed in the kiln and is coupled with a 3-dimensional CFD simulation describing the interstitial gas phase. A PFR-kiln consists of two vertical shafts and a connecting crossover channel. The two shafts periodically switch their function at regular intervals of about 15 min. While one shaft calcines the product in parallel flow with gas temperatures above 900 °C, the other preheats the stones in counter flow. The model has been applied to an industrial PFR-kiln of 18 m height. A realistic particle size distribution of the limestone with particles ranging from 50 to 90 mm has been set. Methane combustion provides the heat for calcination and is simulated by a two-step mechanism. Simulation results show a nearly uniform temperature distribution in the calcination zone but significant inner particle temperature gradients. The calcination degree depends on the particle location within the kiln and decreases towards the outer kiln walls. Measured and simulated temperatures are compared. Maximum temperature values as well as its characteristic oscillation induced by the periodic kiln operation could be reproduced by the simulation, especially keeping in mind the difficulties of thermocouple measurements under the harsh conditions in industrial reality.

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