Coupled three dimensional DEM–CFD simulation of a lime shaft kiln—Calcination, particle movement and gas phase flow field

In lime shaft kilns the limestone is heated in counterflow with the flue gas from multiple burners, commonly fired by fossil or alternative fuels. After preheating, the limestone enters the calcination zone with gas temperatures above approximately 900 °C. To achieve high thermal efficiencies and calcination degrees, a homogeneous gas flow through the packed bed is essential. Spatial pressure differences and energy sinks and sources resulting from the calcination process and the fuel combustion make it even more difficult to predict the actual three-dimensional temperature and flow distribution in the kiln. Furthermore the particle size distribution is important for the system design, because limestone with larger size needs significantly higher residence time in the calcination zone for full conversion. This work presents the application of a novel 3-dimensional particle-mechanics based, numerical tool on an industrial scale. The tool allows the simulation of a moving and reacting limestone bed in a shaft kiln coupled with the 3-dimensional CFD-simulation describing the interstitial gas phase. Convective heat transfer between gas phase and particles, radiative and contact heat transfer between particles as well as calcination reaction are accounted for. With this combined approach the calcination of the limestone bed can be simultaneously described on the system scale while resolving detailed thermo-chemical processes on the particle scale.