8 - Heat Transfer Processes in the Rotary Kiln Bed

Abstract

The combined radiation and convection heat transfer is the total heat input into the bed. A concern arises regarding what happens to the energy after it is directed to the bed. This chapter reviews the heat transfer processes in the rotary kiln bed. A complete evaluation includes the subsequent distribution of energy within the bed where the temperature dependent process takes place, at least for materials processing. The computational fluid dynamics (CFD) modeling of pulverized fuel combustion shows that the temperature in the flame region, one meter around the centerline, does not dramatically differ from the centerline temperature. The assumption that the freeboard is well mixed is applied to rotary kilns and allows the use of one-dimensional modeling of the axial temperature. The IFRF CEMFLAM modeling of rotary kiln flames is based on one-dimensional representation. By assuming that the bed is well mixed and axially moves in plug flow, the axial gradients of bed temperature and gas temperature could be related to the local rates of gas-to-exposed bed and wall-to-covered bed heat transfer by ordinary differential equations. This allows one to establish a representative “averaged” bed temperature at each axial location.