Dynamic CFD Modelling of Calcination in a Rotary Lime Kiln with an External Dryer

Abstract
Mid-kiln ring formation is a problem in lime kilns, which may be related to the fluctuations of
the position of the start of calcination. Therefore, a dynamic 2D axisymmetric CFD gas model
with a methane burner implemented in ANSYS Fluent, coupled by mass and heat balances to a
1D bed model, is developed to calculate the fluctuations in bed and gas temperature profiles
within a lime kiln with an external dryer. The dynamic model is used to calculate changes in the
location at which calcination starts, with fluctuations in operational conditions using data from a
pulp mill.
The model simulates radiative, convective and conductive heat transfer between the gas, wall,
and bed to determine the axial bed temperature in the kiln. The calcination reaction is described
using a shrinking core model, that allows for the prediction of the location at which calcination
begins and the degree of calcination achieved. The solid motion within the kiln is modelled using
Kramer’s equation, modified for transient response. Steady-state and dynamic simulation results
are compared to data from an industrial dry lime kiln, and good agreement is found. A sensitivity
analysis is also presented, to obtain insight on how operating conditions and model variables
impact the calcination location and degree of calcination. Of the variables examined, the fuel rate
and the feed temperature have the largest impact on both the calcination location and degree of
calcination in the kiln. Model predictions of a period of ring formation in the industrial kiln show
that the start location of calcination fluctuates by more than 2 m on either side of the mean of
regular operation, warranting further investigation of the importance of these fluctuations on
mid-kiln ring formation. 

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