Comparison of three combustion models for simulating anode baking furnaces

Carbon anode blocks used in the Hall-Héroult process for primary aluminum production have to be baked up to 1100 °C in dedicated furnaces. These furnaces are equipped with burner ramps to heat the air circulating in the flues at 1200 °C, so that the anodes reach the required temperature. It is therefore mandatory to include the heat provided by the burners in a numerical model of an anode baking furnace. In this work, we modeled the heat input at the burners in three ways: the Eddy-Dissipation model, the Mixture Fraction/PDF approach and a simplified approach consisting in injecting an equivalent calorific value at the burners' inlets. Results obtained with the first two models are very similar in terms of anode baking prediction but slightly different in terms of flame temperature prediction. Results obtained with the simplified approach show that the model can replace combustion model to predict anode baking, but calibration of boundary conditions is necessary in order to match more elaborate combustion models. The importance of other elements of the model in the flue channel of the furnace has been verified: radiation (cannot be ignored, large influence on the spatial temperature distribution), heat transfer due to species diffusion (negligible influence on the baking, but slight effect on flame shape and temperature), and buoyancy (no significant effect on the results in the furnace firing sections).