Predicting sodium release in recovery boilers in conjunction with CFD furnace modelling

Reliable prediction of sodium release from black liquor char is necessary to model fume formation and its consequences in the recovery boiler furnace. Here, a new method for modelling sodium release, which is based on the idea that release is locally possible only if the equilibrium partial pressure of NaOH on the surface of burning black liquor char is greater than the partial pressure of NaOH in the bulk gas flow, was implemented in VTT's recovery boiler CFD model and used to simulate a boiler which was previously the object of an extensive measurement campaign. Results were compared to those obtained with an earlier method, which relies on tuning of the overall release to the actual performance of similarly operated recovery boilers. The new method provides a more realistic prediction of the spatial distribution of sodium release in the furnace, and the predicted total release is well in line with the results of the earlier method and thereby with the level actually observed in recovery boilers. Therefore, the new method, which appears to be capable of predicting sodium release accurately as a function of furnace operating parameters, should prove to be a useful tool for assessing heat transfer, corrosion, fouling phenomena, and other processes in recovery boilers.