Use of a Computer Model for Evaluation of Combustion and No2 Control Alternatives in a Kraft Recovery Boiler, 1998 International Chemical Recovery Conference Proceedings

A three-dimensional computer model to simulate the flow, heat transfer, combustion and NOx emission in a kraft recovery boiler has been developed. In this study, the model is used to evaluate the combustion and NOx control alternatives in a utility kraft recovery boiler in Sweden. The air system of this boiler features a rotation firing configuration in the lower part of the furnace and double overfire air in the upper part. The predicted results from two cases are presented and discussed. The discussion focuses on the application of air staging for NOx reduction. It is shown that NOx emissions can be effectively reduced by double overfire air. The NOx model employed in the recovery boiler model considers NO formation from fuel-NO and thermal-NO mechanisms. The formation of fuel-NO and thermal-NO are calculated using different turbulence-chemistry interaction models. The Eddy-Dissipation Concept (EDC) model is used to calculate the formation of fuel-NO because the Damköhler number is high. A study of the Damköhler number has shown that the formation of thermal-NO is a slow reaction compared to turbulent mixing. Thus, a presumed PDF model is used to calculate the mean reaction rate for thermal-NO formation. This work demonstrates that the developed recovery boiler model can be used as a predictive tool to improve the operation and design of kraft recovery boilers.