CFD simulations of steam cracking furnaces using detailed combustion mechanisms

Abstract

A three-dimensional mathematical model has been developed for the simulation of flow, temperature and concentration fields in the radiation section of industrial scale steam cracking units. The model takes into account turbulence–chemistry interactions through the Eddy Dissipation Concept (EDC) model and makes use of Detailed Reaction Kinetics (DRK), which allows the detailed investigation of the flame structure. Furthermore, simulation results obtained with the EDC-DRK model are compared with simulation results obtained with a simplified model combining the Eddy Break Up (EBU)/finite rate formulation with Simplified Reaction Kinetics (SRK). When the EBU-SRK model is used, much faster fuel oxidation and products formation is predicted. The location of the peak temperature is shifted towards the burner, resulting in a smaller flame and the confinement of the combustion process into a smaller area. This is most likely because of the inherent deficiency of the simplified model to correctly predict the overall (effective) burning rate when the turbulent mixing rate and the reaction rate are comparable. It is shown that when neither the “fast-chemistry” nor the “slow-chemistry” approximation is satisfied, the overall burning rate is overpredicted. The smaller flame volumes obtained with the EBU-SRK model have important effects on the predicted temperature distribution in the furnace as well as on other significant design parameters like the refractory wall and tube skin temperatures. It is suggested that more sophisticated turbulence–chemistry interaction models like the EDC model and more Detailed Reaction Kinetics should be used for combustion modeling in steam cracking furnaces under normal firing conditions.