Use of CFD in Evaluating Pyrolysis Furnace Design

This paper examines the usefulness of CFD modeling in evaluating burner and furnace performance in new or retrofit pyrolysis furnaces. A CFD model of a new furnace is often the first evaluation of how the furnace will perform when integrated with burners and process coils. Model results allow burner and furnace vendors to review performance and identify any sub-optimal design elements before construction or retrofits.

 The paper includes a brief review of the CFD model capabilities required for accurate ethylene furnace simulation, followed by three examples of furnace simulations drawn from a decade of modeling experience demonstrating how predictions have been used to identify and improve sub-optimal designs. Examples will include:

 1)      Guidance for John Zink ultra-low NOx premixed burner arrangement in a Technip furnace. Model results were used to position burners to improve uniformity of heat flux profiles and minimize burner- burner interactions. Predictions were compared with test furnace data from John Zink’s Test Center. Predicted heat flux profile (for the process furnace) compared well with the measured flux profile (in the test furnace). CFD results predicted NOx levels at the furnace exit of 15-16 ppm (dry 3% O₂) whereas test furnace measurements showed NOx levels of 17-21 ppm (dry 3% O₂).

2)      Assessment of burner-burner spacing and burner-end wall spacing on flame behavior. Initial designs showed burners that were spaced too far from end wall resulted in sub-optimal flame profiles and heat flux profiles. Adjusted burner spacing improved heat flux profiles.

3)      Assessment of three burner retrofit options in a large ethylene furnace. Objective was to improve flame quality and maintain acceptable heat flux profiles and low NOx emissions over original burners.  Three burner designs from three different burner manufacturers were evaluated.