Computer simulation played a key role in solving a difficult combustion problem in a vertical cylindrical furnace used in an oil refinery. Computational fluid dynamics (CFD) simulation by Tulsa-based John Zink LLC helped identify the solution.
As part of a retrofit, low-NOX burners, arranged in a circle, were fitted in the furnace floor. Each burner has both primary and secondary stage fuel injection ports. A draft tube, concentric with the burner block, is used to improve flow uniformity. Once retrofitted, it was apparent from observing the furnace that the flames were coalescing into a single flame, producing a large ball of fire with high temperature gradients.
The burner interactions significantly increased the length of the flame. Engineers wanted to make the temperature distribution more uniform by separating the flame into its individual components.
Engineers at John Zink created a CFD simulation of the original design. Zink engineers used the model to look at the flow velocities, pressure distributions and chemical species concentrations predicted by the simulation. Following an analysis of the model, the engineers identified a high proportion of unburned fuel within the circular area formed by the burners, indicating poor fuel and air mixing.
The unmixed fuel did not burn until it had migrated toward the end of the flame, creating the flame definition problem. Based on this insight, John Zink engineers reduced the amount of fuel and increased the air provided to this area to achieve a balance. They accomplished this by biasing the primary and secondary fuel injectors.
Engineers showed the CFD results to the refinery personnel, who approved the new design. The new design worked as predicted in the CFD model, reducing flame length to the desired levels and lowering NOX emissions to acceptable levels.