Optimizing air distribution in floor and wall burners of an industrial steam cracking firebox: a CFD study

A 3D computational fluid dynamics model for an industrial scale steam cracking firebox is combined with a 1D reactor model for the cracking process within the reactor coil. This framework is validated using industrial tunable diode laser absorption spectroscopy (TDLAS) measurements in a naphtha steam cracking furnace. Industrial O2, H2O, CO and temperature data show good agreement with simulation results at different locations within the firebox. The influence of varying the primary-to-secondary air flow distribution to the floor and wall burners separately is investigated using the validated framework. Altering the primary-to-secondary air flow distribution of the floor burners from 75/25 to 25/75 results in a 66% increase in furnace run length and a 35% decrease in NOx formation. This shift in air ratio ensures a lower rollover height of the flame and a large recirculation zone of the flue gas, spreading the radiative heat flux on the reactor coil more evenly. Altering the primary-to-secondary air flow distribution of the sidewall burners from 50/50 to 100/0 results in a 13% increase in run length and a 30% decrease in NOx formation. More primary air towards the sidewall burners is found to lead to more evenly spread out flames along the wall, resulting in flame interactions between the different burners and a combined flow towards the center of the firebox. These interactions result in a more uniform tube metal temperature profile along the different passes of the coil.