(207e) Furnace Firing Control: The Key to Extending Your Runlength

Control and regulation of furnace firing is essential to extend the runlength of new and existing furnaces. For several decades now, computational fluid dynamics (CFD) has been used for the design, troubleshooting and optimization of steam cracking furnaces. It has e.g. been shown to be a valuable tool to detect flame-rollover ¹ and evaluated burner designs ². Nowadays advanced temperature control is possible using BASF’s Exactus® optical thermometers. On the other hand the Zolo Technologies’ ZoloSCAN-ETH allows to obtain more uniform temperature profiles across the furnace using its innovative laser-based combustion diagnostic system. This system simultaneously measures temperature, O₂, CO and H₂O in real-time, directly in an ethylene cracking furnace along multiple laser paths set in critical areas of the furnace. In this work the principles of these technologies are assessed using furnace CFD simulations. The latter requires among others accounting for the detailed geometry of the burners. These details are often omitted in industrial ethylene furnace simulations ^3-5, although they are accounted for during burner design. Also the so-called “shadow effect” ⁶ arising from the projected shadows between adjacent reactors leads to significant heat flux non-uniformities. This phenomenon is important but its effect on product yields is often ignored as only a single reactor is simulated. Lastly the non-gray, composition-dependent radiative properties of the flue gas have to be properly accounted because this phenomenon can lead to temperature underpredictions of more than 100 K ⁷. Therefore a “nine-band model” is developed from the exponential wide band model (EWBM) ⁸ and implemented in a user-defined function for FLUENT 14.0. The model’s validity was tested against Leckner’s correlation ⁹ for the simulation of a small test furnace. It was then used as a non-gray gas radiative property model and compared to the routinely-used gray gas implementation of the Weighted Sum of Gray Gas Model (WSGGM).

Different CFD simulations have been carried to assess all these assumptions and how these results can be used to significantly extend the runlength of an existing USC furnace. For example the simulations show that the conventional gray gas implementation of WSGGM underpredicts the flue gas outlet temperature by about 70 K compared to the non-gray gas radiation model, resulting in a 3.6% higher thermal efficiency. For the USC furnace simulated in this work, shadow effects cause a maximum difference in COT of 29 K and a difference in propene-over-ethene of 0.1 between two different U-coils in the furnace. In order to obtain more uniform TMT’s, COTs and olefin yields for the individual reactors, three different furnace optimization methods were compared and their results will be discussed. Suggestions are made on how to apply the optimal furnace/reactor design using advanced furnace control technology.

Acknowledgment

This work was carried out using the STEVIN Supercomputer Infrastructure at Ghent University, funded by Ghent University, the Flemish Supercomputer Center (VSC), the Hercules Foundation and the Flemish Government – department EWI. The financial support from the BOF Bilateral Scientific Cooperation (ECUST/LCT), the Long Term Structural Methusalem Funding by the Flemish Government, the ‘111_ Project by the Chinese Government, and the China Scholarship Council (CSC) are acknowledged. CMS acknowledges financial support from a doctoral fellowship from the Fund for Scientific Research Flanders (FWO).

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