(136b) Combustion System Modelling of a Cracking Furnace to Reduce NOx Emissions

Linde was contracted to design and erect a new furnace section by BP in Gelsenkirchen. One of the key drivers for the project was compliance with the German clean air regulation "TA Luft" respective to NOx emission (100 mg/Nm3 equiv. 0.058 #/MM BTU). The burners had shown good emission performance in the test facilities of the burner supplier. However, at start-up the emissions were significantly above the permitted levels. Shut down of the furnaces to remedy was not an option due to high cost of lost production. A CFD model was set up and tuned to understand the differences between the test facilities and the furnace installation at site. It could be shown that the high NOx level was due to a "flame roll-over" which, in turn, was caused by excessive internal flue gas recirculation. Simulations showed that changes in burner tip drillings would provide improvement. As soon as the NOx predictions could qualitatively be re-produced at the furnace, additional adaptations at the burner tile were investigated. A promising forecast was simulated for a contraction of the recirculation channels within the burner tiles; BP and Linde decided to apply this approach to one of the furnaces during a scheduled furnace shut-down. The combination of both, adapting the burner tips and contracting the recirculation flow path solved the NOx problem and has been applied to all five furnaces in 2008. The NOx figures recorded at the furnaces are now in the range of 75 ? 90 mg/Nm3 (equiv. 0.04 ? 0.052 #/MM BTU).