Combustion System Modelling of a Cracking Furnace to Reduce NOx Emissions

摘要

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 "TALuft" respective to NOx emission (100 mg/Nm3 equiv. 0.058 #/MM BTU). The burners hadshown good emission performance in the test facilities of the burner supplier. However, atstart-up the emissions were significantly above the permitted levels. Shut down of thefurnaces 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 testfacilities and the furnace installation at site. It could be shown that the high NOx level was dueto 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 soonas the NOx predictions could qualitatively be re-produced at the furnace, additionaladaptations at the burner tile were investigated. A promising forecast was simulated for acontraction of the recirculation channels within the burner tiles; BP and Linde decided to applythis approach to one of the furnaces during a scheduled furnace shut-down.The combination of both, adapting the burner tips and contracting the recirculation flowpath solved the NOx problem and has been applied to all five furnaces in 2008. The NOxfigures recorded at the furnaces are now in the range of 75 – 90 mg/Nm3 (equiv. 0.04 – 0.052#/MM BTU).