Fire but no Flame, FLOX®
A new combustion system "Flameless Oxidation", FLOX®, developed by WS, is attracting a great deal of attention from industry and scientific research. At temperatures above 850°C, thermal NOx is drastically lowered by internally premixing combustion air and exhaust gas and avoiding temperature peaks of a flame. This is even true at high air preheating temperatures.
Lowering the NOx-emissions was and is one of the greatest challenges for modern burner development. Espacially at high temperatures and efforts to increase combustion efficiency by air preheat, peak temperatures in the flame can cause enormous NOx-eemissions.
With flameless oxidation (FLOX®, registered trademark of WS Wärmeprozesstechnik GmbH, Renningen) it was possible, to suppress thermal NO formation even at very high air preheat temperatures. Basic knowledge about flameless oxidation was obtained during expensive research programs and results were converted to burners which can be applied safely and reliable in various applications. Up to day, many hundred FLOX® burners are in operation and their percentage of total sales increases steadily.
Difference between konventional combustion with flames and flameless oxidation will be explained in the figures below.
The following figures shows calculated temperature fields of a burner that is operated in flame- and FLOX mode under same conditions. The large temperature differences in the burner near field are clearly visible. The missing high temperature peaks are the main cause for low NO in FLOX mode. Outside the burner near field, the temperatures are almost identical.
(calculated with PHOENICS CFD code)
The photographs show a test furnace being heated up in flame mode (left photo). After switching to FLOX® mode, there is no visible reaction zone. The fuel is burnt inaudible and invisible.
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In the next figure, the temperatures along the burner axis are shown and also compared with thermocouple measured data. The large difference in temperature can be clearly seen. NOx emissions dropped from 160ppm in flame mode to 6 ppm in FLOX-mode. Air preheat temperature was 650°C.
The following figures show data from laser diagnostics. The upper figure were taken from a turbulent premixed flame, while the others show FLOX combustion. On the left side, temperatures are shown with temperatures, ranging from 300 K to 2100 Kelvin (see scale at the bottom). On the right side, OH distribution is shown. In the middle, line plots for each combustion mode are shown.
The differences are very obvious. Steep gradiants and large temperature and concentration difference are characteristic for flame combustion whereas FLOX combustion is much more homogenious.
(Source: Laseroptical Investigation of Highly Preheated Combustion with Strong Exhaust Gas Recirculation, T. Plessing, N. Peters, J.G. Wünning, 27th International Symposium on Combustion, University of Colorado at Boulder, 2.-7. August 1998)
Recent tests have shown the potential of flameless oxidation for a variety of fuels like heavy oil and even coal. Very promising results were also achieved in the area of gas turbine combustion.
Literature:
(in alphabetical order)
Coelho P.J., Peters N., Numerical Simulation of a Mild Combustion Burner, Combustion and Flame, Volume 124, No. 3, Combustion Institute, February 2001
Plessing T., Peters N., Wünning J.G., Laser Optical Investigation of Highly Preheated Combustion With Strong Exhaust Gas Recirculation, Twenty - Seventh Symposium (International) on Combustion / The Combustion Institute, 1998, pp. 3197-3204
Telger R., Roth W., Betriebserfahrung beim Einsatz von Brennern mit flammloser Oxidation, Gas Wärme International 44 (1995), Heft 7/8, S. 323-337
Wünning J., Flammenlose Oxidation von Brennstoff mit hochworgewärmter Luft, Chem.-Ing.-Tech. 63 (1991), Nr. 12, S. 1243-1245
Wünning J.A., Wünning J.G., Brenner für die flammlose Oxidation mit geringer NO-Bildung auch bei höchster Luftvorwärmung, Gas Wärme International, Band 41 (1992), Heft 10, S. 438-444
Wünning J., Flammlose Oxidation in Strahlheizrohren, 16. Deutscher Flammentag, VDI Berichte 1090, 1993, S. 487-494
Wünning J.G., Flammlose Oxidation von Brennstoff, Dissertation, Aachen, 1996
Wünning J.A., Wünning J.G., Flameless Oxidation to Reduce Thermal NO-Formation, Prog. Energy Combust. Sci. Vol. 23, Seite 81-94, 1997
Wünning J.G., Flameless Combustion in the Thermal Process Technology, 2nd International Seminar on High Temperature Combustion, Stockholm, Sweden, 2000
