Low NOx combustion utilising a Coanda ejector burner.

Current and future pollutant enussion legislation calls for decreased NOx
emissions from combustion systems. A review of techniques used for NOx
abatement led to the choice of combustor redesign to be the most cost effective
method available. This led to the design, construction and development of a
combustion system that utilised a Coanda ejector to generate recirculation of
the exiting high temperature combustion products to mix with the air supply.
Cooling of the burner was integrated into the design through the use of the air
and fuel supplies.
Computational fluid dynamics was used to model and aid development of the
design. The model was used to predict NOx and CO emissions and the fuel-air
mixing pattern. This, along with an analysis of experimental results and
observations led to an understanding of the burner operation with respect to
pollutant emissions and stability.
NOx emissions from the Coanda burner were found to be lowest when using a
0.2 mm Coanda gap width, resulting in 16 ppm NOx being emitted at an air to
fuel ratio of 1.5. However, the use ofa 0.2 mm Coanda gap width required an
air supply pressure of up to 4 bar. The use of a 0.5 mm Coanda gap width
enabled burner operation at lower air supply pressures. The resulting NOx
emissions were measured as 23 ppm at an air to fuel ratio of 1.I, with a
corresponding exit gas temperature of 2200 K.
Flue gas recirculation quantity, flame stability, flame stabiliser shape and
operational limits proved to be inter-linked in the reduction of NOx emissions.
It was found that fuel-air mixing was controlled by the entrainment properties
of the Coanda ejector and the flame stabiliser. The average oxygen
concentration entering the combustion chamber when using a 0.2 mm and 0.5
mm Coanda gap width was 13.7 % and 16.6 %, respectively. Due to the
position of the fuel injector, a fuel rich region formed behind the flame
stabiliser. With a suitable flame stabiliser geometry and the use of 'fingers',
low NOx combustion and flame stability was achieved near stoichiometric
conditions.
It was shown that the design of the burner enabled very low pollutant emissions
near stoichiometric conditions, resulting in high exit gas temperatures.
Conceivable applications of this type of burner could lie in small and
intermediate furnaces where low NOx emissions are required. Additionally,
very high temperature applications, such as glass furnaces could benefit in both
cost and pollutant emissions from such a burner.