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Reducing NOx Emission from Gas-Fired Furnaces
Posted Wed July 31, 2002 @09:08AM
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Application By Sudarshan Kumar, CGPL, IISc, India

One of the oldest and finest establishments of its kind in India, the Indian Institute of Science (IISc) has grown into a premier center of research and advanced instruction. Its Combustion Gasification and Propulsion Laboratory works mostly on fundamental research in combustion and its applications in industry.

A recent research project focused on investigating the effects of flow recirculation on the efficiency of a gas furnace, the goal being to optimize the position of the fuel nozzles. CFD simulations were conducted in parallel with experimental work on a typical preheated-air furnace.


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The laboratory-scale furnace consists of a cylinder in which preheated air enters from the bottom and fuel is injected from four nozzles concentric to the air jet. The furnace outlet is also at the bottom to aid the re-entrainment of combustion products into the inlet streams. The resulting dilution of fuel and oxidizer reduces the reaction rate and makes the whole combustion process mixing-independent.

For the CFD modeling we chose CFX-TASCflow because of its advanced reaction modeling capabilities. Using CFX-TASCflow's Flamelet combustion model we could investigate the influence of the recirculated combustion products on the temperature distribution, NOx emissions and heat transfer characteristics.

The preheated air reduces the fuel consumption but, because of the resulting higher flame temperature, also increases the NOx emissions. We used CFX-TASCflow to study many design variations and to optimize the position of the fuel injection such that the combustion proceeds very slowly in the furnace. This produces very small temperature gradients and fluctuations, and consequently, the NOx production rates are 10-20 times lower than with conventional combustion techniques. This behavior was subsequently verified experimentally on the laboratory-scale furnace.

Our future plans are to develop a burner that simultaneously achieves good performance in terms of fuel consumption, heat transfer and NOx emissions. Since chemistry and fluid dynamics both play an important role in the pollutant formation, temperature distribution and heat transfer characteristics, all the features need to be examined for an efficient design.

Test furnace
Laboratory-scale furnace.

Test furnace
Temperature distribution on the adiabatic furnace, Nox emissions, Vector Plot and velocity distribution.

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