Comparison of Different Global Reaction Mechanisms for MILD Combustion of Natural Gas

Abstract

Emissions of nitrogen oxides from fossil fuel combustion are a major environmental problem because they have been shown to contribute to the formation of acid rain and photochemical smog. MILD (Moderate and Intensive Low oxygen Dilution) combustion is a promising technology to decrease pollutant emissions and to improve combustion efficiency. A combination of air preheating and fuel dilution with combustion products of low oxygen concentration are the main features of this technique. In MILD combustion mode, preheated air and fuel are gradually mixed with large amounts of recirculated exhaust gas.

The objective of the present work is to investigate a global reaction mechanism for natural gas combustion to predict the observed nitrogen oxide and carbon monoxide levels in MILD combustion mode. For this purpose, the comparison of several global reaction mechanisms during the combustion process is paramount. The interaction between turbulence and chemistry is modeled by an advanced Eddy Dissipation Concept (EDC) in order to compare some global reaction mechanisms. For validation purposes, this model is applied to a non-premixed turbulent jet flame of a pilot methane-air mixture (Sandia flame D), and to the semi-industrial scale experiments at the IFRF Research Station. The modeling results are discussed and compared with measurements.

Keywords:

• CFD-Modeling
• Global reaction mechanisms
• MILD combustion
• NO emissions

The financial support of the German Academic Exchange Service (DAAD, Germany) is gratefully acknowledged.

Notes

x _k  = mole fraction of species k (in kmol/kmol).